The study presents the experimental and numeric heat transfer investigations in flow boiling of water through an asymmetrically heated, rectangular and horizontal minichannel, with transparent side walls. A dedicated system was designed to record images of two-phase flow structures using a high-speed video camera with a synchronous movement system. The images were analyzed with Matlab 2019a scripts for determination of the void fraction for each pattern of two-phase flow structures observed. The experimental data measured during the experimental runs included inlet and outlet temperature, temperature at three internal points of the heater body, volume flux of the flowing water, inlet pressure, pressure drop, current and the voltage drop in the heater power supply. The flows were investigated at Reynolds number characteristic of laminar flow. The mathematical model assumed the heat transfer process in the measurement module to be steady-state with temperature independent thermal properties of solids and flowing fluid. The defined two inverse heat transfer problems were solved with the Trefftz method with two sets of T- functions. Graphs were used to represent: the boiling curves, the local void fraction values, the boiling heat transfer coefficients and the errors of both of them for selected mass fluxes and heat fluxes.

The paper presents a novel twofold use of the photographic technique for flow boiling heat transfer investigation in the horizontal mini-channel. The dedicated measurement system was constructed to record basic thermal and flow parameters, i.e., boiling liquid inlet and outlet temperatures and pressures, and also temperatures inside the heating block to calculate heat flux going into the liquid. A high-speed video camera equipped with synchronous movement system was applied to combine the recording of two-phase flow images with simultaneous local void fraction measurements both based on the same photographic data set. The data were collected, managed, and refined with the scripts developed in the MathWorks Matlab 2019b environment. The synchronous use of two intelligent techniques in the scripts, i.e., the background subtraction technique and the statistical analysis of individual pictures allowed obtaining reliable experimental results. The proposed method of the void fraction determination ensures high measurement accuracy.

The study deals with experimental investigations into heat transfer flow boiling in rectangular, vertical and horizontal mini-channels. The dedicated measurement system was designed to observe flow boiling in the mini-channel. The main task of the system is to record images of two-phase flow with a high-speed video camera, which is equipped with the synchronous movement system. The data, in the form of the footage of two-phase flow structures, were subjected to the analysis with the scripts, developed for that purpose in the MathWorks Matlab 2010b environment. The scripts allowed the determination of void fraction for each pattern of two-phase flow structures observed, at various heat fluxes and various volume fluxes, for two spatial orientations of the mini-channel: vertical and horizontal.

The study discusses experiments on ethanol flow boiling in mini-channels. Pre-set thermal and flow parameters (pressure and temperature at the inlet, mass flux, the heat flux delivered to the heater) and temperature, pressure and void fraction measurements obtained in the experiment provided data for further simulation computations. On the basis of high-speed camera images, void fraction in the mini-channel was determined. Thermograms of the heater surface and the surface in the adiabatic part of the mini-channel allowed producing temperature profiles along the channel length. The read and computed parameters satisfy the expected dependences (especially that for void fraction as a function of volume flux and heat flux). Those were used to solve inverse heat conduction problems.