Numerical investigation on detonation cell evolution in a channel with area-changing cross section

The two-dimensional cellular detonation propagating in a channel with area- changing cross section was numerically simulated with the dispersion-controlled dissipative scheme and a detailed chemical reaction model. Effects of the flow expansion and compression on the cellular detonation cell were investigated to illustrate the mechanism of the transverse wave development and the cellular detonation cell evolution. By examining gas composition variations behind the leading shock, the chemical reaction rate, the reaction zone length, and thermodynamic parameters, two kinds of the abnormal detonation waves were identified. To explore their development mechanism, chemical reactions, reflected shocks and rarefaction waves were discussed, which interact with each other and affect the cellular detonation in different ways.     

PIV and LDA measurements of secondary flow in a meandering channel for overbank flow

Secondary flow in a compound meandering channel with straight floodplain banks for overbank was investigated by a visualization method and velocity measurement using three-component laser Doppler anemometor (LDA). The secondary flow in a cross section was visualized by the neutral buoyant tracer method with a submergible video camera. Secondary flow vectors in a cross section were obtained by using PIV software with captured frames from video source through PC and also by LDA measurements. From the comparison of the PIV and LDA results, it is found that PIV data show good agreement in quality with LDA measurements when the secondary flow is strong and stable as shown in this paper.     

Computation of the gas mass and heat fluxes in a rectangular channel in the free molecular regime

The problem of heat- and mass transfer in a long rectangular channel of a constant cross section is solved in the free molecular regime. The distributions of the mass flow rate and the heat flux vector over the channel cross section are calculated. The specific gas mass flux and heat flux are calculated. The results are compared with those obtained for nearly free molecular flows.     

A theoretical study of a special detonation regime of operation of a pulse detonation device with a variable cross section detonation combustion chamber and a valveless supply system

The results of a theoretical study of the special detonation regime that arises in a pulse detonation device with a variable cross section combustion chamber are presented. The problem is solved analytically within the framework of the one-dimensional approximation. In contrast to the standard version of arbitrary discontinuity disintegration, the problem is solved for a variable cross section at the location of the diaphragm. Since the pulse detonation device operated with a valveless system of fuel and oxidizer supply, a fuel flowed through it over the entire period before spark ignition. This peculiarity of the operation of the combustion chamber was also taken into account. The proposed approach was used to perform calculations for methane-oxygen mixtures, for which extensive experimental data exist. The calculation results are compared to the available experimental data.     

Comments on “hydromagnetic slip flow with heat transfer in an inclined channel”

The hydromagnetic slip flow of a viscous incompressible and electrically conducting fluid through an inclined channel of rectangular cross section in the presence of a transverse magnetic field has been analysed. The walls of the channel are assumed to have prescribed temperatures and finite conductivities. The boundary conditions for both velocity and temperature are properly rectified. The expressions for the velocity, induced magnetic field and the temperature are obtained both analytically and numerically.     

Initiation and propagation of detonation waves in combustible high speed flows

The initiation and propagation of detonation waves in combustible high speed flows were studied experimentally. A planar detonation wave traveling in an initiation tube was transmitted into a test section where a combustible high speed flow was induced by an incident shock wave generated in a shock tube. In this study, the flow Mach numbers were obtained as 0.9 and 1.2. The experimental results show that depending on the flow velocity, the apparent propagation velocity of a detonation wave is higher in the upstream and lower in the downstream direction than the CJ velocity. Smoked plate records reveal cellular patterns deformed in the flow direction, and the calculated aspect ratios of the cell were found to agree well with the experimental ones on the basis of the assumption that the velocity of the transverse wave is not affected by the flowing mixture. By analyzing the shock-wave diffraction at the position where there is an abrupt change in the area, on the basis of Whitham’s theory, it was deduced that in the present experimental set-up, the detonation was initiated by the reflection of the diffracted shock waves on the sidewalls of the test section. The agreement between the experimental and calculated results regarding the position of the cellular patterns on the smoked plate record indicated that the position of detonation initiation in high speed flows is shifted downstream due to the flow velocity.     

Formation of detonation waves in channels and interaction of the waves with penetrable screens

Two-dimensional channel flows with shock waves resulting from the detonation of a combustible gas mixture are considered. Conditions for detonation and the parameters of the shock waves are determined. The feasibility of reducing the shock wave intensity and loads on the structure by mounting a set of mesh screens in the channel is investigated. The numerical computation of detonation initiation in an air-hydrogen mixture and subsequent passage of shock waves through the mesh screens is carried out. Basic quantitative characteristics of shock wave reduction depending on the mesh screen penetrability and mutual arrangement of variously penetrable screens are obtained.     

Distribution of guided mode power flow between the layers of circular and elliptical multilayer optical fibers

An invariant expression in the form of a contour integral for guided mode power flow via an arbitrary area of the cross section of an optical fiber with constant permittivity is presented in polar and elliptical coordinates. Expressions for guided mode power flows via the cross sections of circular and elliptical multilayer optical fibers are obtained.     

