The effects of inlet restriction and tube size on boiling instabilities and detection of resulting premature critical heat flux in microtubes using data analysis

In order to achieve high heat removal rates for micro scale cooling, it may be necessary to exploit boiling heat transfer. The size of corresponding heat sinks is continuously decreasing from mini size to micro size, and one of the most practical and extensive cooling methods is boiling heat transfer in plain microchannels and microtubes, which might be limited by inherent boiling instabilities. This study provides useful information about boiling instability phenomena in microtubes and offers a parametric comparative investigation. Experimental data are obtained from microtubes having 254 μm and 685 μm inner diameters, which were tested at low mass fluxes (78.9-276.3 kg/m ² s) to reveal potential boiling instability mechanisms. De-ionized water was used as working fluid, while microtubes were heated by Joule heating. Configurations prone to boiling instabilities (low system pressures, low mass fluxes) were imposed to observe boiling instabilities in microtubes. Fine restriction valves were introduced to the system for providing flow restriction at the inlet. Alongside the experiments without any inlet restriction, experiments were conducted with configurations having inlet restrictions, where pressure drops over inlet restriction elements were 4 and 8 times as much as pressure drop over the microtube to suppress boiling instabilities. Temperature and pressure drop fluctuation signals were recorded and processed before premature CHF (Critical Heat Flux) conditions and at impending premature CHF conditions. Furthermore, Fast Fourier Transform (FFT) of the recorded data is performed for revealing the frequency correlations of the obtained fluctuations for observing the change in the FFT behavior. A significant rise in energy of the side lobes, which are basically the high frequency spectral regions, was observed from FFT profiles for impending premature CHF conditions implying that FFT could be used as a detection tool for premature CHF.