Filtration Combustion Of Gases Research Articles

Some Stability Aspects of Gas Combustion in Porous Media

Some aspects of the stability of hot-spot and oblique gas-combustion fronts in porous media are considered using a thermal model. A general expression for the velocity of a curved wave front of filtration gas combustion is obtained taking into account the curvature and local slope of the front. It is shown that curvature always promotes stabilization of the front. The slope of the front, in principle, can have a destabilizing effect. However, because this effect is weaker than the curvature effect, it does not cause development of instability. The roles of convective heat and mass transfer between the hot spot and the ambient gas flow and selective diffusion in the development of local instability are analyzed. Criteria are found for the development of hot-spot instability, and the ranges of system parameters in which instability is possible are determined. The effect of variation in the length of the front during wave propagation on the development of instability of an oblique front is considered. Accounting for this factor gives a criterion for instability development that coincides with the experimental one.

Transition Processes in Filtration Gas Combustion

The behavior of gas filtration combustion waves in the low‐velocity regime has been studied experimentally with variation in the parameters of the gas flow and porous medium. It is shown that in transition processes there may be quenching or formation of a stable combustion‐wave structure that does not correspond to the initial or final conditions. A nontrivial type of transition process accompanied by spatial transfer of the combustion zone is found.

Filtration Combustion in Hydrocarbon Desorption from a Porous Medium

We have investigated theoretically and experimentally the process of filtration combustion with hydrocarbon desorption from a porous skeleton realized, in particular, in heat cleaning of porous media out of residues of organic impurities. The problem on the wave of such a combustion in a system with three phases — gaseous, liquid, and inert solid — has been solved analytically. The specific features of this process as compared to the traditional process of filtration combustion of gases have been revealed. The dependences of the basic characteristics of the waves on the content of methane in the gas mixture as well as of oil and water in a porous medium have been obtained. A comparison with the experimental data has been made. Recommendations for optimizing the operation of decontamination plants on the basis of the process considered are given.

Analysis of Nitrogen-Oxide Formation Mechanisms in Filtration Combustion of Methane–Air Mixtures

The kinetics of nitrogen‐oxide formation under conditions of filtration combustion of lean and rich methane–air mixtures is analyzed. Intense interphase heat exchange in the wave of filtration combustion of gases leads to a drastic decrease in the gas residence time in the high‐temperature region and also to a decrease in the maximum temperatures in the reaction zone. As a result, the processes of filtration combustion of gases are characterized by reduced emission index of nitrogen oxides as compared to diffusion flames. Under these conditions, the thermal mechanism does not make any significant contribution to NO formation, and the dominating channels are the NNH channels for lean mixtures and the Fenimore mechanism for rich mixtures. Nitrogen oxides in lean mixtures are mainly represented by NO and N2O, whereas nitrogen‐containing components HCN and NH3 are typical of rich mixtures. NO decomposition in rich mixtures occurs in “afterburning” reactions with participation of HCCO, CH3, and CH2 radicals.

Effect of Kinetic Properties of a Mixture on Wave Macrocharacteristics of Filtration Combustion of Gases

On the basis of the latest experimental data using a detailed kinetic model, the influence of kinetic features of ultrarich methane‐air mixtures (in comparison with ultralean ones) on the main characteristics of superadiabatic waves of filtration combustion of gases is considered. It is shown that substantially lower concentrations of O, OH, and H radicals are typical of ultrarich mixtures, which results from effective inhibition of atomic hydrogen participating in the chain branching reaction H + O2 = OH + O by methane in the reaction H + CH4 = CH3 + H2. Therefore, extension of the preheating zone and noticeable expansion of the heat‐release region are typical of rich compositions. A decrease in generation of the main radicals in ultrarich mixtures (as compared to ultralean compositions) leads to an increase in the maximum skeleton temperature by ≈ 300– 350 K and to a significant increase in velocity of wave‐front propagation.

