An experimental study on flow boiling in microchannels under heating pulses and a methodology for predicting the wall temperature fluctuations

Abstract

Flow boiling in microchannels is a particularly interesting cooling method to maintain great temperature uniformity in applications exhibiting transient and/or non-uniform heat dissipation. The two-phase flow heat transfer coefficient is directly proportional to the local heat flux and higher heat loads are counterbalanced by lower thermal resistance. This study provides a fundamental understanding of how the temperature fluctuations caused by transient heat flux are affected by the mass velocity, local vapor quality, average heat flux and the heating pulses waveform, amplitude and its frequency. Transient wall superheat temperature data were obtained from flow boiling experiments for R134a at a saturation temperature of 31 °C and 0.5 and 1.1 mm diameter stainless steel circular channels. The results revealed that the average wall superheat temperature during heating pulses behaves similarly to its respective time-averaged conditions. Also, the amplitude of temperature fluctuations decreased for increasing average wall superheat temperature. A methodology for predicting the temperature fluctuations based on first-order transfer functions is proposed. In this approach, methods developed for steady heating conditions are extrapolated to estimate the heat transfer coefficient corresponding to the average and the fluctuating component of the wall superheat temperature. The accuracy of the methodology was evaluated for seven well-known methods from literature for estimating the flow boiling heat transfer coefficient in small diameter channels.