圆形自由射流冲击曲面的换热特性

采用直接表面温度测量的方法对水喷射高温曲面的传热过程进行实验研究.通过实验得到了驻点区及附壁射流区的对流传热系数的分布情况,并且系统地研究了射流出口速度、喷嘴至加热面的间距等参数对对流传热系数的影响.结果表明,圆形射流冲击曲面的局部传热系数沿传热面随X/D的增大而逐渐减小,驻点处的传热系数最大并且随射流速度的增大而增大,但是当射流速度增大到一定值时,驻点的对流传热系数的增大比较缓慢.在射流速度较低的情况下,喷距对局部传热系数的影响较为显著.     

A numerical study of particle wall-deposition in a turbulent square duct flow

The deposition of dense solid particles in a downward, fully developed turbulent square duct flow at Re_τ = 360, based on the mean friction velocity and the duct width, is studied using large eddy simulations of the fluid flow. The fluid and the particulate phases are treated using Eulerian and Lagrangian approaches, respectively. A finite-volume based, second-order accurate fractional step scheme is used to integrate the incompressible form of the unsteady, three-dimensional, filtered Navier-Stokes equations on an 80 × 80 × 128 grid. A dynamic subgrid kinetic energy model is used to account for the unresolved scales. The Lagrangian particle equation of motion includes the drag, lift, and gravity forces and is integrated using the fourth-order accurate Runge-Kutta scheme. Two values of particle to fluid density ratio (ρ_p/ρ_f = 1000 and 8900) and five values of dimensionless particle diameter (d_p/δ × 10⁶ = 100, 250, 500, 1000 and 2000, δ is the duct width) are studied. Two particle number densities, consisting of 10⁵ and 1.5 × 10⁶ particles initially in the domain, are examined.Variations in the probability distribution function (PDF) of the particle deposition location with dimensionless particle response time, i.e. Stokes number, are presented. The deposition is seen to occur with greater probability near the center of the duct walls, than at the corners. The average streamwise and wall-normal deposition velocities of the particles increase with Stokes number, with their maxima occurring near the center of the duct wall. The computed deposition rates are compared to previously reported results for a circular pipe flow. It is observed that the deposition rates in a square duct are greater than those in a pipe flow, especially for the low Stokes number particles. Also, wall-deposition of the low Stokes number particles increases significantly by including the subgrid velocity fluctuations in computing the fluid forces on the particles. Two-way coupling and, to a greater extent, four-way coupling are seen to increase the deposition rates.     

Study of four-way coupling on turbulent particle-laden jet flows

This paper investigates the influence of inter-particle collisions on the particle phase variables in the configuration of a free turbulent round jet laden with solids, i.e., considering the so-called four-way coupling. As a result, and due to the absence of walls confining the flow, the effects of particle-particle interactions start to be relevant for larger mass loading ratios when compared with confined flows, such as those occurring in pipes or channels. Interestingly, the particle phase fields are modified similarly to what happens in the previous confined flows: the profile of mean axial velocity flattens and the turbulence tends to be more isotropic when inter-particle collisions are taken into account. However, the associated mechanisms are slightly different.     

Deformation and removal of viscous thin film by submerged jet impingement

In this work, for the first time, we characterize the deformation and removal dynamics of viscous thin films under submerged jet impingement using direct numerical simulation. The impinging jet deforms the thin film near the impingement region at the beginning until local removed region is formed. Subsequently, the removed region expands due to the shear stress imposed by the jet. At later times, we find that the film is removed in a quasi-adhesive removal mechanism, and its removal radius as a function of time, follows a scaling law with a universal removal rate for films over a wide range of viscosities. However, the kinetic constant of the removal scaling is highly dependent on the viscosity of the film. We also find that larger initial film thickness may change the removal mechanism, although the scaling law describing the removal rate still applies.     

