Abstract: A thermoelectric cooling system usually consists of a thermoelectric cooler (TEC) and heat exchangers at the cold side and the hot side. Heat exchangers in TEC systems are designed to minimize their thermal resistance under restrictions such as the size of the system and heat transfer method, because as the thermal resistances of heat exchangers increase, the performance of TEC systems decreases. In the TEC system in which thermal resistance of heat exchangers is minimized, the optimum TEC is analyzed to maximize the coefficient of performance (COP) and the exergetic efficiency in absorbing a given heat load. The effects of the thermal resistance of heat exchangers on the performance of the TEC system and the design parameters of that system are also investigated. To discuss these matters, dimensionless entropy flow equations of the TEC system are introduced, and the COP and the exergetic efficiency are expressed as a function of dimensionless quantities by using these dimensionless equations.

Abstract: Based on heat transfer experiments with tubes of different inside geometry, correlations for forced convection heat transfer at near- and supercritical pressure are presented. They are of the Dittus-Boelter typeNu=CRe m Pr n and over the whole enthalpy range. The simple form is made possible by a new method of determining a representative specific heat capacity. At five reference temperatures heat capacities are computed with semi-empirical equations. The two highest values closest to the (pseudo-)critical state are then sorted out, because they are not relevant for turbulent flow. The average of the remaining three values represents a reasonable characteristic heat capacity. Comparisons to measurements show the very good accuracy of the new correlations. The procedure can be recommended whenever strong dependencies of fluid properties on temperature or even singularities occur.

Abstract: The local heat transfer coefficient distribution over all four walls of a large-scale model of a gas turbine cooling passage have been measured in great detail. A new method of determining the heat transfer coefficient to the rib surface has been developed and the contribution of the rib, at 5 percent blockage, to the overall roughened heat transfer coefficient was found to be considerable. The vortex-dominated flow field was interpreted from the detailed form of the measured local heat transfer contours. Computational Fluid Dynamics calculations support this model of the flow and yield friction factors that agree with measured values. Advances in the heat transfer measuring technique and data analysis procedure that confirm the accuracy of the transient method are described in full.

Abstract: A principle of uniformity of temperature difference field (TDF) in heat exchangers is advanced.It states that the more uniform the temperature difference field,the higher the effectiveness of heat exchanger for a given NTU and C,.Analytical and numerical results on the uniformity of TDF and effectiveness of thirteen types of heat exchangers show the validity of the uniformity principle.Its further verification is given by the asymptotical solution of TDF in terms of a recurrence formula of heat transfer area distribution.The analyses of entropy generation caused by heat transfer indicate that the uniformity principle is based on the second law of thermodynamics.Two ways,redistributing heat transfer areas and varying the connection between tubes,are presented for the improvement of the uniformity of TDF and the consequent increase of effectiveness for crossflow heat exchangers.

Abstract: The absorber is one of the most expensive components of an absorption heat pump or chiller, respectively In order to reduce the cost of a heat exchanger, much effort is invested into searching for additives for heat transfer enhancement Another way to reduce heat exchanger cost, especially for machines with low capacities, is to use an adiabatic spray absorber The basic principles of the spray absorber is to perform heat and mass transfer separated from each other in two different components In this way the heat can be rejected effectively in a liquid-liquid heat exchanger, whereas the mass transfer occurs subsequently in a simple vessel The spray technique can not only save heat exchanger cost in conventional absorption systems working with water and lithium bromide, it also allows the use of quite different working fluids such as hydroxides, which have lower heat transfer coefficients in falling films Moreover, the separated heat transfer can easily be performed in a liquid-to-air heat exchanger Hence it is obvious to use hydroxides that allow for a high temperature lift for building an air-cooled chiller with spray absorber In this presentation theoretical and experimental investigations of the spray absorber as well as the setup willmore » be described Finally, possible applications will be outlined« less

Abstract: In recent years, there has been considerable interest in the development of solid-liquid fluidized bed heat exchangers for efficient utilization of energy and for the control of heat transfer surface fouling. However, the design for optimum heat transfer remains uncertain and essentially empirical. In this study, a data bank containing a large number of measured heat transfer coefficients over a wide range of operational parameters (solid and liquid phase physical properties and heat transfer surface configurations) are compared with the prediction of almost all available correlations in the literature. A unified model for the prediction of heat transfer coefficients for wall-to-bed and immersed heater-to-bed systems is presented which predicts the published experimental data with good accuracy.

