EFFECT OF SURFACE ROUGHNESS ON POOL BOILING HEAT TRANSFER DUE TO EXPONENTIAL STEP HEAT INPUT
This paper presents the experimental investigation of pool boiling heat transfer (PBHT) from the rough surface due to exponentially varying step heat input (q α eγ) at atmospheric condition. The unidirectional scratches are made on circular surface of the 20 mm diameter copper specimen. The average roughness value (Ra) of the test surface is 0.106 µm, 1.87 µm and 3.17 µm. The rate of exponential transient (γ) is varied from 1 to 6. It is noticed that the heat transfer from the rough surface during the steady-state heat supply is higher than quasi-steady heating. It is observed that the incipient boiling temperature and onset of nucleate boiling (ONB) heat transfer increases with increase in the rate of transient. The moment of ONB, bubble coalescence and FDNB during each trial is observed using the high speed camera. The heat transfer enhancement during fully developed nucleate boiling (FDNB) is also reported in this paper. The boiling heat transfer increases with increase in the surface roughness and decrease in the rate of transient. The ONB temperature of Ra=0.106 µm, Ra=1.87 µm, Ra=3.17 µm at γ=1 has increased by 3.9%, 17.58% and 32.4% respectively from corresponding steady-state ONB temperature. The CHF of Ra=1.87 µm and Ra=3.17 µm, during exponential heat supply (γ=1) is found to be 39.65% and 47.36% higher than that of Ra=0.106 µm, respectively.