Showing papers in "International Journal of Refrigeration-revue Internationale Du Froid in 1997"

A small capacity steam-ejector refrigerator: experimental investigation of a system using ejector with movable primary nozzle

Abstract: This paper describes an experimental study of a steam-ejector refrigerator using an ejector with a primary nozzle that could be moved axially within the mixing chamber section. The effects on coefficient of performance and cooling capacity produced by adjusting the position of the nozzle were studied. The experimental rig and method are described and results are presented which clearly show the benefit of using such a primary nozzle.

Comparison of the working domains of some compression heat pumps and a compression-absorption heat pump

Abstract: The working domains of a model of a compression heat pump using different fluids and a model of a compression-absorption heat pump using water-ammonia mixtures are defined, plotted and discussed. These domains are defined by means of limiting values for their electrical coefficient of performance, volumetric heating capacity, and low and high pressure. In the case studied in the present paper, the disappearance from use of CFC and HCFC fluids leaves only one alternative for the implementation of high temperature electric heat pumps: hydrocarbons in compression devices or water-ammonia mixtures in compression-absorption devices. Problems relating to the implementation of these systems are also mentioned.

Design optimization of thermoacoustic refrigerators

Abstract: Thermoacoustic refrigeration was developed during the past two decades as anew, environmentally safe refrigeration technology. The operation of thermoacoustic refrigerators employs acoustic power to pump heat. Nowadays, as commercial applications are sought, it is important to be able to obtain fast and simple engineering estimates for the design and optimization of prototypes. This paper provides such estimates by implementing the simplified linear model of thermoacoustics - the short stack boundary layer approximation - into a systematic design and optimization algorithm. The proposed algorithm serves as an easy-to-follow guideline for the design of thermoacoustic refrigerators. Performance calculations applying the algorithm developed in this paper, predict values of 40–50% of Carnot's efficiency for the thermoacoustic core, the heart of a thermoacoustic refrigerator. One reason that these efficiencies have not yet been achieved in devices built to date, is the poor performance of the heat exchangers in thermoacoustic refrigerators. This issue and other remaining challenges for future research are also addressed in the paper. Solving these problems in the near future, we believe, will bring an environmentally safe refrigeration technology a step closer to commercial use.

Adsorption refrigerator using monolithic carbon-ammonia pair

Abstract: This paper presents the description of a laboratory prototype of an adsorption cooling machine which uses an activated monolithic carbon-ammonia pair. The design is focused on the generator consisting of aluminium and monolithic carbon discs. A numerical model of the generator based on the mass and energy conservation equations is developed. The heat transfer model (between aluminium fin and activated carbon-ammonia pair) is validated by comparing computer simulation and experimental results at ice maker operating conditions corresponding to tropical use (the evaporating and condensing temperature ranges are −20°C to 0°C and 20°C to 45°C, respectively).

An improved extended corresponding states method for estimation of viscosity of pure refrigerants and mixtures

Abstract: The extended corresponding states method for calculating the viscosity of pure refrigerants and mixtures is investigated. The accuracy of pure fluid viscosity values is significantly improved by introducing a third shape factor evaluated using available pure fluid viscosity data. A modification to the method of Huber and Ely (Fluid Phase Equilibria, 1992, 80 , 45–46) is proposed for estimation of the viscosity of mixtures; this modification eliminates the possibility of discontinuities at the critical point, ensures that the pure component viscosity is provided in the limit of a component mole fraction approaching 1, and improves the overall accuracy of the method. The method has been applied to 12 pure refrigerants including three hydrocarbons and mixtures. The average absolute deviations between the calculated and experimental viscosity values are within 4% for all of the pure fluids and most of the mixtures investigated.

