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[en] In this study, thermodynamic performance of R170/R290 mixture is measured on a heat pump bench tester in an attempt to substitute R22. The bench tester is equipped with a commercial hermetic rotary compressor providing a nominal capacity of 3.5 kW. All tests are conducted under the summer cooling and winter heating conditions of 7/45 °C and -7/41 °C in the evaporator and condenser, respectively. During the tests, the composition in R170/R290 mixture is varied from 0% to 10% with an interval of 2%. Test results show that the coefficient of performance (COP) and capacity of R290 are up to 15.4% higher and 7.5% lower, respectively than those of R22 for two conditions. For R170/R290 mixture, the COP decreases and the capacity increases with an increase in the composition of R170. The mixture of R170/R290 mixture at 4%/96% composition shows the similar capacity and COP as those of R22. For the mixture, the compressor discharge temperature is 17-28 °C lower than that of R22. For R170/R290 mixture, there is no problem with mineral oil since the mixture is composed of hydrocarbons. The amount of charge is reduced up to 58% as compared to R22. Overall, R170/R290 mixture is a good long term 'drop-in' candidate from the view point of energy efficiency and greenhouse warming to replace R22 in residential air-conditioners and heat pumps.
[en] In this study, a smooth flat surface, low fin, Turbo-B, and Thermo excel-E surfaces are used to examine the effect of the flow velocity on the pool boiling heat transfer coefficients (HTSC) and critical heat fluxes (Chefs). HTSC and Chefs are measured on a smooth square heater of 9.53 Χ 9.53 m2 at 60 .deg. C in a pool of pure water at various fluid velocities of 0, 0.1, 0.15, and 0.2 m/s. Test results show that for all surfaces, Chefs obtained with flow are higher than those obtained without flow. Chefs of the low fin surface are higher than those of the Turbo-B and Thermo excel-E surfaces due largely to the increase in surface area and sufficient fin spaces for the easy removal of bubbles. Chefs of the low fin surface show even 5 times higher Chefs as compared to the plain surface. On the other hand, both Turbo-B and Thermo excel-E surfaces do not show satisfactory results because their pore sizes are too small and water bubbles easily cover them. At low heat fluxes of less than 50 k W/m2, HTSC increase as the flow velocity increases for all surfaces. In conclusion, a low fin geometry is good for application to steam generators in nuclear power plants
[en] Nuclear power generation is being discussed in many countries as an alternative method to solving the world's energy crisis. For the safe operation of nuclear power plants, ways for increasing the critical heat flux (CHF) related to a loss of coolant accident are being investigated. In the event that the local heat flux exceeds the CHF, there is an abrupt shift in the boiling curve such that the nucleate boiling ceases and transition boiling and ultimately film boiling occur, finally resulting in a physical break down of the surface. Therefore, it is essential to maximize the CHF for the protection of nuclear power plants with maximum system performance. For the past decade, as a lot of research has been carried out for an improvement of the boiling heat transfer coefficients (HTCs) and CHF, new methods employing nano particles have been proposed. The objectives of this study are to measure the pool boiling HTCs of the water without and with carbon nanotubes (CNTs) on plain and low fin surfaces up to the CHF, and to analyze the effect of CNTs on both nucleate boiling HTCs and CHF. Pool boiling HTCs on all surfaces tested in water without and with CNTs increased as the heat flux increased, which is a typical trend in the pool boiling of pure fluids. For nanofluid with CNTs on low fin surfaces, the surface geometry and nano particles produced a double effect of increasing the CHFs
[en] In this study, external condensation heat transfer coefficients on a horizontal plain tube and four low fin tubes of 19.0 mm outside diameter are measured for an alternative refrigerant of HCFC123 used widely in building chillers. For ease of surface temperature measurement, thick wall test tubes are manufactured with fin specifications similar to the commercial tubes. Heat transfer measurements are carried out for the vapor temperature of 39 deg. C with the wall subcooling of 3-8 deg. C to determine the optimum fin density of the low fin tubes. Tests results show that HTCs obtained on the low fin tubes are up to 5.8 times higher than those on the plain tube. Especially, the enhancement was high at low wall subcooling. HTCs increase up to the fin density of 28 fpi but decrease sharply beyond that density. For four low fin tubes tested, the 28 fpi tube showed the highest HTCs for three refrigerants including HCFC123
[en] In this study, two pure hydrocarbon refrigerants, R1270 (propylene) and R290 (propane), and three binary mixtures composed of R1270, R290 and R152a were tested in a refrigerating bench tester with a scroll compressor in an attempt to substitute R502, which is used in most low temperature and transport refrigeration applications. The test bench provided 3-3.5 kW capacity, and water and water/glycol mixture were employed as the secondary heat transfer fluids. All tests were conducted under the same external conditions, resulting in the average saturation temperatures of -28 and 45 oC in the evaporator and condenser, respectively. The test results showed that all refrigerants tested had 9.6-18.7% higher capacity and 17.1-27.3% higher COP than R502. The compressor discharge temperature of R1270 was similar to that of R502, while those of all the other refrigerants were 23.7-27.9 oC lower than that of R502. For all alternative refrigerants, the charge was reduced up to 60% as compared to R502. There, of course, was no problem with mineral oil, since the mixtures were mainly composed of hydrocarbons. Since some of them are mixtures, one can change their compositions a little to suit various needs in many applications without significant deterioration of the performance. Overall, these alternative refrigerants offer better system performance and reliability than R502 and can be used as long term substitutes for R502 due to their excellent environmental properties
[en] In this study, performance of R430A is examined numerically and experimentally in an effort to replace HFC134a used in refrigeration system of domestic water purifiers. Even though HFC134a is used predominantly in such a system these days, it needs to be phased out in near future in most of the developed countries due to its high global warming potential. To solve this problem, cycle simulation and experiments are carried out with a new refrigerant mixture of 76%R152a/24%R600a using actual water purifiers. This mixture is numbered and listed as R430A by ASHRAE recently. Test results show that the system performance is greatly influenced by the amount of charge due to the small internal volume of the refrigeration system in water purifiers. With the optimum amount of charge of 21-22 g, about 50% of HFC134a, the energy consumption of R430A is 13.4% lower than that of HFC134a. The compressor dome and discharge temperatures and condenser center temperature of R430A are very similar to those of HFC134a for the optimum charge. Overall, R430A, a new long term environmentally safe refrigerant, is a good alternative for HFC134a in domestic water purifiers requiring no major change in the system.
