Numerical investigation of transpiration and ablation cooling

To predict the integral performance of transpiration and ablation cooling during the reentry of hypersonic vehicles, an unsteady numerical model based on the assumption of thermal equilibrium is presented. The non-thermal equilibrium model and the thermal equilibrium model are coupled by the effective thermal properties of the porous matrix and the coolant. The calculation using the thermal equilibrium model shows the influence of the variation of the effective thermal properties on the numerical results by a comparison between constant and variable thermal properties. The comparison indicates that near the melting temperature of the porous matrix, the position of the moving boundary due to ablation is sensitive to the temperature, therefore, the variation of the thermal properties are considered in this paper. The process of ablation and transpiration cooling is simulated under different numerical conditions. The simulations demonstrate that the injection rate of coolant mass flow and initial temperature of cooling are important parameters for the control of the ablation process.     

三点起爆形成尾翼EFP的数值模拟和实验研究

利用LS-DYNA软件对三点起爆形成尾翼EFP的过程进行了数值模拟,深入研究了爆轰波传播过程中的波形结构和强度的变化规律以及药型罩材料在复合爆轰波作用下驱动变形的特性和规律,加深了对三点起爆条件下药型罩形成带尾翼EFP机理的认识。在此基础上,设计了三点同步起爆装置和EFP装药进行实验。研究结果表明：设计的三点起爆装置作用可靠,满足三点起爆EFP装药的设计要求;形成的EFP弹形稳定,与计算结果吻合较好;尾翼EFP飞行过程中的速度降减小,稳定性提高。     

Hydrodynamics of thin liquid films. Experimental investigation of the effect of surfactants on the drainage of emulsion films

In our previous theoretical work on the velocity of thinning of emulsion films we reached the conclusion that this velocity differs only slightly from that for foam films when the surfactant is soluble in the dispersion medium and is equal to that for a system without any surfactant when the surfactant is soluble in the dispersion phase. In the present work these results are checked experimentally by investigating the effect of different concentrations of surfactant in the dispersed phase (a) on the velocity of thinning of films, containing very low concentrations of surfactant soluble in the dispersion medium and (b) on the life time of films from pure liquids. The experiments show that within the precision limits of the experimental methods used, the presence of surfactant in the dispersed phase has no effect on the velocity of thinning and life time of emulsion films in accord with the theoretical conclusions.     

Numerical simulation of secondary vortex chamber effect on the cooling capacity enhancement of vortex tube

A vortex tube with additional chamber is investigated by computational fluid mechanics techniques to realize the effects of additional chamber in Ranque–Hilsch vortex tube and to understand optimal length for placing the second chamber in order to have maximum cooling effect. Results show that by increasing the distance between two chambers, both minimum cold and maximum hot temperatures increase and maximum cooling effect occurs at Z/L = 0.047 (dimensionless distance).     

Numerical simulation of the effect of plasma aerodynamic actuation on improving film hole cooling performance

The primary goal of this paper is to study film cooling performance for a cylindrical hole with plasma aerodynamic actuation. The simulation model of plasma aerodynamic actuation on improving film hole cooling effectiveness was established. The heat effect of plasma aerodynamic actuation model was taken into consideration. It was firstly found that the velocity and blowing ratio greatly affect the film cooling effectiveness. Then, position, power input, and the number of plasma actuators were particularly investigated.     

Numerical simulation of control ablation by transpiration cooling

A transient, one-dimensional numerical model is developed to describe the processes of transpiration cooling and ablation of the porous matrix used for the cooling. This model is based on the assumption of local thermal equilibrium. The problem of moving boundary due to ablation of the porous matrix is treated by the front-fixing method. This paper discusses the results of numerical simulations under different conditions and control parameters of ablation process. It was found that cooling effects and ablation processes are influenced by the coolant mass flow rate, the intensity of the heat flux, and the initial temperature at the start of transpiration cooling. In additional to the above three parameters, the Stefan number and the Biot number can also influence the transient cooling process, control ablative thickness of the porous plate by the reduction of ablative speed and duration, respectively.     

Numerical simulation of the nanoparticle diameter effect on the thermal performance of a nanofluid in a cooling chamber

The thermal performance of a nanofluid in a cooling chamber with variations of the nanoparticle diameter is numerically investigated. The chamber is filled with water and nanoparticles of alumina (Al₂O₃). Appropriate nanofluid models are used to approximate the nanofluid thermal conductivity and dynamic viscosity by incorporating the effects of the nanoparticle concentration, Brownian motion, temperature, nanoparticles diameter, and interfacial layer thickness. The horizontal boundaries of the square domain are assumed to be insulated, and the vertical boundaries are considered to be isothermal. The governing stream-vorticity equations are solved by using a secondorder central finite difference scheme coupled with the mass and energy conservation equations. The results of the present work are found to be in good agreement with the previously published data for special cases. This study is conducted for the Reynolds number being fixed at Re = 100 and different values of the nanoparticle volume fraction, Richardson number, nanofluid temperature, and nanoparticle diameter. The results show that the heat transfer rate and the Nusselt number are enhanced by increasing the nanoparticle volume fraction and decreasing the Richardson number. The Nusselt number also increases as the nanoparticle diameter decreases.     

