Abstract: The studies were conducted for boiling FC-72 in rectangular, vertical and asymmetrically heated minichannels, 0.5, 1.0 and 1.5 mm deep, 20 mm wide and 360 mm long, for increasing and decreasing heat flux qw from 58 to 132 kWm-2, at the absolute pressure from 1.16 to 1.84 bar for the mass flux from 185 to 1139 kg m-2 s-1. The boiling process took place on a flat heating surface made of 0.1 mm thick acid-resistant rolled plate (alloy of Ni-Cr-W-Mo) with the roughness coefficient Ra = 121 m. At the minichannel inlet the liquid had the constant temperature of 288 K.
A high-speed camera mounted on the measurement stand helped determine, for the set cross-sections, the void and liquid fractions as a function of variable thermal and flow parameters such as channel hydraulic diameter, heat flux, pressure, mass flux and the temperature of the liquid subcooling at the minichannel inlet.
Two-phase flow pattern images were obtained for the variable quantities measured in the experiment. The following flow patterns were observed: bubbly flow, bubbly-slug flow, slug flow, wispy-slug flow and mist flow.
For the considered bubbly and bubbly – slug flow the calculations based on the Trefftz method were intended to determine the liquid temperature in the minichannel, with exclusion of a vapor phase. The heat transfer process in a minichannel was described by a two-dimensional energy equation.
The sought-for two-dimensional temperature distribution of the flowing liquid was approximated with a linear combination of the Trefftz functions, assuming a parabolic profile of the liquid velocity. The coefficients in this linear combination were determined by minimizing the error functional that described the mean squared error with which the approximated refrigerant temperature satisfied the known boundary conditions. The resultant approximation was an accurate solution to the energy equation and approximate solution to the known boundary conditions.
The numerical solution found with the Trefftz functions was compared with the simplified solution based on the assumption that the entire heat generated in the heating foil is transferred to the flowing refrigerant and heat conduction does not occur along the minichannel height. Both solutions gave similar results.
