Search

Some new, low-global warming potential refrigerants will be flammable, and the laminar burning velocity is a useful parameter for quantifying fire risk. Laminar burning velocity measurements have been made using a constant volume experiment with dry air and the refrigerant R-152a (CH3CHF2), pure and blended with R-134a (CH2FCF3), or R-1234yf (CF3CFCH2). The resulting burning velocity data deduced from the pressure rise in the chamber are presented for a range of fuel air equivalence ratio and loading of the less flammable refrigerant, for unburned gases at 298 K and 101 kPa as well as at 375 K and 253 kPa. For comparison, the 1-D, planar laminar burning velocity was numerically simulated using a recently developed kinetic mechanism that includes a wide range of refrigerants with air. The predicted burning velocities agree reasonably well with the experimental values, and the numerical results are used to understand the kinetic mechanism of the reaction of the refrigerants. Uncertainties in the experimental data from radiation heat losses as well as extrapolation to ambient conditions are explored. [ABSTRACT FROM AUTHOR]

In present study the effect of nano particle mixing in base refrigerant of R-152 is evaluated. Now a days two biggest environmental issues confronting the world are global warming and ozone depletion. The increasing national and international concern about global warming, as well as the importance placed on the study of this research work has been prompted by energy conservation. Hydro floro carbon is a kind of greenhouse gas that is used in air conditioning and refrigeration and it majorly contribute in global warming issue which is why it is needed to be strictly ban. In this study, we use nano particles mixed with the base refrigerant. Since we know that performance of VCRS is highly depends upon the type of refrigerant used. It is highly recommended to use a refrigerant with good thermo physical properties as they enhance the transfer of heat & increase performance for the system. Nano additives enhance the cross-sectional area of heat transfer which results in better output of the refrigeration system. Different types of metal oxide nano particles are used to compare the performance by various researchers along with carbon nano tubes. Finer the diameter of the particles less is the problem of clogging and chocking arises. Experimentation work is carried out by using different concentration of nano particles to analyze their impact on the system. The efficiency and exergy analysis of alternative refrigerants (hydrocarbon and nano refrigerants) were also investigated. Evaporating temperature, condensing temperature, and refrigerant mass along with varying nano particles concentration are the factors investigated in the refrigeration method. A Computerized refrigeration cycle test rig has been used for these experiments. Trial runs are used to determine the operating ranges of all the chosen parameters. [ABSTRACT FROM AUTHOR]

Multiwalled carbon nanotubes (MWCNTs) exhibit outstanding physical properties, including high thermal conductivity, excellent mechanical strength, and low electrical resistivity, which make them suitable candidates for a variety of applications. The work presented in this paper focuses on the pool boiling performance of refrigerant R-134a on microporous Cu-MWCNT composite surface layers. A two-stage electrodeposition technique was used to fabricate Cu-MWCNT composite coatings. In order to achieve variation in the surface properties of the Cu-MWCNT composite surface layer, electrodeposition was carried out at various bath temperatures (25 °C, 30 °C, 35 °C, and 40 °C). All surfaces coated with the Cu-MWCNT composite demonstrated superior boiling performance compared to the uncoated surface. Heat transfer coefficient (HTC) values for Cu-MWCNT composite surface layers, prepared at bath temperatures of 25 °C, 30 °C, 35 °C, and 40 °C, exhibited improvements of up to 1.75, 1.88, 2.06, and 2.22, respectively, in comparison to the plain Cu surface. [ABSTRACT FROM AUTHOR]

Heat pumps are frequently used for heating, cooling, and air conditioning. It is well known that nanoparticles can improve the coefficients of conduction and convection, increasing heat transfer along with other properties. The considered heat pump was loaded with R-134a. Titanium dioxide (TiO2) and aluminium oxide (Al2O3) were blended with clean water to create a nanoscale solution used to cool the heat pump condensers. A total of three TiO2 and Al2O3 proportions (0.1%, 0.2%, and 0.3%) were used. The study's findings showed that utilizing 0.3% Al2O3 instead of conventional clean water to cool the heat pump condenser boosted the coefficient of performance by 18% while reducing energy consumption by 26%. [ABSTRACT FROM AUTHOR]

