Stabilized gas combustion wave in an inert porous medium

A stabilized gas combustion wave in an inert porous medium with intense internal interphase heat exchange (at low velocities) is considered to elucidate the mechanism underlying the increase in the burning velocity of the homogeneous gas mixture due to the porous medium. It is shown that the major factor of the increase in the burning velocity is conductive heat recuperation from the postflame zone to the region of the fresh mixture through the solid skeleton of the porous medium. Analytical dependences of the degree of increase in the burning velocity of the mixture in the stabilized wave on the determining parameters are obtained. The possibilities and restrictions of the use of the results to analyze the operation of porous burners are discussed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 8–15, September–October, 2008.     

Filtration combustion of hydrogen-air, propane-air, and methane-air mixtures in inert porous media

The filtration combustion characteristics of hydrogen-air, propane-air, and methane-air mixtures in inert porous media have been studied experimentally. It is shown that the dependences of the combustion wave velocity on the fuel-air equivalence ratio are V-shaped. For hydrogen-air mixtures, the velocity minimum is shifted to the rich region, and for propane-air and methane-air mixtures, it is shifted to the lean region. For lean hydrogen-air and rich propane-air mixtures, the measured maximum temperatures in the combustion wave are found to be reduced relative to those calculated theoretically. For methane-air mixtures, a reduction in the measured temperatures is observed over the entire range of the mixture composition. The results are interpreted within the framework of the hypothesis of selective diffusion of gas mixture components. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 8–20, July–August, 2006.     

Filtration combustion of liquid monofuels

A one-dimensional two-temperature model for filtration combustion of liquid monofuels is proposed. The model is used to analyze the filtration combustion of liquid hydrazine in narrow tubes. Two steady-state regimes are found. In regime I, the dominant mechanism of heat transfer from the combustion products in the preflame zone is heat conduction in the gas, and in regime II, this is interfacial convective heat transfer and heat conduction in the solid phase. Parameter ranges for the existence of the regimes are established. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 21–30, July–August, 2008.     

Spin gas combustion in a narrow slot

Combustion in a gap between two thin circular plates with point injection of the gas in the center is experimentally examined. The existence of several combustion regimes is demonstrated. Unstable pulsed combustion is observed in the case of high flow rates. As the flow rate decreases, a transition to spin combustion occurs under certain conditions. If the distance between the plates is sufficiently small, the so-called excess-enthalpy flame can be realized in an expanding gas flow. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 23–26, May–June, 2006.     

Regimes of gas combustion in a porous body of a cylindrical heat generator

A stationary model of filtration combustion of a gas with allowance for conditions at the entrance to the porous body and conditions of heat exchange with the gas phase surrounding the burner and with the heat exchanger is proposed and numerically analyzed. Ranges of parameters where the regime of gas combustion with a narrow reaction zone near the outer surface of the porous body are determined. Mechanical stresses arising in the porous body owing to high temperature gradients and gas pressure in the pores are estimated. The rate of gas combustion and the radiative heat flux from the burner surface are plotted as functions of process-dependent parameters. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 18–29, January–February, 2009.     

Filtration combustion of a gas in a semiinfinite porous medium

An asymptotic analysis of the combustion of a gas moving in a semiinfinite porous medium is performed for large values of the Zel’dovich parameter. The case of a highly porous medium with high gas permeability is considered. The basic terms of asymptotic expansions of the main parameters of the process for combustion and separation regimes are obtained by the method of joined asymptotic expansions. The effect of the rate of motion of the gas and heat transfer from the surface of the skeleton into the ambient medium on the combustion parameters is analyzed. The critical conditions of failure of stationary combustion at the external surface of the layer and the conditions of transition of the process to the separation and induction regimes are determined. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 4, pp. 3–14. July–August, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 98-01-03009).     

Adiabatic premixed combustion in a gaseous fuel porous inert media under high pressure and temperature: Novel flame stabilization technique

This work presents an experimental investigation to study the characteristics of combustion using a premixed methane-air mixture within a non-homogeneous porous inert medium (PIM) under high pressure and temperature. In order to obtain a stable flame under these operating conditions within PIM, a novel flame stabilization technique in porous inert media (PIM) combustion under high pressure and temperature has been developed and evaluated. The proposed technique avoids the draw backs of the hitherto developed techniques by properly matching the flow and flame speeds and, consequently, ensuring a stable combustion, for a wide range of operating pressure and temperature. The success of this technique permits the extension of PIM combustion to new applications such as gas turbines. The validity of this new technique has been assessed experimentally in detail by analyzing combustion inside a prototype burner. The effects of various operating conditions, such as initial preheating temperature and elevated pressure, have been examined for an output power range between 5 and 40 kW. The experiments covered a broad spectrum of operating conditions ranging from a mixture inlet temperature of 20 °C and pressure ratio of 1 up to a temperature of 400 °C and a pressure ratio of 9. Evaluation of the results revealed excellent flame stability with respect to both flashback and blow-out limits throughout all the operating conditions studied, including relative air ratios far beyond the normal lean limit. While the blow-out stability showed no significant dependence on pressure, it was strongly determined by the preheating mixture inlet temperature. A remarkable broadening of the stability range from 0.6 to 1.0 on preheating to 400 °C was observed. This reveals the potential of pre-heat temperature to improve the dynamic modularity of the burner.     

