查看更多>>摘要：Traditional building air conditioning uses electricity or thermal energy to drive the refrigeration cycle. The energy released by the expansion of compressed air can also be used for driving the refrigeration cycle. Moreover, the expanded low-temperature air can be used in the air conditioning system too. In this paper, three forms of CO2 air conditioning systems driven by novel reciprocating piston compressed air engines were designed, namely EAGC (Exhaust Air for Gas Cooler), EAS (Exhaust Air for Sub-cooler), and EASC (Exhaust Air for Space Cooling), to analyze the performance and economics of CO2 air conditioning systems combined with compressed air energy storage technology. Twenty-seven typical cities were selected in different climate regions in China to evaluate the systems' performance and running cost under outdoor temperature changes and different electricity prices. The results show that compared with the electricity-driven CO2 air conditioning system, the three systems can greatly improve the system COP (Coefficient of Performance) but still cannot reach the performance level of the conventional freon refrigeration systems. However, in cities like Beijing and Shanghai with the large peak-to-valley electricity price difference, EASC system compared to the electricity-driven R410A air conditioning system running costs are lower, the average running cost per hour can be saved 11.96% and 16.35%, respectively. This means that the CO2 air conditioning system with compressed air energy storage technology has a certain economic application value and can be popularized on reasonable occasions. This innovative system design can help solve the contradiction between supply and demand of building energy, realize leisure energy consumption, and reduce the economic cost of energy consumption on the demand side.
查看更多>>摘要：The study of constituent separation in T-junction is important to realize the operation of the constituent adjustable system. This paper presents a numerical simulation on the constituent separation and mass transfer of R134a and R600a in T-junction. The multiphase flow model of volume of fluid (VOF) is adopted, and the interphase mass transfer of R134a and R600a is achieved by adding source terms in governing equations. The quality at inlet varies from 0.51 to 0.92. The mass flow rate at inlet and flow rate weighting of branch outlet are 10 g?s?1 and 0.3, respectively. Results show that the error of numerical simulation with mass transfer is lower than that of numerical simulation without mass transfer. When the quality at inlet is 0.81, the error of numerical simulation with mass transfer can be reduced by 50.07% compared with the numerical simulation without mass transfer. The higher the quality at inlet, the higher the separation efficiency of R134a when the quality at inlet is less than 0.81. When the quality at inlet is 0.81, the separation efficiency of R134a in the numerical simulation with mass transfer is the largest, whose value is 1.47%. Most of the flow patterns are annular flow. The pressure and flow pattern have an important influence on the constituent separation and the interphase mass transfer rate. A correlation with 10% error band of interphase mass transfer for annular flow in T-junction is proposed.
查看更多>>摘要：The utilization of fluted inner tube in tube-in-tube heat exchangers is an effective way to improve the heat transfer performance. Based on the segmented model, the performance of a fluted tube-in-tube gas cooler (FTGC) in the transcritical CO2 heat pump water heater was studied in this paper. The simulation model was validated and adopted to numerically investigate the effects of different parameters on the heat transfer performance of the gas cooler, which was compared to the smooth tube-in-tube gas cooler (STGC) with the same geometric parameters. The results showed that the FTGC had higher heat transfer capacity in most conditions. The heat transfer rate first increased and then decreased as the CO2 inlet pressure increased. The temperature difference at the pinch point in the FTGC was lower than that in the STGC. In the FTGC, the pinch point position was mainly influenced by the CO2 inlet pressure and water inlet and outlet temperatures. Regarding the system COP, the improvement of maximum COP could be up to 29.41% by using the FTGC compared to STGC when the water inlet temperature was 10 °C and the water outlet temperature was 80 °C. However, when the water inlet temperature was 40 °C, the improvement of COP could be impaired to 12.98%. Through changing the designs of the FTGC with the fixed total heat transfer area, when the water inlet and outlet temperatures were 10 °C and 60 °C, the highest maximum COP could be obtained with the design used in the experiments.
查看更多>>摘要：This work presents an experimental analysis of alternative fluids for R410A aiming at reducing the environmental impact. Owing to the impact of R410A on the environment, its use has been extensively studied to find a replacement. Some countries have already set goals for the reduction and elimination of this fluid. Experimental analysis through comparison is a method used to determine the best alternatives. An experimental setup was developed and instrumented to evaluate the efficiency of a refrigeration system with evaporation temperatures between -5 and 5 °C. It consists of a compressor designed for the R410A, with a capacity of 17.5 kW, brazed plate heat exchangers, electronic expansion valve and adequate instrumentation for measuring the main operating parameters. Besides the fluid results, the TEWI methodology used allows comparing the environmental impact of each fluid, considering the direct and indirect effect. The findings revealed that, for the conditions tested, alternative fluids have a lower performance than R410A in terms of energy efficiency, however, they have a lower environmental impact due to their lower GWP. It is noteworthy the result of R454B, which had the lowest overall environmental impact.
查看更多>>摘要：The use of re-gasified liquefied natural gas (LNG) and boil-off gas (BOG) as a fuel in LNG fueled ships generally results in a waste of cold energy. Meanwhile, when the ship is docked, BOG should be re-liquefied to minimize the LNG loss using external energy. To efficiently utilize wasted cold energy, a novel BOG treatment concept combined with air liquefaction is proposed and a new process for recovering cold energy of BOG and fueled LNG is designed and analyzed in this paper. The system is divided into three sections: the BOG and LNG regasification into the fuel gas supply system (FGSS), liquid air energy storage and liquid air energy release. The proposed system is analyzed in three ways: energy analysis by the first law of thermodynamics, heat exchanging analysis by the heat flow, and exergy analysis by the second law of thermodynamics. The energy efficiency of the BOG and LNG gasification into the FGSS, air storage and air release sections are 34.2%, 93.4%, and 52.3%, respectively. The overall heat exchange efficiency is 42.28% and the exergy efficiency is 30.4%. The economic analysis of the system is investigated on the lifecycle of 20 years. This study not only combines the BOG treatment process and the FGSS of the LNG ship but also reduces the energy consumption of the system onboard.
