Oxy-methane combustion characteristics in a vertical porous plate reactor

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Introduction
Environmental protection awareness has demanded a global shift from fossil fuels to renewable energy systems (Vanegas Cantarero, 2020).Additionally, increasing renewable share in the energy mix is necessary to ensure a sustainable future (Qadir et al., 2021).However, the current technologies of renewable energy systems suffer from concerns of high cost, limited scale power generation, and intermittent supply (Nemitallah et al., 2020a).It seems unlikely that renewable energy sources will effectively replace traditional power plants (Tahir et al., 2021).
In fact, conventional power plants like gas turbines allow the integration of renewables by compensating for their intermittent power generation (Power Engineering International, 2019).Recent International Energy Agency (IEA) statistics on power generation capacities by fuel type suggest that the installed capacity for gas power plants will increase by 50% in 2040 (IEA, 2019).As a result, it is now more crucial than ever to use carbon capture technology to reduce emissions from conventional energy systems (Imteyaz et al., 2021).
The CO 2 produced in a conventional combustion exhaust stream is separated via carbon capture (Rubin et al., 2012).It is then pressurized for long-term storage or industrial use.The separation can be done before combustion (pre-combustion capture), which involves gasifying the fuel to syngas and separating CO 2 through a shift conversion reaction.It can also be carried out after combustion (called post-combustion capture) by using a solvent to absorb CO 2 and separate the remaining combustion products.These technologies have higher capital and energy expenditures that affects plant feasibility (Imteyaz and Tahir, 2019).Compared to these technologies, oxy-fuel combustion is more competitive in terms of efficiency (Ghoniem et al., 2019) and cost (Nemitallah et al., 2019).Moreover, it can also be implemented in existing infrastructure through some modifications (Rubin et al., 2012).
Oxy-fuel combustion refers to fuel oxidation with pure O 2 as the oxidizer in the absence of N 2 .Thus, the exhaust gas is free from NO x , and only CO 2 and H 2 O are produced in complete combustion.From the exhaust stream, the CO 2 can be separated for carbon capture and storage or sequestration (CCS).
In the case of oxy-combustion, the combustion properties, such as flame speed and adiabatic flame temperature, are significantly higher (Tahir et al., 2019).Consequently, dilution of the O 2 stream is necessary to control the combustion temperature because of the limitations of the existing combustor materials (Ali et al., 2020).A carrier gas, for instance, CO 2 , is usually mixed with the fuel as diluent.This dilution affects the combustion characteristics, so oxy-combustion is an active field of research.For example, dilution by CO 2 affects the stability of oxy-methane flames (Khalil and Gupta, 2017).
The separation of O 2 from the air for feeding into an oxycombustion system can be done in different ways (Nemitallah et al., 2020b).Cryogenic distillation depends on cooling the air to ultra-low temperatures to separate O 2 ; up to 99% purity can be achieved.However, it is accompanied by an energy penalty of 3%-4% of the rated generation (Shah et al., 2013).Additionally, it requires significant capital investment, which is the driving force for the development of alternatives like Ion Transport Membrane (ITM) Separation (Schulze-Küppers et al., 2019).ITM is a polymeric or ceramic membrane with air fed on one side and pure O 2 produced on the other.Selective transport of O 2 occurs through a combination of bulk O 2 convection, molecular diffusion, and transport of O 2 ions.O 2 molecules are adsorbed on the surface, dissociated into O 2 ions, and transported to the other side by filling O 2 vacancies in the membrane material.ITMs are able to produce a high-purity O 2 stream (up to 100%) and are being explored for integration in an oxy-fuel reactor (Nemitallah et al., 2020b).Air is fed around a central combustion zone made of ceramic ITM walls.The fuel is fed in the central zone, and the permeation of O 2 across the ceramic ITM results in oxycombustion.Such arrangements are termed Oxygen Transport Membrane Reactors (OTMRs).The rate of O 2 permeability through the membrane determines the fuel conversion rate (Kirchen et al., 2013).Hong et al. (2013aHong et al. ( , 2012) ) numerically investigated combustion in a methane OTMR and concluded that the O 2 permeation flux increases with membrane temperature, feed inlet flow rate, and feed concentration of O 2 .These results were later validated by Hong et al. (2013b) by performing experiments.Gromada et al. (2010) experimentally studied different ceramic materials for ITM and found the membrane thickness and surface characteristics to play a vital role.Different researchers have investigated the application of OTMR to full-scale power units.For instance, Mansir et al. (2018a) studied the integration of OTMR in a fire tube boiler, and Mancini and Mitsos (2011) explored ITM for a large-scale power plant application.García-Luna et al. (2022) analyzed various oxygen production systems for the oxycombustion application.Their analysis showed that the required energy could be lowered to 100 kWh/ton of CO 2 using OTMR, which is better than the other available systems.Portillo et al. (2021) compared different oxy-combustion cycles with OTMR and conventional air separation units (ASU).They found that the cycles with OTMR have better thermal performance than with the exiting ASUs.Furthermore, the best case shows only a drop of 2.3% efficiency as compared to the base case.
The current ITMs suffer from the low O 2 permeation flux required for practical fuel conversion rates (Nemitallah et al., 2020b).Further research for improving the ITMs' performance is underway.Meanwhile, porous plates are being used to mimic the combustion characteristics of ITMs.They have high permeation rates and offer to investigate the oxy-combustion in OTMR-like reactors practically.Thus, the design of the OTMRs with respect to combustion can be optimized.Additionally, porous plates enable the uniform entrance of oxygen into the reaction area, preventing any hotspots inside the reactor (Habib et al., 2019).Habib et al. (2015) numerically investigated an oxy-methane porous plate reactor in which CO 2 was used as a diluent.The influence of equivalence ratio and CO 2 percentage in CO 2 /O 2 oxidizer on the temperature and species concentration was evaluated.Mansir et al. (2018b) employed particle image velocimetry (PIV) in a rectangular, atmospheric pressure OTMR for cold-flow analysis.In another study of a face-to-face porous plate reactor, flame shapes acquired through visible flame imaging were compared with temperature profiles obtained from computational fluid dynamics (CFD) for a range of CO 2 /O 2 ratios, and good agreement was achieved (Nemitallah et al., 2019).Tahir et al. (2019) numerically investigated a similar rectangular porous plate reactor and compared oxy-methane combustion with conventional air-methane combustion.The effects of reactor geometry and oxidizer ratio on the flame properties were studied.They also suggested the flow conditions for equivalent oxy and air combustion inside the porous plate reactor

