Influence of strain on flame propagation in laminar flow

The influence of velocity gradients, in a laminar approach flow, on the local burning velocity was investigated, to supplement and verify recent theoretical work.Different velocity profiles were obtained at the exit from a circular burner, using pipes of different lengths between the exit and a settling chamber, and these profiles were measured using hot wire anemometry. The burning velocity and the velocity component parallel to the flame front were obtained by means of the “angle method” using direct photographs of the flame.The results, presented as burning velocity versus velocity gradient along the flame front, show that the local laminar burning velocity decreases as the rate of “stretching” of the flame is increased.     

Pressure dependence of mass burning rates in diluent premixed flames of H2/O2 at high pressures

Pressure dependence of mass burning of diluted hydrogen premixed flames is studied numerically over a full range of pressure. Mass burning rate is selected to be a parameter for burning capability of flames. First, positive linear dependence of mass burning rate has been confirmed at low pressures and negative pressure dependence has appeared in the medium range of pressure, which complies with the results reported in previous experimental works. And then, when the pressure range is extended more, positive pressure dependence is recovered or shows up again at high pressures. The flame structures of temperature and species profiles in each pressure regime are demonstrated. They show that the latter two dependences of negative and positive can be explained by enhanced recombination reactions producing HO₂ at high pressures and chain re-branching to OH production via H₂O₂, respectively. There are three distinct dependences of mass burning or global chemistry in hydrogen flames. Two onset pressures, at which pressure dependence changes, depend on equivalence ratio, degree of dilution, diluent species, and unburned-gas temperature. Accordingly, the onset pressure can be used as a parameter characterizing burning of premixed flames.     

节流配汽超超临界汽轮机调门特性与经济运行

汽轮机主蒸汽流量是机组性能指标计算的关键参数之一,采用现有调门模型对其计算面临建模繁琐、模型复杂和计算过程参数难以获取等问题。针对有效面积变化的汽轮机高压调门,提出用椭圆公式对其建模,并用公开文献试验数据对其进行验证。结合阀门模型,以某660 MW节流配汽型超超临界汽轮机组为研究对象,对该机组进行变工况计算,研究不同负荷的不同运行策略下,高压缸效率和机组经济性的变化情况。结果表明,椭圆公式能够表现调门变工况特性,该方法建模过程简单,计算方便,在调门工作区域内计算精度高。机组高压缸效率随着高压调门开度的减小而减小,纯滑压运行方式下热耗率最低,但考虑到电网调频要求,建议机组变工况运行时调门开度控制在30%～45%为宜。所推荐的宽范围变工况运行策略在调门调节裕量满足电网调频的要求下,能够提高机组的经济性和深度调峰能力。研究结果可为同类型机组的运行优化策略提供参考。     

Field measurement and estimation of ventilation flow rates by using train-induced flow rate through subway vent shafts

This study measured and estimated the subway vent shaft air flow rate induced by moving trains in the tunnel. This work estimated the flow rate via the tunnel structure and train movement to determine the quantitative effect of vent shafts as air purification systems of natural ventilation to improve the air quality management of a subway. The amount of air suctioned into the tunnel is significantly larger than that vented from the tunnel. Thus, placing vent shafts near subway stations is desirable for natural ventilation systems. Experimental approaches to measure train-induced flow rates have not yet been published. Results of this study provide useful fundamental data to study the natural ventilation in a subway. Therefore, this study suggested the significant design factors required to control indoor air quality in a subway.     

固定排气口型气囊冲击减缓特性研究

为研究固定排气口型缓冲气囊冲击减缓特性,基于能量守恒及热力学方程建立了固定排气口型气囊缓冲过程的解析模型,并采用LS-DYNA和实验对其有效性进行了验证。基于该模型研究了竖直圆柱式气囊的缓冲特性,开展了初始压力、排气口面积和排气口触发条件对竖直圆柱式气囊缓冲性能的影响研究,并定义了初始极限压力。研究表明:当初始压力小于初始极限压力时,适当增大竖直圆柱式气囊的初始压力、选取合适的排气口触发条件,均可将设备的峰值过载维持在一个合理水平的基础上,减小气囊的体积;采用合理的排气口设计参数,可以提高系统的吸能率,降低峰值过载,同时避免设备的反弹。     

