Numerical simulations applying to the analysis of thermal explosion of organic gel fuel in a hot gas

In this work we investigate theoretically and numerically the nature of the evolution of a hot gas mixture containing organic gel fuel droplets of different radii with oscillatory evaporation within the context of thermal explosion theory. The polydisperse spray is modeled using a probability density function (PDF). In this paper we take into account the time evolution of the size distribution due to the evaporation process by using the kinetic equation. This new model is in the form of singular perturbed system (SPS) of highly nonlinear of ordinary differential equations. This SPS form of the model and the non-dimensionalization of the equations enable us first to apply the methods of integral manifolds (MIM) and second to identify the parameters that play a major role in determining the dynamical regimes of the considered system. Our analytical and numerical simulations results that based on the MIM show that the dynamical behavior is different than that found with the conventional liquid droplets. An explicit expression of the critical condition for thermal explosion limit is derived analytically and represents a generalization of the critical parameter of the classical Semenov theory.     

Numerical simulations of gas-solid flow in tapered risers

Hydrodynamic behavior of gas-solid flow in tapered risers was simulated using the two-fluid model based on the kinetic theory of granular flow representing the constitutive relations of the solid phase. Present numerical model was verified by comparing with experimentally measured solid mass fluxes, particle concentrations and velocities in column risers. Computed results showed that the core-annular flow structure existing in the column riser may disappear in the tapered risers. The distributions of particle concentration tend to be more uniform in the tapered riser than that in the column riser under the same operating conditions. The uniform particle distribution can be achieved by changing the inclined angle of the tapered riser under specific operating conditions.     

Numerical analysis of flow field in the hot gas filter vessel during the pulse cleaning process

The flow field is simulated for a ceramic filter vessel containing three candle filters which are arranged in the form of an equilateral triangle. Grids generated by GAMBIT are adopted for the simulations. The Reynolds stress model provided by FLUENT code is applied to evaluate gas flow and temperature field in the filter vessel. The temperature profiles in the ceramic candle filter cavity during the pulse cleaning process are analyzed under different operating conditions and for different lengths of candle filter. The evolution of radial velocity in the porous wall of the filters being cleaned and the normal working filters as well as around the filters is discussed. Sharp temperature change takes place in the top of the candle filter which is subject to thermal stress. The phenomenon of temperature increase during the pulse cleaning process has been carefully observed and interpreted based on the effect of gas compression. The simulated results show qualitative agreement with the experimental field observations with the filter vessel.     

基于ANSYS软件的热油管路保温的数值模拟

应用ANSYS软件对埋地热油管道沿径向温度进行了数值模拟,得到了不同保温层厚度时管道径向温度及热流量的变化。分析了含蜡层的径向温度变化,随着保温层厚度的增加,保温层内部温降变化减小,管道向外传递的热流密度逐渐减小,保温效果更好。热流量减小的速度随着保温层厚度的增加变得缓慢,模拟结果与编制计算机程序计算结果相吻合。     

Numerical simulations of ultrafine powder coating systems

Numerical simulations for gas–solid two-phase flows were conducted for an experimental coating booth and an industrial coating booth to study the effect of the coating powder size on the performance of the coating process. To optimize coating parameters, simulations were conducted for different coating parameters, such as the size of the coating part, the distance between the coating part and the spray gun, the air flow rate and particle flow rate from the spray gun, the position of the pattern adjust sleeve of the spray gun, and the electrostatic field, in order to increase the coating process efficiency and coating quality.

In numerical simulations, the air flow field is obtained by solving three-dimensional Navier–Stokes equations with standard κ– turbulence model and non-equilibrium wall function. The second phase, the coating powder, consists of spherical particles and is dispersed in the continuous phase, the air. In addition to solving transport equations for the air, the trajectories of the particles are calculated by solving the particle motion equations using Lagrangian method. It is assumed that the particle–particle interaction can be neglected due to low particle volume fraction in coating systems. The electrostatic field is predicted by solving the Laplace equation.     

