On the quenching of steel and zircaloy spheres in water-based nanofluids with alumina, silica and diamond nanoparticles

Quenching experiments of a high temperature sphere in Al2 O3 nanofluids are conducted to investigate the characteristics of film boiling and compared to those in pure water tests. One stainless steel sphere of 10 mm in diameter at the initial temperatures of 1000∼1400 K was tested in the nanofluids of the volume concentrations from 5 to 20% and the degrees of subcooling from 20 to 80 K. The test results show that film boiling heat fluxes and heat transfer rates in nanofluids were lower than those in pure water. The differences of the film boiling heat transfer rates between pure water and nanofluids become larger when the liquid subcooling decreases. Those results suggest that the presence of nanoparticles in liquid enhances vaporization process during the film boiling. The effects of nanoparticle concentrations of more than 5 vol. % on film boiling appear to be insignificant. However, the minimum heat fluxes tend to decrease when the concentration increases. Direct quenching without film boiling was repeatedly observed when an unwashed sphere was employed for quenching tests in nanofluids. It suggests that nanoparticle deposition on the sphere surface prevents the sphere from forming film around the sphere, which consequently promotes the rapid quenching of the hot sphere.Copyright © 2004 by ASME

Film Boiling Heat Transfer on a High Temperature Sphere in Nanofluid

In this paper, numerical analysis of the pool boiling heat transfer on isothermal elliptical tubes with different aspect ratios at saturated condition is presented. Bubbles’ tip velocities and positions, interfacial topological changes as well as convection heat fluxes of five different cases are presented for wide ranges of wall temperatures. Both time-averaged and dynamic behaviors of flow physics and heat transfer are investigated. The departure time of the first bubble, its time-dependent and averaged velocities, and heat fluxes are obtained. Finally, a novel correlation is presented for the Nusselt number that accounts for the elliptical tubes’ diameter and aspect ratio.

Numerical investigation of the natural convection film boiling around elliptical tubes

Pool film boiling from spheres to saturated and subcooled liquids of Freon-12 and Freon-22

Pool film boiling was studied by visualized quenching experiments on stainless steel spheres in water at the atmospheric pressure. The surfaces of the spheres were coated to be superhydrophobic (SHB), having a static contact angle greater than 160 deg. Subcooled conditions were concerned parametrically with the subcooling degree being varied from 0 °C (saturated) to 70 °C. It was shown that film boiling is the overwhelming mode of heat transfer during the entire course of quenching as a result of the retention of stable vapor film surrounding the SHB spheres, even at very low wall superheat that normally corresponds to nucleate boiling. Pool boiling heat transfer is enhanced with increasing the subcooling degree, in agreement with the thinning trend of the vapor film thickness. The heat flux enhancement was found to be up to fivefold for the subcooling degree of 70 °C in comparison to the saturated case, at the wall superheat of 200 °C. A modified correlation in the ratio form was proposed to predict pool film boiling heat transfer from spheres as a function of the subcooling degree.

Y. Kikuchi

Papers

Heat transfer and the low limit of film boiling