Numerical simulations of nucleate boiling and film condensation heat transfer in a fixed-volume chamber in microgravity

Abstract

Pool boiling has been extensively investigated for the last eight decades experimentally and for almost two decades numerically. As a parameter of interest, the system pressure is generally kept constant in numerical simulations of a given set of experimental conditions. Alternatively, the system pressure may be an input parameter that may depend on time. However, in the absence of active cooling of hot liquid especially in microgravity, bubbles can grow to take up a significant portion of a boiling chamber and thus pressure can vary significantly over time in a fixed-volume chamber. In this work, numerical simulations of the boiling process are performed in which the increase in thermodynamic pressure inside a fixed-volume chamber and its feedback on the boiling process is included. The test liquid used in the simulations is R-134a. The results show that the effect of pressure on volume changes due to phase change is strong. The pressure response provides a feedback for the volume changes by altering heat transfer rates due to corresponding changes in saturation temperature. The pressure response to volume change is stronger for a chamber that has comparatively more liquid than vapor due to reduced compressibility. In microgravity bubble dynamics vary greatly depending on chamber fill level and heat transfer rates also depend on chamber fill level during boiling and natural convection.