Numerical simulation of transport phenomena due to array of round jets impinging on hot moving surface

An attempt has been made to numerically evaluate the heat transfer from a moving surface due to impingement of array of round jets. This paper reports the effect of surface velocity on heat transfer. Transition SST model has been used for simulation to predict heat transfer under laminar, transition, and turbulent regimes. This model has been used as it bridges all flow conditions seamlessly. The computational domain considered in this study is a 3D model with symmetric planes on two sides and periodic interface on two sides, so as to represent an array of round jets. The range of Reynolds number adopted here is 100–5,000. Results were first validated with the correlation given by Martin^[1 Martin, H. Heat and Mass transfer between impinging gas jets and solid surfaces. Advances in Heat Transfer 1977, 13, 1–60.[Crossref] , [Google Scholar]^] for array of round nozzles with stationary isothermal surface under turbulent conditions with an average error of 5.88%. It is observed that at higher surface velocities, the heat transfer from the moving surface is more than the case of heat transfer from a stationary surface. The value of surface velocity at which the heat transfers from moving surface is minimum decreases with increase in Reynolds number. An artificial neural network has been trained to accurately predict the Nusselt number for the given Reynolds number and surface velocity.     

Study of vertical upward flame spread on charring materials—Part II: Numerical simulations

Simulation results, obtained by means of application of an enthalpy‐based pyrolysis model, are presented. The ultimate focus concerns the potential of the model to be used in flame spread simulations. As an example we discuss vertically upward flame spread over a charring material in a parallel plate configuration. First, the quality of the pyrolysis model is illustrated by means of cone calorimeter results for square (9.8 cm × 9.8 cm exposed area), 1.65 cm thick, horizontally mounted MDF samples. Temperatures are compared at the front surface and inside the material, for different externally imposed heat fluxes (20, 30 and 50 kW/m²), for dry and wet samples. The mass loss rate is also considered. Afterwards, vertically upward flame spread results are reported for large particle board plates (0.025 m thick, 0.4 m wide and 2.5 m high), vertically mounted face‐to‐face, for different horizontal spacings between the two plates. The simulation results are compared to experimental data, indicating that, provided that a correct flame height and corresponding heat flux are applied as boundary conditions, flame spread can be predicted accordingly, using the present pyrolysis model. Copyright © 2010 John Wiley & Sons, Ltd.     

CFD prediction of fluid flow and mixing in stirred tanks: Numerical issues about the RANS simulations

This work is aimed at verifying the effect of numerical issues on the RANS-based predictions of single phase stirred tanks. In particular, the effect of grid size and discretization schemes on global parameters, mean velocity, turbulent dissipation rate and homogenization is considered. Although contradictory results have been reported so far on the capability of RANS methods in fluid mixing, the most widely accepted conclusion is that adequate values are generally to be expected for the predicted mean flow quantities, while much less confidence must be put on the calculated turbulent quantities and related phenomena. The results obtained in this work partially revise this last statement and demonstrate that firm conclusions on the limits of RANS simulations can be drawn only after careful verification of numerical uncertainties. The simulation results are discussed and compared to the literature experimental data and to original passive tracer homogenization curves determined with planar laser induced fluorescence.     

CFD simulations of quenching process for partial oxidation of methane:Comparison of jet-in-cross-flow and impinging flow configurations

A new quenching process using the cold pyrolysis gas has been proposed for the partial oxidation (POX) of methane to recover the heat.The mixing of hot product gas and cold pyrolysis gas in milliseconds is critical to this new approach.Two most widely-used rapid mixing configurations,i.e.the jet-in-cross-flow (JICF) and impinging flow configurations,are compared in terms of mixing and quenching performances using computational fluid dynamics (CFD) coupled with detailed reaction mechanism Leeds 1.5.The mixedness,residence time distribution,temperature decreasing rate and loss ratio of acetylene during the quenching are systematically studied.The results show that the impinging flow has a more uniform mixing and narrower residence time distribution than the JICF.However,the temperature decreasing rate of the mainstream is faster in the JICF than in the impinging flow.The loss ratio of acetylene in the quenching process is 2.89％ for the JICF and 1.45％ for the impinging flow,showing that the impinging flow configuration is better and feasible for the quenching of POX of methane.     

