Subgrid Linear Eddy Mixing and Combustion Modelling of a Turbulent Nonpremixed Piloted Jet Flame

A linear eddy model for subgrid mixing and combustion has been coupled to a large eddy simulation of the turbulent nonpremixed piloted jet flame (Sandia Flame D). For the combustion reaction, simplified, single-step, irreversible, Arrhenius kinetics are used. The large scale and the subgrid structure of the flow are compared with experimental observations and, where appropriate, with a flamelet model of the flame. The main objective of this work is to demonstrate the feasibility of the LES-LEM approach for determining the structure of the subgrid scalar dissipation rate and the turbulence-chemistry interactions. The results for the large- and subgrid-scale structure of the flow show a reasonable agreement with the experimental observations.     

横流冲击射流尾迹涡结构的实验研究

本文采用LIF（激光诱导荧光）流动显示和PIV（粒子图像速度场仪）测量对横流冲击射流的尾迹涡结构进行了实验研究。水槽实验是在三种流速比和两种冲击高度实验工况下进行的。由实验结果可得到两种明显的尾迹涡结构、，即射流尾迹涡和横流尾迹涡。横流冲击射流中形成的主要尾迹涡结构主要依赖于流速比。本文还对横流冲击射流近区范围内射流尾迹涡和横流尾迹涡的形成机理和演化特征进行了分析。     

Heat Transfer Effects on a Fully Premixed Methane Impinging Flame

A numerical assessment of different thermal conditions for an impinging flame configuration is investigated using large-eddy simulation. The cases of study correspond to a turbulent methane flame at equivalence ratio ER = 0.8 and temperature T = 298 K exiting at 30 m/s with a nozzle-to-plate distance over diameter of H/D = 2. Computational cases based on different thermal conditions are compared to a conjugate case, in which fluid and solid domains are solved simultaneously. A solid material defined with enhanced conductivity properties is used to represent the wall in the conjugate case, so that the characteristic time scales of the solid are reduced. The results indicate that the heat transfer condition influences not only the mean temperature and gradients, but also the temperature fluctuations in the near-wall region. It is found that adiabatic, isothermal and more sophisticated Robin-type boundary conditions contribute to underpredict/overpredict the temperature field near the wall. As the time scales of fluid and solid are of the same order, the use of conjugate approaches is required to predict the correct flow fields near the wall and the skin temperature.     

Effects of Small- and Large-Scale Structures in a Piloted Jet Diffusion Flame

Laser Doppler Anemometry (LDA) and Planar Laser-Induced Fluorescence (PLIF) measurements have been performed in a turbulent nonpremixed jet flame. One of the features of this configuration is a central co-axial fuel jet surrounded by a turbulent annular air flow. The whole is placed within a low-speed coflowing air stream. This three-flow system with turbulent primary air differs from flow systems used for nonpremixed jet flames reported in the literature and is very useful for obtaining information on the mixing process between fuel and primary air. Next to the characterization of the velocity field, special attention has been paid to the conditional seeding of the central fuel jet and of the annular air flow. Together with visualizations of the OH radical, an important combustion intermediate which is formed during combustion, and the NO radical, which is seeded to the central jet flow, the resulting statistics reveal the properties of small- and large-scale structures in the flame.     

Scalar Mixing and Large-Scale Coherent Structures in a Turbulent Swirling Jet

The paper presents large eddy simulations of co-annular swirling jets into an open domain. In each of the annuli a passive scalar is introduced and its transport is computed. If the exit of the pilot jet is retracted strong coherent flow structures are generated which substantially impact on the transport and mixing of the scalars. Average and instantaneous fields are discussed to address this issue. A conditional averaging technique is devised and applied to velocity and scalars. This allows to quantify the impact of the coherent structures on the mixing process.     

On the Capability of PIV-Based Wall Pressure Estimation for an Impinging Jet Flow

A PIV-based pressure estimation methodology is used to compute the wall pressure from the velocity field of a turbulent impinging jet flow. A simplified formulation (2D-2C) is applied to velocity fields issued from PIV data. The ability of the method to qualitatively estimate the wall pressure signature of a 3D unsteady impinging jet flow using only two velocity components in a plane is demonstrated. Nevertheless, the 2D flow assumption used in the context of planar measurements involves an underestimation of the wall pressure values all along the radial direction. The formulation based on the full integral formalism (3D-3C), computed from DNS data without any assumption on the flow, provides a reference solution. The contributions of the surface and volume integrals to the pressure coefficient are assessed. It is shown that the most important contribution to the wall pressure comes from the volume integral. Then the underestimation observed for the simplified formulation is mostly linked with the assumptions considered for the source term computation. The effect of each assumption is quantitatively analysed with the help of the DNS data and some ways to improve the simplified methodology are finally proposed.     

