查看更多>>摘要：The use of cooling garments is emerging as an effective, convenient, and energy-efficient way to maintain thermal comfort, which is crucial for conserving physical and psychological health and for avoiding potentially life-threatening situations. Current cooling garments are generally bulky and have short operating periods, uncontrollable cooling capacity, and low effectiveness. Here, a novel man-portable cooling garment based on thermoelectric refrigeration was designed, and the heat generated by body is absorbed by capillary tube and dissipated in semiconductor refrigeration system. The coolant is circulated in the pipeline to recover residual cold energy and improve the thermal stability of the system. Several performance tests were conducted in a simulated hot environment to evaluate and to optimize refrigeration efficiency and cooling effectiveness. The results indicated that the system was capable of providing a water temperature of 15.7 degrees C and cooling power of 340.4 W, with a coefficient of performance of 3.40 at 100 W of electrical power and a climatic chamber at 30 degrees C. Additionally, the operating environment of the system significantly influenced the cooling capacity of the garment; and the cooling power of the system was significantly improved by increasing the heat dissipation capacity. The increase in electrical power was limited by constant heat dissipation capacity restrictions. The wearing trial confirmed the effectiveness of the thermoelectric cooling garment. Furthermore, our study improved the understanding of performance characteristics for the thermoelectric cooling garment, and a road map for the further development of thermoelectric cooling garments was suggested.
查看更多>>摘要：In-cylinder thermal barrier coatings (TBCs) reduce heat transfer losses and increase thermal efficiency. It has been shown that thick TBCs negatively impact the performance of conventional combustion modes by degrading volumetric efficiency and increasing the propensity for end-gas knock. Low-temperature combustion (LTC) is an advanced combustion strategy that offers high efficiencies and low emissions. Due to the nature of kineticsdriven autoignition, LTC is fundamentally different from the conventional combustion modes, where the benefits and tradeoffs of thick TBCs need to be re-evaluated. Previous experimental studies showed the feasibility and the efficiency gains associated with a 2 mm thick TBC applied to the piston surface, as well as the reduction in the required intake temperature with no observable deterioration on the high load limit. However, the effects of TBCs and their independent thermophysical properties on LTC have not been systematically explored. It is necessary to perform a comprehensive study on the effects of TBC on LTCs from a fundamental thermodynamic perspective, which serves as the motivation for the current study. This study couples a 0D engine thermodynamic model to a 1D transient heat transfer model of the coating and piston. The model was first validated against the metal piston baseline, followed by validation against experimental data of the TBC cases at different engine loads. With confidence established in the model's fidelity, three parameters are investigated independently: thermal conductivity (k), coating thickness, and volumetric heat capacity (s). The results revealed that the volumetric efficiency actually increases by 7.4 percentage points with a thicker coating due to a reduction in heat transfer during the compression stroke, which allows for a lower intake temperature requirement to reach autoignition. However, there is a thickness limit before the intake temperature becomes impractically low. The results show that elevating surface temperature is directly proportional to higher efficiency. Therefore, the optimal coating configuration for kinetically-controlled LTC is a combination of the lowest k, thickest coating before reaching the limit, and the lowest s, where the low k and high thickness contribute the most thermal efficiency gains (4.1 percentage points) and increased exhaust enthalpy (5.7%).
查看更多>>摘要：Three-dimensional swirling impinging jets using the aerodynamic swirl generator are carried out to understand the pure effects of swirl. A shear stress transport model with cross-diffusion correction (SSTCD) is validated for the above flows with the documented experiments. Based on the good performance of the SSTCD model, a detailed flow analysis for Re = 23,000 and swirl numbers of 0 and 0.45 at nozzle-plate spacings of 2 and 6 is presented in terms of the flow structures, mean velocity field, wall shear stress and heat transfer. The results show that the strength of the swirl is reduced with increasing the nozzle-plate spacing near the wall. On the contrary, the effect of swirl on both flow fields and heat transfer is more evident at high nozzle-plate spacing, leading to a broader impact region along the impinging plate. However, downstream, the effect of swirl can be ignored for the above fields at all nozzle-plate spacings. In addition, the swirl motion contributes to high turbulence but the heat transfer rate still decreases with an increase of swirl number. Meanwhile, the uniformity of heat transfer is improved by the swirl motion for both nozzle-plate spacings, except that near the stagnation region at H/D = 2. It is also found that this improvement depends on the Reynolds number.
