室内空气对流的特征与数值模拟

采用雷诺平均的Navier-Stokes方程和K-ε湍流模型,数值模拟了在温度梯度和浓度梯度作用下的房间空气对流结构。通过改变浮升力比Grc/GrT及自然对流与强迫对流强度比Gr/Re^2值的大小,分析了浓度浮升力方向及其大小和Gr/Re^2值对空气流态的影响。结果表明,当Grc/GrT改变时,热和污染源壁面附近的流体流动状态和热边界层厚度发生了改变;随着Gr/Re^2的增大,室内空气流型由机械通风主导逐步变化到自然通风主导。     

风沙环境对翼型边界层厚度的影响研究

为研究风沙环境对风力机翼型边界层的影响,采用无黏模型、转捩模型和离散相模型对三维S809翼型的边界层特性进行数值模拟,并对清洁空气和风沙环境中翼型的边界层特性进行对比分析.结果表明:相较于清洁空气,沙尘颗粒的加入使翼型边界层厚度减小.同一弦向位置,随着颗粒直径的增大,翼型边界层厚度呈先减小后增大的趋势;随着颗粒质量浓度...     

雷诺数改变对S825翼型的影响

文章采用CFD软件对风力机翼型NREL S825在雷诺数分别为1×10^6、2×10^6、3×10^6的情况进行气动性能的数值研究。通过对比数值计算结果与实验数据,确认计算结果的可靠性。在此基础上,详细地分析边界层参数,包括边界层内速度分布、边界层位移厚度和动量损失厚度。结果表明：相同攻角下,随着雷诺数的增加,边界层厚度变薄,分离点后移．进而使升力系数增加、阻力系数减小。     

循环流化床锅炉燃烧室边界层的实验研究

循环流化床锅炉燃烧室的物料浓度影响传热和燃烧,在75t/h循环流化床锅炉上对燃烧室物料浓度分布的测量结果发现,循环流化床锅炉燃烧室的下降流边界层中在接近壁面处存在气体边界层,气体边界层的厚度与距离布风板高度有关,据此给出了下降流边界层厚度和气体边界层厚度的计算公式,并提出了循环流化床燃烧室双环模型。     

盐梯度太阳池双扩散模型及热盐变化规律数值模拟

建立盐梯度太阳池热盐扩散二维简化数学模型,对1.0 m×1.5 m的太阳池进行数值分析,得出太阳池蓄热过程中的热盐瞬时扩散规律,并与实验值相比较,两者吻合较好。研究结果表明:随着太阳辐射时间的增加,上对流层和下对流层的对流微团数逐渐增多,使得非对流层厚度减小而上下对流层厚度均增加,但在一定时间内盐度梯度造成的密度梯度始终大于温度梯度引起的密度梯度,从而抑制非对流层的对流产生,为实际盐梯度太阳池的运行与维护提供参考依据。     

辐射侧加热腔内自然对流传热特性研究

为了明确辐射侧加热封闭方腔内半透明流体的自然对流传热现象及规律,采用有限体积法进行数值模拟研究,分析了瑞利数和光学厚度对流场、温度场以及传热特性的影响。结果表明：与传统侧壁加热腔内自然对流相比,辐射侧加热腔内等温线和流场分布规律不一致;随着瑞利数和光学厚度增加,涡心由中心位置沿直线向辐射入射侧斜上方偏移;随着瑞利数增加,等温线变得更均匀;随着光学厚度增加,等温线变密,努塞尔数Nu与瑞利数RaT的标度律指数减小,当光学厚度增加到一定时标度律不再变化,此时传热标度律与传统恒壁温侧加热腔内自然对流相当,满足Nu~Ra0.29T。     

