Fundamentals

Abstract

The liquid-vapor interface controlled by the surface tension and meniscus radius plays an important role in fluid flow and heat transfer of a heat pipe. In this chapter, the surface tension will be first introduced from three points of view: physical phenomenon, molecular dynamics, and thermodynamics, respectively. The temperature effect on the surface tension will be discussed and its relationship to the Marangoni flow. When the liquid-vapor interface has a curved surface, a pressure difference across the interface exists. This pressure difference can be predicted by the Laplace-Young equation, which will be addressed including the derivation processes and the origin of the capillary force in a heat pipe. The equilibrium vapor pressure of a liquid will be presented including the effects of meniscus radius and electric field on saturation pressure. When the meniscus radius of a bubble decreases, saturation pressure decreases, which can be used to explain the capillary condensation phenomenon. The contact angle, which significantly affects the capillary force, evaporation, and condensation in a heat pipe, will be discussed including the effects of temperature and surface roughness. Following this, advancing and receding contact angles will be addressed including the velocity effect. Finally, this chapter will cover primary factors affecting thin film evaporation which plays a key role in a heat pipe. The goal of this chapter is to provide the fundamentals related to interface surface, surface tension, contact angle, and phase change heat transfer occurring in heat pipes. Surface TensionWhen liquid flows down in a vertical capillary tube, as shown in Fig. 2.1, a small drop with an almost round shape starts to form and hang from the lower end of the capillary tube. This pendant suspended from the lower end of the capillary caused