A 3D Monte Carlo ray tracing model was developed in FORTRAN for a linear Fresnel reflector solar collector in order to investigate and optimize its optical performance. The performance of three typical mirrors, the effects of the mirror aim lines, slope error, and location, etc. were investigated. The results show that the optimal radius and focal length could be found for the cylindrical and parabolic mirrors, respectively. The optimized cylindrical mirror could achieve the same performance as that of the optimized parabolic mirror. The uniformity of the concentrated solar flux on the absorber tube could be improved by designing the aim lines and using the mirror with a certain slope error. In addition, the LFR system whose yearly mean optical efficiency ranges between 52% and 37% from N20° to N50° could be applied in most area of China.
Yu Qiu;雅玲 何;Ze Dong Cheng
Kung Cheng Je Wu Li Hsueh Pao/Journal of Engineering Thermophysics
In this paper, a novel parabolic trough solar receiver-reactor (PTSRR)system of a locally-installed Kenics static mixer (KSM)is proposed for efficient solar thermal hydrogen production. A three-dimensional comprehensive model was established for PTSRRs of the methanol-steam reforming reaction (MSRR)for hydrogen production, by combining the Finite Volume Method and the Monte Carlo ray-tracing method with a MSRR comprehensive kinetic model. The validated model was preliminarily applied to study the effects and mechanisms of the concentrated solar flux nonuniformity and the locally-installed KSM on PTSRR photo-thermal-chemical comprehensive characteristics and performance, taking the methanol flow rate and the catalyst sintering temperature limitation into account. With a preliminary optimization on the concentrated solar flux nonuniformity, the optical efficiency and the solar flux nonuniformity are improved by 6.58% and 30.42% respectively. It is further revealed that these PTSRRs of better concentrated solar flux density nonuniformity also have better thermal-chemical comprehensive characteristics and performance. Novel PTSRRs of the locally-installed KSM have better comprehensive characteristics and performance than corresponding original PTSRRs or even optimized PTSRRs, with a maximum increase in the methanol conversion rate of 6.92%. It thus will operate more safely and more efficiently, by the cost of a little more pump power to overcome corresponding larger flow resistance caused by the locally-installed KSM. From the mechanism, this kind of novel PTSRR of a locally-installed KSM provides a useful option of high potential for improving uniformities of a series of key field variables in the whole photo-thermal-chemical conversion process, and thus improves the comprehensive characteristics and performance of PTSRRs.
Ze Dong Cheng;Jing Jing Men;Shi Cheng Liu;雅玲 何
A numerical study is reported to investigate the liquid film cooling in a rocket combustion chamber. Mass, momentum and heat transfer characteristics through the interface are considered in detail. A marching procedure is employed for solution of the respective governing equations for the liquid film and gas stream together. The standard turbulence k-ε model is used to simulate the turbulence gas flow and a modified van Driest model is adopted to simulate the turbulent liquid film flow. Radiation of gas stream is also considered and simulated with the flux model. Downstream of the liquid film the gaseous film cooling is numerically studied simultaneously. Results are presented for a mixed gases-water system under different condition. Various effects on the liquid film length are examined in detail. There is a good agreement between the numerical prediction and experimental result on the liquid film length.
H. W. Zhang;文铨 陶;雅玲 何;W. Zhang
International Journal of Heat and Mass Transfer
In this paper, an integrated numerical model was established to solve the complex energy transfer in a parabolic trough collector (PTC) by combining Monte Carlo ray tracing and finite volume method. Based on the model, the thermal performance of the PTC using supercritical CO2(s-CO2) as the heat transfer fluid (HTF) was studied and analyzed. The results indicate that the solar fluxes on the receiver walls are non-uniform, which results in the non-uniform temperatures on the walls and in the s-CO2. Moreover, the optical efficiency of 84.19% is achieved at normal incidence. The circumferential temperature difference (ΔTc) of the absorber is found to be within 18–60 K under typical conditions, and it decreases with increasing inlet velocity (vin) and decreasing inlet temperature (Tin). Studies on the flow and heat transfer characteristic indicate that the secondary flow occurs on the cross-section due to the buoyancy. The velocity of the secondary flow in the pseudo-critical region can be one magnitude order larger than that in the high temperature region, which leads to strong heat transfer enhancement in the former. Studies on the energy conversion performance indicate that the PTC can achieve the collector efficiencies of 18.78–84.17% and 81.93–84.17% at the typical conditions for Brayton and Rankine cycles, respectively, where the efficiency increases with decreasing Tinand increasing vin. Additionally, corresponding efficiency equations have also been obtained. The information from this study will provide a reference for the design and operation of the PTCs using s-CO2as HTF.
Yu Qiu;Ming Jia Li;雅玲 何;文铨 陶
Applied Thermal Engineering
In present work, a cavity receiver with quartz glass cover is presented for the dish concentrating system. The quartz glass cover can separate the receiver cavity from the ambient air and its selective coating layer can intercept the infrared radiation emitted from the inner-surface of the cavity receiver, which greatly reduce the natural convection and surface radiation heat losses. To fundamentally understand this design, a two-dimensional model for the heat transfer process that combines the natural convection and the surface radiation is proposed and developed. The simulation results show that the total heat flux of the covered receiver at 0° inclination is only about 36% of that for uncovered receiver. The model is then used to investigate the effects of various system parameters, such as orientation, temperature and emissivity of inner surface, on the heat transfer and fluid flow performance. It is found that, comparing with the natural convection, the surface radiation is the dominant heat transfer pattern. The orientation has significantly influence the convection heat transfer. However, the surface radiation keeps constant for different inclination angles. In addition, it is also found that the radiation heat transfer is significantly affected by the temperature and emissivity of the inner surface. The increase in the surface temperature and emissivity enhances the surface radiation proportion in the total heat loss. However, for the convection heat loss, it changes little and even decreases as the surface temperature and emissivity increases.