The nucleated pool boiling heat transfer experiments for plain and five different reentrant cavity copper tube surfaces in saturated R134a, R32, and R600a have been carried out. The experiments are carried for heat fluxes ranging from 6.92 to 51.71 kW/m2 at saturation temperatures of 7°C and 10°C. A significant increase in the boiling heat transfer coefficient has been observed for all tube surfaces with the reentrant cavity. The nucleate boiling heat transfer coefficient for R134a, R32, and R600a is 1.2–2.8 times more than that for plain tube surfaces. A correlation for reentrant cavity surfaces has also been developed. [ABSTRACT FROM AUTHOR]

Due to ever growing 'demand of the energy' and for the capability to remove large quantities of heat from the heating surface, pool boiling heat transfer becomes the main attraction for research in search of finding the efficient means for transferring heat. Following this trend, the present study also investigated the pool boiling performance of refrigerants, mainly, pure refrigerant (R-134a) and natural refrigerant (R-600a), on the plain aluminium and two graphene nanoplatelets (GNPs) reinforced aluminium matrix composite (Al@GNPs) coated aluminium heating surface with different thickness (50 ± 5 µm and 125 ± 6 µm), which were synthesised by the help of a three-step technique involving mechanical milling, screen printing and sintering process on the plane aluminium surface. The test is conducted at a saturation temperature of 10°C and heat fluxes ranging from 8.28 kW/m2 to 75.61 kW/m2. Test results of R-134a and R-600a were compared. It is observed that thicker Al@GNPs composite coated surface showed better HTC but with reduced 'onset of nucleate boiling' (ONB) wall superheat than the plain Al surface. [ABSTRACT FROM AUTHOR]

The present work explores the pool-boiling performance of refrigerants (R-134 and R-1234yf) on the plain Cu and graphene nanoplatelets (G) reinforced Cu matrix (Cu@G) composite coated heating surface. A two-step electrodeposition technique was employed to prepare microporous Cu@G composite coatings. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) studies confirmed the successful fabrication of microporous structure of Cu@G composite coatings. Surface profilometer investigation was done to know the surface roughness of prepared Cu@G composite coatings. Pool boiling experiments were carried out with increasing heat flux from 8.80 kW/m2 to 61.25 kW/m2 at a saturation temperature of 10°C. Test results of R-134 and R-1234yf were compared. The experimental results revealed that the heat transfer coefficients (HTCs) of R-134a were higher than R-1234yf for plain Cu and Cu@G composite coated heating surfaces. [ABSTRACT FROM AUTHOR]

The detection of gaseous refrigerants, necessary for industrial control and environmental monitoring, is explored using rotational spectroscopy in the THz wave region. Rotational absorption spectra for two widely used commercial hydrofluorocarbon (HFC) refrigerants, 1,1,1,2-tetrafluoroethane (R-134a or HFC-134a) and 2,3,3,3-tetrafluoropropene (R-1234yf or HFO-1234yf), are characterized in the 220–330 GHz frequency range at room temperature (297 K) and modest pressures (0.25–8 Torr) using a compact microelectronics-based THz wave spectrometer. The absorption spectra illustrate complex, broad, and repeating structures that result from the blending of hundreds of pressure-broadened rotational lines. The unique and distinct spectra for R-134a illustrate potential for its detection in the present frequency region with estimated detection limits approaching 15 ppm per meter path length in 1 atm of air. The spectra for R-1234yf are less distinct than those for R-134a and illustrate complex structured rotational absorption features in combination with an underlying quasi-continuous absorption that increases in strength to higher frequencies. To our knowledge, this is the first experimental characterization for the spectra of R-134a and R-1234yf in this frequency region. [ABSTRACT FROM AUTHOR]

