Thermal regimes of a counterflow reactor: a gas-liquid system

A counterflow reactor model for a system of two phases one of which involves an exothermic bimolecular reaction is considered. At a stationary temperature distribution in the reactor, there are high and low heating regions. For the adiabatic case, maximum heating is only possible at the bottom of the reactor. The maximum stationary temperature in the reactor decreases and shifts toward the top of the reactor as the intensity of heat exchange with the environment increases.     

连续搅拌反应器中自催化化学反应的同步与控制

讨论了由两个连续搅拌反应器组成的化学反应体系的同步问题. 基于Lyapunov稳定性理论,提出了设计控制函数的方法,分别探讨了入料溶液的流速恒定和存在周期微扰两种情况下使反应实现同步的耦合强度范围. 用数值模拟验证了方法的有效性. 结果表明该方法不仅适用于自治系统,同时也适用于具有周期扰动的非自治系统. 关键词： 同步与控制 化学反应 连续搅拌反应器 周期扰动     

Experimental analysis of a double-spark ignition system

The spark that ignites the combustible mixtures is a discharge produced between the electrodes of a spark plug, connected to the secondary of a coil at the high voltage. Optimum combustion requires a steady spark, in a volume as large as is possible, and with maximum energy. We propose a solution to increase the plasma volume and present electrical discharge parameters as a function of inter-electrode distances, pressures in the test-reactor and the width of the electrical pulses of the power supply.     

Parallelization of heterogeneous reactor calculations on a graphics processing unit

Parallelization is applied to the neutron calculations performed by the heterogeneous method on a graphics processing unit. The parallel algorithm of the modified TREC code is described. The efficiency of the parallel algorithm is evaluated.     

Analysis of first stage ignition delay times of dimethyl ether in a laminar flow reactor

The combustion chemistry of the first stage ignition and chemistry/flow interactions are studied for dimethyl ether (DME) with a mathematical analysis of two systems: a plug flow reactor study is used to reduce the reaction chemistry systematically. A skeletal reaction mechanism for the low temperature chemistry of DME until the onset of ignition is derived on the basis of the detailed DME mechanism of the Lawrence Livermore National Laboratory – see Curran, Fischer and Dryer, Int. J. Chem. Kinetics, Vol. 32 (2000). It is shown that reasonably good results for ignition delay times can be reached using a simple system of three ordinary differential equations and that the resulting analytical solution depends only on two reaction rates and the initial fuel concentration. The stepwise reduction of the system based on assumptions yields an understanding on why these reactions are so important. Furthermore, the validation of the assumptions yields insight into the influence of the fuel and the oxygen concentration on the temperature during the induction phase. To investigate the influence of chemistry/flow interactions, a 2D model with a laminar Hagen–Poiseuille flow and 2D-polynomial profiles for the radial species concentration is considered. For the 2D model, it is found that only the diffusion coefficients and the reactor radius need to be taken into consideration additionally to describe the system sufficiently. Also, the coupling of flow and chemistry is clarified in the mathematical analysis. The insight obtained from the comparison of the 2D model and the plug flow model is used to establish an average velocity for the conversion of ignition locations to ignition delay times in a laminar flow reactor. Finally, the 2D analytical solution is compared against new experimental data, obtained in such a laminar flow reactor for an undiluted DME/air mixture with an equivalence ratio of φ = 0.835 and a temperature range of 555 to 585 K at atmospheric pressure.     

The impact of residence time on ignitability and time to ignition in a toroidal jet-stirred reactor

Understanding of ignition processes is central to design for reliable and safe aerospace combustor systems. Ignition is influenced by many factors including combustor geometry, flow conditions, fuel composition, turbulence intensity, ignition source, and energy deposition method. A toroidal jet-stirred reactor (TJSR) utilizes bulk fluid motion, presence of recirculation zones, a bulk residence time, and turbulence intensities which emulate characteristics relevant to cavity stabilized and swirl stabilized combustors. In this work, a TJSR was used to quantify ignitability and time-to-ignition of premixed ethylene and air. The effects of inlet temperature, residence time, and reactivity were studied on forced ignition processes. Experimental conditions ranged from residence times of 15–35?ms, mixture temperatures of 340–450?K, and equivalence ratios of 0.5–1 using capacitive spark-discharge ignition. The minimum equivalence ratio for ignition (MER), or the equivalence ratio at 50% probability, shows an inverse relationship with mixture temperature and residence time. Prior theory of real engine combustor performance for lean light off, proposed by Ballal and Lefebvre, was compared to the MER and displayed similar trends to the model. Spatially integrated OH* chemiluminescence was used to measure time to ignition within the reactor. Reduction in ignitibility was experienced as the time-to-ignition approached the residence time stressing the importance of device flow time scales in relation to kernel growth dynamics and ignition probability.     

