CROSS REFERENCE

10001} This application claims the benefit of the filing date of U.S. Provisional Patent Application ^' Serial Mo. 62/256,286, filed November 17, 20! 5, which is hereb incorporated b reference in its entirety.

{00021 This invention was made with government support under grant number ί 335927 awarded by National Science Foundation. The government has certain rights in this invention,

FIELD

{0003} The current disclosure relates to enhanced poo! boi ting: mierostrueture with nucleating regions (NRs) bounded by feeder channels (FCs) such that the nucleating regions are separated by a suffici nt distance to provide separate liquid: and vapor pathways*, which distance is typically within a range of the bubble departure diameter.

BACKGROUND

{00041 Passive boiling heat transfer enhancement offers attractive cooling possibilities hi high powered electronic systems. The present miniaturization trend in microelectronic devices and ME S technology demands effective thermal management. Pool boiling has long served as a means to dissipate large heat flux over small footprint which has led to development of enhanced heat transfer surfaces for a wide range of applications for example,, power generation,, refrigera tion, air conditioning,, petroeheroi cab chemical,, pharmaeeuti eat. and process industries. The main objecti ve of enhanced surfaces- i s to reduce wall, superheat and increase -critical heat flux which offers enhanced performance over a wide operating range. Improvement in heat transfer will result in lo wer sixes of equipment being used, higher efficiency and reduced consumption of fuels. A typical pool boi ling perf nnance is characterised, by the plot of heat flux versus wall superheat. The degree by which the curve shifts to the left, and also higher critical heat flux (CHF) value depicts the extent of enhancement compared to a flat surface _* Thi heat transfer can be enhanced either by using active devices like ultrasonic vibrations, electrostatic fields etc, or passive techniques like porous/rnicroporous ; surfaces, stroetured surf ces like open microchannels (microgrooves), finned or knurled surfaces, and the like. SUMMARY

fOOOSJ In accordance with; ope aspect ^' _: o,f -Jhe- i$ _. c-l0sure.., there is provided a boiling heat- transfer unit incl ding a substrate having a heat exchange region including a plurality of nucleating regions adjacent to feeder channels, wherein adjacent nucleating regions are separated by the feeder channels at a distance of between about 1 mm to about .10 mm, wherein the width of the ^' nucleating region is a distance within a range from about. μ« to about . ^' mm*. the: width of the feeder channel is a distance within a range from about 5 pm to about 4 mm, the depth of the feeder channel is from above -0 -nirrt to about 10 mm, whereby Vapor formed in the ^' nucleating ^' regions is moved away from the nucleating regions influencing liquid flow through the feeder channels towards the adjacent nucleating region thereby establishing continuous self-sustaining separate vapor and liquid flow pathways increasing heat t ansfer due t developing region heat transfer in the feeder channels and enhancing overall boiling performance. The thickness of the walls separating individual FCs, also referred to as fin width, is from about 100 pm to about 2. mm. These dimensions are determined from structural integrity, -fin efficiency and overall hydrodynamic and thermal field considerations.

{0006} In accordance with another aspect of the disclosure, there is provided a boiling heat transfer unit including substrate? having a heat exchange region including a plurality of nucleating regions adjacent to feeder channels, wherein adjacent nucleating regions are separated by the feeder channels -at ,a distance of between: about SO percent to about 2.00 percent of an estimation of the departure bubble diameter, whereby vapor formed in the nuc leating regions is moved away from the nucleating regions influencing liquid fl w throngh the feeder channels towards the adjacent nucleating region thereby establishing continuous self-sustaining separate vapor and liquid flow pathways increasing heat transfer due to developing' region heat transfer in the feeder channels and enhancing overall boiling performance.

|Ό0Θ7| In accordance with another aspect of the disclosure, there is provided a boi ling heat transfer unit including a substrate having a heat exchange region including a plurality of nucleating regions adjacent to feeder channels, wherein adjacent nucleating regions are separated by the: feeder channels at di tance whereby vapor formed into bubbles in the nucleating regions is- moved away from the nucleating regions influencing liquid flow through the feeder channels; towards the nucleating regions thereby establishing continuous self-sustaining separate vapor and liquid pathways increasing heat transfer due to developing region heat transfer in the feeder channels and enhancing overall boiling performance.

[0008] These and other aspects of the present disclosure will become apparent upon a review of the fij l wing detailed description and the claims appended thereto,

BRIEF DESCRIPTION OF THE DRAWINGS

{0009} Fig. 1 is a drawing of various embodiments incorporating the Nucleating Regions with Feeder Channels (NRFC) configuration;

[0010} Fig. 2 is a series of drawings representing mechanisms showing separate liquid and vapor pathways in accordance with heat exchanger configuration embodiments;

[0011] Fig. 3 is a series of 3-D drawings of the NRFC heat exchanger with varies height configurations of Nils and FCs;

[00121 Fig, 4 is a dra g, of a pool boiling experimental setup tor water

{001S| Fig. 5 is a drawing of various test chips with the NRFC configuratio with feede spacing;

{0014} Fig. 6 is a drawing of a schematic of a heater section;

j [0015 J Fig. 7 is a graph of the variation in uncertainty with heat flnx for NRFC test surfaces;

{ 0161 Fig- 8 is a graph of the temperature distribution at different heat fluxes;

|0017] Fig. 9 is a graph showing the pool boiling comparison of NRFC-4.5 and NRFC- 1.6;

[0018J Fig 10 is a graph showing the pool boiling curves for NRFC-4.S, RFC-3, NRFC- 2.1 and NRFC- 1.6;

{0019\ Fig. 1 1 is graph showing the heat transfer performance obtained with RFC-4,5, NRFC-3, NRFC-2.1 and NRFC- 1.6;

[0020} Fig. 12 is a graph showing the effect of nucieaiio region channel width n the pool boiling performance;

[00211 Fig. 13 is a graph showing the effect of feeder channel width on the pool boiling performance;

[0022] Fig. 14 is a graph showing the comparison of pool boiling curves obtained with different mkrostrnc tore, enhancements; [00231 ί 15 is a graph, showing the comparison of heat transfer performance obtained with.- differen rnierastriiottire -enhancements;

| 024| Fig. 16 is a graph showing the hysteresis curves for NRFC-3, 2.1 and 1.6;

[0025} Fig. 17 is a graph showing the normalized poo! boiling- curves- ^' for NRPC-4, 5,

NRFC-3, NRFC-2.1 and NRFC- 1.6:

{002.61 Fig. 18 is a. series of images showing the bubble sequence from nucieatiori to departure for NRFC-2.1 configuration;

[0027 } Fig, i 9 is a graph of the CHF trend obtained using NRFC configurations;,

[0028} Fig. 20 is a graph showing the pool boiling tests with FC-87 for NRFG .5, NRFC-

3, NRFC-2.1 and NRFC-1.6; and

{0O29J Fig. 21. i a graph showing the comparison of pool boiling curves with FC-87.

DETAILED DESCRIPTION

[0030] The present disclosure relates to a heat transfer system including a substrate for liquid pool boiling. The substrate can be any suitable material for boiling applications.,, including copper, copper alloy, aluminum, steel, nickel, titanium, alloys, silicon, germanium, or a composite:: of d ¾re t ^: n¾at als--.iflcludi :,films _^ The heat transfer system in accordance with the present disclosure includes a substrate for liquid pool boiling having a heat exchange region in which heat is transferred betwee the substrate and a fluid in eoninjunieation with, the substrate. The substrate can include a planar surface, curved surface, tubular surface, sloping surface or eombi nations thereof.