[en] In this study, thermodynamic performance of R431A and HCFC22 is measured in a heat pump bench tester under air-conditioning and heat pumping conditions. R431A is an near azeotropic mixture composed of 71% propane and 29% HFC152a by mass. It has no ozone depletion potential and low greenhouse warming potential of 43. R431A also offers a similar vapor pressure to that of HCFC22 for possible 'drop-in' replacement. Test results showed that the coefficient of performance of R431A is 3.5-3.8% higher than that of HCFC22 while the capacity of R431A is very similar to that of HCFC22 under both conditions. The compressor discharge temperature of R431A is 21.1-27.3 deg. C lower than that of HCFC22 while the amount of charge for R431A is 50.0-51.9% lower than that of HCFC22 due to its low density. Overall, R431A is a good long term environmentally friendly 'drop-in' alternative to replace HCFC22 in residential air-conditioners and heat pumps due to its excellent thermodynamic and environmental properties.
[en] In this review work, energy harvesting methods for waste heat with small temperature differences between heat source and sink are discussed. At present, many methods are tried and employed to utilize this type of waste heat. A typical example is found in a conventional power generation system. By utilizing this type of waste heat, additional energy can be produced in regular power generation systems. Up to this point, two energy harvesting methods have been introduced and applied for the use with this type of waste heat. One is a method using an organic Rankine cycle (ORC) while the other is a method using a thermoelectric generation (TEG). An ORC is a Rankine cycle that can be applied to this type of waste heat using organic fluids such as refrigerants as working fluids instead of water used in a typical Ranking cycle. On the other hand, a TEG utilizes Peltier, Seebeck, and Thomson effects caused by the temperature difference between the heat source and sink for energy harvesting. In this work, various aspects associated with the use ORC and TEG for waste heat harvesting with small temperature differences between the heat source and sink.
[en] In this study, thermodynamic performance of R433A and HCFC22 is measured in a heat pump bench tester under air-conditioning and heat pumping conditions. R433A has no ozone depletion potential and very low greenhouse warming potential of less than 5. R433A also offers a similar vapor pressure to HCFC22 for possible 'drop-in' replacement. Test results showed that the coefficient of performance of R433A is 4.9-7.6% higher than that of HCFC22 while the capacity of R433A is 1.0-5.5% lower than that of HCFC22 for both conditions. The compressor discharge temperature of R433A is 22.6-27.9 deg. C lower than that of HCFC22 while the amount of charge for R433A is 57.0-57.7% lower than that of HCFC22 due to its low density. Overall, R433A is a good long term environmentally friendly alternative to replace HCFC22 in residential air-conditioners and heat pumps due to its excellent thermodynamic and environmental properties with minor adjustments
[en] In this study, thermodynamic performance of R432A and HCFC22 is measured in a heat pump bench tester under both air-conditioning and heat pumping conditions. R432A has no ozone depletion potential and very low greenhouse warming potential of less than 5. R432A also offers a similar vapor pressure to HCFC22 for 'drop-in' replacement. Test results showed that the coefficient of performance and capacity of R432A are 8.5-8.7% and 1.9-6.4% higher than those of HCFC22 for both conditions. The compressor discharge temperature of R432A is 14.1-17.3 deg. C lower than that of HCFC22 while the amount of charge for R432A is 50% lower than that of HCFC22 due to its low density. Overall, R432A is a good long term 'drop-in' environmentally friendly alternative to replace HCFC22 in residential air-conditioners and heat pumps due to its excellent thermodynamic and environmental properties