A study on evaluation of effective thermal conductivity for spherical capsules

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.The main goal of the current work is to evaluate specific equations to define effective thermal conductivity for inward melting problems inside spherical containers in terms of the temperature difference and the spacing between the interface and shell. Sets of numerical analyses have been conducted with commercial CFD software ANSYS-FLUENT. In order to check the validity of the method, proposed effective thermal conductivity equation has been implemented into a phase change problem inside a spherical container, and the results are compared with the experimental findings. Comparative results reveal that implementation of the effective thermal conductivity yields reasonable results regarding to the experimental measurementsdc201

Improvement the thermal performance of Myristic acid

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.This study mainly focus on two aspects; the first one is to increase the thermal conductivity of Myristic acid with graphene additives and the second one is to determine the influence of this increment on the melting duration of a particular encapsulated thermal energy storage system with PCM. As a result, thermal conductivity of composite myristic acid increased by 8%, 18% and 38% after graphene loadings of 0.5%, 1% and 2% into the pure MA, respectively. Besides, one-dimensional spherical computational domain has been considered, and the conduction dominated phase change process simulated with implementing temperature transformation method. Validity of the current code has been revealed by reproducing a numerical work in literature. For this particular case, it is found that the increments of 8%, 18% and 38% in thermal conductivity tend to reduce the total time of melting by 5.6%, 15% and 26%.dc201

Heat transfer analysis of phase change process in a finned-tube thermal energy storage system using artificial neural network

In this study, a feed-forward back-propagation artificial neural network (ANN) algorithm is proposed for heat transfer analysis of phase change process in a finned-tube, latent heat thermal energy storage system. Heat storage through phase change material (PCM) around the finned tube is experimentally studied. A numerical study is performed to investigate the effect of fin and flow parameter by the solving governing equations for the heat transfer fluid, pipe wall and phase change material. Learning process is applied to correlate the total heat stored in different fin types of tubes, various Reynolds numbers and different inlet temperatures. A number of hidden numbers of ANN are trained for the best output prediction of the heat storage. The predicted total heat storage values obtained by an ANN model with extensive sets of non-training experimental data are then compared with experimental measurements and numerical results. The trained ANN model with an absolute mean relative error of 5.58% shows good performance to predict the total amount of heat stored. The ANN results are found to be more accurate than the numerical model results. The present study using ANN approach for heat transfer analysis in phase change heat storage process appears to be significant for practical thermal energy storage applications. (C) 2007 Elsevier Ltd. All rights reserved

Investigation of the Liquid-Vapor Separator Efficiency on the Performance of the Ejector Used as an Expansion Device in the Vapor-Compression Refrigeration Cycle

WOS: 000509108300006Ejector expansion refrigeration cycle with reference to the constant pressure mixing theory is investigated to display the effects of the liquid-vapor separator efficiency on the performance, entrainment ratio, and area ratio at various operation conditions. Reversible ejector assumption is used for the highest theoretical performance limit, whereas efficiency of the liquid-vapor separator and all ejector components is added to the model to calculate more realistic performance improvement potentials. R1234yf and R1234ze(E) having low global warming potential values are used in the analyses. Zero-dimensional thermodynamic models are constructed applying the conservation equations between the inlets and outlets of the refrigeration cycle and ejector components. Percentage performance decrease is higher when the mixing section and the separator efficiency is added to the model at higher condenser temperatures compared with the lower evaporator temperatures according to the investigated operation ranges. Vapor and liquid separation efficiency affects not only the performance but also the design of the ejector although it is an external component since it has influence on the area ratio and entrainment ratio. Finally, the difference between the percentage performance improvement of the reversible ejector cycle and the realistic ejector cycle including the separator and ejector components efficiencies is as high as 35% at the highest investigated condenser temperature for R1234yf.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [116M367, 10.13039/501100004410]Scientific and Technological Research Council of Turkey (TUBITAK) (Grant No. 116M367; Funder ID: 10.13039/501100004410)

Preliminary Design of the Two-Phase Ejector under Constant Area Mixing Assumption for 5 kW Experimental System

4th International Conference on Advances on Clean Energy Research (ICACER) -- APR 05-07, 2019 -- Univ Coimbra, Coimbra, PORTUGALWOS: 000503744000002Ejector expansion refrigeration cycle is the modification of the vapour compression refrigeration cycle with the implementation of a two-phase ejector and a vapour-liquid separator to improve the cycle performance. in this study, main geometrical parameters of an ejector, i.e. diameters of the motive nozzle throat, motive nozzle outlet, suction nozzle outlet, and constant area mixing section are calculated in order to provide the preliminary design aspects at various operation conditions. the thermodynamic model of the ejector is established with reference to constant-area mixing assumption. the equations are solved in Matlab (R). the environmentally-friendly refrigerants, R1234yf and R1234ze(E) from the hydrofluoroolefins (HFOs) and R134a which is about to be phased out by the F-gas Regulation are used in the analyses. When compared to the previous literature findings, the current research aims to compare the dimensions of a twophase ejector to be used in anexperimental system having 5 kW coolingcapacity for these three refrigerants.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [116M367]The authors would like to acknowledge the support of the Scientific and Technological Research Council of Turkey (TUBITAK) under Grant No: 116M367

