Experimental pool boiling heat transfer analysis with Zn-Cu micro-structured surfaces developed by electric discharge coating technique

Abstract

Boiling heat transfer has proven to be a suitable method for removing high heat flux generated in equipment such as nuclear reactors, two-phase coolers for electronics components. The heat generated during the boiling process is used to form bubbles through evaporation. Boiling heat transfer improvement involves reducing the surface temperature for boiling as well as increasing the Critical Heat Flux (CHF). The nucleate boiling heat transfer coefficient (NBHTC) is considered to be improved if a certain heat flux occurs at comparatively lower surface superheat. Information available in the existing literature evidently establishes that the micro/nano-structures on the boiling surface serve as nucleation sites for bubble generation which help in improvement of boiling performance. The present experimental study involves fabrication of micro-structured surfaces using Electric Discharge Coating (EDC) process. In this process, a green compact tool (used as electrode) was prepared from zinc and copper powder in predefined proportion employing powder metallurgy technique. During the EDC process in a hydrocarbon oil dielectric medium, the liberated composite particles of zinc and copper from the electrode were deposited on the copper substrate and thereby the intended micro-structured surfaces were generated. The Scanning Electron Microscope (SEM) images of these generated surfaces showed discrete globular structures and cavities that served as nucleation sites. Pool boiling using deionized water showed decrease of wall superheat compared to bare surface. The CHF improved by 26.7 % for the best performing surface while the average NBHTC reached as high as 116.8 kW/m2/°C indicating an improvement of 216 % as compared to bare surface A detailed discussion on the increase in heat transfer with regard to porosity, pore size and the wettability of the coating are presented in this paper. Further, a comparison of the results of the present study with some relevant experimental results available in the literature clearly demonstrates the applicability of the coating method used in the present work in real-life applications as one of the promising alternatives.