Mechanism of Upper Temperature Limits in a Wave of Filtration Combustion of Gases

A physical mechanism is established, responsible for the experimentally observed strong deceleration of the growth rate of the maximum skeleton temperature in a wave of filtration combustion of gases with increasing flow rate. The maximum temperatures of the gas and skeleton become commensurable, and the length of the thermal‐relaxation zone becomes much shorter. A classification of regimes based on the temperature‐heterogeneity criterion ϕ1 is proposed. Explicit analytical solutions are obtained for the wave for ϕ1≪1 and ϕ1→1. A correction to reverse reactions in combustion products is considered. The effect of composition on wave behavior is studied by means of numerical calculations with a detailed kinetic scheme. The activation energy for ultrarich and ultralean methane–air mixtures is evaluated. It is concluded that the limiting efficiency of the heat‐recuperation cycle in the wave is reached as ϕ1→1; methods for maximizing the efficiency are suggested.

Kinetic Analysis of the Chemical Structure of Waves of Filtration Combustion of Gases in Fuel-Rich Compositions

The structure of heat release and the dynamics of formation of radicals and methane oxidation in a wave of filtration combustion of gases in fuel-rich methane–air compositions is studied with the use of skeleton diagrams and sensitivity analysis. Depending on heat release, the wave is divided into a preheating zone, an exothermic zone characterized by partial oxidation of methane in the reaction CH4 + 0.5O2 = CO + 2H2, and an endothermic zone with the conversion processes CO + H2O = CO2 + H2 and CH4 + H2O = CO + 3H2. It is shown that the composition of products in the wave front is essentially nonequilibrium. Several typical regions are also identified from the viewpoint of prevailing reactions of formation of the basic radicals in the wave. Thus, the dominating mechanism of chain branching is the reaction CH3 + O2 = CH3O + O in the “low-temperature” region, H2O2(+M) = 2OH(+M) and HO2 + CH3 = CH3O + OH in the “transitional” region, and H + O2 = O + OH in the “high-temperature” region. Two former regions correspond to the preheating zone and the latter region corresponds to the exothermic peak of the wave of filtration combustion of gases.

Kinetic Analysis of the Chemical Structure of Filtration-Combustion Waves in Fuel-Lean Gases

On the basis of skeleton diagrams and sensitivity analysis, the structure of the heat-release region, special features of formation of radicals, and methane-oxidation dynamics under conditions of filtration combustion of an ultralean methane–air mixture are considered in detail. It is shown that the exothermal region consists of two consecutive peaks, the first one due to oxidation of hydrocarbons and hydrogen by OH radicals and the other due to the CO oxidation reaction. Several specific regions are identified in the wave, depending on the prevailing reactions of branching of radical chains. The main chain mechanism of formation of radicals is the reaction CH3 + O2 = CH3O + O in the region of “low” temperatures, the reaction HO2 + CH3 = CH3O + OH in the “transitional” region, and the reaction H +O2 = O + OH in the region of “high” temperatures. Two former regions correspond to the preheating zone, and the latter corresponds to the reaction zone of the wave of filtration combustion of gases.

Models of Filtration Combustion of Gases with Allowance for Flame Turbulence

The basic system of equations describing the combustion process inside a porous body as well as in the wave regime of filtration combustion of gases is refined on the basis of the Damkohler theory for turbulent burning rate. Better agreement of the models and available experimental data on the mass burning rate in such systems is obtained. The correction for turbulence of the interstitial flow becomes significant for filtration rates higher than 0.5 m/sec. The data analyzed are presented in the form of Borghi's diagrams. It is found that a weakly turbulent regime of a “folded” flame is typical of filtration combustion in gases in porous media with a mean pour size of ≈ 2–3 mm. Key words: turbulence, thermal wave, turbulent combustion regime, filtration wave, superadiabatic effect.

