Inert Porous Media Research Articles

Combustion of Liquid Fuels in a Porous Radiant Burner

Abstract The combustion of a liquid fuel within a highly porous inert medium (PIM) has been investigated numerically and experimentally A one-dimensional laminar flow model is used in this work. Combustion is modeled using a multistep reaction mechanism. Non-local thermal equilibrium between the gas, the liquid, and the solid phases is accounted for by considering separate energy equations for the three phases. Nongray solid radiation and droplet radiation absorption are included in the model. The equations for the liquid phase are formulated using a Lagrangian description and the equations for the gas and solid phases are formulated using an Eulerian description. A burner that allows effective control of the equivalence ratio and the burning rate is built to study the phenomena. Burning rates, temperature, and emissions are measured Numerical results are obtained for droplet size and temperature history as well as flame speed and temperature profile within the burner. Effects of various parameters on the...

The decomposition flame of hydrazine in inert porous media

The propagation of thermal waves of N 2H 4 decomposition in capillary-porous media of two types has been studied with filtrational supply of the reactant to the decomposition zone. Steady-state decomposition regions have been examined. Parameters affecting the burning velocity and thermal wave propagation velocity have been determined. Thermal wave structure has been found to involve an anomalously wide preheat zone, due to the effect of capillarity. In this zone hydrazine appears in both liquid and gaseous states, the liquid and gaseous mass flows being, in fact, non-one-dimensional. The decomposition process causes internal capillary filtration. A physical model of decomposition flame hydrazone in porous media is discussed. The decomposition flame laws and peculiarities have been shown to depend on the properties of both the monofuel and the porous medium. Filtrational flows induced by external and internal (capillary) forces, as well as thermal interaction between porous and reaction media, have been shown to play an important role in the mechanism of wave propagation.

The combustion of liquid fuels within a porous media radiant burner

The combustion of liquid fuels within the porous inert media (PIM) of a radiant burner was examined. Existing designs of porous radiant burners have typically relied on premixed mixtures of gaseous fuels and air. A radiant burner was built, and various design configurations were tested using the following types of porous ceramics: magnesia-stabilized zirconia, silicon carbide, and yttrium-stabilized zirconia. The fuel heptane was impinged on the combustion section using an oil spray nozzle with a fixed flow rate of approximately 0.025 lpm. The burner had an insulated combustion section that was 10.0 cm in diameter and consisted of several 2.5-cm-thick ceramics. Stable complete combustion was achieved for heptane at equivalence ratios of 0.57–0.67. Temperature measurements taken across the exit plane of the combustion section were typically within 50°C of each other, indicating radially uniform combustion. For an equivalence ratio of 0.64, axial temperature measurements taken down the side of the combustion section showed relatively low temperatures, 1170–1370°C. Very low emissions of both CO and NO x were measured for the range of stable equivalence ratios. Corrected for 3% oxygen, CO varied from 3 to 7 ppm and NO x varied from 15 to 20 ppm. Several variations of the burner were tested, including higher fuel flow rates, prevaporized fuel, and a quartz-enclosed ceramic section.

Standing waves of gaseous chemical reactions in inert porous media

Standing waves of gaseous, exothermic, chemical reactions in inert porous media are investigated in the two-temperature and one-temperature approximations. A “semiinfinite” reaction zone method is developed to solve the problem analytically. It is shown that the wave solutions obtained by this method form a large set. Traditional solutions with strong heat transfer at the end of the wave make up but a small fraction of this set.

A Numerical Investigation of Premixed Combustion Within Porous Inert Media

A numerical investigation of premixed combustion within a highly porous inert medium is reported. Specifically, results of a numerical model using detailed chemical kinetics and energy exchange between the flowing gas and the porous solid are presented. The current formulation differs from prior models of this type of combustion in that multistep kinetics is used and a better description of the thermophysical properties of the solid is applied in the present model. It was found that the preheating effect increases strongly with increasing convective heat transfer and with increasing effective thermal conductivity of the solid. The convective heat transfer is expected to increase with increasing number of cells per unit length of porous matrix but the absorption coefficient decreases with increasing cell size and decreasing cell density. Numerical simulations using baseline properties indicate that the lean limit can be extended to an equivalence ratio of about 0.36 for a methane–air flame and that the peak flame temperature is generally higher than the adiabatic flame temperature. The latter effect is predicted to be more pronounced at lower equivalence ratios.