惰气存在下的热敏物料真空精馏

本文针对热敏物系精馏存在的问题,通过加入惰性气体以降低精馏温度,较全面地考察了惰性气体对精馏塔的影响,说明惰气真空精馏可以有效地解决热敏物质的精馏问题。     

Diffusion-thermal stability of gas filtration combustion waves in an inert porous medium

A mathematical model for the filtration gas combustion taking into account thermal conductivity, diffusion, and intense interfacial heat transfer is presented. The temperature dependence of the chemical reaction rate is approximated by a δ-function and the thermal-expansion coefficient of gases behind the combustion front is taken into account. Unsteady combustion regimes are analyzed using the method of small perturbations. The boundaries of the longitudinal and spatial stability for steady regimes of the filtration combustion wave are obtained. The dependence of the Lewis number on the thermal-expansion coefficient of the gas mixture along the boundary of stability is derived, along with other relations.     

Filtration combustion of a carbon-inert material system in the regime with superadiabatic heating

Superadiabatic regimes of combustion of carbon mixed with an inert solid with filtration of the steam-air mixture are studied theoretically and experimentally. The temperature in the combustion wave and the composition of gaseous products are obtained as functions of the fraction of carbon in the fuel and the amount of steam in the gaseous oxidant. In the examined range of the control parameters, the maximum temperature in the combustion wave is shown to depend only slightly on the fraction of carbon in the mixture and the amount of steam in the oxidant gas. Simulations of filtration combustion of carbon with allowance for the kinetics of its oxidation are in good agreement with experimental results. The calculated combustion temperature coincides with that measured in experiments. In calculating the composition of the gaseous products, coincidence with experimental data is observed only for particular compositions with the mass content of carbon under 60%. As the fraction of the fuel exceeds 60%, the yield of CO and H₂ increases in experiments, though such a behavior is not predicted by the theoretical analysis. Hypotheses on the reasons for the disagreement in results are put forward and experimentally checked. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 3, pp. 30–38, May–June, 2008.     

Filtration Combustion of an Energetic Material in a Cocurrent Flow of Its Combustion Products. Critical Combustion Conditions

The paper reports results of a numerical calculation using the threetemperature model developed previously by the authors. In addition to the temperature difference between the gas and the solid phase (skeleton), the model approximately takes into account the temperature distribution in skeleton elements. It is shown that quenching is possible (after burnout of part of the charge) with variation in a number of input parameters: the thermal conductivity of the charge, the ignition temperature, the weight of the igniter, the initial temperature, the porosity of the charge, and the local rate of its decomposition.     

Catalytic combustion in a domestic natural gas burner

Gastec and Vaillant developed a boiler in which the combustion is catalytically stabilised as well as a completely catalytic boiler. The catalytically stabilised boiler emitted about 5 ppm NO_x and 0 ppm CO. In this boiler the burner is replaced by a metal honeycomb. The honeycomb is partly coated with a catalyst washcoat. The coated part is at the flame side of the honeycomb. The coated length of the channels is an important parameter. A too long coating results in CO emissions, a too short coating in higher NO_x emissions.

The catalytic boiler emitted 0 ppm NO_x, 0 ppm CO and 0 ppm CH₄. The gas is combusted catalytically in two metal honeycombs. Most gas is converted in the first few millimetres from the entrance of the monolith. The heat that is produced is radiated to a heat exchanger. The remaining honeycomb and the secondary honeycomb convert the rest of the methane.

Comparing these boilers, the completely catalytic boiler shows lower emissions and a lower sensitivity to the gas quality. The partly catalytic burner is more reliable and can use a conventional security system. Production and development costs are thus smaller.     

Filtration combustion characteristics of low calorific gas in SiC foams

This study investigated the combustion characteristics of low calorific gas in silicon carbide (SiC) foam. The temperature distribution, reaction zone, maximum temperature, and combustion wave propagation velocity were analysed at different inlet velocities, equivalence ratios of premixed gases, and pore densities. The temperature distribution near the reaction zone was determined by a time-based method. Super-adiabatic combustion was obtained in porous media under different conditions. The experimental results showed that higher temperatures were obtained in SiC foams of 30 pores per inch (PPI) than those measured for foam of 20 PPI. Increased equivalence ratio of premixed gases and pore density led to a thicker reaction zone and a higher preheating efficiency in the preheating zone. The combustion wave propagation velocity, which was less than 2 mm/s under these experimental conditions, was increased with increased inlet velocity and decreased equivalence ratio of premixed gases. The combustion wave propagation in foams of 20 PPI had the lowest velocity because of the good match of convection and radiation.     