Experimental and analytical investigation of the gas filtration combustion inclination instability

Investigation on state of the art of thermal-hydrodynamic instability of gas filtration combustion is presented. Development of the model of flow competition which takes into consideration thermal and filtration reorganization of the system as reaction to the front perturbation is proposed. Existence of characteristic and critical front inclination is demonstrated experimentally. It is shown that in the condition of experiments, characteristic inclination can be estimated by the system length and wave dimensionless velocity ΔX ch∼ L· u. The inclination amplitude growth velocity is found to be proportional to wave front velocity Δ X ̇ ≈0.4·u w .

Parallels between the Regimes of Turbulent and Filtration Combustion of Gases in Inert Porous Media

The parallels between the turbulent and the filtration combustion of a gas in an inert medium have been considered. It is suggested that in the case where the thermal interaction between the gas and the porous medium is absent, elementary processes of mixing, chemical reaction, and others, which occur in turbulent and filtration combustion, can be characterized by similar dimensionless parameters setting up a correspondence between the integral scale of turbulence and the size of the pores. The dimensionless parameters have been analyzed and the diagram of the Borghi regimes of filtration combustion has been constructed. A number of typical cases of filtration combustion have been characterized using this diagram. It is shown that the considered cases of filtration combustion concern the range of distorted flames and distributed reaction zones according to the Borghi classification. From this it was assumed that the surface of filtration combustion in the first of the considered regimes is continuous in character. The formulas for estimating the rate of gas‐phase combustion under filtration conditions have been recommended.

Flame Propagation in a Variable–Section Channel with Gas Filtration

A one–dimensional unsteady model is proposed, which describes gas–flame propagation in a narrow variable–section channel with a gas counterflow and takes into account heat propagation over the channel walls. The case of the channel cross section changing slowly at a distance of the order of the thermal thickness of the combustion wave is considered. It is shown that various regimes of flame propagation are possible in such a system: a regime of flame propagation with a high velocity (of the order of the burning velocity of the flame), a regime of flame propagation with a low velocity as in the case of filtrational gas combustion in a porous medium, and an intermittent regime of combustion, where the flame has a high velocity in the wide section of the channel and a low velocity in the narrow section. A simple analytical model of flame oscillations in such a system is constructed. The possibility of these oscillations was predicted by numerical simulation results. The simple model considered is an attempt to take into account the large–scale inhomogeneity of the porous medium in simulation of filtrational combustion of gases.

Analytical and experimental investigation of the transition from low-velocity to high-velocity regime of filtration combustion

Abstract Combustion of gases under filtration in porous media, or filtration combustion (FC), is characterized with complex heat and mass transfer and interphase interaction and may be utilized in multiple thermo-chemical applications. The work studies filtration combustion of gases at transition from low-velocity (LV) to high-velocity (HV) regime under gas pressure growth. As shown, the transition regime of FC is characteristic for conditions of technological and experimental set-ups, at least at body pore size of d 0 >1.5 to 2 mm. When increasing working pressure by 1–2 atmospheres higher than atmospheric one, critical size of pores decreases 2–2.5 times. Theory and estimation method for velocity and temperature of stationary FC waves under the transition regime is proposed, where dimensionless complex Φ≡βPe S u 2 /Re 1/6 , (β=RT ad /E, Pe S u =S u d 0 /κ,Re=u g d 0 /ν , S u – normal laminar flame velocity, u g – gas filtration velocity) constructed on turbulent combustion to thermal relaxation rate, is used as governing parameter. It is shown that satisfactory description of transition regime as well as of high-velocity FC cannot be obtained by using characteristics of normal laminar flames. Hence, theory of turbulent premixed flames is to be used.

Theory of thermohydrodynamic instability of the front of filtration combustion of gases

Based on the filtration equations with account of heat transfer in the heating region, the problem of instability of small perturbations of the wave front of filtration combustion of a gas is solved. Expressions for the growth decrement and critical transverse size of a perturbation are obtained. The problem of the wave-front stability is solved with account of the macroscopic redistribution of filtration in the system. The solution includes macroscopic characteristics such as the transverse size, the width of the high-temperature zone, etc. A method is proposed for taking into account the influence of the characteristics of the system on the evolution of perturbations. It is shown that small deformation perturbations of the front always increase to a certain amplitude; their subsequent development— continuation of their growth or stabilization—is determined by the characteristics of the system.

