Mechanism of enhancement/deterioration of boiling heat transfer using stable nanoparticle suspensions over vertical tubes

Boiling heat transfer using nanofluids has been a subject of a few investigations in the past few years and incongruous results have been reported in literature regarding the same. Conflicting explanations for deterioration of pool boiling heat transfer coefficient at higher concentrations (4-16 wt %) have been presented by various researchers. Recently, a few works have reported a significant enhancement in pool boiling heat transfer coefficient at lower concentrations (0.32-1.25 wt %) and the physical reasons for this have not been explained. The present work is aimed at removing these ambiguities. Experiments have been carried out by using stable water based nanofluids containing alumina nanoparticles (average sizes of 47 and 150 nm) with vertical tubular heaters of various surface roughnesses (48, 98, and 524 nm). It has been observed that with the rough heater (R-a=524 nm), heat transfer is significantly enhanced and the enhancement reaches similar to 70% at a particle loading of 0.5 wt %. With the smooth heater (R-a=48 nm), heat transfer is significantly deteriorated and the heat flux reduction reaches similar to 45% at a particle loading of 2 wt %. Further, it has been observed that a parameter which is the ratio of average size of the particle to the average roughness value of the heater can explain the reported controversy in the pool boiling behavior of these suspensions. (C) 2007 American Institute of Physics.