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Absorption machines are frequently used for cooling and heating. However, they must be improved to achieve high performance at a lower cost. Being a key component of these machines, this paper focuses on the absorber and specifically on falling film absorber configuration due to its compactness and simple geometry. This paper reviews the different analytical and numerical approaches used to analyze the thermal, mass and hydrodynamic phenomena occurring in falling films for both smooth and wavy-laminar regimes. It shows the significant progress made since 2000 in terms of mathematical models developed and coupling effect resolution. Research using thermodynamics to identify, locate and quantify the entropy generation sources in falling film absorbers is also presented. [ABSTRACT FROM AUTHOR]

Abstract: This paper aims at developing an analytical model for the coupled heat and mass transfer processes in a counterflow Reversibly Used Cooling Tower (RUCT) based on operating conditions, which is more realistic than most conventionally adopted Merkel approximations. Temperature and moisture content differences are chosen as the driving forces of heat and mass transfer correspondingly and a system of specific difference equations is developed to solve the model more efficiently. The model is investigated by using an iterative algorithm, which is validated with the experimental data reported. The analytical model also accommodates the direct and quick calculation of air and water temperature profiles, moisture content of air and the water mass flow rate change along the vertical length of the RUCT. With the aid of the developed model, the thermal behavior of the counterflow RUCT under various operating and environmental conditions is also studied in this paper. The results reveal that the proposed model can provide a theoretical foundation for practical design and performance evaluation of counterflow RUCT. [Copyright &y& Elsevier]

Abstract: The mathematical models of evaporative fluid coolers and evaporative condensers are studied in detail to perform a comprehensive design and rating analysis. The mathematical models are validated using experimental as well as numerical data reported in the literature. These models are integrated with the fouling model presented in an earlier paper, using the experimental data on tube fouling. In this paper, we use the fouling model to investigate the risk based thermal performance of these evaporative heat exchangers. It is demonstrated that thermal effectiveness of the evaporative heat exchangers degrades significantly with time indicating that, for a low risk level (p=0.01), there is about 66.7% decrease in effectiveness for the given fouling model. Furthermore, it is noted that there is about 4.7% increase in outlet process fluid temperature of the evaporative fluid cooler. Also, a parametric study is performed to evaluate the effect of elevation and mass flow rate ratio on typical performance parameters such as effectiveness for rating calculations while surface area for design calculations. [Copyright &y& Elsevier]

A paper has been published in International Journal of Refrigeration, second law analysis of vapour absorption refrigeration (VAR) system using different mathematical expressions which obey thermodynamics principles. The present paper attempts to discuss and comment on the paper. Addition or subtraction of exergy of the heat flux (or exergy of heat transfer) in mathematical expression of internal irreversibility, are not meeting to direction principles of exergy flow. Therefore, internal irreversibility expressions must be corrected for future research based on second law in the field of VAR system. [ABSTRACT FROM AUTHOR]

The paper presents an investigation of the thermal performance of an innovative insulation wrap for food freezing, transportation and storage. The wrap is made of two outer polypropylene films and an internal folded polypropylene film welded together to form rectangular internal air cells. A mathematical model of heat transfer is presented and experimentally validated, with discrepancies less than 10% when there is no internal convection effect and generally less than 12% when internal convection is present. A thermal effectiveness of film usage was proposed and analyzed in order to find the best fold geometry. A mathematical model of the effectiveness and its experimental validation showed good agreement (discrepancies less than 13%). By using a criteria map, we recommend the following geometry: wrap thickness more than 5 and less than 10 mm; fold spacing more than 6 and less than 10 mm. [ABSTRACT FROM AUTHOR]

Abstract: The present work presents a model (Fin1Dx3) for air-to-refrigerant microchannel condensers and gas coolers, with any refrigerant circuitry. The model applies a segment-by-segment discretization to the heat exchanger, adding in each segment a novel bi-dimensional discretization to the fluids flow, fin and tube wall. Fin1Dx3 introduces a new approach to model the air-side heat transfer by using a composed function for the fin wall temperature, which allows to take into account more fundamentally the heat conduction between tubes. The proposed model accounts for: 2D longitudinal heat conduction in the tube wall, the heat conduction between tubes along the fin, and the unmixed air influence on performance. The paper presents the heat exchanger discretization, the governing equations, the numerical scheme employed to discretize equations and the solving methodology. The model has been validated against experimental data for both a condenser and a gas cooler, resulting in predicted capacity errors within ±5%. [Copyright &y& Elsevier]

