DEVELOPMENT OF A ELLIPTIC HEAT PIPE HEAT EXCHANGER
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Heat exchanger made of heat pipes with round cross section (Known as round heat pipe heat exchanger) has been widely used in China. But some further improvements are greatly desired both in design and in operation. The two main performance parameters of heat exchanger are its heat transfer coefficient and flow resistance, which often affect each other. For example, the maximum permissible pressure drop which is propotional to the flow resistance limits heat transfer coefficient, especially for the round heat pipe heat exchangers. The self-clean ability of the exchanger is also affected negatively when heat transfer ability is too low.These problems are well solved by a newly developed heat exchanger made of heat pipes with elliptic cross section (Known as elliptic heat exchanger). It is shown that the performances of elliptic heat pipe heat exchanger are much higher than those of round one. The details will are presented and discussed in the following sections. It is believed that the use of elliptic heat pipe heat exchangers indicates a way for further application of heat pipe heat exchangers.
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Heat exchangers are devices whose primary responsibility is to transfer heat, typically from one fluid to another. In such applications, the heat exchangers can be parallel flow, crossflow, or counter flow. An essential part of any heat exchanger analysis is the determination of the effectiveness of the heat exchanger. In the present work, three different types of heat exchangers are investigated. Numerical and experimental performance analyses are applied. The main objective of the present work is to compare the effectiveness of each heat exchanger at different conditions. Six experimental investigations for Plate, shell & tube, and fluidized bed heat exchangers are executed. All experimental tests are reached to steady-state conditions. The results show that the counter flow plate heat exchanger has an effectiveness of 90% compared with the parallel flow of 60% effectiveness for working experimental conditions. Also, the fouling effect in decreasing heat transfer is cleared. In the present work, fouling decreases effectiveness from about 18% to about 4%. In addition, the effectiveness of the fluidized bed heat exchanger depends on the material used for the bed. Finally, the overall heat transfer coefficient is obtained and compared for all experimental tests, and it is directly proportional to the effectiveness of the heat exchanger. The FEHT program is used to get the temperature distribution in all types of present work heat exchangers.
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Abstract Wickless heat pipe heat exchangers (HPHE) consistently showed instabilities and omits many experimental tests. The performance of a heat pipe is governed by many parameters, and the effects of which may influence each other. The objective of this paper is to develop a new approach for an air–air heat pipe heat exchanger that takes into consideration the effect of heat transfer coefficients, saturation temperature, and thermal resistances inside the heat pipes as well as maximum heat transfer limit. The approach is based on analyzing inner heat pipe parameters with HPHE external working conditions. Results are also assessed for higher heat capacities ratio and found that the thermal resistances inside a heat pipe are a limiting factor, leading the HPHE system to perform poorly in particular for Cr ≤ 1. Overall heat transfer coefficients at HPHE sides as well as HPHE effectiveness as a function of Cr are assessed. Trying to follow the Chaudourne (1992, “The Heat Pipe Heat Exchangers: Design, Technology and Applications,” Design and Operation of Heat Exchangers, Springer, Berlin, Heidelberg, pp. 386–396) profile for conventional heat exchangers, HPHE effectiveness is determined and limited to 0.5 number of transferred units. The established model is verified by the existing literature and demonstrates numerical results that agree with the experimental data within a 2.86% error.
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For many years now, heat exchanger optimization has been a field of research for a lot of scientists. Aims of optimization are different, having in mind heat exchanger networks with different temperatures of certain streams. In this paper mathematical model in dimensionless form is developed, describing operation of one heat exchanger in a heat exchanger network, with given overall area, based on the maximum heat-flow rate criterion. Under the presumption of heat exchanger being a part of the heat exchanger network, solution for the given task is resting in a possibility of connecting an additional fluid stream with certain temperature on a certain point of observed heat exchanger area. The connection point of additional fluid stream determines the exchanging areas of both heat exchangers and it needs to allow the maximum exchanged heat-flow rate. This needed heat-flow rate achieves higher value than the heat-flow rate acquired by either of streams. In other words, a criterion for the existence of the maximum heat-flow rate, as a local extremum, is obtained within this mathematical model. Results of the research are presented by the adequate diagrams and are interpreted, with emphasis on the cases which fulfill and those which do not fulfill the given condition for achieving the maximum heat-flow rate.