Purpose of this study
The face-to-face horizontal porous plate reactor designs were the primary focus of all the aforementioned investigations.A vertical reactor has a very different flame structure and buoyancy effect than a horizontal one.The oxygen partial pressure gradient and, consequently, the oxygen flux would be greatly impacted by the temperature and species distribution across the ITM surface.In order to develop efficient OTMRs, it is necessary to research alternative reactor configurations and optimize the reactor architecture.In the present work, oxy-methane combustion in a vertical face-to-face porous plate reactor is the focus of the investigation.Numerical techniques are used to model combustion and steadily laminar flow.The effects of feed inlet flowrates, intake temperature, and oxidizer ratio are presented and analyzed on axial and transverse temperature, as well as species concentration profiles.The maximum temperatures of the membrane and exhaust gases are provided as design limits.Furthermore, the vertical reactor's performance is compared with that of the horizontal porous plate reactor.This study will aid in optimizing the reactor design for OTMRs application.

Problem description
A generic arrangement of the vertical porous plate reactor (500 mm) is shown in Fig. 1.The porous plates are arranged as two face-to-face walls of an inner reaction zone.The porous plates are 1 mm thick and 150 mm in length.Fuel (methane, CH 4 ) along with the carrier gas CO 2 is fed from the bottom inlet with 25 mm width into the central chamber.There are two outer chambers on the exterior of the porous plates, where O 2 enters and passes through the porous plates to the combustion chamber.As the initial temperature is higher than the self-ignition temperature, the combustion starts near the porous plate region, and the reactants converts into water and carbon dioxide.The top of the chamber is where the exhaust gases exit.To assess reactor performance, this work examines temperature and species concentration profiles.

Numerical modeling
The conservation of mass, momentum, energy, and species are applied to determine fluid dynamics, heat transfer, and species conversion.