Abstract: In this paper questions relating to the performance optimization of heat driven pump systems are discussed. For this purpose a mathematical model for LiBr/H2O absorption heat transformer (AHT) operating according to the endo-irreversible cycle is elaborated. The model is exploited to find out the optimal operating parameters using the so called structural analysis. It shows the influence of some operating parameters on the local irreversibility rates with respect to the overall AHT irreversibility rate. At the end of this work a comparison is performed of the COP, first and second law efficiencies obtained for the optimal operating regime to those calculated for real operating conditions for an already existing AHT. It is concluded that for the same thermal powers and heat transfer parameters, an increase is performed of the first and second law efficiencies equal to 10% and 5.3% consecutively at the obtained optimal operating regime. [Copyright &y& Elsevier]

Abstract: This paper presents a mathematical model and its solution algorithm for analyzing the transient supercooling effect of voltage pulse on a thermoelectric (TE) element. Realistic boundary conditions, including the hot-end convection heat transfer coefficient (h) and cold-end cooling load, are considered in solving the model. The numerical simulation results show that the transient supercooling occurs when the steady input voltage experiences a step change. However, the minimum supercooling temperature (T mts) cannot approach absolute zero as reported in previous research due to the co-existence of Joule heat and Peltier effect. There is a cost-effective h to achieve T mts and maximum hold time of the supercooling temperature. However, the cold-end cooling load only affects T mts. Experiments are conducted to test the transient supercooling effect of voltage pulse. The trends of the cold-end temperature responding to voltage pulses obtained in numerical simulation and experiment tests match well. [Copyright &y& Elsevier]

Abstract: The primary objective of this paper is to quantify the impact of the internal magnetic field and reversibility of the magnetocaloric effect on the numerical evaluation of the thermal performance of a parallel-plate active magnetic regenerator. To this end, direct measurements of the adiabatic temperature change of commercial-grade gadolinium samples were carried out for an applied field change of 1.65 ± 0.03 T (for both magnetization and demagnetization) and temperatures between 283 and 303 K. The tests were carried out in a specially constructed apparatus and the results were in good agreement with the literature. The data were incorporated into a mathematical model for solving the fluid flow and conjugated heat transfer in the parallel-plate regenerator. It was found that if demagnetization phenomena (i.e., the demagnetization factor and the reversibility of the magnetocaloric effect) are not take properly into account, the thermal performance of the regenerator can be severely over-predicted. [Copyright &y& Elsevier]

Abstract: With the aim to develop a generalized model for predicting the frost growth on cold flat plate, experiments were conducted in a suction-type open-loop wind tunnel under variant frosting conditions. Based on the Hayashi correlation, a modified correlation of initial frost density was developed by the experimental results. After validating this proposed correlation, dynamic frosting characteristics, such as the frost thickness, density, frost surface temperature and heat flux were predicted under a wide range of frosting conditions. Compared with 25 groups of experimental results taken from this paper or previous open literatures, the average simulating errors were found to be limited within ±10%. The validation results indicate the proposed model is suitable to predict the dynamic heat and mass transfer process of the frost growth under a wide range of frosting conditions. [Copyright &y& Elsevier]

Abstract: In this paper, (Part II of a two-part study), an investigation of void fractions for condensing flows of refrigerant R-134a in minichannels is presented. Custom digital image processing techniques were used to quantitatively analyze these flows in transparent minchannel geometries (square, rectangular and circular) with 2 < D h < 4.91mm. A total of 140 void fraction values in the intermittent flow regime (slug and plug flow), the wavy flow regime, and an intermittent-wavy overlap region were measured. A new model was developed based on these measured values and predicts void fraction better than correlations from the literature. [Copyright &y& Elsevier]

