基于有限元分析的高压喷射式熔断器弧前时间仿真计算

弧前时间是描述熔断器开断性能的重要参数,针对熔断器弧前时间仿真计算研究的不足,结合熔断器在系统中的等值电路和熔丝在短路电流故障下的弧前热电耦合数学方程,以熔丝三维几何模型为基础,基于多物理场仿真软件COMSOL Multiphysics和有限元理论建立了熔丝弧前时间计算模型。计算模型分析了熔丝在短路电流作用下的温度场,绘制了熔丝的最大温度随时间的变化曲线,以熔体熔化到电弧出现瞬间的时间间隔为熔断器的弧前时间,并与试验数据做对比,验证了此计算方法的可行性。研究结果对产品改进提供了依据。     

Numerical simulations of solar chimney power plant with radiation model

A three-dimensional numerical approach incorporating the radiation, solar load, and turbine models proposed in this paper was first verified by the experimental data of the Spanish prototype. It then was used to investigate the effects of solar radiation, turbine pressure drop, and ambient temperature on system performance in detail. Simulation results reveal that the radiation model is essential in preventing the overestimation of energy absorbed by the solar chimney power plant (SCPP). The predictions of the maximum turbine pressure drop with the radiation model are more consistent with the experimental data than those neglecting the radiation heat transfer inside the collector. In addition, the variation of ambient temperature has little impact on air temperature rise despite its evident effect on air velocity. The power output of the SCPP within the common diurnal temperature range was also found to be insensitive to ambient temperature.     

Multi-stress aging of stator bars with electrical, thermal, and mechanical stresses as simultaneous acceleration factors

Accelerated aging tests have been carried on stator bars under simultaneous electrical, thermal and mechanical multi-stress conditions, using a simulated three-phase model stator. Over each thermal load cycle, the partial discharge activity has been found to be a strong function of temperature. Tests at twice rated electrical stress did not result in any bar specimen failures up to a total of 1500 load cycles. However, when the three phase load current was increased above its rated value, current-limited type failures were observed to occur below 1000 load cycles, thereby indicating a marked influence of thermal and mechanical stresses associated with the elevated three phase load current.     

An Efficient Summer Operation Scheme for a Thermal Power Plant Air Cooling System based on Electric Peak Shaving

We proposed a novel efficient operation scheme for a thermal power plant’s air-cooling system based on peak shaving, in order to cope with high ambient temperature in summer. We introduced an absorptiongeneration equipment with water/lithium working pairs into the air cooled condenser(ACC) to reconstruct the traditional thermal power plant, and established a dynamic thermodynamic model adopting Ebsilon code. We studied the thermodynamic performance variation of the reconstructed thermal power plant throughout a 24-hour cycle and found that the fluctuation ratio of the turbine back pressure decreased to 6% from 78%, which is beneficial for the stable and safe operation of the electric power system. The thermal performance improvement benefited from the exploitation of the heat transfer potential of ACC, which realized via cold duty schedule throughout the day, under different ambient temperature conditions. In this system, the generated power was higher at relatively high ambient temperature than that at relatively low ambient temperature, which solved the electricity demand-supply imbalance problem under high ambient temperature. Finally, the same optimization effects for power thermal plants with an indirect air-cooling system were obtained using the same operation scheme.     

Temperature control strategy for polymer electrolyte fuel cells

A polymer electrolyte fuel cell (PEFC) is an electrochemical device that converts chemical energy directly to electrical energy, and its performance greatly depends on its operating temperature. Therefore, in this paper, a novel thermodynamic PEFC model with the airflow cooling method is firstly developed for the PEFC system. Then, a novel model predictive control (MPC) controller is designed to control the stack temperature at an optimal value by adjusting the air flow rate on the basis of the developed thermodynamic PEFC model. The thermodynamic PEFC model and the designed controlling strategies are simulated and analysed in Matlab/Simulink. Three tests are conducted to estimate the reliability of the developed controllers concerning different operating conditions: (a) typical perturbation in the current load, (b) any perturbation in the current load, and (c) variation of the ambient temperature. The simulation results demonstrate that the MPC controller can effectively control the stack temperature at the desired value. Moreover, the MPC controller shows much superior effects compared with the conventional proportional integral derivative (PID) controller. In addition, the developed coolant circuit model can be easily applied to various PEFC systems. The MPC controller shows potential also for other controlling issues of PEFC systems due to its strong robustness and fast response.     