Numerical simulation of transpiration cooling through porous material

Transpiration cooling using ceramic matrix composite materials is an innovative concept for cooling rocket thrust chambers. The coolant (air) is driven through the porous material by a pressure difference between the coolant reservoir and the turbulent hot gas flow. The effectiveness of such cooling strategies relies on a proper choice of the involved process parameters such as injection pressure, blowing ratios, and material structure parameters, to name only a few. In view of the limited experimental access to the subtle processes occurring at the interface between hot gas flow and porous medium, reliable and accurate simulations become an increasingly important design tool. In order to facilitate such numerical simulations for a carbon/carbon material mounted in the side wall of a hot gas channel that are able to capture a spatially varying interplay between the hot gas flow and the coolant at the interface, we formulate a model for the porous medium flow of Darcy–Forchheimer type. A finite‐element solver for the corresponding porous medium flow is presented and coupled with a finite‐volume solver for the compressible Reynolds‐averaged Navier–Stokes equations. The two‐dimensional and three‐dimensional results at Mach number Ma = 0.5 and hot gas temperature T_HG=540 K for different blowing ratios are compared with experimental data. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical investigation of unsteady mixing mechanism in plate film cooling

A large-scale large eddy simulation in high performance personal computer clusters is carried out to present unsteady mixing mechanism of film cooling and the development of films.Simulation cases include a single-hole plate with the inclined angle of 30° and blowing ratio of 0.5,and a single-row plate with hole-spacing of 1.5D and 2D(diameters of the hole).According to the massive simulation results,some new unsteady phenomena of gas films are found.The vortex system is changed in different position with the development of film cooling with the time marching the process of a single-row plate film cooling.Due to the mutual interference effects including mutual exclusion,a certain periodic sloshing and mutual fusion,and the structures of a variety of vortices change between parallel gas films.Macroscopic flow structures and heat transfer behaviors are obtained based on 20 million grids and Reynolds number of 28600.     

Heat transfer and crystallization kinetics during fast cooling of thin polymer films

In this work, the heat transfer phenomena taking place during the cooling of thin films of crystallizable polymers were analyzed. The thermal histories, as recorded during experimental cooling runs carried out at various cooling rates, were compared with the predictions of a general purpose numerical code, which was resulted able to capture all the main features of the process. Thus, the conditions which allow homogeneous cooling (negligible temperature gradient within the sample) or homogeneous cooling history (the same cooling history for all the positions within the sample) were predicted by the simulation code.     

Numerical and experimental investigation of transverse injection flows

The flow field resulting from a transverse injection through a slot into supersonic flow is numerically simulated by solving Favre-averaged Navier–Stokes equations with κ − ω SST turbulence model with corrections for compressibility and transition. Numerical results are compared to experimental data in terms of surface pressure profiles, boundary layer separation location, transition location, and flow structures at the upstream and downstream of the jet. Results show good agreement with experimental data for a wide range of pressure ratios and transition locations are captured with acceptable accuracy. κ − ω SST model provides quite accurate results for such a complex flow field. Moreover, few experiments involving a sonic round jet injected on a flat plate into high-speed crossflow at Mach 5 are carried out. These experiments are three-dimensional in nature. The effect of pressure ratio on three-dimensional jet interaction dynamics is sought. Jet penetration is found to be a non-linear function of jet to free stream momentum flux ratio.     

Magnetoelasticity of highly deformable thin films: Theory and simulation

A non-linear two-dimensional theory is developed for thin magnetoelastic films capable of large deformations. This is derived directly from the three-dimensional theory. Significant simplifications emerge in the descent from three dimensions to two, permitting the self-field generated by the body to be computed a posteriori. The model is specialized to isotropic elastomers and numerical solutions are obtained to equilibrium boundary-value problems in which the membrane is subjected to lateral pressure and an applied magnetic field.     

薄膜破坏过程数值模拟的MPM方法

将连续与不连续本构模型相结合用于薄膜破坏过程的模拟计算. 采用von Mises关联塑性本构模型描述材料的弹塑性变形过程,在塑性硬化和软化阶段通过检查分岔 是否发生, 从而判断是否有破坏出现,即材料出现应变局部化现象. 一旦出现破坏,则在发生 破坏的区域采用位移不连续(decohesion)本构模型进行模拟, 直到破坏面两侧材料完全分离. 分离阶段采用相应的分离算法同时运用黏性边界条件以提高计算效率. 结果表明结合连续与 不连续本构模型的方法, 能很好地模拟材料从局部化到完全破坏的过程,体现了材料破坏连续 建模的思想,可用于预测材料发生破坏的环境和形式, 从而通过改变材料工作环境提高其使用 寿命;同时数值算例也显示了物质点法(material-point-method, 简称MPM)的健壮性和有 效性.     

