Effect of regeneration on the thermo-ecological performance analysis and optimization of irreversible air refrigerators

A thermo-ecological performance analysis of an irreversible regenerative air refrigerator cycle exchanging heat with thermal reservoirs is presented. In the analysis, the external irreversibility effects due to heat transfer across finite temperature differences and the heat leak loss between the external heat reservoirs while the internal irreversibilities are due to the non-isentropic compression and expansion processes and the regenerative loss are taken into account. The effects of regeneration and heat sources temperature ratio are given special emphasis and investigated in detail. A comparative performance analysis considering the objective functions of an ecological coefficient of performance, exergetic efficiency and coefficient of performance is also carried out. The maximum of the objective functions and the corresponding optimal conditions have been derived analytically. The obtained results may provide a general theoretical tool for the thermo-ecological design of regenerative air refrigerators.     

Selection of the most suitable refrigerant for a shell and tube condenser

A theoretical performance study on a shell and tube condenser with various refrigerant blends was conducted for various ratios proposed by other researchers in the literature. The theoretical results showed that all of the alternative refrigerants investigated in the analysis have a slightly lower convective heat transfer coefficient than their base refrigerants. The refrigerant mixture of R290/R600, R152a/R125/R32 and R32/R134a were found to be the most proper replacement refrigerant among the alternatives.     

Efficiency analysis of organic Rankine cycle with internal heat exchanger using neural network

In this study, artificial neural network (ANN) has been used for efficiency analysis of the organic Rankine cycle with internal heat exchanger (IHEORC) using refrigerants R410a, R407c which do not damage to ozone layer. It is well known that the evaporator temperature, condenser temperature, subcooling temperature and superheating temperature affect the thermal efficiency of IHEORC. In this study, thermal efficiency is estimated depending on the above temperatures. The results of ANN are compared with actual results. The coefficient of determination values obtained when the test set were used to the networks were 0.99946 and 0.999943 for the R410a and R407c respectively which is very satisfactory.     

Low-GWP refrigerants flow boiling heat transfer in a 5 PPI copper foam

This paper reports an experimental investigation of the heat transfer performance of the new low-GWP refrigerants, R1234yf and R1234ze(E), during flow boiling heat transfer inside a horizontal high porosity copper foam with 5 Pores Per Inch (PPI). Metal foams are a class of cellular structured materials consisting of a stochastic distribution of interconnected pores; these materials have been proposed as effective solutions for heat transfer enhancement during both single and two-phase heat transfer. R1234yf and R1234ze(E) refrigerants are appealing alternatives of the more traditional R134a by virtue of their negligible values of GWP and normal boiling temperatures close to that of R134a, which make them suitable solution in several different applications, such as: refrigeration and air conditioning and electronic thermal management. This work compares the two-phase heat transfer behaviour of these new HFO refrigerants, studying the boiling process inside a porous medium and permitting to understand their effective heat transfer capabilities. The experimental measurements were carried out by imposing three different heat fluxes: 50, 75, and 100 kW m⁻², at a constant saturation temperature of 30 °C; the refrigerant mass velocity was varied between 50 and 200 kg m⁻² s⁻¹, whilst the mean vapour quality varied from 0.2 to 0.95. The two-phase heat transfer and pressure drop performance of the two new HFO refrigerants is compared against that of the more traditional R134a.     

On the coupled system performance of transcritical CO2 heat pump and rankine cycle

As one of the natural refrigerants, CO₂ is a potential substitute for synthesized refrigerants with favorable environmental properties. In order to improve the performance of rankine cycle (RankC), the coupled system cycle (CSC) was designed and the performance was analyzed in this paper, which the CSC is combined by the RankC and the transcritical CO₂ heat pump cycle with an expander. Based on thermodynamic principles, the performance analysis platform was designed and the performance analysis was employed. The results show that the average efficiency of the RankC is about 30 %, and the extraction cycle is about 32 %, while the CSC is about 39 %, and the last one is better than the others at the same parameters. With increasing of the boiler feed water temperature, the efficiencies of the three kinds of cycles show increasing trend. With increasing of pressure in conderser–evaporator or outlet temperature of gas cooler, the efficiency of the CSC shows a downward trend. Some fundamental data were obtained for increasing the RankC efficiency by waste heat recovery, and play an active role in improvement the efficiency of power plants.     

