• Energy Research

This paper presents results concerning flow boiling heat transfer in three parallel vertically oriented and asymmetrically heated rectangular minichannels. Each minichannel was 1.7 mm deep, 16 mm w...

This article proposes a passive cooling system for photovoltaic (PV) panels to achieve a reduction in their temperature. It is known that the cooling of PV panels allows for an increase in the efficiency of photovoltaic conversion. Furthermore, reducing the high temperature of the surfaces of PV panels is also desirable to ensure their long-lasting operation and high efficiency. Photovoltaic panels were modified by adding copper sheets to the bottom side of the panels. Two types of modification of the outer surface of the sheet were investigated experimentally, which differed in surface roughness. One was characterised by the nominal roughness of the copper sheet according to its manufacturer, while the other was enhanced by a system of pins. Numerical simulations, performed using the Simcenter STAR-CCM+ software, version 2020.2.1 Build 15.04.010, helped to describe the geometry of the pins and their role in the resulting reduction in the temperature of the PV panel surface. As a result, modifying a typical PV panel by adding a copper sheet with pins helps to achieve a higher decrease in the temperature of the PV panel. The addition of a copper sheet with a smooth surface to the bare PV panel improved the operating conditions by lowering its surface temperature by approximately 6.5 K but using an enhanced surface with the highest number of pins distributed uniformly on the copper sheet surface resulted in the highest temperature drop up to 12 K. The highest number of pins distributed uniformly on the copper sheet surface resulted in the highest temperature drop in its bottom surface, that is, on average by more than 12 K compared to the surface temperature of the bare PV panel surface. The validation of the numerical calculations was performed on data from the experiments. An analysis of the quality of the numerical mesh was also performed using a method based on the grid convergence index.

This paper reports the results of heat transfer during refrigerant flow in rectangular mini-channels at stationary conditions. The impacts of selected parameters on boiling are discussed, i.e., thermal and flow parameters, dimensions and orientation of the channels. Four refrigerants (FC-72, HFE-649, HFE-7000 and HFE-7100) were used as the working fluid. Research was carried out on the experimental set-up with the test section with a single rectangular mini-channel of 180 mm long and with a group of five parallel mini-channels, each 32 mm long. The temperature of the mini-channel’s heated wall was measured by infrared thermography. Local values of the heat transfer coefficient at the contact surface between the fluid and the plate were calculated using the 1D mathematical method. The results are presented as the relationship between the heat transfer coefficient and the distance along the mini-channel length and boiling curves. Two-phase flow patterns are shown. Moreover, the results concerning various refrigerants and the use of modified heater surfaces are discussed. The main factors influencing the heat transfer process were: mini-channel inclination to the horizontal pane (the highest heat transfer coefficient at 270° and 0°), using modified heater surfaces (especially electroerosion texturing and vibration-assisted laser No. 2 texturing) and working fluids (FC-72 and HFE-7000).

This work focuses on subcooled boiling heat transfer during flow in a minichannel heat sink with three or five minichannels of 1 mm depth. The heated element for FC-72 flowing along the minichannels was a thin foil of which temperature on the outer surface was measured due to the infrared thermography. The test section was oriented vertically or horizontally. A steady state heat transfer process and a laminar, incompressible flow of the fluid in a central minichannel were assumed. The heat transfer problem was described by the energy equations with an appropriate system of boundary conditions. Several mathematical methods were applied to solve the heat transfer problem with the Robin condition to determine the local heat transfer coefficients at the fluid/heated foil interface. Besides the 1D approach as a simple analytical method, a more sophisticated 2D approach was proposed with solutions by the Trefftz functions and ADINA software. Finite element method (FEM) calculations were conducted to find the temperature field in the flowing fluid and in the heated wall. The results were illustrated by graphs of local heated foil temperature and transfer coefficients as a function of the distance from the minichannel inlet. Temperature distributions in the heater and the fluid obtained from the FEM computations carried out by ADINA software were also shown. Similar values of the heat transfer coefficient were obtained in both the FEM calculations and the 1D approach. Example boiling curves indicating nucleation hysteresis are shown and discussed.

The paper continues the discussion of experimental and numerical investigations of forced convection boiling heat transfer in vertical minichannels covered by two former editions of this conference and our previous papers. Liquid crystal thermography technique was used for measuring the two-dimensional heating surface temperature distribution and boiling front detection. Influence of selected parameters on boiling heat transfer and nucleation hysteresis was observed and discussed. The two-dimensional heat transfer model and the analytic-numerical heat polynomial method were applied to solve the inverse boundary value problem and determine the temperature distributions in the heating foil and protecting glass and the boiling heat transfer coefficient as well. This paper shows how to modify and improve the heat polynomial method if we know the measurement errors and implement them into the numerical procedure. The accuracy of temperature measurements on the heating surface with liquid crystal method was estimated and the analysis of experimental results was given. The functions sought in numerical calculations describe temperature distribution in the protecting glass and the heating foil of the minichannel. They are presented in the form of linear combination of heat polynomials. The adopted boundary conditions and temperature measurements are used to construct error functionals. The latter express the root-mean-square errors, with which computed solutions satisfy relevant boundary conditions. On the basis of functional minimalisation unknown coefficients of linear combinations are determined. The solutions obtained satisfy the differential equations in the exact manner whereas the adopted boundary conditions are met in the approximate fashion. The unknown boiling heat transfer coefficient is the function computed from the boundary condition of the third kind. In the modified method, measurement errors are weights for individual temperature measurements. The more accurate is the measurement, i.e. has a smaller error, the greater is the weight put to it in further calculations. Therefore, it is possible to heighten the accuracy with which glass and foil temperature distributions, determined experimentally, fulfil the assumed equality conditions on the contact surface. Temperature distributions in the glass and the foil, computed on the basis of the modified method, are closer to real values than those obtained with the basic one. Local heat transfer coefficients obtained for two-dimensional boiling heat transfer model with both the basic and the modified heat polynomial methods are also compared.

