Viscito, Luca (2017) Flow boiling in a circular channel and CHF in rectangular minichannels with low-GWP refrigerants. [Tesi di dottorato]

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The correct design of evaporators and heat-spreader systems continuously requests flow boiling heat transfer and pressure drop data, especially for new low-GWP fluids that are about to replace HFC substances in different fields. More data are also required in operating conditions that promote flow asymmetry, for which the classical heat transfer prediction methods are not completely accurate. In addition, the progressive miniaturization of electronic devices and their performance evolution made the two-phase cooling an interesting option. For these systems, the determination of the critical heat flux (CHF) is extremely important to avoid possible overheating and physical burn-out. In this context, the first part of the thesis provides new CHF data for five different refrigerants (R134a, R32, R1234yf, R1234ze and R1233zd) in an aluminum multi-minichannel heat sink in which seven rectangular minichannels are carved. A new operative definition of critical heat flux is firstly given. Then, the effect of geometry (represented by the Lh/D ratio, equal to 19, 27 and 44), mass flux (from 145 to 3000 kg/m2 s) and saturation temperature (from 24.7 to 75.5 °C) is investigated and discussed. Finally, the experimental data are compared to the predicted values obtained from some of the most quoted CHF correlations available in scientific literature. New flow boiling heat transfer and pressure drop data in a single, circular, horizontal smooth stainless steel channel of 6.0 mm internal diameter are instead provided in the second part of the thesis. Specifically, the heat transfer coefficients are measured at the top, bottom, left and right sides of the tube in order to have a more accurate estimation of the peripheral average heat transfer coefficient. Different operative conditions are chosen to promote stratification and to better understand the relative importance of the convective and nucleate boiling contributions on the two-phase heat transfer process. Propane (R290) and R134a are employed as working fluids. Mass flux goes from 150 to 500 kg/m2 s, heat flux from 2.5 to 40 kW/m2 and saturation temperature is set from 20 to 35 °C. The effect of all the operative parameters and also that of the working fluids on local heat transfer coefficients and pressure drop are discussed and the experimental data are compared with some of the available correlations taken from scientific literature.

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