Abstract: Direct heat transfer is an important method in the exchange of heat between two countercurrent process streams within a column. The process can be simulated using either the theoretical stage or the rate based concept. With both concepts, a reliable heat transfer coefficient is needed. Additionally, the rate of the heat transfer coefficient is influenced by the simultaneous mass transfer. A number of application-dependent methods to estimate the heat transfer coefficient have been developed, mainly for random packings. It is the purpose of this paper to extend this work to structured packings. A number of experiments with air/water have been performed in a column of 300 mm inner diameter with Mellapak 250.Y, 250.X and 125.X at ambient conditions. A second group of measurements were done using an oil/air system where only sensible heat was transferred. Based on these experimental results a method was developed to predict the heat transfer coefficient for structured packings. The method is applied to examples of industrial importance, like a gas quench, a gas saturator and a pump-around zone in an atmospheric tower.

Abstract: In this paper we consider initial value problems for heat equation with discontinuous heat flow and concentrated heat capacity in interior points or at the boundary. Convergence of the Crank-Nicolson scheme is analyzed via the concept of elliptic projection. Namely, second order convergence is proved for the corresponding elliptic problems in special norms. Then, splitting the error of the heat problem into two errors we prove second order estimates in space and time in modified L2 norm.

Abstract: The effects of moisture on the performance of thermoelectric air conditioning systems and heat pumps equipped with a heat exchanger were studied. Coefficients of performance and fluid temperature variations were calculated for heat capacity ratios from 1 to 10 and relative humidities ranging from 0 to 100% at the cold fluid inlet. Only the energy effects of the water condensation are considered as it is assumed that the heat transfer coefficients are those of a dry heat exchanger. It was found that different flow arrangements and the energy associated with condensation on the cold fluid side have no strong effects on the variation of the hot fluid temperature. The coefficient of performance decreases and the cold fluid exit temperature increases when condensation occurs. When the moisture content at the cold fluid inlet increases, most of the cases studied show a decrease in the difference between the optimum and uniform current results. The difference among different flow arrangements also becomes smaller as more water vapor condenses in the cold flow.

Abstract: Heat transfer coefficients in nucleate pool boiling of five binary mixtures at atmospheric pressure were measured on an upward-facing circular plate for a wide range of heat fluxes from about 15percent of the critical heat flux (CHF) to close to CHF. As has been observed in many previous measurements, heat transfer coefficients were reduced in comparison with the interpolated values between their constituent pure components. This reduction was dependent on the mixture composition and became more pronounced with increasing heat flux. To provide the simplest and most reliable prediction of the reduction of heat transfer coefficients regarding the mixture composition and heat flux, the Thome correlation was modified so as to include the effect of heat flux in a dimensionless form. The developed correlation succeeds in correlating the present experimental data within ±20 percent accuracy.

Abstract: Experimental results are reported for perforated-plate heat transfer matrices. Friction and heat transfer data were obtained for low-Reynolds-number continuum flow (Red≤100) over a range of geometric parameters, including plate open-area ratio, plate spacing-to-thickness ratio, and hole diameter-to-plate thickness ratio. Friction factors were obtained from isothermal pressure drop tests, and single-blow transient tests were used to obtain heat transfer coefficients. The main trends in the data are identified. A modified London area goodness factor is introduced to account properly for the effect of plate open-area ratio on thermal-hydraulic performance. Implications of the results for heat exchanger design are discussed briefly.