氚增殖剂球床内氦气流动特性的数值模拟

通过计算流体力学(CFD)数值模拟方法,对氚增殖剂球床内部的氦气流动特性进行了初步研究。分析了球床流通长度和流通截面对提氚气体压降的影响,获得了不同入口流速下规则堆积球床和随机堆积球床的压降和阻力系数。      

微通道内气体流动的DSMC方法

基于压力边界条件开展了微尺度低速流动DSMC方法的研究, 定义了两个无量纲参数作为微尺度DSMC方法下网格尺寸与时间步长的约束条件, 通过微尺度Poiseuille流进行了方法的验证与比较, 获得了网格尺寸与时间步长的一般原则。在此基础上, 对变截面的单孔和双孔模型的微通道气体流动进行DSMC模拟, 结果表明, 通道几何形状对微尺度气体流动具有显著影响, 孔口后由于通道收缩, 产生压降, 导致气流加速, 并在孔口下游拐角处发生分离; 双孔口模型的流动结构与单孔口模型相似, 且在相同压差情况下, 经双孔口后的气体流速低于经单孔口后的气体流速; 随着入口压力的增加, 经过孔口压缩后的速度越大, 分离区尺寸也越大。      

Numerical simulation of laminar forced convection of water-CuO nanofluid inside a triangular duct

In this article, distilled water and CuO particles with volume fraction of 1%, 2% and 4% are numerically studied. The steady state flow regime is considered laminar with Reynolds number of 100, and nano-particles diameters are assumed 20 nm and 80 nm. The hydraulic diameter and the length of equilateral triangular channel are 8 mm and 1000 mm, respectively. The problem is solved for two different boundary conditions; firstly, constant heat flux for all sides as a validation approach; and secondly, constant heat flux for two sides and constant temperature for one side (hot plate). Convective heat transfer coefficient, Nusselt number, pressure loss through the channel, velocity distribution in cross section and temperature distribution on walls are investigated in detail. The fluid flow is supposed to be one-phase flow. It can be observed that nano-fluid leads to a remarkable enhancement on heat transfer coefficient. Furthermore, CuO particles increase pressure loss through the channel and velocity distribution in fully developed cross section of channel, as well. The computations reveal that the size of nano-particles has no significant influence on heat transfer properties. Besides, the study shows a good agreement between provided outcomes and experimental data available in the literature.     

Viscoplastic flows in channels with variable cross section and deformable walls

A longitudinal axially symmetric Shvedov-Bingham medium flow in a deformable vessel is studied. A formulation of the problem in terms of the integral characteristics of the channel cross section (radius, flow rate, rigid core radius in the cross section, core velocity, and average pressure in the cross section) is pro- posed. The problem of a steady flow in a confuser and the problem of medium acceleration in a cylinder are solved using the integral method.     

Modeling the flow of liquid-metal coolant in the T-shaped mixer

The results of experimental studies on the structure of the temperature field in the tube cross section at the flow of liquid-metal coolant in a T-shaped mixer are presented. Experiments were carried out using the Rose alloy as the working fluid. To determine temperature distribution on the test section wall, infrared thermography was used; to determine temperature distribution in the channel cross section, a mobile thermocouple was used. Considerable temperature maldistribution in the mixing zone of liquid flows with different temperatures on the tube wall and in the coolant melt is shown.     

Dynamics of detonation waves in a channel with variable cross section and filled with bubbly fluid

The flow of bubbly fluid comprising a mixture of bubbles filled with explosive and inert gases, which is driven through a converging channel, was studied. Depending on the velocity of the hummer hitting the bubbly fluid boundary, the flow may be accompanied by the development of detonation waves which compress the bubbles with inert gas.     

A flame technique to isolate the detonation/product interface relevant to rotating detonation engines

A novel experimental technique is proposed to study the detonation propagation in a layer of non-reacted gas weakly confined by combustion products. This problem is relevant to rotating detonation engines, where transverse detonations are confined by products of a previous rotation cycle, and other applications such as industrial safety. The experimental technique utilizes a flame ignited along the top wall in a long channel. The preferential growth of the flame along the long direction of the channel creates a finger flame and permits to create a narrow layer of unburned gas. A detonation ignited outside of this layer then propagates through the layer. This permits to conduct accurate observations of the detonation interaction with the inert gas and determine the boundary condition of the interaction. The present paper provides a proof-of-concept demonstration of the technique in a 3.4 m by 0.2 m channel, in which long finger flames were observed in ethylene-oxygen mixtures. The flame is visualized by high-speed direct luminosity over its entire travel, coupled with pressure measurements. A direct simulation of the flame growth served to supplement the experiments and evaluate the role of the induced flow by the flame growth, which gives rise to a non-uniform velocity distribution along the channel length. Detonation experiments were also performed at various layer heights in order to establish the details of the interaction. The structure was visualized using high speed Schlieren video. It was found that an inert shock always runs ahead of the detonation wave, which gives rise to a unique double shock reflection interaction.     

