Two-layer Porous Media Research Articles

Experimental and Numerical Study on the Combustion Characteristics of Premixed Methane/Air in Porous Media Burner

ABSTRACT The porous media burner is considered as the promising alternative for residential water heaters. In this work, a two-layer porous media burner was designed and the maximum NOx emission was 48.7 mg/m3 under the firing rate of 666 kW/m2. A two-dimensional model was established; the effect of the pore parameters on the flame stability limit and NOx emission was investigated. The flashback, blow-off and flame instability were observed in the experimental and numerical investigations. Through the numerical optimization, the preferred pore parameters were 0.8 mm & 0.4 (preheating zone) and 1.6 mm & 0.7 (combustion zone); the predicted NOx emission was 32.9 mg/m3. The preferred porous media were produced and studied experimentally. Under the same condition, NOx emission was 28.7 mg/m3, which was less than 35 mg/m3 (the standard requirement, GB 18,111-2021) and reduced by 41% compared with that of the initial burner (48.7 mg/m3). The flash back limit was increased and the flame was stable at φ = 0.71.

Experimental investigation of the mutual influence of micro- and macroscale processes on the displacement of viscous oil by water in a two-layer porous medium

This paper presents the results of an experimental and theoretical study of the immissible displacement of high-viscosity model oil from the two-layer porous medium with different layers permeability. The experiments were performed using cylindrical model of the porous medium having the ratio of transverse size to length D/L equals to 0.05 and ratio of layers permeability equals to 0.46 during forced imbibition and drainage. It is shown that the alignment of a displacement front in the layers is caused by macroscale capillarity-driven flow arising from the curvature of the front. The conditions for stabilized motion of the displacement front for this case are determined using the analytical model and proved using experimental data on the recovery of displaced fluid. Microscale processes in pore level caused by the influence of the capillary forces are taken into account throw the relative phase permeability and capillary pressure curves.

Effect of the Cross-Section of a Porous Burner on the Combustion Stability Limit of Premixed Oxy-Methane Flames.

This study presents the idea of using a porous media burner to improve the oxy-methane combustion reaction rate and broaden the stability limit. Numerical studies on the premixed combustion of CH4/O2/CO2 in a two-layer porous medium burner using a two-dimensional symmetrical volume-average model with the skeleton mechanism based on OpenFOAM. The combustion characteristics of burners with variable cross-sectional (VC) and straight cylindrical (SC) structures were compared, including stable range, temperature field, thermal cycle efficiency, and CO emissions. It is confirmed that the cross-sectional structure is effective for broadening the stable range, and the broadening rate is more than 4 times. As more heat is lost from the outlet due to the increased cross-section, the solid-phase temperature of VC is lower than that of SC. As a result, the flame temperature of VC will be about 200 K lower than that of SC under the condition of insulated walls. It also leads to a reduction of about 2% in thermal cycle efficiency compared with SC. Meanwhile, it is shown that the VC structure slightly increases CO emissions at low thermal power but is smaller than that of SC at high power. When the wall heat loss of the burner is considered, the VC structure is still effective for improving the stable range. In addition, the VC structure slightly affects flame tilt and temperature uniformity at the burner outlet when the thermal power is large. It is confirmed that the VC structure is effective for increasing the power adjustment range and reducing pollutant emissions at high power.

Numerical Simulation of Methane Combustion in Two-Layer Porous Media Under Oxy-Fuel Condition

Numerical Simulation of Methane Combustion in Two-Layer Porous Media Under Oxy-Fuel Condition

Study on heat transfer in two-layer porous media with heat generation in porous media