Numerical Simulation of Two Phase Turbulent Flow of Nanofluids in Confined Slot Impinging Jet

In this article, a numerical investigation is performed on flow and heat transfer of confined impinging slot jet, with a mixture of water and Al₂O₃ nanoparticles as the working fluid. Two-dimensional turbulent flow is considered and a constant temperature is applied on the impingement surface. The k ? ω turbulence model is used for the turbulence computations. Two-phase mixture model is implemented to study such a flow field. The governing equations are solved using the finite volume method. In order to consider the effect of obstacle angle on temperature fields in the channel, the numerical simulations were performed for different obstacle angles of 0° ? 60°. Also different geometrical parameters, volume fractions and Reynolds numbers have been considered to study the behavior of the system in terms of stagnation point, average and local Nusselt number and stream function contours. The results showed that the intensity and size of the vortex structures depend on jet- impingement surface distance ratio (H/W) and volume fraction. The maximum Nusselt number occurs at the stagnation point with the highest values at about H/W = 1. Increasing obstacle angle, from 15° to 60°, enhances the heat transfer rate. It was also revealed that the minimum value of average Nusselt number occurs in higher H/W ratios with decreasing the channel length.     

旋流喷嘴中空旋转射流近区域流动的研究

从理论上分析了旋流喷嘴喷出的中空射流近区域的液膜的运动,在只考虑液膜表面张力的作用下,应用质量守恒和动量定理,建立了描述液膜运动的非线性常微分（积分）方程组,该方程组可以用数值方法方便地求解,结果表明,液体离开旋流喷嘴后在自由空间形成的液膜呈葫芦形状,其速度和液膜厚度等都周期性地变化。本结果是在液厝受拓动失称碎成液滴前的最基本运动状态,可以在射流的近区域内实验观察到,也是进一步从理论液膜破碎雾化过     

Turbulence Resolution Scale Dependence in Large-Eddy Simulations of a Jet Flame

The explicit dependence of LES fields on the turbulence resolution scale Δ implies that LES statistics usually vary with Δ and exhibit different convergence behaviors for different types of statistics, flow variables and subgrid LES models. The present work compares the performance of two popular subgrid models—the dynamic Smagorinsky model and the Vreman model—based on the convergence of their LES statistics with respect to Δ for a piloted methane-air (Sandia D) flame. The Δ-dependence of the LES statistics is studied based on five grids with progressively increased resolution ranging from 3 × 10⁵ to about 10.4 × 10⁶ cells. The simulation results show that the resolved velocity statistics converge for the finest grids with some weak Δ-dependence observed in the variance fields. The mixture fraction statistics are found to be more sensitive to the turbulence resolution scale upstream in the flame signifying the importance of the estimation of the Δ-invariant LES statistics at the DNS limit. For the considered flame the Vreman subgrid model exhibits good performance with the statistics being very close to those given by the dynamic Smagorinsky model, and being rather insensitive to a choice of the model constant.     

Effects of the Local Flow Topologies Upon the Structure of a Premixed Methane-air Turbulent Jet Flame

Local flow topologies have been identified and their interactions with the iso-scalar surfaces geometries have been investigated using the results of a three-dimensional direct numerical simulation (DNS) of a turbulent premixed methane-air flame in a piloted Bunsen burner configuration with tabulated chemistry. The universal teardrop shape of the joint probability density function (jpdf) of the second and third invariants of the velocity-gradient tensor disappears in the different flame regions under study. A ‘canonical’ vortex, which affects the fine-scale structure of the turbulent premixed flame, has been identified and analyzed at three times, differing by increments of the order of the Kolmogorov time micro-scale.     

Fibrous-Suspension Plug Flow in an Annular Pipe

The problem of fibrous-suspension plug flow in a straight annular pipe is solved by expanding the solution in powers of a small parameter. The specific features of the flow are found.     

Annular surface-charged jet in an external electric field

The problem of the shape of an annular charged jet in an external electric field is considered. In the strong-field approximation, the formulation of the problem is reduced to a system of ordinary differential equations. The asymptotics of the solutions at large distances from the point of outflow are investigated. The condition of formation of the jet profile non-increasing along the streamwise coordinate is found.     

Evaluations of SGS Combustion,Scalar Flux and Stress Models in a Turbulent Jet Premixed Flame

A newly developed fractal dynamic SGS (FDSGS) combustion model and a scale self-recognition mixed (SSRM) SGS stress model are evaluated along with other SGS combustion, scalar flux and stress models in a priori and a posteriori manners using DNS data of a hydrogen-air turbulent plane jet premixed flame. A posteriori tests reveal that the LES using the FDSGS combustion model can predict the combustion field well in terms of mean temperature distributions and peak positions in the transverse distributions of filtered reaction progress variable fluctuations. A priori and a posteriori tests of the scalar flux models show that a model proposed by Clark et al. accurately predicts the counter-gradient transport as well as the gradient diffusion, and introduction of the model of Clark et al. into the LES yields slightly better predictions of the filtered progress variable fluctuations than that of a gradient diffusion model. Evaluations of the stress models reveal that the LES with the SSRM model predicts the velocity fluctuations well compared to that with the Smagorinsky model.     

