Gas-phase detonation propagation in mixture composition gradients

The propagation of detonations through several fuel-air mixtures with spatially varying fuel concentrations is examined numerically. The detonations propagate through two-dimensional channels, inside of which the gradient of mixture composition is oriented normal to the direction of propagation. The simulations are performed using a two-component, single-step reaction model calibrated so that one-dimensional detonation properties of model low- and high-activation-energy mixtures are similar to those observed in a typical hydrocarbon-air mixture. In the low-activation-energy mixture, the reaction zone structure is complex, consisting of curved fuel-lean and fuel-rich detonations near the line of stoichiometry that transition to decoupled shocks and turbulent deflagrations near the channel walls where the mixture is extremely fuel-lean or fuel-rich. Reactants that are not consumed by the leading detonation combine downstream and burn in a diffusion flame. Detonation cells produced by the unstable reaction front vary in size across the channel, growing larger away from the line of stoichiometry. As the size of the channel decreases relative to the size of a detonation cell, the effect of the mixture composition gradient is lessened and cells of similar sizes form. In the high-activation-energy mixture, detonations propagate more slowly as the magnitude of the mixture composition gradient is increased and can be quenched in a large enough gradient.     

湍流扩散燃烧数值模拟中几种湍流模型的比较

为了比较湍流模型对火焰模拟结果的影响,应用CFD商业软件Fluent,对以C3H8为燃料,O2为氧化剂的火焰气相沉积法合成TiO2颗粒的火焰温度场进行数值模拟,通过对3种不同的湍流模型进行比较,发现标准κ-ω模型的结果与实验中的火焰相差很大,RSM模型并不能适用于所有的工况,而RNGκ-ε模型的温度场和火焰形状与实验中所观测到的结果吻合。     

Knock characteristics of dual-fuel combustion in diesel engines using natural gas as primary fuel

This paper investigates the combustion knock characteristics of diesel engines running on natural gas using pilot injection as means of initiating combustion. The diesel engines knock under normal operating conditions but the knock referred to in this paper is an objectionable one. In the dual-fuel combustion process we have the ignition stage followed by the combustion stage. There are three types of knock: diesel knock, spark knock and knock due to secondary ignition delay of the primary fuel (erratic knock). Several factors have been noted to feature in defining knock characteristics of dual-fuel engines that include ignition delay, pilot quantity, engine load and speed, turbulence and gas flow rate     

Numerical Simulation and Calculation of the Process of Interaction between a Swirling Jet and Drift Flow

A mathematical model and results of numerical calculation of the process of interaction between a swirling jet and drift flow are presented in the paper. The structure of flow and the process of chemical reactions are investigated under conditions of transverse injection of a swirling jet of combustion products into a flow of fuel-air mixture. Inferences are made of the possibility of using a transversely injected jet to provide for a gasdynamic stabilization of flame in combustors of gas-turbine engines.     

Experimental investigation of saturation effects in ferrite frequency doublers

Saturation of ferrite frequency doublers due to the reaction of the generated second-harmonic field back on the sample, the excitation of unstable spin waves by transverse pumping, and the excitation of unstable spin waves by longitudinal pumping is investigated experimentally. An expression for the second-harmonic power as a function of the power absorbed by the ferrite, which holds for any degree of processional motion ellipticity of the magnetization, is derived including the effect of the reaction field. An experimental method of observing the three saturation effects through the introduction of an externally impressed second-harmonic field is described. Experimental observations of all three saturation mechanisms are reported, and the results are compared with theory. The upper bound on conversion efficiency of a ferrite doubler, assuming adequate output coupling circuits can be devised, is set by longitudinally-pumped spin waves. This upper bound on efficiency is measured experimentally for a typical doubler.     

