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[en] Highlights: • Optimal thermodynamic parameters of two-phase ejector refrigeration system. • Empirical correlation for primary evaporator temperature. • Calculation of optimal condenser and evaporator dimensions. • Comparison of calculated values with experimental study. - Abstract: Air-conditioning is necessary for the comfort of passengers in commercial buses. However, installing an air-conditioning system can add extra load on the engine and result in extra fuel cost. Therefore, an improvement in the air-conditioning system can lower the fuel consumption of the buses and reduce the size of the evaporator and the condenser. It is known that using two-phase ejector as an expansion valve in the air-conditioning system can improve the system performance. This study offers a model to predict the optimal thermodynamic parameters for a two-phase ejector refrigeration system for buses using R134a under various operating conditions. An empirical correlation is derived to determine the optimal thermodynamic parameters of the system. The effect of evaporation and condensation temperatures on the heat transfer surface area are discussed and graphically illustrated. Moreover, an experimental study to validate the developed model has been carried out in a midibus air-conditioning system. The study findings revealed that the heat transfer surface area can be reduced by about 4% and 55% in the condenser and evaporator, respectively.
[en] Graphical abstract: Transformation of K components during combustion. - Highlights: • High temperature promotes the formation of gaseous and insoluble K compounds. • Melting decreases K release due to poor mass transfer effect. • The reactions related to K species in bottom ash of co-firing were studied. • The thermodynamics and chemical reaction rate affect the distribution of products. - Abstract: Co-combustion experiments of rice straw with K, Si-riched rice straw and coal were conducted in a horizontal tube furnace at 600–1000 °C. Potassium migration and transformation during co-combustion was investigated via analysis of combustion products by Inductively Coupled Plasma (ICP), X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM) and thermodynamic calculation software respectively. Results indicated that high temperature improved the release of K to the gaseous phase and promoted the generation of insoluble K compounds in the bottom ash. K in the rice straw mainly formed KCl, K2SiO3, while in both coal combustion and co-combustion was in the form of aluminosilicate. At 1000 °C, a serious melting phenomenon was observed in ash of rice straw, bringing a decrease of K release while co-combustion could relieve this condition via increasing the melting temperature of ash effectively. The primary chemistry reaction during the co-combustion was Al migrated from coal ash to rice straw ash, forming relatively steady and high-melting point potassium aluminosilicate rather than potassium sulfate reported before. Furthermore, KCl(g) and K2CO3 were important predecessors to form K2SO4 and K2SiO3 in combustion by theory analysis.
[en] A considerable increase in thermal separation by the use of radiation induced evaporation of binary base fluids using nanofluids has been reported. It was shown for the water-ethanol system, that the vapor phase composition was enhanced about 20 wt% of ethanol above the thermodynamic equilibrium value. Even the azeotropic point seemed to disappear. This paper aims to reproduce the reported results and it tries to analyze possible mechanisms explaining these findings that appear anomalously at first sight.
[en] Highlights: • AHT and RHT have been modelled with the NH3/LiNO3 mixture by the first time. • Single and double stage absorption and resorption heat transformers were modelled. • Double stage cycles achieved higher GTL but lower COP than single cycles. • AHT achieved higher GTL and COP with regard to RHT. • RHT operate at considerably lower pressures than AHT. - Abstract: This paper presents the modeling and thermodynamic analysis of different heat transformers configurations using the alternative mixture NH3-LiNO3. The analyzed configurations are: (i) Absorption Heat Transformer (AHT), (ii) Double Absorption Heat Transformer (DAHT), (iii) Resorption Heat Transformer (RHT) and (iv) Double Resorption Heat Transformer (DRHT). Coefficients of performance and exergetic efficiencies are reported for each one of the systems as function of the main system temperatures. The results showed that the coefficients of performance and exergetic efficiencies are higher for the conventional cycles without the resorption circuit (AHT and DAHT), however, their operating pressures are considerable higher than those reached with the systems with the resorption circuit.
[en] Highlights: • A micro-radial radioisotope thermoelectric generator is manufactured and tested. • The simulated performance of the RTG are compared with the experimental value. • Performance characteristics were determined in different sizes and numbers. • The designed RTG is expected to be a reliable space power supply for MEMS. - Abstract: To satisfy the flexible power demand of the low power dissipation devices in the independent space electric system, a micro-radial milliwatt-power radioisotope thermoelectric generator (RTG) was prepared and optimized in this research. The overall geometrical dimension of the RTG in the experiment was 65 mm (diameter) × 40 mm (height). The RTG, which was built and tested using simulated radioisotope source, eventually obtained an open-circuit voltage of 92.72 mV, an electric power of 149.0 μW, and an energy conversion efficiency of 0.015% at the ambient temperature of 293.15 K and heat source power from 0.1 W to 1 W. On the basis of the structure used in the experiment, the length and cross-sectional area of the thermoelectric leg and the number of thermoelectric modules were effectively optimized through the COMSOL Multiphysics. With the optimized length of 35 mm and cross-sectional area of 1.2 mm2, the RTG with four thermoelectric modules achieved a 15.8 mW output power under 1 W heat source power. The maximum conversion efficiency calculated using COMSOL code increased to 1.58%. According to the optimized electrical output, the micro-radial RTG is expected to be a reliable space power supply for micro components and could satisfy the low power requirements of space missions.
