The role of surface tension and viscosity of the coolant on spray cooling performance of red-hot inclined steel plate

Abstract

The dissolved surfactant in water droplet alters the surface tension, contact angle and viscosity and thereby heat transfer mechanism changes accordingly. The sliding characteristic of the water and vapor film resulting from the agglomeration of partially evaporated droplets on the hot plate are controlled by the aforesaid properties of the fluid. In addition to the above, the sliding characteristic further depends on the orientation of the plate. For the identification of appropriate conditions to get high heat transfer rates, the effect of viscosity, surface tension and orientation of the plate in the favorable direction of heat transfer needs to be investigated. The literature does not reveal the combined effect of the aforesaid process parameters on heat transfer rate. Therefore, in the current work an attempt has been made to reveal the effect of viscosity, surface tension and the orientation of the plate on heat transfer. To determine the effect of viscosity and surface tension in the presence of the foaming characteristics, three types of surfactants such as Sodiumdodecyl sulfate (SDS), Cetyltrimethylammonium bromide (CTAB) and Polyoxyethylene (20) sorbitan monolaurate (Tween 20) below critical micelle concentration have been used. The result reveals that the reduction of surface tension and viscosity up to 48% and 45% enhances the heat transfer rate (CHF from 1.39 MW/m2 to 1.58 MW/m2) at 30° plate inclination due to reduction in the residence time from 40 ms to 20 ms and enhancement in wettability characteristic (contact angle from 72.34° to 30.96°). The aforesaid result has been achieved at an optimum concentration of SDS (600 ppm), CTAB (180 ppm) and Tween 20 (42 ppm). In addition to the above, to identify the effect of viscosity and surface tension in the absence of foaming, ethanol has been selected as an additive. In comparison with Tween 20, the ethanol added water spray at an inclination of θ = 30° shows insignificant enhancement due to very small reduction and increment in surface tension and viscosity, respectively. The accuracy and enhancement are identified by comparing the obtained results with the information reported in the literature.