Trefftz functions methods in analysis of the effect of enhanced heated surface on FC-72 flow boiling in minichannels

This paper focuses on time-dependent flow boiling heat transfer. The main element of the research stand was the test section with 11 rectangular minichannels of 1 mm depth. The FC-72 was heated by a thin foil during upward flow. The temperature of the outer surface of the foil was measured by infrared thermography. Two types of foil surface were tested: smooth and enhanced with recesses produced using emery paper. The main objective was to verify whether the enhanced surface helps to obtain a better heat transfer efficiency compared to the smooth surface. The 2D mathematical model of heat transfer was proposed, while methods with Trefftz functions were applied to solve the time-dependent problem of heat transfer. The Robin condition was used to determine heat transfer coefficients at the interface between the foil and the fluid. Experimental data on temperature and pressure and their validation were presented and discussed. Results based on the heat transfer coefficients were provided. They covered the calculation procedure, the determination of the mean relative error and results validation with using the 1D approach. The heat transfer coefficients as a consequence of application of the Trefftz method and the hybrid Picard-Trefftz method in the calculation procedure with comparison analysis of the results were shown. Furthermore, comparisons aimed at obtaining the results characteristic for smooth and enhanced surfaces were also conducted. In the experiment with an enhanced heated surface, the values of the heat transfer coefficient appeared to be higher. Boiling curves were illustrated and discussed.