Transient oscillatory saturated flow boiling heat transfer and associated bubble characteristics of FC-72 over a small heated plate due to heat flux oscillation

Abstract

An experiment is carried out here to investigate the transient oscillatory boiling heat transfer and associated bubble characteristics of FC-72 flow over a small circular plate subject to a time varying heat flux with the plate flush mounted on the bottom of a horizontal rectangular channel. At the inlet the flow is maintained at saturated liquid state with zero vapor quality. The imposed heat flux oscillates periodically with time in the form of rectangular waves. In the experiment the FC-72 mass flux G varies from 300 to 400 kg/m(2)s, mean imposed heat flux (q) over bar ranges from 0 to 10 W/cm(2) and the amplitude of the heat flux oscillation Delta q is fixed at 10-50% of (q) over bar with the period of the heat flux oscillation varied from 10 to 30 s. The experimental results show that the time-averaging FC-72 oscillatory boiling heat transfer characteristics resemble that for stable flow boiling. However, the imposed heat flux oscillation causes significant temporal oscillations in the heated plate temperature, boiling heat transfer coefficient, bubble departure diameter and frequency, and active nucleation site density. These physical quantities oscillate at the same frequency as the heat flux oscillation and at a higher (q) over bar, a larger Delta q/(q) over bar, and a longer t(p) they exhibit stronger oscillations. Besides, a slight time lag in T-w oscillation is seen. Moreover, the size of departing bubbles, active nucleation site density and bubble departure frequency decrease as the heat flux is reduced to the low level of (q) over bar - Delta q. The opposite processes take place for the heat flux raised to the high level of (q) over bar + Delta q. Furthermore, at the mean imposed heat flux close to that for the ONB in the stable boiling we observe intermittent boiling in the flow. A regime map is provided to delineate the boundaries among single-phase liquid flow, intermittent boiling and persistent boiling. (C) 2011 Elsevier Ltd. All rights reserved.