Experimental study on forced convective boiling heat transfer of pure refrigerants and refrigerant mixtures in a horizontal tube

Abstract: Convective boiling heat transfer coefficients of pure refrigerants (R22, R32, R134A, R290, and R600a) and refrigerant mixtures (R32R134a, R290R600a, and R32R125) are measured experimentally and compared with Gungor and Winterton correlation. The test section is made of a seamless stainless steel tube with an inner diameter of 7.7 mm and is uniformly heated by applying electric current directly to the tube. The exit temperature of the test section was kept at 12°C ± 0.5°C for all refrigerants in this study. Heat fluxes are varied from 10 to 30 kW m−2 and mass fluxes are set to the discrete values in the range of 424–742 kg m−2 s−1 for R22, R32, R134a, R32R134a, and R32R125; 265–583 kg m−2 s−1 for R290, R600a, and R290R600a. Heat transfer coefficients depend strongly on heat flux at a low quality region and become independent as quality increases. The Gungor and Winterton correlation for pure substances and the Thome-Shakil modification of this correlation for refrigerant mixtures overpredicts the heat transfer coefficients measured in this study.

Enhancement of pool boiling

Abstract: This review considers the many techniques that have been developed to enhance boiling heat transfer. After introducing the techniques, the applications to pool boiling are described. Comments are offered regarding commercial introduction of this technology and the generations of heat transfer technology; advanced enhancement represents 3rd generation heat transfer technology.

Comparative simulation and investigation of ammonia-water: absorption cycles for heat pump applications

Abstract: Several recent programs in absorption research have focused on technology for domestic heating and cooling utilizing natural gas. In residential and small commercial size applications, ammonia-water cycles offer the possibility of a gas-fired heat pump for both winter heating and summer cooling, at better year-round COPS than currently available by various alternatives. Several cycles have been considered for this purpose, ranging from the simplest single effect to the GAX (Generator-Absorber heat eXchange) with its different variations. Detailed calculations of ammonia-water systems are rather difficult, particularly in complex cycles such as the GAX. This may be the reason that relatively few simulation studies have been published to date that give more than design point performance. The objective of the present study has been a comprehensive investigation of various ammonia-water cycles, with operating conditions and different design parameters varied over a wide range to compare their performance. To this end, a modular simulation program (ABSIM) was employed, which makes it possible to simulate absorption cycles in varying configurations. The cycles investigated increase in complexity from the basic single-effect cycle, through the same cycle with an added precooler, through an added solution-cooled absorber and solution-heated generator, to the GAX and branched GAX cycles with different types of branching. Each cycle was formed on the basis of the previous one by adding one or two components at each stage, resulting in increasing complexity rewarded by improved performance. This kind of investigation enabled determination at each stage of the influence of the added components on the cycle. The results of the investigation show cooling CON ranging from about 0.5 for the simplest single-effect cycle to about 1.0 for the GAX cycle.

Thermal stability of R-134a, R-141b, R-13I1, R-7146, R-125 associated with stainless steel as a containing material

Abstract: An experimental apparatus for assessing the thermal stability threshold of refrigerant working fluids is described and results for R-134a (1,1,1,2-tetrafluoroethane), R141b (1,1-dichloro-1-fluoroethane), R-13I1 (trifluoromethyl iodide), R-7146 (sulphur hexafluoride), R-125 (pentafluoroethane) are presented. The information is a concern for the design of refrigeration systems, high temperature heat pumps and Organic Rankine Cycles (ORC), for which the above refrigerants are proposed. The method aims to identify a maximum temperature for plant operation in contact with stainless steel and involves the evaluation of four indicators: (1) pressure variation while the fluid is maintained at set temperature; (2) saturation pressure comparison after heat treatment; (3) chemical analysis; and (4) vessel visual inspection after the test session. The highest temperatures at which no evident degradation occured are: 368°C for R-134a; 102°C for R-13I1; 90°C for R-141b; 204°C for R-7146; and 396°C for R-125.