Conservation of Mass
Conservation of Energy : where p is the pressure (Pa), U is the velocity (m/s), T is the temperature (K), ρ is the density (kg/m 3 ), µ is the viscosity (N s/m 2 ), Y i is the concentration of ith species, D is the mass diffusion constant, k is the thermal conductivity (W/m K), and g is the acceleration due to gravity (m/s 2 ).In the simulation of a combustion problem, heat transfer by radiation plays a vital role due to the high temperatures involved.Therefore, a radiative transfer equation needs also to be solved in addition to the above conservation equations, which for the Discrete Ordinates radiation model is given as: where I is the radiant intensity in r − s coordinates, K is the absorption coefficient, σ s is the scattering coefficient.In the porous plate reactor, laminar non-premixed combustion takes place in which the methane fuel is converted into CO 2 and H 2 O.The global reaction can be written as follows: For stoichiometric conditions, 4 kg of CH 4 requires 1 kg of O 2 for complete combustion.When the combustion mixture is diluted with a carrier gas, another important parameter called Oxidizer Ratio (OR) is used.It is defined as the mass of O 2 in the mixture of CO 2 + O 2 : A finite rate chemistry model in which the rate of reaction k r is governed by Arrhenius law and was used to model the fuel conversion: where A is the pre-exponential coefficient, β is the temperature exponent, E is the activation energy, and R is the gas constant.The values of these parameters depend upon the chemical reaction.involved, along with the coefficients for the Arrhenius law, are given in Table 1.The third reaction in this mechanism accounts for the effect of CO 2 reactivity on the combustion characteristics.The equations mentioned above are discretized using the finite volume method (FVM) (Versteeg and Malalasekera, 2007).For this purpose, a commercial software package, ANSYS Fluent 19, was used to simulate the laminar flow and combustion problem numerically.
A 2D rectangular chamber with a symmetry-preserving dimension of unity in the z-direction makes up the simulation domain.Its whole height is 500 mm, and its breadth is 25 mm.Alumina-based porous plates are used with thermal conductivity of 3.85 W/m-K and fluid porosity of 0.5.For numerical investigation, porous plates were simulated as 1 mm thick 2D porous zones with an inertial resistance of 100 m −1 and viscous resistance of 2.44 × 10 13 m −2 .The boundary conditions used are summarized in Table 2.The domain has a mass-inlet boundary condition for the CH 4 + CO 2 mixture at the bottom and a massinlet condition for O 2 at the porous plate inlet.The fuel and porous inlet velocities for a given operating case are calculated from their mass flow rates.A fixed-pressure (ambient) boundary condition was specified at the outlet for the exhaust gases left at the top.The walls have zero velocity conditions representing a no-slip phenomenon at the walls.For simplicity, the walls were assumed to be insulated and considered adiabatic.Moreover, a fixed uniform temperature was specified at the two inlets.The initial conditions for the inner volume were set as atmospheric pressure, zero velocity, and a high temperature (1200 K) above the self-ignition temperature to initiate the combustion process.

Solution procedure
For the numerical solution, a pressure-standard built-in solver of Fluent is used with double precision.As suggested in Versteeg and Malalasekera (2007), a staggered grid is used for the pressure-velocity solution.The pressure-velocity coupling is solved using the Semi-Implicit Method for the Pressure-Linked Equations (SIMPLE) algorithm.Second-order upwind discretization schemes are employed with high orders of relaxation factors.The thermophysical properties of the fluids are derived using polynomials-based models.The effect of gravity is incorporated by specifying acceleration due to gravity g in the negative ydirection (Fig. 1).Since all the walls are considered adiabatic, adopting the radiation model compensates for the radiative heat transfer inside the participating gases.The inner emissivity of all the walls is taken as 0.8, and the absorption coefficient is determined by the domain-based Weighted Sum of the Gray-Gas Model (WSGGM).

Operating conditions
The simulations were conducted for various operating conditions for the vertical upward configuration, as summarized in Table 3.A stoichiometric mixture with OR = 0.25 and ambient fuel & porous inlet temperatures is used as a base case against which others are compared.First, the impact of raising the fuel inlet flow rate while keeping the oxidizer and equivalence ratios is investigated.The influence of the oxidizer ratio on combustion properties is then investigated.The effect of the oxidizer ratio was evaluated by varying the percentage of CO 2 in the CO 2 + O 2 mixture while keeping the equivalence ratio fixed at 1.0.Afterward, the effects of fuel and oxygen inlet temperatures are analyzed by raising the inlet temperature to 600 K and 900 K.
Different configurations concerning reactor orientation representing the influence of gravity on fuel conversion and temperature distribution are tested.Case 9 represents a horizontal porous plate reactor.The reaction zone is expected to manifest asymmetric temperature and species distribution.

