Enhancement of pool boiling heat transfer with electrohydrodynamics and its fundamental study

Abstract

The enhancement of heat transfer in the realm of pool boiling refrigerants, using the concepts of electrohydrodynamics(EHD), has been actively researched in the past decade. This research aims at studying the effect of EHD on new refrigerants (R-123, R-134a, R407c, and R-410a) with two types of evaporator tube surfaces. One surface is a smooth tube while the other is an enhanced 19 fins per inch tube. The electrode configuration is straight wire and the applied voltage levels are 0, 5, and lOkV. To conduct this study, an existing low pressure pool boiling apparatus was modified and another high pressure apparatus was designed and built. The fluids were tested on the smooth tube at operating temperatures of 4C and 20'C, and 19 fins per inch tube at 4C. Results of the experiments show that the enhancements of pool boiling heat transfer varied within the range of 6% to 570% depending upon the type of refrigerant, pool temperature, applied voltage and type of evaporator tube surface. Fundamentally the relation among the fluid thermodynamic properties, electric properties, and the applied electric field was investigated to understand the enhancements observed. The comparison of charge relaxation time (a function of the electric properties of the fluid) to the bubble departure time (a function of fluid thermodynamic properties) proved the required condition that the charge relaxation time needs to be less than the bubble departure time for the fluid to exhibit enhancement. A study was also conducted to estimate the contribution of enhancements of free convection due to EHD in the isolated bubble nucleate boiling regime to the total enhancements observed. This study led to the interesting conclusion that this total enhancement due to EHD depending upon fluid and operating conditions.