科创中国

In this paper, experimental and numerical study were both conducted to investigate the effect of pore size and porosity distribution on radiation absorption and thermal performance of porous solar energy absorber. Ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer was used to measure the transmittance of porous media to reflect its radiation absorption capabilities. Numerical model was established based on the assumption of thermal nonequilibrium condition as well as using P1 model to consider the radiation heat transfer. The UV-Vis-NIR spectrophotometer measurement showed that: (1) With smaller pore size, the spectral transmittance of the porous media would be lower and the solar radiation absorption would be better; (2) Among the materials with different pore size distributions, pore-size-decreased combo and pore-size-increased combo have almost equal absorption coefficient which are higher than that of uniform structure. Numerical simulation demonstrated that: (3) For materials with different pore size distributions, pore-size-decreased structure has the best radiation absorption due to its ability of maximizing the volumetric absorption effect, which is agreed with the UV-Vis-NIR spectrophotometer experimental results; (4) For materials with different porosity distributions, porosity-gradually-increased structure has the highest mean fluid/solid temperatures because it can utilize the enhanced convective/conductive heat transfer to improve the overall thermal performance of porous receiver; (5) Porous structure with pore-size-decreased distribution and porosity-gradually-increased distribution has the best thermal performance of which the mean temperatures of fluid/solid phases are the highest among all the studied cases.

Tao Xie;Kai Di Xu;Bo Lun Yang;雅玲 何

Science China Technological Sciences

2019-12-1

A transient, two-dimensional two-phase mass transport model is applied to investigate the cell dynamic operating behaviors of a liquid-feed direct methanol fuel cell (DMFC). The influences of various processes on the cell dynamics in response to sudden change of cell current density, methanol feed concentration, oxygen feed concentration, and the transient gas-slug blocking in the anode channel are studied. The results reveal that in response to the sudden drop of cell current density and methanol concentration, the cell voltage exhibits overshooting behavior as a result of the interaction between cathode and anode overpotentials with different time responses. The dominant factor causing the long response of cell voltages is the methanol rebalance in the membrane electrode assembly, which usually takes tens of seconds because of the sluggish methanol transport process. Also, it is indicated that in response to temporary blocking of anode diffusion layer surface with gas slug, the cell can still operate normally for a while because the anode diffusion layer serves as the fuel reservoir. It takes over a minute for the cell to break down in this case studied, implying that the cell output can be maintained stable if the gas bubbles or slugs in the anode channel can be removed quickly. However, too long residence time of gas slug in the channel definitely degrades the cell performance.

W. W. Yang;雅玲 何;印实 李

International Journal of Hydrogen Energy

2012-12

3D numerical simulations are performed for laminar heat transfer and fluid flow characteristics of a flat-plate channel with longitudinal vortex generators. The effects of two different type longitudinal vortex generators (LVGs), rectangular winglet pairs (RWP) and delta winglet pairs (DWP), with two different configurations, common-flow-down and common-flow-up, are examined. The Reynolds number based on the height of the channel varies from 190 to 1125. The numerical results show that the applications of the LVGs obviously enhance the heat transfer of the channel, the maximum enhancement of average Nusselt number is 46%. Compared by the performance evaluation parameter, (Nu_m/Nu_m0)/(f/f0), the DWP shows a better overall performance than RWP, the common-flow-down and common-flow-up configurations of DWP have almost same overall performance, the common-flow-down configuration has a better overall performance than the common-flow-up configuration for RWP. The numerical results are also analyzed from the view point of field synergy principle, and it is found that the inherent mechanism of heat transfer augmentation by LVGs is that the secondary flow generated by LVGs results in the reduction of the intersection angle between velocity and temperature gradient.