A dynamic model of air-breathing polymer electrolyte fuel cell (PEFC): A parametric study

A dynamic model for an air-breathing PEFC has been built to investigate the transient response of the fuel cell to load changes. The sensitivities of the dynamic response, as well as the steady state performance, to: the ambient temperature and relative humidity; the thickness and the thermal conductivity of the cathode GDL; and the fuel utilisation, have been studied. A previously-developed steady-state model of the fuel cell was linked to the dynamic model to feed the latter with the data of the cell temperature as it changes with the current density. It was found that, when there are sudden changes to high loads, there exist optimum values for the ambient temperature and GDL thickness at which the overshoots are mitigated and the steady state performance is improved. Further, the transient and steady state performance were found to improve with increasing the ambient relative humidity and GDL thermal conductivity. Finally, the fuel utilisation was found to have no impact on the dynamic response of the fuel cell. All the above findings have been presented and discussed in the paper.     

Degradation analysis of a heavy-duty gas turbine engine under full and part load conditions

This paper describes the degradation analysis and the performance diagnostics of the gas-turbine (GT) cycle of a combined cycle power plant (CCPP). Three different operating loads, which are 100%, 75%, and 50%, were tested at different ambient conditions, namely, temperature, pressure, and humidity. First, a degradation model to simulate the GT performance with these various operating loads and conditions has been developed. It is then demonstrated how this degradation affects the GT and its components. The degradation analysis has been performed on 2500 readings obtained during 2 years of operation. After applying the load determining criteria, 60 readings were obtained to represent the full load operation and 40 reading points for each part-load operation. The degradation analysis has been carried out on the basis of actual data obtained from a CCPP; this differentiates this study from the others in the same area. Based on the commissioning test performance of the GT cycle, the model has been validated. The results show that the rate of degradation increases dramatically as the load increases. Moreover, the degradation rate also increases with an increase in the ambient air temperature. However, the degradation rate for the various studied parameters, namely, polytropic efficiencies, GT exhaust mass flow rate, and the overall GT efficiency, has been found to decrease with time. The maximum degradation percentage has been estimated to be −1.71% at full load conditions in comparison with −1.33% and −1.16% at 75% and 50% load, respectively.     

Thermal load deviation model for superheater and reheater of a utility boiler

The extreme steam temperature deviation experienced in the superheater and reheater of a utility boiler can seriously affect its economic and safe operation. This temperature deviation is one of the root causes of boiler tube failures (BTF), which causes about 40% of the forced power station outages. The steam temperature deviation is mainly due to the thermal load deviation in the lateral direction of the superheater and reheater. This variation is very difficult to measure in situ using direct experimental techniques. In this paper, we propose a thermal load model that is based on the power plant thermodynamic parameters, thermal deviation theory, and flow rate deviation theory. It is found that the calculated results from our model agree well with the in situ experimental results. The predicted BTF positions are the same as that in the reheater of a 300 MW utility boiler at Wujing Power Plant. The proposed model has been used to improve the design of utility boiler in Boiler Works, predict the possible BTF in the design stage, and assess the existing designs. This model can also be applied to utility boilers of different manufactures, and has been successfully applied to the BTF prediction and prevention in the Power Station.     

Simulation of instantaneous thermal fluxes in solar walls by polynomial approximations

By solving the heat-conduction equation with polynomial driving functions, the time evolution of the thermal fluxes for external surface excitations representative of different ambient conditions has been obtained. The error involved in the determination of the initial thermal profile of the wall and the ‘numerical coupling’ between successive periods, such as the diurnal and nocturnal ones, have been considered. Also, the case in which the surface temperature is assumed to be ‘floating’ with a specified inner ambient temperature has been analysed. The proposed method is well adapted for the treatment of non-repetitive ambient conditions. We suggest applications for conventional walls.     

Anion exchange membrane fuel cell modelling

A parametric model predicting the performance of a solid polymer electrolyte, anion exchange membrane fuel cell (AEMFC), has been developed, in Matlab environment, based on interrelated electrical and thermal models. The electrical model proposed is developed by modelling an AEMFC open-circuit output voltage, irreversible voltage losses along with a mass balance, while the thermal model is based on the energy balance. The proposed model of the AEMFC stack estimates its dynamic behaviour, in particular the operating temperature variation for different discharge current values. The results of the theoretical fuel cell (FC) stack are reported and analysed in order to highlight the FC performance and how it varies by changing the values of some parameters such as temperature and pressure. Both the electrical and thermal FC models were validated by comparing the model results with experimental data and the results of other models found in the literature.     

Transient behavior of a radiative distiller/condenser system

In this work, a mathematical model is proposed to describe the thermal performance of a radiative distiller under transient conditions. The parameters which cause the dynamic variation in the condenser performance are the finite thermal capacity of the radiative condenser panel, effective sky-temperature, ambient temperature, humidity ratio and the condensers overall heat transfer coefficient. The presented model is solved numerically and the effects of the design and operating conditions on the condensers performance are investigated.     