浅析GB150．1—2011附录B《超压泄放装置》

对GB150附录B前后两个版本（1998版和2011版）从超压限度、动作压力、泄放面积计算三个方面进行了比较分析。通过对附录B压力容器超压限度相关规定的分析,根据超压原因及所设置的超压泄放装置的不同归纳出了8种超压事故工况,并给出了各工况下超压泄放装置动作压力及泄放面积确定原则,为设计人员选择超压泄放装置提供依据。     

Thermodynamic model for prediction of performance and emission characteristics of SI engine fuelled by gasoline and natural gas with experimental verification

In this study, a thermodynamic cycle simulation of a conventional four-stroke SI engine has been carried out to predict the engine performance and emissions. The first law of thermodynamics has been applied to determine in-cylinder temperature and pressure as a function of crank angle. The Newton-Raphson method was used for the numerical solution of the equations. The non-differential form of equations resulted in the simplicity and ease of the solution to predict the engine performance. Two-zone model for the combustion process simulation has been used and the mass burning rate was predicted by simulating spherical propagation of the flame front. Also, temperature dependence of specific heat capacity has been considered. The performance characteristics including power, indicated specific fuel consumption, and emissions concentration of SI engine using gasoline and CNG fuels have been determined by the model. The results of the present work have been evaluated using corresponding available experimental data of an existing SI engine running on both gasoline and CNG. It has been found that the simulated results show reasonable agreement with the experimental data. Finally, parametric studies have been carried out to evaluate the effects of equivalence ratio, compression ratio and spark timing on the engine performance characteristics in order to show the capability of the model to predict of engine operation.     

Effect of spark plug protrusion on the performance and emission characteristics of an engine fueled by hydrogen-natural gas blends

Mixtures of hydrogen and natural gas are promising for improving efficiency and reducing harmful emissions in spark ignition engines, since limits of flammability can be extended while stable combustion is secured. In this research, the combustion characteristics of long electrode spark plugs were evaluated in a hydrogen blended with natural gas (HCNG) engine. Decreases in the flame propagation distance through the use of spark plugs can lead to increased burning rates and further improvement of fuel economy in HCNG engines. An 11-liter heavy duty lean burn engine was employed and performance characteristics including emissions were assessed according to the spark timing of the minimum advance for best torque (MBT) for each operating condition. Retarded MBT spark advance timing with long electrode spark plugs due to increased burning speed supported increases in engine efficiency and reductions of nitrogen oxide (NO_x) emissions. The lower positions of initial flame kernels due to the use of long electrode spark plugs were preferable to improvements of cyclic variability due to reduced flame front quenching, and carbon monoxide (CO) emissions at the flammability limit were also improved.     

Chemical,thermal and dilution effects of carbon dioxide in oxy-fuel combustion of wood in a fixed bed

Experimental and numerical modeling was performed on eucalyptus wood combustion under oxy-fuel conditions using a fixed bed reactor in order to isolate the role of various carbon dioxide effects on the burning rate. Wood combustion was investigated under four different mixtures of O₂ and Ar/CO₂/N₂: 21 % O₂/79 % N₂; 21 % O₂/22.5 % CO₂/56.5 % Ar; 40 % O₂/60 % CO₂; and 40 % O₂/47 % CO₂/13 % Ar. The first three mixtures were designed to have the same peak temperatures in order to isolate chemical and dilution effects of CO₂. This was achieved by substituting some percentage of CO₂ with Ar in O₂/CO₂ mixture while maintaining a constant concentration of O₂. The fourth mixture was meant to isolate the thermal effect of CO₂. The results were obtained from both the experimental rig and numerical simulation for a fixed bed configuration. Wood combustion in the fixed bed was modeled using Lagrange-Euler method, where gas-phase was calculated using computational fluid dynamics (CFD), that is Euler phase, while solid-phase was tracked in Lagrange phase using discrete element method (DEM). The results show that ignition time in CO₂ environment decreases gradually as O₂ concentration is increased. On the other hand, burning rate and flame front speed increase as O₂ concentration is increased. It was established that dilution effect is the most influential parameter on the burning rate of wood combustion in an oxy-fuel system.

