Lattice Boltzmann simulations for flow and heat/mass transfer problems in a three‐dimensional porous structure

The lattice Boltzmann method (LBM) for a binary miscible fluid mixture is applied to problems of transport phenomena in a three‐dimensional porous structure. Boundary conditions for the particle distribution function of a diffusing component are described in detail. Flow characteristics and concentration profiles of diffusing species at a pore scale in the structure are obtained at various Reynolds numbers. At high Reynolds numbers, the concentration profiles are highly affected by the flow convection and become completely different from those at low Reynolds numbers. The Sherwood numbers are calculated and compared in good agreement with available experimental data. The results indicate that the present method is useful for the investigation of transport phenomena in porous structures. Copyright © 2003 John Wiley & Sons, Ltd.     

On the relative impact of subgrid‐scale modelling and conjugate heat transfer in LES of hot jets in cross‐flow over cold plates

This work describes numerical simulations of a hot jet in cross‐flow with applications to anti‐ice systems of aircraft engine nacelles. Numerical results are compared with experimental measurements obtained at ONERA to evaluate the performances of LES in this industrial context. The combination of complex geometries requiring unstructured meshes and high Reynolds number does not allow the resolution of boundary layers so that wall models must be employed. In this framework, the relative influence of subgrid‐scale modelling and conjugate heat transfer in LESs of aerothermal flows is evaluated. After a general overview of the transverse jet simulation results, a LES coupled with a heat transfer solver in the walls is used to show that thermal boundary conditions at the wall have more influence on the results than subgrid scale models. Coupling fluid flow and heat transfer in solids simulations is the only method to specify their respective thermal boundary conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental study of heat transfer characteristics due to confined impinging two-dimensional jets

Experimental results are presented for characteristics of impingement heat transfer caused by three slot jets. Experimental values were obtained for the dimensionless distance H = 0.5−3, dimensionless pitch P = 6−16, and Reynolds number Re = 500−8000. For laminar impinging flow, they were compared with numerical results. For turbulent impinging flow, two peaks of the local Nusselt number were obtained behind the second nozzle. The position of the second peak approached the nozzle as the space between nozzle and impinged surface decreased. The average Nusselt number between the central and second nozzles was determined from the ratio P/H and the Reynolds number based on the pitch of the nozzles.     

ON THE MECHANISM OF TURBULENT COHERENT STRUCTURE (III)──A STATISTICAL AND DYNAMICAL MODEL OF COHERENT STRUCTURE AND ITS HEAT TRANSFER MECHANISM

Following Tsai & Ma^[1] and Tsai & Liu^[2], a statistical and dynamical near-wall turbulent coherent structural model with separate consideration of two different portions: locally generated and upstream-transported large eddies has been established. With this model, heat transfer in a fully developed open channel in the absence of pressure gradient is numerically simulated. Database of fluctuations of velocity and temperature has also been set. Numerical analysis shows the existence of high-low temperature streak caused by near-wall coherent structure and its swing in the lateral direction. Numerical results are in accordance with the computations and experimental results of other researchers.     

二维水平通道内流动沸腾换热的格子Boltzmann模拟

利用格子Boltzmann方法模拟二维水平通道内水的流动沸腾过程,获得不同壁面过热度下流型特点和不同因素对换热过程的影响规律。结果表明,随着壁面过热度升高,流道内流型依次经历从泡状流、弹状流到反环流的转变,平均热流密度和平均换热系数先增大后减小。入口流速降低会使流道内出现受限气泡流,核态沸腾受到抑制。提高入口流速能够有效促进气泡脱离,壁面平均换热系数随入口流速增大而增大,但增长速率有所减小。减小通道宽度有利于汽化现象发生,核态沸腾得到强化,壁面平均换热系数有所提高。     

Single-phase convection and boiling heat transfer: Confined single and array-circular impinging jets

In the present study, the effects of diverse situations of confinement on heat transfer from single and array-circular jet impingements are carefully investigated over various heat transfer regimes of single-phase convection and fully developed nucleate boiling. For the single, circular, unconfined free-surface jet, the transition to turbulence was observed to start around x/d = 5.5 and end around x/d = 9. For the array-circular jet, however, the wall jet structure yielded no transition to turbulence for all the tested cases, instead monotonically decreasing the convection coefficient. Conversely, the single-circular jet experienced the transition for V ? 6.1 m/s. For the confined submerged jet, the transition length was very short due to the vigorous mixing driven by lateral velocity components, and the locus of the secondary peak moved downstream as velocity increased. The temperature distributions of the confined array-circular jet were fairly uniform over the whole heated surface. The averaged single-phase convection coefficients indicated that the confined jet provided the most uniform convection in the lateral direction.     

