Investigation of Pool Boiling Heat Transfer from Rough Surface and Microchannel Geometry Under Variable Heat Supply

Abstract

Enormous amount of heat is generated in the Economic Simplified Boiling Water Reactor (ESBWR) due to exponential heat generation from the fuel rod. A core melt accident occurs when the heat generated in the nuclear reactor exceeds the heat removed by the coolant to the point where at least one nuclear fuel element exceeds its melting point temperature. Critical Heat Flux (CHF) is the phase of boiling after which heat transfer coefficient drops resulting in the rapid increase in temperature of core. Hence, understanding the mechanism of CHF is important to control loss of coolant accident (LOCA). CHF enhancement may retard the LOCA in ESBWR. Passive enhancement techniques are the most suitable for the nuclear reactor application. In view of the facts discussed above, the CHF enhancement by two passive techniques namely, rough surfaces and microchannel geometries is investigated. The transient CHF enhancement is compared with the steady-state CHF upto 10 bar pressure. The experimental setup is designed to study the pool boiling of water at 1 bar, 5 bar and 10 bar pressures. The pool boiling experiments are conducted on the thick copper sample of 20 mm diameter at saturated condition of distilled water. The unidirectional scratches are made on the sample to obtain wide range of surface roughness varying from Ra=0.106 μm to Ra=4.03 μm. It is found that steady-state CHF increases with increase in the Ra. Improved wettability and increased nucleation site density resulted in the CHF enhancement by rough surface. The microchannel geometries namely, square (SM-1.0), parabolic (PM-1.6) and stepped (SM-1.6) were fabricated by VMC machining. The improved liquid supply through the channel space and significant bubble growth resulted in the CHF enhancement by the microchannel geometry. The CHF enhancement by SM-1.6 is highest among all the microchannel geometries. The experimental setup is commissioned with programmable power supply to compare the CHF of water during pool boiling on rough surface and microchannel geometry under steady-state and exponential heat supply. The time constant (γ) of exponential heat supply is varied from 1 to 6. It is found that both, rough surfaces and microchannel geometries enhance the transient CHF. However, transient CHF gradually decreasedwith increase in γ due to liquid-vapor instability during exponential heat supply. CHF increased with increase in the pressure at both the condition viz. steady and transient. Steady-state CHF for Ra=4.03 μm and SM-1.6 at P=10 bar is found to be 71.43% and 47.37% higher compared to the CHF at P=1 bar, respectively. The correlation for heat transfer coefficient is developed for prediction of transient boiling performance which includes the non-dimensional time constant γ. Present correlation predicts the experimental values of transient HTC with MAE of 14.91%. CHF model for rough surface, based on force balance approach, is developed incorporating the effect of time constant, bubble angle and roughness parameter viz. Ra, Sm to predict the boiling crisis during pool boiling. It predicts the experimental transient CHF with MAE of 11.89%. Boiling videos are recorded at 1000 fps using high speed camera during the experiments to study the bubble dynamics during pool boiling on rough surface and microchannel geometries upto 10 bar pressure. Bubble dynamics during pool boiling of saturated water is significantly affected by the surface characteristics i.e. surface roughness and microchannel. Prolonged nucleated boiling regime is noticed for rough surface at high pressure due to the capillary wicking in the unidirectional scratches which retards the horizontal coalescence. Forces acting vertically on the growing bubble are considered to predict the bubble departure diameter. The MAE between measured and predicted bubble departure diameter for the rough surface and microchannel geometries at all pressure is 17.09% and 13.30%, respectively.