液体阵列射流冲击冷板工质与传热结构研究进展

液体阵列射流冲击冷却是解决高热流密度散热问题的最有效技术之一,能够有效地对目标表面进行散热,具有散热能力高、能效比高和噪声低的优点,在散热方面具有巨大优势。本文简述了国内外对阵列射流冲击的研究进展,从换热工质和射流冲击冷板的换热结构两个方面,指出了其对液体阵列射流冲击换热特性的影响,并介绍了倾斜射流和旋流射流两种新型阵列射流方式。综合分析了常用的液体换热工质和纳米流体换热工质在射流冲击过程中强化换热的原理,介绍了喷嘴孔型、喷嘴的排列方式和冲击表面结构三种阵列射流结构。分析表明,不同孔型的喷嘴会影响流体的射流速度和湍流特性,不同的喷嘴排列方式会对射流流体的相互作用和有效冲击面积产生影响,不同的冲击表面会影响射流工质的循环混合,这些都将对射流冷板的换热特性产生很大影响。指出了解影响液体阵列射流冲击效果的主要因素,是改善和提高射流换热性能的根本方法。     

磁场对熔盐射流冲击传热的影响

以永磁铁构建定磁场,进行外加磁场作用下熔盐射流冲击传热的实验研究,并得到Nusselt数Nu驻点关联式和径向分布。结果表明,在驻点区范围内,Nusselt数较无磁场作用时增大,传热得到比较明显的增强,而在壁面射流区,这种强化传热效果逐渐减弱。此外,当Reynolds数Re一定时,熔盐Nusselt数随着磁场强度的增加而增大,且驻点处强化传热效果最为显著。在Reynolds数Re=6400与磁场强度B=2800 Gs条件下,熔盐驻点Nusselt数Nu₀提高约6％,可见磁场作用对熔盐射流冲击传热具有一定的强化效果。     

Influence of magnetic field on jet impingement heat transfer with molten salt

Experimental study of jet impingement heat transfer with molten salt under the influence of external constant magnetic field was generated by permanent magnets. Both stagnation correlation and radial distribution of Nusselt number under magnetic field were obtained. The results showed that the Nusselt number with magnetic field became higher than that without magnetic field at stagnation region and jet impingement heat transfer was comparatively enhanced, while in wall jet region, the enhancement of heat transfer was gradually weakened. In addition, when the Reynolds number was constant, the Nusselt number of molten salt increased with increasing of the intensity of magnetic field, and the most enhanced heat transfer existed at the stagnation point. Under the conditions of Reynolds number Re=6400 and the intensity of magnetic field B=2800 Gs, the stagnation Nusselt number of molten salt was about 6 % higher than that without magnetic field. It can be seen that the magnetic field may promote the jet impingement heat transfer of molten salt.     

Impingement heat transfer under a confined slot jet. Part II: Effects of surface motion and throughflow

Local and average heat transfer coefficients were measured for a confined turbulent slot jet impinging on a moving surface at which there may be throughflow. Profiles of the local convective coefficient at the impingement surface were obtained using a fast responding, highly sensitive porous heat flux sensor. The decrease in average heat transfer with surface motion is not negligible, ～20%, at values of the surface motion parameter, M_vs, comparable to those used in industry. The enhancement of heat transfer by throughflow at a moving impingement surface is linearly additive and, when expressed as δSt, is proportional to only the throughflow parameter, M_us, with a proportionality constant of 0.17 which is independent of Re, M_vs or extent of the heat transfer surface.     

Modelling of instabilities in turbulent swirling flames

A large eddy simulation-based data analysis procedure is used to explore the instabilities in turbulent non-premixed swirling flames. The selected flames known as SM flames are based on the Sydney swirl burner experimental database. The governing equations for continuity, momentum and mixture fraction are solved on a structured Cartesian grid and the Smagorinsky eddy viscosity model with dynamic procedure is used as the sub-grid scale turbulence model. The thermo-chemical variables are described using the steady laminar flamelet model. The results show that the LES successfully predicts the upstream first recirculation zone generated by the bluff body and the downstream second recirculation zone induced by swirl. Overall, LES comparisons with measurements are in good agreement. Generated power spectra and snapshots demonstrate oscillations of the centre jet and the recirculation zone. Snapshots of flame SM1 showed irregular precession of the centre jet and the power spectrum at a downstream axial location situated between the two recirculation zones showed distinct precession frequency. Mode II instability defined as cyclic expansion and collapse of the recirculation zone is also identified for the flame SM2. The coupling of swirl, chemical reactions and heat release exhibits Mode II instability. The presented simulations demonstrate the efficiency and applicability of the LES technique to swirl flames.     