Pool boiling has been widely employed in electronic, power production and refrigeration systems due to its high efficiency in heat transfer. However, the investigation and application of microporous Al matrix composite coatings to enhance the pool boiling heat transfer (BHT) are very limited. In this study, graphene nanoplatelets reinforced Al matrix (Al@GNPs) composite coatings are fabricated by combining mechanical milling, screen printing and sintering techniques to investigate the pool boiling heat transfer using R-134 as working fluid. The experimental data were obtained at a saturation temperature of 10 °C for heat fluxes ranging from 9.04 kW·m−2 to 73.57 kW·m−2. The effect of various coating thicknesses on boiling characteristics and heat transfer coefficient (HTC) of R-134a were studied and presented in detail. Our results demonstrate that the HTC obtained for Al@GNPs-4 composite coated heating surface is 143% higher than the plain Al heating surface. Enhanced nucleation sites and increased bubble pumping action are the main reasons for the augmented BHT performance on the Al@GNPs composite coated heating surfaces. [ABSTRACT FROM AUTHOR]

Two-phase closed thermosiphon system for cooling high heat flux electronic devices was constructed and tested on a lab scale. The performance of the thermosyphon system was investigated using R-134a as a working fluid. The effect of heat flux and the refrigerant pressure on the evaporator side heat transfer coefficient were investigated. It was found that the heat transfer coefficient increases by increasing the heat flux on the evaporator or by reducing the inside pressure. The effect of heat transfer mode of the condenser (natural or forced) also affected the overall heat transfer coefficient in the cycle. At the 200W heating load, the values of the heat transfer coefficients were 32 and 1.5 kW/m². °C, for natural and forced convection modes, respectively. The temperature difference between the evaporator and the refrigerant saturation pressure was found to be dependent on heat flux and the pressure inside the system. At 40W heating load, the heat transfer coefficient was calculated to be 500, 3000 and 7300 W/oC.m2 at 0.152, .135 and 0.117 reduced pressure, respectively. It can be concluded that such a thermosyphon system can be used to cool high heat flux devices. This can be done using an environmentally friendly refrigerant and without any need for power to force the convection at the condenser. [ABSTRACT FROM AUTHOR]

In this study, heat transfer coefficients and pressure drops of R-134a inside round and flat tubes are investigated experimentally with mass flux of 450, 550, and 650 kg m−2 s−1 at saturation temperatures of 35°, 40°, and 45°C. The effects of mass flux and saturation temperature on heat transfer coefficient and pressure drop are examined. The maximum enhancement factor and pressure drop penalty are obtained by flat tube (FT-2) up to 2.101 at 450 kg m−2 s−1 and 3.01 at 650 kg m−2 s−1, respectively. The correlation for flat tubes is proposed to predict the heat transfer coefficient within ±20% error. [ABSTRACT FROM AUTHOR]

This work presents the experimental evaluation of energy consumption and refrigerant charge reduction when a commercial direct expansion refrigeration system is converted into an indirect system. The evaluation (with R-134a and R-507A) used a commercial cabinet with doors for medium temperature and a single-stage refrigeration cycle using a semi-hermetic compressor and electronic expansion valve; 24-h energy consumption tests were performed at laboratory conditions for each refrigerant and configuration at three heat rejection levels (23.3, 32.8 and 43.6 °C), maintaining an average product temperature inside the cabinet of 2 °C. The work analyses the impact of the conversion on temperature and pressure indicators, as well as, in the energy performance of each element. For R-134a the refrigerant charge was reduced in a 42.9%, but the energy consumption rose by 22.0%-22.8%; for R-507A the charge reduction was of 32.8% with an increase in energy consumption of between 27.7% and 38.7%. [ABSTRACT FROM AUTHOR]