Condensing heat transfer characteristics of hydrocarbon refrigerants in 9.52 and 12.7 mm smooth tube

Experimental results for heat transfer characteristic and pressure gradients of hydrocarbon (HC) refrigerants and R-22 during condensing inside horizontal double pipe heat exchangers are presented. The test sections which have one tube diameter of 12.70 mm with 0.86 mm wall thickness, another tube diameter of 9.52 mm with 0.76 mm wall thickness are used for this investigation. The local condensing heat transfer coefficients of HC refrigerants were higher than that of R-22. The average condensing heat transfer coefficient increased with the increase of the mass flux. It showed the higher values in HC refrigerants than R-22. Comparing the heat transfer coefficient of experimental results with that of other correlations, the presented results had a good agreement with most of the Cavallini’s correlations.     

Condensation pressure drop of R22, R134a and R410A in a single circular microtube

The condensation pressure drop characteristics for pure refrigerants R22, R134a, and a binary refrigerant mixture R410A without lubricating oil in a single circular microtube were investigated experimentally. The test section consists of 1,220?mm length with horizontal copper tube of 3.38?mm outer diameter and 1.77?mm inner diameter. The experiments were conducted at refrigerant mass flux of 450–1,050?kg/m²s, and saturation temperature of 40°C. The main experimental results showed that the condensation pressure drop of R134a is higher than that of R22 and R410A for the same mass flux. The experimental data were compared against 14 two-phase pressure drop correlations. A new pressure drop model that is based on a superposition model for refrigerants condensing in the single circular tube is presented.     

Performance analysis of solar powered absorption refrigeration system

The present work provides a detailed thermodynamic analysis of a 10 kW solar absorption refrigeration system using ammonia-water mixtures as a working medium. This analysis includes both first law and second law of thermodynamics. The coefficient of performance (COP), exergetic coefficient of performance (ECOP) and the exergy losses (ΔE) through each component of the system at different operating conditions are obtained. The minimum and maximum values of COP and ECOP were found to be at 110 and 200°C generator temperatures respectively. About 40% of the system exergy losses were found to be in the generator. The maximum exergy losses in the absorber occur at generator temperature of 130°C for all evaporator temperatures. A computer simulation model is developed to carry out the calculations and to obtain the results of the present study.     

Prediction of refrigerant void fraction in horizontal tubes using probabilistic flow regime maps

A state of the art review of two-phase void fraction models in smooth horizontal tubes is provided and a probabilistic two-phase flow regime map void fraction model is developed for refrigerants under condensation, adiabatic, and evaporation conditions in smooth, horizontal tubes. Time fraction information from a generalized probabilistic two-phase flow map is used to provide a physically based weighting of void fraction models for different flow regimes. The present model and void fraction models in the literature are compared to data from multiple sources including R11, R12, R134a, R22, R410A refrigerants, 4.26–9.58 mm diameter tubes, mass fluxes from 70 to 900 kg/m² s, and a full quality range. The present model has a mean absolute deviation of 3.5% when compared to the collected database.     

