Abstract: We consider the flow of humid air over fin-tube multi-row multi-column compact heat exchangers with possible condensation. Previously published experimental data are used to show that a regression analysis for the best-fit correlation of a prescribed form does not provide an unique answer, and that there are small but significant differences between the predictions of the different correlations thus obtained. It is also shown that it is more accurate to predict the heat rate directly rather than through intermediate quantities like the j-factors. The artificial neural network technique is offered as an alternative technique. It is trained with experimental values of the humid-air flow rates, dry-bulb and wet-bulb inlet temperatures, fin spacing, and heat transfer rates. The trained network is then used to make predictions of the heat transfer. Comparison of the results demonstrates that the neural network is more accurate than conventional correlations. @DOI: 10.1115/1.1351167#

Abstract: Cooling towers are one of the largest heat and mass transfer devices that are in common use. In this paper, we present a detail model of counter flow wet cooling towers. The authenticity of the model is checked by experimental data reported in the literature. The values of number of transfer units (NTU) and tower effectiveness (e) obtained from the model were compared with the commonly described models. Appreciable difference in NTU and e values is found if the resistance to heat transfer in the water film and non-unity of Lewis number is considered in the calculations. The results demonstrate that the errors in calculating the tower effectiveness could be as much as 15 percent when considering the effect of air-water interface temperature. A procedure for the use of the model in designing and rating analyses of cooling towers is demonstrated through example problems. The limiting performance of the cooling towers; that is effectiveness equal to one, is explained in terms of air-approach temperature. The model is also used for obtaining the maximum possible mass-flow rate ratio of water-to-air, for different operating conditions.

Abstract: This paper presents results of 178 experiments evaluating the effects of internal heat exchange (IHX) on the performance of a well-instrumented prototype transcritical mobile air-conditioning system using R-744 (carbon dioxide) as the refrigerant. The effect on cycle efficiency is substantial, up to 25%, because of the relatively high irreversibility at the expansion device in the standard transcritical cycle. Three coaxial internal heat exchangers of various lengths, having identical cross sections, were used in the experiments in both parallel and counterflow configurations. These data were used to develop a simulation model that predicted accurately the IHX performance. Finally the model was used to develop an optimal design for a COP-maximizing IHX, which reduces material requirements by 50% while increasing effectiveness by 10%. The cycle optimization was guided by the trade-off between heat exchanger effectiveness and suction pressure drop.

Abstract: In this paper, the power density, defined as the ratio of power output to the maximum specific volume in the cycle, is taken as the objective for performance analysis and optimization of an irreversible regenerated closed Brayton cycle coupled to variable-temperature heat reservoirs from the viewpoint of finite time thermodynamics (FTT) or entropy generation minimization (EGM). The analytical formulae about the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the regenerator, the irreversible compression and expansion losses in the compressor and turbine, the pressure drop losses at the heater, cooler and regenerator as well as in the piping, and the effect of the finite thermal capacity rate of the heat reservoirs. The obtained results are compared with those results obtained by using the maximum power criterion, and the advantages and disadvantages of maximum power density design are analysed. The maximum power density optimization is performed in two stages. The first is to search the optimum heat conductance distribution corresponding to the optimum power density among the hot- and cold-side heat exchangers and the regenerator for a fixed total heat exchanger inventory. The second is to search the optimum thermal capacitance rate matching corresponding to the optimum power density between the working fluid and the high-temperature heat source for a fixed ratio of the thermal capacitance rates of two heat reservoirs. The influences of some design parameters, including the effectiveness of the regenerator, the inlet temperature ratio of the heat reservoirs, the effectiveness of the heat exchangers between the working fluid and the heat reservoirs, the efficiencies of the compressor and the turbine, and the pressure recovery coefficient, on the optimum heat conductance distribution, the optimum thermal capacitance rate matching, and the maximum power density are provided by numerical examples. The power plant design with optimization leads to a smaller size including the compressor, turbine, and the hot- and cold-side heat exchangers and the regenerator. When the heat transfers between the working fluid and the heat reservoirs are carried out ideally, the pressure drop loss may be neglected, and the thermal capacity rates of the heat reservoirs are infinite, the results of this paper then replicate those obtained in recent literature.