     

开孔率对竖井型城市隧道阻滞工况自然通风影响的分析

为了分析开孔率对竖井型城市隧道在交通阻滞工况下对自然通风的影响,本文采用无量纲分析和模型试验的方法对其速度场、温度场和污染物浓度场进行了研究。通过对阻滞工况下隧道内空气所受的热压浮升力与重力的无量纲分析,得到了模型试验所需的热压通风相似准则数和温度分布相同所需散热量比例常数。利用所得相似条件搭建了开孔率分别为2.8%、3.7%、4.5%的3组试验模型进行试验。试验结果表明,随着开孔率的增大隧道内最不利通风段的通风效果得到改善,但竖井的建设费用大幅度提高。选择合适的竖井开孔率既能满足阻滞工况下隧道的通风要求,也能避免开孔率盲目增大导致的初投资浪费,对城市隧道建设具有重要意义。     

氢气-空气预混火焰传播特性的数值模拟研究

应用Fluent流体计算软件,基于标准湍流κ-ε模型和EDC燃烧模型,采用SIMPLE格式算法对常温常压下对常温常压下氢气／空气预混火焰在光滑管道中的传播特性进行二维数值模拟,获得火焰传播速度、火焰结构、表面积、火焰到达位置和时刻沿管道变化情况。结果表明：火焰传播速度先增加后逐渐减小。在离点火端105mm处到达最大值,之后逐渐降低。在t=9．1ms时,火焰开始逐渐呈现典型的郁金香结构。火焰在初始加速阶段的主要物理机理是前方未燃气体受到前驱压缩波作用而被加热和压缩的正反馈微分加速机制,此后在管右端的反射压缩波影响下火焰传播速度略有降低。     

Effects of additive and preheat on the partially premixed CH4-air counter flow flames considering non-gray gas radiation

Detailed structures of the counterflow flames formed for different inlet fluid temperatures and different amount of additives are studied numerically. The detailed chemical reactions are modeled by using the CHEMKIN-II code. The discrete ordinates method and the narrow band based WSGGM with a gray gas regrouping technique (WSGGM-RG) are applied for modeling the radiative transfer through non-homogeneous and non-isothermal combustion gas mixtures generated by the counterflow flames. The results compared with those obtained by using the SNB model show that the WSGGM-RG is very successful in modeling the counterflow flames with non-gray gas mixture. The numerical results also show that the addition of CO₂ or H₂O to the oxidant lowers the peak temperature and the NO concentration in flame. But preheat of fuel or oxidant raises the flame temperature and the NO production rates. O₂ enrichment also causes to raise the temperature distribution and the NO production in flame. And it is found that the O₂ enrichment and the fuel preheat were the major parameters in affecting the flame width.     

HTPB/镁铝合金含量对含硼富燃推进剂压强指数影响

通过推进剂的燃速、热重(TG-DTG)分析和高压差示扫描(DSC)分析手段研究了端羟基聚丁二烯(HTPB)/镁铝合金(MA)含量变化对含硼推进剂压强指数的影响。采用燃速u与压强P的关系u=apn,求出该推进剂的压强指数n(压强范围0.5 MPa~1.5 MPa)。燃速测试实验结果表明,当HTPB/MA含量为27.3%/3%时,压强指数为0.280,而当HTPB/MA含量为20.3%/10%时,压强指数升高到0.420。通过推进剂的TG-DTG和DSC热分析可知,含硼推进剂中HTPB含量较低、MA含量较高时,推进剂失重较快,且推进剂的热分解受到压强的影响显著增强,特别是氧化剂高氯酸铵(AP)的低温分解受压强影响非常显著,因而燃速较高,压强指数也高。     

含ACP的无烟改性双基推进剂燃烧特性研究

快燃物ACP作为一种有效含能助剂用来提高推进剂的燃速具有一定的效果。通过DSC和TG-DTG研究了ACP对无烟改性双基推进剂(CMDB)热分解特性的影响,采用靶线法研究了不同含量的ACP对无烟改性双基推进剂燃速和燃速压力指数的影响,用燃烧火焰单幅照相技术和微热电偶测温获得了含ACP无烟改性双基推进剂在稳态燃烧条件下的火焰结构和燃烧波温度分布,分析了该推进剂中主要组分对燃烧性能的影响。结果表明:含ACP推进剂的燃速随ACP含量的增加而增大,推进剂燃烧火焰结构随ACP含量增加而更加明亮。     

