Clustered Nucleate Boiling Cavity Grid

Technical Field

This disclosure relates to cooling arrangements and cooling methods in which liquid coolants may be prevented from changing boiling state from a controlled boiling state to a film boiling state.

Background

Coolant circuits designed to cool down certain parts of equipment, machinery, or, in general, devices and apparatuses that are exposed to high temperatures, are widely known in the art. For example, coolant circuits are adopted in the automotive industry for cooling internal combustion engines. The coolants may be generally in the form of heat-resistant liquids, which may be circulated in the coolant circuit to remove heat from hot surfaces, at a certain thermal transfer rate.

The thermal transfer rate, which may determine the efficiency and performance of a coolant circuit, can be increased by a process known as "nucleate boiling", which may be used to improve the cooling of specific portions of a cooling system or of the entire cooling system itself. In such a process, small steam bubbles form in correspondence to cavities disposed on the surface of the cooling system. Formation of bubbles begins when the temperature of the coolant reaches the so-called saturation temperature, which temperature is first reached in correspondence to cavities. Eventually, the bubble departs the cavity, separates from the surface and floats into the main fluid stream. Since the temperature of the main fluid stream is lower than the temperature of the bubble, the steam bubble collapses.

The nucleate boiling thermal transfer may reach a condition known as "critical heat flux", or CHF. In this condition, an increase in the heat flux may cause nucleate boiling to change into "film boiling". This change is known as "departure from nucleate boiling", or DNB. Film boiling can occur when the bubbles departing from a surface of the cooling circuit merge together and build up an evenly distributed film, covering the surface that should be in contact with the coolant. This film actually builds up a barrier that keeps the liquid coolant separated from the surface. Due to the suddenly diminished heat flux in this area, a dramatic increase of the wall

temperature of the surface can occur. Depending on the environment and materials used in of the cooling system, film boiling may cause the temperature to rise rapidly, a situation that in turn may cause damage to the components of the cooling system itself or failure of the system to be cooled.

EPl 428997 in the name of Perkins Engines Company Ltd discloses a cooling arrangement for controlled nucleate boiling, the arrangement having cavities distributed on the surface in a way that may prevent film boiling. Particularly, cavities are spaced at a distance such that bubbles departing thereof do not interfere one with the other. According to EP 1428997, an arrangement of cavities having mutual distance about three times greater than one bubble departure diameter results in a controlled boiling state.

Even though the cooling arrangement of EP 1428997 may be satisfactory in terms of thermal transfer and heat flux for various applications, yet it would be desirable to further increase the performance and efficiency of cooling systems. Accordingly, a need exists for a cost-effective and flexible solution in which a surface configuration may enhance the thermal transfer rate or the heat flux in liquid coolant circuits.

Brief Summary of the Invention

In a first aspect, the present disclosure describes a nucleate boiling grid, in particular for liquid cooling systems, comprising a plurality of micleation cavities grouped in one or more clusters.

In another aspect, the disclosure describes a method of cooling a surface, in particular in liquid cooling systems, comprising the steps of grouping a plurality of nucleation cavities in one or more clusters.

Distribution and shape of cavities in the clusters and distribution of the clusters in the nucleate boiling grid as described in the present disclosure promotes activation of nucleation cavities while inhibiting film boiling.

Other features and advantages of the present disclosure will be apparent from the following description of various embodiments, when read together with the accompanying drawings.

Brief Description of the Drawings

Fig. 1 is a partial plan view of a first embodiment of a boiling grid comprising clustered nucleation cavities according to the present disclosure;

Fig. 2 is an isometric view of the boiling grid of Fig. 1, illustrating bubble creation;

Fig. 3 is a partial plan view of a second embodiment of the boiling grid in according to the present disclosure;

Fig. 4 is an isometric view of an insert having the clustered nucleation cavities in according to the present disclosure;

Fig. 5 - 8 are cross-sectional views of different embodiments of the nucleation cavities according to the present disclosure.

Detailed Description

This disclosure generally relates to a nucleate boiling grid for liquid cooling systems and, in one embodiment, to the process of cooling a surface in liquid cooling systems providing enhanced thermal transfer characteristics.

Fig. 1 illustrates a first embodiment of a nucleate boiling grid according to the present disclosure. A surface 1 in a cooling circuit comprises a plurality of nucleation cavities 2 arranged thereon. Multiple cavities 2 are grouped together so as to form a plurality of clusters 3.

Cavities 2 may be spaced apart at a spacing or distance 10. In general, each cavity 2 gives rise to a bubble 4, which bubble is therefore referred to, in this disclosure, as "cavity bubble" 4. The spacing 10 between nucleation cavities 2 in a cluster 3 may be lesser than a diameter of cavity bubbles 4 departing thereof.

Clusters 3 may be spaced apart at a distance 12 one from the other and may be arranged in substantially regular distribution on surface 1. Each cluster 3 may contain a same amount of cavities 2 or, according to requirements, different amounts of cavities, for instance in case different parts of the surface to be cooled are exposed to different temperatures.

