Abstract: Heat transfer devices are provided in many refrigeration systems to exchange energy between the cool gaseous refrigerant leaving the evaporator and warm liquid refrigerant exiting the condenser. These liquid-suction or suction-line heat exchangers can, in some cases, yield improved system performance while in other cases they degrade system performance. Although previous researchers have investigated performance of liquid-suction heat exchangers, this study can be distinguished from the previous studies in three ways. First, this paper identifies a new dimensionless group to correlate performance impacts attributable to liquid-suction heat exchangers. Second, the paper extends previous analyses to include new refrigerants. Third, the analysis includes the impact of pressure drops through the liquid-suction heat exchanger on system performance. It is shown that reliance on simplified analysis techniques can lead to inaccurate conclusions regarding the impact of liquid-suction heat exchangers on refrigeration system performance. From detailed analyses, it can be concluded that liquid-suction heat exchangers that have a minimal pressure loss on the low pressure side are useful for systems using R507A, R134a, R12, R404A, R290, R407C, R600, and R410A. The liquid-suction heat exchanger is detrimental to system performance in systems using R22, R32, and R717.

Abstract: This work presents an experimental investigation of the thermal performance of a flatplate heat pipe during startup and shutdown operations. Using the analytical model developed in a previously study, analytical and experimental results on the effect of input power and cooling heat transfer coefficient on the thermal performance of the heat pipe are presented and discussed. The results indicate that the wick in the evaporator section provides the largest resistance to the heat transfer process followed by the wick in the condenser section. It is found that the heat transfer coefficient has an insignificant effect on the maximum temperature difference across the heat pipe where this difference refers to the maximum difference on the outside surfaces of the flat-plate heat pipe. However, as expected, the input heat flux has a substantial effect on the temperature rise where the temperature rise refers to the temperature increase on the outside surface of the heat pipe. It is found that the temperature difference across the heat pipe depends mainly on the input power. The heat transfer coefficient strongly affects the time it takes to reach steady state while input power has a substantially smaller effect. Empirical correlations for the maximum temperature rise, the maximum temperature difference and the time constants are obtained. The experimental results are compared with the analytical results and are found to be in very good agreement.@S0022-1481~00!01803-X#

Abstract: The geothermal or ground-source heat pump (GHP) has been shown to be a very efficient method of providing heating and cooling for buildings. GHPs exchange (reject or extract) heat with the earth by way of circulating water, rather than by use of circulating outdoor air, as with an air-source heat pump. The temperature of water entering a GHP is generally cooler than that of outdoor air when space cooling is required, and warmer than that of outdoor air when space heating is required. Consequently, the temperature lift across a GHP is less than the lift across an air-source heat pump. The lower temperature lift leads to greater efficiency, higher capacity at extreme outdoor air temperatures, and better indoor humidity control. These benefits are achieved, however, at the cost of installing a ground heat exchanger. In general, this cost is proportional to length of the heat exchanger, and for this reason there is an incentive to install the minimum possible length such that design criteria are met. The design of a ground heat exchanger for a GHP system requires, at a minimum, the operating characteristics of the heat pumps, estimates of annual and peak block loads for the building, and information about the properties of the heat exchanger: the size of the U-tubes, the grouting material, etc. The design also requires some knowledge of the thermal properties of the soil, namely thermal conductivity, thermal diffusivity, and undisturbed soil temperature. In the case of a vertical borehole heat exchanger (BHEx) these properties generally vary with depth; therefore, in the design, effective or average thermal properties over the length of the borehole are usually sought. When the cost of doing so can be justified, these properties are measured in an in situ experiment: a test well is drilled to a depth on the same order as the expected depth of the heat pump heat exchangers; a U-tube heat exchanger is inserted and the borehole is grouted according to applicable state and local regulations; water is heated and pumped through the U-tube (using a field generator to power the equipment, or line voltage where available); and the inlet and outlet water temperatures are measured as a function of time. Data on inlet and outlet temperature, power input to the heater and pump, and water flow rate are collected at regular intervals--typically 1 to 15 min--for the duration of the experiment, which may be as long as 60 h. Two common methods for determining soil thermal properties from such measurements are the line source method and the cylinder source method. Both are based on long-term approximate solutions to the classical heat conduction problem of an infinitely long heat source in an infinite homogeneous medium. Although there are some differences in the way the two methods are implemented, the only difference between the two models is whether the heat source is considered to be a line or a cylinder. In both methods, power input to the water loop is assumed to be constant. The simplicity of these methods makes them attractive, but they also have some disadvantages. First of all, because the line source and cylinder source approximations are inaccurate for early time behavior, some of the initial data from the field test must be discarded. The amount of data discarded can affect the property measurement. Also, both methods assume that the heat transfer to the ground loop is constant. In practice, heat input to the loop may vary significantly over the course of a field test due to rough operation of the generator or short-term sags and swells in power line voltage. Presumably, this variation affects the accuracy of the thermal property measurement, but error analysis is rarely performed. This report presents a new method for determining thermal properties from short-term in situ tests using a parameter estimation technique. Because it is based on numerical solutions to the heat conduction equation, the new method is not affected by short-term variations in heat input. Also, since the model is accurate even for short times, there is no need to discard initial data. The parameter estimation technique used to determine the properties is based on statistical principles that provide quantitative estimates of measurement accuracy. The parameter estimation method has now been tested with a laboratory test rig at Oklahoma State University and in field tests at two elementary schools in Lincoln, Nebraska. Using our estimation algorithms, and building on the validation achieved during testing, we have developed a computer program, the Geothermal Properties Measurement (GPM) model, that allows users to determine thermal properties from short-term in situ field tests. This program is currently available free of charge.