Theoretical Prediction of Differential Cross Sections for Reaction pp → pK^＋A in a Resonance Model

The reaction of pp → pK^＋A is a very good channel to study N^＊ resonances through their KA decay mode, because there is no mixing of isospin I = 1/2 and I = 3/2 due to isospin conservation. In this work, we extend a resonance model, which can reproduce the total cross section very well, to offer differential cross section information about this reaction. It can serve as a reference to build the scheduled hadron detector at Lanzhou Cooler Storage Ring （CSR）. Experiment measurement of these differential cross sections in the future will supply us more constraints on the model and help us understanding the strangeness production dynamics better.     

一种非矩形截面对浅刻蚀SOI脊形波导侧向泄漏损耗的影响

为了进一步探索用绝缘体上晶体硅制作的浅刻蚀脊形波导侧向泄漏损耗的规律,提出并研究了一种非矩形截面浅刻蚀绝缘体上晶体硅脊形波导.用光的干涉理论建立该波导的周期性损耗模型并推导出损耗周期公式,然后通过完美匹配层边界条件下的频域有限元法仿真观察该特殊波导类TM0模的侧向泄漏损耗周期的变化与最大损耗点的偏移现象.周期大小的仿真结果与理论计算符合度较高,其平均相对误差仅0.56%.此外,发现该类波导在某些沟槽宽度下可以通过改变截面来实现对类TM0模损耗从最大到最小的调节,而在另外一些沟槽宽度下,类TM0模损耗对截面变化不敏感.研究结果可以简化波导加工并提高制作容差,为该类型波导的设计与制作提供参考.     

The pulsating laminar flow in a rectangular channel

The finite difference method is used to solve the task of the developed pulsating laminar flow in a rectangular channel. The optimum of the difference scheme parameters was determined. Data on the amplitude and phase of the longitudinal velocity oscillations, the hydraulic and friction drag coefficients, the shear stress on the wall have been obtained. Using the dimensionless value of the frequency pulsations two characteristic regimes — the quasisteady-state regime and the high-frequency regime have been identified. In the quasi-steady-state regime, the values of all hydrodynamic quantities at each instant of time correspond to the velocity value averaged over the cross section at a given moment of time. It is shown that in the high-frequency regime, the dependences on the dimensionless oscillation frequency of oscillating components of hydrodynamic quantities are identical for rectilinear channels with a different cross-sectional form (round pipe, flat and a rectangular channels). The effect of the aspect ratio of the rectangular channel sides channel on the pulsating flow dynamics has been analyzed.     

悬浮RDX炸药粉尘爆轰的数值模拟

用两相流模型对悬浮RDX炸药粉尘爆轰波进行了数值模拟。RDX炸药颗粒在爆轰波阵面后的高温高速气流中加速并升温,颗粒表面发生熔化。参考液滴在高速气流作用下剥离的效应,假设炸药熔化部分在高速气流的作用下发生剥离,破碎成极小的颗粒,瞬时发生分解反应,释放出能量支持爆轰波传播。数值模拟了在不同粒径和浓度的悬浮RDX炸药粉尘中爆轰波的发展与传播过程,得到了爆轰波流场中气-固两相的物理量分布,并确定了爆轰波参数。在较低的RDX粉尘浓度条件下,爆轰波阵面压力的峰值曲线出现振荡。当RDX粉尘浓度在80~150 g/m3时,数值模拟得到的爆轰波阵面压力峰值曲线的振荡是规则的;当RDX粉尘浓度为70 g/m3时,爆轰波阵面压力峰值曲线出现不规则振荡。     

Studies on the valveless scheme to produce high-frequency detonations with different purge methods

Producing high-frequency detonations is an important topic for pulse detonations which has received considerable attentions. The valveless scheme has been verified to be able to obtain high-frequency detonations more than 100 Hz. This work has been conducted to investigate the possibility to achieve a higher detonation frequency and clarify the limits of stable operations preliminarily for the valveless scheme with different purge methods. Oxygen, ethylene, and nitrogen or liquid water are utilized as oxidizer, fuel, and purge medium in the experiments while two injection configurations are employed. The maximum detonation frequencies of 180 Hz and 330 Hz have been achieved in stable operations for two different injection configurations when nitrogen is used as the purge gas. The ceiling frequency for stable detonations is 300 Hz if nitrogen is replaced by liquid water, which indicates that water vapor is capable to create an efficient buffer zone to ensure stable operations. The results imply that the injection configuration also has a great impact on the ceiling stable detonation frequency. Three operating modes have been observed in this study, i.e., a stable detonation mode, an unstable detonation mode, and a deflagration mode. In the unstable mode, failure of detonation initiation occurs frequently and one interesting phenomenon is that the detonation frequency is reduced by half exactly when insufficient filling happens. The supply pressure ratios of oxidizer to fuel and purge to fuel are obtained for different operating modes when the purge method is changed. Furthermore, the equivalence ratios have been also studied for different operating modes which reveals that the range will change when different purge methods and injection configurations are employed. According to the equivalence ratio and the mass flow rates, an equivalent volume fraction of oxygen is defined and its range for the stable detonation mode is clarified.