Coherent contribution to turbulent mixing of a jet in cross flow

In this work, an experimental study of a jet issuing in cross flow is reported. Support is offered to a theory on the origin of Karman-like vortices, in the wake of jets issuing in boundary layers. Further, a coherent structure with strong contribution to the energetics of the flow field is identified here. This structure has not been reported earlier, according to the present authors' knowledge, and it is shown to be related to engulfment of external fluid at the bottom of the jet, and to interesting stochastic and spectral characteristics of the flow field. Three-dimensional plots of the coherent quantities, based on experimental data, reveal a double-helical morphology of the coherent structure. The same morphology has recently been proposed for the far field of jets issuing in stagnant fluid (i.e., without cross flow). The results of this study are expected to support theoretical and numerical work on jets issuing in cross flow.     

Numerical simulation of vortical and coherent structures in compressible jet flows

The compressible flows of plane free jets and jets of the intake-stroke of a rectangular piston-engine model are investigated by numerical simulations. The observed vortical structures appear to be the well-known coherent structures of turbulent shear layers. The simulated structures are compared to experimental data by means of density fields and turbulent statistics taken from different authors. The computed flow depends on physical as well as on numerical parameters. The good agreement with the experimental data is obtained by direct simulation without any turbulence model.     

Multiscale simulation of flow and heat transfer of nanofluid with lattice Boltzmann method

Based on the lattice Boltzmann (LB) approach, a novel hybrid method has been proposed for getting insight into the microscale characteristics of the multicomponent flow of nanofluid. In this method, the whole computational domain is divided into two regions in which different-sized meshes are involved for simulation (fine mesh and coarse mesh). The multicomponent LB method is adopted in the fine mesh region, and the single-component LB approach is applied to the coarse mesh region where the nanofluid is treated as a mixed single-component fluid. The conservation principles of mass, momentum and energy are used to derive a hybrid scheme across the different scaled regions. Numerical simulation is carried out for the Couette flow and convective heat transfer in a parallel plate channel to validate the hybrid method. The computational results indicate that by means of the present method, not only the microscopic characteristics of the nanofluid flow can be simulated, but also the computational efficiency can be remarkably improved compared with the pure multicomponent LB method.     

脉冲流在Y型和倒Y型微反应器中强化传质的格子Boltzmann模拟

连续流微反应器的迅速发展为化学合成技术提供了一条可精准控制的路径。微反应器中流体的流动、混合和传质是反应的物理基础,因此强化传递就能够合理改善混和效果,增加相接触面积,减小微通道尺寸,以缩短分子扩散距离。由于微反应器中对流传递数量级非常低,不容易控制。通过改进微混合器构型和引入脉冲流动等主被动强化措施,可以有效改善对流和传质,影响化合反应。如何量化分析这些影响,是微反应器强化传质和优化控制反应过程的基础。本文基于有效的虚拟串行竞争反应格子Boltzmann模型,通过对Y型和倒Y型的微反应器的流场结构、混合传质和化合反应进行数值研究,定量分析了脉冲流动在不同流场构型下传质和化学反应的影响,所得结论可为连续流微反应器设计以及微反应精准控制提供有意义的参考。     

燃气射流冲击传热特性的数值模拟

针对射流传热问题,利用基于RNGk-ε湍流模型的数值方法模拟了射流垂直冲击平板的流动过程,并与实验数据比较,验证了模型的可行性。在此基础上,以火箭喷管入口参数为入口条件,建立了超音速燃气射流垂直冲击平板和冲击浸没平板的计算模型,分析了不同冲击条件下努塞尔数分布规律和温度分布规律, 论述了超音速射流传热的特性及影响传热特性的因素。得到了冲击距离为(14~18)D的努塞尔数取值范围,并表明冲击距离和射流温度是影响传热效率的关键因素;冲击距离增加,传热效率降低,冲击平板表面的射流温度越高,传热效率越高。     

微通道人工泡状流的Lattice Boltzmann数值模拟

采用高频电控热激发汽泡的方式构造微通道人工泡状流,可以有效抑制微通道沸腾流动的不稳定性和强化传热。本文基于Lattice Boltzmann大密度比多相流复合模型,数值研究了通道内人工泡状流的流动和传热,通过比较分析不同发泡频率的泡状流,量化分析了汽泡运动和增长对微通道流动与传热的相互影响。一方面着重分析了汽泡运动对微通道运动边界层以及汽泡相变增长对热边界层的影响,另一方面也研究了边界层对汽泡动力行为的影响,所得结论对研究抑制微通道沸腾流动不稳定性和强化传热有参考意义。     

Periodic flow and heat transfer using unstructured meshes

A numerical scheme has been developed for computing fluid flow and heat transfer in periodically repeating geometries. Unstructured solution-adaptive meshes are used in a cell-centred finite volume formulation. The SIMPLE algorithm is used for pressure‒velocity coupling. For periodic flows the static pressure is decomposed into a periodic component and one that varies linearly in the streamwise direction. The latter is computed from the imposition of overall mass balance at the periodic boundary. A subiteration between the periodic pressure correction equation and the correction to the linear component is used. For heat transfer a formulation using the physical rather than the scaled temperature is employed. The scheme is applied to both laminar and turbulent computations of periodic flow and heat transfer in a variety of heat exchanger geometries; comparison with published computations and experimental data is found to be satisfactory. © 1997 John Wiley & Sons, Ltd.     