CFD study of non-premixed swirling burners: Effect of turbulence models

This research investigates a numerical simulation of swirling turbulent non-premixed combustion. The effects on the combustion characteristics are examined with three turbulence models: namely as the Reynolds stress model, spectral turbulence analysis and Re-Normalization Group. In addition, the P-1 and discrete ordinate (DO) models are used to simulate the radiative heat transfer in this model. The governing equations associated with the required boundary conditions are solved using the numerical model. The accuracy of this model is validated with the published experimental data and the comparison elucidates that there is a reasonable agreement between the obtained values from this model and the corresponding experimental quantities. Among different models proposed in this research, the Reynolds stress model with the Probability Density Function (PDF) approach is more accurate (nearly up to 50%) than other turbulent models for a swirling flow field. Regarding the effect of radiative heat transfer model, it is observed that the discrete ordinate model is more precise than the P-1 model in anticipating the experimental behavior. This model is able to simulate the subcritical nature of the isothermal flow as well as the size and shape of the internal recirculation induced by the swirl due to combustion.     

Study on shape deviation and crack of ultra-thin glass molding process for curved surface

The demands towards high precision and surface quality of ultra-thin glass for curved screens are continuously rising in the field of smart mobile terminals. Although the ultra-thin glass molding process (UTGMP) has the advantage of the shorter production cycle and higher efficiency, there are still typical forming defects in the molding process, namely crack, shape deviation, and large surface roughness. This paper aimed to investigate the influence mechanism of UTGMP molding temperature and pressure on the shape deviation, crack area, and surface quality of ultra-thin glass. In this study, a finite element model (FEM) was established to study typical forming defects of curved surfaces, and the effects of molding temperature and pressure on the shape deviation and crack area for ultra-thin glass were studied by the FEM simulation method. The simulation results revealed the molding temperature has a significant effect on the shape deviation, crack area and surface quality, while the molding pressure is only strongly correlated with shape deviation and crack area. In addition, the reliability of the model was verified by a series of five-level single factor experiments, and the shape deviation and crack area of ultra-thin glass were discussed in detail. Under the appropriate molding pressure and temperature range (0.45 MPa, 802–806 °C), the accuracy of curvature was improved by 33%, the roughness was reduced by 21%, and the probability of crack was also reduced. Thus, this study contributes to improving UTGMP's molding accuracy and reducing molding defects, and plays a positive role in reducing production costs and improving production efficiency.     

Discussion on second-order dispersed phase Eulerian equations applied to turbulent particle-laden jet flows

In this work a Reynolds stress two-phase flow model is presented. Equations for the second-order statistical moments are considered for both phases, continuous and dispersed, in the limit of high-inertia non-colliding particles. A simplified version of this model is used for studying axisymmetric particle-laden gas jets. Emphasis is made in the analysis of the dispersed phase equations only. The equations for radial and axial momentum and normal Reynolds stresses for the particulate phase are divided into their basic terms and analysed separately. The modelling of the corresponding shear stresses relies on a Boussinesq closure, consistent with the theoretical work of Reeks (Phys. Fluids A 5(3) (1993) 750) and Zaichik (J. Appl. Math. Mech. 61 (1997) 127) in the limit of high-inertia particles. By this procedure a global picture of the momentum and fluctuating energy transfer in the flow is attained. Moreover, the dispersed phase and fluid-particle velocity correlation, that enter the interaction terms describing the exchange of fluctuating energy between the phases, are compared with the result of Reeks’ and Zaichik's theoretical expressions in the case of simple shear flow.     

