Comparative study on steady and unsteady heat transfer analysis of a spherical element using air/water mist two-phase flow

Abstract

In this study, two approaches to investigating the process cooling of a heated sphere were performed using air as well as air/water mist two-phase flow. Steady-state and unsteady heat transfer analysis were compared in the terms of the aver-age surface temperature and heat transfer rate between the sphere surfaces and the cooling fluid. When the Bi < 0.1, the temperature variation inside sphere can be neglected and the wildly known lumped capacitance model can be applied to estimate the heat transfer coefficient by measuring the sphere surface temperature. The effect of the different factors such as the inlet Reynolds numbers, surface temperature and water mist rate on heat transfer characteristics are examined. The experimental results showed that the presence of water mist leads to a significant increase in heat transfer rate over the use of air coolant alone. Also, the un-steady thermal behaviors of the water mist impingement on the heated surface and dynamic-state of cooling process changing over the sphere surface were analyzed experimentally based on the unsteady surface temperature measurements. The experimental results of the unsteady heat transfer were compared to the results obtained from steady-state estimation under the corresponding surface temperature of the sphere. Moreover, the new proposed empirical correlation for the Nusselt number based on the present experimental data are given for practical uses. Results reasonably agree well within ?3.8%.