[0031 j A boiling system includes a heat exchange region in contact with the boiling liquid. The heat . exchange region includes cavitie at which nucleation occurs in the liquid,

preferentially when the surface temperature is greater than the saturation temperature of the liquid. In an embodiment this is accomplished by selectively suppr ssing nucleating cavities by returning liquid either flowing along the surface: of the substrate or impinging on the surface of the substrate from the bulk liquid. Nucleating regions can be clearly defined b adding nucleating cavities in the selected regions. The current disclosure describes heat exchange region having a network of nucleating regions and adjacent feeder channels in contact with the boiling liquid. [00321 At the beginning of the- boiling process, bubbles nucleate randomly over the heat exchange region before a spatial ordering of liquid and vapor prevails for the remainder of the operating range. The continuous self-sustained spatial ordering of separate liquid and vapor flow paths is reached when the preferential sueieatiou ccurs through cavities located n the nucleating region through which vapor is removed, while liquid feeder channels adjacent the riueleatisg region .promote liquid transport towards the nueleaiing region. The liquid flow within individual FCs provides a cooling effect. This cooling effect combined with the liquid velocity suppresses nueleafion in the FCs. The spatial ordering is promoted by the efficacy of the heat exchange region having a designed network of nucleating regions and adjacent feeder channels to establish continuous an ^' self-sustaining separate liquid and vapor pathways.

100331 As: vapor hubbies depart from the nucleating regions vapor pathways are created, liquid is returned to these regions along the heated substrate surface creating liquid ^{' '} pathways. ^' In one embodiment a nucleating region is designed as a lengthwise strip having feeder channels on each side along the length of the nucleating region. As the liquid flows over the substrate ^■ returning liquid enters the feeder channels and flows towards the adjacent. nucleating. reg on Jjroni the sides of the lengthwise nucleating region strip.

[Q03 J The system is designed sueh that the nueleating region has nneieation. cavities which turn superheated liquid contacting this region into vapor. The nucleated vapor bubbles move, nto the nucleating regions and depart: from the surface. The departed bubbles create liquid motion over the adjacent regions. The vapor generation rate increases at higher heat fluxes which amplify the effects due to increased liquid velocity created over the adjacent regions.

003S A feeder channel is defined as a region on the substrate ^' made-up -of ^' walls ^' or ^' fin-like structures capable Of having liquid flowing through the open feede channel flow area formed by them. The feeder channels direct the liquid flow towards the nucleating region during pool boiling. The feeder channels in an embodiment are open microchannels. Feeder channels may tafce other and the like. The returning liquid flowing in feeder channels experience developing hydtodynanrie and thermal bou dar 1 avers. The short flow distances in such deyelopingrregions leads to enhancement in heat transfer. This enhancement may be higher than the enhancement provided by the growth and departure of bubbl es in thei immediate vicinity sometimes ^referred to as an influence region. This presents an enhancement scenario wherein the CHF is increased as compared to the CHF obtained on a plain surface. Surfaces incorporating mierostrnetures described herein are also: able to enhance flow boiling, heat transfer when the flow does not Ave sely affect formation of continuous self-sustained separate liquid and vapor pathways.

[0036] The s eeific: eometricaI arrangement of nucleating regions and adjacent feeder channels leads to nucleation over a substantial portion of the substrate surface at the start-up or low heat flus.es closer to the onset of nucleate boiling conditions:. As the heat flux, increases the returning liquid through the feeder channels suppresses nucleation in the feeder channel regions. The nucleating region produces bubbles: from cavities located: in these regions. The base of the feeder channels adjoining the nucleating region provides additional cavities due to typical manufacturing processes. Such cavities: occur naturally at comers.. Nucleation cavities may also be intentionally introduced in the nucleating regions by using techniques such as but not limited to etching; porous coating, laser drilling, mechanical abrasion, and the like. Some of the bubble which may he generated in the feeder regions, experience the flow of returning liquid which, is cooler than the liquid adjacent to the bubble. This flow suppresses bubble growth and leads to suppression of nucleation in the feeder channel regions _* A few bubbles may. still grow In the feeder channel regions due to local variations in the thermal field. However, the overall effect of such babbles m y not be significant .in ^■ ..reducing the performance. The introduction of reiiirning liquid can cause enhancement of heat transfer due to one of the different effects such as jet impingement, flow: through open iiiieroeiiannels, flow through an array of raicroscale features, developing flow through the feeder channels, and disruption of hydrodynamle and thermal boundary layers in the liquid flow throug¾ the feeder channels by additional mieroseale fe tures.

[0037 j In an embodiment, a heat transfer system includes a heat transfer substrate having a heat exchange region and enhancement nnerostfuctures inclndlng a nucleating region ^' (NFCJ feeder channels (FC) such that the NR channel are bounded by feeder channels. The corner regions at the intersection of PCs and channels serve as nucleation sites arid the NR channels serve as preferential vapor removal pathways with subsequent liquid, addition through the PCs. (0038} The FCs can he fabricated by conventional milling machine, Computer Numerical Control (CMC) .machining, embt ssing: ieehnic|ue, powder compaction, bonding, electrical discharge ^■ machining (EDM), raicrofabrieation techniques such as deep reactive ion etching (DRIE), laser ablation, and the like, or any other additive or suhtraciive technique _* . [00391 The ^' PCs can be planar, curved, rad al, sloping or any suck configuration thai promotes: lif aid transport towards the MR channels..

[004(1} The FCs can be coated further with nnero/ ^' narso structures thai promote liquid transport alone. Shortmnowlres promote wicking and enhance liquid: transport, which is desirable, whereas tail nauowires result in bunching and act as rmcleation sites, which is

undesirable for FCs, The selection of niicro/nan structures when used is also important to maintain bubble departure diameters close to the desirable range obtained through experimental observations or theoretical calculations such as specified: in equation (1).

[0041} MR. channels can be fabricated using CNC milling, laser ablation, conventional milling machine, GNG machining, embossing technique, powder compaction, microf¾bricatiori techniques such as DR1E, laser ablation, chemical etching, sintering, electroplating, and the like.

[0042 } The width of the NR channel can be between about 5 μηι t about 4 mm , Moreover, it is determined through experiments wi th water that a width of 500 μηι can be selected. The width selection depends on the considerations s uch as: (1) the generation of adequate number of bubbles: to induce the liquid motion through, the feeder channels, (b) avoiding using much. larger widths that would reduce the heat transfer coefficient ove longer developing region flow lengths under the curren mechanism, of separate liquid and vapor ath ays, a»<d (e) reduction k the area fraction available in the PC region since this region in general provides a more efficient heat transfer mechanism as compared: to that in the NR regions.

