Fluid microjet system

The present invention relates to a process and device for generating a plurality of fluid microjets.

A device with multiple fluid microjets is described in US 5,902,543 for cooling an article. The device comprises a plurality of micro channels with diameters from 30 to 100 μm formed as grooves in annular plates that are stacked one against the other so as to form a plurality of micro channels between adjacent plates, the cooling liquid being supplied through a central opening of the circular plates. This allows a dense arrangement of very fine jets of cooling liquid to be projected onto the surface of the article to be cooled, resulting in a well controlled and efficient cooling.

Although the above-mentioned microjet cooling system has a high cooling efficiency compared to many other conventional systems, the pressure drop through the micro channels for creating the microjets is quite high and the desire to have essentially laminar flow limits the velocity of the microjet. Manufacturing of the stacked circular plates with grooves is also quite costly.

There is a continuous need for more efficient and effective cooling systems.

In certain cooling applications, such as metal quenching, a high degree of uniformity of cooling over or through the article is desired to minimize the generation of internal stresses.

The use of fluid microjets may also be envisaged for applications that are not limited to cooling alone, such as for heating, degassing various liquids such as molten metals, cooling of combustion gases, and chemical reactions between the microjets and a medium onto which they are sprayed.

A fluid jet nozzle for blowing air with a wide target area, for use in the textile field, is disclosed in US 2006/0186229. The nozzles are formed by cut-outs in a pair of shims with the cut-outs in the upper shim being offset with respect to the cut-outs in the lower shim such that the nozzle head is provided with two layers of rectangular shaped nozzles. Each nozzle layer requires a separate gas inlet i.e. there are gas inlets into the nozzle head on opposed outer sides of the pair of stacked shims. Such an arrangement allows generation of a thin substantially planar wide air jet, but is not appropriate for a nozzle jet system with a large surface area or a plurality of nozzle layers greater than two since it is not designed for stacking more than two shims. Such a nozzle system is also not well adapted for generating high pressure liquid jets in view of the pour sealing between the pair of stacked shims due to leakage between the cut-outs and the interface between the two shim plates. Such a nozzle arrangement would not be appropriate for example for implementation in a liquid microjet system as described in the above-mentioned document US 5,902,543.

An object of this invention is to provide a device for generating a plurality of fluid microjets that is compact, and that has a high uniformity of treatment and a high efficiency in terms of contact between the fluid and an article or medium to be treated.

It would be advantageous to provide a fluid microjet device that is compact and cost effective to manufacture.

It would be advantageous to provide a fluid microjet device that minimizes pressure loss for generating the microjets.

It would be advantageous to provide a fluid microjet device that is economical on fluid usage.

It would be advantageous to provide a fluid microjet device that is able to generate a high density of very fine microjets of essentially laminar flow, at high speed.

Other objects of this invention are to provide apparatus's implementing fluid jets for :

(i) cooling an article, liquid, or gas very rapidly and with a high efficiency and uniformity;

(ii) for inducing an efficient chemical reaction between a gas and a liquid;

(iii) for efficient degassing of liquid metals.

Another object of this invention is to provide a microjet cooling device that enables rapid freezing of articles, for example for cryogenic freezing of processed food.

Objects of this invention have been achieved by providing a micro stream device according to claim 1.

Disclosed herein is a device for generating fluid micro streams comprising a body portion with a fluid supply cavity therein and a plurality of fluid micro channels interconnecting the fluid supply cavity with an external outlet face or plurality of external outlet faces of the body portion, the body portion being formed from a plurality of stacked plates, said micro channels being formed at interfaces of at least certain of said stacked plates, wherein said micro channels have an oblong cross-sectional profile over a certain section leading to the outlet face with a major width that is greater than two times a minor width thereof, where the minor width is in the range of 1 to 200 μm.