Meshing
The meshing of the simulation domain is conducted using ANSYS Meshing software; a specimen is shown in Fig. 2. Quadrilateral elements are used for meshing, and refinement was done  near the walls, porous zones, and start/end of porous zones where high gradients are expected.Mesh sensitivity analysis was performed by increasing the number of cells stepwise and recording the simulation results changes.Three grids were tested with 24,000, 30,000, and 36,000 cells.A negligible change was observed while comparing the meshes with 30,000 and 36,000 cells.Therefore, the mesh with 30,000 cells was selected for further study.

Validation
To assure the accuracy of the numerical model, the experimental data by Nemitallah and Habib (2013) is used for validation purposes.Both modified Westbrook and Dryer (WD) 2-step and Jones and Lindstedt (JL) 4-step reaction kinetics models were implemented and compared with the experimental results, as shown in Fig. 3a and b.It can be seen that the numerical results follow the same trend as experimental results, and the JL model predicts well than the WD model.Except for one location, where the error is 20.3%, the WD model's error is less than 12%, while the maximum error for the JL model is 13.5%.The variations could be related to the kinetics, turbulence, and radiation model's correctness, 2D approximation, discretization, and experimental data errors.The two-step mechanism's deviations are within acceptable bounds; therefore, it can be utilized for numerical experiments to save computational time and effort.

General combustion characteristics-Base case
The fuel (CH 4 ), along with the carrier gas (CO 2 ), enters the reactor at x = 0 mm, and at x = 125 mm, the oxygen enters from the top and bottom porous plates in the cross-flow fashion.The fluid is above the self-ignition temperature; hence, the combustion starts near the porous plates.The temperature and velocity contours for the base case (fuel flow rate of 0.75 × 10 −3 kg/s with a stoichiometric ratio of 1 and oxidizer ratio of 0.25) are shown in Fig. 4a and b.The temperature contours show that the temperature is maximum where combustion begins.However, at this point, the average temperature of the fluid is low.As the combustion continues, the methane converts into water and carbon dioxide, raising the flow's average temperature.The maximum and outlet temperatures are 1891 K and 1718 K, respectively.Furthermore, the velocity starts to rise during combustion as the respective densities of species get lower at a higher temperature, resulting in a higher velocity field.
The transverse temperature distributions at different axial locations for the base case are shown in Fig. 5.The temperature is high near the porous plates and low at the middle indicating two flames at the top and bottom by the transverse temperature distribution at x = 150 mm (25 mm from the start of the porous plate).As fluid moves forward, the flames tend to move towards the center because of the force exerted on the flow of oxygen  from the top and bottom.At x = 300 mm, the centerline temperature also rises because of the combustion and radiative heat transfer.Because the temperature close to the top and bottom edges are higher than the temperature in the center, the massweighted average temperature is more appropriate for analysis than the centerline temperature.The mass-weighted temperature and velocity distribution along the axial direction are shown in Fig. 6a.Both follow the same trend: a temperature rise raises the fluid velocity proportionately.
The species distribution inside the porous plate reactor is shown in Fig. 6b.The methane concentration at the entrance is 0.0769, and it starts to decline as the reaction begins at x = 125 mm, forming CO, CO 2 , and H 2 O.By the end of the reactor, almost all of the methane is combusted.The oxygen concentration starts rising from zero at the start of the porous plate as it reaches the reaction zone; it grows at first, then falls as the reaction progresses, and all of the oxygen is consumed before the outlet.The CO 2 is formed as the combustion product; however, the concentration starts to decline as the combustion starts because of the additional O 2 mass flow rate that lowers the overall concentration in the middle zone.Afterward, some CO converts into CO 2 , increasing the CO 2 concentration near the reactor exit.