Each cavity 2 in each cluster 3 may be spaced apart from adjacent cavities in the same cluster at a substantially same distance 10. In one embodiment, cavities 2 inside a cluster 3 may be arranged according to an equilateral distribution, wherein adjacent cavities 2 are spaced away one from the other at a same or similar distance. For any selection of three adjacent nucleation

cavities 2, each cavity 2 may form the vertex of a substantially equilateral triangle. In general, nucleation cavities 2 may be arranged in a substantially regular distribution in a cluster 3.

Similarly, clusters 3 may be arranged according to an equilateral distribution, wherein adjacent clusters 3 may be spaced one from the other at a same or similar distance. For any selection of three adjacent clusters 3, each cluster 3 may form the vertex of a substantially equilateral triangle.

In operation, adjacent cavity bubbles 4 are formed under the effect of the high temperature proximate to the cavities. The arrangement of cavities 2, spaced at a distance smaller than one cavity bubble 4 diameter, may promote cavity activation, i.e. the activation of bubbles due to vapour from departing bubbles 4 of adjacent cavities 2. Particularly, vapour released from each of the cavities 2 may seed other neighbouring cavities 2, lowering the required superheat wall temperature and initiating nucleation activity at lower temperatures.

Additionally, an increased nucleation activity may improve the heat flux, since the cycle of liquid coolant entering a cavity 2, building up a cavity bubble 4 and departing from the cavity 2 may occur more frequently. As a result, more fluid coolant can enter each cavity 2 over a certain period of time.

Cavity bubbles 4 may get in reciprocal contact and merge together to form a larger bubble 5. In general, each cluster 3 may give rise to a bubble 5, which bubble is therefore referred to, in this disclosure, as "cluster bubble" 5. The expression "clustered nucleate cavity boiling grid" indicates, in the present disclosure, a configuration of cavities 2 grouped in clusters 3.

The spacing 12 between two adjacent clusters 3 may be greater than the diameter of a cluster bubble 5 departing the cluster 3, so that cluster bubbles 5 do not get into reciprocal contact and film boiling is inhibited.

Clustered nucleate cavity boiling grids according to the disclosure may be distributed across all the surfaces of a cooling system or arranged in specific areas only, wherein clusters 3 may be applied in positions where high heat fluxes may be expected.

In general, nucleation cavities 2 can be applied to surface 1 during manufacturing of the cooling system. The application of such cavities may be obtained by any suitable known process, for instance through laser treatment, stamping or edging.

In one embodiment, clustered nucleate cavity boiling grids can be retrofitted as separate surfaces, adapted to be applied to the surface of a cooling system where an improved thermal heat transfer is desired. When a separate surface is used, cavities 2 can also be formed as through holes in such separate surface, with one face in tight contact to the surface to be cooled and the opposite face exposed to the liquid coolant.

The area between cavities 2 and the clusters 3 may be substantially planar and smooth, in particular to avoid that scratches or irregularities in the surface may initiate nucleation. Therefore, materials having a specific surface roughness, may be employed for surface 1, to prevent nucleation in the plan areas between cavities 2 and clusters 3, respectively.

Fig. 2 illustrates an embodiment of the nucleate boiling grid of Fig.1 showing cavity bubbles 4 protruding out of cavities 2 in a first stage of nucleation. Once the cavity bubbles 4 have grown sufficiently, they may get in contact with adjacent cavity bubbles inside a cluster and merge to form a cluster bubble 5.

This process of combining cavity bubbles 4 into a cluster bubble 5 allows for a controlled film boiling effect inside clusters 3. The maximum diameter of cluster bubbles 5 is proportional to the distance of two cavities 2 located farthest to each other in the cluster.

Fig. 3 illustrates another embodiment of a nucleate boiling grid according to the disclosure, wherein nucleation cavities 2 in clusters 3 are arranged in the form of a matrix. The distance 11 between adjacent cavities 2 in a cluster 3 may substantially be the same. The surface 1 between clusters 3 as shown in Fig. 3 may be substantially planar.

Inside cluster 3, cavities 2 may be arranged in rows and columns, wherein adjacent cavities 2 in each row and column may be spaced apart at a substantially same distance. Clusters 3 may be distributed so that a selection of neighbouring clusters forms a substantially rectangular shape, adjacent clusters 3 being spaced apart at a distance 13. The distance 13 may be of a dimension greater than one bubble diameter departing from a cluster 3, to avoid merging of cluster bubbles 5, which could originate the undesired film boiling effect.

In one embodiment, neighbouring cavities 2 in a clusters and/or clusters 3 themselves may be arranged to form a substantially square matrix. Again, the surface not covered by cavities or clusters may be substantially planar and adjacent clusters 3 may be spaced apart by at least one diameter of a cluster bubble 5 departing a cluster.

