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

Abstract

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 (4–16wt%) have been presented by various researchers. Recently, a few works have reported a significant enhancement in pool boiling heat transfer coefficient at lower concentrations (0.32–1.25wt%) 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 150nm) with vertical tubular heaters of various surface roughnesses (48, 98, and 524nm). It has been observed that with the rough heater (Ra=524nm), heat transfer is significantly enhanced and the enhancement reaches ∼70% at a particle loading of 0.5wt%. With the smooth heater (Ra=48nm), heat transfer is significantly deteriorated and the heat flux reduction reaches ∼45% at a particle loading of 2wt%. 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.

A theoretical model for nucleate boiling of nanofluids considering the nanoparticle Brownian motion in liquid microlayer

“…In this study, their correlation [10] was further improved (Eq. (6)) to take into account the fact that heat transfer deterioration occurs when the ratio of surface roughness to nanoparticle size is near 1.0, otherwise heat transfer is enhanced as R a /d p is away from 1.0, as reported by Narayan et al [21] and Das et al [22]. …”

“…12(a) illustrates that with the increasing nanoparticle size, the heat transfer is deteriorated at first, but then enhanced after the nanoparticle size exceeds the average surface roughness (100 nm). Narayan et al [21] and Das et al [22] suggested that when the nanoparticles size equals roughly to the surface roughness, the deposited nanoparticles could settle in the cavity and thus significantly reduce the active site density. On the contrary, when the nanoparticles are obviously larger or smaller than the surface roughness, the deposited nanoparticles could create more active sites and thus enhance the heat transfer.…”

A comprehensive review on pool boiling of nanofluids

“…phenomenon can be also explained by using the surface particle interaction parameter (ψ), which is the ratio of average surface roughness (R a ) to average particle diameter (d p ) reported by Narayan et al [104]. They stated that at low nanoparticle concentrations for ψ≫1, the heat transfer coefficient increases, multiplying cavities to smaller sites [80].…”

Effects of pressure, orientation, and heater size on pool boiling of water with nanocoated heaters

“…They explored some prevailing CHF theories that showed a direct link between the CHF value and the wettability of the heater surface. Das et al [8] and Narayan et al [9] looked into the surface roughness, particle size, and nanoparticle concentration to see the role each parameter played in the nucleate boiling heat transfer (BHT). Sefiane [10] tried to explain the CHF enhancement created by the nanoparticle deposition due to structural disjoining pressure and contact angle pinning.…”