{0043J Microstructures are arranged such that there is spacing between the structures t permit eonveetive liquid flow aboui me stmetures induced by bubble departure from the Mils. The channels with nucleating regions, referred to as NR channels, provide the same function as nucleating regions. This spacing between multiple NR channels can be estimated usin -available departure bubble diameter equations or by experimentally measuring the departure diameter. j©044| The width of the PC channel can be between about 5 μ«α to about 4 mm . The length of the FC can be froin about 1 mm to about 10 mm- The width of the microchannel wall can be from abou 100 μηι to about 2 mm. The length of the FC channels can be smaller or larger for different fluids, such as refrigerants or any other boiling liquids, depending on their respective, departure bubble diameters. These diameter can depend on the specific geometry of the heating surface with microsiritetures and also the heat flux, levels. The bubble departure diameter for a particular geometry and fluid combination may be obtained from the experimental or theoretical considerations. An optimum spacing is determined ^' from the heat transfer in the developing region of the feeder channels along: its , length. As an example, it is estimated, to be about 50 percent to 200 percent of the theoretically estimated departure diameter. In many cases, experimental determination may be

best performance.

j 0045J The heat transfer aieehanistn- in the FC is enhanced: by the entrance regi on effect. Similar effect is seen when the liquid jet impinges on the heated surface in the FC region. When the liquid enters the open channels the liquid: front is ^■ assumed to have uniform velocity. As it advances in the PCs. the liquid contacting the wails conies to a complete stop which induces high shear stress. This layer further causes the adjacent layers in the liquid to slow down as a result of friction between the layers. To maintain a constant mass flow rate inside the channel the velocity reduction at the walls is compensated by the liquid speed up in the center of the channel. These two effects induce a velocity gradient from zero; velocity at the wall which gradually develops to the center of the channel as the liquid advances. The region of the channel from entry of the liquid to the development: of the velocity ^' profile towards the center is called the hydrodynamic entrance (developing) region which dictates the liquid flow in the FCs of the NRFC heat exchangers. Simi lar effect is observed for the temperature profile, with a .very high heat transfer coefficient in this region, which decreases as the flow length increases. The liquid transport in the feeder channels may he assisted by the. capillary' flow when tire channel dimensions are small enough to provide a capillary effect. The liquid flowing toward the heater surface may travel laterally over the feeder channels before t¾ri»g ^' theSe:thaime ^: lS ^''' arid -flowing towards the R. channels. Another mechanism for bubble motion emerging from the NR is present, when the bubble motion, is dictated by the evaporation momentum force. The bubble .ebullition cycle is goveraed by evaporation momentum force ( EMF). The asymmetric temperature conditions in the region containing- the enhancement structure influences the bubble trajectory in such a way that the bubble moves towards the NR. This sets up the eonveeiive flow such that the vapor generated, on. the heat exchange region is removed through the NR channels with subsequent liquid addition to the NR through the FC.

{00461 The current disclosure deals with boiling liquid contacting the heat exchange region and turning into vapor. A nucfeation site is defined as the location, of bubble origination, In one of the embodiments of the NRFC configurations, the intersection of the FCs and NRs serve as preferential nueleation sites, in another embodiment, porous structures are incorporated to promote nueleation in the NR.

[0047 j The FCs can be coated with any structure that promotes heat transfer and liquid transport. Hydrophilic coatings will further enhance liquid wetting and delay HF. When nanowires are coated it is advised that their heights remain small such that they promote liquid transport only and not se ve as nueleation sites.

[0048J This disclosure deals with boiling of liquids wherein bubble nueleation occurs and is accompanied with transfer of heat from a surface to the: hailing liquid.

[0049} A microstructure is defined as a structure generally from about 1 micrometer to about 4 millimeters in one of its dimensions on the heat transfer srirfeee, it also -forms part f the heat transfer surface. It includes, but is not limited to microchannels, pin fins, and elevated or depressed regions. The FC can be flush with the heat transfer surface, or It may he elevated or depressed from the heat transfer surface.

{0050| Wall superheat is defined a the difference between the substrate surface temperature and: the saturation temperature of the liquid. H eat transfer coefficient is defined as the ratio of heat flux dissipated by the substrate to the wall superheat. The CHF condition is initiated by a blanket of va or over the heater surface: preventin the liquid from coming in contact with the heater surface. Under constant heat flux heating condition, reaching CHF leads t -a rapid increase in heate sarfaee tetnperature and may cause thermal damage, to the surface {also known as burnout). In a boiling curve, the wall superheat is plotted on the x~axis against the heat flux on the y-axi:s. CHF represents the highest heat flux beyond hich an increase in heat flux is accompanied with a Significant rise in wall temperature and a dramatic reduction in heat transfer coefficient as compared to the boiling prior to reaching the CMF condi tion. Vapor blankets the heater surface at and beyond CHF condition.

[0051 j The channel dimensions of bot the N channels and FCs have an important role to play in determining the pool boiling performance, The channel dimensions: affect the liquid and vapor transport in fundamentally different ways. Nueleation region includes the areas near corners between the NR. and adjacent FC regions. Liquid flow paths along: the heat exchange regions occur through the FCs. Having a very wide FC reduces the available nueleation sites due to the: reduced areas of the corners between the NR:;and EC regions, and also ..reduces: the heat transfer coefficient and available surface area from the walls or fins forming the channels. whereas reducing the channel width increases nueleaik sites but may affec the liquid transport to the .n.ueleation sites. The NR channels ar hou d by PCs which influences the liquid from, th bulk to enter the PCs and feed to the nudeation sites. Liquid is supplied to the nueleation sites t roug the FCs and also due to the counter flow influenced by the chaotic liquid vapor disturbance. In other embodiments, nucleating may occur over the entire NR and liquid flow toward the nueleatifig cavities in the NR regionls affected by the width of the NR channels. {0052} The spacing between two adjacent NR channels also i ays an important role in enhaneing the erfr3rniaii.ce. Making the spacing too small or too wide in comparison -to the departure bubble diameter will deteriorate the performance. Small spacing might disrupt the cOi ective flow as the chaotic nature of boiling at high heat fluxes causes the vapor to b removed in different pathways than desired. A wide spacing may not set up the desired liquid- vapor pathways and sometimes may result In uhdesired nueleation in the PC.

[00531 In another enibqdinient, the bubble escape pathway is m the vertically upward direction from the NR from a horizontal heat exchange surface in a gravity driven system.

However, the bubbles may be removed under the influence of any other force away from the nueleation sites in this or other orientations such that the do not interfere with the convective liquid flow pathways in the FC.

[0054} In another embodiment, the bubble motion is influenced by the evaporation rnornentum force. This force arising due to specific, microstracture ^' ■geometry or due to asymmetric temperaiure conditions on the heat exchange region of the microsiruetures causes the bubble to ^■■ move towards the NR. regions. At higher heat fluxes this force dominates -the mechanisms such that both the CHF and HTC at CHF is enhanced by about 50% or more when compared to a heat exchange region without any mierosiriieiures, coating or any structure that improves the pool boiling performance.

{0Q55J In ar»othei embodiment, the CHF is enhanced at least by 100 percent and the HTC at 80% of the CH F is enhanced by at least 100 percent.

(0056} In another embodiment, the CHF is enhanced at least by 100 percent and the HTC at 80% of the HF is enhanced by at least .150 percent..

|00S7] In another embodiment, the CHF is enhanced at least by 200 percent and the HTC at 80% of the CFfE is enhanced by at least 50 peree . {0058J In another embodiment, the CHF is enhanced at least by 200 percent and the HTC at 80% of the CHF is enhanced by least 100 percent.

