前级叶片安装角对对旋轴流风机的性能影响

为研究两级动叶安装角可调的对旋轴流风机的前级叶片安装角与风机整体性能的匹配,本文利用CFD软件,对风机模型进行了三维全流场数值模拟与分析。结果表明:在小流量工况下,适当调小风机的前级叶片安装角可以提高风机的效率;风机在运行过程中,根据当前工况选择合适的安装角,可以有效提高对旋轴流风机的整体性能,改善各流动参数在风机中的分布情况,减少流动损失,扩大工作范围,延迟风机失速和喘振的发生。

板式无蜗壳离心风机内部流动分析及分流叶片影响

本文采用一种分流叶片结构提高无蜗壳后向离心风机出口静压及效率,来改善风机的性能,达到设计目的。研究表明,无蜗壳离心风机的损失在低流量工况下主要由叶片吸力面分离涡导致,在高流量工况下主要由二次流等因素引起。通过研究不同长度系数的分流叶片对风机内部流动的影响,发现分流叶片长度为叶片长度的一半,在整个流量范围内,均能达到减少分离涡和抑制二次流的效果,使得低流量和高流量下的静压效率均有大幅提高。

Liutex similarity in turbulent boundary layer

Kolmogorov's 1941 theory(K41)of similarity hypotheses and the-5/3 law for energy spectrum are considered as the most important theoretical achievement in turbulence research and the success of the modem turbulence theory.The assumptions of sufficient high Reynolds number and isotropy of turbulence that K41 based upon,however,cannot generally be met in practice,and thus discrepancy is often observed between the f law and direct numerical simulation(DNS)results of boundary layers in wall bounded turbulence,especially for moderate to low Reynolds number flows.Liutex vector is a recently defined new physical quantity which is extracted from turbulent flow to represent the rigid rotation part of fluid motion.Actually,Liutex is free from viscous dissipation and thus independent of Reynolds number,relaxing the very high Reynold number assumption of K41.Liutex similarity has been solidly demonstrated by DNS for a moderate Reynolds number turbulent boundary layer(Reθ≈1000),both the frequency and wavenumber spectrum of Liutex accurately matches the-5/3 law,which is obviously much better than the turbulence energy spectrum,while vorticity and other popular vortex identification methods,Q criterion for example,do not possess such a distinguished feature due to stretching and shearing contamination.

Investigation of Turbulent Transition in Plane Couette Flows Using Energy Gradient Method

The energy gradient method has been proposed with the aim of better understanding the mechanism of flow transition from laminar flow to turbulent flow.In this method,it is demonstrated that the transition to turbulence depends on the relative magnitudes of the transverse gradient of the total mechanical energy which amplifies the disturbance and the energy loss from viscous friction which damps the disturbance,for given imposed disturbance.For a given flow geometry and fluid properties,when the maximum of the function K(a function standing for the ratio of the gradient of total mechanical energy in the transverse direction to the rate of energy loss due to viscous friction in the streamwise direction)in the flow field is larger than a certain critical value,it is expected that instability would occur for some initial disturbances.In this paper,using the energy gradient analysis,the equation for calculating the energy gradient function K for plane Couette flow is derived.The result indicates that K reaches the maximum at the moving walls.Thus,the fluid layer near the moving wall is the most dangerous position to generate initial oscillation at sufficient high Re for given same level of normalized perturbation in the domain.The critical value of K at turbulent transition,which is observed from experiments,is about 370 for plane Couette flow when two walls move in opposite directions(anti-symmetry).This value is about the same as that for plane Poiseuille flow and pipe Poiseuille flow(385-389).Therefore,it is concluded that the critical value of K at turbulent transition is about 370-389 for wall-bounded parallel shear flows which include both pressure(symmetrical case)and shear driven flows(anti-symmetrical case).

Influence of Magnetic Force on the Flow Stability in a Rectangular Duct

The stability of the flow under the magnetic force is one of the classical problems in fluid mechanics.In this paper,the flow in a rectangular duct with different Hartmann(Ha)number is simulated.The finite volume method and the SIMPLE algorithm are used to solve a system of equations and the energy gradient theory is then used to study the(associated)stability of magnetohydrodynamics(MHD).According to the energy gradient theory,K represents the ratio of energy gradient in transverse direction and the energy loss due to viscosity in streamline direction.Position with large K will lose its stability earlier than that with small K.The flow stability of MHD flow for different Hartmann(Ha)number,from Ha=1 to 40,at the fixed Reynolds number,Re=190 are investigated.The simulation is validated firstly against the simulation in literature.The results show that,with the increasing Ha number,the centerline velocity of the rectangular duct with MHD flow decreases and the absolute value of the gradient of total mechanical energy along the streamwise direction increases.The maximum of K appears near the wall in both coordinate axis of the duct.According to the energy gradient theory,this position of the maximum of K would initiate flow instability(if any)than the other positions.The higher the Hartmann number is,the smaller the K value becomes,which means that the fluid becomes more stable in the presence of higher magnetic force.As the Hartmann number increases,the K value in the parallel layer decreases more significantly than in the Hartmann layer.The most dangerous position of instability tends to migrate towards wall of the duct as the Hartmann number increases.Thus,with the energy gradient theory,the stability or instability in the rectangular duct can be controlled by modulating the magnetic force.

Numerical Simulation of Deflagration to Detonation Transition in a StraightDuct: Effects of Energy Release and Detonation Stability

Numerical simulation based on the Euler equation and one-step reaction model is carried out to investigate the process of deflagration to detonation transition(DDT)occurring in a straight duct.The numerical method used includes a high resolution fifth-order weighted essentially non-oscillatory(WENO)scheme for spatial discretization,coupled with a third order total variation diminishing Runge-Kutta time stepping method.In particular,effect of energy release on the DDT process is studied.The model parameters used are the heat release at q=50,30,25,20,15,10 and 5,the specific heat ratio at 1.2,and the activation temperature at Ti=15,respectively.For all the cases,the initial energy in the spark is about the same compared to the detonation energy at the Chapman-Jouguet(CJ)state.It is found from the simulation that the DDT occurrence strongly depends on the magnitude of the energy release.The run-up distance of DDT occurrence decreases with the increase of the energy release for q=5020,and increases with the increase of the energy release for q=205.This phenomenon is found to be in agreement with the analysis of mathematical stability theory.It is suggested that the factors to strengthen the DDT would make the detonation more stable,and vice versa.Finally,it is concluded from the simulations that the interaction of the shock wave and the flame front is the main reason for leading to DDT.

Analysis of Two-Phase Cavitating Flowwith Two-Fluid Model Using Integrated Boltzmann Equations

In the present work,both computational and experimentalmethods are employed to study the two-phase flow occurring in a model pump sump.The twofluid model of the two-phase flow has been applied to the simulation of the threedimensional cavitating flow.The governing equations of the two-phase cavitating flow are derived from the kinetic theory based on the Boltzmann equation.The isotropic RNG k−ε−kca turbulence model of two-phase flows in the form of cavity number instead of the formof cavity phase volume fraction is developed.The RNG k−ε−kca turbulence model,that is the RNG k−εturbulence model for the liquid phase combined with the kca model for the cavity phase,is employed to close the governing turbulent equations of the two-phase flow.The computation of the cavitating flow through a model pump sump has been carried out with this model in three-dimensional spaces.The calculated results have been compared with the data of the PIV experiment.Good qualitative agreement has been achievedwhich exhibits the reliability of the numerical simulation model.