Transition from the regime of low rates of filtration combustion of gases to the regime of high rates with a pressure increase

Regularities of the transition from the regime of low rates (RLR) of filtration combustion (FC) of gases to the regime of high rates (RHR) with a pressure increase are investigated. It is shown that allowance for gas-phase transfer is important for description of FC, at least when the size of the skeleton pores d>1.5–2.0 mm and the calorific power of the fuel exceeds the limit of propagation of the flame. With an increase of 1–2 atm in the working pressure as compared to atmospheric pressure the critical pore size decreases by a factor of 2.0–2.5. A theoretical model that makes it possible to evaluate the velocities and temperatures of FC stationary waves under conditions of the transition from RLR to RHR and to establish realization limits for these regimes is proposed. It is shown that a more adequate description of the transient regime and the regime of high rates is achieved when the turbulent character of the combustion is allowed for.

Propagation of spherical gas filtration combustion waves in inert porous media

The behavior of spherical combustion waves of gases in inert porous media is described in terms of one- and two-temperature models. Parametric dependences are obtained for the wave velocity and acceleration and for the temperature of the gas and porous medium in the wave. It is shown that in a diverging spherical gas flow, combustion waves initiated at different radii of the sphere converge to the standing wave coordinater*, and in a converging flow, on the other hand, they diverge from it. The experimentally observed propagation behavior of spherical combustion waves is well described by the proposed models.

Momentum loss as a mechanism for deflagration-to-detonation transition

A reduced model for premixed gas filtration combustion where the nonlinear effects are discarded everywhere but in the reaction rate term and where the only accounted for effect of the porous medium is its resistance to the gas flow is explored. While ruling out formation of shock waves the model appears rich enough to cover detonation-like phenomenon with barodiffusion acting as a driving agency. It is shown that depending on the initial conditions this creeping detonation mode is evoked either immediately or emerges after some time delay as a product of an abrupt transition from the low-velocity deflagration. The transition is triggered by a localized thermal explosion in the extended (friction-induced) preheat zone gradually formed ahead of the advancing deflagration.

Combustion wave instability in the filtration combustion of gases

Theoretical and experimental approaches to the problem of combustion wave stability in gas filtration combustion are described. An approximate criterion of instability for an initially plane combustion wave front is suggested within the framework of a hydrodynamic model. It is shown that the wave instabiliry cannot be observed if the reactor diameter is under a critical value. The critical value of the diameter increases with increasing rate of gas filtration. A combustion wave propagating in the direction opposing the filtration flow is more stable than a cocurrent wave, which is consistant with experimental results.

Theory of filtrational combustion of gases

1. A two-temperature adiabatic model of filtrational gas combustion has been investigated; the model takes account of molecular transfer in the gas phase and the finite width of the chemical reaction zone, thereby permitting a general analysis of the role of thermal interphase interaction, including the limiting cases. Using a modified extrapolational model, approximate relations have been obtained for determining the basic characteristics of the process. 2. The possibility that two cases of combustion — LVC and HVC — may be realized has been demonstrated. The parameter ranges corresponding to these two cases have been determined. The nonuniqueness region has been identified. 3. The possibility and conditions of realization of three types of flame propagation have been shown: combustion-wave motion in the direction of the combustion products, in the direction of the fresh gas, and in the direction opposite to that of the fresh gas, in particular, with zero filtration velocity. 4. The possibility of slowing of the reaction on account of internal thermal interactions leading to incomplete burnup in the region of maximum gas temperatures has been shown.

Effect of heat losses on propagation of stationary waves in filtration combustion of gases

Effect of heat losses on propagation of stationary waves in filtration combustion of gases