附加阳光间型被动式太阳房传热量简化计算方法研究

附加阳光间型被动房传热量受室外空气温度和太阳辐射双波动扰动影响,导致其热负荷具有显著波动特征,传统稳态计算方法难以适用,而动态方法过于复杂,不适用于工程设计使用。基于此,该文采用周期反应系数法研究结构参数和被动房传热量之间的定量关系,并结合结构参数,提出附加阳光间型被动房热负荷简化计算新方法。结果表明阳光间总传热量受公用墙体构造变化影响较大,内门传热量与公用墙体热阻呈正相关,墙体传热量与墙体热阻呈负相关;同类围护结构下,不同参数组合下公用墙导热量与阳光间空气温度变化规律趋势一致,进而提出附加阳光间导热变化系数和温度变化系数,以该参数为基础建立了关键结构参数与传热量之间的多元回归模型,该简化计算方法与实测值对比结果表明吻合度较大,简化计算方法精度较高,可供实际工程设计使用。     

An improved thermal control of open cathode proton exchange membrane fuel cell

Proton exchange membrane fuel cell is a well-known technology that has shown high efficiency and performance as a power system compared to conventional sources such as internal combustion engines. Especially, open cathode proton exchange membrane is growing more popular thanks to its simple structure, low cost and low parasitic losses. However, the open cathode fuel cell performance is highly related to the operating temperature variation and the airflow rate which is adjusted through the fan voltage. In this regard, the present study investigates the thermal management of an open cathode proton exchange membrane fuel cell. The objectives are the stack performance improvement and the stack degradation prevention. Indeed, a safety and optimal operating zone governed by the load current, the stack temperature and the air stoichiometry, is designed. This optimal operating zone is defined based on the system thermal balance and the operating constraints. Hence, the proposed control strategy deals concurrently with the stack temperature regulation and the air stoichiometry adjustment to guarantee the goals achievement. The performance of the proposed control strategy is verified through experimental studies with different operating conditions and results prove its efficiency. To properly design an appropriate control strategy, a multiphysic fuel cell model is developed based on acausal approach by mean of Matlab/Simscape and experimentally validated.     

Oil strategies benefits over different driving cycles using numerical simulation

95 g/kin is the allowed quantity of CO2 emission normalized to NEDC to be set in 2020.In addition,NEDC will be replaced by more severe driving cycles and will be united worldwide.To respond to those criteria,automotive industries are working on every possible field.Thermal management has been proved to be effective in reducing fuel consumption.Cold start is a primordial reason of overconsumption,as the engine highest efficiency is at its optimal temperature.At cold start,the engine's oil is at its lowest temperature and thus its higher viscosity level.A high viscosity oil generates more friction,which is one of the most important heat losses in the engine.In this paper,hot oil storage is studied.Numerical simulations on GT-suite model were done.The model consists of a 4-cylinder turbocharged Diesel engine using a storage volume of 1 liter of hot oil.Ambient temperature variation were taken into consideration as well as different driving cycles.Furthermore,different configurations of the thermal strategy (multifunction oil sump) were proposed and evaluated.Lubricant temperature and viscosity profiles are presented in the article as well as fuel consumption savings for different configurations,driving cycles and ambient temperatures.     

Cell and stack‐level study of steady‐state and transient behaviour of temperature uniformity of open‐cathode proton exchange membrane fuel cells

The steady‐state temperature uniformity and thermal transients of open‐cathode proton exchange membrane fuel cell (PEMFC) at cell and stack level are researched experimentally in this study. The local temperatures are obtained by 30 thermocouples contacting the surfaces of cathode gas diffusion layers (GDL). The s temperature homogeneity under different load currents and air flow rates are investigated. The results reveal that the fluctuation of temperature distribution under different currents is small under the lowest air flow rate set in the experiments. Comparatively, the temperature is less uniform when the load current is higher under other air flow rates. The evaluation indicator, temperature uniformity index (TUI), varies nearly linearly with the current. And the maximum variation is 55.6% to 59.0%. This distinct behaviour is probably related to the existence of liquid water and its nonuniform distribution which can enlarge the temperature difference at high current. With respect to thermal transients, there is rapid deterioration in temperature uniformity when the load current is stepped up. It may arise from the uneven liquid water distribution which can lead to different temperature variation rates. Further, the research gives direction for optimization of cooling strategy and thermal management of open‐cathode PEMFC stack in application.     