Working fluids for an absorption system based on R124 (2-chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents

Abstract: The possibility of using R124 (2-chloro-1,1,1,2,-tetrafluoroethane, CHClFCF3) and organic absorbents as working fluids in absorption heat pumps was investigated. Various classes of organic compounds, all commercially available, were tested as absorbents for possible combination with R124; the absorbents included DMAC (N′, N′-dimethylacetamide, C4H9NO), NMP (N-methyl-2-pyrrolidone, C5H9NO), MCL (N-methyl ϵ-caprolactam, C7H13NO), DMEU (dimethylethylene urea, C5H10N2O), and DMETEG (dimethylether tetraethyleneglycol, C10H22O5). To evaluate the performance of a candidate refrigerant-absorbent pair in a refrigeration or heat pump cycle, the thermophysical properties of the pure components and the mixture and the equilibrium and transport properties have to be determined, either from experimental data or by prediction methods. The thermal stability of the refrigerant-absorbent must also be tested. A method for the calculation of the concentration in the liquid and gas phases and the excess thermodynamic properties of the mixture as a function of the system temperature and pressure based on our experimental setup is described. On the basis of vapor-liquid equilibrium measurements, density and viscosity measurements and thermostability testing, enthalpy-concentration diagrams were constructed. The performance characteristics of the investigated working fluids in terms of the coefficient of performance (COP) and the circulation ratio (f) were calculated for a single-stage absorption cycle. In terms of overall performance (COP, f and stability) R124-DMAC was found to be the superior combination, followed by R124-NMP, R124-DMEU and R124-MCL (the three pairs for which stability problems were found at high temperatures), and finally by R124-DMETEG.

Freezing processes in high-pressure domains

Abstract: Experimental freezing of water in high-pressure domain is studied considering temperature reduction (TRF) as well as high-pressure-assisted freezing (HPAF). The most important advantage of HPAF is that the whole volume of the sample is subcooled when an expansion is made, so a rapid and uniform nucleation and growth of ice crystals are produced. In this work through mathematical modelling the amount of ice appearing instantaneously in the latter freezing, is predicted.

Pool boiling heat transfer from horizontal tubes with different surface roughness

Abstract: The influence of the roughness of a heating surface on the heat transfer coefficient has been investigated by experiments with propane boiling on single horizontal copper and steel tubes with different surface roughnesses (emery ground, fine or rough sandblasted). The results can be correlated in a preliminary manner by no longer separating the influence of surface roughness from the other parameters in an empirical prediction method. A detailed description of the three surfaces on the steel tube examined is given, with surface profiles, quasi three-dimensional topography presentation, SEM photographs, and with a statistical analysis of a selected roughness parameter. The size distributions of active nucleation sites have been determined using the model assumptions of Schomann of the heat transfer near growing and departing bubbles. The calculated results have been compared to experimental investigations by high speed video techniques. New measurements of the departure diameter dA and frequency f of the bubbles at the same conditions as for the heat transfer measurements show that dA and f are influenced by the azimuthal position of the active nucleation sites on the tube, and that the calculated results react sensitively on the input data for dA and f.

Fundamental thermodynamics of ideal absorption cycles

Abstract: Starting from the representation of a real absorption refrigeration cycle on a temperature-entropy diagram, step-by-step idealisations of the binary mixture, together with the thermodynamic transformations are considered, in order to derive the ideal thermodynamic absorption cycle performance and temperature formulae It is demonstrated that the ideal absorption cycle is the combination of a Carnot driving cycle with a reverse Carnot cooling cycle The resorption cycle is analysed in the same manner Information is included on absorption cooling with heat recovery cycles, heat pumps and temperature amplifiers From the analysis of single-stage cycles, and by superimposing absorption cycles operating at different temperatures and utilising specific residual heat of the higher temperature sub-cycles, the performance and temperature relations of double, triple and multistage cycles are derived Special attention is given to three types of triple-stage cycle and their ideal equivalence is demonstrated and represted on the pressure-temperature-concentration (PTX) diagram A simple hybrid absorption-compression cycle is analysed and the results are compared with those of ideal cold generation cycles (combinations of driving and cooling cycles) Consideration is also given to cold generation systems Finally, the validation of the fundamental thermodynamics of absorption cycles is presented by applying an exergy analysis This paper presents the thermodynamic principles involved to obtain simple formulae, in a similar way to the Carnot cycle in order to convey the ideal theoretical limitations

