Experimental Study of Pool Boiling Heat Transfer Enhancement over Microchanneled Surfaces.

摘要

Pool boiling is of interest in heat transfer applications because of its potential for removing large amount of heat resulting from the latent heat of evaporation and little pressure drop penalty for circulating coolant through the system. However, the heat transfer performance of pool boiling systems is still not comparable to the cooling ability provided by enhanced microchannels operating under single-phase conditions. This investigation focuses on the bubble dynamics and heat transfer on plain and structured microchanneled surfaces under various heat fluxes in an effort to understand the underlying heat transfer mechanism through the use of a high speed camera.;In a preliminary study, silicon chips have been tested in the nucleate boiling regime, and beneficial microchannel geometries have been identified. It is determined that heat transfer enhancement occurs because of (i) an increase in surface area and (ii) an improvement in the heat transfer mechanism through the channels functioning as liquid conduits for three side heating. The range for channel size in which the greatest enhancement occurs has been identified as being 200 -- 400 mum width and 300 -- 500 mum depth.;The second study has been investigated with copper chips, with improvements to the test setup for accurate measurement of surface temperature. Ten chips, in addition to a plain chip have been evaluated for heat transfer performance. It has been determined that surfaces with many, small hydraulic diameter channels enhance the heat transfer as well as surfaces with wide and deep channels. The best performing chip had a record heat transfer coefficient of 269 kW/m2K. The large heat fluxes of over 240 W/cm2 were attained without reaching the critical heat flux condition, because of the open channels on the surface acting as conduits for liquid supply to the nucleation sites. The microchannels prevent surface dryout and critical heat flux (CHF), while the channel width controls the size of the departing bubbles.