The necessity of using detailed kinetics in models for premixed combustion within porous media

Models for premixed combustion within porous inert media (PIM) are complicated by the highly nonlinear radiative exchange terms in the energy equation for the solid matrix in addition to the stiffness of the set of gas phase equations. Therefore, prior researchers have simulated the gas-phase reactions using single-step chemistry. In the present work, predictions are made using both single-step and multistep kinetics mechanisms. It is concluded that it is essential to use multistep kinetics if accurate predictions of the temperature distributions, energy release rates, and total energy release are sought. Obviously, this is also true if predictions of the composition profiles and emissions are sought. Single-step kinetics is shown to be adequate for predicting all the flame characteristics except the emissions for the very lean conditions under which equilibrium favors the more complete combustion process dictated by global chemistry. The first predictions of NO and CO emissions from PIM burners are presented and compared with experimental data. The model predicts the CO emissions very accurately and predicts the NO trend correctly but overpredicts the NO emissions for φ > 0.8. The present multistep PIM burner model does not accurately reproduce the data for the burning speed and NO emissions for nondilute mixtures. These discrepancies can be only partially attributed to experimental uncertainties and/or imprecise knowledge of the properties of the solid matrix. Thus, it is concluded that important aspects of the physical processes within PIM combustors are not well simulated at present.

Theory of ice premelting in monosized powders

The gradual freezing of water in inert porous media is considered to be a consequence of interfacial and grain boundary melting combined with curvature-induced depression of the melting point. The unfrozen fraction is calculated for monosized powders of spherical particles. It is found that the unfrozen fraction contains separate terms associated with interfacial and curvature melting, with distinctive dependencies on temperature and particle size. The calculated fraction is in excellent agreement with measurements on pure water in graphitized carbon black and polystyrene micron-sized spheres. The introduction of dissolved impurities adds a third term, with an intermediate functional dependence.

Thermal explosion in an inert porous medium

We have investigated thermal explosion (autoignition) of a reaction mixture situated in an inert porous medium. We consider a two-temperature model in the adiabatic formulation. We isolate the following regimes for the process: explosive, explosive with delay, slow regime with a discontinuity between the temperature of the reaction mixture and the temperature of the inert medium, slow and fast regimes without such a discontinuity. We determine the limits of the regions of existence of these regimes.

Structure of gas-film and gas detonation in an inert porous medium

Structure of gas-film and gas detonation in an inert porous medium

An experimental and theoretical study of porous radiant burner performance

We have studied experimentally the stability and heat transfer characteristics of lean premixed, methane-air flames embedded in a porous layer. The work is directly relevant to understanding the performance and operating behavior of porous radiant burners (PRB). Flame speed and radiant output data were obtained for different stoichiometries and flame locations in porous ceramic foam. The results indicate that stable combustion at elevated flame speeds can be maintained in two different spatial domains: one spanning the upstream half of the porous region and the other in a narrow region near the exit plane. The heat release and radiant output are also found to increase as the flame is shifted toward the middle of the porous layer. A one-dimensional laminar premixed flame model incorporating a radiatively participating inert porous medium was used to describe the test conditions. Calculations using a one-step reaction confirmed the observation of two stable flame regions. The predicted flame speeds and radiant output agree favorably with the experimental trends.

The effect of volatilization on the mass flow of a non‐aqueous pollutant liquid mixture in an inert porous medium: experiments with kerosene

SUMMARYKerosene, an industrial petroleum product characterized by the presence of a large number of petroleum hydrocarbons (C9‐C15), was selected as an example of a non‐aqueous pollutant liquid (NAPL) mixture for our studies. Three inert materials (fine, medium and coarse sand) were used in the experiments and the initial amount of kerosene applied ranged from residual to saturated retention capacity.Volatilization in the air phase and saturated mass flow of kerosene in the three sands were studied in the laboratory under controlled conditions. The volatilization was the major physicochemical process affecting the fate of kerosene in the inert porous medium. During volatilization the liquid kerosene changed its composition by gradually losing its light components (C9‐C13), and the viscosity of the remaining liquid kerosene increased. The increase in viscosity led to a decrease in the infiltration rate, for example, by about 20% when the viscosity increased from 1.3 × 10−3 to 2.0 × 10−3 Pa s. The relationship between the composition of the residual kerosene following volatilization and its mass flow in a sand illustrates the behaviour of non‐aqueous pollutants in inert porous media.

Self-sustaining high-velocity gas burning regime in inert porous media with poured density

Self-sustaining high-velocity gas burning regime in inert porous media with poured density

Flame Stabilization and Multimode Heat Transfer in Inert Porous Media: A Numerical Study

Abstract This paper presents a numerical analysis of combustion and multimode heat transfer in inert porous media. The work is directly relevant to the understanding of premixed flame stabilization in porous radiant burners. The influence of the flame location, the radiative properties of the porous material, the solid thermal conductivity, and stoichiometry on the flame speed and stability are determined using a one-dimensional conduction, convection, radiation, and combustion model. The porous medium is allowed to emit, absorb, and scatter radiant energy, Non-local thermal equilibrium between the solid and gas is accounted for by introducing separate energy equations for the two phases. Heat release is described by a single-step, global reaction. The results indicate that stable combustion at elevated flame speeds can be maintained in two different spatial domains. Flame propagation near the edge of the porous layer is controlled mostly by solid-phase conduction; whereas, in the interior both solid cond...