Abstract: A new, fast and flexible, time-dependent, one-dimensional numerical model was developed in order to study in detail the operation of an active magnetic regenerator (AMR). The model is based on a coupled system of equations (for the magnetocaloric material and the heat-transfer fluid) that have been solved simultaneously with the software package MATLAB. The model can be employed to analyze a wide range of different operating conditions (mass-flow rate, operating frequency, magnetic field change), different AMR geometries, different magnetocaloric materials and heat-transfer fluids, layered and single-bed AMRs, etc. This paper also presents an optimization of the AMR’s geometry, where the AMR consists of a packed-bed of grains (spheres) of gadolinium (Gd). The optimization of the mass-flow rate and the operating frequency of the AMR were performed by studying five different diameters of Gd spheres. [Copyright &y& Elsevier]

Abstract: In order to obtain a better reliability, consistency and accuracy of results obtained with a numerical simulation of an AMRR (active magnetic regenerator refrigerator), a dimensionless numerical model was developed, which can equally be used for determination of regenerator’s heat transfer coefficient and simulation of passive heat regenerators or AMRR operation. Regenerator’s heat transfer coefficient α f, is a crucial input parameter in the simulation of AMRR operation and has a primal effect on the outcome of a solution. This paper deals with a derived dimensionless model and discusses errors involved when using different models for heat transfer coefficient and AMRR operation simulation. [Copyright &y& Elsevier]

Abstract: This paper presents a numerical model conceived for a multi-physics evaluation of a magnetic refrigeration system. The model takes into account the variable magnetic field, the characteristics of the magnetocaloric materials (MCM) as well as the efficiency of the fluid flow heat transfer between the hot and the cold sources via the MCM. This model allows us to optimize these elements and to determine their impact on the overall performance of a magnetocaloric device. We present here a finite-difference model of AMRR cycles for a sub-unit of MCM crossed by micro-channels of fluid connected to cold and hot regenerative receivers. Thus, the calculation of the output parameters such as the temperature span, and thermal power passing from the cold receiver to the hot receiver has been performed. The influence of the geometry of the MCM and the fluid flow were investigated over the working domain highlighting the current technological achievements and indicating possible improvements. [Copyright &y& Elsevier]

Abstract: This paper presents the results of a predictive two-dimensional mathematical model of an adsorption cooling machine consisting of a double consolidated adsorbent bed with internal heat recovery. The results of a base-case, taken as a reference, demonstrated that the COP of the double bed adsorption refrigeration cycle increases with respect to the single bed configuration. However, it was verified that, in order to maximize also the specific power of the machine, the adsorbent beds must have proper thermo-physical properties. Consequently, a sensitivity analysis was carried out, studying the influence of the main heat and mass transfer parameters on the performance of the machine. The results obtained allowed us to define the adsorbent bed design that maximizes its heat and mass transfer properties, as well as the most profitable heat recovery conditions. [Copyright &y& Elsevier]

Abstract: A state-of-the-art review of horizontal-tube, falling film evaporation is presented; the review is critical, in an attempt to uncover strengths and weaknesses in prior research, with the overall purpose of clearly identifying gaps in our understanding. The review covers flow-pattern studies, and the experimental parameters that affect the heat transfer performance on plain single tubes, enhanced surfaces and tube bundles. In addition, this paper presents a comprehensive review of the significant efforts to develop mathematical models, and empirical correlations for the heat transfer coefficient. Emphasis is placed on studies that are related to refrigeration applications. [Copyright &y& Elsevier]

In this paper a new mathematical model for calculating the indicator diagram of twin screw refrigeration compressor is presented firstly. The geometric parameters related to the rotation angle of male rotor are used in the model such as groove volume, suction and discharge port area, slide valve bypass port area, leakage area etc. The effects of internal leakage through five paths, oil injection, gas–oil heat transfer, refrigerant property and partial loading etc. are taken into account simultaneously and separately in the theoretical study. To verify the model and the calculated p–V indicator diagram, experimental recording of p–V indicator diagram of twin screw refrigeration compressor is carried out. Various indicator diagrams are recorded successfully by a small pressure sensor embedded in the bottom of female rotor at the discharge side. The results of theoretical calculation are in good agreement with the measured data, which lead to conclusion that the model can be used as a powerful tool for performance prediction and product development. [Copyright &y& Elsevier]