查看更多>>摘要：Currently there still exist a large base of subcritical coal-fired boilers remaining in operation worldwide. Improving their thermal efficiency is critical for these units to reduce the cost of electricity and meet the increasingly stringent efficiency regulations. This paper presented an economic high temperature upgrading retrofit solution for these subcritical units in which the final superheat and reheat steam temperatures are elevated from 540 degrees C to 600 degrees C level while the steam pressures remain at subcritical such that the retrofit can be made with minimum modifications to the boiler to reduce retrofit cost and technical risk. A systematic analysis was first conducted to deduce the best boiler heating surface modification option to achieve such significant increase of boiler steam temperatures. The analysis shows that replacing part of the furnace water wall below the furnace exit with wall superheater is the most viable and economic boiler modification design. A threedimensional computational fluid dynamics (CFD) simulation was then conducted for a typical 320 MW subcritical boiler design to verify the feasibility of this design and determine the area of furnace wall that needs to be modified to achieve the target steam temperatures. The results show that with only 3.6 m height of furnace wall replaced with wall superheater both the superheat and reheat steam temperatures can reach the target 600 degrees C level even without any modification made to the reheaters. With this heating surface modification design, the efficiency of the existing subcritical power plants can be improved considerably with minimum boiler retrofit work. More importantly, this technology is going to help the plant owners meet the increasingly stringent efficiency and emission regulations mandated by the government and place them in a better position to continue operating these units.
Park, Seong HyunHa, Man YeongKim, Kyung ChunYoon, Sang Youl...
12页
查看更多>>摘要：A numerical investigation of a design modification of a two-dimensional supersonic gas ejector was undertaken via an adjoint method, with a focus on enhancing the performance of the ejector. Maximization of the secondary mass flow rate was used as an objective function. The entrance geometry and throat height of the primary nozzle were fixed, but the other geometries were allowed to vary during the design modification. The shape of the divergence of the primary nozzle, the nozzle exit position (NXP), and the entrance region of the mixing chamber were modified simultaneously using the adjoint method. The height of divergent section of primary nozzle (Hne), NXP, and the inlet height of the mixing chamber (Hmi) were enlarged by about 13.33%, 27%, and 49%, respectively, resulting in an entrainment ratio (ER) that was enhanced by about 37.17% compared to that obtained for the baseline geometry. Two simplified geometries from the adjoint modification were suggested, and have the same performance with the adjoint-modified geometry. Further investigation was performed by varying the height of the straight section of the mixing chamber of the simplified geometry. The results show that the ER increased by about 46.77% compared with that obtained using the simplified geometry.
查看更多>>摘要：The application of the dynamic integrated method, implemented in the form of a linear regression based on averages over a certain period for the evaluation of the thermal performance of the building envelope, is presented and discussed. Previous researches and results on this method are corroborated and extended. A data set obtained from two well insulated twin houses, located in Germany, measured during the winter period, was analysed. Both dwellings were unoccupied but the heating and ventilation systems were operated to simulate the behaviour of a family with two children. The analysis was focused on obtaining accurate heat transfer coefficient (HTC) and solar aperture (gA) estimates characterising the behaviour of the building envelope regarding heat loses and solar gains respectively. To do so, different models for the main heat exchange phenomena were evaluated, finding precise models for the solar gains and the ventilation system. Different options were considered for the dependent variable in the regression equation and the most suitable was determined. The optimum average period was also identified. Finally, the selected model was tested to find the minimum number of days required to obtain accurate results. The consistency of the results obtained from different data series is considered as key validation criteria. With a test duration of 18 days, HTC and gA estimates were within +/- 5% and +/- 8% from the mean, respectively, in both houses.