A mathematical model involving the mechanism of heat transfer in two-layer porous materials with the heat generation between two porous materials was studied. Open-cellular material with porosity (ϕ) ranging from 0.70 to 0.8 and a porous thickness of 0.02 m was used. The porosity of the first layer was kept at 0.70 but the second layer was varied in the range of 0.73 to 0.79. The heat generation (q̇gen) was a heat flux of 100 to 10,000 W/m2. The air velocity (uf) of 0.4 m/s flowed through the porous material in the x-direction. The governing equation used in the calculation was a two-phase (gas or fluid and solid) energy conservative equation. The heat radiative equation in porous materials was determined by the P1 approximation method. In the model, the temperature profiles (the gas (Tf) and the solid (Ts) phases) and the local energy balance (LEB) of both phases were reported. From the studies, the profile trend of the gas (Tf) and the solid (Ts) temperatures depended on the heat generation (q̇gen). However, the inverse results were obtained by the effect of the porosity of the porous material (ϕ). In the local energy balance (LEB) of the gas phase (or fluid), the main effective terms were the convection (CVF) and the interaction (INT) with almost no thermal conduction (CDF). The interaction (INT) and the heat radiation (RAS) in the solid phase gave a significant effect and there was insignificant conduction (CDS). This was described by both the gas (fluid or air) and solid phases becoming low thermal conduction for the heat transfer mechanism in the case of heat generation between the two-layer porous media. In both phases at the same position, the total energy balance becomes zero according to thermodynamic law.

The traveling wavefront for foam flow in two-layer porous media

The traveling wavefront for foam flow in two-layer porous media

Three-dimensional pore-scale simulation of non-equilibrium in a two-layer porous burner

A pore-scale numerical simulation for combustion of methane-air mixtures in a two-layer porous media burner made of staggered arrangement of connected particles is preformed. Combustion is modeled by a one-step reaction mechanism, and solid surface radiation heat transfer is computed by discrete ordinates (DO) model. The bridge approach is used to deal with the point contact between adjacent particles. The present study is intended to investigate the non-equilibrium characteristics of gas velocity and temperature under two operating parameters such as the inlet velocity and equivalence ratio. A comparison of the numerical values and experimental values was carried out to prove the effectiveness of numerical model, and a good agreement is shown. The results showed considerable non-equilibrium of velocity and temperature along the space coordinate due to the inhomogeneous porous structure. The extent of flow non-equilibrium increases as inlet velocity and equivalence ratio are increased. The extent of gas temperature non-equilibrium decreases slightly with the inlet velocity, but the equivalence ratio effect on thermal non-equilibrium can be neglected.

Dispersion tensor in stratified porous media.

Dispersion in porous media is of great importance in many areas of science and engineering. While dispersion in porous media has been generally well discussed in the literature, little work has been done regarding a generalization of Taylor dispersion in stratified media. In this work, we generalized the Taylor dispersion theory and Stokes flow in porous media to derive a reduced-order model for tracer dispersion in stratified porous media. Our findings revealed that for a simple case of two-layer porous media, the hydrodynamic coupling between the two layers leads to the tensorial nature of dispersion and advection. The results showed that the obtained dispersion tensor and advection are not symmetric unless both porous layers have similar thickness, porosity, and molecular diffusion. We found that the main elements of the coefficient of the dispersion tensor remain positive while the off-diagonal elements can take negative values. On the contrary, all elements of the advection matrix may take negative values. On the basis of these observations, we report the manifestation of the dispersion barrier, uphill dispersion and advection, and osmotic dispersion during tracer transport in stratified porous media. In particular, the identified uphill advection reveals that the injected tracer in one layer could be transported countercurrent to the adjacent layer. Furthermore, we have shown that in the limiting case of Darcy flow, the Taylor dispersion is absent, and the tracer mixing between the two layers is restricted to the cross-diffusive flux between them. The results revealed that the field scale mixing may not necessarily originate from the Taylor dispersion and could be due to the modified advection terms and the cross-diffusive flux between the two layers.

Development of an Air/Fuel Premixer for a Gas-Fueled Two-Layer Porous Media Burner

ABSTRACT Air-fuel premixers are integrated parts of the premixed burner systems. The present paper describes the modeling, design optimization, and testing of an air-fuel premixer for a 5 kW natural gas porous media burner. The design was aimed at decreasing NOx and CO emissions by forming a homogeneous mixture of air and fuel. The design parameters were optimized employing CFD analysis coupled with a design of experiments (DOE) method. Then, a test model, based on the optimized design, was built and installed in a porous media burner testing apparatus. The test results have shown that the new premixer successfully reduced NOx emissions. It also improved flame stability by preventing flashback in several cases which revealed a new flashback mechanism in such burners. This improvement is achieved by preventing locally near-stoichiometric zones with high flame speeds from being formed. The effect of the new premixer on CO emissions was shown to depend on the firing rate and equivalence ratio. While reduced CO emissions are achieved at high firing rates and equivalence ratios, the level of CO emissions is increased for low firing rate and equivalence ratio magnitudes.