[0059J In another embodiment, the CHF is enhanced at least by 200 percent and the HTC at 80%· of the CH F is enhanced by at least 150 percen t,

[0060J In another embodiment, the CHF is enhanced at least by 100 percent and the HTC at 80% of the CHF is enhanced by at least 100 percent.

{0061} in another embodiment, the CHF is enhanced at least by 100 percent and the HTC at 80% of the CHF s enhanced by at least 150 percent

[0062} In another embodiment, the CHF is enhanced a least by 200 percent and the HTC at 80% of the CHF ts enhanced by at least 50 percent.

[0063} In another embodiment, the CHF is enhanced at least by 200 percent and the HTC at S0% of the CHF is enhanced by at least L00 percent.

[0064} In another embodiment, the CHF is enhanced at leasi by 200 percent and the HTC at 80% of the CHF is enhanced by at least 150 percent,

[00651 la another embodiment, the CHF is enhanced .at leas by 100 percent and the HTC at 80% of the CHF is enhanced by at least 50 percent.

j [Q0661 in another embodiment, the CHF is enhanced at least b 100 percent and the HTC at 80% of the CHF is enhanced by at least 100 percent.

[00671 Irs another embodiment, the CHF is enhanced a least b 100 percent and the HTC at 80% of the CHF is enhanced by at least 150 percent.

[00681 Ih another embodime , the CHF is enhanced at least by 200 percent and the HTC at 80% of the CHF is enhanced by at least 50 percent.

[0069} in another embodiment, the CHF i enhanced at least by 200 percent and the F!T at 80% of the CHF is enhanced by at least 100 percent.

[0070] In another embodiment, the CHF is enhanced at least by 200 percent and the HTC at 80% of the CH F is enhanced by at least 150 percent.

[0071} In another embodiment, the CHF is enhanced at least by 100 percent and the HTC at 60% of the CHF is enhanced by at least 50 percent,

[0072} In another embodiment, the CHF is enhanced at least by 100 percent and the HTC at 60% of the CHF is enhanced by at least L00 percent. |0073] In another embodiment, the CHF is enhanced at least by 100 percent and the HTC at 60% of the -CHF is enhanced by m least 150 percent.

[0074] In another embodiment, the CHF is enhanced at least by 200 percent and the HTC at 60% of the ..CHF is enhanced by at least 5 percent,

{ 75J In another embodiment, the CHF is enhanced at least by 200 percent and the HTC at 60% of the: CHF is enhanced by at least 100 percent.

{0076} In another embodiment, the CHF is enhanced at least by 200 percent and the HTC at 60% of the CHF is enhanced by at least 150 percent

[0077} The desired spacing betwee adjacent nucleating regions separated by feeder channels is approximately equal to the departure babble diameter, in one embodiment, due to other effects such as local fluid Oow fields around growing bubbles, the desired spacing between adjacent nucleating regions separated by feeder channels may be between 50 percent smaller to 200 percent larger than the departure bubble diameter. As the feeder channel length increases, the heat transfer rate in the feeder channels decreases due to the increasing lengths of the developing regio in the FCs,- This may also be used in deciding the appropriate spacing between the NR channels. The desired spacing between the NR. channels will depend on the liquid properties and: the bubble nucleation characteristics. The nueleation, characteristics will al o depend on the fabrication method employed, in an embodiment, it is demonstrated thai, for water at atmospheric pressure the Fritz equation can be employed to determine the desired spacing : between the NR channels. For other liquids, the departure bubble diameters can be found from experiments and applied t : calculate th desirable spacing between th NR. channels oh the heat; exchange region. These equations provide guidance for determining the desired spacing. At higher heat fluxes, including hear ^' CHF conditions, the ^' bubble departure diameters are different due to rapid growth and departure. This value may be different from the theoretical estimation of the departure dianieter. in ah embodiment a spacing range from 50 percent id 20 percent of the estimated departure bubble diameters can be used, The spacing may he smaller if the bubbles: leave rapidly, or may be larger due to bubble coalescence.

1 81 I».. nother embodiment the enhancement structure can include plurality of NR channels separated by the bubble departure diameter. The separation is incorporated with liquid feeder channels thai enhance the heat transfer performance. The number of NR channels is determined by the length and width span of the heat exchange region. It is preferred- that the NR channels ^' be -bounded by the FCs to permit continuous self-sustained eonvective flow of liquid and vapor in separate path ways. The: R in another embodiment may be formed as isolated regions from other NR separated by adjacent FCs.

$ 79} Fig. i illustrates- .ait example: of various...embodiments of MRFC configurations placed on a heat exchange region. Fig. 1 (a) shows a microstructore with the NR placed

hor^zontally^and the. FCs -placed, perpendicular o ^: ihe R channels. Fig. -1- b shows the FCs placed horizontally and the NR perpendicular to the FCs, Fig. 1 (c) shows the NRFC placed on the -outside .-su faee ^' of a-tubular stmcfure. Fig, 1. (f) shows a radial arrangement of the NRFC configuration. Fig. 1 (d) and (e) shows the NR channels as a rectangular and triangular groove with FCs running between them.

|0t 80] A heat exchange system including niicrochannels as feeder channels presents unique opportunities to enhance the perfbrmanee. In accordance with the present disclosure, when bubbles are formed. at the intersection of NR ehanneSs and FC they influence the liquid transport through the feeder channels which sets up the eonvective flow in that fashion

permitting: self-sustained separat liqu d and vapor flow fields over the enhanced; geometry. {OOSI J In another embodiment, the substrate can be the outside surface of a tube subjected to liquid boiling over it The: outside tube surface is covered with raicrostraetures composed of NR and FC such that the NR channels are separated by the bubble departure diameter and are bounded by the feeder channels.

{0082J Fig. 2 shows an embodiment of a mechanism to enhance performance by providing separate liquid and vapor pathways ^' , (a Schematic view, and (h) 3-D: view. Fig. 2 illustrates an: embodiment with the schematic representation of the mechanism prevalent in the NRFC configuration. The FCs and NR channels are fabricated on smooth surfaces such that bubbles are removed through the NR channels as a result of evaporation momentum force while liquid supply occurs through the FCs. Evaporation momentum force appears On a liquid- apor interface as liquid evaporates into vapor which moves away from the interface at high velocity. The spacing between the NR channels is approximately equal to bubble departure diameter, which may be determined through equations known to those skilled in the art or can also be obtained ^• experimentally; The Fig. 2 identifies the bubble removal pathways through the ^' NR. channels- with liquid supply in both, the vertical and lateral directions through the. FCs, [00831 in one of the embodiments, NRFC configuration is developed on a copper heat exc an e region. The NR channels and FCs: are anufactured using CNC .milling process. The feeder channels can be considered as open: microehannels that influence liquid flow through capillary ; motion towards the preferred nueiea ion sites located at the intersection of channels and FGs.

j 0O8 J The heat exeh arjge region is composed of MR channels arid FCs, The NR eharffisls and FC can be on the same plane on the heat exchange region or they can be elevated or

depressed with respect to each other. The preferred, depth of the FC and NR channels is estimated to be in the range of about 0 to about 10 mm. In another embodiment, the preferred depth is from about 1 jim to about 10 μηα. For example, the NR channel can have a depth f 8 mm while the FC ca be 6 mm deep or the N channel can be 6 mm deep while the FC can be 8 mm deep. These confi urations are aeceplable as long as the arrangement permits self-susiaihed convective flow of liquid through me FCs and bubble departure through the NR eharmels,

{00851 Fig- 3 shows various depth configurations of NRFC heat exchangers. Fig. 3 illustrates the possible variations of tbe _: depth of NR channels and FCs _: such the preferred vapor removal pathway occurs through the NR channel and liquid addition through the FC. The possibilities are illustrated: with respect to the bottom plane: of the NR and FC cSiaanels.. Fig. 3(a) shows the NR. channels depressed relative to the feeder channels. Fig. 3(b) shows the FC

channels depressed with respect to the NR channels. Fig, 3(e) shows the NR channels and FC in the same plane with respeci to each other.

