BOILING HEAT TRANSFER IN A VERTICAL MICROCHANNEL: LOCAL ESTIMATION DURING FLOW BOILING WITH A NON INTRUSIVE METHOD

This paper summarizes the results of experimental and numerical studies concerning boiling heat transfer inside vertical minichannels. The objective here is to provide basic knowledge on the systems of biphasic cooling in minichannels for several gravity levels (μg, 1g, 2g). To fully understand the high heat transfer potential of boiling flows in microscale’s geometry, it is vital to quantify these transfers. To achieve this goal, an experimental device has been set up and in order to study the influence of gravity, the device has been embarked on board A300 Zero-G to make Parabolic Flights experiments. Analysis is made up by using an inverse method in order to estimate the local heat coefficient while boiling occurs inside a minichannel. The results concern two-phase flow during convective boiling and this investigation leads to solve an inverse heat conduction problem (IHCP). The estimation consists in inversing experimental data measurements located in the heating rod of our experiment to obtain the wall temperature and the heat flux on the minichannel surface. Images and video sequences have been performed with a high speed camera. The experiments are conducted with HFE-7100 as this fluid has a low boiling temperature at the cabin pressure of the A300. This choice was governed by the capillary length which is 4.3 mm in microgravity (0.05 m.s). The experimental loop allows the control of the heat flux and the liquid flow rate for three hydraulic diameters (DH): 0.49 mm; 0.84 mm and 1.18 mm. The influence of three different parameters on the heat transfer coefficient are carried out: the gravity level, the Reynolds number and the vapor quality. First results show that whatever the gravity level, the local heat transfer decreases sharply from the inlet to the outlet channel. In the annular slug regime, the average heat transfer is found around 6000 W.m.K and we observe that the heat transfer coefficient is higher in microgravity (30 %) compared to normal and hypergravity conditions.

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