Nonlinear regimes of binary mixture convection in a two-layer porous medium of different configurations

The results of modeling the convection of a binary mixture of liquid hydrocarbons in a two-layer porous medium are presented. The simulation area is a horizontally elongated rectangular cavity divided into two layers. In one of the considered cases, these sublayers are of equal height, and in the other case, the interface between the layers has the shape of a convex downward circular arc, which imitates a synclinal geological fold. The layers have different permeability. In the cavity, there is a geothermal temperature gradient with an average temperature characteristic of a depth of 2000 m, which corresponds to the average oil depth. The composition of the mixture, the thermal conditions and the geometry used make up the reservoir model of the hydrocarbon deposit. The problem is solved in terms of the Darcy–Boussinesq model with allowance for the effect of thermal diffusion. The emergence and establishment of nonlinear convection regimes, as well as the distribution of the concentration of mixture components for different values of the permeability of the layers and their dependence on which of these layers is more permeable, are considered in simulations. It is found that in the case of a small difference in the permeabilities of the layers, a stationary regime is established in the cavity of any of the considered configurations. With a significant difference in permeability of the layers in the cavity, either quasi-periodic oscillations of a complex shape or irregular oscillations can be observed. A "local" occurrence of convection is shown for a large permeability difference, followed by a weak penetration of the flow into a less permeable layer, as well as a "large-scale" occurrence of convection in the case of an insignificant permeability difference.

Multi-scale flow patterns during immiscible displacement of oil by water in a layer-inhomogeneous porous media

This paper presents the results of numerical study of the relationship between micro-and macroscale flows during immiscible displacement in a two-layer porous medium. A feature of the proposed approach is the allowance for large-scale capillarity induced flow due to curvature of the displacement front in macro-inhomogeneous porous medium. The physical mechanisms determining the development of viscous instability in a layer-inhomogeneous porous medium are considered, the methods for suppressing viscous fingers formation based on the stabilization of the displacement front due the action of capillary forces are proposed.

Combustion performance of a two-layer porous inert medium burner at different low powers

Abstract- Improving the performance of modern combustion systems in many engineering application fields has great favorable impact on the energy conservation and global environmental pollution. This can be done through developing state of the art techniques for this purpose. One of them is the combustion within porous media burners. The two-layer porous inert medium burner (PIM) has significant merits such as high burning ratios, very low emission levels, high levels of the combustion stability, and operation at ultra-lean excess air ratios. The main work is to examine a new PIM design has a square cross-section area with two porous layers namely, aluminum oxide as the subcritical layer (quenching layer) and silicon carbide (SiC) as the supercritical layer each of 130 x 130 mm. The burner performance was estimated by recording the axial temperature profile at the selected operated thermal powers. The extent of the combustion stability limits between the flashback limit to the blow-off limit was improved. The excess air ratios of the stability limits at powers 0.7, 1.5, 2.5 kW were deduced to be (1.9-2.8), (1.8-2.6), and (1.5-2.4), respectively. The exhaust emissions were measured by a gas analyzer. The results revealed low levels of the CO emissions and decreased with increasing the operated thermal power. The CO ppm recorded the order of hundreds at powers 0.7 and 1.5 kW but reached the order of tens at power 2.5 kW. The NOx emission recorded ultra-low levels and increase with increasing the thermal power but not exceed 2 ppm.

Dual-phase flow in two-layer porous media: a radial composite Laplace domain approximation

The main purpose of this work is to present an interpretation method for injectivity test in a two-layer reservoir that can be extended to a multilayer approach, based on new analytical solutions to the well pressure response. The developed formulation uses a radially composite reservoir approach and considers that the water front propagation may be approximated by a piston-like flow displacement. The reservoir is assumed to be laterally infinite and properties such as permeability and porosity may be different in each layer. The solutions were developed in the Laplace domain and then inverted to real domain using the Stehfest Algorithm. The proposed formulation was then validated by comparison with a numerical flow simulator. Results showed a good agreement between the numerical simulator and the analytical model. Also, a sensitivity study was done by comparing the results of different scenarios varying oil viscosities and injection flow rate to assess how these properties affect the pressure and pressure derivative profiles.