Abstract: This paper presents a steady-state model for predicting the performance of vapour-compression liquid chillers over a wide range of operating conditions. The model overcomes the idealisations of previous models with regard to modelling the heat exchangers. In particular, it employs an elemental NTU- e methodology to model both the shell-and-tube condenser and evaporator. The approach allows the change in heat transfer coefficients throughout the heat exchangers to be accounted for, thereby improving both physical realism and the accuracy of the simulation model. The model requires only those inputs that are readily available to the user (e.g. condenser inlet water temperature and evaporator water outlet temperature). The outputs of the model include system performance variables such as the compressor electrical work input and the coefficient of performance ( COP ) as well as states of the refrigerant throughout the refrigeration cycle. The methodology employed within the model also allows the performance of chillers using refrigerant mixtures to be modelled. The model is validated with data from one single screw chiller and one twin-screw chiller where the agreement is found to be within ±10%.

Abstract: The field validation of a short time step temperature response factor model is presented using actual operational data from an elementary school building in Lincoln, Nebr. The short time step model is used to predict hourly temperature changes of vertical ground-loop heat exchangers as used in ground-source heat pump applications. The model was based on an analytically validated numerical borehole model that simulates the heat transfer in and around the ground heat exchanger, using its thermal response to unit heat pulses. The short time step model was cast as a modular component and used to simulate the thermal behavior of the borefield using the TRNSYS environment. In order to assess the potential influence of errors associated with predicted temperatures, a series of sensitivity analyses are also presented. The sensitivity analyses focus on the impact of the heat pump entering fluid temperature on the system energy consumption, since the uncertainty in the predicted temperature is expected to have a corresponding uncertainty in the system energy use. System energy consumption, based on predicted and measured heat pump entering fluid temperatures, is compared and discussed.

Abstract: This paper presents a theoretical analysis and an experimental test on a shell-and-tube latent heat storage exchanger. The heat exchanger is used to recover high-temperature waste heat from industrial furnaces and off-peak electricity. It can also be integrated into a renewable energy system as an energy storage component. A mathematical model describing the unsteady freezing problem coupled with forced convection is solved numerically to predict the performance of the heat exchanger. It provides the basis for an optimum design of the heat exchanger. The experimental study on the heat exchanger is carried out under various operating conditions. Effects of various parameters, such as the inlet temperature, the mass flow rate, the thickness of the phase-change material and the length of the pipes, on the heat transfer performance of the unit are discussed combined with theoretical prediction. The criterion for analyzing and evaluating the performance of heat exchanger is also proposed. Copyright © 2001 John Wiley & Sons, Ltd.

Abstract: This paper considers the coupling of a finite element thermal conduction solver with a steady, finite volume fluid flow solver. Two methods were considered for passing boundary conditions between the two codes - transfer of metal temperatures and either convective heat fluxes or heat transfer coefficients and air temperatures. These methods have been tested on two simple rotating cavity test cases and also on a more complex real engine example. Convergence rates of the two coupling methods were compared. Passing heat transfer coefficients and air temperatures was found to give the quickest convergence. The coupled method gave agreement with the analytic solution and a conjugate solution of the simple free disc problem. The predicted heat transfer results for the real engine example showed some encouraging agreement, although some modelling issues are identified. Copyright © 2001 by ASME.

Abstract: In power cycles using ammonia–water mixtures as the working fluid, several heat exchangers are used. The influence of different correlations for predicting thermophysical properties on the calculations of the size of the heat exchangers is presented. Different correlations for predicting both the thermodynamic and the transport properties are included. The use of different correlations for the thermodynamic properties gives a difference in the total heat exchanger area of 7%, but for individual heat exchangers, the difference is up to 24%. Different correlations for the mixture transport properties give differences in the predicted heat exchanger areas that are, at most, about 10% for the individual heat exchangers. The influence on the total heat exchanger area is not larger than 3%. A difference in the total heat exchanger area of 7% would probably correspond to less than 2% of the total cost for the process equipment. Experimental data and correlations developed for the ammonia–water mixture transport properties are very scarce. The evaporation and condensation processes involving ammonia–water mixtures are also not fully understood.

Abstract: Combined conduction and radiation heat transfer with and without heat generation is investigated. Analysis is carried out for both steady and unsteady situations. One-dimensional gray Cartesian enclosure with an absorbing, emitting, and isotropically scattering medium is considered. Enclosure boundaries are assumed at specified temperatures. The heat generation rate is considered uniform and constant throughout the medium. The Crank?Nicholson scheme is used to solve the transient energy equation. The radiative part of the energy equation is solved using the collapsed dimension method. Transient and steady state temperature and heat flux distributions are found for various radiative parameters. Results are found for situations with and without heat generation. Heat generation is found to have significant bearing on temperature and heat flux. Results are compared with the data reported in the literature. Excellent agreement has been found.