Elastic liquid jet impaction on a high‐speed moving surface

In the railroad industry a friction‐modifying non‐Newtonian liquid, showing elastic behavior, may be applied to the rail in the form of a liquid jet. The interaction of this elastic liquid jet and the moving surface—specifically whether it splashes or adheres without splash—is important in this industrial application. Twelve different elastic liquids with widely varying relaxation times were tested to isolate the effect of elasticity from other fluid properties. Using high‐speed imaging, the interaction between the impinging jet and the moving surface could be captured and analyzed. Although similar to Newtonian jets, for which the Reynolds number plays a major role, the Deborah number was also salient to the splash of elastic liquids. At the elevated Weber numbers of the testing, the Weber number had a much smaller impact on splash than did the Reynolds or Deborah numbers. The ratio of the surface velocity to the jet velocity has only a small effect on the splash. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Impingement heat transfer under a confined slot jet. Part I: Effect of surface throughflow

Local and average heat transfer coefficients were measured for a confined turbulent slot jet impinging on a permeable surface at which there may be throughflow. Local Nusselt number was obtained using a unique porous sensor designed for measurement of local heat transfer at a permeable surface which is subjected to rapidly and widely varying heat transfer. Measurements were performed for a wide range of jet Reynolds number and throughflow rates. Convective heat transfer coefficients was found to be enhanced by throughflow, and the enhancement factor in terms of Stanton number to be independent of jet Reynolds number and of extent of heat transfer area.     

Effect of Operating Parameters on Gas‐Solid Hydrodynamics and Heat Transfer in a Spouted Bed

Through a combined computational fluid dynamics and discrete element method approach, the effect of the operating parameters on the hydrodynamics and heat‐transfer properties of gas‐solid two‐phase flows in a spouted bed are extensively investigated. Considering the high velocity in the fountain region, gas turbulence is resolved by employing the large‐eddy simulation. The rolling friction model is adopted for more precise predictions of solid behavior near the wall. Subsequently, the gas‐solid flow patterns, gas‐solid velocities, and temperature evolution are investigated. Moreover, different operating conditions and geometry configurations are evaluated with respect to heat‐transfer performance. The results provide a fundamental understanding of heat‐transfer mechanisms in spouted beds.     

Fully coupled LES-DEM of particle interaction and agglomeration in a turbulent channel flow

Coupled large eddy simulation and the discrete element method are applied to study turbulent particle–laden flows, including particle dispersion and agglomeration, in a channel. The particle–particle interaction model is based on the Hertz–Mindlin approach with Johnson–Kendall–Roberts cohesion to allow the simulation of van der Waals forces in a dry air flow. The influence of different particle surface energies, and the impact of fluid turbulence, on agglomeration behaviour are investigated. The agglomeration rate is found to be strongly influenced by the particle surface energy, with a positive relationship observed between the two. Particle agglomeration is found to be enhanced in two separate regions within the channel. First, in the near-wall region due to the high particle concentration there driven by turbophoresis, and secondly in the buffer region where the high turbulence intensity enhances particle–particle interactions.     

超亲水表面上滞止区水喷流沸腾的临界热通量

引言二氧化钛(TiO2)作为一种光催化剂,由于其独特的特性近年来引起了人们的重视.研究者发现,TiO2有着极其特殊的超亲水特性[1].当具有TiO2涂层的表面受到紫外线的辐射时,水在其表面的接触角会一直减小,最后接近于0°.对于水沸腾这类有气液相变现象的传热过程来说,这一特性有着重要的应用价值.利用超亲水传热表面,相变传热过程可以被大大强化.高温壁面上液体喷流和钢铁工业中应用十分广泛的一种高效方法.有关滞止区内喷流沸腾的研究,以往主要集中在对沸腾区域内非稳态换热特性的研究上,而对滞止区内喷流沸腾的稳态实验研究则很少[2～4].最…     

磨料射流清除压力容器氧化皮的研究

介绍了一种新型的、用于清除压力容器表面氧化皮的工作系统。通过研究和试验确定了合适的工作参数。试验结果表明，该系统清除氧化皮的效率达６～７ｍ２／ｈ，射流的能耗仅为６ｋＷｈ。     

Bubble dynamics and its effect on the performance of a jet fluidised bed gasifier simulated using CFD

A computational fluid dynamic modelling study of a jet fluidised bed gasifier has been carried out. The modelling was based on the Eulerian-Eulerian models for gas and solid flows, which take into account the hydrodynamics, mass and heat transfer, and heterogeneous and homogeneous reactions. The bubble dynamics was simulated in detail, enabling its effect on temperature distributions, gasification reactions and gas compositions in the bed to be examined. The results revealed that jet growth, bubble rise, and the associated convective flow play a significant role in the heat exchange and mass transfer, and in turn, affect the gasification reactions.