Experimental investigation of flame stability in the premixed propane-air combustion in two-section porous media burner

The characteristics of combustion and the flame stabilization within a two-layer porous media burner were investigated experimentally in the present study. Lean propane-air mixtures were used with the equivalence ratio values ϕ in the range between 0.1 and 0.5 and the flow rate Q of the mixture in the range between 10 and 45 l/min. It was experimented with three porous ceramics: reticulated foam alumina, honeycomb foam alumina and SiC foam. The axial temperature distribution, reaction zone, maximum temperature, CO and NOx emissions, and combustion wave propagation rate were analyzed. The experimental results showed that, in the three ceramics studied, the combustion wave propagation rate increases with increased inlet velocity and decreased equivalence ratio of premixed gases. Result of the analysis of the information obtained, a porous burner with flame stabilized between two porous ceramics was built that works permanently with the equivalence ratio ϕ = 0.5 and the flow rate of the mixture in the range between 20 and 35 l/min. The honeycomb foam was placed on top of the burner and the reticulated foam on the bottom. The combustion temperatures were in the range between 1160 and 1380 K, where a higher flow rate resulted in higher temperatures. The CO and NOX emissions did not exceed a few ppm showing that combustion was almost complete.

Nonlinear convection regimes of a ternary mixture in a two-layer porous medium

Numerical experiments have been performed to study the onset and nonlinear regimes of convection of a ternary mixture of methane (35%), ethane (35%), and butane (30%) in a horizontally elongated rectangular region of a porous medium under the action of a geothermal gradient. The region has rigid, impermeable boundaries and is divided into two horizontal layers with equal porosity but different permeability and having the heights in ratio 1:3. The values of medium porosity and permeability are chosen close to those of real media, such as sands, sandstones or limestones. The components of the mixture belong to the main groups of chemical compounds present in oil and gas fields. The configuration under study is a model of hydrocarbon field. The calculations are carried out for the cases when the value of permeability of the upper layer is higher than that of the lower one and, conversely, when the lower layer is more permeable than the upper one. The rest of the parameters of the porous medium are considered to be the same in the entire computational domain. The problem is solved within the framework of the Darcy-Boussinesq model taking into account the effect of thermal diffusion. The temporal evolution of the local characteristics of the flow, the structure of the forming flow, and the distribution of the mixture components is evaluated. Analysis of the data shows that in the case of a more permeable narrow layer, the onset of convection has a local character. The flow arises in a more permeable layer and, as soon as convection develops, it begins to penetrate into a less permeable layer. However, the centers of the forming vortices are noticeably shifted towards the more permeable layer. Similar vortex displacements are observed in a thick layer with higher permeability, yet convection, in this case, is of the "large-scale" character.

Study of Polymer Flooding Behavior in Heterogeneous Two-Layered Porous Media

In this paper, a numerical study was conducted to investigate the effect of spatial heterogeneity of multiple porosity fields on oil recovery, residual oil saturation, polymer retained, and polymer adsorption. The generated porosity fields were applied to UTCHEM for simulating polymer and water flooding in heterogeneous two-layered porous media. From the analysis, the increase of reservoir heterogeneity resulted in higher polymer retention and lower polymer adsorption. In general, polymer flooding results in more balance residual oil saturation in the upper and lower layer than water flooding. This indicated that the vertical sweep efficiency of polymer flooding was better than water flooding. Residual oil saturation ratio between layers after water or polymer flooding was about equal along with the increase of reservoir heterogeneity. Spatial heterogeneity of multiple porosity fields had only a small effect on recovery factor. The variation of the recovery factor of polymer and water flooding due to the reservoir heterogeneity was under 1%.

The combustion characteristics and performance evaluation of DME (dimethyl ether) as an alternative fuel in a two-section porous burner for domestic cooking application

Towards enhancing the thermal performance and fuel flexibility of existing domestic cooking stoves, the present work employs the heat recirculation mechanism of the porous medium (PM) combustion to these burners offering greater fuel compatibility for both liquefied petroleum gas (LPG) and renewable fuel dimethyl ether (DME). To establish the advantages of DME combustion than that of LPG within the stove, experimental measurements and numerical modeling are performed in a two-layer PM burner. The numerical model is used to investigate the dynamics of DME flame in the PM through reaction path analyses. Both experiment measurements and numerical predictions show lower CO emissions for DME flame than that of LPG flame inside the PM stove. With the use of DME instead of LPG, following the guideline of World Health Organization, the maximum allowable equivalence ratio can be extended from 0.4 to 0.5 and the thermal load from 4.0 kW to 5.0 kW. Moreover, the total heat generation rate, the gas- and solid-phase temperatures and radiant efficiencies of the burner with DME flame are higher than that with LPG flame at the same input conditions. However, the stability ranges of DME flame are found to be less than that of LPG flame.