The stacked plates include a first set of microchannel forming plates and a second set of spacer or sealing plates interleaved between the first set of

plates. Advantageously, the alternating first and second plates enable the stacked plates to have excellent sealing of the microchannels as well as improving the ease of manufacturing microchannels with low resistance (e.g. polished) surfaces to reduce flow resistance and thus the pressure required to generate a given number of fluid jets for a specified surface area. The laminar flow properties and velocity of the jets are thus also improved especially at critical subsonic or supersonic velocities. The surface properties of the stacked plates may be optimized not only by manufacturing techniques, but also by the choice of materials, depending on the application.

Advantageously, the oblong cross-sectional profile of the micro stream improves efficiency of treatment of an article or medium with respect to the conventional microjets that have a cross-section approximately square or circular, the oblong microjet having a greater surface area for a given laminar or turbulent flow rate.

The micro channels may advantageously be arranged such that their long axes are essentially parallel and essentially orthogonal to a direction of relative movement of an article or medium to be treated with respect to the device.

In a preferred embodiment, cross-section of micro channels at the outlet have a major width greater than three times the minor width, where the minor width is preferably less than 100 μm, more preferably less than 50 μm. In a preferred embodiment, the distance between adjacent micro channels in a direction of stacking of the plates is preferably less than 10mm. Preferably the distance between adjacent micro channels in the same plane is 0,5 to 10mm. Advantageously the density of micro channels in the device according to this invention may be greater than 4 micro channels per cm ² , preferably more than

10 micro channels per cm ² up to as many as 1000 micro channels per cm ² .

The position of micro channels in adjacent plates may advantageously be offset in a direction orthogonal to the plate stacking direction thus giving a better surface coverage by the micro streams of the article to be treated.

The micro channels may advantageously have a non-constant major width, the major width being larger towards the input side and narrower towards the outlet face. This advantageously allows the body portion to have the necessary mechanical integrity and good sealing between stacked plates, while reducing flow resistance and therefore the pressure drop across the micro channels. In an embodiment, the micro channels may advantageously be formed as slits in a first set of plates sandwiched between plates of a second set, central cavities of the second set of plates configured such that they overlap ends of the slits opposite the outlet end of the micro channel. The first set of plates also comprise a central cavity for passage of the supply fluid therethrough, the cavity however being separated from ends of the slits by a certain width of material ensuring structural integrity of the first set of sheets. The minor width of the micro channels may thus be determined by the thickness of plates of the first set, the width of the slits that are cut through the plate determining the major width of the micro channels, at least over a longer section thereof.

The slits through the plates of the first set may be cut by various known cutting techniques, such as by laser cutting, by means of a die, high-pressure water jet cutting, electro-erosion and other known manufacturing techniques for cutting through thin plates as well as etching. The first set of plates may be made of the same material as the second set of plates, or of a different material whereby the combination of materials may be optimized for sealing effectiveness between the stacked faces, ease of cutting and forming the slits, and for cost reasons. The plates of the first set may for example be made of a ceramic, metal or plastics material, depending on the application and the environmental temperatures whereas the plates of the second set could be

made of steel or plastic or ceramics depending on the application and operating temperature range.

Plates of the second set may have smooth surfaces, for example polished surfaces with a low roughness thus reducing flow resistance in a fairly economical manner. Advantageously, the density of micro streams can also be easily varied by varying the thickness of the plates of the second set, without affecting the micro channel geometry or the manufacturing process for the micro channels.

A fluid micro stream device according to an embodiment of this invention may advantageously be used in an apparatus for rapid freezing of food stuffs and other perishable goods, the fluid micro stream device being installed in an apparatus having a supply of cryogenic liquid, in particular liquid nitrogen, that is injected by the micro streams onto the article to be frozen. The articles to be frozen may advantageously be transported on a mesh conveyor belt, fluid micro stream devices being positioned either side of the conveyor belt such that jets of the cooling liquid are projected upon opposing sides of the article. The extremely rapid and efficient cooling resulting from the micro stream geometry and arrangement according to this invention enables particularly rapid cooling with minimal use of the cryogenic liquid thus reducing freezing cycle time, and use of cooling liquid, thus reducing overall costs as well as improving product quality. Very rapid freezing of food stuffs reduces dehydration of the product during the freezing process, among other advantages.

