The study of boiling heat transfer in vertically and horizontally oriented rectangular minichannels and the solution to the inverse heat transfer problem with the use of the Beck method and Trefftz functions

Abstract

The paper presents experimental studies of boiling heat transfer in a rectangular, asymmetrically heated minichannel which is 1.0mm deep, 60mm wide, and 360mm long. It is oriented vertically with the bottom-up flow and horizontally at two different positions (180° difference). The investigations focus on the transition from single-phase forced convection to nucleate boiling, that is, from the zone of boiling incipience further to developed boiling. The experiment is carried out with FC-72 at a mass flow rate range of 165kg/(m2s) and a pressure of 120–140kPa at the inlet to the minichannel. Owing to the liquid crystal layer located on the heating surface contacting the glass, it is possible to measure the heating wall temperature distribution while increasing the heat flux transferred to the liquid flowing in the minichannel. The flow structure is observed simultaneously on the opposite side of the minichannel through another piece of glass. The first objective of the calculations is to evaluate a heat transfer model and numerical approach to solving the inverse boundary problem, and to calculate the heat transfer coefficient. The inverse problem is solved with the use of sensitivity coefficient method (Beck method) in combination with Trefftz functions. Calculations are supplemented with an error analysis focused on determining the errors including those of heating foil temperature measurements with liquid crystals thermography and of heat transfer coefficient for the transition from a single phase to boiling incipience. The second objective of the paper is to determine the void fraction for cross-sections of selected images for increasing heat fluxes supplied to the heating surface. These results are presented as void fraction dependence along the minichannel length for the selected cross-sections.