Effect of mass flow rates
The effects of mass flow rates on temperature and methane concentration distributions inside the reactor are shown in Fig. 7.
It can be seen from these trends that as the flow rate increases, convective heat transmission takes precedence over radiative heat transfer, and upstream flow has a slightly lower temperature.The increase in mass flow rate positively impacts the exit temperature.However, the field velocity increases when the mass flow rate increases, decreasing the methane depletion rate.As for the fuel flow rate of 0.5 g/s and 0.75 g/s, the methane is completely converted, but in the 1 g/s case, the reaction is still not complete at the reactor's outlet.The maximum and outlet temperature variations with respect to flow rates are shown in Fig. 8.With a higher mass flow rate, more fuel is present near the porous zone, resulting in higher temperatures.The outlet and maximum temperatures increase from 1478 K to 1894 K and 1653 K to 2160 K, correspondingly, by increasing the fuel flow rate from 0.5 g/s to 1 g/s.

Effect of oxidizer ratio
The effects of the oxidizer ratio on temperature and methane concentration distributions inside the reactor are shown in Fig. 9.The oxidizer ratio of 0.20 to 0.35 is considered by varying the CO 2 flow rate for this parametric study.As the oxidizer ratio increases, there is less carrier gas, resulting in a lower velocity field.The higher percentage of oxygen causes the methane to deplete faster, as can be seen by the slope of methane concentration in the porous plate region.The lower CO 2 concentration is the reason for the higher average temperature field in the reactor.Furthermore, the temperature decreases for OR = 0.35 before the outlet, which is because of radiative heat transfer that distributes the energy to the low-temperature region (upstream flow).As a higher OR, the reaction is intense and complete oxidation is achieved way before the outlet of the reactor, resulting in gradual thermal losses through radiative and convective heat transfer downstream of the reactor.
The maximum and outlet temperature variation concerning OR is shown in Fig. 10.In the case of higher OR, velocity is low, which lowers the convective heat transfer.The outlet and maximum temperatures change from 1587 K to 1621 K and 1751 K to 2341 K, correspondingly, by increasing OR from 0.20 to 0.35.Higher oxidizer ratios result in a faster CO conversion rate.A greater maximum temperature in the reactor results from a higher methane depletion rate and lower carrier gas flow.There is little effect on the outlet temperature as the radiative heat transfer increases, affecting the reactor temperature field, as shown in Fig. 9.These findings show that increasing the velocity field in the reactor to meet outlet temperature will improve convective effects.This can be done by either raising flow rates or decreasing reactor geometry.

Effect of inlet temperature
The effects of inlet temperature on temperature and methane concentration distributions inside the reactor are shown in Fig. 11.The inlet temperatures of fuel, carrier gas, and oxygen are varied from 300 K to 900 K with 300 K intervals.The rise in inlet temperature has a negligible effect on the methane depletion rate.However, methane depletes slightly faster at 900 K.The higher temperature results in a lower localized reaction rate.Also, it raises the flow velocity, which affects the methane conversion.The temperature distribution at higher inlet temperatures is higher than that of lower.However, the change in outlet temperature is not of the same order of magnitude as inlet temperature.
The maximum and outlet temperature variation and maximum reaction rate with respect to inlet temperature are shown in Fig. 12.The higher inlet temperature also increases flow velocity, affecting methane conversion and reducing the localized reaction rate.The maximum reaction rate decreases from 0.838 kgmol/(m 3 s) to 0.427 kgmol/(m 3 s) when the inlet temperature is increased from 300 K to 900 K.The outlet and maximum temperatures change from 1718 K to 1943 K and 1891 K to 2205 K, correspondingly increasing the inlet temperature from 300 K to 900 K. Radiative heat transfer affect the outlet temperature at higher inlet temperatures.The higher radiative heat transfer at a higher inlet temperature distributes the energy throughout the reactor.

Comparison of configurations
The same conditions as the base case are adopted for comparing the combustion process inside the horizontal and vertical configurations.The only difference is the direction of acceleration due to gravity (g), which acts downward in the horizontal and opposite to the flow in the vertical conditions.The change in the g direction affects the flow field and hence the combustion process.The temperature contours in Fig. 13a and b  the temperature distribution is symmetric for the vertical configuration while it is asymmetric for the horizontal configuration along the centerline.Moreover, it is observed that the maximum temperature is more for the horizontal than the vertical configuration.

exhibit that
The traverse temperature distribution for the subject configurations can be observed in Fig. 14.It can be found that the temperature is much higher in the horizontal configuration case at x = 150 mm as compared to the vertical one and the flame shape is more towards the center.Furthermore, the temperature in the top half is higher than in the bottom half.However, the temperature distributions differ as the flow moves toward the outlet direction.The main parameters for horizontal and vertical configurations are listed in Table 4.The maximum and outlet temperatures for the horizontal configuration are higher than the vertical one by the difference of 82 K and 53 K, respectively.The maximum reaction rate is lower for the horizontal configuration, while the O 2 species concentration at the outlet is comparable.The flow field and convective and radiative heat transfer play a vital role in the distribution of energy inside the reactor.This study concludes that the reactor design should consider the mass flow rates, oxidizer ratios, and inlet temperature.It affects the temperature inside the reactor, which may exceed the operating temperature limit for the reactor.