查看更多>>摘要：To evaluate the potential of inhomogeneous thermoelectric materials, the hybrid system model comprised of a molten carbonate fuel cell, a thermoelectric generator with inhomogeneous heat conduction and a regenerator is established. In particular, thermal conductivity of thermoelectric materials is regarded as a spatial dependence coefficient. Taking electrochemical and thermodynamic irreversible losses into account, the expressions of hybrid system's equivalent output power and efficiency are deduced and optimal operating current density interval of proposed system is determined. The maximum output power density and efficiency of the hybrid system is 35.4% and 6.9% higher than stand-alone molten carbonate fuel cell, respectively. It is proved that inhomogeneous thermoelectric materials are capable of significantly enhancing the hybrid system performance, since the efficiency can be improved by 25.4% and the optimal operating interval is shifted to a lower current density direction compared with that with homogeneous materials. Finally, several critical parameters on the system performance are analyzed through further discussions of the established model. The results obtained can sever as a theoretical guidance for the optimization of thermoelectric integrated system.
查看更多>>摘要：Nuclear power plants mostly use the multimodule once-through steam generator (OTSG) parallel system as heat exchange equipment, but the current research mostly focuses on a single OTSG. The huge system brings great challenges to modeling and control system design. In this paper, the ODE method is used to model the main equipment of a multimodule parallel system based on mechanism. The results show that the maximum error is 3.77%. Based on the characteristics of parallel system, a new control strategy is proposed and compared with other control strategies by introducing four typical disturbances. The results show that the proposed feedwater control scheme can effectively solve the coordination between loop valve and module valve of multimodule parallel OTSG system, and the coordination between main feedwater pump and loop valve. Compared with Method 1, the response time of Method 2 in sodium temperature disturbance is shortened by 319 s and the temperature fluctuation is reduced by 1.67 times, but Method 2 has potential safety hazards. Method 1 can improve the system response speed when the sodium flowrate and water flowrate change. Compared with Method 3, the outlet sodium temperature fluctuation of Method 1 is reduced by 10 times and the adjustment time of Method 1 shortens 141.9 s in the case of feedwater flow disturbance.
查看更多>>摘要：To improve the heat transfer performance in the anti-gravity direction, the multi-layer wick heat pipe with three pore sizes was fabricated. The wick consisted of a coarse powder layer and a nanoporous fine copper powder layer. The critical heat load of the multi-layer wick was as high as 79 W in the 90 degrees direction, while the homogeneous wick sintered from coarse powder and nanoporous fine powder is only 23 W and 30 W, respectively. Multi-layer wick heat pipes with high heat transfer capacity will broaden their applications in terrestrial environments.
查看更多>>摘要：In this study, a novel variable-length intake manifold is designed to increase the volumetric efficiency of a four cylinder gasoline engine. The proposed intake manifold is designed with four independent throttle bodies on its inlet duct, which are separately connected to the atmosphere. By opening and closing each one of these throttle bodies, a different length of the intake manifold is obtained. The novelty of this study lies in the fact that in addition to the variable-length system at each engine speed, it uses a different combination of opened throttle bodies to further increase volumetric efficiency. To demonstrate the efficiency of the novel design, a onedimensional model of the Stock engine is developed using the GT-Power software in wide-open throttle steady-state condition and then validated with experimental results. Five vital criteria are defined for evaluating all of the thermo-fluid, heat, and mass transfer characteristics of the engine. The result of using the novel proposed design indicates that the volumetric efficiency, brake torque, brake specific fuel consumption, brake thermal efficiency, and heat release rate of the engine compared to the stock intake manifold were improved by 6.33%, 7.23%, 0.83%, 1.77%, and 11.79% respectively. Based on the obtained results, the best performance is achieved at 3500 rpm almost in all conditions. At this condition, the air and fuel mass flow rates and brake power are 0.0574 kg/s, 0.0041 kg/s, and 56.42 kW, respectively.
NSTL
Elsevier