EVAPORATION FROM AN EXTENDED MENISCUS FOR NONISOTHERMAL INTERFACIAL CONDITIONS

A study is presented to determine the effects of evaporation from the thin film region of a liquid-vapor meniscus within the micropores of a heat pipe porous or grooved wick on the interfacial shape, temperature distribution, and pressure distribution. Wayner's theoretical treatment of evaporating thin films is applied to the problem of an evaporating extended meniscus within circular or slotted pores. In this application of Wayner's model, the nondimensional momentum equation is uniquely scaled in terms of the capillary number, to justify the use of the static meniscus curvature as a boundary condition for the extended meniscus profile even for the dynamic evaporating conditions studied herein. This boundary condition for small capillary numbers is consistent with (he observation of the nearly constant meniscus curvature with evaporation rate that the thin film must asymptotically approach. From these basic tenets, the mechanical and thermal behavior of a stably, evaporating, nonisothermal extended meniscus is predicted. The resulting predictions are qualitatively consistent with the experimental findings of Wavner and the previous theoretical studies. They further support the claims that for the cases studied herein, both thermocapillary stresses and vapor recoil stresses at the liquid-vapor interface are negligible. However, scaling arguments are presented that identify the conditions necessary for these terms to be important.