汽油机爆震强度的分析

分析了汽油机缸内气体爆震燃烧压力的频率特性并推导了爆震压力共振频率计算公式。研究表明：在爆震工况下，缸内压力在高频域中有几个共振峰，峰的大小表明了汽油机爆震强度。汽油机的表面振动在这些共振频率峰处也出现相应振动峰。表面振动峰与缸力压内峰值近似成正比。汽油机表面振动峰值可作为爆震强度的评价指标，该结论在ＣＡ１１０２汽油机上得到了证实。     

A review of factors controlling the gas turbine hot section environment and their influence on hot salt corrosion test methods

Environmental and operational factors that influence the formation and deposition of corrosive species on hot section components in gas turbine engines are reviewed. In addition to air and fuel impurities, combustion gas chemistry, velocity, pressure and temperature are identified as key operational factors affecting the formation and deposition of Na₂SO₄ salt (the primary corrosive species) when trace amounts of sodium and sulphur are entrained into combustors. Test methods for ranking the resistance of different materials to hot salt corrosion are reviewed and compared in terms of the type of damage they produce. The methods considered range from a simple furnace or crucible test to burner rig and engine tests. It is shown that high velocity burner rigs, running at atmospheric pressure, allow all the relevant operational factors to be simulated, and they produce realistic hot corrosion damage similar to that observed on engine parts in service.     

生成项模型在二氧化钛颗粒合成中的应用

为了更好地研究二氧化钛纳米颗粒合成过程中生成项模型以及表面氧化反应对颗粒尺寸的影响，用CFD商业软件Fluent，对湍流扩散火焰中的颗粒合成过程进行了详细的数值模拟。在没有通入TiCl4先驱物的情况下，计算出火焰的温度场。然后通过对五种不同工况的对比，讨论了火焰场温度和浓度对颗粒生长的影响。在此基础上考虑了表面氧化反应对颗粒尺寸变化的影响。结果表明：表面氧化反应对颗粒尺寸的分布具有一定影响，但是温度才是颗粒尺寸变化的最主要影响因素。     

Thermodynamic and numerical analysis of intake air humidification of a turbocharged GDI engine

Reducing the temperature and increasing the specific heat capacity of working medium of gasoline engines are the most efficient methods of mitigating knock tendency. The charge cooling effect of intake air humidification is helpful for decreasing the initial temperature of intake air, and the increase of the specific heat capacity of working medium can reduce the temperature rise in the in-cylinder process. This study established a mathematical model of intake air humidification of gasoline engines, and analyzed the effects of the technique on the thermodynamic process of a turbocharged gasoline engine with Ricardo WAVE Code. The results indicated that the intake air humidification is an isenthalpic process; the vapor influences the working process of the engine by altering the thermodynamic parameters of the working medium. A decrease in the initial temperature and adiabatic index and an increase in the specific heat capacity of working medium lowered the in-cylinder temperature and pressure, hence suppressing the knock occurrence. After the humidification of intake air, the engine performance slightly increased, and the thermal efficiency showed different levels of improvements at all the working conditions.     

Effect of polymer additives on wall turbulence

The average and fluctuation velocities associated with turbulent flow of a weak polyox solution in smooth and rough ducts have been investigated experimentally. The suppression of wall pressure fluctuations is discussed on the basis of the data obtained on the influence of polymer additives on the shear and transverse velocity fluctuations.     

Influence of magnetic field on electromagnetic instabilities in semiconductor superlattices

Electromagnetic instability criteria in semiconductor superlattices (SL) in the region of a negative differential conductivity (NDC) are studied in the presence of a static transverse field. The regimes with vertical constant electric field in a "short" plate of SL and with vertical current density vector in a "long" plate are considered. The effective differential conductivity tensors for both these regimes are derived and the influence of the magnetic field on the NDC threshold and the growth increment is investigated. The frequencies of the excited waves and the dependence of the growth increment on the direction of propagation for the extraordinary waves are determined.     

Incompressible micromorphic fluid model for turbulence

A theory of incompressible micromorphic fluids is introduced as a rational model for turbulence studies. Balance laws and constitutive equations are given. The theory is then applied to obtain the solution of the turbulent channel flow problem. Turbulent velocity, gyrations, Reynolds stresses, root-mean squares of longitudinal and transverse turbulent velocities, and turbulent shear stress are given.     

The asymptotic structure of a counterflow premixed flame for large activation energies

The asymptotic structure of a steady counterflow, premixed flame is analyzed in the limit of a large ratio of the activation energy to the thermal energy in the flame for the overall combustion process. The governing conservation equations are similar to those for the counterflow diffusion flame analyzed previously by Liñán. The Damkohler number, defined as the ratio of a characteristic diffusion time to a characteristic chemical time is a parameter in this analysis. The characteristic $\text{S}$ curve emerges when the maximum flame temperature is plotted as a function of the Damkohler number. Segments of this curve represent a nearly frozen ignition regime, an unstable partial burning regime, and a near equilibrium regime. The structure of these regimes are analyzed in detail. Critical conditions for ignition and extinction of a premixed flame are also given.     