[00861 The preferred width of the FC and NR. is estimated to be i the range of about 5 : urn to about 4 mm.

{00871 The preferred spacing between two FCs, !.¾,, wall widths can be in the range of about. 100 pm to about 2 mm,

[00881 Liquid re-eire lation in microch nnels is ah important factor to enhance

performance and delay CFfF, In order to delay CHF and facilitate continuous liquid supply, disjunctions in the fins are proposed to serve as vapor removal pathways as shown in Fig. 2.

[00891 Furthermore, the bubble departure diameter is an important parameter in. enhancing; the performance which facilitates separate liquid- vapor flow fields. The bubble mergence in the lateral, and vertical direction at higher hei¾ .fluxes can be identified as the chief eontribtuhig factor for the vapor blanket to appear on the heater surface. It. is postulated that for water boiling on a copper heater surface at atmospheric conditions the feeder channel bank width on the heat exchange u fa e should approximately be equal-to: the departure bubble diamieter.. Th ;: Fritz, equation, relates the contact angle and surface tension forces to the bubble departure diameter as, d _of = 0.0208ff L . ^σ _λ A ^X h

where _¾ . fi is the receding contact angle,, σ is the surface tension force, gis the gr vitational fo ce and pi and p _G are the liquid and vapor densities respectively. The departure bubble diameter (d _ft p is calculated using eq. (1) and is equal to 2,213 rnrrt f r β -42.5 ^' °, σ ^; ~ 58.91 * \ ®~ ^~ N/ for water at 100 °C. The theory provided by Fritz is generally seen to be valid but there are deviations observed in experimental data.

10090 J A hea transfer surface made o nucleating regions on a heated substrate, subjected to pool boiling of a liquid over it, and liquid feed regions on the heated surface directing liquid toward the ^■■ nucleating region is in vestigated for. pool boiling, enhancement. Nucleating: region includes channels incorporating nueleation cavities, either ^■■ artificial or naturally occurring from a maniiiactnring process, and liquid feed region includes feeder microchannels in one of its embodiment. The spacing between the nucleating regions is a factor that k important in the boiling performance. In ¾ne of i ts embodirne ts, whe¾ the spacing between adjacent nucleatin regions is in the vicinity of the bubble departure diameter, either estimated using a theoretical equatio or Measured from experiments, it provides : a higher degree of boiling perfonftance ehhancement. This spacing avoids vapor flow from being blocked by opposing liquid flow and leads to a high critical heat fluxes, The liquid is fed to the nucleating regions through the feeder channels. This results in separate liquid pathways toward and vapor pathways away from the nucleating regions. The boiiing process behig chaotic, other liquid vapor pathways, such as liquid flowing aloug the heater surface it in the nucleating region, o liquid flowing directly from the bulk into the nucleating channels in some areas is also present to a varying degree. Similarly, some degree of nueleation may als be: present, in areas other than the nucleating regions, including the feeder regions. However, ovemli effect is to enhance the critical heat flux through the boiling process based o the main mechanism of separate liquid-vapor pathways provided by incorporating the above structure. The heat transfe coefficient is als improved ove a plain surface dne to this enhancement mechanism,- [00931 Liquid re wetting en a heated surface is a factor in delaying critical heat flux (CHF) for enhancing pool boiling performanc Pool boiling enhancement is achieved by ^■■ providing: improved liquid supply pathways to rmeleatiors sites. Heat transfer surfaces composed of

Nucleating Regions/ ith Feeder Channels (NRFC) was experimeiitally investigated, with distilled water at atmospheric pressure. The bubble departure diameter is used as guidance in designing the s acing between ^' the nucleating channels on a copper chip. CHF of 394 W/ern ² at a wail superheat of 5.5 °C with a heat transfer coefficient (HTC) of 13 kW/rr °C was obtained with: a feeder bank width of 2.125 mm and a nucleating channel widt of 0.5 mm. High speed images suggested thai separate liquid and vapor pathways were the driving mechanism. Such arrangements of NR and FGs in hniltiple networks can lead to CHF enhancement between 50-250% and a simultaneous heat transfer coefficient enhancement between 50-1000%.

[0092} Self¾ustained separate I iquid-vapOf - athways are Men tsfied as enhanced

mechanisms which can be viewed- in high speed images of boiling indicating vapor leaving the surface from the NRs and the liquid from the bulk approaching the surface toward the NRs through the FCs in separate pathways.

[0093] The disclosure will be further illustrated with reference to the following speci ic examples. It is- understood that these examples are given by way of illustration and are not meant to limit the disclosure or the claims to follow.

10094} EXAMPLE 1 :

[0095]: In an embodiment of the enhancement structure, NRs and adjacent FGs are

lafcrteatsd i& t¾eiJitate-Se^aifateli¾rad and vapor pathways, as shown in Fig. 2. Disjunctions on an open microchannel surface with feeder channel bank width equal to the departure bubble diameter is proposed as a means to enhance performance by minimising bubble coalescence in the lateral direction. These disjunctions added to an open microchannel can be considered as ucleating Regions with Feeder Channels (NRFC). The nucleating channels will serve as preferential, vapor removal pathway with liquid addition through the niicrochannels.

[0096} Fig. 4 shows the schematic of e pool boiling test setup for water. The bottom garoliie plate is composed of a ceramic chi holder to hold the heater surface over which a quartz glass water bath measuring 14 mm x 14 mm x 38 mm is assembled by means of 4 stainless steel socket bead. cap screws; A glass water hath is used chiefly to aid visualization and is sealed to the ceramic chip holder by means of a rubber gasket w hich co vers the area outside the boiling surface. A -.reservoir water bath is mounted between the middle garolite plate and top aluminum pi¾te. The water reservoir is sealed wit rubber :ga$ksts o ither side to ensur against leakage at ail tiroes. The top aluminum plate is provided with two circular openings for the saturation thermocouple probe and a 6 ^" 0~VPC,: 200 W auxiliary: cartridge: heater to maintain, wate n the reservoir at saturation by boiling continuously.

j 0O 7J The bottom section of the setup consists of a 120- V C,- 4 κ 20 W capacity cartridge heater inserted into a copper heater block similar to Kalani and Kandlikar. The copper block is composed of a. truncated portion measuring 10 mm κ 10 mm 40 mm that fits int the groove on the bottom side of the ceramic chip holder. This ensured, that 10 nim x 10 mm surfac of the heater is in contact with the test chip which also has a base section measuring 10 mm x 10 mm which facilitated I D conduction f om the heater to the test chip. A grapfoil® paper attached to the heater surface ensured good contact between the test chip and the heater block.