[en] Graphical abstract: The effects of CO2/R41 as an azeotropy refrigerant on system performance are analyzed under various working conditions. The results show that CO2/R41 mixture is a good choice to substitute for pure CO2 in the studied systems because of its low optimal high pressure and high system COP. - Highlights: • The CO2/R41 mixture is an azeotropy refrigerant applied in refrigeration systems. • Comparing with pure CO2, CO2/R41 mixture can decrease the optimal high pressure. • Comparing with pure CO2, CO2/R41 mixture can increase the system COP. - Abstract: In this study, blend of CO2 with R41 is evaluated for application in three different systems including a refrigerated cabinet, an air-source heat pump water heater and a water-source heat pump water heater. The effects of CO2/R41 mixture as an azeotropy refrigerant on system performance are analyzed under various working conditions. The results show that CO2/R41 mixture may be a good choice to substitute for pure CO2 in the studied systems because of its stable chemical properties, low optimal high pressure, high system COP, low compression ratio, low discharge temperature, high refrigerating capacity per unit, and high heating capacity per unit. Furthermore, the exergy efficiencies of the three studied systems are improved to more than 23% with the proposed CO2/R41 (0.5/0.5) mixture refrigerant.
[en] Highlights: • Wetness fraction of the low pressure cylinders is identified. • Condenser thermodynamic characteristics under varying working conditions are investigated. • The objective function of the condenser pressure optimization is presented. • The set value of the mass flow rate of the circulating water is obtained with the manipulation strategy. - Abstract: The operation conditions of the cold-end system have significant impact on the unit thermal economy. However, due to the difficulty of online determination of some key parameters, the condenser pressure optimization has been a challenging task for a long time. This paper proposes an online applicable approach to optimize the condenser pressure with variable speed pumps, taking the mass flow rate of the circulating water as the manipulating variable, to achieve better thermal economy. After the exhaust steam wetness fraction is online identified, the condenser thermodynamic characteristics under varying working conditions are investigated based on the effectiveness and steady-state energy balance of the condenser. By maximizing the net power benefit, defined as the difference between the unit power increment and the pump power consumption increment, the optimal mass flow rate of the circulating water is derived. To validate the approach, pseudo-online simulations are conducted with the history data from an ultra-supercritical unit. The retention time during which the set value of the mass flow rate of the circulating water remains constant is studied in context with the implementation of the manipulation strategy under on-site scenario. Simulation results reveal the energy-saving potential of condenser pressure optimization with the proposed approach.
[en] Highlights: • The middle-temperature solar-driven Kalina cycle was investigated. • The parabolic trough collector has a variable concentration ratio. • Kalina cycle basically operates at design points with varying solar irradiance. • Cycle can efficiently utilize a border range of direct normal irradiance to power. • Operation method for off-design conditions improves the annual efficiency. - Abstract: Solar thermal power generation is currently an attractive solar electricity technology. Currently, we face an important issue of lower annual solar-to-power efficiency (approximately 10.0%) using parabolic trough technology because the direct normal irradiance instantly varies, and the solar thermal power cycle always derivates from the designed operation. Here, we investigate a middle-temperature solar-driven Kalina cycle that uses a parabolic trough collector with a variable concentration ratio. From lower to higher direct normal irradiance, both the aperture area of collector and the flow process of the Kalina cycle can be changed. As a result, a much border direct normal irradiance of 100–1000 W/m2 achieves a solar-to-power efficiency of 4–20%, resulting in an annual solar-to-power efficiency of approximately 14%. Furthermore, the interactions are analyzed among direct normal irradiance, the aperture area of the collector, and the flow process of the thermal cycle. An operation method for off-design conditions is proposed to greatly improve the annual solar-to-power efficiency, offering a pathway to efficiently utilize a border range of direct normal irradiance.
[en] Highlights: • Chaotic flow in a 2D multi-turn PHP was investigated. • Non-linear temperature oscillations were analyzed. • Optimal filling ration and minimum thermal resistance were obtained. - Abstract: Numerical study has been conducted for the chaotic flow in a multi-turn closed-loop pulsating heat pipe (PHP). Heat flux and constant temperature boundary conditions have been applied for heating and cooling sections respectively. Water was used as working fluid. Volume of Fluid (VOF) method has been employed for two-phase flow simulation. Volume fraction results showed formation of perfect vapour and liquid plugs in the fluid flow of PHP. Non-linear time series analysis, power spectrum density, correlation dimension and autocorrelation function were used to investigate the chaos. Absence of dominating peaks in the power spectrum density was a signature of chaos in the pulsating heat pipe. It was found that by increasing the filling ratio and evaporator heating power the correlation dimension increases. Decreasing of the autocorrelation function with respect to time showed the prediction ability is finite as a result of chaotic state. An optimal filling ratio of 60% and minimum thermal resistance of 1.62 °C/W were found for better thermal performance of the pulsating heat pipe. It is notable that two dimensional simulations in current study lead better understanding of the mechanism and validating the numerical method for full three dimensional modeling.