Direct numerical simulations of a planar jet laden with evaporating droplets

A direct numerical simulation (DNS) study is conducted on the various aspects of phase interactions in a planar turbulent gas-jet laden with non-evaporative and evaporative liquid droplets. A compressible computational model utilizing a finite difference scheme for the carrier gas and a Lagrangian solver for the droplet phase is used to conduct the numerical experiments. The effects of droplet time constant, mass-loading and mass/momentum/energy coupling between phases on droplet and gas-jet fields are investigated. Significant changes in velocity, temperature, density and turbulence production on account of the coupling between the liquid and gas phases are observed in non-isothermal jets with evaporating droplets. Most of these changes are attributed to the density stratification in the carrier gas that is caused by droplet momentum and heat transfer.     

Simulation numérique des jets turbulents subsoniques à masse volumique variable par le modèle k–ε

A numerical investigation is reported for round free turbulent non-isothermal binary mixing incompressible jets discharging into a quiescent atmosphere. The standard k–ε model is used. The standard closure schemes in Favre averaged variables are first introduced. The parabolic numerical simulation method of Patankar and Spalding [Heat and Mass Transfer in Boundary Layer, Intertext Books, London, 1970] is followed. The numerical simulations show a satisfactory agreement with the experimental results of Chassaing [Mélange turbulent de gaz inertes dans un jet de tube libre, Thèse d'état, INPT, 1979], Birch et al. [J. Fluid Mech. 88 (3) (1978) 431–449] and Panchapakesan et Lumley [J. Fluid Mech. 246 (1993) 197–223, 225–247]. The developed numerical code is used to study the sensitivity of turbulent characteristics to the density ratio between the jet and the ambient air. The decay rate of the mean axial velocity, temperature and mass fraction are shown to increase with decreasing density ratio. This confirms a higher mixing efficiency (parameter which determines the quantity of mass or heat injected at the jet exit and found further from the axis) when the density ratio between the jet and the quiescent air decreases. Finally, it is shown that the density effects are affected by the buoyancy terms in the similarity region of the jet.     

Heat transfer in a turbulent particle-laden channel flow

The objective of this paper is twofold: (i) to present and analyze particle temperature statistics in turbulent non-isothermal fully-developed turbulent gas–solid channel flow for a large range of particle inertia in order to better understand particle heat transfer mechanisms; (ii) to examine the performance of a recent Probability Density Function (PDF) model provided by Zaichik et al. (2011) [1]. In order to achieve such objectives, a Direct Numerical Simulation (DNS) coupled with a Lagrangian Particle Tracking (LPT) was used to collect fluid and particle temperature statistics after particles reach a statistically stationary regime. A non-monotonic behavior of particle temperature statistics is observed as inertia increases. The competition between different mechanisms (filtering inertia effect, preferential concentration, production of fluctuating quantities induced by the presence of the mean velocity and/or mean temperature gradients) are responsible for such a behavior. This competition is investigated from the exact transport equations of particle temperature statistical moments, fluid statistics conditionally-averaged at particle location, and instantaneous particle distribution in the flow field. Using these data, the accuracy of a PDF model is also assessed in the second part. From this assessment, it is seen that, despite the assumptions made, the model leads to a satisfactory prediction of most of the particle temperature statistics for not too high particle inertia.     

Classification of turbulent jets in a T-junction area with a 90-deg bend upstream

Many nuclear power plants report high cycle thermal fatigue in their cooling system, which is caused by temperature fluctuation in a non-isothermal mixing area. One of these areas is the T-junction, in which fluids of various temperatures and velocities blend. The objective of this research is to classify the turbulent jet mechanics in order to examine the flow-field structure under various operating conditions. Furthermore, this research discovers the optimum operating conditions of the mixing tee in this piping system. An experimental model, including the T-junction with a 90-deg bend upstream, is operated to analyze this mixing phenomenon based on the real operation design of the Phenix reactor. The temperature and velocity data show that a 90-deg bend has a strong effect on the fluid mixing mechanism and the momentum ratio between the main velocity and the branch velocity of the T-junction, which could be an important parameter for the classification of the fluid mixing mechanism. By comparing their mean velocity distributions, velocity fluctuations, and time-series data, the behavior of the branch jet is categorized into four types of turbulent jets; sorted from the highest to the lowest momentum ratios, the jets are categorized as follows: the wall jet, the re-attached jet, the turn jet, and the impinging jet. Ultimately, the momentum ration of the turn jet was selected as the optimum operating condition because it has the lowest velocity and the lowest temperature fluctuations near the wall of the mixing tee.     

Numerical investigation of non-isothermal phase change phenomena in vertical Bridgman crystal growth

The non-isothermal phase change phenomena during the vertical Bridgman growth process for HgCdTe are numerically investigated using an interface capturing finite element scheme. The influence of the growth parameters such as Bi, Ste, U and the flow parameters including Gr_T and Gr_S on the non-isothermal phase change phenomena are obtained. Some new features about the melt/crystal interface shape, the temperature field near the interface and the flow field are revealed by comparing the non-isothermal phase change with the isothermal phase change. Furthermore, the comparison of the non-isothermal interfacial characteristics between the pure diffusion, the natural convection and the double-diffusive convection is made and the obvious differences are presented.     

