查看更多>>摘要：Micro-encapsulated phase change material with metallic core can work under high temperature for energy storage purpose, making it an appealing candidate for renewable energy technologies, such as thermal energy storage for concentrating solar power plants. The objective of this work is to study the impact of encapsulation on the thermal performance of the micro-encapsulated phase change materials under various operating conditions. To evaluate the thermal performance of the micro-encapsulated phase change material particles, the governing equation of transient heat conduction with phase change in spheres with composite walls is solved numerically. The impact of encapsulation shell on the convective heat transfer rate, total energy absorption, latent heat ratio, energy density, and duration of phase change is presented and analyzed under different operating conditions. Three different core materials including tin, aluminum and copper with distinct Stephan numbers are investigated. In addition to the encapsulation shell effect, this model can be used to study micro-encapsulated phase change materials with different material compositions, core to shell ratios, and it can be used to analyze the impact of air gaps in between materials. Understanding the thermal performance at particle level is essential, and the results of this study could potentially be used as input for thermal energy storage system level analysis, such as the charging and discharging processes.
查看更多>>摘要：Finding a supersonic nozzle minimum length is a classical application of the method of characteristics (MOC) for aerospace propulsion technology among many other applications, such as air and vapor handling processes. That methodology allows design contoured nozzle free of oblique shock waves created at sharp area changes in short nozzles. While the designing technique is well established in textbooks for ideal gas flow, the use of the MOC technique for real gas is problematic due to the highly complex fluid behavior, captured by modern real gas equations-of-state (EOS). This work presents the MOC devised for real gases using multi-parameter equations-of-state (MPEOS) for different substances and compositions. This paper also compares the MPEOS solution obtained from other classical EOS, such as Peng–Robinson, and the ideal gas equation to establish the optimal application range for each EOS and their effect on the nozzle wall construction. The study was carried out for a commercial fluid refrigerant, pure carbon dioxide, and a CO2?CH4 mixture. The methodology can be used for designing industrial pieces of equipment, such as turbo-machineries and supersonic gas separators or supersonic ejectors, to evaluate the isentropic flow expansion within those devices.
查看更多>>摘要：A mechanically-driven loop heat pipe heat recovery system by booster and refrigerant pump was proposed to match the all-year fresh air load varying greatly with ambient temperature in an energy recovery ventilation unit and enhance its energy-saving potentials. The system prototype was developed and the experimental setup established in which the booster and pump can operate together or separately. Namely, the prototype could be running in pump-driven loop heat pipe (PLHP) mode, booster-driven loop heat pipe (BLHP) mode or booster combining with pump-driven loop heat pipe (CLHP) mode. The heat transfer characteristics of the prototype running in these three modes under winter and summer conditions were studied, respectively. The results showed that the temperature effectiveness of CLHP was greater than that of PLHP or BLHP under all-year conditions. When outdoor temperature is ?15 °C, the temperature effectiveness of CLHP is 78.0% and 52.5% higher than that of PLHP and BLHP, respectively, and the heating EER of CLHP is 29.6% higher than that of BLHP. When outdoor temperature is 40 °C, the CLHP has 19.5% higher of temperature effectiveness and 21.7% higher of cooling EER comparing with the BLHP, respectively. In winter, BLHP performs better when outdoor temperature is greater than 7.5 °C while CLHP performs better when outdoor temperature is lower than 7.5 °C. And BLHP has better performance when outdoor temperature is lower than 35 °C in summer while CLHP performs better when outdoor temperature is higher than 35 °C. The composite system can switch its operating mode according to the fresh air load, which can improve effectively the year-round performance of the system to recover heat in building ventilation.
查看更多>>摘要：In the present study, a theoretical assessment of an adsorption-assisted cascade compression refrigeration system powered by photovoltaic/thermal collectors is performed. The proposed system is intended to produce refrigeration and electricity simultaneously. Herein, mathematical modeling of the proposed system is developed using MATLAB/SIMULINK and validated with the open literature. The impact of collector area, compressor displacement volume, cooling water temperature, and brine temperature on the system performance are quantitatively investigated and analyzed. The economic analysis of the proposed system has also been conducted. It is observed that decreasing the compressor displacement volume and cooling water temperature could substantially increase the overall coefficient of performance and energy saving. Furthermore, the results demonstrate that the system performance enhances with the increase of the collector area and the brine temperature. It is found that the proposed system can concurrently attain a daily average cooling capacity and electricity yield of 1.7 kW and 103 kWh at cooling and brine temperatures of 30 °C and ?20 °C, respectively. Simulation results indicate that the proposed system can generate annual refrigeration production and electricity production of 34.4 kWhc m?2 and 213 kWhel m?2, respectively, with an annual average coefficient of performance of 2.38. Moreover, the economic analysis exhibited that the photovoltaic/thermal collectors-assisted cascade adsorption-compression system is economically competitive for refrigeration applications, revealing an annual energy saving of 24.8% and a payback period of ～3.9 years. In this regard, cascade adsorption-compression systems could significantly extend the potential application and energy savings for cooling systems and heat pumps.