Additionally, the copper block is housed on a ceramic sleeve to minimize heat losses. Four compression springs supported the bottom aluminum plate that provided the required degree of movement to establish contact between the test chip and the heater and to accommodate for any expansion during the testing. A- shaft ^: pin . {3 ^: /8'-" diameter) connected the middle garolite plate, bottom garoHte plate and the work desk which ensured stability of the setup.

[§098J A National. Instnnrients cDaq-9172 data acquisition system with Ni-9213 temperature module was used to record the temperature. A LabVIEWVR virtual instrument displayed and calculated the surface temperature and heat flux.

(00 9! ¾ ^e test section used in this stud was a 17 mm. x 17 mm x 9 mm. as shown is Fig. 6. The heater side consisted of a 10 mm 10 mm x 9 mm protrusion with three 0.56 mm holes drilled- 3 mm apart to accommodate the thermocouples. The effect of contact resistance in the heat flux calculation is eliminated by holes drilled in the test chip. Three K-type thermocouples are inserted into these holes to read the temperature at different location on the chip.

00100] The location of the channels (channel width ^~ 500 μ,ιη) are as shown in Fig 5. On the 10 ram ¾< 10 mm boiling surface, spacing of 4.75 mm, 3 mm, 2,125 ram and i,$ mm are machined such that the NR channels (channel width ^! ~ .500 μηι) are bounded by FCs< NRFO-2. is expected to have the highest performance in the test surfaces investigated here as it is in close correspondence to the bubble departure theory discussed, preyi ously. This departure diameter was estimated by using Fritz equation (Eqn. 1 ). NRFC-4.5, 3 and 1 ,6 contribute in identifying a trend in the performance providing further insights into the self-sustained separate liquid-vapor pathway mechanisms.

[OOIOIJ Fig. 5 represents various test chips with NRFC configurations. Fig. 5 shows an assembly of the heater section. The mt fl» ^' ¾: ^: ih!6-tes - ^: seetiaft was calculated using ID

conduction equation where, the temperature gradient άΤ/dx was calculated using the three point backward Taylor's series approxi inati on where, Γ ₁ , Τ _{2 ι} Τ ₃ are the temperatures corresponding to the top, middle and bottom of the test chip under study.

{00102] The boiling surface temperature was obtained by using eq. (2) and is given by, where, T _waj i is the boiling surface temperature and xj- is the distance between the boiling surface and thermocouple ¾ which i§ equal to 1 ,5: mm.

[00103] Fig. 6 shows a schematic of the heater section. In this study, heat exchange surface wit nucleating regions and feeder channels is tested for its pool boiling performance . This configuration has not been investigated in pool boiling with water and FC-87 as the working liquid. The NRs and FGs were fabricated using CN€ machining on the central 1.0 mm κ 10 mm area of the boiling surface of the chip. Table 1 shows the microchanoel dimensions used in the study after measuring under a confbeal laser scanning microscope. The pool boiling performance was obtained to study the effect of feeder spacing, nucleating channel width and feeder channel width.

{00.104] Bias and precision errors are the two main errors that arise during an

experimentation process. Errors from calibration and sensitivit are the mai source of bias and precision errors respectively. Ι00Ϊ Θ5] The general expression for the bias and precision errors are. where, U _y is th uneeminiy or error, iSlhe ^' bi- s error arsd Py s the precisio error.

|00106| Individually, each error that propagated dim to measurement i temperatiire (both heater and surface temperature) and also distance between thermocouple ^' spacing are calculated using the equation below, where U _p is the uncertainty in the parameter p, and i½ is the uncertainty Of measured

parameter %, The uncertainty n the heat flux can thus be expressed by the following equation.

[00107 _. 1 Fig. 7 shows the variation of heat flux with uncertainty. A maximum uncertainty of 40% is observed at a low hear flux but this value plateaus to less than. 5% at higher heat fluxes which is the main region of interest in this study.

[00108] Fig. 8 shows the variation of temperature alon the length of the. thermocouples. Based on Fouriers's law of conduction, the temperature profile across the test section is expected to be linear, The temperature distribution: plot for 39 W/em2, 100 W/crh2 and 171 W/cr 2 showed linear progression with. R, quared value close to ,1 which ensured minimal heat, loss during the experimental process.

[00109] Fig. 8 illustrates- ' em e ature distribution at differen heai flux value. The entipj test setup was assembled and distilled water was tilled n both the water bath and reservoir and visually inspected for leakage. The auxiliary heater ;and the cartridge: heater were power d b two independent power supplies. The power was increased in periodic intervals once the working liquid (distilled water) attained saturation temperature. Data was recorded at each interval when th¾: thermocou le flu&iiiat on a¾ not greater than. ±0.1% ^: f¾>r duration of approximately I Cf min.

[00110] Pool boiling experiments were conducted with distilled water at atmospheric pressure for the test matrix shown in Table t , To serve as a baseline for enhancement

comparisons distilled water was boiled over a plain surface. This chip reached a CHF of 128 W7crrr at a wall superheat of 20 ^:0 G. Three sets of tests are conducted on the enhanced surfaces to study the (i) effect of feeder channel spacin (or feeder bank widths) (ίί) effect of nucleating region channel width, and { iii) ef ect of feeder channel widths.

Table 1: Nucleating channels microchannel dimension

(190111] The results are presented in ^■ terms of a pool boil ing curve w hich depleted the heat, flux dissipated, at a certain wall superheat. The heat flux is computed individually wing both the projected area and normalized surface area of the test chips. The wall superheat is defined as the difference in temperature between the surface of the "chip and saturation temperature of the fluid (distilled waier). The surface temperature is calculated as the temperature at the fin tops of the configuration. he heat transfer performance curve which is a plot of heat transfer ooe eient versus heat flux revealed the heat dissipating capability of the chip compared to a plain chip.

[0011 ] The plain chip hat! a toughness of 1.2 m which, was measured using a cohfoeal laser scanning microscopy. To draw a first-hand conclusion on the design of NRFC heat exchanger and to identify: its boiling potential, NRFC-4,5 was boiled in distilled water to draw an estimate on the degree of enlianeenteni and the boiling curves are shown in Fig. 9. The RFC- 4.5 chip showed sigftifieant enhancement with a CHF' of 350 W ern2 at a wall superheat of 13 ^Q C as against a plain, chip which had a CHF of 128. YY7cro2 at a wall superheat of 20 ¾ which corresponded to an enhancement of 182% in CFIF. The results suggested that the design of the heat exchanger resulted in, increased CHF with a simultaneous increase in MTC. [00113] Fig. shows a pool boiling comparison ^■■ between NRFC-4.5 and plain chip, 10011 ] Effect of Feeder Spacing

{00I15J The objective of this section was to identify the effect of feeder spacing on the pool boiling per ¾OT¾ee, Four chips identified: as NRFC-4.5, MRTC~3: _: ¾1RFC-2J , were tested for their pool boiling performance as shown in Fig 10. The nomenclature for naming the surfaces was; such that the beat exchanger (NRFC) was followed by the feeder spacing distance: in mm. The analysis is targeted at identifying the optimum spacing between feeder banks to enable separate liquid-vapor athways.- The test chips ere pushed to CHF to determine the maximum performance of these surfaces from which the operating range of the heat exchanger can be established.

