(36a) A Micro Heat Exchanger for High Heat Flux

This publication describes the development of a new microstructure to transfer higher heat fluxes. Design details of the devices will be explained and experimental results will be presented. Microstructure devices are considered as interesting tools not only for lab scale applications but also for industry. In general, they provide superior performance in heat and mass transfer and are, depending on the design, suitable for mass flow ranges from some kg per hour up to several tons of liquids per hour.

With a simple mathematical model based on heat conduction theory for the heat transfer in a micro channel at laminar flow conditions it was deduced that for the transmission of higher heat fluxes only the initial part at he beginning of the micro channels is of importance, i.e. the micro channels may be short.

Two microstructure devices, a heat exchanger and a surface micro cooler providing the newly developed micro structure have been presented. Experimental test of the heat exchanger showed that the new design gives superior heat transfer performance compared to standard micro devices with straight long micro channels. By changing the material of the microstructure devices from polymer to aluminum and ceramic, no significant effect with regard to heat transfer could be observed.

Micro Heat exchanger

The micro heat exchanger consists of two plastic plates for the cold and hot passage. The plates contain the newly developed structure with short micro channels. The heat transfer took place via a 0,2 mm metal foil. The inlet temperature of the cold water side was 10°C, while the inlet temperature of the hot water side was 95°C. Heat fluxes up to 500 W/cm² were achieved at a pressure loss of 0.16 MPa and a mass flow of the cooling water of 200 kg/h per passage. Because of the weakness of the plastic, higher mass flows were not possible.

Due to the use of materials with a higher temperature resistance and higher stability like aluminium or ceramic, higher water throughputs and higher flow velocities could be realized in the micro channels. Thus it was possible to increase the heat flux up to approx. 800 W/cm² at a pressure loss of approx. 0.35 MPa and a mass flow of 350 kg/h per passage. The heat transfer coefficient calculated according to the conventional Nusselt theory for such a device was about 70 kW/m²K.

A potential operational area of such micro heat exchangers is the employment in chemical industry, i.e. the effective cooling of highly exothermal chemical reactions.

Surface-micro-cooler

The important focus of investigation of the surface-cooler was set on the examination of the surface temperatures for different heat fluxes and different velocities of the cooling water. Several prototypes of surface-micro-coolers using the same principle were manufactured and examined. One prototype consisted of five individual stainless steel foils with dimensions of 1 cm * 1 cm. Two foils, which formed the newly designed heat transfer structure with the short micro channels, had a heat transfer surface of 1 cm². The remaining three foils served as flow distributors for the cooling water. The metal foils were micro machined by mechanical precision milling and stacked on top of each other. The foil stack was then diffusion bonded to generate a leak-tight connection between the foils. The tube connections for the cooling water were laser welded to the base body of the surface-micro-cooler.

In the case of this indirect cooling of the electrically heated surface the heat flows via the gap between the heated surface and the cooling foil, which is filled with a thermal conductance paste, then via the cooling foil of the surface-micro-cooler into the cooling water. The characteristic heat transfer behaviour of the device was determined by calculation of every single thermal resistance. The experimental results of these surface-micro-coolers are summarized to characteristic maps. With this characteristic maps it is possible to determine whether a micro-surface cooler can be used for a specific application.

The heat exchanger designs may be useful tools in, e.g., evaporation or condensation of liquids. Here, high heat flux has to be transferred to perform the phase transition. Other applications like highly exothermal chemical reactions are also feasible. The ability to transfer large amounts of thermal power and to keep the surface temperature reasonably low possibly opens applications in surface cooling, like cooling of high-power electric devices, laser diodes or semiconductors, e.g. processors of future computing devices.