In this paper, the issue of flow boiling heat transfer in an annular minigap was discussed. The main aim of the paper was determining the boiling heat transfer coefficient at the HFE-649 fluid–heater contact during flow along an annular minigap. The essential element of the experimental stand was a test section vertically oriented with the minigap 2 mm wide. Thermocouples were used to measure the temperature of the heater and fluid at the inlet and the outlet to the minigap. The mathematical model assumed that the fluid flow was laminar and the steady–state heat transfer process was axisymmetric. The temperatures of the heated surface and of the flowing fluid were assumed to fulfill energy equations with adequate boundary conditions. The problem was solved by the Trefftz method. The local heat transfer coefficients at the fluid–test surface interface were calculated due to the third kind boundary condition at the saturated boiling. Graphs were used to illustrate: the measurement of the heater surface temperature, 2D temperature distributions in the pipe and fluid, and the heat transfer coefficient as a function of the distance from the minigap inlet. The measurement uncertainties and accuracy of the heat transfer coefficient determination were estimated.

Purpose This paper aims to focus on flow boiling heat transfer in an asymmetrically heated minichannel. Two-dimensional inverse heat transfer problem was solved using the Trefftz and Beck methods. The primary purpose was to find an enhanced surface that could help intensify heat transfer. Design/methodology/approach The experimental set-up and methodology for FC-72 boiling heat transfer in two parallel vertical rectangular minichannels with smooth or enhanced heated surfaces was presented. The heat transfer coefficient was calculated using the Trefftz and Beck methods. Findings The results confirm that considerable heat transfer enhancement takes place when selected enhanced heated surface is used in the minichannel flow boiling and that it depends on the type of surface enhancement. The analysis of the experimental data revealed that the values and distributions of the heat transfer coefficient obtained using the Beck and Trefftz methods were similar. Practical/implications Many studies have been recently devoted to flow boiling heat transfer in minichannels because of the rapid development of high-performance integrated systems generating large amounts of heat. Highly efficient small-size cooling systems for new-generation compact devices are thus in great demand. Originality/value The present results are original and new in the study of cooling liquid boiling in minichannels with enhanced heated surfaces that contribute to heat transfer enhancement. The paper allows the verification of state-of-the-art methods of solving the inverse problem by using empirical data from the experiment. The application of the Trefftz and Beck methods for finding a solution of the inverse heat transfer problem is promising.

This study investigated FC-72 boiling heat transfer in minichannels using two non-contact temperature measurement techniques: liquid crystal thermography (LCT) and infrared thermography (IRT). These methods were applied simultaneously to measure temperature distributions on the heated wall surface of minichannels, formed by a thin metal foil. The temperature data facilitated the calculation of local heat transfer coefficients at the foil–working fluid contact surface. Calibration of the liquid crystal colour response to temperature was conducted prior to the use of LCT. According to a comparison of the heat transfer coefficients and Nusselt numbers determined using LCT and IRT measurements, comparable temperature distributions are provided, with the average relative differences in heat transfer coefficients determined using these techniques remaining below 15%. The findings highlight the advantages of non-contact temperature measurement in minimising system disturbances while providing precise data for understanding flow boiling heat transfer mechanisms. Such results can contribute to the design of minichannel heat exchangers.

This paper presents the results of research on heat transfer during fluid flow in a heat exchanger with two rectangular mini-channels. There was Fluorinert FC-72 flow, heated by the plate in the hot mini-channel, and co-current flow of distilled water in the cold mini-channel. Both fluids were separated by the copper plate. A thermal imaging camera was used to measure the temperature distribution of the outer surface of the heated plate. The purpose of the calculations was to determine the heat transfer coefficients at the contact surfaces: the heated plate—FC-72 and FC-72—the copper plate. Two mathematical models have been proposed to describe the heat flow. In the 1D approach, only the heat flow direction perpendicular to the fluid flow direction was assumed. In the 2D model, it was assumed that the temperature of the heated plate and FC-72 and the copper plate meet the appropriate energy equation, supplemented by the boundary conditions system. In this case, the Trefftz functions were used in numerical calculations. In the 1D model, the heat transfer coefficient at the interface between FC-72 and the copper plate was determined by theoretical correlations. The analysis of the results showed that the values and distributions of the heat transfer coefficient determined using both models were similar.