100116] NRFC-2.1 dissipated 394 W/cm ² at a wall superheat of 5.5 °C which translated to an enhancement of 217% in. CHF compared to a plain surface. MRFC-4.5, 3 and l .& had CHFs of 349 W/em ² . 285 cm ² and 252 W/cm ² at wall superheats of 13 C, 1 1 °C and 1 ,9 ^C, respectively. NRFC-t .6 with the lowest CHF of 255 W/em ² amongst the enhanced surfaces was comparable to the best per rming chip reported, by Coofee and Kandlikar with a CHF of 244 W/cnt , From a simplistic approach, although the area enhancement was reduced compared to open microehannelSj the performance was enhanced which ^suggested that self-sustained separate-liquid vapor pathway was the driving mechanism here. The results are supplemented with high speed images which reveal bubble dynamics in the enhanced configuration at Sow heat fluxes which is described in the discussion section.

[001171 Fig. 10 shows boiling curves for plain, and nucleating channels niieroehannel.

surfaces with water at atmospheric pressure with fin top temperature.

{ Οί 18] Fig. 1 1 illustrates heat transfer coefficient comparison for plain and nucleating channels microchannel surfaces using fin top temperature. Fig. 11 shows the variation o HTC with heat flux. Similar trend t Fig. is observed in HTC At CHF, HTG of 267 KW/tn2 ^¾ C-, 257 kW/m2°C and 168 li W/m2°G is observed with; NRFC-4.5, 3 and 1.6, respectively. NRTC~2. I had the best performance with a FITC of 713 kW m^C representing an enhancement of 996% in HTC at CHF over a plain, chip. Table 2: Test matrix and results

[001191 Effect of R Channel Width

(00120] Fi g . 12 sho ws th effect of nucleating- region channel width on the pool boiling performance, of NRFC-2, 1 -configuration. Since this configuration offered the highest CHF, the effect of nucleating region width and feeder channel width were studied on this geomein'. Three channel wi dibs - 300 μιη, 500 μ ^η ¾ 762 μηι were chosen based on the ranges reported: in literature. The 300 μηι, 500 μνα, 762 μ.π» NR. channel width surfaces had a CHF of 199 W/cm2, 394 W/crr and 212 ^" W/crfi ^* . respectively. The 500 μιη channel width surface is. borrowed front the previous test matrix to facilitate better understanding of the trend. The main observation in this pirn is that an optimal MR channel width exists for an NRFC _^ S. heat exchanger. A drastic reduction in CHF is observed for a narrow (300 μη ) and a wide (762 μιη). The HTC at CHF for the 300 unt, 500 μηι and 762 urn NR channel widths were 173 fcW/m^C, 713 kW/m ^¾ e and 126 ^' kW/rn ^2o C, respectively. This further ^' reinstates that the performance of NR FC is influenced by the channel dimensions which affects the. relative merits .-of the ^' liquid and vapor transport in fundamentally different ways.

[00121] Fig. 12 s ows boiling curves for plain and NRFC with three MR. channel widths ~ 300 gm, 500 μη.ι and 762 μτη with water at atmospheric pressure with 11.» top temperature. 100122] In further analyzing the trend, ^' Kandlikar reported a CHF of 30 W7c-m2 using channel width of 500 um in a. contoured fin ffiicrosirueture. The motion of the bubble on the contoured fin surface was governed by E F. This is indicative that using a 500 μηι channel width with distilled water as the working fluid seems to amplify the -performance betierthan other chamiel dimensions in the tested range for water. However it is expected that the working fluid will determine the optimal channel width. |00ϊ 23] Effect of Feeder Channel Width

|00124] The effec of feeder ch nnel wi th was also investigated, in this study and is shown in Fig. 13. Similar to the width range selected i the previous section, three channel widths 300 μηι, 500 μ-m and 762 μτϊι) w¾r« ^' ehoses io^etejtniaethe- iarfQttTisnc^. The: 300 μυι, 500 μι¾ 762 μηι feeder channel width surfaces had a CHF of 270 W/em ² , 394 W/cm ² and 205 W/cm ² , respectively. The corresponding HTCs at CHF for the three surfaces were 173 fcW n °C _J 713 kW/m2°C and 194 kW/m2°C, respectively. A similar trend in CHF is observed here were a narro (300 μήι) and wide channel (762 μ¾η) show a reduction in performance compared to a 500 μ,ηι feeder channel width surface.

| W125] Fig. 13 s ws boiling curves for plain and NRFC with three FC channel widths ~ 300 μηι, 500 μηι and 762 μηι with water at atmospheric pressure with fin top temperature. |00126| Comparison with literature

[00127] Fig, 14 shows, the comparison between the best erforating surface reported in this study (chip 3) and performance plots available in literature for other enhancement techniques, The _: CHF of the chip, reported in this study is higher than all values presented in the comparison. Fo exam le. Pistil and Kandlikar reported a CHF value of 325 W/eirf at a wall superheat of 7.3 °C..A a wall superheat of 5.5 °C .a higher CHF (394 Wi cm ² ) is observed for NRFC-2J . This surface also had a higher CHF compared to Kandlikar, however the wall superheat was slightly increased. The NRF can he, looked at as an rih ftcemΦnt ^· ^ eΐ ^ ftlicrø ^' e BneίS· ^· ·w i ^' ch has shown stark improvemeot in providing higher CHF at lower wall superheats. Furihennore, CHF reported by Mori and Okayuma and Li and Peterson are in the range of 250 W/cm* and 325 W/cm ^* respectively but at very high wall superheats, in excess of 50 °C, which is undesirable. The wait superheats for ail surfaces investigated in this study are in the range of 5.5 °C to IS ^a C A recently reported enhancement structure involving bi-eonduciive heat exchange region is also compared. This chi reached a CHI of 228: W/errr ^3' with a corresponding ETC of 2 ί 0 k m ^io' C.

[00128 ] Fig, 14 shows a comparison of best perforrning chip with other enhancements available in literature using fin top temperature.

{(Mil 29] Fig. .15 shows comparison heat transfer performance with other enhancement techniques available in literature.

[0013(1] Hysteresis study: [00131 ] A -hysteresis study was performed to understand any losses that may occu during the cool down cycle. Fi , .I _: 6:Sho s:the-:he¾:ltii?g _. -2pd. eo©ling _: #uC;leate. boiling curves: for FC-3, 2.1 and 1.6. firstly, NR.FC-3 and 1.6 were pushed to a eat flux of ^" around 230 W enr and ^• subjected to: redweiag -heatiln* to obser e any deviation in the two curves. This was done because of the design of the experimental setup used in this study. Upon reaching CHF, the setup demands that the contact between the copper heater block and test chip be removed to prevent: thermal damage of the components associated. In order to safeguard the setup, hysteresis study was conducted by pushing the chip to around 80-90% of the CH F value which was determined in the first test run. All the curves indicate minimal heat loss (hysteresis) ensuring repeatability at different ..surface temperatures within experimental errors discussed previously In the uncertainty analysis.

90:132 _: | Fig. 16 illustrates a heat transfer study to analyze hysteresis for 2, 3 and 4 nucleating channels.