HCCI发动机多区燃烧模型的比较研究

多区模型作为现阶段均质压燃（HCCI）发动机高效准确的数值模型得到了世界范围的广泛关注。讨论了不同子模型对多区模型预测性能的影响。以实验为基准,比较了多区模型中区间划分、缸壁传热模型、区间热量交换模型、区间质量交换模型和边界层模型对HCCI发动机燃烧和排放模拟结果的影响,全部计算均基于异辛烷的详细化学动力学机理。结果表明：在区间划分时对温度较低的区域细化可以提高排放的计算效果,而对高温区域的细化对计算结果影响不大;改进的Woschni传热模型更准确地模拟了缸壁的传热过程;区间的质量和热量交换对计算结果影响显著,特别是质量交换模型的加入使CO排放的预测与实验值更为接近;而边界层厚度模型对整个结果影响不大。     

层流脉动流中平行圆柱体的温度边界层

采用数值模拟方法研究了一个平行圆柱体在层流脉动流中的温度边界层特性。数值模拟结果与实验数据一致。研究发现脉动流中平行圆柱体形成了形状不规则但相对稳定的温度边界层,并在流动方向上周期性脉动。脉动流中平行圆柱体的温度边界层平均厚度小于稳定流动下的温度边界层平均厚度,并以脉动流的频率进行脉动。此外, 脉动流中平行圆柱体的壁面温度小于稳定流动下的壁面温度,表明脉动流下圆柱体的对流传热得到了强化。在一个脉动周期内,圆柱体在后半周期的温度边界层厚度和热阻均小于前半周期的温度边界层厚度和热阻。     

同速对转涡轮盘腔内流动和换热特性

为研究对转盘旋转速度对盘腔内换热效果的影响,应用RNG k-ε湍流模型对中心进气、径向出流的同速对转涡轮盘腔内的流动和换热特性进行了数值模拟。揭示了盘腔内气体的流动结构、两盘壁面边界层内的径向速度、换热效率以及壁面温度分布特征,并进一步探究了转盘转速对上述流动和换热特征的影响规律。结果表明:对转盘腔内存在两个反向的回流涡胞,这两个涡胞的相对大小以及相遇的滞止点位置取决于进气惯性力和旋转力的相对大小;转盘近壁面流体径向流动速度与盘壁面的对流传热系数呈正相关变化;对于上游盘,低转速范围时转速对壁面换热影响取决于旋转径向力和进气惯性力对上游盘近壁面流体流动驱动作用的相对大小,高转速范围内增大转速,上游盘壁面传热系数增大,壁面温度降低;对于下游盘,壁面低半径区域的对流传热系数随转速增大而减小,而高半径区域的对流传热系数随转速增大而增大。     

工业供热中汽汽再热器换热强化数值研究

为了强化工业供热中的汽汽换热,通过CFD技术对不同类型换热管的流动及换热特性进行了研究。结果显示：光滑壁面时管壁两侧的高、低温蒸汽的温度梯度沿着流向逐渐变化,对流换热逐渐增强;相比于光滑管,采用内波节管和内螺纹波节管时,高温蒸汽侧的温度梯度增大,而低温蒸汽侧的壁面温度梯度明显增大;采用壁面异型结构能够改变管壁内温度梯度,采用内螺纹波节管尤甚。采用内波节管和内螺纹波节管的平均Nu相比于光滑管显著提高,最大值分别提高了26%和30%。     

On the relative influence of convection in serpentine flow fields of PEM fuel cells

In polymer electrolyte membrane fuel cells, power losses associated with slow reaction kinetics and mass-transport limits can be strongly influenced by convective flow characteristics. Specifically, convection in the form of channel bypass may be utilized to simultaneously increase reactant concentration and reduce product concentration in the catalyst layer, thus reducing the activation and mass-transport overpotentials. An analytical model is developed here to predict the flow pattern and pressure field in general single-serpentine flow field geometries. The model predicts that a significant portion of the total flow through the fuel cell can occur as in-plane convective flow through the gas diffusion layer under realistic operating conditions. Further, by comparing the in-plane rates of diffusive and convective transport it is found that the dominant mechanism depends on the geometric and material parameters of the flow field. In particular, it is found that the relative influence of convection depends highly on in-plane permeability of the gas diffusion layer and channel length, and is relatively independent of gas diffusion layer thickness. By designing fuel cells to utilize enhanced in-plane convection, it is suggested that losses associated with low oxygen content as well as liquid water buildup in the catalyst layer can be reduced.     