查看更多>>摘要：Phase change materials (PCMs) are widely used in thermal energy storage systems as they can absorb and release a large amount of heat during the phase change process. The numerical study of PCM heat exchangers is an efficient method to analyze PCM behaviors. In this paper, a solid–liquid phase change model is developed based on the lattice Boltzmann method (LBM) to simulate transient phase change in the porous media. This model combines the axisymmetric porous LBM with an enthalpy-updating scheme, which enables it to simulate the axisymmetric porous PCM phase change efficiently. The enthalpy-based LBM at the scale of representative elementary volume is adopted for the modeling of the PCM phase change and the porous media. Moreover, double distribution functions coupled with a multi-relaxation-time scheme are utilized in LBM for the simulation of the fluid flow and temperature field. This improved model is validated using experimental results for a copper-foam enhanced PCM heat exchanger. Validation results indicate that the new model can successfully simulate the temperature glide of the PCM and the effect of natural convection on the PCM temperature field with an error of 10%. A parametric study is then conducted to evaluate the effect of natural convection on PCM melting and results indicate that the average acceleration of PCM melting due to the natural convection can be up to 10%. Based on the validation and parametric study, the new model can predict the performance of the cylindrical PCM heat exchanger. The new model is expected to be applied in a wider field of PCM phase change, which can benefit the design and improvement of PCM heat exchangers in thermal energy storage systems.
查看更多>>摘要：Intake oxygen enrichment has a promising potential to improve the combustion performance of a small-scaled rotary engine (RE) operating at high altitudes. However, the process is complicated because the intake temperature and pressure are sensitive to altitudes as well as the elongated rotor chamber. In this study, a three-dimensional CFD simulation model coupling with a suitable turbulent model and a reduced chemical kinetic mechanism was established and validated. The combined effects of intake oxygen enrichment as well as intake pressure and temperature under different altitudes on early flame growth, combustion behavior, and emission characteristics of small-scaled REs were numerically investigated. Results showed that intake oxygen enrichment promotes the formation of OH, O, H radicals, which leaded to the significant improvements of the early flame growth, especially for the flame near leading spark plug. The promotion effect due to intake oxygen enrichment became weak with increasing altitude. Moreover, the early flame growth duration at high altitudes was more dramatically shortened by intake oxygen enrichment than that at low altitudes. It is interesting to note that when oxygen concentration of 25% and 29% were applied to the small-scaled RE working at 2000 m and 3000 m altitude, respectively, the engine performance could be restored to the level of that working at sea level at the price of a smaller increment of NO emissions. Under high intake oxygen concentration conditions, the effect of altitude on indicated thermal efficiency of the engine could be ignored. Besides, increasing oxygen concentration significantly reduces HC and CO emissions, especially more significant reduction for HC emissions at higher altitudes.
查看更多>>摘要：Accurate performance evaluation of a wrapped-around condenser for heat pump water heaters (WHPWH) is critical since the COP of the system depends heavily on thermal stratification and condenser design. An analytical, quasi-steady state heat balance method has been developed to determine the optimal spacing between adjacent channels, tube diameter, tube shape, and total refrigerant charge amount for the condenser. The heat transfer rate is compared among three approaches: the refrigerant thermodynamic model, the condenser-wrap fin model, and the analytical natural convection model on the inside of the tank for the three regions based on the refrigerant phase (e.g., super-heated, saturated, or subcooled). The heat transfer rate was predicted by a combination of slug-plug and annular flow for a D-shaped helical tube. Experimental data were used as boundary conditions for validating the model. The rate of heat transfer based on tube shape and tank wall temperatures was compared by CFD analysis. Parametric analysis indicates a tradeoff between refrigerant mass, pressure drop, tube diameter, and tube length to maximize the heat transfer rate. The model suggests the saturated region length can be extended by 400%, and the condenser pressure drop can be reduced by 23% with an optimal spacing pattern.
