Effect of Surface Active Agents in Boiling Heat Transfer

Abstract

The boiling heat transfer phenomenon has presented a state o ambiguity regarding the role of solid-liquid-vapour interface in the mechanisms of heat transfer. Recent studies (S1, M1) have given an indication to the possibility of the vaporization of a micro layer at the boiling surface as a alternative to the well known theories based purely on the hydrodynamic factors. This study is an attempt to understand the boiling heat transfer mechanism at solid-liquid-vapour interface and to study the effect of interfacial properties like surface tension and contact angle on the maximum (critical) heat flux. The present studies use the technique of changing the solid-liquid-vapour interface characteristics of water through the use of surface-active agent as additive, to study the boiling heat transfer under changed interface conditions. Four different surfactants were used at three levels of concentration in water. Surface tension and contact angle measurements were carried out using the shadow photographs of pendant cops and sessile drops. Boiling heat flux measurements of these surfactant solutions in water were carried out using heat transfer surface. Experiments involving pool boiling and the boiling of thin liquid films were carried out over the transition and nucleate boiling regimes. It has been observed that the solid-liquid-vapour interface characteristics play a cry important role in the boiling heat transfer mechanism. By a suitable choice type of surfactant and concentration, the critical heat flux and heat transfer coefficients can be improved markedly. It is suggested that the spreading wetting characteristic improves the heat transfer rate whereas the increased viscosity and decreased thermal conductivity of the liquid microlayer under the vapour masses may cause the heat flux to decrease. The present study shows significant possibilities for future studies in the nucleate boiling, transition boiling and film boiling regimes using surfactant solutions.