Comparative studies on accuracy and convergence rate between the cell-centered scheme and the cell-vertex scheme for triangular grids

The truncation errors of the cell-centered and the cell-vertex schemes for unstructured triangular grids are theoretically derived respectively, showing that the truncation error of the cell-vertex scheme is smaller than that of the cell-centered scheme for the same triangular grids. The theoretical derivation is validated by some numerical examples. In addition, it is also shown by both of theoretical analysis and numerical examples that the convergence rate of the cell-vertex scheme is faster than that of the cell-centered scheme.     

Large eddy simulations of wall heat transfer and coherent structures in mixed convection over a wavy wall

In this numerical study the mixed convective flow of water over a heated wavy surface over a range of Reynolds and Richardson numbers, including transitional and turbulent flow regimes (20 ≤ Re ≤ 2000 and 0.5 ≤ Ri ≤ 5000) is investigated. A dynamic Large Eddy Simulation (LES) approach is applied where the thermal buoyancy effects are represented by the Boussinesq approximation. The LES results show good agreement with available measurements including first and second order statistics of velocity and thermal fields. We focus our investigation on the thermal buoyancy effects on the wall heat transfer and the spatial reorganisation of the vortical flow structures. In order to characterise the reorganisation of the mean flow features, the vortical coherent structures are identified and extracted according to the swirling strength criteria. Interesting reorganisation of flow structures takes place between Re = 20 and Re = 200 where the initially spanwise oriented large coherent structures start to be streamwise oriented. With further increase of Re, these large structures disappear from the central part of the simulated domain and reappear in the proximity of the horizontal wavy wall for Re ≥ 1000. The imprints of this flow reorganisation are clearly visible in the distributions of the local heat transfer coefficient (Nusselt number) along the horizontal wavy wall. The integral heat transfer for the wavy wall configuration is significantly enhanced (≈2.5 times) for Re = 1000, 2000 in comparison with the standard flat horizontal wall configuration.     

Heat transfer with flow and evaporation in loop heat pipe’s wick at low or moderate heat fluxes

An axisymmetric two-dimensional mathematical model of the cylindrical evaporator’s wick of loop heat pipes is developed to simulate heat transfer with flow and evaporation in the capillary porous structure. Effect of the interaction between the flow field and the liquid–vapor interface on the position of the interface and the curvature of menisci is adequately considered in this model. The flow fields of transient and steady states are obtained at low or moderate heat fluxes. The dynamic and thermodynamic behavior is discussed in this paper. The auto-driving mechanism of “inverted meniscus type” evaporators is validated. Effect of heat flux is investigated in detail.     

Transport properties of Ar–Kr binary mixture in nanochannel Poiseuille flow

Transport properties, including thermal conductivity and shear viscosity, of the Ar–Kr binary mixture confined in a nanochannel under Poiseuille flow are calculated by equilibrium molecular dynamics (EMD) simulation through Green–Kubo formula. An external force is applied in the x-direction to drive the Poiseuille flow. Thermal conductivity of the confined mixture in the x- and y-direction is obviously higher than that in macroscale, as a result of the strong interacting potential between the fluid atoms and the wall atoms. Thermal conductivity of the flowing binary mixture is obviously anisotropic. With increasing the external driving force, in the x-direction the thermal conductivity increases, whereas in the y-direction it keeps constant. The xz- and yz-component of the shear viscosity of the confined mixture are enhanced comparing with the xy-component owing to the collisions between the fluid atoms and the wall atoms in the z-direction. They are higher than the results in macroscale and decrease with the external driving force increasing. For the binary mixture, thermal conductivity and shear viscosity vary with the mole fraction of the Kr atoms. The interactions between the fluid atoms and the wall atoms play a key role in the transport properties of the binary mixture confined in the nanochannel.     