The objective of this study is to design and briefly investigate the cooling performances of an air conditioning system for a special purpose vehicle under various experimental conditions. An air conditioning system with two parallel refrigeration cycles consisting of two compressors and two condensers for satisfying the required cooling performance of the special purpose vehicle was tested under extremely hot weather conditions and high thermal load conditions and then optimized by varying the refrigerant charge amount. The optimum refrigerant charge amount of the tested air conditioning system was 1200 g with the consideration of the cooling speed and cooling capacity. The indoor temperatures of the suggested air conditioning system at the refrigerant charge amounts of 1200 g, 1400 g, and 1600 g were 24.7 °C, 25.2 °C, and 26.4 °C, respectively, at the elapsed time of 300 s. The cooling time required to reach a 15.0 °C inner temperature in the suggested air conditioning system increased by 13.3% with the decrease of the refrigerant charge amount from 1600 g to 1200 g. The cooling capacity and the coefficient of performance (COP) of the suggested air conditioning system increased by 37.9% and 10.9%, respectively, due to a decrease of the refrigerant charge amount from 1600 g to 1200 g. The observed cooling performance characteristics of the air conditioning system with two parallel refrigeration cycles means it could be suitable for cabin cooling of special purpose vehicles. In addition, the designed special air conditioning system with two parallel refrigeration cycles for a special purpose vehicle was built to ensure a sufficient cooling performance for equipped passengers. [ABSTRACT FROM AUTHOR]

The effects of operating conditions on the air-side heat transfer, and pressure drop of a micro-channel heat exchanger under wet surface conditions were studied experimentally. The test section was an aluminum micro-channel heat exchanger, consisting of a multi-louvered fin and multi-port mini-channels. Experiments were conducted to study the effects of inlet relative humidity, air frontal velocity, air inlet temperature, and refrigerant temperature on air-side performance. The experimental data were analyzed using the mean enthalpy difference method. The test run was performed at relative air humidity's ranging between 45% and 80%; air inlet temperature ranges of 27, 30, and 33 0C, refrigerant-saturated temperatures ranging from 18 to 22 0C, and Reynolds numbers between 128 and 166. The results show that the inlet relative humidity, air inlet temperature, and the refrigerant temperature had significant effects on heat transfer performance and air-side pressure drop. The heat transfer coefficient and pressure drop for the micro-channel heat exchanger under wet surface conditions are proposed in terms of the Colburn factor and Fanning factor. [ABSTRACT FROM AUTHOR]

Enhanced tubes are widely used in shell and tube condensers of refrigeration, air-conditioning and process industries because of their high heat transfer performance. In this study, condensation heat transfer tests were conducted for four three-dimensional enhanced tubes having different fin density and fin height using R-134a. The satuartion temperature was 40C. The heat transfer was significantly enhanced by the present enhanced geometry. At 5K wall subcooling, the enhancement ratio is 6.3 for 1654fpm, 4.6 for 1575fpm, 4.0 for 1496fpm and 3.3 for 1102fpm tubes. Within the geometric variation of the present study, the condensation heat transfer coefficient increased with the increase of fin density and of fin height. The heat transfer coefficients of the 1654fpm tube were approximately the same as those of the commercial three-dimensional enhanced tube Turbo-C. [ABSTRACT FROM AUTHOR]

In the literature, the modeling of saturated flow boiling heat transfer characteristics of R134a flowing through small channels is still questionable since it is based on very limited operational data. This study proposes a new evaporative heat transfer model based on R134a for micro- and macro-tubes. The proposed correlation is developed from 3594 data points, which are obtained from 19 different studies. Ranges of the database cover mass fluxes between 50.0 and 1500.0 kg/m s, heat fluxes between 3.0 and 150.0 kW/m, hydraulic diameter between 0.5 and 13.84 mm, saturation temperatures between −8.8 and $${52.4 ^{\circ} {\rm C}}$$ , and vapor qualities up to 1.0. Findings of the proposed method are compared with those of the most quoted flow boiling heat transfer correlations developed for micro- and macro-tubes. Results of the comparison indicate that new method, which has a mean absolute deviation of 19.1 % and captures 66.7 and 83.2 % of the experimental data within ±20 and ±30 % error bands correspondingly, outperforms the available flow boiling correlations in the literature in terms of prediction accuracy. [ABSTRACT FROM AUTHOR]