查看更多>>摘要：Advanced PVE was developed as the coupled lubricant oil for the alternative refrigerant R32 and R32/PVE will be a potential working pair of R32 refrigeration systems. This paper presented modeling study of physical properties of PVE lubricant oil with viscosity grade 68 and R32/PVE mixture. Thermal conductivity, specific heat and surface tension of PVE oil were measured experimentally. New calculation models for properties of pure oil were developed based on the experimental data. Moreover, the solubility, miscibility and viscosity of R32/PVE mixture were also well correlated with the new correlations. Further, a parametric analysis was performed to study oil transport and retention characteristics in compressor suction line, based on the force balance method. A new consolidated database for oil retention was amassed from six existing literatures, including 187 data points. A new correlation for oil retention was established by incorporating influence factors derived from the parametric analysis. The validation result showed that the oil retention correlation exhibits satisfactory prediction accuracy, which achieved mean absolute error of 14.43% and 87.70% of data fell within ±30% error bands. The property models and oil retention correlation could serve as tools to accurately predict oil retention amount of suction line with R32/PVE mixture.
查看更多>>摘要：Propositions of vapor compression cycles for microgravity applications have a long history. In contrast, experimental efforts and implemented applications are rare in general and in recent years even less than in the 1980s and 1990s. The paper shows experimental results from a vapor compression cycle on parabolic flights. The focus lies on the effects of varying gravity forces on the condenser and evaporator. Flow visualizations clearly showed a change to an annular or slug-annular flow regime at the evaporator inlet shortly after the onset of gravity. The discharge pressure was found to consistently increase sharply by 1–3% across widely varying operating conditions. Oscillations were observed in the evaporator for some parabolas due to microgravity. Overall, the cycle response to varying gravity based on the cooling capacity can be categorized in four different groups, which appear to have a dependence on both the mass flow rate and the evaporator superheat.
查看更多>>摘要：Hydrofluidisation (HF) is an original method of food freezing that allows very high heat transfer coefficients to be reached and significantly reduces the freezing time of small foods. This paper presents a numerical study of HF for food products immersed in different liquids: binary solutions of ethanol (30%) and glycerol (40%) and a ternary solution of ethanol and glucose (15% and 25%). The particle movement (0.02 m diameter) was simulated using the Macroscopic Particle Model (MPM) approach to extend the CFD simulation by the particle flow, which affects the liquid phase velocity field. The numerical model developed in that study fully describes the hydrodynamics of HF process and is the most realistic out of all the numerical studies performed so far due to the ability to resolve the fluid flow locally on a smaller scale than in other models where the movement of foods in HF was introduced. Experiments with high-speed camera recordings validated the numerical model. A wide range of liquid mass flow rates in the range from 0.1 kg?s?1 to 2.0 kg?s?1 was investigated. As a result, heat transfer coefficients from 1 000 W?m?2?K?1 to 4 500 W?m?2?K?1 were reached, depending on the fluid mass flow rate. The liquid type had a minor effect on the heat transfer coefficient (HTC) but affected the behaviour of food samples. The effect of the food product suspension within the HF tank was noticed only for the ethanol solution with a moderate mass flow rate.
查看更多>>摘要：Powering ammonia-water absorption refrigeration cycles with solar energy demands an operating temperature above 170 °C for the proper generator operation when conventional flooded generator technologies are used. However, the falling film technology operates at a lower temperature due its superior heat and mass transfer performance. Therefore, an experimental investigation focused on the energy balance along with a heat and mass transfer analysis between liquid and vapor ammonia-water mixtures in the generator and the rectifier have been developed. Four experimental sets of runs were carried out for oil temperatures at 111 and 136 °C, strong solution mass fraction between 0.37 and 0.47, two rectification temperatures at 34 and 63 °C, and a strong solution mass flow rate of 0.016-0.027 kgs?1. Heat transfer rates for both components were computed by overall energy balances over the components. Moreover, the latent heat and sensible heat rate were calculated. The results indicated that the heat transfer process in the rectifier was lower for the minimum generation temperature. The maximum heat transfer coefficients for the liquid and vapor phase were respectively 5476 and 26Wm?2°C?1. Analogously, the maximum mass transfer coefficients between the liquid film and vapor phase were 1.27·10?4 and 3.25·10?2ms?1.
查看更多>>摘要：As a typical absorption device, falling film absorber has a significant effect on the performance of the ammonia water absorption refrigeration system. In this paper, a novel absorber with atomizer installed at the bottom of the absorber is proposed. The atomizer is introduced to increase the effective mass transfer area between ammonia vapor and the solution. A theoretical model in established and simulation investigation is carried out to obtain the absorption enhancement by the atomizer. The result indicates that the enhancement is 10.5% with the 1 mm initial droplet diameter in the design point of this paper, and it can reach 34.2% when the initial droplet diameter is reduced to 0.2 mm. Furthermore, the optimization of enhancement effect by other droplet parameters has been analyzed.
NSTL
Elsevier