查看更多>>摘要：Screening potential vaccine formulations during freeze-drying is a time-consuming task. High-throughput systems, consisting of small vials inside aluminium well plates, can accelerate formulation selection. However, heat transfer variations among vials due to the edge effect can entail deviations in the final product quality and bias results. This work investigates how the vial position in the well plate impacts the heat flow received during the primary drying step of freeze-drying. Two 3D steady-state models were proposed and compared to evaluate the effect of time passing. One model, called the distilled water model, represents vials containing only a frozen layer at the beginning of primary drying. A second model, called the product model, represents vials containing frozen and dried product layers after drying has progressed (up to half of the product dried). Heat transfer models were validated using heat flows determined by gravimetric analysis during sublimation tests (shelf temperatures ?40 and ?15 °C, chamber pressures from 4 to 65 Pa). At the beginning of primary drying, the distilled water model indicated that vials facing a chamber wall received heat flows up to 25% greater than those in the centre of the well plate. As sublimation progressed (product model), the dried product layer resistance to mass transfer tended to counterbalance the impact of the chamber wall.
查看更多>>摘要：Fogging cooling is one of the most effective methods for cooling gas turbine inlet air in hot areas. Most of the previous studies focused only on the influence of the fogging cooling effect on the gas turbine performance. Nevertheless, due to the limited space available for installing a system, the fogging system requires special attention, notably in micro gas turbines, to avoid the thermal stress and blade erosion risks at a compressor intake. Hence, in the current paper, the performance of the fogging cooling system for micro gas turbines has been investigated experimentally and numerically to enhance the cooling efficiency, temperature stability and accomplish complete droplets evaporation. An experimental setup has been designed and installed to study the fogging cooling effect through a cylindrical duct. Eulerian-Lagrangian CFD model was also adopted to simulate the trajectories of the injected droplets and to predict the evaporation cooling process through the spray duct. The CFD model was validated with the current experiments, and there is good agreement between the experimental data and simulation results with an average deviation of less than 6.4%. The droplets' size and injected mass flow were measured at different nozzles orifice diameters and injection pressures to select the appropriate spray nozzles at various test conditions. The fogging performance has been tested under various operating conditions, including nozzle arrangement and flow directions, to identify the crucial factors for obtaining fully saturated air and stable temperature from the spray duct. The results reveal that employing a single nozzle to achieve full saturation is ineffective. On the other hand, increasing the number of nozzles with the same input water flow rate improves droplets distribution across the duct and enhances the evaporation rate. It was found that the temperature drop can be reduced by 1.5 °C due to dividing the required water mass flow on four nozzles instead of one. Furthermore, it was observed that the cooling performance of the counter-current flow case could be improved by 3.4% compared to the co-flow one with a high uniform temperature profile through the duct. Finally, a counter-current spray design with four nozzles is proposed for micro gas turbines since it meets the design constraints best compared to other operating scenarios.
查看更多>>摘要：In this present paper, an experimental research was carried out to investigate the influence of various motion conditions on the flow and heat transfer characteristics in natural circulation loops. A natural circulation loop was built on the six-degree-of-freedom motion experiment platform at Xi'an Jiaotong University (XJTU). First, it was found that the small acceleration heaving would not cause significant changes on flow rate of the natural circulation loop. Then, the influence mechanism of rolling motion on the natural circulation loop is different as the central axis of rolling is changed. The increase of additional inertial force along the coolant flow direction causes larger flow fluctuation. The periodic average frictional resistance coefficient under different working conditions is obtained, which is divided into flow fluctuation region (Re < 5000) and turbulence region (5000 < Re < 35000). Last, according to the experimental data, a mathematical model to predict the periodic average frictional resistance coefficient which is suitable for the turbulent region is obtained, and the deviation between the predicted value and the experimental data is less than 20%. Beyond that, the period average Nu under different working conditions is also achieved. It shows that the rolling motion promotes the heat transfer with the small flow rate, while the rolling motion affects the heat transfer characteristics mainly by affecting the flow rate under the large flow rate conditions. Based on our research, some suggestions on the layout of marine natural circulation loop were given.
NSTL
Elsevier