Abstract: In this paper, the power density, defined as the ratio of power output to the maximum specific volume in the cycle, is taken as objective for performance optimization of an irreversible regenerated closed Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of finite time thermodynamics (FTT) or entropy generation minimization (EGM). The analytical formulae about the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the regenerator, the irreversible compression and expansion losses in the compressor and turbine, and the pressure drop loss in the piping. The maximum power density optimization is performed by searching the optimum heat conductance distribution corresponding to the optimum power density among the hot- and cold-side heat exchangers and the regenerator for the fixed total heat exchanger inventory. The influence of some design parameters, including the temperature ratio of the heat reservoirs, the total heat exchanger inventory, the efficiencies of the compressor and the turbine, and the pressure recovery coefficient, on the optimum heat conductance distribution and the maximum power density are provided. When the heat transfers between the working fluid and the heat reservoirs are carried out ideally, the analytical results of this paper become those obtained in recent literature. The power plant design with optimization leads to smaller size including the compressor, turbine, and the hot- and cold-side heat exchangers and the regenerator.

Abstract: A finite element method is applied to evaluate the performance of microchannel heat exchangers that are used in electronic packaging. The present approach is validated against the CFD data available in the literature. A comparison of the predicted results with other available results obtained from different concepts shows that the present method is able to predict the surface temperature, the fluid temperature and thus the total thermal resistance of the microchannel heat sink satisfactorily. The present methodology has an added advantage in that non‐uniform surface heat flux distribution over the package base can also be analysed easily. The method used in the present analysis is an alternative to massive CFD calculations.

Abstract: A heat transfer medium is shown, having a very high heat transfer rate that is simple in structure, easy to make, environmentally sound, rapidly conducts heat, and preserves heat in a highly efficient manner. Also shown is a heat transfer surface and a heat transfer element utilizing the heat transfer medium.

Abstract: Practical, relatively simple methods are presented to enhance heat transfer with condensation in a compact, polymer plate heat exchanger. Tilting the exchanger, while adapting the spacers, increases the condensate and heat fluxes by 6–8%, as a function of the inlet mass fraction of the moist air at ambient conditions. Polymer inserts increase convective heat transfer and yield a total heat flux improvement of 20%, at the cost of increased pressure drop.

Abstract: Airside performance for tube bank having N = 1 ~6 are reported in this study. Test results indicated that the heat transfer coefficients for N = 1 and N = 2 are comparable and the heat transfer coefficients increase with the number of tube row. For the same P t, P t the heat transfer coefficients for the tube bank containing smaller diameter tube are higher than those of larger tube diameter for N≥2 and is comparable for N = 1. Comparisons of the heat transfer performance were made with several correlations. It is found that the Gnielinski correlation gives the best predictive ability. For multiple-row heat exchangers, the predictive ability of the well-known Žukauskas correlation is also good. But the correlation under-estimate the present I-row heat exchangers. As a consequence, a slight modification to the Žukauskas correlation is proposed that can extend its applicability in N = 1 and ReD < 1000. In addition, modification to the frictional performance of Žukauskas correlation is also made. Th...

Abstract: This chapter reviews the various methods that are available for the measurement of heat flux. These include methods based on temperature difference, heat balance, energy supply, and a mass transfer analogy. Heat flux can be defined as the energy in transit due to a temperature difference per unit cross-sectional area normal to the direction of the flux. The measurement techniques available for determining heat flux can, for convenience, be broadly arranged into four categories, such as differential temperature, calorimetric methods, energy supply or removal, and mass transfer analogy. The basis of differential temperature heat flux measurement techniques is to monitor the difference in temperature between locations in a component and, with knowledge of the thermal properties, to use a conduction analysis to determine the heat flux. Calorimetric method for determining heat transfer rates is realized through the measurement of the rate of change of temperature with time at a location near to or on the surface of interest. Analysis of these results, with an appropriate form of the conduction equation or heat balance and accurate knowledge of material properties, allows the heat flux to be quantified. In energy supply techniques, a physical balance between incoming heat and heat loss is achieved by actively cooling or heating. In mass transfer analogy, there is a mathematical similarity between the governing partial differential equations that describe heat transfer and the transport of mass.