Impact of the mixing theories on the performance of ejector expansion refrigeration cycles for environmentally-friendly refrigerants

WOS: 000452347300020Ejector expansion refrigeration cycle was investigated theoretically to compare the constant pressure mixing and constant area mixing ejector theories making use of thermodynamic models. Performance improvement was presented according to optimum and constant value pressure drop assumptions in order to simulate the design and off-design conditions, respectively. The agreement between the results with respect to both pressure drop assumptions is better at higher condenser and lower evaporator temperatures for each ejector mixing theory. Optimum area ratio (phi(opt)) is less for constant area mixing approach since the ratio of the secondary flow velocity to the primary flow is higher for this ejector theory at each operation point. Although a group of low-global-warming-potential refrigerants were compared in terms of the performance and expansion losses, deep investigations were presented for R1234yf in main and sometimes R1234ze(E). Finally, when compared to other ejector sections, the mixing section efficiency has the most critical effect on the optimum performance improvement ratio of R1234yf with reference to constant pressure mixing assumption. (C) 2018 Elsevier Ltd and IIR. All rights reserved.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [116M367]The authors would like to acknowledge the support of the Scientific and Technological Research Council of Turkey (TUBITAK) under Grant No: 116M367

Experimental assessment of energy storage via variable speed compressor

23rd IIR International Congress of Refrigeration -- AUG 21-26, 2011 -- Prague, CZECH REPUBLICWOS: 000294398000011In this study, usage of a variable speed refrigeration system in latent heat thermal energy storage (LHTES) system is investigated to increase energy storage efficiency. Four different compressor speed control cases are compared to obtain a constant heat transfer fluid (HTF) temperature at the inlet of the energy storage tank. These control cases are (i) control with evaporation temperature, (ii) control with ethylene glycol temperature at the outlet section of evaporator, (iii) control with suction pressure of the compressor and (iv) on/off control. By means of the experimental analysis the best control strategy is obtained as control with Case (ii), in terms of stability of inlet temperature of heat transfer fluid, variations of energy efficiency of LHTES and coefficient of performance (COP) of the system. While the Case (ii) provides the most stabile inlet temperature of HTF, Case (iv) represents the worst stability. Besides, the highest energy efficiency (99.0%), exergy efficiency (87.0%) and COP (2.05) values observed in Case (ii). Additionally, the time period to reach the set value is nearly 50 min in Case (i), Case (iii) and Case (iv) on the other hand this unstable initial time period becomes nearly 25 min for Case (ii). As a result, variable speed compressor should be controlled with Case (ii) to stabilize ethylene glycol-water solution temperatures as well as increase efficiency and COP of the system. (C) 2011 Elsevier Ltd and IIR. All rights reserved.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [106M418]The authors would like to acknowledge the support of the Scientific and Technological Research Council of Turkey (TUBITAK) under Grant No: 106M418

THERMODYNAMIC PERFORMANCE OF THE TRANSCRITICAL REFRIGERATION CYCLE WITH EJECTOR EXPANSION FOR R744, R170, AND R41

WOS: 000459361300010For more than a decade, there is a great demand for finding environmentally-friendly refrigerants obeying the global warming potential value restrictions of the tough environmental legislation. Among the candidate working fluids, R744 (carbon dioxide or CO2), R170 (ethane), and R41 (fluoromethane) are selected to be investigated parametrically in this paper. Performance comparison is made for these three working fluids individually in both transcritical (supercritical) refrigeration cycle and modification of this cycle with ejector expansion. As the first step, the effects of the gas cooler outlet temperature, evaporator temperature, and evaporator outlet superheat temperature difference on the overall performance and percentage expansion losses are investigated within a specific gas cooler pressure range. Evaporator outlet superheat temperature difference is found to be the least effective parameter on the performance; hence, secondly, the transcritical ejector expansion refrigeration cycle is analyzed considering only evaporator temperature and gas cooler outlet temperature based on the same gas cooler pressure ranges. Thermodynamic models are constructed in Matlab (R) and the ejector equations for the ejector expansion refrigeration cycle are established with reference to constant pressure mixing assumption. Comparisons of the performance, percentage expansion losses, and performance improvement potential through the implementation of the ejector instead of the expansion valve among these three refrigerants having low critical temperatures represent the main objective of the paper in order to make contributions to the previous researches in the literature.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [116M367]The authors would like to acknowledge the support of the Scientific and Technological Research Council of Turkey (TUBITAK) under Grant No: 116M367