Abstract
This paper summarizes the procedure of experimental determination of flow boiling heat transfer coefficient in the rectangular minichannel 2 mm wide, 1.8 mm deep and 193 mm long. In this experiment, the basic thermal and flow parameters were measured and the temperature distribution on the foil insulating the heater was recorded using an infrared camera. A high speed video camera observed the vapor patterns forming during the boiling process. The void fraction was determined with the image processing technique. Stationary heat transfer process in the insulating foil, the heater and the flowing liquid was described using two-dimensional Laplace’s equation (for the foil), the Poisson equation (for the heater) and energy equation (for the liquid) – all complemented with an appropriate system of boundary conditions. The system of the differential equations with adopted boundary conditions gives the three consecutive inverse problems in elements of the test section containing minichanel. Trefftz method was used to determine two dimensional temperature distribution of the foil and the heater. The liquid temperature was calculated using the Picard-Trefftz method. The values of the experimentally obtained heat transfer coefficients with the values of these coefficients calculated on the basis of correlations known from the literature were also compared.
Highlights
Recent years have been abundant with experimental and theoretical works on thermal processes occurring in flow boiling in mini- and micro channels
2.2 Experimental results The experimental setup described above and in [9] enabled to collect the folowing parameters:the flow of the liquid at a set rate, temperature and pressure at the inlet to the minichannel, to heat the liquid in the minichannel to the set temperature, to measure and record the temperature and pressure at the inlet and outlet of the minichannel, liquid volumetric flow rate, and the electric current supplied to the heating foil, and to record the flow structures and two-dimensional temperature distributions on the outer surface of the insulation foil
The temperature distributions recorded with the thermal imaging camera on the insulating foil were important for further calculations, Fig. 2. They formed the basis for determining heat transfer coefficients in the minichannel using the Picard-Trefftz method
Summary
Recent years have been abundant with experimental and theoretical works on thermal processes occurring in flow boiling in mini- and micro channels. The most general model, that is, two-fluid four-field model that at least theoretically allows continuous modelling of flow changes, has been discussed in [2]. This model comprises a system of several partial differential equations, even when related to developed nucleate boiling. A suitable set of boundary conditions has to be defined Solving such a complex system of equations is not always possible, especially in the case of inverse problems. The Trefftz method was first described by Trefftz [3] and was addressed to linear equations This method was generalized by many authors [4,5,6,7,8] who expanded its applicability. The paper presents application of the Trefftz method and the Picard-Trefftz
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