The influence of radiation absorption on solar pond stability

This paper presents a study of the gravitational stability of a salty layer of a fluid subject to an adverse temperature gradient as a result of heat absorption. This is intended to model solar ponds where an artificial gradient of salt concentration in water is used to prevent convective motions induced by the absorption of solar radiation. The stability of the Boussinesq approximation of the Navier-Stokes equations is analysed for perturbations of a certain kind imposed on the stationary solution. The marginal states for the onset of convection are obtained using a Galerkin method based on a weak formulation of the governing equations. The analysis considers solar energy absorption in the layer and assumes prescribed heat flux values as boundary conditions for the temperature equation. Results are compared with those obtained earlier by different authors for a layer of fluid, heated from below, with linear profiles of both salt concentration and temperature.     

Prandtl number scaling of the unsteady natural convection boundary layer adjacent to a vertical flat plate for Pr>1 subject to ramp surface heat flux

It is found in the literature that the existing scaling results for the boundary layer thickness, velocity and steady state time for the natural convection flow over an evenly heated plate provide a very poor prediction of the Prandtl number dependency of the flow. However, those scalings provide a good prediction of two other governing parameters’ dependency, the Rayleigh number and the aspect ratio. Therefore, an improved scaling analysis using a triple-layer integral approach and direct numerical simulations have been performed for the natural convection boundary layer along a semi-infinite flat plate with uniform surface heat flux. This heat flux is a ramp function of time, where the temperature gradient on the surface increases with time up to some specific time and then remains constant. The growth of the boundary layer strongly depends on the ramp time. If the ramp time is sufficiently long, the boundary layer reaches a quasi-steady mode before the growth of the temperature gradient is completed. In this mode, the thermal boundary layer at first grows in thickness and then contracts with increasing time. However, if the ramp time is sufficiently short, the boundary layer develops differently, but after the wall temperature gradient growth is completed, the boundary layer develops as though the startup had been instantaneous.     

Inverse Radiation Problem of Boundary Incident Radiation Heat Flux in Semitransparent Planar Slab with Semitransparent Boundaries

INTRODUCTI0NInverseradiati0nproblemshavedefinedasubjectofinterestf0rthepast3Oyears0nsoandthereex-istsac0nsiderablebody0fknowledgesurroundingthesubjectthathasbeenextensivelyreviewedinaseries0fpapersbyM.C.rmick[1-4].Theyarecon-cernedwiththedeterminati0noftheradiativepr0p-ertiesandthetemperaturedistributionsofmediaus-ingvari0ustypesofradiationmeasurements.Despitetherelativelylargeinterestexpressedininverseradia-tionproblems,mostoftheworkfocusedontheinverseestimati0noftemperaturedistributions…     

Effect of a magnetic field on heat and mass transfer by natural convection from vertical surfaces in porous media-an integral approach

The study deals with natural convection heat and mass transfer from vertical plates embedded in electrically conducting fluid saturated porous media with constant surface temperature and concentration. The flow is exposed to a transverse magnetic field. The integral method is applied to obtain the analytic solution. Results are illustrated graphically for various features of the solutions. The application of a transverse magnetic field normal to the flow direction decreases the Nusselt number and Sherwood number. Moreover, the thickness ratio of the thermal boundary layer to the concentration boundary layer is found to be independent of the intensity of the magnetic field.     