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Heat exchangers are main equipmants in the process industries . Heat exchangers having high heat transfer surface area per unit volume(725m 2 /m 3 ) are called compact heat exchangers. Plate fin heat exchanger is one of the type of compact heat exchanger which is widely used in automobiles, cryogenics, liquifires and chemical industries need to be highly efficient becausein gas liquification,effectiveness is the main parameter which denotes the performance of the heat exchangers in terms of heat transfer rate between the lowing fluids.The effectiveness of thesecompact heat exchangers before putting them in to operation should be checked and it should not be below 90 percent in case of gas lequification. The trial is conducted under steady condition i.e the mass flow rate for both sides of fluid stream is same ,and the experiment is carried out at different mass flow rates. Various correlations are available in the literature for estimation of heat transfer and flow friction characteristics of the plate fin heat exchanger, so the various performance characteristics like effectiveness, heat transfer coefficient and pressure drop obtained through experiments are compared with the values obtained from different correlations . A heat exchanger is a device to transfer heat from a hot flu- id to cold fluid across an impermeable wall. Fundamental of heat exchanger principle is to facilitate an efficient heat flow from hot fluid to cold fluid. This heat flow is a direct function of the temperature difference between the two fluids, the area where heat is transferred, and the conductive/convective properties of the fluid and the flow state. In order to increase the heat transfer in, assuming that the heat transfer coeffi- cient cannot be changed, the area or the temperature dif- ferences have to be increased. Usually, the best solution is that the heat transfer surface area is extended although in- creasing the temperature difference is logical, too. In reality, it may not be much meaningful to increase the temperature dif- ference because either a hotter fluid should be supplied to the heat exchanger or the heat should be transferred to a colder fluid where neither of them are usually available. For both cases either to supply the hot fluid at high temperature or cold fluid at lower temperature extra work has to be done. Fur- thermore increasing the temperature difference more than enough will cause unwanted thermal stresses on the met- al surfaces between two fluids. This usually results in the deformation and also decreases the lifespan of those materials. As a result of these facts, increasing the heat transfer surface area generally is the best engineering approach. A large amount of study has been conducted to analyze the heat transfer and pressure drop characteristics of compact heat exchangers in the past few decades. But this study mainly fo- cuses on the Offset strip fins type of plate fin heat exchanger. And therefore the emphasis has been given on the literatures related to the prediction of j and f factors and the thermal per- formance testing of heat exchangers.
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An computational study of heat transfer in a compact heat exchanger is reported, performed with regard for the heat and mass transfer of the humid-air flow in the cold and hot circulation circuits of the heat exchanger. The computational study was performed using a homogeneous, two-dimensional theoretical model of heat transfer in a cross-flow compact heat exchanger with plate ribbing. In the model, the heat and mass transfer in the cold and hot flows, and also the heat transfer over the initial lengths of the channels, were taken into account. The model was validated by comparing the predicted data with experimental values. As a result of the study, main factors have been identified providing for enhanced heat transfer in the heat exchanger. The temperature fields on the heat-exchanging surface and in the heat-transfer medium in various operating regimes of the heat exchanger were examined.
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All process industries involve the usage of heat exchanger equipment and understanding its performance during the design phase is very essential. The present research work specifies the performance of a pure cross flow heat exchanger in terms of dimensionless factors such as number of transfer units, capacity rate ratio, and heat exchanger effectiveness. Steady state sensible heat transfer was considered in the analysis. The matrix approach that was established in the earlier work was used in the study. The results were depicted in the form of charts, tables, and performance equations. It was observed that indeterminately increasing the number of transfer units past a threshold limit provided very marginal improvement in the performance of a pure cross flow heat exchanger. Likewise, flow pattern in a heat exchanger is usually assumed either as mixed or unmixed. However, due to various operating conditions, partially mixed conditions do exist. This work considers partially mixed conditions in the tube side of the heat exchanger. The correction factor for heat exchanger effectiveness was developed to accommodate partially mixed flow conditions in the pure cross flow heat exchanger.
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With the introduction of an index of available energy destruction rate a thermodynamic performance analysis and evaluation is conducted of a system of heat exchangers connected in series. A general calculation formula for the available energy destruction rate was obtained. In addition, also discussed is the effect on the available energy destruction rate of a whole variety of factors. They include: the general flow tendency of the heat exchanger system, cold and hot fluid heat capacity flow rate ratio, the number of heat transfer units and the flow pattern of a single heat exchanger, heat transfer effectiveness, and the number of connected heat exchangers, etc.
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Separate type heat pipe heat exchangers are often used for large-scale heat exchanging. The arrangement of such a heat exchanger conveniently allows heat input to and output from the heat exchanger at remote locations. The traditional method of designing an ordinary HPHE (heat pipe heat exchanger) is commonly applied in the separate type exchanger design, but the calculations have to be carried out separately, which makes it very complicated. In this work, the e-NTU (effectiveness-Number of Transfer Units) method was applied for optimization analysis of single-or multi-level separate type heat pipe heat exchangers. An optimizing formula for single-level separate type heat pipe heat exchangers was obtained. The optimizing principles of effectiveness-NTU and heat transfer rate by the equal distribution method for multi-level separate type heat pipe heat exchanger are presented. The design of separate type heat pipe heat exchangers by the optimizing method is more convenient and faster than by the traditional method.
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