Skip Nav Destination
Filter
Filter
Filter
Filter
Filter

Update search

Filter

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- ISBN-10
- ISSN
- EISSN
- Issue
- Journal Volume Number
- References
- Conference Volume Title
- Paper No

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- ISBN-10
- ISSN
- EISSN
- Issue
- Journal Volume Number
- References
- Conference Volume Title
- Paper No

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- ISBN-10
- ISSN
- EISSN
- Issue
- Journal Volume Number
- References
- Conference Volume Title
- Paper No

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- ISBN-10
- ISSN
- EISSN
- Issue
- Journal Volume Number
- References
- Conference Volume Title
- Paper No

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- ISBN-10
- ISSN
- EISSN
- Issue
- Journal Volume Number
- References
- Conference Volume Title
- Paper No

- Title
- Author
- Author Affiliations
- Full Text
- Abstract
- Keyword
- DOI
- ISBN
- ISBN-10
- ISSN
- EISSN
- Issue
- Journal Volume Number
- References
- Conference Volume Title
- Paper No

### NARROW

Format

Article Type

Conference Series

Subject Area

Topics

Date

Availability

1-8 of 8

Karam R. Beshay

Close
**Follow your search**

Access your saved searches in your account

Would you like to receive an alert when new items match your search?

*Close Modal*

Sort by

Proceedings Papers

*Proc. ASME*. HT2007, ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 3, 193-202, July 8–12, 2007

Paper No: HT2007-32064

Abstract

The present paper is concerned with the numerical computation of flow, heat transfer and chemical reactions in porous burners. The porous solid matrix acts as a host for redistributing the thermal energy transferred to it from the hot reacting gases. Inside the porous matrix, heat is transferred down stream by conduction and radiation. This thermal energy is then transferred to the incoming cold fuel/air mixture to initiate the chemical reaction processes and thus stabilize the flame. One of the important features of porous burners is its presumed low levels of NO concentration. In the present work, the computed NO x is compared with experimental data and open premixed flames. In order to accurately compute the nitric oxide levels in porous burners, both prompt and thermal NO x mechanisms are included. In the present work, the porous burner species mass fraction source terms are computed from an ‘extended’ reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e., the porosity is not uniform over the entire domain. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to insure that the influence coefficients are always positive. Finite-difference equations are solved, iteratively, for velocity components, pressure correction, gas enthalpy, species mass fractions and solid matrix temperature. A non-uniform (80×80) computational grid is used. The grid used to solve the solid energy equation is extended inside the solid annular wall of the porous burner, to improve its modeling. A discrete-ordinate model with S4 quadrature is used for the computation of thermal radiation emitted from the solid matrix. The porous burner uses a premixed CH 4 -air mixture, while its radiating characteristics are required to be studied numerically under equivalence ratios 0.6 and 0.5. Twenty-five species are included, involving 75 elementary chemical reactions. The computed solid wall temperature profiles are compared with experimental data for similar porous burners. The obtained agreement is fairly good. Some reacting species, such as H 2 O, CO 2 , H 2 , NO and N 2 O increase steadily inside the reaction zone. However, unstable products, such as HO 2 , H 2 O 2 and CH 3 , increase in the preheating zone to be depleted afterward.

Proceedings Papers

*Proc. ASME*. HT2008, Heat Transfer: Volume 3, 119-128, August 10–14, 2008

Paper No: HT2008-56229

Abstract

The present paper is concerned with the numerical computation of flow, heat transfer and chemical reactions in porous burners. One of the important features of porous burners is their presumed low levels of nitrogen oxides. In the present work, the computed NO x is compared with similar conventional premixed burners and measured nitrogen oxides in porous burners. In order to accurately compute the nitrogen oxides levels in porous burners, both prompt and thermal NO x mechanisms are included. In the present work, the porous burner species mass fraction source terms are computed from an ‘extended’ reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e. the porosity is not uniform over the entire domain. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to ensure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. Finite-difference equations are solved iteratively for velocity components, pressure correction, gas enthalpy, species mass fractions and solid matrix temperature. The grid used to solve the solid energy equation is extended inside the zero-porosity solid annular wall of the burner porous disk. This was found useful for computing the solid wall temperature with high accuracy. A two-dimensional, discrete-ordinate, model is used for the computation of thermal radiation emitted from the solid matrix. The porous burner uses a premixed CH4-air mixture, while its radiating characteristics are studied numerically under equivalence ratio ranging from 0.5 to 0.8. Twenty-one species are included, involving 55 chemical reactions. The computed solid wall temperature profiles are compared with experimental data of similar porous burners. The obtained agreement is fairly good. The present numerical results show that as the equivalent ratio decreases, the reaction zone moves downstream. Moreover, as the flame speed increases, the NO x mole fraction increases. Some reacting species, such as H 2 O, CO 2 and H 2 increase steadily inside the reaction zone; they stay appreciable in the combustion products. However, unstable products, such as HO 2 , H 2 O 2 and CH 3 , first increase in the preheating region of the reaction zone; they are then consumed in the remaining part of the reaction zone. The numerical results show that most of the formed NO x is composed of nitric oxide. The velocity and temperature profiles were accurately predicted using a grid of 80×80 while the nitrogen oxides were computed accurately utilizing a finer grid of 160×160.