Flow boiling of halogenated refrigerants at high saturation temperature in a horizontal smooth tube

Several correlations are available in the open literature for computing the heat transfer coefficient during flow boiling inside plain channels. With respect to halogenated refrigerants, these correlations are usually compared to data taken in a limited range of evaporation temperature and reduced pressure. More recently, the adoption of new refrigerants, such as high pressure HFCs and carbon dioxide, requires to largely extend the pressure range of application of such correlations. Besides, the design of evaporators for some heat pumping applications, where temperatures are set at higher values as compared to usual evaporating temperatures in air-conditioning equipment, requires proper validation of the computing methods.The present paper aims at comparing four well-known predicting models to a new database collected during flow boiling of HCFC (R22) and HFC refrigerants (R134a, R125 and R410A) in a horizontal 8 mm internal diameter tube. This database is characterized by saturation temperature ranging between 25 °C and 45 °C, reduced pressure spanning between 0.19 and 0.53. Mass velocity ranges between 200 and 600 kg m^?2 s^?1 and heat flux between 9 and 53 kW m^?2.Evaporating heat transfer coefficients of halogenated refrigerants at such high temperatures have not been reported in the open literature so far. The discussion of the results will enlighten some similarities with experimental trends presented in the literature for evaporating carbon dioxide.Two models tested here show good prediction capabilities of the present experimental data, but not for all the data sets in the same way. For the purpose of practical use, a simple modification of the correlation by Gungor and Winterton [1] is proposed, showing that this is able to catch the experimental trends of the present database with good agreement.     

Condensation of a zeotropic CFC 114-CFC 113 refrigerant mixture in the annulus of a double-tube coil with an enhanced inner tube

Local heat transfer and pressure drop measurements were made during condensation of a zeotropic CFC114-CFCll3 refrigerant mixture in the annulus of a double-tube coil consisting of three U-bends and four straight lengths. The inner tube is a 19.1-mm O.D. corrugated copper tube with wire fins soldered onto the outer surface and the inner diameter of the outer duct is 25.0 mm. The vapor-phase mass transfer coefficient exhibited a sawtooth behavior with the U-bends showing higher coefficients than the straight lengths. The frictional pressure gradient data agreed well with a previously developed empirical equation for the condensation of pure refrigerants. A prediction method for the condensation heat transfer rate was proposed on the basis of the correlations of the vapor-phase mass transfer coefficient and heat transfer coefficient of the condensate film. The heat transfer data were correlated by the present method to a mean absolute deviation of 12.9%.     

Thermodynamic modelling of a double-effect LiBr-H2O absorption refrigeration cycle

The goal of this paper is to estimate the conductance of components required to achieve the approach temperatures, and gain insights into a double-effect absorption chiller using LiBr-H₂O solution as the working fluid. An in-house computer program is developed to simulate the cycle. Conductance of all components is evaluated based on the approach temperatures assumed as input parameters. The effect of input data on the cycle performance and the exergetic efficiency are investigated.     

Exergy analysis for quasi two-stage compression heat pump system coupled with ejector

Ejectors are simple mechanical components, can utilize high pressure energy from liquid in the quasi two-stage compression heat pump system coupled with scroll compressor, and the performance of the heat pump system can be further improved. According to thermal analysis model based on the first and second law of thermodynamics, the heat pump prototype has been developed and comprehensively tested, the influence of ejector on the heat pump system was exergetically analyzed using experimental data of the prototype. The results show that, compressor has the greatest exergy loss, amounts to about 77% of the total exergy; ejector can recover the fluid pressure exergy in supplementary circuit compared with the throttling element in the quasi two-stage compression heat pump system, decreases the exergy loss of compressor; and the exergetic efficiency can be improved about 3-5%, while the exergy output remains nearly constant.     

Heat transfer and two-phase flow characteristics of refrigerants flowing under varied heat flux in a double-pipe evaporator