Abstract: A mathematical model is developed to study the performance of a parallel-flow heat exchanger in which both fluid streams are interacting thermally with the surroundings The fluid temperatures are found to be dependent on the magnitude of the ambient temperature relative to fluid inlet temperatures, the ratios of conductances between the fluids and the ambient and the interfluid conductance, the ratio of minimum to maximum fluid capacities, and the number of transfer units, NTU, for the heat exchanger Two heat exchanger effectiveness criteria, one each for the hot and cold fluids, are used to study performance The effectiveness is found to be adversely affected by increasing conductance ratios, increasing NTU, and increasing temperature difference between the ambient and the fluid of interest For very high values of the conductance ratios, the heat exchanger will not perform as expected and both fluid temperatures will approach that of the ambient The parallel-flow arrangement is compared to counterfl

Abstract: An application for an integrated array of small (mesoscale) cooling devices is introduced and a survey of related research is provided. A test apparatus appropriate for experiments with small-scale, low-capacity evaporators is described. Two-phase pressure drop and heat transfer data are presented for R-134a in a heat exchanger consisting of an inlet manifold, 52 parallel channels, and an exit manifold. Each individual channel has a cross-sectional flow area 800 {micro}m by 800 {micro}m and 74 {micro}m long. Experiments are conducted over a range of conditions, with flow rates up to 0.48 g/s, inlet qualities from 7% to 15%, and an evaporating temperature of approximately 10 C. The heat exchanger operated with a pressure drop of less than about 8 kPa and provided a heat transfer coefficient greater than 8,000 W/(m{sup 2}{center_dot}K). The heat transfer data suggest that nucleate boiling dominates for flow rates below an equivalent Reynolds number (Re{sub eq}) of about 40 in a channel. A comparison of the pressure drop and heat transfer results to related data from the literature shows general agreement and supports these promising results for mesoscale heat exchangers.

Abstract: Adsorption cycles for refrigeration or heat pumping can be environmentally friendly. Moreover, when they use the process of heat-regeneration, they can be energetically efficient provided the adsorbers are correctly designed, which requires correct modelling. In such a model, all the heat exchanges undergone by the heat transfer fluid are considered. The temperature changes of this fluid induce variations in its physical properties. The influence of these variations on the cycle performance are numerically investigated and thermodynamically analysed. The effects of the changes in density are negligible, but those of the changes in heat capacity cannot be neglected. The analysis shows that this result is due to the interdependence of all the heat quantities involved in the cycle via the first principle of thermodynamics.

Abstract: The rotary process presented here is designed for continuous operation and to use the concept of a heat regeneration cycle developed for solid sorption cold production systems. Based on the analysis of the thermodynamic cycle followed by the reagent, the system is modeled in the form of counter-flow heat exchangers in series. This allows an estimate of the energy performance of the process in terms of coefficient of performance (COP) and cold production capacity. It appears that for a given set of thermodynamic operating conditions, the number of transfer units (NTU) of the heat exchangers is the parameter, which conditions the value of the COP. A comparison between the rotary system by adsorption and by chemical reaction helps to select the ideal reagent according to the temperature level for cold production.

