Showing papers on "Subcooling published in 2012"

Waste heat recovery system

Abstract: The waste heat recovery system includes a Rankine cycle device in which working fluid circulates through a pump, a boiler, an expander and then through a heat exchanging device, heat exchange occurs in the boiler between the working fluid and intake fluid that is introduced into an internal combustion engine while being cooled. The heat exchanging device includes a condenser condensing the working fluid, a receiver connected downstream of the condenser and storing liquid-phase working fluid, a subcooler connected downstream of the receiver and subcooling the liquid-phase working fluid, and a selector device serving to change the ratio of the condenser to the subcooler. The waste heat recovery system further includes a determination device for determining required cooling load for the intake fluid, and a controller for controlling the selector device depending on the required cooling load determined by the determination device.

Review of Pulsating Heat Pipe Working Fluid Selection

Abstract: The pulsating heat pipe (PHP) is a novel, simply formed, wickless heat pipe that relies on the phase change induced motion of a contained working fluid to transport heat between the evaporator (the hot end) and condenser (the cold end). The improved heat transfer capability, simplicity, and reduced mass of PHPs have lead to great interest in the PHP. This paper reviews one crucial, yet often overlooked, aspect of PHP operation: the working fluid. Analytical analysis is used to show that R-134a and its replacement, HFO-1234yf, are particularly well suited as PHP working fluids. The following five conclusions are reached: 1) surface tension and density play an important role in sizing PHPs for operation in varying gravitational environments; 2) at low vapor pressures, noncondensable gas considerably increases the PHP system pressure, thus subcooling the system; 3) dynamic contact angle and surface tension significantly affect the capillary resistance force acting to damp PHP operation; 4) large viscosities act to damp PHP operation; and 5) latent heat of vaporization, surface tension, and density play a significant part in PHP startup.

Hybrid vapor compression refrigeration system with an integrated ejector cooling cycle

Abstract: A refrigeration system was developed which combines a basic vapor compression refrigeration cycle with an ejector cooling cycle. The ejector cooling cycle is driven by the waste heat from the condenser in the vapor compression refrigeration cycle. The additional cooling capacity from the ejector cycle is directly input into the evaporator of the vapor compression refrigeration cycle. The governing equations are derived based on energy and mass conservation in each component including the compressor, ejector, generator, booster and heat exchangers. The system performance is first analyzed for the on-design conditions. The results show that the COP is improved by 9.1% for R22 system. The system is then compared with a basic refrigeration system for variations of five important variables. The system analysis shows that this refrigeration system can effectively improve the COP by the ejector cycle with the refrigerant which has high compressor discharge temperature.

Exergetic analysis of a vapour compression refrigeration system with R134a, R143a, R152a, R404A, R407C, R410A, R502 and R507A

Abstract: This communication deals with the exergetic analysis of a vapour compression refrigeration system with selected refrigerants. The various parameters computed are COP and exergetic efficiency in the system. Effects of degree of condenser temperature, evaporator temperature and sub-cooling of condenser outlet, supper-heating of evaporator out let and effectiveness of vapour liquid heat exchanger are also computed and discussed. In this study, it was found that R134a has the better performance in all respect, whereas R407C refrigerant has poor performance.

The effect of refrigerant combinations on performance of a vapor compression refrigeration system with dedicated mechanical sub-cooling

Abstract: Performance characteristics due to use of different refrigerant combinations in vapor compression cycles with dedicated mechanical sub-cooling are investigated. For scratch designs, R134a used in both cycles produced the best results in terms of COP, COP gain and relative compressor sizing. In retrofit cases, considering the high sensitivity of COP to the relative size of heat exchangers in the sub-cooler cycle and the low gain in COP obtained due to installation of a dedicated sub-cooling cycle when R717 is the main cycle refrigerant, it seems that dedicated mechanical sub-cooling may be more suited to cycles using R134a as the main cycle refrigerant rather than R717. With R134a as the main cycle refrigerant, no major difference was noted, by changing the sub-cooler cycle refrigerant, in the degradation of the performance parameters such as COP and cooling capacity, due to equal fouling of the heat exchangers.

