Refrigeration – Using electrical or magnetic effect – Thermoelectric; e.g. – peltier effect
Reexamination Certificate
2002-01-22
2004-10-05
Jones, Melvin (Department: 3743)
Refrigeration
Using electrical or magnetic effect
Thermoelectric; e.g., peltier effect
C165S180000, C165S185000
Reexamination Certificate
active
06799428
ABSTRACT:
The present invention relates to heat exchangers and to methods for producing them.
The prior art includes heat exchangers used as cooling bodies in the form of rib profiles for instance, which are preferably produced from aluminium. However, their cooling performance is limited by the maximum achievable ratio of cooling-body surface area to cooling-body volume.
Heat exchangers made of porous foamed metal that are intended to remedy this deficiency have furthermore been disclosed by EP 0 559 092, for instance. Admittedly, they achieve a better ratio of cooling-body surface area to cooling-body volume but this is at the expense of poorer heat conduction with larger pores. However, if smaller pores are chosen, resistance to the flow of the incoming cooling medium increases too sharply.
Sintered blocks of the type known from German Utility Model G 91 02 117 have similar disadvantages. Here too, good thermal conductivity can only be achieved through sufficiently fine-grained sintered material and this in turn leads to a heat exchanger with an undesirably high flow resistance.
These solutions are therefore only feasible if use is made of a liquid cooling medium that provides adequate heat dissipation even with relatively large pores. However liquid cooling entails a not inconsiderable outlay on bringing in and carrying away the cooling fluid and cleaning it or holding it ready in an intermediate circuit. Owing to the associated costs, their use is therefore generally restricted to cases where this is unavoidable.
WO 99/09594 attempts to obviate these disadvantages by means of a particular geometric configuration by proposing a sintered cooling body that has a sintered body with a meandering structure and thereby enlarges the areas of inflow and outflow of the cooling medium to and from the sintered body, thus reducing the flow resistance of the heat exchanger and thereby enabling it to be used for gaseous cooling media as well.
However, the subject matter of WO 99/09594 has a crucial disadvantage. The meandering shape of the actual sintered body leads to adjacent passages that are separated only by the sintered body itself both on the side where the cooling medium flows in and on the side where it flows out. The passages on the side where the cooling medium flows in leads to an unwanted effect here. These passages are not only bounded by a wall of the sintered body on their sides facing the outflow passages but are also separated from the outflow side of the heat exchanger at their respective ends by a piece of sintered body. These inflow passages thus form a kind of nozzle, in which the inflowing medium first of all reaches the end of the passage and there tries to pass through the wall of sintered material with which the passage ends. Only if the flow resistance there is too high does cooling medium also pass through the side walls of the passage, first of all through the side pieces closest to the end piece of the passage and then through the parts further away from the latter as the pressure increases.
This is extremely unsatisfactory since, although the heat exchanger according to WO 99/09594 thus provides a favourable ratio of cooling-body surface area to cooling-body volume overall, an inflowing cooling medium cannot use this overall surface area made available for heat dissipation at all since only a small part of it comes into contact with this overall surface area. Instead, the small end piece of the sintered body in the inflow passage receives the majority of the inflowing cooling medium but cannot release as much heat to it as it could absorb owing to the small contact surface in its area. The side walls of the sintered body in the outflow passage, on the other hand, receive only an inadequate supply of cooling medium and, for their part, cannot therefore make use of the surface area available here for heat dissipation.
This disadvantage thus restricts the utility of the heat exchanger according to Wo 99/09594, especially where the inflow velocity of the cooling medium used is considerable.
DE-B 1 639 436 has furthermore disclosed a solution that attempts to improve the thermal conductivity of sintered bodies by disclosing a sintered body that is composed of a compacted mixture of metals comprising metals of poorer and better thermal conductivity. Here, however, the improvement in thermal conductivity is achieved at the expense of reducing the cooling surface area since the sintered body is compacted and its porosity severely reduced or even eliminated, thus of course also reducing the surface area of the body.
However, eliminating the porosity is disadvantageous for other reasons too. This is because the porous structure leads to turbulent flow of the cooling medium in the cooling body since it leads to an adequate increase in the velocity of flow. With increasing velocity, there is a transition from initially laminar flow to turbulent flow at what is referred to as the critical Reynolds' number. Without this structure, on the other hand, the velocity of flow would not rise and the flow would remain laminar. In laminar flow, the cooling medium particles move on parallel paths. The streamlines run adjacent to one another. There is no mixing along the flow path. Friction leads to a velocity profile in each flow. Particles in contact with the wall adhere to it. Velocity rises with increasing distance from the wall. The profile of laminar flow rises gently. For laminar flow in a tube, it is parabolic. In the case of the desirable turbulent flow, on the other hand, the particles perform additional random and irregular transverse movements superimposed on the forward motion. They interweave in the flow. The medium is continuously being mixed along the flow path. The type of flow thus has a decisive effect on heat transfer by convection. In the case of laminar flow, the heat passes from within the substance flow to the wall or vice versa purely by heat conduction in the medium. In turbulent flow heat transport is not dependent solely on conduction. The mixing motion of the particles results in a material exchange of liquid or medium transversely to the flow and hence to direct heat transport. This is many times more than that for pure heat conduction within the medium. Pure heat conduction allows only the laminar boundary layer to be bridged. Heat transfer is therefore many times better with turbulent flow than with laminar flow. It increases as the velocity of flow rises. It can be improved by measures (e.g. roughening the surface) that disrupt or reduce the laminar boundary layer.
Compacting a cooling body by means of pressure as described in the abovementioned publication is thus not desirable but is counter productive since it eliminates or reduces the porosity of the cooling body and, as a consequence, also hinders the formation of turbulent flow that promotes heat dissipation and reduces the area available for cooling to an undesirable extent.
The object for the person skilled in the art is therefore to specify a heat exchanger which, while having a favourable ratio of cooling-body surface area to cooling-body volume and as low a flow resistance as possible, nevertheless has good thermal conductivity.
This object is achieved by means of a heat exchanger, which has a cooling body that is in contact with a medium to be cooled, on the one hand, and with a cooling medium, on the other hand, and transfers the heat of the medium to be cooled to the cooling medium, characterized in that the cooling body is composed of at least two materials, of which one is a better conductor of heat than the other, and the material of better thermal conductivity is essentially in contact with the medium to be cooled and dissipates the heat from the latter to the material of poorer thermal conductivity, which is essentially in contact with the cooling medium, to which for its part it transfers the heat, and the cooling body furthermore has sintered parts that form a porous sintered structure through which the cooling medium can flow, sintered parts being composed of the material of bette
Jones Melvin
Kasper Horst M.
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