Proceedings Papers

*Proc. ASME*. HT2005, Heat Transfer: Volume 1, 723-734, July 17–22, 2005

Paper No: HT2005-72439

Abstract

The present work is a numerical simulation of the, piloted, non-premixed, methane–air flame structure in a new mathematical imaging domain. This imaging space has the mixture fraction of diffusion flame Z1 and mixture fraction of pilot flame Z2 as independent coordinates to replace the usual physical space coordinates. The predications are based on the solution of two–dimensional set of transformed second order partial differential conservation equations describing the mass fractions of O 2 , CH 4 , CO 2 , CO, H 2 O, H 2 and sensible enthalpy of the combustion products which are rigorously derived and solved numerically. A three–step chemical kinetic mechanism is adopted. This was deduced in a systematic way from a detailed chemical kinetic mechanism by Peters (1985). The rates for the three reaction steps are related to the rates of the elementary reactions of the full reaction mechanism. The interaction of the pilot flame with the non-premixed flame and the resulting modifications to the structure and chemical kinetics of the flame are studied numerically for different values of the scalar dissipation rate tensor. The dissipation rate tensor represents the flame stretching along Z1, the main mixture fraction, and in the perpendicular direction, along Z2, the pilot mixture fraction. The computed flame temperature contours are plotted in the Z1-Z2 plane for fixed values of the dissipation rate along Z1 and Z2.These temperature contours show that the flame will become unstable when the dissipate rates along Z1 and Z2 increase, simultaneously, to the limiting value for complete flame extinction of 45 s −1 . However, the diffusion flame will extinguish for dissipate rates less than 20 1/s, if unpiloted. It is also noticed that the flame will remain stable if the dissipation rate along Z2 is increased to the limiting value, while the dissipation rate, along Z2, remains constant at a value less than 30 s −1 .

Proceedings Papers

*Proc. ASME*. HT2005, Heat Transfer: Volume 3, 683-692, July 17–22, 2005

Paper No: HT2005-72008

Abstract

The present paper presents, numerical computations for flow, heat transfer and chemical reactions in an axisymmetric inert porous burner. The porous media re-radiate the heat absorbed from the gaseous combustion products by convection and conduction. In the present work, the porous burner species mass fraction source terms are computed from an ‘extended’ reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e. the porosity is not uniform over the entire domain. Therefore, it has to be included inside the partial derivatives of the transport governing equations. Finite-difference equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to insure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. Finite-difference equations are solved, iteratively, for U, V, p’ (pressure correction), enthalpy and species mass fractions, utilizing a fine grid of (80×60) nodes. The eighty grid nodes in the axial direction are needed to resolve the detailed structure of the thin reaction zone inside the porous media. The radial grid is extended inside the annular solid wall of the porous burner, to compute the wall temperature. The porous burner uses a premixed CH 4 -air mixture, while its radiating characteristics are computed numerically, using a four-flux radiation model. Sixteen species are included, namely CH 4 , CH 3 , CH 2 , CH, CH 2 O, CHO, CO, CO 2 , O 2 , O, OH, H 2 , H, H 2 O, H 2 O, H 2 O 2 , involving 49 chemical reaction equations. It was found that 1000 iterations are sufficient for complete conversion of the computed results with errors less than 0.1%. The computed temperature profiles of the gas and the solid show that, heat is conducted from downstream to the upstream of the reaction zone. Most stable species, such as H 2 O, CO 2 , H 2 , keep increasing inside the reaction zone staying appreciable in the combustion products. However, unstable products, such as HO 2 , H 2 O 2 and CH 3 , first increase in the preheating region of the reaction zone, they are then consumed fast in the post-reaction zone of the porous burner. Therefore, it appears that their important function is only to help the chemical reactions continue to their inevitable completion of the more stable combustion products.

