Heat transfer mechanism, pressure drop and flow patterns during FC-72 flow boiling in horizontal and vertical minichannels with enhanced walls

The paper discusses the results of the flow boiling heat transfer in horizontal (in two positions) and vertical minichannels of 1 mm depth, 40 mm width and 360 mm length. A single-sided enhanced foil with various depressions heats FC-72 laminar flow. Liquid crystal thermography was used for measuring the temperature distribution on the plain side of the foil. The observations of the flow structures were carried out on the enhanced side of the foil contacting fluid in the minichannel, which enabled the identification of flow structures and presentation of flow pattern maps. The void fraction was determined for some cross-sections of two-phase flow images and the vapour quality was determined on this basis. The study employed heat transfer analysis and results regarding local heat transfer coefficient, and boiling curves were constructed. In addition to the typical shapes of boiling curves, the untypical boiling curves with several “stepped” courses of nucleation hysteresis were found. The suitability of classical methods of the two phase pressure drop determination using the experimental verification was confirmed. The comparison of the total measured pressure drop with data obtained from the theoretical models shows that all predicted results fall within the range of ±30%, but only for the vertical minichannel. In most cases the experimental data reached values approaching approximate values in comparison with values from the theoretical models. Calculation of the pressure drop on the basis of the thermodynamic vapour quality and the vapour quality calculated from the experimentally determined void fraction showed a similar dependence.