燃烧室结构对天然气转子发动机燃烧过程的影响

以一台由端面进气汽油转子发动机改装而来的预混天然气转子发动机为研究对象,在FLUENT软件的基础上通过编程实现转子发动机三维网格的偏心运动,并选择合适的湍流模型、燃烧模型以及详细的CHEMKIN化学反应机理,建立基于化学反应动力学的端面进气天然气转子发动机三维动态数值模拟模型。通过与试验数据进行对比和分析,验证模型的可靠性。在此基础上,研究燃烧室结构对端面进气天然气转子发动机的缸内流场、温度场和中间产物浓度场的影响。结果表明,当燃烧室凹坑布置于转子曲面长度方向的前端和转子曲面宽度方向的中心时,燃烧过程同时利用了燃烧室后部的滚流以及燃烧室中部高速流区对火焰的加速作用,缸内整体燃烧速率最大。同时,其缸内压力最大以及中间产物OH的生成量也最大,其压力峰值比中置凹坑燃烧室提高了19.9%,但其NO质量分数仍在0.5%以内。     

Modeling of aluminum particle combustion with emphasis on the oxide effects and variable transport properties

A simplified analytical modeling of single aluminum particle combustion was conducted. Ignition and quasi-steady combustion (QSC) were separately formulated and integrated. Both the heat transfer from the hot ambient gas and the enthalpy of heterogeneous surface reaction (HSR) served to cause the particle ignition. Conservation equations were solved for QSC parameters in conjunction with conserved scalar formulation and Shvab-Zeldovich function. Limit temperature postulate was formulated by a sink term pertinent to the dissociation of the aluminum oxide near the flame zone. Effective latent heat of vaporization was modified for the thermal radiation. Ignition and QSC of the aluminum particle were predicted and discussed with emphasis on the effect of the aluminum oxide and variable properties. The model was validated with the experiments regarding ignition delay time, burning rate, residue particle size, flame temperature, QSC duration, and stand-off distance of the envelop flame. Agreement was satisfactory and the prediction errors were limited within 10%.     

煤粉云火焰传播特性实验研究

煤矿开采中存在煤粉爆炸的重大安全隐患,本实验主要研究煤粉云的火焰传播,为预防和控制煤粉爆炸事故提供理论依据.实验采用粉尘云点火装置对煤粉在燃烧玻璃管中进行试验,通过改变煤粉云浓度探讨其对煤粉云火焰传播过程的影响,并利用高速摄影记录煤粉火焰传播过程.结果表明,点火电极在点燃煤粉云后,火焰沿着玻璃直管管壁竖直传播,火焰在传播过程中发出明亮的黄光并在玻璃直管上端端口形成蘑菇云状的火焰.当煤粉云的浓度为250 g/m3时,其燃烧时间远远小于浓度为500 g/m3和750 g/m3时的燃烧时间.当浓度分别为250 g/m3、500 g/m3和750 g/m3时,其火焰前锋阵面最大传播速度分别在100 ms、353 ms和310 ms时达到相应的最大值1.51 m/s、2.00 m/s和1.61 m/s.火焰前锋阵面达到最大速度的时间和火焰传播的最大速度随浓度的增加先增大后减小.     

压力容器大开孔补强设计的压力面积法与ASME法的分析比较

针对压力容器两种大开孔的补强计算方法———压力面积法和ASME法 ,分析了两种方法的异同 ,考证了ASME法计算公式的理论依据和由来 ,通过对某容器大开孔结构的两种方法进行计算比较 ,显示了其间的重大差异并由此对大开孔有限元应力分析结果的应力分类评定提出见解。     

Large eddy simulation of turbulent premixed combustion flow around bluff body

Large eddy simulation (LES) of turbulent premixed flame stabilized by the bluff body is developed by using sub-grid scale combustion model based on the G-equation describing the flame front propagation. The basic idea of LES modeling is to evaluate the filtered-front speed, which should be enhanced in the grid scale by the scale fluctuations. The dynamic sub-grid scale models newly introduced into the G-equation are validated by the premixed combustion flow behind the triangle flame holder. The objective of this study is to investigate the validity of the dynamic sub-grid G-equation model to a complex turbulent premixed combustion such as bluff body stabilized turbulent premixed flames for realistic engineering application. A new turbulent flame speed model, introduced by the sub-grid turbulent diffusivity and the flame thickness, is also proposed and is compared with the usual model using sub-grid turbulent intensity and with the experimental data. The calculated results can predict the velocity and temperature of the combustion flow in good agreement with the experiment data.