Thermodiffusion,chemical reaction,and Hall and ion-slip impacts on MHD rotating flow past an infinite vertical porous plate

The current scrutiny explores the impacts of thermodiffusion, chemical reaction, and Hall and ion-slip impacts lying on unsteady heat and mass transport of free convective hydromagnetic flow enclosed past a semi-infinite porous plate within a gyratory frame under the accomplishment of a transverse magnetic field and convective boundary conditions. The nondimensional governing equations are solved systematically by means of the finite element method. Through the facilitation of graphical profiles, the outcomes of a variety of significant parameters within the boundary layer are addressed. In addition, the local skin friction coefficient and rates of heat and mass transports in expressions of the local Nusselt number and local Sherwood number are presented digitally in tabulation form, although it is originated that the Nusselt number and Sherwood number remain constant with varying all pertinent parameters. It is found that the porous medium impact on the boundary layer growth is significant due to the increase in the thickness of the hydrodynamic boundary layer and the decrease in the thickness of the thermal and concentration boundary layers. The resultant velocity enhances with increasing rotation, Hall and ion-slip parameters.     

Stability of thermal convection in basin-type solar stills

The onset of thermal convection in a basin-type solar still is studied using linear stability theory. The system is idealized as a horizontal layer of an absorbing, scattering and emitting liquid with a lower insulated rigid boundary and an upper convective free boundary subjected to uniform solar radiant heat. Radiative transfer in the liquid is modelled by the differential approximation. At low optical thickness values, the lower boundary emissivity is the dominant factor controlling the fluid instability with the mirror boundary cases being more stable than the corresponding cases for a black boundary. Generally, lowering the following parameters, Planck number, scattering albedo, weighted non-grayness and increasing the heat losses from the top surface, all delay the onset of convection.     

Inverse Natural Convection Problem with Radiation in Rectangular Enclosure

Inverse thermal problem is applied to natural convective flow with radiative heat transfer. The bottom wall temperature in the 2-D cavity domain is estimated by using gas temperature measurements in the flow field. The inverse problem is solved through a minimization of an objective function using the conjugate gradient method with adjoint problem. The effects of functional form of bottom wall temperature profile, the number and the position of measurement points, and the measurement errors are investigated and discussed. The conjugate gradient method is found to work well in estimating the bottom wall temperature, even when natural convection with radiation phenomena is involved.     

Electrothermal convection in a rotating dielectric fluid layer: Effect of velocity and temperature boundary conditions

The simultaneous effect of a vertical AC electric field and rotation on the onset of thermal convective instability in a horizontal rotating dielectric fluid layer is studied by performing linear stability analysis. The lower and upper boundaries of the fluid layer are considered to be either rigid or free and either isothermal or insulated to temperature perturbations. The resulting eigenvalue problem is solved exactly for free–free isothermal boundaries. It is observed that the oscillatory convection is not a preferred mode of instability for dielectric fluids and the necessary conditions for its occurrence are independent of applied vertical AC electric field. For the other combinations of velocity and temperature boundary conditions, the problem is solved numerically using the Galerkin method. The similarities and differences between the results of isothermal and insulated boundaries are highlighted. It is noted that the effect of increasing AC electric Rayleigh number is to increase the transfer of heat more effectively and hence to hasten the onset of convection. To the contrary, the effect of rotation is to delay the electrothermal convection for a fixed type of boundary conditions. Although the rigid–rigid boundaries enhance the stability when compared to rigid–free and free–free boundaries up to moderate values of Taylor number, the situation is reversed at high Taylor number domain. This trend depends on the temperature boundary conditions as well.     

管内填充多孔介质强化换热的数值研究

管内填充多孔介质强化换热的基本原理是构造热边界层,增大壁面附近流体的温度梯度,并且流动阻力增幅不大。本文运用数值模拟的方法,模拟填充多孔介质管内的流场和温度场,探讨填充比例φ、渗透率Da以及空隙率ε对管内对流换热的影响规律。研究表明,提高填充比例φ和减小渗透率Da都能明显提高换热效果,但也增加了管内流动阻力。空隙率ε对强化换热作用不大,但高空隙率可以明显降低管内流动阻力,在实际中应选用空隙率较大的多孔介质。