A mathematical model based on the annular flow pattern is developed to simulate the evaporation of refrigerants flowing under varied heat flux in a double tube evaporator. The finite difference form of governing equations of this present model is derived from the conservation of mass, energy and momentum. The experimental set-up is designed and constructed to provide the experimental data for verifying the simulation results. The test section is a 2.5 m long counterflow double tube heat exchanger with a refrigerant flowing in the inner tube and heating water flowing in the annulus. The inner tube is made from smooth horizontal copper tubing of 9.53 mm outer diameter and 7.1 mm inner diameter. The agreement of the model with the experimental data is satisfactory. The present model can be used to investigate the axial distributions of the temperature, heat transfer coefficient and pressure drop of various refrigerants. Moreover, the evaporation rate or the other relevant parameters that is difficult to measure in the experiment are predicted and presented here. The results from the present mathematical model show that the saturation pressure and temperature of refrigerant decrease along the tube due to the tube wall friction and the flow acceleration of refrigerant. The liquid heat transfer coefficient increases with the axial length due to reducing the thickness of the liquid refrigerant film. Due to increase of the liquid heat transfer coefficient, increasing wall heat flux is obtained.Finally, the evaporation rate of refrigerant increases with increasing wall heat flux.     

Entropy generation of radial rotation convective channels

The exchange of heat between two fluids is established by radial rotating pipe or a channel. The hotter fluid flows through the pipe, while the cold fluid is ambient air. Total length of pipe is made up of multiple sections of different shape and position in relation to the common axis of rotation. In such heat exchanger the hydraulic and thermal irreversibility of the hotter and colder fluid occur. Therefore, the total entropy generated within the radial rotating pipe consists of the total entropy of hotter and colder fluid, taking into account all the hydraulic and thermal irreversibility of both fluids. Finding a mathematical model of the total generated entropy is based on coupled mathematical expressions that combine hydraulic and thermal effects of both fluids with the complex geometry of the radial rotating pipe. Mathematical model follows the each section of the pipe and establishes the function between the sections, so the total generated entropy is different from section to section of the pipe. In one section of the pipe thermal irreversibility may dominate over the hydraulic irreversibility, while in another section of the pipe the situation may be reverse. In this paper, continuous analytic functions that connect sections of pipe in geometric meaning are associated with functions that describe the thermo-hydraulic effects of hotter and colder fluid. In this way, the total generated entropy of the radial rotating pipe is a continuous analytic function of any complex geometry of the rotating pipe. The above method of establishing a relationship between the continuous function of entropy with the complex geometry of the rotating pipe enables indirect monitoring of unnecessary hydraulic and thermal losses of both fluids. Therefore, continuous analytic functions of generated entropy enable analysis of hydraulic and thermal irreversibility of individual sections of pipe, as well as the possibility of improving the thermal–hydraulic performance of the rotating pipe consisting of n sections. Analytical modeling enabled establishing of a mathematical model of the total generated entropy in a radial rotating pipe, while the generated entropy of models with radial rotating pipe were determined by experimental testing, with comparisons of the achieved results.     

Suppression of nucleate boiling of pure and mixed refrigerants in turbulent annular flow

The following issues are addressed: (a) Is suppression of nucleate boiling an appropriate physical mechanism to explain experimental heat transfer coefficient behavior? (b) Is suppression possible under reasonable conditions with refrigerants? (c) Is suppression more or less likely with a binary mixture of refrigerants as compared with either pure fluid? Issue (a) is resolved in favor of traditional suppression theory by critical review of the literature. Issue (b) is resolved by experimental evidence suggesting one must have rather low pressures to suppress nucleation with refrigerants. Issue (c) is resolved by experimental and correlative evidence suggesting systematic differences with mixtures.     

Working fluids selection for fishing boats waste heat powered organic Rankine-vapor compression ice maker

To utilize waste heat from fishing boats, an organic Rankine cycle/vapor compression cycle system was employed for ice making and a thermodynamic model was developed. Six working fluids were selected and compared in order to identify suitable working fluids which may yield high system efficiencies. The calculated results show that R600a is most suitable working fluid through comprehensive comparison of efficiency, size parameter, pressure ratio, coefficient of performance, system pressure and safety.     

