Ultra high heat flux cooling provided by flow boiling in microscale with enhancements using nanostructured surfaces
Due to their heat transfer efficiency and compact implementation methods, the use of flow boiling via plain and modified microchannels for cooling solutions gained a significant importance in the last decade. The increasing need for more efficient cooling solutions in various fields of micro scale cooling such as aerospace, microreactors, automotive industry, micropropulsion, fuel cells, drug delivery systems, biological and chemical applications is motivating researchers to investigate the physics behind the micro scale flow boiling phenomena. The proposed study aims to make a contribution to the literature in the related field by filling the gap of scientific knowledge about the microchannel flow boiling heat transfer capabilities at ultra high mass fluxes, under unstable boiling conditions and with microchannels having inner wall surface enhancements via nanostructure coating. The present thesis study and results of related experiments are divided into three main parts: ultra high mass flux flow boiling experiments, the effect of inlet restrictions and tube size on premature critical heat flux in microchannels and flow boiling heat transfer enhancement via coating polyhydroxyethylmethacrylate (pHEMA) on inner microtube walls. In the first part, microchannels having ~250 mm and ~500 mm hydraulic diameters were tested at various ultra high mass fluxes values and different heated length for forcing the conventional heat removal limits of flow boiling via microchannels. De ionized water was used as working fluid and test section was heated with Joule heating. Wall temperatures for each case were recorded and exit qualities were calculated. The resulting CHF boiling curve demonstrates cooling rates >30000W/cm2) that were never achieved by flow boiling in microchannels could be obtained. In the second part of the present study, useful information about premature CHF phenomena was provided. The study offers a parametric comparative investigation. Experimental data are obtained from microtubes having 250~mm and 685~ mm inner diameters, which were tested at low mass fluxes (78.9 276.3 kg/m2s) to reveal potential boiling instability mechanisms. Moreover, inlet restrictions were introduced to the system for observing their effect in mitigating unstable boiling conditions and extending the boiling curve. De ionized water was used as a coolant, while microtubes having 5,65 cm heated length were heated by Joule heating. Furthermore, Fast Fourier Transform (FFT) of the deduced data is performed for revealing the frequency correlations of the every obtained temperature and pressure oscillations before and just before the premature dryout condition. The results show the inlet restrictions have a significant effect on reducing the unstable boiling fluctuations and the proposed FFT method was proved to be a useful tool to detect premature dryout before it occurs. In the third part, flow boiling heat transfer experiments were conducted on microtubes (inner diameter of 250 mm, ~500 mm and ~1 mm) with a constant heated length 2 cm and with enhanced inner surfaces having deposited polyhydroxyethylmethacrylate (pHEMA) which extends the boiling curve, increases the heat transfer surface area, and provides additional nucleation sites. De ionized water was utilized as the working fluid and test section was heated by Joule heating in this study. Nanostructures on the microtube walls were coated through initiated chemical vapor deposition (iCVD) technique. A significant extension in CHF boiling curve and increase in heat transfer were observed with nanostructure enhanced surfaces compared to the plain surface counterparts for two relatively high mass velocities, namely, 10000 kg/m2s and 13000 kg/m2s.