Thermodynamic optimization of free convection film condensation on a horizontal elliptical tube with variable wall temperature

This study focuses on the thermodynamic analysis of saturated vapor flowing slowly onto and condensing on an elliptical tube with variable wall temperature. An entropy generation minimization (EGM), technique is applied as a unique measure to study the thermodynamic losses caused by heat transfer and film-flow friction for the laminar film condensation on a non-isothermal horizontal elliptical tube. The results provide us how the geometric parameter ellipticity and the amplitude of non-isothermal wall temperature variation affect entropy generation during filmwise condensation heat-transfer process. The optimal design can be achieved by analyzing entropy generation in film condensation on a horizontal elliptical tube with further account for the amplitude of non-isothermal wall temperature variation.     

The creeping flow of a polymeric fluid through bent square ducts with heat dissipation

The 3D non-isothermal creeping flow of nylon-6 in a bent square duct with uniform temperature is studied numerically. The non-Newtonian characteristics of this fluid polymer are represented by a differential-type non-isothermal White-Metzner model. Computational results are obtained by the elastic-viscous split-stress (EVSS) finite element method, incorporating the streamline-upwind Petrov-Galerkin (SUPG) scheme. The generated thermal field is entirely due to viscous heating. Essential flow characteristics, including temperature distribution in the flow field, are predicted. The resulting average Nusselt numbers along the walls are obtained. Subsequently, the effects of flow-rate and geometry are investigated.     

Predictions of concentration in the pyrolysis of biomass materials—I

The present work involves the prediction of the concentration profiles in the case of pyrolysis of different lignocellulosic materials in isothermal and non-isothermal conditions. The operative temperature range is from 573 to 973 K for isothermal conditions, and for non-isothermal conditions, the heating rate ranges from 5 to 80 K/min (5, 20, 40, 60 and 80 K/min).

The concentration for the above mentioned conditions is predicted for various biomass components, viz. cellulose, hemicellulose and lignin. Based on the concentration profiles of different biomass materials, it is possible to predict the pyrolysis behavior over a wide range of temperatures under isothermal and non-isothermal conditions for a large number of biomass materials, provided the activation energy and the frequency factor for the various reaction steps are known. It is also possible to ascertain the degree of combustibility of different biomass materials.

The simulation model utilizes a 4th order Runge-Kutta Predictor-Corrector method to solve the coupled ordinary differential equations. Based on thermogravimetric analysis done elsewhere, it is considered that temperature and time have a linear relationship. The above technique enables us to predict concentration profiles of different biomass materials for the entire range of pyrolysis. The concentration vs time data is plotted graphically for both isothermal and non-isothermal conditions utilizing the Harvard Graphics package on a PC-A/T personal computer.     

格子Boltzmann方法模拟湍流流动

概述了国内外利用分子动力学研究流动的方法，主要介绍一种新的数值计算方法——格子Boltzmann方法。对此法的原理、模拟的模型及其在湍流流动中的应用进行了综述。分析这种方法在模拟湍流时存在的问题。为湍流流动研究指出了一条新的途径——用分子动力学理论研究湍流流动。     

基于小波分析的湍流参数提取方法

在定容燃烧弹内，利用孔板平动法生成湍流，用热线风速仪测量容弹内的速度，用小波分析技术将湍流分解为具有不同频带的组分，方便地获得了湍流强度、湍流尺度、湍流能谱等湍流特征参数。对具有不同频带的湍流积分时间尺度进行详细研究，结果表明，整个湍流的积分时间尺度代表所有湍流涡的平均寿命，与湍流频率和湍流能量分布有关。     

紊流工况下的滑动轴承性能分析

大机组轴承越来越多运行于紊流工况下,采用不同的紊流模型计算分析机组轴承的性能存在差异。该文目的是分析采用不同的紊流模型计算结果之间的差异,为选择紊流下轴承性能分析提供依据。该文对比了采用四种紊流模型对椭圆轴承性能的分析结果,包括最小膜厚、温升,流量、刚度阻尼等,并采用单元盘弹性转子一轴承模型讨论轴承的稳定性。分析结果显示,应该以分析目的和分析范围的不同来选择紊流模型。在稳定性分析中,采用青木弘模型计算结果比其他三种模型更趋保守;在轴承流量分析,对于纯紊流下分析可采用青木弘模型,对于层紊流过渡区可采用Ng-Pan模型。     

Analytical fuel cell modeling; non-isothermal fuel cells

The isothermal fuel cell model, given in an earlier publication, will be generalized to describe the behaviour of non-isothermal fuel cells of co-flow type. To this end the temperalure distribution inside a fuel cell in steady state is investigated analytically. A simplified relation between the local temperature and the fuel utilization is derived and its practical significance elucidated. Furthermore, it is shown that the solution of the non-isothermal model is accurately approximated by analytical expressions obtained from a so-called quasi-isothermal approach. This new approach yields a similar expression for the cell voltage as derived from the isothermal model. The quasi-isothermal approach is also used to make a clear comparison between the isothermal and the non-isothermal fuel cell model.     

