Boiling and condensation

Two-phase heat transfer, which involves conversion of liquid into vapor, referred to as boiling, or conversion of vapor into liquid, referred to as condensation, takes place in equipment such as boilers and condensers in thermal power plants, evaporators and condensers in refrigeration systems, water-cooled nuclear reactors, major equipment in the process industry, and modern heat sinks for thermal management of electronics. Of particular importance in the design of the equipment are the rates of heat transfer or heat transfer coefficients and the associated pressure losses. In boiling and condensation, the coupling between the fluid dynamic process and the heat transfer process is stronger than what exists in single-phase flows. In a two-phase flow with phase change, there is a continuous variation in the fraction and distribution of each phase and hence the flow pattern, which influences the local heat transfer processes. Therefore, the flow at any axial location in the tube can never be fully developed thermally or hydrodynamically, unlike a single-phase flow. In this chapter, the commonly observed regimes of pool boiling and flow patterns in flow boiling are discussed. Prior to the presentation of the correlations used to predict the heat transfer coefficients and the critical heat flux, the chapter presents a brief discussion on the wall superheat required for nucleation from a heating surface. This is followed by a discussion on the film condensation that occurs on flat plates and horizontal tubes and a presentation of the respective heat transfer coefficient correlations. The chapter ends with an introduction to the prediction of pressure drop in two-phase flows with phase change.