Abstract: This paper describes the performance of plate heat exchangers in residential water radiator heating systems receiving their heat from geothermal resources. Radiator theory is reviewed and determination of annual hot water requirements for space heating is discussed. Performance evaluation is made of plate heat exchangers and results obtained by means of two equations commonly used for this purpose, the Sieder-Tate and the Dittus-Boelter equations, compared to results obtained with a simplified equation where heat transfer in the heat exchanger is assumed to depend only on the fluid mass flow on both sides. It is found that for prevailing temperature ranges in Icelandic geothermal systems the mass flow approximation gives results very close to those determined by the more complicated conventional equations.

Abstract: In the ammonia-water generator-absorber heat exchange (GAX) absorption heat pump, the heat and mass transfer processes which occur between the generator and absorber are the most crucial in assuring that the heat pump will achieve COPs competitive with those of current technologies. In this study, a model is developed for the heat and mass transfer processes that occur in a counter-current vertical fluted tube absorber (VFTA) with inserts. Correlations for heat and mass transfer in annuli are used to model the processes in the VTA. Experimental data is used to validate the model for three different insert geometries. Comparison of model results with experimental data provides insight into model corrections necessary to bring the model into agreement with the physical phenomena observed in the laboratory.

Abstract: Hundreds of stacked wire screens are used in the regenerator matrix of a common cryocooler. The number of transfer units of such a matrix (denoted as NTUm) may well exceed 60. However, most of the earlier studies reported are limited to studies of regenerators with NTUm values less than 60, as the single-blow method was employed to measure the NTUm value of the regenerator matrix. Furthermore, in these earlier studies, the effect of heat transfer from the working fluid to the external tube and the Joule-Thomson effect were neglected.In the present study, three regenerators having high NTUm values have been constructed and a transient single-blow technique has been employed to measure the friction factor and the heat transfer performance of these regenerators. In addition, an improved model has been adopted to correct the shortcomings of the earlier studies.Empirical correlations have been provided for the relation between the friction factor and Reynolds number and between the Nusselt number and Reynolds ...

Abstract: In chemical plants shell-and-tube heat exchangers are among the most frequently used apparatuses and costs of heat transfer usually determine the cost of a whole process. The definition of the optimum series of heat exchangers (J. Buzek, M. Buzanowski and J. Podkafiski, Chem. Eng. Proc., (submitted), and the application of elements from the series lead to the decrease in costs of heat transfer. Optimum series of heat exchangers can be created only if there is market demand for apparatuses suitable for various process parameters and costs of heat exchanger production and operation are known. During calculations combinatorial explosion can be avoided owing to certain properties of the cost functions of apparatuses forming the series.

Abstract: Capillary tube-suction line heat exchanger performance with refrigerant HFC-134a was experimentally evaluated for a range of heat exchanger geometries and operating conditions typically found in household refrigerators. Due to the number of variables that could potentially influence heat exchanger performance, statistical methods were employed in designing the experimental test plan and in reducing the data. Key variables and their respective ranges tested included: capillary tube inner diameter (0.66 to 0.79 mm), capillary tube length (2,440 to 3,300 mm), heat exchanger length (1,000 mm to 1,780 mm), condensing temperature (29.4 to 55.6 C, which corresponds to a condensing pressure of 758 kPa and 1,516 kPa), evaporator pressure (131 to 165 kPa), capillary tube inlet condition (2.8 to 5.6 C subcool), and oil concentration level (0 to 3%). For these experiments, the measured HFC-134a mass flow rates ranged between 2.7 and 9.5 kg/hr, and measured effective subcooling level ranged between 13 and 33 C. Based on the resulting performance database, design correlations predicting mass flow rate and the heat exchanger effectiveness were developed and verified. The HFC-134A results were also compared to measured heat exchanger performance with refrigerants HFC-152a and CFC-12.