Methane partial oxidation in a two-layer porous media burner with Al2O3 pellets of different diameters

In this study, the fuel-rich combustion of methane in a two-layer porous media burner with Al2O3 pellets of different diameters was experimentally investigated. The upstream layer consisted of 2.5-mm diameter packed Al2O3 pellets, and the downstream layer consisted of 5-mm, 6.5-mm, 7.5-mm, and 9.5-mm diameter packed Al2O3 pellets. The effects of pellet diameter on the temperature distribution, exhaust composition, and the syngas energy conversion efficiencies were studied at a fixed operation condition with an equivalence ratio of 1.6 and a gas velocity of 0.13 m/s. An optimal downstream pellet diameter of 7.5 mm was determined for the partial oxidation of methane corresponding to the highest syngas energy conversion efficiency. Stabilized methane fuel-rich flames were realized in the optimized burner for various gas velocities (0.09 m/s–0.15 m/s) and equivalence ratios (1.2–1.7). The effects of operation conditions (gas velocities and equivalence ratios) on the combustion performance were also tested. We found that 50.0% of the methane was converted to H2 and CO at an equivalence ratio of 1.7 and an inlet gas velocity of 0.15 m/s with burner energy conversion efficiency based on lower heating values.

Highly Thermal-Integrated Flame Fuel Cell Module with High Temperature Heatpipe

Flame fuel cell(FFC) is a novel kind of fuel cell coupling rich-flame and solid oxide fuel cell(SOFC) together. With rich-flame as both heater and reformer, FFC takes advantages of quick start-up, no sealing and low cost. However, for the temperature distribution is determined by flame, temperature nonuniformity is a significant problem, especially along the axis direction of the tubular SOFC or within SOFC stacks. Due to great temperature gradient and insufficient thermal shock resistance, micro cracks occur inside the cell, resulting in a low open circuit voltage and failure of FFC. Relatively low temperature in partial regions of SOFC also leads to a low power density. In this paper, a high temperature heatpipe was introduced to a flame fuel cell module to lower the temperature gradient and improve the performance of the whole SOFC. A highly thermal-integrated FFC module with a two-layer porous media burner, micro-tubular SOFCs and an annular high temperature heatpipe was designed and tested. The temperature difference along the axial of the tubular SOFC decreased from 34°C/10 mm to 12°C/10 mm due to the high heat conductivity of the heatpipe. By integrating with a heatpipe, the power output for a single fuel cell increased from 0.88 W to 1.54 W at 0.7 V with a methane-rich flame at an equivalence ratio of 1.6. The FFC module with four cells showed considerable performance and had the potential for fabrication of FFC stacks.

Flame stability analysis of the premixed methane-air combustion in a two-layer porous media burner by numerical simulation

The flame stabilization within a two-layer porous burner was investigated numerically in the present study. Two-dimensional model was implemented to simulate the premixed methane-air combustion process. The governing equations including the radiative transport equation and separate energy equations for the solid and gas phases were solved by using the finite volume method. In order to validate the utilized model, the gas and solid temperature profiles within the burner were compared to the experimental temperature distribution and good agreement was observed. The results proved that the stability limit and flame temperature within a porous burner can be controlled by the equivalence ratio of the incoming mixture. Heat recirculation study showed that high convective heat recirculation occurred at low equivalence ratios caused higher stable inlet velocities. The radiative recirculation efficiency was almost constant over the stability limit at constant equivalence ratio while this parameter decreased with increasing equivalence ratio. Also, the effect of outlet diameter of the burner on the flame stabilization suggested that the optimum outlet diameter eventuating the highest flame stability limit was double the amount of inlet diameter. The study of the effect of the length of first porous layer on the flame status showed that the optimum ratio of preheat zone to combustion zone length was 0.33 to reach the highest burner operating range.