Fuqing Cui;雅玲 何;Zedong Cheng;印实 李
The pressure-based method shows the potential to develop the procedure for the flow of entire spectrum of Mach numbers. In the paper the pressure-based CLEAR algorithm is adopted to compute the compressible flows. Meanwhile the performance of different schemes for density and convection terms in compressible flows is investigated. OFCUI, ADBQUICKEST and HLPA schemes are used to discretize the convection term. FUD scheme, the second-order center difference scheme blended first upwind scheme and SMART scheme are used to calculate the density. Flow in a converging-diverging nozzle and flow over a bump are taken as the test cases. Results show that CLEAR algorithm is reliable to compute the compressible flows. The convergence rate accelerates by using SMART scheme to interpolate the density. ADBQUICKEST convection scheme shows the superiority on stability compared with the other two convection schemes.
Jian Fei Zhang;Jin Ping Wang;治国 屈;雅玲 何;文铨 陶
Applied Thermal Engineering
In this paper, a 1 kW ORC experimental system is built. Using R123 as the working fluid, transient responses of Basic ORC (BORC) and ORC with a regenerator (RORC) are both tested under critical conditions. A total of four experiments are carried out, including: (1) Case 1: the working fluid pump is suddenly shut down; (2) Case 2: the working fluid is overfilled or underfilled; (3) Case 3: the torque of the expander is suddenly loss. (4) Case 4: the cooling water pump is suddenly shut down. All the major quantities such as the output power and torque of the expander, temperatures and pressures at the inlet and outlet of the expander, temperatures at the inlet and outlet of the condenser are measured. The transient responses of the two systems under the controlled critical conditions are tested and compared, some physical explanations are provided. It is found that RORC is more stable than BORC because of the regenerator. Regenerator should act as a “pre-heater” or “pre-cooler” under the critical conditions thus improving the stability of RORC. When the working fluid in the system is underfilled or leaked, the system performance is extremely unstable. Otherwise, when the working fluid is overfilled, the trend of the curves are similar to the optimal working condition but with weaker performances. We also find that if the working fluid pump is shut down when working fluid is overfilled, the rotation speed and shaft power output of the expander will increase significantly, the unique phenomenon can be used to estimate whether the working fluid in the system is overfilled.
Huan Xi;雅玲 何;Jinhua Wang;佐华 黄
International Journal of Hydrogen Energy
A non-isothermal two-phase mass transport model is developed in this paper to investigate the heat generation and transport phenomena in a direct methanol fuel cell with anisotropic gas diffusion layers (GDLs). Thermal contact resistances at the GDL/CL (catalyst layer) and GDL/Rib interfaces, and the deformation of GDLs are considered together with the inherent anisotropy of the GDL. Latent heat effects due to condensation/evaporation of water and methanol between liquid and gas phases are also taken into account. Formulation of the two-phase mass transport across the membrane electrode assembly (MEA) is mainly based on the classical multiphase flow theory in the porous media. The numerical results show that the overall heat flux in MEA is mainly contributed to heat generation in anode and cathode CLs. And the anisotropic factors of the GDLs, including the inherent anisotropy, the spatially varying contact resistances, and the deformation of GDLs, have strong impacts on heat transport processes in the DMFC by altering the distribution of temperature across the MEA.
Zheng Miao;雅玲 何;天寿 赵;文铨 陶
Frontiers in Heat and Mass Transfer
Three-dimensional numerical simulations of a typical unit (TU) of Polymer Electrolyte Membrane Fuel Cell (PEMFC) and a 27-channel entire-cell (EC) were respectively conducted with a steady and nonisothermal and two-phase model. Results comparisons were conducted focusing on the distribution of the liquid water saturation, phase-change source terms and temperature in TU and EC. It is found that, there exist significant differences between the numerical predictions either in variation trend or in parameter range of TU and EC. For example the water saturation in TU is far less than that in EC under the same operational and computational conditions. This might be mainly resulted from the set up of boundary conditions in the computational domains. Thus the computational results by the methods of TU (which is widely adopted in present literatures) and EC represent different processes even under the same computational conditions.
Hong Lin;Xing Wei;春华 闵;增耀 李;雅玲 何;文铨 陶
Kung Cheng Je Wu Li Hsueh Pao/Journal of Engineering Thermophysics
We propose an implicit-explicit finite-difference lattice Boltzmann method for compressible flows in this work. The implicit-explicit RungeKutta scheme, which solves the relaxation term of the discrete velocity Boltzmann equation implicitly and other terms explicitly, is adopted for the time discretization. Owing to the characteristic of the collision invariants in the lattice Boltzmann method, the implicitness can be completely eliminated, and thus no iteration is needed in practice. In this fashion, problems (no matter stiff or not) can be integrated quickly with large CourantFriedricheLewy numbers. As a result, with our implicit-explicit finite-difference scheme the computational convergence rate can be significantly improved compared with previous finite-difference and standard lattice Boltzmann methods. Numerical simulations of the Riemann problem, Taylor vortex flow, Couette flow, and oscillatory compressible flows with shock waves show that our implicit-explicit finite-difference lattice Boltzmann method is accurate and efficient. In addition, it is demonstrated that with the proposed scheme non-uniform meshes can also be implemented with ease.
Y. Wang;雅玲 何;天寿 赵;G. H. Tang;文铨 陶
International Journal of Modern Physics C