Refrigerant conversion of auto-refrigerating cascade (ARC) systems

Abstract: This paper describes converting Auto-Refrigerating Cascade (ARC) ultra-low temperature systems from a multicomponent zeotropic working fluid containing CFS to a CFC-free mixture. The differences between azeotropes, mixtures and blends with minimal glides used in conventional refrigeration systems and the steep glide (wide span) zeotropes required for ARC system operation are compared. The design tools used include the Carnahan-Starling-DeSantis and the Peng-Robinson equations of state and relationships of components based upon Raoult's law. Some comparisons of prep and post-conversion operating data are presented. A brief history of ARC cycle cooling systemsis included.

Optimizing chiller operation based on finite-time thermodynamics: universal modeling and experimental confirmation

Abstract: The efficiency of chillers (refrigeration and heat pump devices) is limited by the dissipation from their principal components: compressor, throttler, and heat exchangers at the condenser and evaporator. Developing a generalized finite-time thermodynamics model for reciprocating chhillers, we derive analytic formulae for how the fixed finite resources of cycle time and heat exchanger inventory should be allocated so as to optimize chiller performance. Our predictions for optimal operating schemes are compared with detailed experimental data from two different commercial chillers. The agreement between theory and actual performance data attests to the empirical wisdom that has evolved in chiller manufacture. Besides quantitatively documenting the individual sources of irreversibility, we show how the limitations of currently-available chiller components affect optimal chiller design, as well as how potentials steps to improve chiller efficiency can be evaluated within a universal thermodynamic framework.

Critical heat flux of binary mixtures in pool boiling and its correlation in terms of Marangoni number

Abstract: Critical heat flux (CHF) in nucleate pool boiling of binary mixtures was newly measured with a horizontal platinum wire, 0.5 mm in diameter, and heated by DC, over the full range of concentrations. Seven mixtures were selected with the intent to cover various types of mixtures: methanol/water, ethanol/water, methanol/ethanol, ethanol/n-butanol, methanol/benzene, benzene/n-heptane and water/ethylene glycol, each in the saturated state at atmospheric pressure. Total 311 raw CHF data were obtained at 75 concentrations including pure components. Aqueous mixtures of methanol and ethanol revealed significant increase of CHF compared to either CHF linearly interpolated between pure components or CHF predicted from a single component correlation with use of the mixture properties. Three organic mixtures showed more or less the same level as an interpolated CHF, while the remaining two mixtures of methanol/benzene and water/ethylene glycol gave the reduced CHF by 20% and 50% at most, respectively. Marangoni number was introduced as a controlling variable to explain the observed increased, invariable, or reduced CHF, and an empirical correlation was developed.

CYRANO: a computational model for the detailed design of plate-fin-and-tube heat exchangers using pure and mixed refrigerants

Abstract: A computational model for the detailed design of finned coils has been developed. This programme discretises heat exchangers into tube elements for which the governing equations are solved using local values of temperature, pressure, physical properties and heat transfer coefficients. Single-phase, condenser and evaporator cases can be automatically treated using water, R22, R134a, and refrigerant mixtures based on R32, R125, and R134a. The software can handle non-conventional coil circuits with different numbers of inlets and outlets, non-uniform air distribution at the coil inlet face, using smooth, wavy and louvered fins, and smooth and internally finned tubes. The programme has been validated on seven finned coils using pure fluids, with and without moisture condensation on the fins. Comparisons with tests show errors of less than 5% on the coil duty and of the order of 30% on the refrigerant pressure drop. A performance simulation of a coil using R22 and a ternary mixture is presented to validate the programme algorithms developed for mixtures.