Computation of a turbulent natural convection in a rectangular cavity with the elliptic-blending second-moment closure

A numerical study of a turbulent natural convection in an enclosure with the elliptic-blending second-moment closure (EBM) is presented. The primary emphasis of the study is placed on an investigation of the accuracy and numerical stability of the elliptic-blending second-moment closure for the turbulent natural convection flow. The turbulent heat fluxes in this model are treated by the general gradient diffusion hypothesis (GGDH). The model is applied to the prediction of a natural convection in a rectangular cavity and the computed results are compared with the experimental data commonly used for a validation of the turbulence models. The results are also compared with those by the two-layer model, the SST model, the V2-f model and the second-moment differential stress and flux model. It is shown that the elliptic blending model predicts as good as or better than the existing models for the mean velocity and turbulent quantities although this model employs a simpler GGDH for treating the turbulent heat fluxes.     

Analyses of heat and water transport interactions in a proton exchange membrane fuel cell

A two-phase non-isothermal model is developed to explore the interaction between heat and water transport phenomena in a PEM fuel cell. The numerical model is a two-dimensional simulation of the two-phase flow using multiphase mixture formulation in a single-domain approach. For this purpose, a comparison between non-isothermal and isothermal fuel cell models for inlet oxidant streams at different humidity levels is made. Numerical results reveal that the temperature distribution would affect the water transport through liquid saturation amount generated and its location, where at the voltage of 0.55 V, the maximum temperature difference is 3.7 °C. At low relative humidity of cathode, the average liquid saturation is higher and the liquid free space is smaller for the isothermal compared with the non-isothermal model. When the inlet cathode is fully humidified, the phase change will appear at the full face of cathode GDL layer, whereas the maximum liquid saturation is higher for the isothermal model. Also, heat release due to condensation of water vapor and vapor-phase diffusion which provide a mechanism for heat removal from the cell, affect the temperature distribution. Instead in the two-phase zone, water transport via vapor-phase diffusion due to the temperature gradient. The results are in good agreement with the previous theoretical works done, and validated by the available experimental data.     

Laminar flow distribution in solar systems

This work investigates the isothermal and non-isothermal laminar flow distribution through water solar systems. Reverse, as well as direct return circuits are studied. First and second order algebraic equations were developed for isothermal and non-isothermal, fully developed laminar flow distribution, taking into consideration experimental results on junction pressure losses. The effect of the various parameters appearing in the derived equations is also investigated. The flow uniformity was found to be greater in reverse return circuits rather than in centrally-fed direct return circuits.     

Creeping flow of a polymeric liquid passing over a transverse slot with viscous dissipation

Numerical simulations for the non-isothermal flow of a nylon-6 fluid passing over a transverse slot with heat dissipation are considered with a differential-type non-isothermal White-Metzner model describing the non-Newtonian behavior of the melt. The results obtained in the study are computed by using the elastic-viscous split-stress finite element method incorporating the non-consistent streamline-upwind scheme. As a verification of the numerical scheme, the algorithm is first applied to compute the corresponding isothermal flow of the upper-convected Maxwell fluid, a special case of the melt, characterized by constant viscosity and relaxation time. Hole pressure was evaluated for various Deborah numbers (De), and compared with that derived from the Higashitani-Pritchard (HP) theory. The agreement between the two is found to be satisfactory for creeping flow in the De range for which the HP theory is valid. Subsequently, hole pressure and other flow characteristics were predicted. Furthermore, the effects of heat-transfer, shear-thinning, and slot geometry on hole pressure were also investigated.     

Computation of turbulent natural convection with the elliptic-blending differential and algebraic flux models

A computation of turbulent natural convection in enclosures with the elliptic-blending based differential and algebraic flux models is presented. The primary emphasis of the study is placed on an investigation of accuracy of the treatment of turbulent heat fluxes with the elliptic-blending second-moment closure for the turbulent natural convection flows. The turbulent heat fluxes are treated by the elliptic-blending based algebraic and differential flux models. The proposed models are applied to the prediction of turbulent natural convections in a 1:5 rectangular cavity and in a square cavity with conducting top and bottom walls. It is shown that both the elliptic-blending based models predict well the mean velocity and temperature, thereby the wall shear stress and Nusselt number. It is also shown that the elliptic-blending based algebraic flux model produces solutions which are as accurate as those by the differential flux model.     

Simulations of post-filling process considering the cooling effect of the mold cooling systems

Simulation of the post-filling process has been developed and performed to study the compressible polymer melt flow during the packing phase and the pressure development inside the mold cavity for the entire post-filling process. A mixed finite-element/finite-difference numerical scheme is implemented to solve the non-isothermal, compressible viscous flow equations. The transient, non-isothermal mold cavity surface temperatures which depend on the mold cooling channel arrangement and coolant flow conditions are also incorporated during simulation using hybrid finite-element/shape-factor method. It has been found that the difference in the pressure profile variation during the post-filling stage is quite distinguished for cases assuming constant mold wall temperature and cases with the consideration of the cooling effect of the cooling system configuration.