Proceedings Papers

*Proc. ASME*. HT-FED2004, Volume 2, Parts A and B, 31-39, July 11–15, 2004

Paper No: HT-FED2004-56012

Abstract

The present paper presents, numerical computations for flow, heat transfer and chemical reactions in an axisymmetric inert porous burner. The porous media re-radiate the heat absorbed from the gaseous combustion products by convection and conduction. In the present work, the porous burner species mass fraction source terms are computed from an ‘extended’ reaction mechanism, controlled by chemical kinetics of elementary reactions. The porous burner has mingled zones of porous/nonporous reacting flow, i.e. the porosity is not uniform over the entire domain. Therefore, it has to be included inside the partial derivatives of the transport governing equations. Finite-difference equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to insure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. Finite-difference equations are solved, iteratively, for U, V, p’ (pressure correction), enthalpy and species mass fractions, utilizing a grid of (60×40) nodes. The sixty grid nodes in the axial direction are needed to resolve the detailed structure of the thin reaction zone inside the porous media. The porous burner uses a premixed CH 4 -air mixture, while its radiating characteristics are computed numerically, using a four-flux radiation model. Sixteen species are included, namely CH 4 , CH 3 , CH 2 , CH, CH 2 O, CHO, CO, CO 2 , O 2 , O, OH, H 2 , H, H 2 O, HO 2 , H 2 O 2 , involving 49 chemical reaction equations. It was found that 900 iterations are sufficient for complete conversion of the computed results with errors less than 0.1%. The computed temperature profiles of the gas and the solid show that, heat is conducted from downstream to the upstream of the reaction zone. Most stable species, such as H 2 O, CO 2 , H 2 , keep increasing inside the reaction zone staying appreciable in the combustion products. However, unstable products, such as HO 2 , H 2 O 2 and CH 3 , first increase in the preheating region of the reaction zone, they are then consumed fast in the post-reaction zone of the porous burner. Therefore, it appears that their important function is only to help the chemical reactions continue to their inevitable completion of the more stable combustion products.

Proceedings Papers

*Proc. ASME*. FUELCELL2004, 2nd International Conference on Fuel Cell Science, Engineering and Technology, 23-29, June 14–16, 2004

Paper No: FUELCELL2004-2447

Abstract

The produced water vapor in the vicinity of the membrane, of a proton exchange membrane fuel cell (PEMFC), may condense into liquid water, if the water mass fraction is higher than the saturation value corresponding to the local temperature. In this case the flowing fluid inside the layers of the PEMFC is a 2-phase flow. The present mathematical model is based on the locally homogeneous flow model (LHFM) where the slip velocity between the two phases is assumed negligibly small. Therefore, the governing gas-phase and liquid-phase, for each dependent variable, can be economically added together. The resulting equations contain a ‘mixture density’, which is a function of the void fraction, species mass fractions, pressure and temperature. The resulting governing equations for u, v, T and species mass fractions together with the electric potential and mass continuity equations are solved iteratively using the SIMPLE algorithm. One solution domain is superimposed over all the layers of the PEMFC with appropriate boundary conditions applied at inlet, exit and sidewalls of the fuel cell. Special care is devoted to the electric potential, ‘Poisson-type’, equation boundary condition to prevent any escape of protons through the two diffuser layers and simultaneously insuring a non-singular matrix of finite-difference coefficients. This is because Poisson equation is notoriously known for having problems with zero gradient boundary condition. Numerical computations are carried out for a typical proton exchange membrane fuel cell that has experimental data. In order to obtain complete performance results, the computations are repeated for increasing fuel cell electric current densities until the voltage vanishes. The obtained 2-phase and 1-phase simulations are compared with the corresponding experimental and numerical data available in the literature. Systematically, the 1-phase current density is under predicted especially for values of the cell potential less than 0.8 V. On the other hand, the two-phase simulation current density, of the parallel geometry FC, is in very good agreement with the corresponding experiment data. The 2-phase flow simulations show that most of the liquid phase is concentrated in the cathode, reaching maximum value near the cathode catalyst layer-membrane interface. A new design of the serpentine PEMFC is suggested and is tested numerically. The new design involves blocking the outlet sections, either partially or fully, of the anode and/or the cathode gas channels to force the flowing fluids to diffuse into the catalyst layers at rates higher than a typical parallel geometry PEMFC. The new serpentine PEMFC design is expected to increase the concentrations of the hydrogen fuel and the oxidant in the catalyst layers and hence increase the transfer current densities. In order to obtain full simulation of the enhanced geometry of the fuel cell, the end boundary condition of the gas channels is adjusted using zero porosity to prevent any flow through the blocked area which automatically reduces the local velocity to zero value. The two-phase flow numerical results, for the modified serpentine PEMFC, indicate that the performance of the fuel cell could be enhanced appreciably.