Predicting the dynamic product heat load and weight loss during beef chilling using a multi-region finite difference approach

Abstract: A model for predicting the dynamic product heat load and weight loss during beef chilling has been developed using the finite difference method, in which the irregular beef geometry is approximated by a combination of seven cylinders and slabs. The model was tested against data from 55 industrial beef chilling trials covering a wide range of carcass size, fatness and chilling conditions. The predicted heat removed during the first 2 h in the model was on average 12.6% higher than the experimental value. The predicted weight loss after 20 h was on average 0.0194 kg (0.00046%) higher than the experimental value. Improvements in accuracy would probably have arisen if slaughter floor and chiller conditions had been more accurately known. The model allows for time-variable chiller conditions, making it widely applicable to meat industrv applications.

Study of different internal vapour transports for adsorption cycles with heat regeneration

Abstract: A ‘three-temperature’ model of the adsorption cycles with heat regeneration is used for investigating and analysing the influence of different parameters on the performance of such cycles. The influence of the heat source temperature on the thermodynamic efficiency (COP/COPCarnot) is investigated. The result is that the thermodynamic efficiency of the cycle is always limited. In order to reduce the internal irreversibilities, different internal vapour transports for pressurising (depressurisation) the adsorber are investigated: first, adiabatic direct pressurisation (depressurisation) with the condenser (evaporator) instead of pressure changes by heat transfer; second, adiabatic internal vapour recovery between the adsorbers (partial pressurisation/depressurisation); third, separation of the adsorber into separate compartments between which vapour cannot be redistributed during pressurisation or depressurisation. Results show that the first process significantly reduces the COP, while the second one enhances the cooling power, and the third one does not change the performance. Analysis gives satisfactory explanation of these results.

Radiation and natural convection heat transfer from wire-and-tube heat exchangers in refrigeration appliances

Abstract: The air-side heat transfer from wire-and-tube heat exchangers of the kind widely used in small refrigeration appliances has been studied. Radiation and free-convection components have been separately investigated. The radiation component was theoretically computed using a diffuse, gray-body network with interactions between each part of the heat exchanger and the surroundings. For the free-convection heat transfer component, a semiempirical correlation was developed on the basis of experimental tests conducted on a set of 42 low-emittance exchangers with various geometrical characteristics. Comparisons between overall heat transfer predictions and a second, independent set of experiments on eight high-emittance exchangers showed satisfactory agreement. The proposed analysis is suitable either to determine the heat transfer performance of an existing (already sized) exchanger or to design a new one for prescribed heat duty and working temperatures.

Replacement of R22 in tube-and-shell condensers: experiments and simulations

Abstract: An investigation of the change in condenser overall heat transfer coefficient when replacing R22 with one of the three mixtures R407C, R404A and R410B was made, both experimentally and theoretically. Measurements have been carried out on a full-scale test plant consisting of a horizontal shell-side condenser. According to the measurements the decrease in overall heat transfer coefficient for the non-azeotropic mixture R407C was very large, up to 70% compared to R22, while for the near-azeotropic mixture R404A the decrease was less than 15%. Simulations of the condenser were done with a comprehensive computer program, calculating the condensation heat transfer with an approximate method including a correction for mass resistance. The calculation model was not able to predict this large degradation for the non-azeotropic mixture, while the predictions agreed rather well with the measurements for the pure fluid and the near-azeotropic mixtures.

The surface tension of HFC refrigerants and mixtures

Abstract: The surface tension of the refrigerants R32, R125, R134a, R143a and R152a, as well as the binary refrigerant mixtures R32-R125, R32-R134a, R125-R134a, R125-R143a, R125- R152a, R143a-R134a and R134a-R152a, and the commercially available ternary mixtures R404A and R407C was measured across the temperature range from −50 to 60°C using a measuring unit based on the capillary rise method. Different formulations for calculation of the surface tension of the binary and ternary mixtures on the basis of the surface tension of the pure refrigerants were tested. With an approach based on mass proportions in the mixture, a good correspondence between the measured and calculated values was achieved.

