Mechanism of nucleate boiling heat transfer in heater sizes, working fluids, and micro-porous surface effects

The present study involves an experimental investigation of nucleate boiling heat transfer mechanism during boiling from the surfaces of wire heaters immersed in saturated FC-72 coolant and water. The vapor volume flow rate from the wire heater surface during nucleate boiling was determined by measuring the volume of bubbles with varying the wire diameter (25 mum, 75 mum, and 390 mum), using the consecutive-photo method. The effects of the wire size on the heat transfer mechanism during nucleate boiling were investigated by measuring the vapor volume flow rate and the frequency of bubbles departing from a wire immersed in saturated FC-72. One wire diameter (390 mum) was selected and tested in saturated water to investigate the fluid effect on the nucleate boiling heat transfer mechanism. Results of the study showed that the observed increase in nucleate boiling heat transfer coefficients with reductions in wire diameter was related to the decreased latent heat contribution. The latent heat contribution of boiling heat transfer during the water test was found to be higher than that of FC-72. Moreover, generally the frequency of the bubbles escaping from the surface of the wire correlated with bubble diameter. The present study also includes an experimental investigation of the nucleate pool boiling heat transfer enhancement mechanism of microporous surfaces immersed in saturated FC-72. Measurements of bubble size, frequency, and vapor flow rate from a plain and microporous coated 390-mum diameter platinum wire using the consecutive-photo method were taken to determine the effects of the coating on the convective and latent heat transfer mechanisms. Results of the study showed that the microporous coating augments nucleate boiling performance through increased latent heat transfer in the low heat flux region and through increased convection heat transfer in the high heat flux region.