GB2124742A - Promoting nucleate boiling in heat exchangers - Google Patents

Abstract

A surface (3) to be cooled by a liquid coolant is provided with an electrical heating element (8), preferably deposited upon the surface and thereby closely conforming to the surface contour, so that the establishment and maintenance of nucleate boiling is promoted. <IMAGE>

Description

SPECIFICATION Heat transfer means This invention relates to means for improving the transference of heat between a heated surface and a fluid coolant with which the surface is in contact, of the type utilising the phenomenon known as nucleate boiling.

In general, before the boiling point of the coolant is reached, heat transfer occurs by convection and the coolant remains in its liquid phase. Where the boiling point of the coolant is reached, the single phase convection is succeeded, with a proviso discussed below, by nucleate boiling in which bubbles form on nucleation sites on the surface to be cooled. It is found that the heat flux (that is to say the heat transfer per unit area of surface - Watts/cm2) is much increased by nucleate boiling. As temperature increases nucleate boiling is succeeded by the phenomenon known as film boiling. In this a film is formed over the surface and once it is established it is found that the heat flux levels off or even reduces.

The proviso to which reference is made above is that super heating can occur (i.e. the temperature increases beyond the boiling point of the coolant) without nucleate boiling being established. In this situation although the temperature rises the high heat flux associated with nucleate boiling is not present and the temperature of the surface can soar.

Naturally, when nucleate boiling is established, then the associated high heat flux reduces the surface temperature.

Thus the present invention has for an objective the provision of improved means by which nucleate boiling is established and maintained during operation so that temperature excursions of the surface are reduced.

According to the present invention cooling means includes a surface to be cooled and a cooland liquid immersing that surface, the arrangement being characterised in that there is provided heating means at the surface to be cooled whereby that coolant adjacent the surface to be cooled is brought to a temperature at which the establishment and maintenance of nucleate boiling is aided.

Preferably, where the surface has been treated to enhance its nucleate boiling characteristics, for example as described in U.S. Patent Specification No. 4,312,012 the coating forming the element is chosen such that it conforms with any intricacies of the treated surface; it is thus conveniently a deposited coating.

One embodiment of the invention is described with reference to the accompanying drawings in which :- Figure 7 is a typical graph of the heat transfer performance of known means including an immersed surface to be cooled and a coolant. Heat flux, that is to say heat transfer per unit area (Watts/cm2) is plotted vertically. Temperature, that is to say the difference between surface temperature and coolant temperature (AT0) is plotted horizontally.Since the coolant temperature is maintained as near boiling as possible and is therefore practically constant, AT is an indication of surface temperature, Figure 2 is a cross sectional elevation of heat transfer means according to the invention, Figure 3 is a sectional view on lines Ill-Ill of Figure 2, and, Figure 4 is a vastly enlarged cross sectional elevation of a surface of the heat transfer means of Figures 2 and 3.

Referring initially to Figure 1, as the value of AT rises the heat flux increases only slowly as shown by curve A - B. At a value of AT well above that at which nucleate boiling can occur, the curve shows that, in certain circumstances, ordinary convection heat transfer is still occurring and since this is accompanied by only a low heat flux the temperature AT is seen to increase. Once nucleate boiling becomes initiated, as shown by the reverse curve B - C the temperature begins to fall as the heat flux associated with that phenomenon. The curve D - E illustrates the film boiling succession to nucleate boiling. Heat flux increase levels off substantially.

In that band of values of AT in which both convection and nucleate boiling heat transfer can take place, it appears that some areas of the surface may have convection and some may have nucleate boiling predominating; since the latter gives a higher heat flux, the areas of the former can develop as hot spots; in some embodiments thus is not tolerable.

In Figure 2 heat transfer means include a coolant container 1 having a base 2 and a vent 4. The base 2 has an insert wall 3 formed of alumina, one face of which is immersed in the coolant. The vent 4 is fluid cooled so that any vapour issuing therefrom is condensed back into the container.

The coolant is a fluorocarbon liquid (e.g. ARCTON or FLUTEC). The vessel contains baffles 5, 6 and a sustainer heater 7. The latter maintains the coolant at a temperature conducive to nucleate boiling.

In heat transfer association with the insert wall 3 is mounted a device or devices (not shown) which in service require to be cooled.

The immersed face of the insert wall 3 is provided with a thinly deposited nichrome coating 8 to provide an auxiliary heating element. In Figure 4, the close conformity of a deposited coating 8 as illustrated, such that any nucleation sites, such as 9, are not rendered ineffective. Electrical connector strips are provided at 10. Although illustrated as deposited upon the wall 3, the auxiliary heating element may be located closely adjacent the wall 3. Where this is the case, and the wall 3 is specially prepared to provide nucleation sites, then the element may be of a mesh or of a porous nature.

The heat output of this element, and its location at the interface between the wall 3 and the coolant, is chosen to help ensure that nucleate boiling is established and once established is maintained.

Thus the heat flux is about 2 Watts/cm2. In terms of the graph of Figure 1, the region C - D is held by the presently described arrangement so that near isothermal heat transference is obtained in operation. In this context the term 'isothermai' is used to describe both heat transfer at constant temperature, e.g. that shown in curve C - D which approaches that condition.

Additionally, the described arrangement provides a short time constant, that is to say the high heat flux of nucleate boiling is continued even though the devices associated with the wall 3 are not generating sufficient heat (for example on switch-off) to maintain nucleate boiling.

If the devices to be cooled are themselves immersed in the coolant, then their surfaces are provided with the conformal coating 8.

The described and illustrated device enables nucleate boiling to be established and maintained during operation so that temperature excursions of the relevant surfaces are reduced and possible, but not necessarily or certainly, a relatively uniform heat flux is maintained over the surface.

Similarly, temperature excursions of the coolant are reduced since nucleate boiling is readily established at the surface to be cooled and the bulk of the liquid needs not to be excessively heated, in order to effect such nucleate boiling at said surface.

Claims (5)

1. Cooling means including a surface to be cooled and a coolant liquid immersing that surface, the arrangement being characterised in that there is provided heating means at the surface to be cooled whereby that coolant adjacent the surface to be cooled is brought to a temperature at which the establishment and maintenance of nucleate boiling is aided.

2. Cooling means according to Claim 1, wherein the heating means is an electrical element formed upon or closely adjacent the surface to be cooled.

3. Cooling means according to Claim 2, wherein the element is deposited upon the surface to be cooled.

4. Cooling means according to Claim 3, wherein the deposited element is about 1 x 1 0-8m thick and the heat transferred is about 2 Watts/cm2.

5. Cooling means substantially as described with reference to the accompanying Figures.