Fluid micro stream devices may advantageously be used in many other cooling applications, such as for cooling of metal articles in material treatment processes (high precision extrusion quenching, sheet and plate uniform quenching, roll cooling, cooling of polymer extrusions), or for cooling hot gases, such as combustion gases.

Inventors have further realised, surprisingly, that fluid micro stream devices may also be advantageously implemented in applications for: inducing a chemical reaction between a liquid and a gas; for chemical reactions on liquid metals; and for degassing liquid metals.

Also disclosed herein is an apparatus for cooling a gas or inducing a chemical reaction between a liquid and a gas, comprising a chamber with a gas inlet and a gas outlet configured for flow of said gas through the chamber, and one or more fluid micro stream devices configure for generating liquid micro streams in the chamber through the gas. The micro stream device comprises a body portion with a fluid supply cavity therein connected to a liquid supply system and a plurality of fluid micro channels interconnecting the fluid supply cavity with an external outlet face of the body portion, the body portion being formed from a plurality of stacked plates. The micro stream device used in this application may advantageously comprise the features of the micro stream device according to the embodiments described above.

Also disclosed herein is an apparatus for chemical reactions on liquid metals comprising a fluid micro stream device comprising a body portion with a fluid supply cavity therein connected to a fluid supply system and a plurality of fluid micro channels interconnecting the fluid supply cavity with an external outlet face of the body portion, the body portion being formed from a plurality of stacked plates, said micro channels having minor widths less than 200 μm and being formed at interfaces of at least certain of said stacked plates, wherein the liquid metal is supplied as the fluid for generating fluid micro streams and the device is immersed in a reaction medium, in particular a reaction gas. The device for generating fluid micro streams may advantageously further comprise features of the micro stream described above.

Also disclosed herein is an apparatus for degassing liquid metals, comprising a fluid micro stream device comprising a body portion with a fluid supply cavity

therein connected to a fluid supply system and a plurality of fluid micro channels interconnecting the fluid supply cavity with an external outlet face of the body portion, the body portion being formed from a plurality of stacked plates, said micro channels having minor widths less than 200 μm and being formed at interfaces of at least certain of said stacked plates, wherein the device is immersed in the liquid metal and a degassing medium, such as an inert gas, is supplied as the fluid for generating fluid micro streams. The device for generating fluid micro streams may advantageously further comprise features of the micro stream described above.

Further objects and advantageous features of the invention will be apparent from the claims and the following description and drawings in which:

Figure 1 a is a perspective view of a portion of a fluid micro stream device formed of a stack of plates according to this invention;

Figure 1 b is an exploded perspective view of a portion of a fluid micro stream device formed of a stack of plates according to this invention;

Figure 2a is a view of a plate of a second set of plates of an embodiment of the invention;

Figure 2b is a view of a plate with micro channels of a first set of plates of the device according to this invention;

Figure 2c is a view of the stacked assembly of the plates of figures 2a and 2b;

Figure 3a is a perspective view of a section of an embodiment of the device according to this invention;

Figure 3b is a perspective representation of fluid micro streams according to this invention;

Figure 3c is a partial view of a configuration of fluid microchannels according to an embodiment of this invention;

Figure 4a is partial detailed view of first and second plates of a device according to another embodiment of this invention, showing microchannels according to a second embodiment;

Figure 4b is a view similar to figure 2a, of a third embodiment;

Figure 5 is a perspective schematic view of part of an apparatus for cryogenic freezing of processed food products according to this invention;

Figure 6 is a schematic cross-sectional view of an apparatus suitable for cooling heated gas, or for reacting a gaseous medium with a liquid, according to this invention.