Physical properties of the lithium bromide + 1,3-propanediol + water system

Abstract: The experimental measurements of the basic four physical properties (solubility, vapor pressure, density and viscosity) of the lithium bromide + 1,3-propanediol + water system (LiBr/HO(CH2)3OH mass ratio = 3.5), a possible new working fluid for absorption heat pump, were carried out. Solubility was measured by the visual polythermal method in the temperature range from 264.65 to 358-95 K, and vapor pressure by the boiling point method from 330.75 to 408.45 K. Densities and viscosities were also measured by using a set of hydrometers and Ubbelohde-type capillary viscometers in the temperature range from 283.15 to 343.15 K. Each measured data set was correlated with a proper equation, and all the correlation results showed good agreement between measured and calculated values. Using the correlation results the Duhring chart was constructed and thus the system was found to be able to have a high absorber temperature, which is essential for the design of air-cooled absorption chiller.

A review of pulse tube refrigeration

Abstract: The pulse tube belongs to the class of miniature cryogenic refrigerators usually referred to as cryocoolers. In common with Stirling and Gifford-McMahon machines, operation depends on a regenerative gas expansion cycle but unlike these coolers the pulse tube has no moving parts at low temperature and hence offers the potential for high reliability. Although comparisons may be drawn between the operation of a pulse tube and that of a Stirling cooler, the exact nature of the working cycle is far from clear and the device continues to intrigue. We provide here an introduction to the device and attempt to explain it's operation as a conventional second law system. Resume Le tube a pulsion appartient a la categorie des refrigerateurs cryogeniques miniatures communement appeles cryorefroidisseurs. Comme pour les machines Stirling et Gifford-McMahon, leur fonctionnement depend d'un cycle regenerateur a detente gazeuse; mais a la difference de ces machines, le tube a pulsion ne possede pas de partie mobile a basse temperature ce qui lui permet une grande fiabilite. Bien que l'on puisse comparer le fonctionnement d'un tube a pulsion et celui d'un refroidisseur Stirling, la nature exacte de son cycle frigorifique est loin d'etre elucidee et cet appareil suscite encore des interrogations. Nous presentons ici le dispositif et tentons d'en expliquer le fonctionnement d'une facon traditionnelle fondee sur le deuxieme principe.

A heat exchanger model for mixtures and pure refrigerant cycle simulations

Abstract: This paper describes a heat exchanger simulation developed for transient and steady state cycle simulations of mixtures and pure components. The simulation focuses on air to refrigerant condensers and evaporators found in residential heat pumps. The refrigerant differential momentum, continuity, species and energy equations are solved for these components and the steady state results are verified experimentally. Ten different heat transfer correlations for condensation and evaporation are evaluated to determine which best reproduces experimental data. Of those tested Jung and Radermacher's (1989, Int. J. Heat Mass Transfer 32 2435–2446) heat transfer correlation worked the best for evaporation while Dobson et al.'s (1994, ACRC Project 37) correlation worked the best for condensation. The experimentally determined capacity of four cross flow heat exchangers operating as condensers and evaporators with four different refrigerants is compared to the simulation results. The capacities predicted by the simulation agreed with the experimental results within ±8.0%. Furthermore, the simulation is used to quantify the effects of using a zeotropic mixture, R-407C, with cross, parallel and counter flow heat exchangers. As compared to a typical cross flow heat exchanger at typical heat pump operating conditions, the simulation predits that a pure parallel flow heat exchanger can decrease capacity by as much as 8.3% while a pure counter flow heat exchanger can increase performance by up to 4.4%.