Numerical simulation of steady flow past a liquid sphere immersed in simple shear flow at low and moderate Re

This work presents a numerical investigation on steady internal, external and surface flows of a liquid sphere im-mersed in a simple shear flow at low and intermediate Reynolds numbers. The control vol...     

数值模拟中低Reynolds数下简单剪切流流过单液滴的稳态流场(英文)

This work presents a numerical investigation on steady internal, external and surface flows of a liquid sphere im-mersed in a simple shear flow at low and intermediate Reynolds numbers. The control volume formulation is adopted to solve the governing equations of two-phase flow in a 3-D spherical coordinate system. Numerical re-sults show that the streamlines for Re=0 are closed Jeffery orbits on the surface of the liquid sphere, and also closed curves outside and inside the liquid sphere. However, the streamlines have intricate and non-closed struc-tures for Re≠0. The flow structure is dependent on the values of Reynolds number and interior-to-exterior vis-cosity ratio.     

Synergistic and interference effects in coaxial mixers: Numerical analysis of the power consumption

This paper is concerned with the design and application of coaxial mixers with the aid of analysis of interaction between each individual impeller.Two types of coaxial mixers pitched blade turbine (PBT)-helical ribbon (HR) and inner-outer HR operated in laminar regime were studied experimentally and numerically.The interaction implies synergistic and interference effects,which was revealed through the investigation of axial circulation rate,energy dissipation rate and power consumption.The influence factors including rotational speed ratio,rotating mode and impeller configuration were explored systematically.Quantitative analysis of power consumption involves three parameters:rate of variation in power consumption,interactive mode and ratio of power consumption.Analysis indicated that some important properties were embodied in the power curve.These properties are one-way and two-way interactions,critical speed ratio and dominant impeller.Finally,a new suggestion for power estimation was given.     

Numerical investigation of bend and torus flows—Part II : Flow simulation in torus reactor

Flow in a torus reactor with straight parts fitted with a marine impeller is investigated. The laser Doppler anemometry (LDA) is first employed to achieve experimental measurements of mean velocity profiles. Next, a numerical resolution of the steady-state flow is performed using a multiple reference frames (MRF) approach to represent the particular flow induced by the marine impeller in the geometry. A comparison of predictions using different turbulence models to LDA measurements is made, and a k-ω model is assessed.The numerical tool is used to investigate in more details the particular flow induced in the torus geometry. Evolution of the axial and rotating motions when moving away from the impeller is especially investigated, showing the complex hydrodynamical interaction between the main rotating swirl motion involved downstream the impeller, and bend curvature effects. Two different flow conditions can be considered in the torus geometry, with a main swirling motion close to the impeller, which freely decays and vanishes when Dean vortices appear in bends. Simulations for two rotation velocities of the impeller and comparison with the study with simple bends (first part) reveal pertinence of the swirl number Sn to describe the change of flow conditions along the reactor axis. When this parameter decreases below a threshold value around 0.2 in a bend entry, centrifugal effects due to bend curvature are more important than the swirl motion, and Dean vortices appear in bend outlet. One main consequence is the axial distance of the swirl motion persistence, which is found to be smaller for the higher impeller rotation velocity, due to the dual effect of the marine impeller that generates simultaneously both axial and rotating motions.     

Numerical simulations of bubble formation on submerged orifices: Period-1 and period-2 bubbling regimes

The process of bubble formation is involved in several gas-liquid reactors and process equipment. It is therefore important to understand the dynamics of bubble formation and to develop computational models for the accurate prediction of the bubble formation dynamics in different bubbling regimes. This work reports the numerical investigations of bubble formation on submerged orifices under constant inflow conditions. Numerical simulations of bubble formation at high gas flow rates, where the bubble formation is dominated by inertial forces, were carried out using the combined level set and volume-of-fluid (CLSVOF) method and the predictions were experimentally validated. Effects of gas flow rate and orifice diameter on the bubbling regimes and in particular, on the transition from period-1 to period-2 bubbling regime (with pairing or coalescence at the orifice) were investigated. Using the simulation data on the transition of bubble formation regimes, the bubble formation regime map constructed using Froude and Bond numbers is presented.