Fundamental laws governing subsonic and detonating gas combustion in inert porous media

Fundamental laws governing subsonic and detonating gas combustion in inert porous media

Seepage gas combustion

Seepage gas combustion is defined as the propagation of a region of gaseous exothermic reaction in an inert porous medium, as the gaseous reactants seep into the region of chemical transformation. Such combustion is heterogeneous because of the active participation of two phases (a solid porous medium and a reacting gas) in the propagation of the combustion waves. The study of these waves revealed the existence of two subsonic steady-state combustion regimes, one with low propagation velocity and one with high propagation velocity. In this paper the theoretical study of seepage gas combustion is based on a system of one-dimensional equations describing the propagation of heat in the inert medium and the gas, the mass transfer of the deficient component, and the conservation of the total amount of matter, together with the equation of state of an ideal gas. The parameters of seepage combustion, including the stationary and velocity of the combustion wave, the wave structure, and the wave propagation mechanism and limits, were derived for both velocity regimes. Supersonic regimes in porous media were also analyzed.

Fast subsonic combustion of gases in an inert porous medium with a smooth rise in the pressure in the wave

Fast subsonic combustion of gases in an inert porous medium with a smooth rise in the pressure in the wave

Gas combustion in a vessel with a highly porous inert medium

The authors investigate the basic properties of the gas combustion process in a porous medium differing significantly (in porosity and heat capacity) from the medium of packed spheres. The porous medium used was thermally reticualted polyurethane foam. The mixtures were ignited by a single spark in the upper foam-free region of the tube. An illustration shows a typical oscillogram of flame propagation in a tube filled with polyurethane. It was found that the basic principles of gas combustion in highly porous media remain the same as in low-porosity media. A unique feature is the possibility of significant heating of the medium due to its relatively low heat capacity. As a consequence, system pressure increases and the direction of motion of gas flows relative to the combustion wave changes.

Characteristics of a stationary combustion wave in gas-free systems with varying thermophysical parameters

Thus, in this work we have investigated theoretically and experimentally the conditions for propagation of a wave of filtration combustion of gases in the presence of heat losses and we have demonstrated the existence of limits on the propagation of a combustion wave in the LVP regime. We obtained an equation that permits determining the critical parameters. We established that in contrast to combustion of homogeneous gases and condensed systems, the behavior of the velocity of the wave of filtration combustion of gases both at the limit and far from it is determined not only by the kinetic and thermophysical properties of the reacting phase, but also by the thermophysical properties of the inert porous medium, as well as by the permeability velocity of the gas. The theoretical and experimental results agree quantitatively, which indicates the validity of the assumptions made.

Filtration combustion of gases

1. The process of filtration combustion of a gas in an inert porous medium has been carried out experimentally and investigated. The possibility is shown for the propagation of combustion waves counter to the flow and in the direction of the flow, and for the “standing wave” state. The significant role in the process of such factors as the flow velocity, heat content of the mixture, and the size of the elements of the medium is established. 2. A two-temperature model of the filtration combustion of gases has been constructed and validated which describes the effect of the parameters on the characteristics of the process. The model predicts, in particular, the existence of a temperature peak in the zone of chemical reaction and of the existence of anomalously low and high combustion temperatures. The theoretical conclusions coincide satisfactorily with experiment. 3. The presence of two regimes of filtration combustion of gases is established: low velocities with a strong thermal interphase interaction, and high velocities with a weak thermal interaction. The possibility is shown for the existence of limits of combustion wave propagation, due to thermal losses. 4. An important element of the mechanism of filtration combustion of a gas in the regime of low velocities is the recuperation of heat in the cycle: combustion products-porous medium-fresh gas-combustion products. In this case the thermophysical characteristics of the medium play a decisive role in the structure and propagation of combustion waves.

Flame zone in gas combustion in an inert porous medium

The paper describes an experimental method for determining the propagation characteristics of a flame in a porous medium. The apparatus comprises a vertical tube filled with a medium of porosity 0.4. Photodiodes are provided for determination of the combustion zone boundaries, and photomultipliers are used to measure the extent of the combustion zone. The whole is filled with a stoichiometric methane air mixture and ignited with a single spark at the top end of the tube. Experiments are performed for mixtures at various initial pressures, and deductions are made concerning the simultaneous procession of the chemical transformations and combustion product cooling within the combustion zone. The flame propagation in a porous medium is determined to be turbulent in nature on the basis of observations concerning the relationship between the extent of the combustion zone and the thickness of a laminar flame front.