Li Ting Tian;Yong Gang Lei;雅玲 何

Kung Cheng Je Wu Li Hsueh Pao/Journal of Engineering Thermophysics

2008-12

In this paper a detailed one dimensional nonuniform thermal model of a parabolic trough solar collector/receiver is presented. The entire receiver is divided into two linear halves and two inactive ends for the nonuniform solar radiation, heat transfers and fluid dynamics. Different solar radiation and heat transfer modes can be taken into consideration for these four different regions respectively. This enables the study of different design parameters, material properties, operating conditions, fluid flow and heat transfer performance for the corresponding regions or the whole receiver. Then the nonuniform model and the corresponding uniform thermal model are validated with known performance of an existing parabolic trough solar collector/receiver. For applications, the uniform thermal model can be used to quickly compute the integral heat transfer performance of the whole PTC system while the nonuniform thermal model can be used to analyze the local nonuniform solar radiation and heat transfer performance characteristics and nonuniform heat transfer enhancements or optimizations. Later, it could also be effectively used with an intelligent optimization, such as the genetic algorithm or the particle swarm optimization, to quickly evaluate and optimize the characteristics and performance of PTCs under series of nonuniform conditions in detail.

Ze Dong Cheng;雅玲 何;Yu Qiu

Renewable Energy

2015-2

Nonlinear propagation of dust-ion-acoustic (DIA) waves is investigated in a one-dimensional, unmagnetized plasma containing positive ions, negative ions, trapped electrons featuring vortex-like distribution, and immobile dust grains having both positive and negative charges. Via reductive perturbation method, Agrawal's method, and Euler-Lagrange equation, the time-fractional Schamel-KdV equation under the sense of Riesz fractional derivative is derived to describe nonlinear behavior of DIA waves. The approximate solution of the time-fractional Schamel-KdV equation is constructed in terms of Jacobi elliptic functions by variational iteration method. The effect of the plasma parameters on the DIA solitary waves is also discussed in detail.

Shimin Guo;Liquan Mei;雅玲 何;Yibao Li

Physics Letters, Section A: General, Atomic and Solid State Physics

2016-3-6

The effects of wall axial heat conduction in a conjugate heat transfer problem in simultaneously developing laminar flow and heat transfer in straight thick wall of circular tube with constant outside wall temperature are numerically investigated. The results show that the heat transfer process is most sensitive to wall-to-fluid conductivity ratio k_sf, and when k_sf≤25 the increasing tube thickness and the decreasing k_sf could make the inner wall surface approaching the uniform heat flux condition. It turns out that the basic function of the wall axial heat conduction for the cases studied is to unify the inner wall surface heat flux.

Sun Xiao Zhang;雅玲 何;Guy Lauriat;文铨 陶

International Journal of Heat and Mass Transfer

2010-9

A gradually-varied porous structure is designed to increase the thermal performance of the porous volumetric solar receiver. Based on the replica method and multilayer recoating technique, the silicon carbide porous ceramic with linear-changed geometrical parameters is fabricated. The performances of the uniform and gradually-varied porous volumetric solar receivers are studied by both experiment and numerical simulation. An optimization method combining genetic algorithm and computational fluid dynamics analysis is applied to determine the optimum porosity distribution. The results present that porous volumetric solar receiver with linear-changed geometrical parameters exhibits better thermal performance than the uniform porous volumetric solar receivers, especially when the thickness of the receiver is small. Larger porosity in the front is beneficial for increasing the solar radiation penetration depth, which limits the reflectance and thermal radiative losses. Smaller porosity in the rear traps more solar radiation and increases the convective heat transfer. When the receiver’s thickness is larger, the performance of the gradually-varied volumetric solar receiver is nearly identical to that of the uniform receiver with largest porosity. The double-layer configuration is found to be the optimized structure of the gradually-varied porous volumetric solar receiver. The thermal efficiency could be further improved using genetic algorithm with an 11 K increase of the outlet temperature.