Heat transfer and enhancement analyses of flow boiling for R417A and R22

The experimental study on heat transfer of R417A and R22 flow boiling inside a horizontal smooth and two internally grooved tubes with different geometrical parameters was conducted. Based on the experimental results, evaporation heat transfer characteristics of R417A and R22 flowing in different tubes, the influence of micro-fin geometrical parameters, vapor quality and mass flux of refrigerants on heat transfer enhancement factors, and the difference of enhancement factors between R417A and R22 were analyzed and discussed. The result indicates: whether for R22 or for R417A, the enhancement effect of Tube III having the narrower distance between micro-fins excels than Tube II. The influence of vapor qualities and mass fluxes on enhancement factors for R417A is different from R22. And the difference of enhancement factors between R417A and R22 appears different cases at different vapor quality regions.     

Two-phase flow of refrigerants in 85 μm-wide multi-microchannels: Part I – Pressure drop

This article is the first part of a study on flow boiling of R236fa and R245fa. This part presents pressure drop measurements obtained on a silicon multi-microchannel evaporator with 85 μm wide and 560 μm high channels separated by 46 μm wide fins. The 135 microchannels were 12.7 mm long. Dielectric refrigerants R236fa and R245fa were used as the evaporating test fluids. The inlet saturation temperature was maintained at 30.5 °C while the mass fluxes were varied from 499 to 1100 kg/m² s and the base heat flux was tested from 130 to 1400 kW/m². A new experimental technique was developed to measure the outlet pressure losses, which represented up to 30% of the total pressure drop and thus cannot be neglected. The microchannel pressure drop measurements were very well predicted by the method of Cioncolini et al. (2009).     

Zum Wärmeübergang beim Blasensieden von Kohlenwasserstoffen und Halogen-Kältemitteln an einem Glattrohr und einem Hochleistungs-Rippenrohr

Zusammenfassung Mit einem Glattrohr und einem Hochleistungs-Rippenrohr aus Kupfer wurde der Wärmeübergang beim Blasensieden von fünf Kohlenwasserstoffen und vier Halogenkältemitteln, darunter die zwei z.Zt. noch in Erprobung befindlichen Stoffe R134a und R227, in einem großen Bereich der Wärmestromdichte und des Siededruckes gemessen. Das Rippenrohr besitzt aus dem Vollen gewalzte Rippen, die nachträglich zum Teil wieder plattgewalzt wurden und am Grund der Rippenzwischenräume zusätzlich Kerben zur Verbesserung des Wärmeübergangs enthalten (Typ: GEWA-TX). Während sich der Einfluß der Wärmestromdichte, des Siededruckes und der Stoffeigenschaften auf den Wärmeübergangskoeffizienten beim Glattrohr für alle neun Stoffe in guter Näherung einheitlich darstellen läßt, sind die entsprechenden Zusammenhänge bei den Rippenrohren wegen der besonderen Form der Blasenbildung komplexer, als von herkömmlichen Rippenrohren bekannt ist. Im Vergleich zum Glattrohr verspricht dieses Rippenrohr besonders bei nicht zu hohen Siededrücken und beim Einsatz mit Kohlenwasserstoffen und den neuen Kältemitteln R134a und R227 Vorteile.

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Heat transfer with pool boiling of hydrocarbons and halogeneated refrigerants at a plain and a high performance finned tube

Heat transfer with pool boiling of five hydrocarbons and two halogenated refrigerants was measured for a plain and a high performance finned tube over an extended range of heat fluxes and saturation pressures. The finned tube with T-shaped cross sections of the fins contains additional cavities at the bottom of the channels between the fins to enhance heat transfer (type: GEWA-TX). For the plain tube, the influence of heat flux, saturation pressure and thermophysical properties of the boiling liquid can be represented in a rather uniform manner with all of the nine substances. In the case of the finned tube, the corresponding relations are more complex than is known from traditional finned tubes, because of the special form of bubble release with this tube. Compared to the plain tube, the finned tube investigated seems to be favourable when used for hydrocarbons and the new refrigerants, and for boiling conditions at comparatively low saturation pressures.

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Herrn Prof. Dr. K. Bier zum 65. Geburtstag gewidmet