Transition from nucleate boiling to convective boiling in compact heat exchangers

Abstract: The boiling of pure fluids has been experimentally studied in several types of compact heat exchanger channels. Plate fin and corrugated heat exchangers have been studied (seven geometries). Controlling the flow parameters (mass flux and vapour quality), the heat flux and measuring the wall temperature, have allowed characterization of the local heat transfer coefficient. The results clearly show that the dominant mechanisms occurring could be nucleate or convective boiling. The transition between these two mechanisms depends on the flow characteristics and also on the channel geometry. Based on these measurements, an objective criteria can be established to identify the flow boiling regime. The knowledge of such a criteria is useful if we want to extend the use of compact heat exchanger to boiling of mixtures.

Influence of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134A

Abstract: The effect of ultrasound on pool boiling heat transfer to mixtures of the refrigerants R23 and R134a has been investigated in a wide range of heat flux and saturation pressure. The enhancement of the heat transfer coefficient, which can be achieved by ultrasound, is much more pronounced for mixtures than for pure substances. It is, however, limited to rather small heat fluxes (q < 10 kW m−2). Especially remarkable is the fact, that the maximum influence of ultrasound on the heat transfer coefficient of the mixtures occurs at medium saturation pressures (ppc ≈ 0.2); the effect is markedly less for higher and for lower saturation pressures. Obviously, the improvement of the heat transfer to mixtures is mainly caused by a decrease of the local saturation temperature near the heating wall, due to a better mixing in the liquid boundary. This explanation is supported by evaluating important parameters of bubble formation from high-speed photographs of the heating surface. It is further noticeable, that the well known hysteresis effect at the beginning of pool boiling is reduced to a great extent by exposure to ultrasound.

Pool boiling heat transfer at finned tubes : influence of surface roughness and shape of the fins

Abstract: Pool boiling heat transfer from finned tubes with different shapes of fins (trapezoid-shaped, T-shaped, or Y-shaped) to various hydrocarbons and partly fluorinated hydrocarbons has been investigated at the Laboratorium fur Warme- und Kaltetechnik, Universitat-GH Paderborn during the recent past. Compared to corresponding measurements on plain tubes, heat transfer on traditionally finned tubes with trapezoid-shaped fins is considerably improved, and even better results are achieved with T-shaped or Y-shaped fins. The influences of the macrostructure (i.e. fin geometry) or microstructure (i.e. surface roughness) on the heat transfer coefficient have been studied separately, in order to evaluate the improvement of heat transfer by either influence.

Potassium formate as a secondary refrigerant

Abstract: The properties of aqueous fluid of potassium formate were studied theoretically and experimentally in order to find an improved secondary refrigerant for indirect refrigeration systems. The most important advantages compared to the traditional aqueous solutions of alcohols and glycoles are good thermodynamic properties, low toxicity and non-flammability. A remarkable benefit is a reduced change of a laminar flow. Volumetric heat is lower, which brings about a higher rate of mass flow, if the temperature change of the fluid is maintained constant. However, because of the clearly lower values of viscosity, the pumping power demand is at same level. Good thermal conductivity promotes good heat transfer.

Sliding and sticking vapour bubbles under inclined plane and curved surfaces

Abstract: In flow boiling heat is transferred by the combined effects of nucleate boiling, with local generation of bubbles, and evaporative and convective cooling by the passage of bubbles generated elsewhere. In this study, nucleate boiling was eliminated by measuring the heat transfer near injected steam bubbles sliding under an inclined plate heated to low superheats, using liquid crystal thermography combined with high speed video recording and computerised image analysis. Heat was transferred by evaporation of the thin liquid film between the bubble and the wall and by enhanced convection in a wake region wider than the bubble and many bubble diameters long. Evaporation was the dominant mechanism for large, easily deformed, slow-moving bubbles. For small, faster-moving bubbles the reduction in evaporation was offset by an improvement in convection.