Refrigeration systems require optimal amount of refrigerant for maximum system performance. Undercharged or overcharged systems experience reduced efficiency and accessories deterioration. Optimal amount of refrigerant to be charged in a refrigeration system depends on the physical and thermal dynamic properties of the evaporator and the refrigerant. This paper presents formulation of a numerical model that can be used in determination of optimal amount of refrigerant charged in a system for maximum cooling rate as hence maximum system performance. Rayleigh's method of dimensional analysis was used obtain the relationship between the maximum cooling rates of direct expansion evaporators as a function of thermodynamic properties of refrigerant R-134a, Different sizes of evaporator were fitted in the refrigeration system and charged with systematically varying amount of refrigerant until a maximum cooling rate was determined. The variation of pressures and temperatures both at the inlet and exit of the evaporator were observed and analyzed. The cooling rate of the numerical model formulated was compared with the cooling rate of the actual physical refrigeration system. A t-test of 95% confidence interval indicated no significance difference between the numerical model, and the physical refrigeration system. [ABSTRACT FROM AUTHOR]

The accurate estimation of convection coefficients constitutes a crucial issue in designing and sizing any type of heat exchange device. The Wilson plot method and its subsequent modifications deliver a suitable procedure to estimate the convection coefficients from the post-processing of experimental data in a multitude of convective heat transfer processes. Uncertainty analysis is a powerful tool not only for handling the data and reporting coherent results of a certain experimental program, but also is a valuable tool in those stages devoted to the experimental design. This paper details the application of an analytical methodology for calculating the uncertainty associated with experimental data obtained by the Wilson plot method. Results based on a representative Wilson plot experiment to measure the condensation coefficients of R-134a over a horizontal 19 mm diameter smooth tube are shown. A parametric analysis was carried out sequentially to investigate the influence of the uncertainties in the measured variables and design parameters of the Wilson plot experiment in the results uncertainties. Although the example presented in this paper relates to a specific heat transfer process, the technique turns out to be rather general and can be extended to any heat transfer problem. [ABSTRACT FROM AUTHOR]

The article presents a measurement of the complete global phase behavior of the refrigerant gas 1,1,1,2-tetrafluoroethane (R-134a) and 1-n-alkyl-3-methyl-imidazolium ionic liquids. The inclusion of the anions hexafluorophosphate [PF6], tetrafluoroborate [BF4] and bis(trifluoromethylsulfonyl)imide [Tf2N] from approximately 0 to 105 degrees Celsius and to 33 megapascals (MPa). It is noted that all of the systems studied were classified Type V according to the scheme of Scott-van Konynenburg.

This article directly investigates the effect of a cooling medium's coolant temperature on the condensation of the refrigerant R-134a. The study presents an experimental investigation into condensation heat transfer, vapor quality, and pressure drop of R-134a flowing through a commercial annular helicoidal pipe under the severe climatic conditions of a Kuwait summer. The quality of the refrigerant is calculated using the temperature and pressure obtained from the experiment. Measurements were performed for refrigerant mass fluxes ranging from 50 to 650 kg/m2s, with a cooling water flow Reynolds number range of 950 to 15,000 at a fixed gas saturation temperature of 42°C and cooling wall temperatures of 5°C, 10°C, and 20°C. The data shows that with an increase of refrigerant mass flux, the overall condensation heat transfer coefficients of R-134a increased, and the pressure drops also increased. However, with the increase of mass flux of cooling water, the refrigerant-side heat transfer coefficients decreased. Using low mass flux in a helicoidal tube improves the heat transfer coefficient. Furthermore, selecting low wall temperature for the cooling medium gives a higher refrigerant-side heat transfer coefficient. [ABSTRACT FROM AUTHOR]

The condensation heat transfer coefficient and the two-phase pressure drop of refrigerant R-134a in a vertical plate heat exchanger were investigated experimentally. The area of the plate was divided into several segments along the vertical axis. For each of the segments, local values of the heat transfer coefficient and frictional pressure drop were calculated and presented as a function of the mean vapor quality in the segment. Owing to the thermocouples installed along the plate surface, it was possible to determine the temperature distribution and vapor quality profile inside the plate. The influences of the mass flux and the heat flux on the heat transfer coefficient and the pressure drop were also taken into account and a comparison with previously published experimental data and literature correlations was carried out. [ABSTRACT FROM AUTHOR]