Effect of intense turbulence on turbulent premixed V-flame

The influence of free stream turbulence on the statistics of turbulent premixed V-flame is numerically investigated in this paper. The flame front is tracked using the level-set algorithm with the effect of exothermicity and baroclinicity. Results indicate that free stream turbulence affects the statistics greatly including the conditioned and unconditioned mean axial and transverse velocities, fluctuation velocities and Reynolds stresses. The unconditioned and conditioned fluctuating velocities, unconditioned mean axial velocities and the flame brush thickness increase with increased free stream turbulence. The maximum unconditioned mean axial velocity along the centre line increases linearly with turbulence. The peaks of unconditioned transverse fluctuation velocity smoothens with the increase of free stream turbulence level, indicating that the effect of intermittency decreases with the increase of free stream turbulence. These results show that free stream turbulence is a major influence on turbulent statistics in premixed V-flame.     

Organised structures in wall turbulence as deduced from stability theory-based methods

In earlier work, we have explored the relevance of hydrodynamic stability theory to fully developed turbulent wall flows. Using an extended Orr-Sommerfeld Equation, based on an anisotropic eddy-viscosity model, it was shown that there exists a wide range of unstable wave numbers (wall modes), which mimic some of the key features of turbulent wall flows. Here we present experimental confirmation for the same. There is good qualitative and quantitative agreement between theory and experiment. Once the dominant coherent structure is obtained from stability theory, control of turbulence would be the next logical step. As shown, the use of a compliant wall shows considerable promise. We also present some theoretical work for bypass transition (Klebanoff/K-modes), wherein the receptivity of a laminar boundary layer to a vortex sheet in the freestream has been studied. Further, it is shown that triadic interaction between K-modes, 2D TS waves and 3D TS waves can lead to rapid algebraic growth. A similar mechanism seems to carry over to inner wall structures in wall turbulence and perhaps this is the “root cause” for sustenance of turbulence.     

Laminar and Turbulent Transition of Fine-Grained Slurries

An experimental investigation of flow behavior of fine-grained highly concentrated slurries, i.e., water mixtures of kaolin and fly ash from a fluidic-type combustion chamber produced during the process of desulphurization, in horizontal straight pipes is presented. A pipe loop with hydraulically smooth stainless steel pipes was used to measure the slurry flow parameters. Kaolin slurry has time-independent, yield pseudo-plastic response for volume concentrations higher than about 6%. In contrast, fluidic fly ash-gypsum water mixture is time dependent and showed substantial decrease of flow resistance due to the effect of shearing during the initial period of pumping. An intensive shearing of concentrated fluidic fly ash-gypsum slurry results in a substantial reduction of the hydraulic gradient in the laminar region and in a marked shift of the laminar/turbulent transition point towards a lower velocity value. After shearing in a turbulent regime a reduction in the hydraulic gradient at the transition point reached about 50% of its original value. The transition from laminar to turbulent regime results in an abrupt increase of flow resistance. The flow patterns become fundamentally different for the two regimes. It was found that pressure fluctuation could well indicate the laminar/turbulent transition. The transition from laminar to turbulent flow is very important for accurate and efficient design and operation of dense slurry pipelining. The optimum operational condition is slightly above the laminar/turbulent transition point, where flow conditions are often very attractive from an economic point of view.     

Occurrence and Development of Vortices in Turbulent Jets under the Effect of Sawtooth Sound Waves of Finite Amplitude

Consideration is given to the formation of vortices in turbulent jets under the effect of sawtooth sound waves of finite amplitude in the case of internal longitudinal acoustic action. The convection velocity and the rate of rise of the disturbances are determined. It is shown that the transverse dimensions of the disturbances increase linearly on the initial portion of the flow.     

Direct resonance analysis and modeling for a turbulent boundary layer over a corrugated surface

Summary Effects of surface corrugation on turbulent flow in a boundary layer are studied using a model based on the direct resonance theory. The induced mean flow due to weakly nonlinear waves, superimposed on the mean and fluctuating components of turbulence, is determined. The mean turbulent flow is affected by the surface corrugation throughout the boundary layer. The corrugated surface generates higher harmonics and affects the streamwise vortices generated by the waves superimposed on turbulence whose mean flow includes secondary induced mean flow components due to the corrugation.