Shen Du;增耀 李;雅玲 何;Dong Li;Xiang Qian Xie;Yang Gao

Science China Technological Sciences

2020-7-1

Black silicon (nanostructured front surface) can significantly improve the efficiency of the silicon solar cell by absorbing more solar irradiance. However, due to the band gap of the semiconductor material, most part of the absorbed solar irradiance is converted into heat to increase the temperature of solar cell. Especially under the concentrating condition, the waste heat is more. Besides, multi-physics coupling problem makes it difficult to study the effects of nanostructures on the performances of the low concentrated black silicon photovoltaic system. In this study, a multi-physics model of a low concentration photovoltaics (LCPV) that employs an all-back-contact black silicon solar cell is built. As for the multi-physics problem in LCPV, a multi-physics coupling mathematical model is developed. Its advantage is to couple near-flied optics, photoelectric conversion, and heat transfer. Furthermore, through parameter sensitivity analysis, it is proved that the coupling mathematical model is more accurate, because it takes full account of the nonlinear correlation between the output power and the temperature, and the effects of series resistance under concentrating condition. Hence, the mathematical model is used to investigate the coupling effects of nanostructures, and it is found that the nanostructure with lower reflectance may not be beneficial for the low concentrated black silicon photovoltaic due to the increased temperature. According to different circumstances, the maximum output power with the lower cost could be achieved by choosing the nanostructure with appropriate reflectance. At last, the dynamic analysis proves that the coupling effects of nanostructure lead to the result that the nanostructure with lower reflectance reduces the annual power generation per square meter of the low concentrated black silicon photovoltaic system by about 116 kW h/m² when the concentration ratio is 10.

Yi Peng Zhou;Ming Jia Li;雅玲 何;印实 李

Energy Conversion and Management

2018-3-1

Dynamic response characteristics of a PEM Fuel cell were experimentally studied in this paper. The parameters subject to abrupt change include current, voltage and gas flow rate. It is found that with an abrupt change of current and voltage at low gas flow rate a steady state can be reached at much slower speed than high gas flow rate, while with an abrupt change of gas flow rate, the fuel cell can reach its steady state sooner at small current than at large current.

Xing Wei;Hong Lin;Jin Xuan Fan;雅玲 何;文铨 陶

2009

In this paper, the studies on the characteristics with time and failure criterion in the solar molten salt cavity receiver are carried out based on the coupled photon-thermal-stress integration method. Especially, the truly non-uniformed solar heat flux in the cavity receiver with different time are simulated using the optical simulation approach coupling the Monte Carlo ray tracing and Gebhart methods proposed by our team. Besides, the complex coupled heat transfer process of conduction, convection and radiative in the receiver are also comprehensively considered. Based on the integrally model, the solar flux, temperature and thermal stress on the absorber surfaces with different time are obtained. Then, the influences of different tube materials, flow layouts of molten salt on the coupling performances of photon-thermal-stress conversion process are discussed. Finally, the stress failure criterion in the molten salt cavity receiver is proposed based on the fracture mechanics theory. The results show that the temperature and thermal stress are influenced significantly by the non-uniformed solar flux, the distribution of temperature and thermal stress are also extremely non-uniformed. The solar flux and thermal stress change remarkably with different time, the peak heat flux and stress are maximum at 12:00 noon, and the peak thermal stress could increase by 11% and 10% compared with 11:00 and 13:00. It is also found that the peak thermal stress would be reduced effectively when the receiver tube with better thermal conductivity property and the molten salt flow layout which is from low-flux area to high-flux area are adopted. For example, compared with 800H receiver tube, the peak thermal stress of 316H tube could be reduced by 37%. Meanwhile, the peak thermal stress of flow layout which is from low to high flux area would be 20% lower than that with the opposite layout. The fracture toughness of receiver tube, which is influenced by the molten salt corrosion, is one of the key parameters to assess stress failure. Based on the stress failure criterion of critical solar flux proposed by this paper, the critical heat flux of stress failure would exhibit a decreasing trend when the fracture toughness degradation is serious in the molten salt receiver.

雅玲 何;Baocun Du;Kun Wang;Yu Qiu;Zhanbin Liu

Kexue Tongbao/Chinese Science Bulletin

2017-12-25