Numerical simulations of the ignition of n-heptane droplets in the transition diameter range from heterogeneous to homogeneous ignition

Spontaneous ignition of single n-heptane droplets in a constant volume filled with air is numerically simulated with the spherical symmetry. The volume is closed against mass, species, and energy transfer. The numerical model is fully transient. It continues calculation even after the droplet has completely vaporized, and therefore can predict pre-vaporized ignition. Initial pressure and initial air temperature are fixed at 3 MPa and 773 K, respectively. The droplet is initially at room temperature, and its diameter is between 1 and 100 μm. When the overall equivalence ratio is fixed to be sufficiently large, there exists no ignition limit in terms of initial droplet diameter d₀, and the ignition delay takes a minimum value at certain d₀. In such a case, transition from the heterogeneous ignition to the homogeneous ignition with decreasing d₀ is observed. When d₀ is fixed to be so small that the ignition would not occur in an infinite volume of air, the ignition delay takes a minimum value at certain , which is less than unity. Two-stage ignition behavior is investigated with this model. Ignition delay of a cool flame has the dependence on d₀ that is similar to that of ignition delay of a hot flame when is unity. When is almost zero, the ignition limit for cool flame in terms of d₀ is not identified unlike that for hot flame.     

Catalytic ignition of light hydrocarbons over Rh/Al2O3 studied in a stagnation-point flow reactor

The ignition (light-off) temperatures of catalytic oxidation reactions provide very useful information for understanding their surface reaction mechanism. In this study, the ignition behavior of the oxidation of hydrogen (H₂), carbon monoxide (CO), methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) over Rh/alumina catalysts is examined in a stagnation-point flow reactor. The light-off temperatures are identified by means of the sudden increase of the catalyst temperature when linearly heating the catalyst for various fuel/oxygen ratios. For hydrogen and all hydrocarbons studied, the results show a rise of ignition temperature with increasing fuel/oxygen ratio, whereas the opposite trend is observed for the light-off of CO oxidation. Hydrogen oxidation, however, shows an opposite trend compared to previous investigations, performed on platinum [1], [2].     

Structure-oriented computer model of a carbon-loaded heterogeneous system

We present an algorithm describing the structure of a heterogeneous, carbon-loaded system. After establishing the validity of the structure and computer model for a given growth scale, we show it is possible to solve the problem of producing materials with predicted properties. Tomsk Polytechnical University. Altaiskii Technical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 95–99, June, 1998.     

混响室复合搅拌方式的搅拌效率分析

为了评价混响室在不同搅拌方式下的搅拌效率,提高相关测试的精确度,对独立采样数这一重要指标的计算方法做了简单介绍,并在某大型混响室内,对200, 500, 1000 MHz频点下的机械搅拌与频率搅拌方式的搅拌效率进行了试验分析。结果表明:二者的搅拌效率均与工作频率成正比;在机械搅拌方式下,多个搅拌器在不同转速比下能够显著提高搅拌效率;频率搅拌的搅拌带宽选取越大搅拌效率越高;两种搅拌方式提供的独立采样点数均存在一个上限值。通过分析对比,提出了基于机械搅拌与频率搅拌相结合的复合搅拌方式,并对该方式的搅拌效率进行了重点分析,给出了该方式下独立采样点的计算方法,试验结果表明该复合搅拌方式可显著提高测试样本的独立采样数。     

Liquid-solid phase transition in a heterogeneous system of solid spheres

A molecular dynamics method is used to study the influence of a heavy dispersed particle on a liquid-solid phase transition in a molecular system of solid spheres. It is shown that the presence of a dispersed particle shifts the transition toward higher densities and pressures. In addition, in the liquid state the heterogeneous system has a lower pressure, whereas in the solid state the pressure is higher than that in the corresponding states of a homogeneous medium. It is established that the pressures of the heterogeneous mixture in the regions before and after the phase transition can be converted to the pressure of the single-component system by introducing various scaling factors (effective densities of the medium). Zh. éksp. Teor. Fiz. 116, 2006–2011 (December 1999)     

Effect of feedstock water leaching on ignition and PM1.0 emission during biomass combustion in a flat-flame burner reactor

