查看更多>>摘要：The concentrated hybrid power/thermal system is proposed and discussed in this paper. This system, which includes the concentrator, the solar thermal collector, and thermoelectric generators, not only provides the heated fluid but also can generate electrical power by thermoelectric generators, thus, two energy conversions are realized: heat recovery and thermoelectric power generation. Theoretical investigations found that the variation trends of heat gain and electrical power of the system have a good correlation with solar radiation. During the all-day operation, the thermal and electrical performance of the concentrated hybrid power/thermal system using linear Fresnel lens is better than that of the compound parabolic concentrating power/thermal system with the all-day overall efficiency of the former of 75.59%, 5.12% higher than the latter. Also, the electrical power thermoelectric generators generate in the two systems with the maximum of 0.60 W and 0.54 W under the conditions of 4 concentration ratio and 15 celcius initial water temperature, respectively. Additionally, the performance of the system from both viewpoints of energy and exergy analysis is investigated under conditions of different parameters of thermoelectric generators, concentration ratios, and fluid properties. The valuable conclusions provide a new thought for the application of TEG in solar systems.
查看更多>>摘要：Towards fundamental investigation of key physical aspects of pre-chamber combustion, the current work utilizes computational fluid dynamics to comprehend the effect of the throat diameter in an engine operated with methane. Previous studies showed that this parameter is dominant in pressure build-up and flow pattern inside the pre-chamber, suggesting that a detailed characterization is necessary. This pre-chamber type is composed of an upper conical part that lodges the spark plug and fuel injector, followed by a straight and tubular region called throat, which tip accommodates the nozzles responsible for the charge exchange between pre and main chambers. Two types of pre-chamber having distinct throat diameters are investigated, while utilizing consistent experimental data for validation of the model. The combustion process is modeled with the G-Equation model; the laminar flame speed was tabulated from a methane oxidation mechanism reduced from the GRI 3.0; the turbulent flame speed was computed using Peters' relation. The simulations were run for a full cycle, starting at exhaust valve opening. A homogeneous charge of methane is considered at the intake port, maintaining a global lambda = 1.8, while 3% of total energy fuel is added through the pre-chamber. The results show that the throat changes the flow field inside the pre-chamber, impacts the air-fuel ratio, stratification, turbulence, jet dynamics, and ultimately the pre and main chambers combustion processes and heat fluxes. The combustion regime according to the Borghi-Peters diagram were found to lay in the thin reaction zone and in the flamelet regime.
查看更多>>摘要：Modern electric arc furnace (EAF) steelmaking relies on the coherent jet burner to melt scrap at the cold spots inside the furnace, but the relevant computational fluid dynamics (CFD) modeling of this process has not been reported and systematically studied. The present work established an innovative comprehensive CFD model including a scrap melting model and a coherent jet model based on the detailed physical principles, which couples the solid-liquid-gas three-phase system with coherent jet and scrap melting/re-solidification for the investigations of key evaluation factors including burner efficiency, scrap heating rate, and scrap melting cavity. An experiment was designed and implemented specifically in an industrial-scale EAF for validating the proposed model. The model was employed to explore the impacts of burner power, scrap porosity, scrap preheat temperature, and scrap blockage on burner energy utilization. An understanding and guidance of coherent jet burner operations were provided based on the studies for more effective scrap melting.
查看更多>>摘要：The aim of this research is to examine the off-design performance of H+SOFC based on numerical simulations. The model used in the study was discussed in our previous papers thus the study focuses only on the characteristics generated by the model. In the first part of our study, we selected the design point of the cell where the highest possible effectiveness is achieved in view of key parameters: efficiency, power density, operating temperature, and electrolyte thickness. The next stage was to develop off-design maps of the cell where we marked the ranges of fuel cell operation i.e. ranges in which there are no unfavorable parameters - mainly temperature and voltage. The results of the study will be useful for selecting the operational strategy for micro-CHP units based on H+SOFC.
查看更多>>摘要：In this paper, the transient thermal analysis theory of isogeometric dual reciprocity boundary element method (IG-DRBEM) of the multi-patch inhomogeneous complex model is established and the non-iterative inversion method of load identification is proposed based on the IG-DRBEM. The establishment of the multi-patch analysis theory framework extended the ability of IG-DRBEM to analyze complex geometry to some extent. The multi patch analysis method adopted herein is simple and operable, and still retains the advantages of seamless connection between CAD generated geometry and response analysis of isogeometric boundary element method (IGBEM), which is very beneficial to expand IG-DRBEM to practical engineering application fields in the future. The inversion method based on IG-DRBEM makes full use of the good geometrical and field representation ability of NURBS, and further expands the application scope of IG-DRBEM. Furthermore, the introduction of the precision integration method for both forward and inverse problems improves the accuracy and stability of calculation to a certain extent. The numerical results show that the proposed method has good stability and accuracy even when solving complex geometric problems such as a fighter model. Moreover, the implementation of basis function expansion and regularization scheme further improves the accuracy and noise resistance of boundary condition inversion.
