MICROSCALE HEAT TRANSFER MEASUREMENTS DURING SUBCOOLED POOL BOILING OF PENTANE: EFFECT OF FLUID PROPERTIES AND BUBBLE DYNAMICS

Abstract

Pool boiling heat transfer measurements were made in earth gravity using a 2.7 2.7 mm2 microheater array during subcooled pool boiling of pentane. The microheater array consists of 96 independent heater elements that were maintained at an isothermal boundary condition using control circuitry. Experiments were made to investigate the dominant heat transfer mechanisms and to study the effect of fluid properties and bubble dynamics.

The semitransparent nature of the heater allowed the high speed images of the bubbles, and thus the bubble contact area, to be taken from underneath. The contact area movement on the heater was then correlated to heat transfer variation on the heater and provided a basis to investigate heat transfer mechanisms (e.g. microlayer evaporation and transient conduction).

Heat transfer related to single bubbles was studied primarily. Boiling at atmospheric pressure resulted in short and moderate bubble growth times. Boiling at higher pressures (1.34 atm and 1.5 atm) generally resulted in larger bubble growth times. Single phase heat transfer mechanisms (transient conduction and/or microconvection) were found to be dominant for bubbles with shorter growth time; two phase heat transfer mechanisms (contact line evaporation and/or microlayer evaporation) were found to be dominant heat transfer mechanisms for bubbles with longer growth time.

The proposed hypothesis is that when the bubble grows rapidly, the majority of the required heat for bubble growth originates from the superheated liquid layer and not from instantaneous heat from the wall. For bubbles that grow more slowly, however, the bubble growth is limited by the wall heat transfer--the energy stored in the superheated liquid layer has been depleted (perhaps by previously departing bubbles) leaving the wall as the only source of energy. This energy must be transferred through the microlayer evaporation or contact line heat transfer.

Finally, the heat transfer characteristics related to a more complicated case i.e. lateral merger of two bubbles on the heater was investigated. The heat transfer variation was found to be closely related to the change in the contact area and the contact line movement on the heater.