Numerical and experimental investigation of variable phase transformation number effect in porous media during freezing process

A model is proposed to investigate heat and moisture transfer in porous media during freezing process based on Luikov’s model by considering the effect of variation of phase transformation number, ε. This parameter has been mostly used as a constant by researchers. Three-dimensional Luikov’s equations are considered and solved numerically. The model is compared with obtained experimental data. It is shown that the effect of variable phase transformation number is noticeable in heat and moisture transfer process.     

Numerical and experimental investigation of a serpentine inlet duct

This article presents a numerical and experimental investigation of the flow inside an ultra-compact, serpentine inlet duct. The numerical analysis used two flow solvers: FLUENT®, a commercial code, and UNS3D, an in-house code. The flow was modelled using the Reynolds-averaged Navier-Stokes equations. The turbulence effects were modelled by using the shear-stress transport k–ω model. The numerical investigation was compared against experimental data obtained in an open-circuit, low-speed wind tunnel in the Fluid Dynamics Laboratory at Texas A&M University. The numerical simulations and experimental testing were performed to reveal the separation points and the strong secondary flow phenomena within the inlet. UNS3D overpredicted the location of the first separation point by 9 mm and the location of the second separation point by 1 mm, while the area-averaged pressure loss coefficient was 5% higher than in the experiment. The numerical results of UNS3D agreed better with the experiment than those of FLUENT.     

Numerical investigation of the aerodynamics of the near‐slot film cooling

Fluid injection from slot or holes into cross‐flow produces highly complicated flow fields. Physical situations encountering the above problem range from turbine blade cooling to waste discharge into rivers. In this paper, the flow field created by a two‐dimensional slot cooling geometry is examined using the finite volume approach with a second‐order upwind differencing scheme. The time‐averaged Navier–Stokes equations were solved on a collocated Cartesian grid with a two‐equation model of turbulence. Attempting to solve the flow field by assuming a uniform velocity profile at the slot exit leads to inaccurate results, while extending the solution domain improves significantly the results, but proves to be costly, both in memory and in computing time (particularly in the case of multiple holes). A pressure‐type boundary condition, based on uniform total pressure, is developed for the slot exit (easily applied to a three‐dimensional geometry), which yields more accurate results than the widely used uniform velocity assumption. It is also found that the implementation of low Reynolds number turbulence models on this geometry provides no significant differences from the standard k–ε model. Copyright © 2000 John Wiley & Sons, Ltd.     

Numerical and experimental investigation of the laminar boundary layer over riblets

One of the main aims of this work is to show to what extent drag reduction in a turbulent boundary layer can be ascribed to a purely viscous effect. A numerical and experimental study is performed in a laminar boundary layer over triangular riblets. The 2-D parabolic equations of motion are integrated using an x marching method and the discretised system is solved with the MSI algorithm. The influence of the riblet geometrical parameters and of the number of grid points is studied. Measurements are carried out in a water tunnel with forward scatter and backscatter laser-Doppler velocimetry extending within the riblets. The longitudinal velocity component measurements and computations are practically identical. Numerical results presented herein show that a slight drag reduction is obtained for s/h=1.2. It appears that, as far as friction is concerned, the wetted area is not the surface to be considered. Thus, the boundary layer over riblets would behave like a boundary layer on an equivalent smooth plate located beneath the crest plane. The numerical study in terms of the riblet height h shows best results are for h tending to zero, with the ratio s/h being equal to 1.2.     

Numerical and experimental investigation of turbulent flow in a rectangular duct

Details of the turbulent flow in a 1:8 aspect ratio rectangular duct at a Reynolds number of approximately 5800 were investigated both numerically and experimentally. The three-dimensional mean velocity field and the normal stresses were measured at a position 50 hydraulic diameters downstream from the inlet using laser doppler velocimetry (LDV). Numerical simulations were carried out for the same flow case assuming fully developed conditions by imposing cyclic boundary conditions in the main flow direction. The numerical approach was based on the finite volume technique with a non-staggered grid arrangement and the SIMPLEC algorithm. Results have been obtained with a linear and a non-linear (Speziale) k–ε model, combined with the Lam–Bremhorst damping functions for low Reynolds numbers. The secondary flow patterns, as well as the magnitude of the main flow and overall parameters predicted by the non-linear k–ε model, show good agreement with the experimental results. However, the simulations provide less anisotropy in the normal stresses than the measurements. Also, the magnitudes of the secondary velocities close to the duct corners are underestimated. © 1998 John Wiley & Sons, Ltd.