查看更多>>摘要：Gas turbine blades require high-performance cooling structures to meet the increasing turbine inlet temperature requirements. The matrix cooling structure has high heat transfer performance, but at the same time brings a large resistance loss. In basic research, the cooling structure is usually modeled according to the similarity criterion, but there is doubt about the generalization of the results obtained from the scaled model to the real-size cooling structure in engineering applications. Therefore, numerical simulation was applied to study the similarity characteristics of matrix cooling performance and the effects of geometric scaling factor in this paper. Through data fitting, it was found that the heat transfer performance and resistance performance increased exponentially with the increase of the scaling factor. As geometric scaling factor shrank 10 times, the heat transfer and resistance loss decreased by 21.9% and 34.7% respectively. The heat transfer and resistance loss of each sub-duct were also markedly affected by the geometric scaling. In small scaling factor cases, the heat transfer was weak in the downstream areas of the cooling channel, and there was an obvious gap in resistance loss for each sub-duct. Due to the same geometric characteristics, the flow characteristics of the channels were less affected by geometric scaling factor, but the relative velocity increased with the decrease of the scaling factor. The exponential regression equations of the heat transfer and resistance with respect to the scaling factor were established by machine learning, providing an accurate and appropriate prediction for engineering application.
查看更多>>摘要：Heat pump drying technology has broad application prospects in sludge drying due to its clean, low-carbon, and high-efficiency features. A field experiment in an electroplating factory was carried out to reveal the variations and distribution of the circulating air in the sludge heat pump dryer (SHPD). Subsequently, the influence of crucial operating parameters on the performance of SHPD was investigated based on the simple variable method. Field data showed that the condensed water generated at the air precooling dehumidifier (APCD) accounted for approximately 20% of the total. The bottom and middle conveyor belts were the major locations for sludge drying, and the top conveyor sludge was mainly in the preheating stage. Increasing the supply air temperature and circulating fan frequency can obtain sludge with lower moisture content. The increased circulation fan frequency raised the specific moisture extraction rate (SMER) from 1.98 kgw/kWh to 2.07 kgw/kWh, while the supply air temperature was reversed. Moreover, extending the sludge residence time in the drying chamber reduced the water removal, which indicated that the adverse effect of increasing the sludge thickness was greater than the beneficial effect of extending drying time. This full-size SHPD field experiment provided reference and support for the application.
查看更多>>摘要：A two-stage absorption heat exchanger (AHE), which consists of two absorption heat pumps (AHP) and a plate heat exchanger (HEX), is designed and utilized in a pressure isolation station in Taian, China. The AHE is usually used to solve the mismatching problem of the heat transfer when there is a high flow rate ratio of the secondary network water to the primary network. Unlike in previously studied case, the flow rate ratio of the AHE in Taian is relatively low (1.7-3.7), giving rise to the research values to test the performance of this special unit and to simulate its practical operating conditions with a numerical model. The heat transfer effectiveness of the two stage AHE is 0.954-1.196 while the heat load rate ranges from 46.8% to 94.1%, and the outlet water temperature of the primary network is averagely 4.93 K lower than the inlet temperature of the secondary network. The AHE system shows better performance than normal PHE system even at a quite low flow rate ratio. Moreover, the numerical model is established, and the simulation results show that the standard deviation of the simulated values to the measured values is less than 1.2 K. Furthermore, some optimization strategies of the AHE are given and several performance characteristics at different flow rate ratios are found based on tested data and simulation results. And a new connection method of the secondary network through the AHE is proposed and three connection methods are compared in this research. The work could further expand the scope of application of the AHE, and contribute to reducing energy consumption in not only the district heating systems but also other energy systems.
查看更多>>摘要：Capillary-driven evaporation of liquids play an important role in the thermal management of electronic devices. The recently developed nano-porous evaporator supported by microchannels proved to be promising in heat dissipation and energy saving. In this work, we proposed novel modifications to the nano-porous evaporator structure which could simultaneously enhance the dry-out heat flux and heat transfer coefficient. These include a non-uniformly distributed nanopore size and a membrane with shortened length. By conserving the farthest nanopore size and enlarging the closer nanopores, or simply removing part of the membrane, the pressure drop and thermal resistance of the evaporator were both minimized. Numerical approach was developed for simulating the heat transfer and fluid flow in the nano-porous evaporator. Kinetic boundary conditions were applied at the liquid-vapor interface to simulate evaporation. Results of temperature and pressure fields were obtained. It was found that the evaporative thermal resistance and the pressure drop in microchannel played dominant roles in determining the evaporator performance. The structure with partial membrane 10 mu m in length had the best heat dissipation performance, which improved the dry-out heat flux and heat transfer coefficient by 22.3% and 139.5%, respectively, compared to the benchmark evaporator structure.
NSTL
Elsevier