An Experimental Investigation of Pool Boiling Heat Transfer from a Micro-finned Cylindrical Surface

Abstract

When a large amount of energy transfer is required in a small space, the pool boiling heat transfer is preferred compared with the single phase heat transfer. Pool boiling is a phenomenon in which energy transfer or energy conversion takes place due to the liquid-vapor phase change (latent heat). The pool boiling process is widely used for high heat removal and it is used for cooling of various engineering applications in HVAC industry, steam generators, nuclear power reactors in power plant, cooling of electronic components, power generation in marine ships, etc. In this work, Pool boiling experiments were performed on micro-finned cylindrical test surface at various pressure values of 1 bar, 1.25 bar, 1.5 bar, and 1.75 bar with refrigerant R-141b and R-123. The experiments were carried out for the heat flux range varying from 10 to 100 kW/m2. The outside diameter and effective heating surface length of the test tube are 20 mm and 46 mm respectively. Also the bubble departure diameters were studied in pool boiling heat transfer over plain and micro-finned cylindrical surfaces with different refrigerant. The bubble departure diameters were measured from the images captured by a using Canon EOS 1200D camera at different values of heat fluxes with various pressure values. It is found that the boiling heat transfer coefficient increases in the range of 12 % to 270 % with a micro-finned surface over a plain surface with R-141b at atmospheric pressure and increases in the range of 24 % to 310 % with R-123 at atmospheric pressure. The boiling heat transfer coefficient is higher for R-123 compared to R-141b at given system pressure. The experimental data of the boiling heat transfer coefficient are compared with the plain cylindrical surface at different pressure with different refrigerant.And also the experimental values of boiling heat transfer coefficient are compared with different available correlations. A new correlation for boiling heat transfer coefficient over a micro-finned surface is developed using the dimensional analysis. For the presently developed correlation, the R-square value obtained is 0.947, and the standard error of data is 0.169, and MAE is 13.75 % for the whole range of data points. From the bubble departure diameter analysis it is found that with an increase in Ja (Jacob Number) at a given pressure, the bubble departure diameter decreases. The experimental data of the bubble departure diameter are compared with the plain cylindrical surface at different pressure. And also the experimental values of bubble departure diameter are compared with different available correlations. A new correlation for bubble departure diameter over a micro-finned surface is proposed which predict the experimental value with satisfactory accuracy.