Motion and heat and mass transfer in a disperse admixture in turbulent nonisothermal jets of a gas and a low-temperature plasma

The motion and heat and mass transfer of particles of a disperse admixture in nonisothermal jets of a gas and a low-temperature plasma are simulated with allowance for the migration mechanism of particle motion actuated by the turbophoresis force and the influence of turbulent fluctuations of the jet flow velocity on heat and mass transfer of the particle. The temperature distribution inside the particle at each time step is found by solving the equation of unsteady heat conduction. The laws of scattering of the admixture and the laws of melting and evaporation of an individual particle are studied, depending on the injection velocity and on the method of particle insertion into the jet flow. The calculated results are compared with data obtained with ignored influence of turbulent fluctuations on the motion and heat and mass transfer of the disperse phase. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 3, pp. 95–108, May–June, 2008.     

Shear-opposed mixed-convection flow and heat transfer in a narrow, vertical cavity

A combined experimental and theoretical study has been carried out to investigate mixed-convection heat transfer in a narrow, vertical cavity. The shear force is produced at the left side of a cavity by a belt moving upward that constitutes the sixth wall of the cavity. The left wall of the cavity was cooled and the opposite (right wall) was heated. Hence, the buoyancy force tries to bring the fluid down, and the shear force tends to induce upward fluid flow. The test cell was equipped with two heat exchangers and three thermocouple racks for measuring the temperature distributions at 12 different positions. The temperature field was scanned in the cavity for various flow and temperature boundary conditions. Three-and-two-dimensional laminar models were used to analyze the problem theoretically. The experimental measurements were found to be in good agreement with the numerical predictions.     

Forced‐convection heat transfer from tandem square cylinders in cross flow at low Reynolds numbers

This paper presents the results of a numerical study on the flow characteristics and heat transfer over two equal square cylinders in a tandem arrangement. Spacing between the cylinders is five widths of the cylinder and the Reynolds number ranges from 1 to 200, Pr=0.71. Both steady and unsteady incompressible laminar flow in the 2D regime are performed with a finite volume code based on the SIMPLEC algorithm and non‐staggered grid. A study of the effects of spatial resolution and blockage on the results is provided. In this study, the instantaneous and mean streamlines, vorticity and isotherm patterns for different Reynolds numbers are presented and discussed. In addition, the global quantities such as pressure and viscous drag coefficients, RMS lift and drag coefficients, recirculation length, Strouhal number and Nusselt number are determined and discussed for various Reynolds numbers. Copyright © 2008 John Wiley & Sons, Ltd.     

Effects of temperature-dependence of viscosity and viscous dissipation on laminar flow heat transfer in circular tubes

Heat transfer effects of variable viscosity and viscous dissipation for heated developing laminar flows in circular tubes have been investigated. Three studies are reported covering a comprehensive range of input data for the case of constant wall heat flux. Initially the program was used to predict the effect on heat transfer of temperature-dependent viscosity via a general temperature power relation. In addition, predictions were made for nine particular fluids covering a range of Prandtl numbers from 0.025 to 12 500, and a range of Brinkman numbers from 1.8 × 10^?10 to 6.8 × 10³. A more detailed study was made for two particular oils covering a range of practical interest. For the liquids considered their viscosity temperature-dependence resulted in enhancement of heat transfer, whereas for fluids with a Prandtl number <200 the effect of viscous dissipation was negligible, and for fluids of a Brinkman number > × 10^?2 the outcome was a reduction of heat transfer. A numerical instability problem occurred for situations of very high viscous dissipation which limited the length of duct that could be examined.     

具有壁面射流激励的圆管内气相流动大涡模拟

采用大涡模拟方法和单方程亚格子模式对小尺度量进行模拟。研究了不同强度壁面射流激励对圆管内气相流动的影响,模拟结果给出了射流对瞬态拟序结构发展、时平均流向速度分布的影响。随着射流强度的增加,射流入口附近流体的回流现象增强。射流强度足够大时可以减小管壁处的切应力值,同时会减小壁面附近流动速度,这种速度分布会导致气体夹带颗粒的能力下降,从而在实际两相流动中容易造成壁面附近的气粒返混现象。