Temperature and wind speed impact on the efficiency of PV installations. Experience obtained from outdoor measurements in Greece

Although efficiency of photovoltaic (PV) modules is usually specified under standard test conditions (STC), their operation under real field conditions is of great importance for obtaining accurate prediction of their efficiency and power output. The PV conversion process, on top of the instantaneous solar radiation, depends also on the modules' temperature. Module temperature is in turn influenced by climate conditions as well as by the technical characteristics of the PV panels. Taking into consideration the extended theoretical background in the field so far, the current study is focused on the investigation of the temperature variation effect on the operation of commercial PV applications based on in-situ measurements at varying weather conditions. Particularly, one year outdoor data for two existing commercial (m-Si) PV systems operated in South Greece, i.e. an unventilated building-integrated (81 kW_p) one and an open rack mounted (150 kW_p) one, were collected and evaluated. The examined PV systems were equipped with back surface temperature sensors in order to determine module and ambient temperatures, while real wind speed measurements were also obtained for assessing the dominant effect of local wind speed on the PVs' thermal loss mechanisms. According to the results obtained, the efficiency (or power) temperature coefficient has been found negative, taking absolute values between 0.30%/°C and 0.45%/°C, with the lower values corresponding to the ventilated free-standing frames.     

Effect of ambient conditions on the first and second law performance of an open desiccant cooling process

An open desiccant cooling process is presented and applied to ventilation and recirculation modes of the system operation. The cooling system consists of a desiccant wheel, a rotary regenerator, two evaporative coolers, and a heating unit. Certain ideal operating characteristics based primarily on the first law of thermodynamics are assumed for each component. The system with indoor and outdoor ARI conditions has a thermal coefficient of performance (COP) of 1.17 in ventilation mode and 1.28 in recirculation mode. A second law analysis is also performed and at ARI conditions, the reversible COP of the system is determined to be 2.63 in ventilation mode and 3.04 in recirculation mode. Variation of the first and second law based COP terms and cooling load with respect to ambient temperature and relative humidity are investigated in both modes of the system operation. The results of the analysis provide an upper limit for the system performance at various ambient conditions and may serve as a model to which actual desiccant cooling systems may be compared. As an additional study, a non-ideal system operation is considered and it is determined that both the COP and cooling load decrease with increasing ambient temperature and relative humidity, and they approach zero at high values of ambient temperature and humidity.     

Field study of various air based photovoltaic/thermal hybrid solar collectors

In the present work a comparative study for thermal and electrical performance of different hybrid photovoltaic/thermal collectors designs for Iraq climate conditions have been carried out. Four different types of air based hybrid PV/T collectors have been manufactured and tested. Three collectors consist of four main parts namely, channel duct, glass cover, axial fan to circulate air and two PV panels in parallel connection. The measured parameters are, the temperature of the upper and the lower surfaces of the PV panels, air temperature along the collector, air flow rate, pressure drop, power produced by solar cell, and climate conditions such as wind speed, solar radiation and ambient temperature. The thermal and hydraulic performances of PV/T collector model IV have been analyzed theoretically based on energy balance. A Matlab computer program has been developed to solve the proposed mathematical model.The obtained results show that the combined efficiency of collector model III (double duct, single pass) is higher than that of model II (single duct double pass) and model IV (single duct single pass). Model IV has the better electrical efficiency. The pressure drop of model III is lower than that of models II and IV. The root mean square of percentage deviations for PV outlet temperature, and thermal efficiency of model IV are found to be 3.22%, and 18.04% respectively. The calculated linear coefficients of correlation (r) are 0.977, 0.965 respectively.     

A spherical cell model for multi‐component droplet combustion in a dilute spray

A model for sphericosymmetric thin‐flame combustion of a multi‐component fuel droplet in a dilute spray has been developed using a unit cell approach. The gas‐phase transport has been modelled as convective–diffusive while the liquid‐phase processes as transient–diffusive. Convective heat and mass transfer condition has been used at the cell surface. The results indicate that evaporation and combustion characteristics of the droplet are strongly affected by the variation of both ambient conditions and convective transfer coefficients. Using the model, the effects of droplet spacing in spray, ambient oxidizer concentration, ambient temperature and pressure have been considered. Droplet life increases with decrease in droplet spacing, ambient temperature and ambient oxidizer concentration. However, droplet life has a weak dependence on ambient pressure. Copyright © 2001 John Wiley & Sons, Ltd.     

600 MW超临界锅炉高温过热器T23和T91金属氧化膜热应力分析

以某600 MW超临界锅炉高温过热器为研究对象,分析了氧化膜厚度、生长温度和热膨胀系数等参数对金属管内蒸汽侧氧化膜热应力的影响.基于有限元方法建立了圆管数值计算模型,计算得到不同厚度和不同生长温度下T23和T91金属蒸汽侧氧化膜从生长温度冷却至常温时的热应力变化,并依据热应力的变化特征确定了这2种金属管材蒸汽侧氧化膜易发生脱落的温度范围.结合锅炉出口蒸汽温度与高温过热器指定位置处氧化膜温度的关系,提出用出口蒸汽温度判别氧化膜是否处于易发生脱落的温度范围的方法,给出了该锅炉停炉时的出口蒸汽温度值.结果表明：2种金属管材在冷却过程中低于一定温度时,其氧化膜的应力达到或接近最大应力值,可以依此判断氧化膜是否处于易发生脱落的危险区域.