Experimental investigation on the packed bed of rodlike particles

Rodlike particles have been usually found in industrial applications, such as the straw and needle catalyst in energy and chemical engineering. Compared to spherical particles, rodlike particles exhibit different behaviour in the packing structure due to their rotational movement. In this work, we have experimentally explored the packing structure and its friction factor for fluid flow. The porosity of packing structure generated by two packing methods is measured for four kinds of rodlike particles. The experimental results show that the porosity of bed of rodlike particles in the poured packing is not a monotonic function of the aspect ratio of particles. This is due to the competition between the “self-fitting” effect and excluded effect. The porosity of bed of rodlike particles is more sensitive to the packing method than that of spherical particles. To describe the pressure drop of fluid flow through the packing structure, the Ergun equation is further modified by introducing the modified Reynolds number and Galileo number. By combing the experimental data for packed bed generated by the fluidised packing method, and other experimental work in current literature, a new empirical equation is proposed to predict the friction factor of the packing structure of rodlike particles, in which the effects of the particle orientation and particle shape are both considered by the equivalent sphericity. These experimental results would be of interest from applied standpoints as well as revealing fundamental effects of the aspect ratio of rodlike particles on the packing structure.     

Unburned carbon measurement in fly ash using laser-induced breakdown spectroscopy with short nanosecond pulse width laser

The unburned carbon in fly ash is one of the important factors for the boiler combustion condition. Controlling the unburned carbon in fly ash is beneficial for fly ash recycle and to improve the combustion efficiency of the coal. Laser-induced breakdown spectroscopy (LIBS) technology has been applied to measure the fly ash contents due to its merits of non-contact, fast response, high sensitivity, and real-time measurement. In this study, experimental measurements have been adopted for fly ash flows with the surrounding gases of N₂ and CO₂, while the CO₂ concentration varified to evaluate the CO₂ effect on the unburned carbon signal from fly ash powder. Two kinds of pulse width lasers, 6?ns and 1?ns, were separately adopted to compare the influence of laser pulse width. Results showed that compared with that using 6?ns pulse width laser, plasma temperature was lower and had less dependence on delay time when using 1?ns pulse width laser, and spectra had more stable background. By using 1?ns pulse width laser, the emission signal from surrounding CO₂ also decreased because of the less surrounding gas breakdown. The solid powder breakdown signals also became more stable when using 1?ns pulse width laser. It is demonstrated that 1?ns pulse width laser has the merits for fly ash flow measurement using LIBS.     

超临界机组锅炉20%负荷深度调峰水动力实炉试验研究

为挖掘现有燃煤机组锅炉的深度调峰能力,将华能陕西秦岭发电有限公司7号超临界660 MW机组锅炉各类集箱和大量水冷壁管分别等效为压力节点和流量回路,建立基于流动网络系统法的非线性数学模型。根据守恒定律和传热关联式,通过迭代直接求解得到660 MW（BMCR）负荷时该锅炉的水动力特性,并将计算结果与实炉数据进行比较,验证了模型的可靠性。在实炉试验研究的基础上,计算得到132 MW（20%BMCR）深度调峰干态运行负荷时锅炉水冷壁压力沿流动方向的变化、回路流量分配、炉膛出口汽温分布、管壁金属温度沿炉高方向的变化趋势。实炉试验和计算结果表明,采用内螺纹螺旋管圈布置的超临界660 MW机组锅炉水动力突破了30%BMCR最低启动负荷的限制,实现了20%BMCR深度调峰负荷时的水动力运行安全,且流动稳定特性良好。     

A mixed P1–DP0 diffusion theory for planar geometry

A variational analysis is used to derive a mixed P₁–DP₀ (P₁ spherical harmonics–double P₀ spherical harmonics) angular approximation to the time-independent monoenergetic neutron transport equation in one-dimensional planar geometry. This mixed angular approximation contains a space-dependent weight factor α(x) that controls the local angular approximation used at a spatial point x: α(x) = 1 yields the standard P₁ (diffusion) approximation, α(x) = 0 gives the standard DP₀ approximation, and 0 < α(x) < 1 produces a mixed P₁–DP₀ angular approximation. The diffusion equation obtained differs from the standard P₁ diffusion equation only in the definition of the diffusion coefficient. Standard Marshak incident angular flux boundary conditions are also obtained via the variational analysis. We examine the use of this mixed angular approximation coupled with the standard P₁ approximation to more accurately treat material interfaces and vacuum boundaries. We propose a simple but effective functional form for the weight factor α(x) that removes the need for the user to specify the value. Numerical results from several test problems are presented to demonstrate that significant improvements in accuracy can be obtained using this method with essentially no computational penalty.