The heat transfer and instabilities results during the onset of flow boiling in minichannels

Abstract

The paper discusses the results of flow boiling heat transfer in minichannels obtained on the basis of time-dependent experiments. The main interest of the work was to investigate the occurrence of the accompanying instabilities during the boiling incipience. The essential part of the experimental standwas a test section with two minichannels, each of 1.7 mm depth. The heated element for FC–72flowing along the minichannels was a thin foil. In the tested minichannel, the temperature of the outer surface of the foil was measured due to thermoelements. The onset of flow boiling in minichannels was induced by increasing the heat flux supplied to the heater. The main aims of the investigation were to determine the heat transfer coefficient by means of the FEM with time—dependent Trefftz–type basis functions based on the Hermite interpolation and to recognize dynamic instabilities during boiling incipience. The results were illustrated as: the heat transfer coefficient, the mass flow rate and the inletpressure versustime and as boiling curves.

Pressure and Thermal Characterisation of Dynamic Instabilities During Flow Boiling in Micro/Mini-channels at Different Azimuth Orientations

“…Only 𝜃 = 180° had non-zero heat transfer coefficients at a mass flux of 10 kg/m 2 s. 𝜃 = 0° had non-zero heat transfer coefficients at a heat flux of 4.3 kW/m 2 and 7.4 kW/m 2 but they were within the measurement error. 𝜃 = 0° and 180° both had non-zero heat transfer coefficient improvements at a mass flux of 20 kg/m 2 s. At the highest mass flux tested (40 kg/m 2 s), 𝜃 = 0°, 30°, 60° and 180° had non-zero heat transfer coefficient improvements.At a mass flux of 10 kg/m 2 s, the maximum heat transfer coefficient improvement was up to 35% in the twophase region at a rotation of 𝜃 = 180°, which is in agreement with the work of Li et al[27]. This improvement was up to 250% higher than the maximum improvement of 8% and 10% at a mass flux of 20 kg/m 2 s and 40 kg/m 2 s.All heat transfer coefficient improvements at a heat flux greater than 4.4 kW/m 2 in the two-phase region of 𝜃 = 180° was more than the maximum heat transfer coefficient improvements at a mass flux of 20 kg/m 2 s and 40 kg/m 2 s. It is expected this is because the effect of vapour on the heated wall at the 10 kg/m 2 s was more pronounced, allowing a greater improvement to be achieved.…”

“…However, in some instances flow instabilities have been found to enhance the heat transfer performance of a flow boiling system. For instance: Maciejewska, et al [27] investigated flow boiling in two parallel minichannels at a mass flux of 163 kg/m 2 s and found that flow instability improved heat transfer performance at the onset of nucleate boiling. Improvements in heat transfer have also been found in arrays with 60 parallel microchannels at mass fluxes between 11 kg/m 2 s and 44 kg/m 2 s [28].…”