100133] Normalized curves:

[00134] Fig, 17 illustrates mrrnialized pool boiling curves to show area enhancement of nucleating channels microchannel surfaces using base temperature,

100135 J Table 2 shows the surface are enhancement facto fo each NRFC configuration used in this study. The contributing factors to the area enhancements are number of channels, channel width, flu width and channel depth. The heat ftu*es ^: fo thfc ^aeel- uncle* investigation are divided by their respective area enhancement factors neglecting fin efficiency. The base temperature of the surfeee is considered while reporting wall superheat as shown in Fig. 17. NRFC -2.1 had a CHF of 1 3 W/cm ^* at a wall superheat of 9.8 ^C 'C which resulted in an enhancement of 56% in CHF over a plain surface.

(00136] Discussion

[00137] High speed images were taken ^' to facilitate ^' detailed understanding of the underlying mechanisms. Bubbles were formed ^; at the corner regions {intersections of feeder channels and nucleating channels ) on the Bottom of channels and departed from the top of the fins whic induced liquid inflow into the channels which was the chief contributor to low wail superheats and delayed CHF. Feeder channels provided better irrigation pathways to the nucleating region channels setting up. the conveeiive flow aiding in liquid re-weiting in the channels.

[00138] Visualization images: {00139] The architecture of the surface was such the feeder channels were able to continuously supply liquid to the nueleating channel regions. The liquid supply was heavil influenced by the feeder channel bank width. High speed images were obtained using a Photrou fastcam at a high frame rate of 4000 f s.

100140] The bubble nucleation and departure sequence is shown in Fig. 18(a-c) for NRFC-3, The bubbles ate seen to nucleate m the nucleating channel region before it grows to the channel width and finally departs from the fin tops of feeder channels. Fig. 18 (d) identifies additional nucleation sites thai become active inside the nucleating channels. Fig. 18 (e) shows the coalescence of bubbles in the vertical direction. Fig, 18 (f) distinctly shows separate liquid-vapor parbways. ¾por columns are observed over the nucleating channel region with subsequent liquid addition through the feeder charmel regions. In the videos captured, some bubbles were seen to nucleate inside the: feeder channels. ^" However, these ^" bubbles are proposed to -create reverse pathways in the liquid due to the increased agitation, improving the heat transfer in the region.

{00141] Fig. 18,- Bubble sequence obtained with C-3 surface, (a) A bubble nucleates inside the nucleating channel region (b) Bubble growing to channel width fc) Bubble departing from the fin top of the feeder channels (d) Additional nucleation site become active in the nucleating channel (e) Bubbles coalesce in the vertical direction (f) Distinct vapor columns in the nucleating channels and: liquid supply pathwa s ^; through the. channel regions,

100142] CHF trend;

[00143] A deflni te trend in€MF is observed from: the test surfaces investigated, i t s study. Fig. 19 shows a plot of CHF versus pitch of nucleating channels. As mentioned previously, the bubble departure diameter obtained from Fri.t¾ equation resulted in a value of 2.12 mm and NRFC-2,1 and NRFC-4,5 have bank widths that are 1 and 2 integer multiples of the bubble departure -diameter suggesting- a. trend in CHF as reported here. These value indicate that bank widths that are inulii pi es of the bubble diameter enhance the pool boils ng performance better than bank widths that are not multiples.

{00144] Fig. 1 illustrates the CHF trend in -nucleating channels microehannel showing odd number of channels performing better than even number of channels.

[00145] Results - Pool boiling with FC-87 [00146} The temperature limit 85 ^Q C imposed for electronics cooling inhibits the use of water as; a potential cooling fluid in these devices. Instead fluorineit series fluids and/refrigerants: are suitable liquids due to their lower saturation temperatures. In this study, pool boiling tests of nucleating channels surfaces with FC-87 at atmospheric conditions is conducted.

{(10147] Pool boiling tests with FC-87 is performed with the test setup designed and fabrica ed by Kalani and Kandlikar. Fig. 20 shows the pool boiling curve for nucleating channels mieroehanneis with FC-87 at atmospheric pressure. Firstly, FC-87 is allowed to boil on a plain chip which is prepared by rubbing on 2000 grit sandpaper. A CHF of 10 /cn is observed and this will serve as baseline comparison tor all enhancements reported in this study, A CHF of 16 /enr, 17 Wctrf, 20 W/c MH 13 W/enr is reported for NRFG-4.5, N FG-3, N FG-2, 1 and NRFC- L6, respectively. Large wall superheats are expected due to the poor ihermal properties of tluorinert fluids. The wall superheats are 30 °C 46 °C, 35 °C, 40 °C and f *€ for plain, N FC- 4.5, FC-3-, RFC-2,1 and NRFC- 1.6 respectively. The nucleating channels chips show minimal enhancement at lower heat finxes compare to a plain chip whereas at higher heat flux this en|ja ^ ie t:ii --KiQr -;pro»otmeed.. M tested; chips follow similar pool boiling- pattern in which natural convection is dominant in the initial phase ti ll the onset of nucleate boiling where more enucleation sites become available which are responsible for increased heat dissipation rates. The experiments were stopped once CHF is attained which is seen by a sudden spike in the surface, tem erature indicating existence of a thin vapo blanket on the . surface: inhibiting heat- transfer.

[00148] Fig. 2.1. shows pool boiling test results: with FC-87 at atmospheric pressure.

[0014 J Comparison to literature

{00150] Fig. 21 sh s pool boiling performance comparison with similar enhancements available in literature.

[00151] Fig. 21 shows the pool boiling comparison with similar enhancement techniques available in literature, M!IFC»2,1 imderperforms when compared to other enhancement techniques reported here. However, the enhancement techniques correspond to tali fins in the order of 2 mm or more, ft has been established in ..literature that tall fins, due t& additional surface area have shown significant enhancement. This study aims to improve the heat transfer performance, with low fins in the order of 400 xm, At Off,, the best performing chi has a ..CHF of 21 W/cm ^* which is higher than that reported by Mudawar and Anderson and Chang and You. Due to poor themiai properties of FC-87 -high wall siiperheats are expected which is consistent with all. the corves reported in this: study,

[00152] Validation of mechanism

[0 153 High speed, i .stages were inhibited- b the: design of the experimental, setu used In this study. A similar trend is observed in CHF values when compared to water. The Fritz equation does not hold good - or -FC-87 as the effect of Sirid flow resistance in the channels have to be accounted for high ly wetting fluids like FC-87. Experimental determination of bubble departure diameter is suggested with FC-S7,

[00154] Additional Commeiits

[00155} The spacing between the nucleating channels were derived from the departure bubble diameter information using Fritz equation. However, this is used as a guidance. The optimum spacing may be different because of deviations in departure bubble diameter due to effects arising from localized, ge metr ;, conditions and heat flux levels. An optimum spacing Tor specific -heater surface and fluid may be obtained through experiments.

[00156] The NR channels shown here were normal to the mieroehanaels. The mi crochannel shape, size and profile may be different from what is shown here. Also, the nucleating ^' channels may be of different widths, and at different angles titan 90 degrees. The mieroeha iels and the nucleating channels may be not be straight as shown and may be curved or geometrically patterned such as. square patterned nneroehannels with nucleating channels in squ re pattern or circular pattern, etc.

[001571 Although various embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the life can be made Without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the claims which follow.