Characterization of the combustion of biomass producer gas in a constant volume combustion bomb

In this article a methodology is presented for studying the influence of the biomass moisture content and the biomass/air ratio on the producer gas composition and on the chemiluminescent emissions during the combustion process.Firstly, a mathematical model based on the thermo-chemical processes inside the gasifier is used to predict the composition of the producer gas as a mixture of CO, CO₂, H₂, N₂, CH₄ and H₂O gases. Secondly, the predicted composition is introduced into a constant volume combustion bomb (CVCB), which simulates the typical conditions inside the combustion chamber of an internal combustion engine, in order to characterize its combustion under gasoline-type conditions through the instantaneous pressure and the detection of the spontaneous luminous radiation (chemiluminescence) emitted by chemical radicals (OH* and CH*) excited by the front flame propagation. Thirdly, the pressure curve is introduced into a two-zone analysis model based on the conservation equations to determine the parameters useful for making a diagnosis of the combustion process, such as the temperature in each zone, the laminar burning velocity, the burned mass fraction and the rate of heat release during the combustion process.     

富氧燃烧中气体辐射模型对燃烧与换热数值模拟的影响

富氧燃烧过程中,由于使用再循环烟气代替空气中N₂作为稀释剂,烟气中存在大量CO₂和H₂O。CO₂和H₂O作为非极性三原子分子,具有N₂没有的辐射能力,导致富氧燃烧中气体辐射特性发生变化。在数值模拟过程中,气体辐射模型是一个重要的子模型。前人提出多种修改后适用于富氧燃烧的气体辐射模型,但不同气体辐射模型在不同富氧燃烧工况数值模拟中的影响尚未有统一研究。为了研究不同炉型下,气体燃烧和煤粉燃烧中气体辐射模型对燃烧换热模拟结果的影响,通过编程,将一种考虑CO影响的气体辐射模型以及文献中的6种典型气体辐射模型耦合入数值模拟计算。结果表明,在气体富氧燃烧中,气体辐射模型影响了火焰结构。同时,燃烧温度分布有所变化,不同模型结果之间差别最高可到500 K。气体与壁面之间的辐射换热受到影响。气体辐射模型对炉膛中心火焰区域影响较大,而对非火焰区域影响较小。在煤粉富氧燃烧过程中,当有效辐射层厚度在0.3 m左右时,如在100 kW下行炉中,气体辐射模型对煤粉燃烧数值模拟结果几乎没有影响。这可能是由于颗粒辐射在辐射换热计算中占主导地位。而当有效辐射层厚度在16 m左右时,如1000 MW塔式炉中,气体辐射模型对炉内切圆燃烧火焰温度以及组分浓度影响较大,温度差别可到100 K左右。而气体辐射模型对炉膛中心模拟结果没有影响。     

天然气催化燃烧在燃气轮机和锅炉中的应用

综述了天然气催化燃烧技术在燃气轮机和热水锅炉中的应用和研究现状及其发展方向,介绍了几种不同形式的催化燃烧器和一些典型的设计实例,指出催化燃烧技术仍存在的问题。     

天然气燃烧中NOx减排的数学模型

在燃烧过程氮氧化物生成的基元反应和反应动力学基础上,推导并建立了新的NOx生成化学动力学模型,并利用流体力学计算软件及湍流模型分析手段,对天然气燃烧过程NOx生成进行了模拟计算,预测结果与实验结果吻合良好。研究表明,新模型比平衡模型和部分平衡模型预测更加接近实验结果,实现了对NOx生成数学模型的优化。     

手动加样-惰气熔融-热导法测定氮化铬铁含氮量

研究了惰气熔融-热导法测定氮化铬铁含氮量的分析条件:采用手动加样方式,分析功率为5.0k W,称样量为30~60mg,加入0.5g纯铁做助熔剂,以0.5g钢铁标样校准仪器,可以得到满意的分析结果。对含氮量为5%~11%的氮化铬铁,本方法的相对标准偏差(RSD,n=5~10)不大于3%,NO-3中氮的回收率为94%~101%。最后,讨论了手动投样-惰气熔融-红外吸收/热导法联测氮化铬铁含氧/氮量的可行性。     

Flame stabilization in a narrow slot in an expanding gas flow

Conditions of flame stabilization in a narrow gap between two circular plates with gas injection through the center of one plate are considered. Lean hydrogen-air mixtures are used as a combustible gas. It is shown that the flame can become stabilized under certain conditions either at the tube exit, at the point of injection of the combustible mixture into the slot, or at a certain distance from the point of injection. Stable existence of such flames is observed even if the slot width is smaller than 0.1 mm. The burner walls are substantially heated in both cases. The basic trends of the flame behavior are found for the second case with variation of the flow rate of the combustible gas, composition of the mixture, and slot width. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 21–25, July–August, 2006.