Abstract: The heat transport to a talin/water ice slurry in a cylindrical heat exchanger was determined by theory and experiment. The theory based on perturbation analysis is only valid for small heat transfer rates. Direchlet and Neumann boundary conditions are applied to numerically calculate temperature profiles at different distances downstream. For the constant-heat-flux-density Neumann boundary condition, numerical results were compared with measured profiles. For laminar and low-Reynolds-number turbulent flows, heat transfer coefficients are presented as a function of the Hedstrom number.

Abstract: Average boiling and condensation heat transfer coefficients were determined experimentally for a coaxial tube-in-tube heat exchanger used in hot water heat pumps. During manufacturing, the heat exchanger geometry used for the experiments changed from round tubes to elliptical tubes as no spacers were used to keep the inner tube from touching the outer tube. The refrigerant used was two different mixtures of R22 with R142b in mass ratios of 80 percent/20 percent and 60 percent/40 percent. The results were compared to theoretical results for straight tubes. It was concluded that the theoretical modes do not predict the heat transfer coefficients very well in coaxial tube-in-tube heat exchangers where the annulus touches the inside of the outer tube.

Abstract: A novel moving and stirred bed reactor with a high heat transfer capacity has been operated to achieve the thermal decomposition of used tyre particles under vacuum. The overall heat transfer coefficient determined in this reactor reaches 200–250 W m−2K−1, a value exceeding the levels obtained in conventional rotary kilns and multiple hearth furnaces. In order to design large scale stirred bed vacuum pyrolysis reactors, both experimental and theoretical studies were carried out to understand the heat transfer mechanism and to determine the heat transfer coefficient in the reactor as a function of the operating conditions. In this work, the heat transfer coefficients under different agitation speeds up to 22.5 rpm were measured. The heat transfer coefficient was found to increase with the agitation speed, proportionally to (1/tmix)1/2. A Schliinder's modified model was used to describe the correlation between the heat transfer coefficient and the operating conditions. Calculation of the partial heat transfer coefficients during the three pyrolysis evolution periods revealed the influence of the chemical reactions, the phase change and the feedstock thermal property variation on the overall heat transfer coefficient during the vacuum pyrolysis of tyre particles.

Abstract: The effect of condensation on air-side heat transfer perfonnance has been studied for plain-fin-and-tube and wavy-louvered heat exchangers with fm spacings of2.12, 1.57, and 1.27 tnm. Wet and dry heat exchanger experiments have been conducted to obtain sensible heat transfer, pressure drop, and condensate retention data to help understand the effects of retained condensate on heat exchanger perfonnance. Condensate retention behavior was characterized by measurements of the real-time and steady-state mass of retained condensate. Sensible j and friction factor correlations have been developed using dimensional analysis and the experimental data for the heat exchangers under wet and dry conditions. A condensate retention model has been developed to help predict the quantity of retained condensate on plain-fin-and-tube heat exchangers based on geometry, contact angles, and orientation.

Abstract: The results of an experimental study on the heat transfer characteristics of two-phase flow condensation of some azeotropic refrigerant mixtures, proposed as alternatives to R-22, on air/refrigerant horizontal enhanced surface tubing are presented. The condensation data indicated that the heat transfer coefficient of the blend R-408A has the highest heat transfer rate among the blends under investigation. The condensation data also showed that R-507 and R-404A have similar heat transfer rates to that of R-22 when plotted against the refrigerant mass flow rate. It can also be observed that, as the mass flux increases, the heat transfer coefficient increases. Correlations were proposed to predict the heat transfer characteristics such as average heat transfer coefficients as well as pressure drops of alternatives to R-22 such as R-507, R-404A, R-407C and R-408A, as well as R-410A in two-phase flow condensation inside enhanced surface tubing. In addition, proposed correlations were found to fairly predict the two-phase flow heat transfer condensation data. Copyright © 2000 John Wiley & Sons, Ltd.

Abstract: In this paper the analysis of an experimental test campaign to characterise the heat transfer and pressure drop through Brazed Plate Heat Exchangers is presented, for two different refrigerants, R22 and propane. The study have been performed using an air to water reversible heat pump, in such a way that the BHPE could be tested as the condenser or the evaporator of the cycle. In the paper, first the characteristics and features of the experimental loop are presented, and the range of the test conditions is described. Then a simple model of the heat exchanger is described which takes into account the local variation (ID) of the pressure, temperature and properties, and which allows, by comparison between the calculated and the measured results, the estimation of the average heat transfer coefficient of the Two-Phase region. Then a comparative analysis of the results generated in that way is performed for R22 and propane. A discussion of the results mainly focussed to explain the differences in behaviour between R22 and propane is given. Finally a set of conclusions, concerning both, the analysis and interpretation of these kinds of data, and the differences in behaviour between both refrigerants, are included.