Proceedings Papers

*Proc. ASME*. FUELCELL2005, 3rd International Conference on Fuel Cell Science, Engineering and Technology, 129-139, May 23–25, 2005

Paper No: FUELCELL2005-74142

Abstract

The proton-exchange membrane (PEM) fuel cell works under low temperatures and hence is suitable for the automotive industry. The produced water vapor in the vicinity of the membrane may condense into liquid water, if the water mass fraction is higher than the saturation value corresponding to the local temperature. In this case the flowing fluid inside the layers of the PEMFC is a 2-phase flow. The locally homogeneous flow (LHF) model has been previously used for modeling the 2-phase flow in PEM fuel cells, with limited success. This model could not predict the blocking effect of the liquid phase, since both phases flow locally with the same velocity, according to the LHF model. In contrast to complete coupling of the two phases, assumed by the LHF model, a blocking model was used by some investigators where the liquid is totally uncoupled to the gas phase. This assumption causes the liquid to become essentially stationary inside the pores of the GDL and catalyst layers and hence only the gas phase equations need to be considered. Both of these extreme models were only successful to a limited extent. The present work considers a two-fluid mathematical model for the gas-liquid flow in PEM fuel cells. One fluid represents the continuous gas phase flow through the layers of the fuel cell. For this fluid, the governing equations of momentum, energy, mass continuity and species mass fractions, are considered with additional inter-fluid exchange source terms. The second fluid represents the dispersed liquid phase that is formed from the condensed water vapor inside the layers of the PEM fuel cell. For this fluid only the momentum, energy and mass continuity equations need to be included, as no electrochemical reactions are essentially possible. The dispersed fluid is made up of small droplets in the gas channel. However, in the porous layers of the fuel cell, the flowing layers of water represent the dispersed fluid over the solid matrix. The thickness of the creeping water layers is controlled by the wetability of the solid matrix of the porous layers of the PEM fuel cell. Numerical computations are carried out for a typical proton exchange membrane fuel cell that has experimental data. In order to obtain complete performance results, the computations are repeated for increasing fuel cell electric current densities until the limiting current is reached. The obtained two-fluid and single-phase simulations are compared with the corresponding experimental and numerical data available in the literature. The 2-fluid model shows that the blocking effect of the liquid phase starts to dominate, for cell voltage less than 0.65 V; in this case, the flowing 2-phase flow produces faster drop in cell voltage as the loading electric current increases. This phenomenon was partially hindered by the LHF model but essentially completely bypassed by the single-phase simulations. The 2-fluid simulations show that most of the liquid dispersed phase is concentrated in the cathode, reaching maximum value near the cathode catalyst layer- membrane interface. This behavior results from the lack of mobility of the liquid water inside the pores.

Proceedings Papers

*Proc. ASME*. FUELCELL2006, ASME 2006 Fourth International Conference on Fuel Cell Science, Engineering and Technology, Parts A and B, 1-12, June 19–21, 2006

Paper No: FUELCELL2006-97003

Abstract

The present work considers a two-fluid mathematical model for the gas-liquid flow in PEM fuel cells. One fluid represents the continuous gas phase flow through the layers of the fuel cell. For this fluid, the governing equations of momentum, energy, mass continuity and species mass fractions, are considered with additional inter-fluid exchange source terms. The second fluid represents the dispersed liquid phase that is formed from the condensed water vapor inside the layers of the PEM fuel cell. For this fluid only the momentum and mass continuity equations need to be included, as no electrochemical reactions are essentially possible. The dispersed fluid is made up of small droplets in the gas channel. The mean droplet diameter can be computed from a balance equation of the forces acting on the emerging liquid water from the pores of the GDL into the gas channels. The droplet diameters are found to range between 150 and 170 microns for the present PEM fuel cell at 0.8 V. In the present work, the full momentum conservation equations are invoked, in the layers of the fuel cell, for the two fluids. The resulting governing equations for u, v, T and species mass fractions together with the electric potential and mass continuity equations are solved iteratively, using a modified SIMPLE algorithm, for the two fluids. One solution domain is superimposed over all the layers of the fuel cell. Special care is devoted to the electric potential, ‘Poisson-type’, equation boundary condition to prevent any escape of protons through the two gas diffusion layers and simultaneously insuring a non-singular matrix of finite-difference coefficients. The obtained two-fluid and single-phase numerical simulations are compared with the corresponding experimental and numerical data available in the literature. The 2-fluid model shows that the blocking effect of the liquid phase starts to dominate, for cell voltage less than 0.65 V; in this case, the flowing 2-phase flow produces faster drop in cell voltage as the loading electric current increases. This phenomenon was partially hindered by previous LHF model results and essentially completely bypassed by the single-phase simulations.