Referring to the figures, a device for generating fluid micro streams 7 comprises a body portion 4 comprising an outlet face 6 through which micro streams of fluid 7 are projected, a fluid supply cavity 8 within the body portion connected to fluid supply system (not shown), and a plurality of micro channels 10 in fluid communication between the fluid supply cavity 8 and the outlet face 6.

In a preferred embodiment, the body portion 4 comprises a stack of plates 12, 14 between which the micro channels 10 are formed.

In a preferred embodiment, the micro channels 10 are formed in plates 12 of a first set as slits that are cut through the entire thickness of said plates of the

first set and plates 14 of a second set without slits are interposed therebetween. The micro channels 10 are thus formed by the slits in the first set of plates 12 sandwiched between the plates 14 of the second set in an alternating manner. The first plates 12 have openings 16 that form part of the boundary of the fluid supply cavity 8 in the body, the slits 10 extending from an outlet edge 18 that forms part of the body portion outlet face 6 to a closed end 20 that is separated at a certain distance R from an edge 22 of the opening 16, the distance between the end of the slit 20 and the edge 22 being sufficient to ensure mechanical integrity of the first plate 12 during handling and assembly between the second plates 14.

The second plates 14 also have openings 24 that form part of the fluid supply cavity 8, an edge 26 of the opening adjacent the outlet face 6 overlapping a portion 10a of the slits such that the closed ends 20 of the slits are in fluid communication with the cavity 8 as best seen in figure 2c, 4a and 4b. The second plates 14 may be of a simple planar construction with smooth surfaces thus lowering flow resistance in the micro channels 10.

The slits 10 may be produced by various conventional techniques, such as laser cutting, water jet cutting, electro-erosion, die stamping, etching, or by means of circular saws, depending on the material of the first plate 12 and the channel dimensions. The slit manufacturing method may also be chosen as a function of the surface smoothness of the micro channel and manufacturing costs. The alternate sandwich construction of first and second plates with the micro channels formed by slits in one of the two plates provides a large versatility in the choices of materials and manufacturing techniques for the plates to optimize the performance and cost for various applications. For high temperature applications, the plates 14 may for example be made of a high temperature stainless steel whereas the first plates 12 could also be made of steel, or a thin ceramic such as mica. For low temperature applications the first plates with slits 12 could be made out of a sheet of thin polymer or composite

material. Depending on the desired density of micro streams, the second plates 14 may be from various thicknesses, without affecting the manufacturing of the micro channels. The flexibility in material choice and the separate alternate first and second plates also enable the surface properties of the microchannels to be optimized, in particular to reduce flow resistance and improve laminar flow, and also to provide excellent sealing between plates and around the microchannels.

The stack of plates forming the body portion 4 may be held together sealingly by means of compression bolts 11 extending through bolt holes 13 in the body portion, clamping the stack of plates together. Other clamping means may however be used. It is also possible, within the scope of the invention, to weld, glue, or otherwise bOnd the stacked plates together.

Referring to figure 4a, the micro channels 10 may advantageously have non- constant width, with a large width W ₁ towards the closed end 20 in order to increase the channel cross-section at the fluid inlet, the channel width reducing to a narrow section W ₂ corresponding essentially to the desired micro stream cross-sectional profile at the outlet face 6. The latter configuration reduces flow resistance and pressure drop through the micro channels without compromising on the structural integrity of the device and the sealing between adjacent plates. The enlargened closed ends of the slits may be easily manufactured, for example by a die stamping etching or electro-erosion process.

As shown in figure 4b, the micro channels may have non-constant profiles 27 also at the outlet end for example to create a laval shaped channel for the creation of supersonic fluid jets.

