The image analysis consists in extracting from the information which is available to the observer of the part that is important from the perspective of the investigated process. This process usually accompanies a considerable reduction in the amount of information from the image. In the field of two-phase flows, computer image analysis can be used to determine flow and geometric parameters of flow patterns. This article presents the possibilities of using this method to determine the void fraction, vapor quality, bubble velocity and the geometric dimensions of flow patterns. The use of computer image analysis methods is illustrated by the example of HFE 7100 refrigerant methoxynonafluorobutane condensation in a glass tubular minichannel. The high speed video camera was used for the study, and the films and individual frames received during the study were analyzed.
The paper presents four 1-dimensional models of thermal resistance of walls in a heat exchanger with rectangular minichannels. The first model is the simplest one, with a single wall separating two fluids. The second model of the so called equivalent wall takes into account total volume of intermediate walls between layers of minichannels and of side walls of minichannels. The next two more complicated models take separately into account thermal resistance of these walls. In these two models side walls are treated as fins. The results of models comparison are presented. It is shown that thermal resistance may be neglected for metal walls but it should be taken into account for the walls made of plastics. For the case of non-neglected wall thermal resistance the optimum wall thickness was derived. Minichannel heat exchangers made of plastic are larger than those built of metal, but are significantly cheaper. It makes possible to use of such exchangers in inexpensive microscale ORC installations.
The paper describes issues related to pressure drop that accompanies the phenomenon of maldistribution of working fluid between the channels of a model minichannel plate heat exchanger. The research concerns a single exchanger’s plate containing 51 (in every geometry) parallel rectangular minichannels of hydraulic diameters 461 μm, 571 μm, 750 μm, and 823 μm. In addition, more complex geometry has been investigated, equipped with additional diagonal channels (so called extended geometry). The moment of the liquid phase transition through the heat exchanger was recorded at the flow rates ranging from 0.83 g/s to 13.33 g/s in the inlet manifold. The paper discusses the total pressure drop as a function of the flow rate and the characteristic dimension of minichannels, as well as the pressure drop as a function of the time of the fluid passage through the main part of the measuring section in which measurements were done. The resulting profiles correlate with the images of the flow distribution between channels recorded using the fast shutter speed camera, that allows to draw a further conclusions about the specifics of the maldistribution process. The impact of the total pressure drop on the actual range of optimum operating conditions of the heat exchanger was analyzed.
The aim of the study was the implementation of a numerical simulation of the air-water two-phase flow in the minichannel and comparing results obtained with the values obtained experimentally. To perform the numerical simulations commercial software ANSYS FLUENT 12 was used. The first step of the study was to reproduce the actual research installation as a three-dimensional model with appropriate and possible simplifications - future computational domain. The next step was discretisation of the computational domain and determination of the types of boundary conditions. ANSYS FLUENT 12 has three built-in basic models with which a two-phase flow can be described. However, in this work Volume-of-Fluid (VOF) model was selected as it meets the established requirements of research. Preliminary calculations were performed for a simplified geometry. The calculations were later verified whether or not the simplifications of geometry were chosen correctly and if they affected the calculation. The next stage was validation of the chosen model. After positive verification, a series of calculations was performed, in which the boundary conditions were the same as the starting conditions in laboratory experiments. A satisfactory description of the experimental data accuracy was attained.
Analysis of the state of-the-art in research of minichannel heat exchangers, especially on the topic of flow maldistribution in multiple channels, has been accomplished. Studies on minichannel plate heat exchanger with 51 parallel minichannels with four hydraulic diameters, i.e., 461 μm, 574 μm, 667 μm, and 750 μm have been presented. Flow at the instance of filling the microchannel with water at low flow rates has been visualized. The pressure drop characteristics for single minichannel plate have been presented along with the channels blockage, which occurred in several cases. The impact of the mass flow rate and channels’ cross-section dimensions on the flow maldistribution were illustrated.
Gas-liquid two-phase flow in minichannels has been the subject of increased research interest in the past few years. Evaluation, however, of today's state of the art regarding hydrodynamics of flow in minichannels shows significant differences between existing test results. In the literature there is no clear information regarding: defining the boundary between minichannels and conventional channels, labelling of flow patterns. The review of literature on the hydrodynamics of gas-liquid flow in minichannels shows that, despite the fact that many research works have been published, the problem of determining the effect of diameter of the minichannel on the hydrodynamics of the flow is still at an early stage. Therefore, the paper presents the results of research concerning determination of flow regime map for the vertical upward flow in minichannels. The research is based on a comprehensive analysis of the literature data and on the research that has been carried out. Such approach to the mentioned above problems concerning key issues of the two-phase flow in minichannels allowed to determine ranges of occurrence of flow structures with a relatively high accuracy.
Technology advancements entail a necessity to remove huge amounts of heat produced by today’s electronic devices based on highly integrated circuits, major generators of heat. Heat transfer to boiling liquid flowing through narrow minichannels is a modern solution to the problem of heat transfer enhancement. The study was conducted for FC-72 boiling in a rectangular, vertical and asymmetrically heated minichannel that had depths of 0.5-1.5 mm, a width of 20 mm and a length of 360 mm. The heat flux increased and decreased within the range of 58.3-132.0 kWm−2, the absolute pressure ranged from 0.116 to 0.184 MPa and the mass flux was 185-1139.2 kgm−2s−1. The boiling process took place on a flat vertical heating surface made of Haynes-230 0.1 mm thick acid-proof rolled plate with the surface roughness of 121 μm.