Conclusion
A two-dimensional CFD model is employed to analyze the combustion process in the vertical porous plate reactor in this work.The mathematical model is discretized and solved using a finite volume method in ANSYS Fluent.The general combustion characteristics and the effects of oxidizer ratio, inlet temperature, and mass flow rate on the combustion process in the porous plate reactor are presented and analyzed.Further, the combustion process in the vertical and horizontal configurations is equated and discussed.Some of the key findings are as follows:  • Higher oxidizer ratios result in a faster methane conversion rate.For OR = 0.20, the methane converts completely near the outlet of the reactor; however, as the OR is increased to 0.35, methane depletes completely at x = 350 mm.Increasing the velocity field in the reactor to meet output temperature will improve convective effects.This can be done by either raising flow rates or decreasing reactor geometry.
• Higher maximum temperature in the reactor results from a higher methane depletion rate and lower carrier gas flow.While the maximum localized temperature was found to be higher at higher ORs, for instance, the maximum temperature of 2341 K is found at OR = 0.35.Furthermore, the outlet temperature tends to lower due to intense and complete reaction before the exit.
• The lower CO 2 concentration is the reason for the higher average temperature field in the reactor at higher ORs.
• The higher inlet temperature results in a lower localized reaction rate as it also raises the flow velocity, affecting methane conversion.The maximum reaction rate decreases from 0.838 kgmol/(m 3 s) to 0.427 kgmol/(m 3 s) when the inlet temperature is increased from 300 K to 900 K.
• The temperature contours exhibit that the temperature distribution is symmetric for the vertical configuration while it is asymmetric for the horizontal configuration along the centerline.• It is observed that the maximum temperature level is higher for the horizontal than for the vertical configuration.The maximum and outlet temperatures for the horizontal configuration are higher than the vertical one by 82 K and 53 K, respectively.
• The flow field and convective and radiative heat transfer play a vital role in the distribution of energy inside the reactor.
• The reactor design should consider the mass flow rates, oxidizer ratios, and inlet temperature.It affects the localized temperature distribution inside the reactor, which may exceed the operating temperature limit for the reactor.
The study shows that the vertical configuration offers uniform temperature distribution and has a lower maximum temperature inside the reactor than the horizontal configuration, which is beneficial.However, the outlet temperature is slightly lower, which can be adjusted by inlet flow rates.This study will contribute to improving OTMR reactor design.For a deeper understanding and insight into the reactor design, additional research on the flame structure and static flame stability is needed.

Fig. 6 .
Fig. 6.Axial distribution of (a) temperature and velocity, and (b) species concentration inside the reactor.

Fig. 7 .
Fig. 7. Effect of mass flow rates on axial (a) temperature and (b) CH 4 species distribution.

Fig. 8 .
Fig. 8. Effect of mass flow rates on reactor's maximum and outlet temperatures.

Fig. 10 .
Fig. 10.Effect of oxidizer ratio on reactor's maximum and outlet temperatures.

Fig. 14 .
Fig. 14.Transverse temperature distribution at different axial location for horizontal (dashed lines) and vertical (solid line) configurations.

Table 1
Reaction equations and Arrhenius constants for modified Westbrook-Dryer mechanism.
(Andersen et al., 2009) reported that the flame speed and temperature are reduced when a mixture of O 2 + CO 2 is used as an oxidizer instead of air (O 2 + N 2 ).Therefore, a modified 2-step Westbrook and Dryer reaction mechanism proposed by Andersson et al. was used(Andersen et al., 2009).The reactions F.Tahir, B. Imteyaz, M. Yasir et al.

Table 2
Summary of boundary conditions.

Table 3
Operating conditions for the cases investigated.

Table 4
Comparison of combustion parameters for horizontal and vertical reactor configurations.