The micro channels could alternatively be formed as grooves on the surface of plates that are stacked one on the other where a side of the plate opposite the grooves is stacked against the side with grooves of the adjacent plate. Such

plates could advantageously be made by injection moulding of a polymer or other injectable materials such as certain metal alloys, whereby the die imprinting the micro channels could be made by photo lithography and etching. For example the injection moulding die with the micro channel profiles could be made out of silicon in a standard etching process. This would allow micro channels of particularly small and well controlled dimensions to be produced. The injected micro channels could have non-constant profiles as discussed in relation to figures 4a and 4b above.

According to the invention, the micro channels advantageously have an oblong profile at or proximate the outlet face 6, defined by a minor width W _mιn and a major width W _maj where the major width is advantageously more than two times the value of the minor width. The micro channels are configured preferably such that the major width W _maj is defined by the narrow width W ₂ of the slit 11 and the minor width is defined by the thickness of the first sheet 12. It is however possible to have a first plate thickness superior to the slit width W ₂ such that the micro channel major width W _maj is defined by the plate thickness and the slit width W ₂ defines the minor width W _mιn .

The micro channels in a first plane may be offset in a direction orthogonal to the stacking direction of the plates, which may advantageously correspond to a direction of relative movement between the fluid micro stream device and an article to be treated such that the oblong micro streams have a better impact coverage across the surface to be treated.

The oblong shape of the micro streams, and in addition the offset arrangement of adjacent micro streams provides more uniform and more efficient cooling of the surface to be treated than the conventional microjets. As shown in Fig. 3c, a plurality of successive micro stream layers may be offset by a distance Os with respect to a first layer depending on the major width and spacing S (see figure 3) between micro streams and the cooling efficiency required.

Referring to figure 5, an apparatus for cryogenic freezing of articles, for example food stuffs, comprises a conveyor system 30 for conveying the articles through the apparatus, and one or more fluid micro stream devices 2 arranged along the conveyor system and configured to project micro streams on the articles as they are transported along the conveyor system. The conveyor system may comprise a conveyor belt that is preferably in the form of a mesh or grill in order to allow projection of micro streams from above and below the articles for more efficient and rapid all round cooling of the articles. Other conveyor systems may however be used, depending also on the articles to be cooled.

In the embodiment shown, fluid micro stream devices are placed on opposite sides of the conveyor belt 32, across the width of the conveyor belt and project micro streams of fluid on top and bottom sides of the articles 34 to be cooled. Depending on the amount cooling required, a plurality of fluid micro stream devices can be arranged along the conveyor belt as shown in figure 3. For cryogenic freezing of articles, the cooling fluid supplied to the devices is advantageously liquid nitrogen although other very low temperature cooling liquids could be used, such as liquid helium. In view of the relatively high cost of liquid nitrogen and other cryogenic cooling liquids, the very rapid and efficient cooling provided by the micro streams enables minimum use of cooling fluid and moreover the velocity of micro streams may be easily varied with a direct effect on the rate of cooling. For example, the micro streams velocity may be reduced, by reducing pressure in the supply cavity towards the end of the cooling cycle. In the arrangement of figure 5, the most downstream fluid micro stream device 2c may for instance have a lower supply pressure of cooling liquid than the most upstream device 2a.

The invention may advantageously be used in various applications not limited to cooling, such as for heating, degassing various liquids such as molten

metals, cooling of combustion gases, and chemical reactions between the micro streams and a medium onto which they are sprayed.

Referring to figure 6, an apparatus for cooling combustion gas comprises one or more fluid micro stream devices 2 mounted in a cooling chamber 38 having an inlet 40 for the inflow of hot combustion gas at one end of the cooling chamber, and an outlet 42 for exit of the cooled combustion gas at the other end of the chamber. A cooling liquid source 43 connected to the fluid micro stream device 2 preferably supplies de-ionised or distilled water. The fluid micro stream device used in this application may advantageously be provided with features of the fluid micro stream device described hereinabove in relation to figures 1 to 5.