Three-dimensional Linear Eddy Modeling of a Turbulent Lifted Hydrogen Jet Flame in a Vitiated Co-flow

A new methodology for modeling and simulation of reactive flows is reported in which a 3D formulation of the Linear Eddy Model (LEM3D) is used as a post-processing tool for an initial RANS simulation. In this hybrid approach, LEM3D complements RANS with unsteadiness and small-scale resolution in a computationally efficient manner. To demonstrate the RANS-LEM3D model, the hybrid model is applied to a lifted turbulent N₂-diluted hydrogen jet flame in a vitiated co-flow of hot products from lean H₂/air combustion. In the present modeling approach, mean-flow information from RANS provides model input to LEM3D, which returns the scalar statistics needed for more accurate mixing and reaction calculations. Flame lift-off heights and flame structure are investigated in detail, along with other characteristics not available from RANS alone, such as the instantaneous and detailed species profiles and small-scale mixing.     

Focusing of Strong Shocks in an Annular Shock Tube

Focusing of strong shock waves in a gas-filled thin convergence chamber with various forms of the reflector boundary is investigated experimentally and numerically. The convergence chamber is mounted at the end of the horizontal co-axial shock tube. The construction of the convergence chamber allows the assembly of the outer chamber boundaries of various shapes. Boundaries with three different shapes have been used in the present investigation—a circle, an octagon and a smooth pentagon. The shock tube in the current study was able to produce annular shocks with the initial Mach number in the range M _s = 2.3 − 3.6. The influence of the shape of the boundary on the shape and properties of the converging and reflected shock waves in the chamber has then been investigated both experimentally and numerically. It was found that the form of the converging shock is initially governed by the shape of the reflector and the nonlinear interaction between the shape of the shock and velocity of shock propagation. Very close to the center of convergence the shock obtains a square-like form in case of a circular and octagonal reflector boundary. This is believed to stem from the instability of the converging shock front triggered by the disturbances in the flow field. The outgoing, reflected shocks were also observed to be influenced by the shape of the boundary through the flow ahead as created by the converging shocks.     

Physical and Numerical Experiments on an Under-Expanded Jet

The axial characteristics of axisymmetric under-expanded sonic jets are dealt with, both experimentally and numerically. Experiments were carried out in a test rig where a sonic nozzle discharged into an ambient which could be kept at different pressures. The density measurements were taken by means of a differential interferometer. The numerical simulation was obtained by adapting a code by Bird (1994) which is based on the MonteCarlo method. The comparison between experimental data and numerical results was carried out by considering the density distribution along the axis of the jet. The direct simulation was then applied to explore the jet characteristics in a rather wide range of the dimensionless products upon which the phenomenology depends. New expressions are presented for the density distribution along the jet axis and for the location of the Mach disk. Sommario. Si trattano sia sperimentalmente che numericamente le caratteristiche assiali di getti assialsimmetrici sonici sottoespansi. Gli esperimenti sono stati eseguiti con ugelli sonici in ambienti a pressione variabile. La densità è stata misurata mediante un interferometro differenziale e le simulazioni sono state condotte mediante un codice basato sul metodo MonteCarlo. Il confronto tra dati sperimentali e numerici è basato sulla distribuzione di densità lungo l'asse del getto. La simulazione diretta è stata poi applicata allo studio del getto in un ampio campo di valori dei numeri caratteristici. Si discutono la distribuzione della densità lungo l'asse e la posizione del disco di Mach, ottenute sia dai dati sperimentali che dai risultati numerici e si propongono per esse nuove espressioni.     

Multiphysics Simulation Combining Large-Eddy Simulation,Wall Heat Conduction and Radiative Energy Transfer to Predict Wall Temperature Induced by a Confined Premixed Swirling Flame

A multi-physics simulation combining large-eddy simulation, conjugate heat transfer and radiative heat transfer is used to predict the wall temperature field of a confined premixed swirling flame operating under atmospheric pressure. The combustion model accounts for the effect of enthalpy defect on the flame structure whose stabilization is here sensitive to the wall heat losses. The conjugate heat transfer is accounted for by solving the heat conduction within the combustor walls and with the Hybrid-Cell Neumann-Dirichlet coupling method, enabling to dynamically adapt the coupling period. The latter coupling procedure is enhanced to determine statistics (mean, RMS, \(\ldots \)) in a permanent regime accurately and efficiently thanks to an acceleration technique which is derived and validated. The exact radiative heat transfer equation is solved with an advanced Monte Carlo method with a local control of the statistical error. The coupled simulation is carried out with or without accounting for radiation. Excellent results for the wall temperature are achieved by the fully coupled simulation which are then further analyzed in terms of radiative effects, global energy budget and fluctuations of wall heat flux and temperature.     

振荡频率对振荡脉冲射流水下穿透特性的影响

振荡脉冲射流大大增加了流场的紊乱程度，射流有效作用靶距和体积冲蚀速度都有显著的提高，提高了采矿效率。