In this work, the effects of feedstock water leaching on ignition and PM_1.0 emission during biomass combustion were studied, for the first time, in a Hencken flat-flame burner reactor (HFFBR). A high-speed video camera and high-resolution electrical low-pressure impactor were respectively employed to diagnose ignition and PM_1.0 along the height of the burner. The mineral composition of PM₁₀₊ was measured as a function of height to demonstrate the potassium release during the early stage of biomass combustion. The results show that water leaching does not change the functional group of the biomass (straw), but increases the BET surface area and pore volume. Water leaching removes 90% of the potassium and all the chlorine, reducing the same amount of PM_1.0 emission. The effect of water leaching on ignition delay observed in the flat-flame burner reactor agrees with the delay of biomass-devolatilization in TGA. Profiles of mineral composition in the PM₁₀₊ with height shows that a large amount of the potassium is released before biomass ignition. This indicates that, at realistic heating rates, the catalytic promotion of water-soluble minerals on biomass ignition is primarily through promoting devolatilization. The ignition delay of biomass particles caused by water leaching is more significant at lower temperature, e.g., ignition is delayed from 20 to 24?ms at 1000?°C, and from 9.2 to 10.2?ms at 1300?°C.     

混响室复合搅拌方式的搅拌效率分析

为了评价混响室在不同搅拌方式下的搅拌效率,提高相关测试的精确度,对独立采样数这一重要指标的计算方法做了简单介绍,并在某大型混响室内,对200, 500, 1000 MHz频点下的机械搅拌与频率搅拌方式的搅拌效率进行了试验分析。结果表明:二者的搅拌效率均与工作频率成正比;在机械搅拌方式下,多个搅拌器在不同转速比下能够显著提高搅拌效率;频率搅拌的搅拌带宽选取越大搅拌效率越高;两种搅拌方式提供的独立采样点数均存在一个上限值。通过分析对比,提出了基于机械搅拌与频率搅拌相结合的复合搅拌方式,并对该方式的搅拌效率进行了重点分析,给出了该方式下独立采样点的计算方法,试验结果表明该复合搅拌方式可显著提高测试样本的独立采样数。     

Diffusion degradation model for a heterogeneous photoconducting system

It is shown that the excitation of the electronic subsystem in heterogeneous semiconductors causes the gettering (accumulation) of defects in regions where the diffusion coefficient of defects is low. This is because this parameter in the wide-gap (active) region of the material increases, remaining low in narrow-gap areas (drains). This effect improves the radiation hardness of the heterogeneous material, as demonstrated with the CdS-PbS system, which exhibits a limited series of solid solutions.     

Thermal fluctuations in a Knudsen flow system

Spatial correlations of thermal fluctuations in a model system are examined. The system is a one-dimensional chain of cells containing a dilute gas and connected by Knudsen apertures. A Monte Carlo simulation is described in which the long range correlations observed agree quantitatively with a general master equation formulation.     

Thermal conduction in a homogeneous two-phase system

Assuming a regular geometry of dispersed phase (λ ₂) an integrated theory for the effective thermal conductivityλ _e of all kind of two-phase materials (including loose materials) is developed. The flux modification is carried out by considering the effective neighbouring interactions in the solution of Poisson’s equation. A comparison of calculatedλ _e values with the reported experimental results over a wide range of two-phase materials shows a good agreement.     

Effects of the heterogeneous landscape on a predator-prey system

In order to understand how a heterogeneous landscape affects a predator-prey system, a spatially explicit lattice model consisting of predators, prey, grass, and landscape was constructed. The predators and preys randomly move on the lattice space and the grass grows in its neighboring site according to its growth probability. When predators and preys meet at the same site at the same time, a number of prey, equal to the number of predators are eaten. This rule was also applied to the relationship between the prey and grass. The predator (prey) could give birth to an offspring when it ate prey (grass), with a birth probability. When a predator or prey animal was initially introduced, or newly born, its health state was set at a given high value. This health state decreased by one with every time step. When the state of an animal decreased to less than zero, the animal died and was removed from the system. The heterogeneous landscape was characterized by parameter H, which controlled the heterogeneity according to the neutral model. The simulation results showed that H positively or negatively affected a predator’s survival, while its effect on prey and grass was less pronounced. The results can be understood by the disturbance of the balance between the prey and predator densities in the areas where the animals aggregated.     

Consecutive reactions in a countercurrent plug-flow reactor. Gas-liquid system

A mathematical model of a bimolecular two-step consecutive exothermic reaction in the liquid phase of a gas-step consecutive exothermic reaction in the liquid phase of a gas-flow reactor is proposed. Spatial profiles of the heatings and relative concentrations of the reactants in both phases for the steady state of the system are presented. The behavior of the reaction intermediate product is examined. The kinetic characteristics of the reaction are demonstrated to substantially influence the behavior of the system in the reactor.