Subcooled flow boiling heat transfer of ethanol aqueous solutions in vertical annulus space

Abstract: The subcooled flow boiling heat-transfer characteristics of water and ethanol solutions in a vertical annulus have been investigated up to heat flux 132kW/m2. The variations in the effects of heat flux and fluid velocity, and concentration of ethanol on the observed heat-transfer coefficients over a range of ethanol concentrations implied an enhanced contribution of nucleate boiling heat transfer in flow boiling, where both forced convection and nucleate boiling heat transfer occurred. Increasing the ethanol concentration led to a significant deterioration in the observed heat-transfer coefficient because of a mixture effect, that resulted in a local rise in the saturation temperature of ethanol/water solution at the vapor-liquid interface. The reduction in the heat-transfer coefficient with increasing ethanol concentration is also attributed to changes in the fluid properties (for example, viscosity and heat capacity) of tested solutions with different ethanol content. The experimental data were compared with some well-established existing correlations. Results of comparisons indicate existing correlations are unable to obtain the acceptable values. Therefore a modified correlation based on Gnielinski correlation has been proposed that predicts the heat transfer coefficient for ethanol/water solution with uncertainty about 8% that is the least in comparison to other well-known existing correlations.

Heat Transfer to Suspensions of Microencapsulated Phase Change Material Flowing Through Minichannels

Abstract: The heat transfer to water-based suspensions of microencapsulated phase change material (MEPCM) flowing laminarly through rectangular copper minichannels was investigated both experimentally and numerically The MEPCM-particles had an average size of 5 μm and contained as phase change material n-eicosane, which has a theoretical melting temperature of 364 °C Water and suspensions with particle mass fractions of 10% and 20% were considered While the experiments result in rather global values such as wall temperatures at certain points, suspension in- and outlet temperatures, and the pressure drop, the numerical simulations allow additionally a more detailed insight, for example, into the temperature distribution in the flowing suspension The results show that MEPCM suspensions are only advantageous in comparison to water in a certain range of parameter combinations, where the latent heat is exploited to a high degree The available latent heat storage potential, which depends on the particle fraction in the suspension and on the mass flow rate, has to be in the same order of magnitude as the supplied heat Moreover, the mean residence time of the particles in the cooling channels must not be considerably shorter than the characteristic time for heat conduction perpendicular to the flow direction Otherwise, the particles in the center region of the flow leave the cooling channels with still solid cores, and their latent heat is not exploited Furthermore, the benefit of the added MEPCM particles depends on the inlet temperature, which has to be slightly below the theoretical melting temperature, and on the subcooling temperature after the heat supply, which has to be sufficiently low to guarantee that the entire phase change material solidifies again before it re-enters the cooling channels The suspensions showed Newtonian behavior in the viscosity measurement The actual pressure drop determined in the experiments is smaller than the pressure drop estimation based on the measured viscosities The difference between the two values increases with increasing particle mass fraction This shows that the particles are not evenly distributed in the flowing suspension, but that there is a particle-depleted layer close to the channel walls This reduces the required pumping power, but makes it even ntore important to provide conditions, in which a sufficiently large amount of the supplied heat is conducted to the center region of the channels

A Performance comparison of vapour compression refrigeration system using various alternative refrigerants

Abstract: A performance analysis on a vapour compression refrigeration system with various refrigerants mixture of R152a, RE170, R600a, and R290 were done for various mixture ratios and their results were compared with R134a as possible alternative replacement. The results showed that all of the alternative refrigerants investigated in the analysis except R431A, (R 152a (29%), R290 (71%)) have a slightly higher performance coefficient (COP) than R134a for the condensation temperature of 50 0 C and evaporating temperatures ranging between -30 0 C and 10 0 C.Refrigerant blend of R152a/RE170 (20/80 by wt%) instead of R134a was found to be a replacement refrigerant among other alternatives. The effects of the main parameters of performance analysis such as refrigerant type, degree of sub cooling and super heating on the refrigerating effect, coefficient of performance and volumetric refrigeration capacity were also investigated for various evaporating temperatures.

Simulation study on a novel vane-type expander with internal two-stage expansion process for R-410A refrigeration system

Abstract: Large volumetric expansion ratio of HFCs is the major obstacle to develop positive displacement expander for HFCs air-conditioning system. The paper presented a novel vane-type expander with two internal expansion stages for R-410A system. Simulation models were established to estimate the characteristics and performances of the expander and the system. The structure and operating principle of the expander were presented, the major influences of structural parameters on the built-in volumetric ratio of the expander were analyzed, and the unique mass transition process between stages was discussed. The study showed that the proposed expander obtained built-in volumetric ratio up to 7.6 with isentropic efficiency of 55.9% at 2000 rpm and theoretically improved the coefficient of performance from 4.0 to 4.56, by 14.2%, under design operating condition. The expander outperformed in vapor compression refrigeration system under conditions of relatively higher condensation temperature, higher rotating speed, lower evaporation temperature and smaller degree of subcooling.