The fluid micro stream devices 2 are preferably mounted at or near the end of the combustion chamber proximate the inlet 40 and configured to generate a large plurality of densely distributed and very fine micro streams 7 towards the outlet end 42 of the chamber 38. The density of micro streams, the dimensions of each jet, the velocity of the jet and flow rate may be optimized empirically for optimal cooling of the combustion gas, depending on the combustion gas temperature and flow rate through the cooling chamber. The configuration of the micro streams may advantageously be configured to ensure complete or almost complete evaporation of the injected cooling water to avoid accumulation of cooling liquid in the cooling chamber or outlets of the cooling chamber. The region near the outlet end of the cooling chamber 38 thus represents an evaporation zone 46 of the micro stream cooling liquid. The cooling chamber may however also be provided with a collector for collecting cooling liquid and circulating it to a waste water treatment installation.

The above-described cooling system may be applied to other hot gases, not just combustion gases, and the cooling liquid used for the micro streams may be a liquid other than water.

The apparatus illustrated in figure 6 and described above may however also be configured for use in an application for inducing a chemical reaction between the micro streams and a gaseous medium. In this case the apparatus in figure 6 would represent a reaction chamber 38 in lieu of the cooling chamber and the reaction gas would replace the heated combustion gas. Such a configuration would be particularly advantageous with the micro stream device 2 according to the invention in view of the high density, high velocity and very fine fluid streams that can be generated over a large surface.

In applications for chemical reactions on liquid metals, the device for generating fluid micro streams is supplied with liquid metal as the fluid for generating fluid micro streams, and the device is immersed in a reaction medium, in particular a reaction gas.

In applications for degassing liquid metals, the device for generating fluid micro streams is immersed in the liquid metal and a degassing medium, such as an inert gas such as Argon, is supplied as the fluid for generating fluid micro streams and injected into the liquid metal.

In apparatuses of the above applications, the stacked plates of the micro stream devices may be made from high temperature materials, or chemically inert materials, such as ceramics. In such applications, the intermediate plates with slits could be omitted, and the micro channels provided as grooves directly on one side of each of the stacked plates. The micro stream devices in these applications could however advantageously be provided with any of the advantageous features of the preferred embodiments of the micro stream devices described herein, provided high temperature resistant, respectively chemically inert materials are used, depending on the application.

Examples

Example 1 : Quenching Aluminium Alloy

Cooling of Alluminium alloy strip, thickness of 0.5 to 5 mm, typical specific flow 20 Iiter/m ² /s. A typical set of values for micro streams for this application are: W _maj = 120μm, W _mιn = 50μm, S = 3mm, H = 3mm.

Advantages compared to conventional cooling technology: More homogeneity in cooling, and consequently high mechanical properties homogeneously distributed, less residual stresses.

Example 2: Freezing Food

For cryo-freezing of foodstuffs such as meat, fish, fruits Typical specific flow 0.5

Iiter/m ² /s. A typical set of values for micro streams for this application are: W _maj

= 150μm, W _mιn = 20μm, S = 5mm, H = 5mm.

Advantages compared to conventional freezing technology: economy of cryogenic fluid (which is expensive). The quantity of cryo-coolant needed is minimal, since the freezing is optimally conducted and the heat-transfer is optimal. Rapidity of freezing and small jet impingement forces preserves also the integrity of the products as they are being frozen.

Example 3: Heating, degassing and chemical reactions on liquid metals.

In this example, liquid metal, for example aluminium, is sprayed as micro streams. The ambient gas around the streams interacts with the liquid metal micro streams. The gas may be used to heat the liquid metal, or to interact with it chemically, or for degassing. The liquid metal is thus supplied as the fluid for generating fluid micro streams and the device immersed in a reaction medium, in particular a reaction gas. Typical specific flow 5 Iiter/m ² /s. A typical set of values for micro streams for this application are: W _maj = 1000μm, W _mιn = 180μm, S = 6mm, H = 4mm. Advantages compared to conventional technology: The surface of interaction and of contact between gas and liquid is maximal. Therefore the chemical reaction as well as the heat transfer is highly efficient.