Weak temperature gradient effect on the stability of the circular Couette flow

We have investigated the influence of a weak radial temperature gradient in a wide gap and large aspect ratio Couette-Taylor system. The inner cylinder is rotating and can be heated or cooled, the outer cylinder is at rest and immersed in a large thermal bath. We found that a radial temperature gradient destabilizes the Couette flow leading to a pattern of traveling helicoidal vortices occurring only near the bottom of the system. The size of the pattern increases as the rotation frequency of the cylinder is increased. We have characterized the spatiotemporal properties of the pattern and we have shown that it behaves as a wall mode found in the simulation of the complex Ginzburg-Landau equation with homogeneous boundary conditions.     

Kinetic modeling of a high power fast-axial-flow CO2 laser with computational fluid dynamics method

A new computational fluid dynamics (CFD) method for the simulation of fast-axial-flow CO2 laser is developed.The model which is solved by CFD software uses a set of dynamic differential equations to describe the dynamic process in one discharge tube.The velocity,temperature,pressure and turbulence energy distributions in discharge passage are presented.There is a good agreement between the theoretical prediction and the experimental results.This result indicates that the parameters of the laser have significant effect on the flow distribution in the discharge passage.It is helpful to optimize the output of high power CO2 laser by mastering its kinetic characteristics.     

Application of quartz glass tubes as the track detectors for chemical identification of short-lived isotopes of elements 104, 105 and 106

The quartz glass open chromatographic columns with thermal gradient, which usually were used for the investigation of chemical properties of short-lived Z104 nuclides, were first used as the track detectors of spontaneously fissioning isotopes of elements 104, 105 and 106. The recoil nuclei of these elements were captured by the flow of gaseous chlorides then go to the quartz tube with reducing thermal gradient 400°C-100°C. The quartz tube was etched in 40% HF. The tracks due to spontaneous fission of isotopes ²⁵⁹104, ²⁶²105 and ²⁶³106 which were etched at the internal wall of quartz tubes were counted under optical microscope.     

Calculation and Optimization of the Effective Extinction Cross Section of the Spherical Cold Obscuring Smoke

The smoke is often generated in the battlefield by the smoke ammunition to protect the personnel and equipment against the enemy's optoelectronic systems. The effective extinction cross section is an important parameter of the infrared smoke ammunition. In this paper, the relationships between the effective extinction cross section of a cloud of spherical smoke and the mass of the smoke agent, the radius of the screening smoke and the mass extinction coefficient are discussed. The expressions of the maximal effective extinction cross section is derived. This research will help us to achieve optimal design of the smoke ammunition and to appropriately evaluate the effectiveness of the smoke. Examples are given for two kinds of obscurants.     

Visualization of flameholding mechanisms in a supersonic combustor using PLIF

Laser-induced fluorescence of OH and CH₂O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH₂O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal.     

直流纯氩层流等离子体射流的长度变化

采用主要由阴极、阳极以及介于阴极和阳极之间的中间段组成的直流非转移式电弧等离子体发生器,在大气压条件下,比较系统地研究了纯氩层流等离子体射流的长度随着弧电流、气体流量以及发生器结构而变化的规律。结果表明:层流射流的长度随弧电流和工作气流量的增加而增长;层流向湍流流动转变的临界气流量值随弧电流增大而提高;在发生器的伏安特性呈大梯度变化的情况下,射流长度随弧电流的变化幅度增大。     

Gas kinetic algorithm for flows in Poiseuille-like microchannels using Boltzmann model equation

~~Gas kinetic algorithm for flows in Poiseuille-like microchannels using Boltzmann model equation1. Feynman, R., There's plenty of room at the bottom, Journal of Microelectromechanical Systems, 1992, 1: 60 -66. 2. Piekos, E. S., Breuer, K. S., Numerical modeling of micromechanical devices using the direct simulation Monte Carlo method, Transactions of the ASME, Journal of Fluids Engineering, 1996, 118: 464-469. 3. Beskok, A., Karniadakis, G. E., Trimmer, W., Rarefaction and …     

改进风冷螺杆热泵机组的分液器性能实验研究

风冷螺杆机组冬季风侧换热器作为蒸发器时存在分液不均问题,换热器集气管温度波动,影响制热量。通过实验测试找出原因:翅片换热器上下进风量不均;通过换热器各路铜管的制冷剂质量流量不均。调整换热器管路布置;计算调整混合室的内径获得雾状流。通过性能更好的分液器和适当的换热器设计,取得了较好的分液效果,提高了机组制热量。     

Time-dependent and outflow boundary conditions for Dissipative Particle Dynamics

We propose a simple method to impose both no-slip boundary conditions at fluid-wall interfaces and at outflow boundaries in fully developed regions for Dissipative Particle Dynamics (DPD) fluid systems. The procedure to enforce the no-slip condition is based on a velocity-dependent shear force, which is a generalized force to represent the presence of the solid-wall particles and to maintain locally thermodynamic consistency. We show that this method can be implemented in both steady and time-dependent fluid systems and compare the DPD results with the continuum limit (Navier-Stokes) results. We also develop a force-adaptive method to impose the outflow boundary conditions for fully developed flow with unspecified outflow velocity profile or pressure value. We study flows over the backward-facing step and in idealized arterial bifurcations using a combination of the two new boundary methods with different flow rates. Finally, we explore the applicability of the outflow method in time-dependent flow systems. The outflow boundary method works well for systems with Womersley number of O(1), i.e., when the pressure and flowrate at the outflow are approximately in-phase.     

Digital flow control of electroosmotic pump: Onsager coefficients and interfacial parameters determination

Electroosmosis and streaming potential are two complementary electrokinetic processes related by the Onsager relation. In particular, an electroosmotic pump (EOP) is potentially useful for a variety of engineering and bio-related applications. By fabricating samples consisting of dry-etched cylindrical pores (50 μm in length and 3.5 μm in diameter) on silicon wafers, we demonstrate that the use of digital control via voltage pulses can resolve the flow regulation and stability issues associated with the EOP, so that the intrinsic characteristics of the porous sample medium may be revealed. Through the consistency of the measured electroosmosis (EO) and the streaming potential (SP) coefficients as required by the Onsager relation, we deduce the zeta potential and the surface conductivity, both physical parameters pertaining to the liquid-solid interface.