Other embodiments, not shown, may have different cavity arrangements inside clusters. As an illustrative example, cavities 2 may be distributed in a star-like formation, in an annular arrangement, or even distributed pseudo-randomly, provided that the distances between adjacent cavities is such that adjacent cavity bubbles 4 can get in contact and merge into a cluster bubble 5 before departing the respective cavities.

Fig. 4 shows another embodiment of a surface 1 having nucleation cavities 2 arranged in clusters 3. Surface 1 may be substantially curved for usage in specific regions of the cooling circuit, for instance when the cooling system or the piece to be cooled is angled or curved. Clusters 3 and cavities 2 may be distributed in a same stibstantially regular distribution as shown in Fig.l . Particularly, cavities 2 and clusters 3 may substantially form equilateral triangles.

Cavities 2 in clusters 3 may also have different sizes of opening and depth for achieving different cluster bubble activation. Figs. 5 through 8 illustrate several embodiments of nucleation cavities 2 in cross-section. Fig. 5 shows a cavity 2 having a substantially square cross-section with cavity walls facing each other. The cavity 2 in Fig. 6 has a substantially trapezoid cross- section, with the bottom of the cavity being larger than the opening at the surface 1. In the embodiment shown in Fig.7, the shape of cavity 2 is opposite to the one of Fig. 6, wherein the trapezoid cross-section is reversed, with the opening at the surface 1 wider than the bottom of the cavity. In the embodiment of shown in Fig. 8, the cavity 2 is substantially in the form of a semicircular cross-section.

The depth of the cavities may be selected, at least in one dimension, such that the surface tension of the liquid coolant is overcome, allowing the liquid coolant to be in contact with the cavity walls. For examples, satisfactory results are achieved, but not limited to, when cavity- opening diameter is substantially equal to the depth of the cavity.

In another embodiment, not shown in the Figures, a surface configuration at the cavity surface may contain tines or have a corrugated shape, to increase the surface of the cavity that may be in contact with the liquid coolant, without changing the diameter of the cavity.

Other configurations having the same effect of increasing the effective surface inside a cavity can be applied, for instance cavities having triangular cross-sections, as far as the diameter of the opening of nucleation cavities 2 is suitable to allow a liquid coolant to enter the cavity, so that the surface tension of the liquid is overcome.

A controlled boiling state may be maintained by selecting the shape, size and pattern of the cavities and of the clusters in a manner that the bubble departure size, the amount of bubbles created in a set timeframe, and the bubble activation temperature meets specific requirements. Optimal cavity spacing and size, amount of cavities in the clusters and size and spacing of the clusters can be determined by analysis and limited experimentation and/or using as a starting point formulas concerning cavity activation temperatures, superheat temperatures and prediction of bubble department diameters which are well known to the skilled in the art.

The material used for constituting parts of a cooling system may be suitable to withstand heat and may be corrosion-resistant to be substantially unaffected by contact with the liquid coolant. Metals, like steel and aluminium, polymers or any other material having characteristics similar to such metals may be used.

Industrial Applicability

This disclosure describes an enhanced nucleate boiling grid, wherein nucleation cavities 2 are grouped in clusters 3. The mutual spacing between cavities 2 in clusters 3, which may be smaller than the diameter of one cavity bubble, can lower the superheat temperature to achieve forming of cavity bubbles 4 and cluster bubbles 5, departing from cavities 2 and clusters 3 respectively. The vapour acting in a cavity 2 may seed adjacent cavities, therefore reducing the temperature required for creating a cavity bubble 4 and a cluster bubble 5.

Particularly, tight spacing of cavities 2 in clusters 3 promotes cavity activation, while wider spacing of clusters 3 inhibits film boiling.

The industrial applicability of the nucleate boiling grid manufactured as part of a cooling system or as an add-on element as described herein will have been readily appreciated from the foregoing discussion.

The present disclosure is applicable to cooling systems for, but not limited to, combustion engines, for instance internal combustion engines. Other applications include cooling of large or critical surfaces, for instance in nuclear reactors.

An item comprising nucleation cavities 2 imprinted thereon may be manufactured as a part that is separate to the cooling system. In such embodiment, existing cooling systems can be retrofitted with such an item and benefit from the improved heat transfer provided by the

disclosure. Additionally, individual items or separate surfaces may allow for greater flexibility in application of the nucleate boiling grid according to the disclosure in areas that may be subjected to high heat. The skilled in the art easily appreciates that cavities and clusters according to the disclosure can be applied to any shape of an element in a cooling system.

Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein.

Where technical features mentioned in any claim are followed by references signs, the reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, neither the reference signs nor their absence have any limiting effect on the technical features as described above or on the scope of any claim elements.

One skilled in the art will realize the invention may be embodied in other specific forms without departing from the invention or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting of the invention described herein. Scope of the invention is thus indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalence of the claims are therefore intended to be embraced therein.