LOCAL HEAT FLOW AND TEMPERATURE FLUCTUATIONS IN WALL AND FLUID IN NUCLEATE BOILING SYSTEMS

摘要

Recent numerical and experimental investigations to improve the understanding of the nucleate boiling heat transfer process mainly concentrate on the description or measurement of local transport phenomena. It is known from these investigations that the interaction between microscale evaporation and macroscale transient heat flow in the wall and the thermal boundary layer is a key aspect for our physical understanding of boiling processes. However reliable quantitative data on the local and transient heat distribution and storage in the heater wall and thermal boundary layer is rare. In this paper we summarize recent developments and present new numerical and experimental results in this specific field of research. A fully transient numerical model has been developed based on a previous quasi stationary model of Kern and Stephan [3]. It allows describing the transient heat and fluid flow during the entire periodic cycle of a growing, detaching and rising bubble including the waiting time between two successive bubbles from a single nucleation site. It contains a multiscale approach ranging from the nanometer to the millimeter scale for the detailed description of the relevant local phenomena. The detailed analysis of the computed transient temperature profiles in wall and fluid gives accurate information about the heat supply, temporal energy storage and evaporation. It is shown that during the bubble growth and detachment period more heat is consumed by evaporation than heat supplied to the overall system. Thus the wall-and liquid thermal boundary layer cool down. After detachment, during the bubble rise period and waiting time, the evaporative heat flow decreases. In this period more heat is supplied to the overall system than consumed by evaporation, thus the wall and liquid thermal boundary layer heat up again. Experimental investigations with high resolution wall temperature measurements underneath a vapor bubble were performed in a micro-g environment and qualitatively confirm these numerical observations.