The Effect of Transient Power Hotspots on the Heat Transfer Coefficient during Flow Boiling Inside Single Microscale Channels

摘要

This paper presents flow boiling experimental results concerning the transient behavior of the heat transfer coefficient (HTC) of a hotspot region undergoing through dynamic heat flux variations. The experimental data were obtained for R134a flowing inside a horizontal 1.1-mm internal diameter stainless-steel tube at a saturation temperature of 31 degrees C, mass velocities of 400 and 600kg m(-2) s(-1), local vapor quality of 8 and 40% and time-averaged heat flux of 80 and 120kW m(-2). The hotspot region undergoes over-sinusoidal, square, and sawtooth heat pulsation at frequencies of 0.5, 1, and 2Hz and half amplitudes of 20 and 40kW m(-2). A fast response type-K thermocouple of 13-mu m wire diameter was used to capture the wall temperature fluctuations during the tests. Parametric analyses are performed and the effects of heat pulses severity, waveform, frequency, and amplitude on the local HTC and wall superheat temperature are assessed. Also, the influences of mass velocity and vapor quality on the transient behavior of the HTC are analyzed.