Aluminum Multi Flow Channel Flat Tube (MFC) Condenser with micro-fins is the recent development in the field of automobile air conditioning (AC) systems. The condenser rejects heat to the atmosphere. Various correlations, based on physics of condensation process are available for determining pressure drop and heat transfer coefficients. Efforts are made to measure pressure drop and heat transfer coefficient by experimental as well as analytical methods. In order to study the behavior of MFC tube condenser, for an automobile AC unit, using R-134a as a working medium, and for developing heat transfer coefficient correlations for the same, a project work is undertaken. The available correlations to determine heat transfer coefficient for the MFC geometry are studied and Yang & Webb (1997) correlation is found to be the best fit for the job. Such correlations are being tested for the boundary conditions of interest and the results are discussed in this paper. [ABSTRACT FROM AUTHOR]

In this study, pool boiling test results are provided for the structured enhanced tubes having pores with connecting gaps. The surface geometry of the present tube is similar to that of Turbo-B. Three tubes with different pore size (0.20 mm, 0.23 mm and 0.27 mm) were manufactured and tested using R-11, R-123 and R-134a. The pore size which yields the maximum heat transfer coefficient varied depending on the refrigerant. For R-134a, the maximum heat transfer coefficient was obtained for the tube having 0.27 mm pore size. For R-11 and R-123, the optimum pore size was 0.23 mm. One novel feature of the present tubes is that their boiling curves do not show a ‘cross-over’ characteristic, which existing pored tubes do. The connecting gaps of the present tube are believed to serve an additional route for the liquid supply and delay the dry-out of the tunnel. The present tubes yield the heat transfer coefficients approximately equal to those of the existing pored enhanced tubes. At the heat flux 40 kW/m2 and saturation temperature 4.4° C, the heat transfer coefficients of the present tubes are 6.5 times larger for R-11, 6.0 times larger for R-123 and 5.0 times larger for R-134a than that of the smooth tube [ABSTRACT FROM AUTHOR]

A mixture model explicit in Helmholtz energy has been developed that is capable of predicting thermodynamic properties of refrigerant mixtures containing R-32, R-125, R-134a, and R-152a. The Helmholtz energy of the mixture is the sum of the ideal gas contribution, the compressibility (or real gas) contribution, and the contribution from mixing. The contribution from mixing is given by a single equation that is applied to all mixtures used in this work. The independent variables are the density, temperature, and composition. The model may be used to calculate thermodynamic properties of mixtures, including dew and bubble point properties and critical points, generally within the experimental uncertainties of the available measured properties. It incorporates the most accurate published equation of state for each pure fluid. The estimated uncertainties of calculated properties are ±0.25% in density, ±0.5% in the speed of sound, and ±1% in heat capacities. Calculated bubble point pressures are generally accurate to within ±1%. [ABSTRACT FROM AUTHOR]

In this report, we have examined the behavior of heat capacities and Joule–Thomson coefficients in low- and moderate-density regions based on recent theoretical studies of the ideal-gas heat capacity and virial coefficients of R-32, R-125, R-134a, R-143a, and R-152a. The results have been compared with those derived from empirical equations of state which have been recently developed, based on a large quantity of experimental data for these refrigerants. Both results are in good agreement. Proper behaviors for these second-derivative properties justify the use of the empirical equations of state in low-temperature and low-density regions where no experimental data are available. [ABSTRACT FROM AUTHOR]

An 18-coefficient modified Benedict–Webb–Rubin equation of state has been developed for R-404A, a ternary mixture of 44% by mass of pentafluoroethane (R-125), 52% by mass of 1,1,1-trifluoroethane (R-143a), and 4% by mass of 1,1,1,2-tetrafluoroethane (R-134a). Correlations of bubble point pressures, dew point pressures, saturated liquid densities, and saturated vapor densities are also presented. This equation of state has been developed based on the reported experimental data of PVT properties, saturation properties, and isochoric heat capacities by using least-squares fitting. These correlations are valid in the temperature range from 250 K to the critical temperature. This equation of state is valid at pressures up to 19 MPa, densities to 1300 kg·m−3, and temperatures from 250 to 400 K. The thermodynamic properties except for the saturation pressures are calculated from this equation of state. [ABSTRACT FROM AUTHOR]