Performance of a Heat Exchanger Based on Enhanced Heat Diffusion in Fluids by Oscillation: Experiment

The results of a study on the performance of a heat exchanger that takes advantage of enhanced heat diffusion in oscillated fluids are presented. In this heat exchanger, the fluid occupies a bundle of capillary tubes that connects two reservoirs at different temperatures; a piston in each reservoir drives the oscillation. The experimental findings are compared with predictions based on the assumptions that (a) a capillary tube does not exchange heat with the neighboring tubes, (b) the pressure in the reservoirs undergoes an ideal sinusoidal motion, and (c) each reservoir has an infinite heat capacity such that the fluid entering the tubes is at a constant temperature. Good agreement has been found between the actual performance of the heat exchanger and the idealized analysis for low and high frequencies. However, around the frequency corresponding to optimum performance, i.e., where the thermal boundary layers occupy the entire cross section of the capillary tubes, agreement is only fair. The measurements show that there is a temperature variation across the bundle and that the fluid entering the tubes has a nonsteady temperature due to weak, nonuniform mixing within the reservoirs (therefore, a spatial/temporal average was taken). This lateral and temporal variation in the temperature distribution appears to be the leading cause of the difference between the experimental and predicted results. As with any heat pipe, the reduction in the resistance to heat flow in the pipe must be accompanied by a similar ease of heat flow to and away from the ends of the pipe. Therefore, the reservoir fluid dynamics is of paramount importance in these heat exchangers. Some numerical modeling of the fluid flow in the reservoirs, as well as some velocity measurements (using a laser-Doppler anemometer), are also presented.