Abstract: In this paper a model for heat exchangers working as evaporator or condenser of a refrigeration system is presented, including a comparison between calculated and measured results for a plate heat exchanger and a tube & fins coil. In the paper, the main characteristics of the model and a comparison between experimental and calculated results is presented and discussed. The flow inside the pipes or channels is considered to be one-dimensional, and discretized in as many elements as required. The basis of the numerical procedure is to de-couple the calculation of the fluid flows from each other. Then, both fluid flows evolution along the heat exchanger are calculated through the integration of the 1-D conservation equations. For the refrigerant, the two fluid (liquid & vapour) separated model under equilibrium is considered. Specific correlations for evaporation and condensation heat transfer in pipes and plate heat exchangers have been implemented into the model. Concerning the air, both sensible heat transfer and dehumidification are considered with appropriate correlations for the heat transfer coefficient and friction factor. A b,. cr Dh e f g G h he ho 1 k m p p q

Abstract: A new procedure for estimating area and capital cost targets of constrained heat exchanger networks is presented. The method allows for match constrained networks and exchangers with more than one tube pass. The procedure is based on modelling the problem as a non-linear formulation where the forbidden exchanger matches are included as constraints and the temperature difference correction due to multipass exchangers is included in the model. The difficulty of converging to a solution due to the additional non-linear constraints imposed by the multipass exchangers required the use of a two-level approach: at the inner level, the area targets for simple pass exchangers are obtained, and at the outer level the temperature difference required for multipass exchangers are computed and fed back to the inner level. The procedure is repeated until an appropriate tolerance between two iterations was achieved. A comparison between the estimated exchanger areas and costs estimated by the new procedure and the area and costs obtained from the final heat exchanger design shows a very good agreement.

Abstract: The forced convective heat transfer of heat exchangers used in gas boilers withsteam condensing was investigated experimentally. Three kinds of the heat exchangers wereinvestigated, including column trays, single finned circular tubes and stagger filnned circulartubes heat exchanger. The experimental system and investigating method are presented inthe papers. The experimental results indicate that, compared with the heat transfer withoutcondensation, the heat transfer coefficient of heat exchangers with steam condensation isseveralfold higher than that of the heat exchangers without condensation. Comparing theinvestigated condensing heat exchangers, the heat transfer coefficient of the column heatexchangers is higher than that of the finned circular tubes heat exchangers.

Abstract: Heat transfer to single particles in the transport zone of circulating fluidized beds and heat transfer to membrane tube walls have been investigated. The heat transfer coefficient to spheres in the transport zone of a circulating fluidized bed boiler has been measured with dark and light calorimeters. A model was developed for the heat transfer mechanisms involved: radiation, gas and particle convection. The model predicts the measured data within 30 %. The gas convective constituent is 50% larger than in a single-phase non turbulent flow, which is explained by turbulence induced by bubbles in the bottom bed. The heat transfer model and a correlation for mass transfer were used to calculate the temperature of burning char particles. The calculation confirmed previous measurements, showing particle temperatures of several hundred degrees above the average bed temperature. Heat transfer from the core to the wall-layer was investigated by a heat balance of the wall-layer. It was found that the computation underestimates the measurements with in average 25%. This is explained by a horizontal in-flow and out-flow of particles, instead of the net-flow used in the model. Heat transfer and suspension density data from circulating fluidized bed boilers were averaged over the entire heat transfer surface of the furnace. The averaged data forms the basis for a correlation, which estimates the heat transfer coefficient with a standard deviation of 15 %. An attempt to separate radiation from convection was made, but it did not improve the accuracy of the correlation. Heat transfer experiments in a laboratory unit under thermally scaled conditions has been investigated theoretically and experimentally. Criteria for thermal similarity were derived from the energy equations of gas and particles. It was found that the ratio of the heat capacity of the particles and the gas should be constant in addition to the fluid-dynamic scaling. Measurements in a boiler and a scaled laboratory rig showed that the Nusselt number was not the same in the two units. However, the heat transfer coefficients from the two units coincided if they were compensated for the difference in thermal conductivity of the gas.