Computation of Nitrogen Oxides in Radiant Porous Burner Flows

2008 ◽  
Author(s):  
Timothy W. Tong ◽  
Mohsen M. Abou-Ellail ◽  
Yuan Li ◽  
Karam R. Beshay
Keyword(s):  
Nitrogen Oxides ◽  
Reaction Zone ◽  
Chemical Reactions ◽  
Wall Temperature ◽  
Solid Wall ◽  
Numerical Results ◽  
Solid Matrix ◽  
Reacting Flow ◽  
Temperature Profiles ◽  
Porous Burner

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 NOx 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 NOx 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 NOx mole fraction increases. Some reacting species, such as H2O, CO2 and H2 increase steadily inside the reaction zone; they stay appreciable in the combustion products. However, unstable products, such as HO2, H2O2 and CH3, 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 NOx 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.

Numerical Prediction of Nitrogen Oxides in Radiant Porous Burner Flows

2007 ◽  
Author(s):  
Timothy W. Tong ◽  
Mohsen M. Abou-Ellail ◽  
Yuan Li ◽  
Karam R. Beshay
Keyword(s):  
Experimental Data ◽  
Chemical Reactions ◽  
Thermal Energy ◽  
Solid Wall ◽  
Porous Matrix ◽  
Solid Matrix ◽  
Reacting Flow ◽  
Elementary Reactions ◽  
Influence Coefficients ◽  
Porous Burner

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 NOx 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 NOx 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 CH4-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 H2O, CO2, H2, NO and N2O increase steadily inside the reaction zone. However, unstable products, such as HO2, H2O2 and CH3, increase in the preheating zone to be depleted afterward.

Numerical Simulation of Reacting Flows in Radiant Porous Burners

2005 ◽  
Author(s):  
Timothy W. Tong ◽  
Mohsen M. M. Abou-Ellail ◽  
Yuan Li ◽  
Karam R. Beshay
Keyword(s):  
Porous Media ◽  
Finite Difference ◽  
Reaction Zone ◽  
Chemical Reactions ◽  
Difference Equations ◽  
Solid Wall ◽  
Combustion Products ◽  
Fine Grid ◽  
Finite Difference Equations ◽  
Porous Burner

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 CH4-air mixture, while its radiating characteristics are computed numerically, using a four-flux radiation model. Sixteen species are included, namely CH4, CH3, CH2, CH, CH2O, CHO, CO, CO2, O2, O, OH, H2, H, H2O, H2O, H2O2, 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 H2O, CO2, H2, keep increasing inside the reaction zone staying appreciable in the combustion products. However, unstable products, such as HO2, H2O2 and CH3, 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.

Numerical Computation of Reacting Flow in Porous Burners With an Extended CH4-Air Reaction Mechanism

2004 ◽  
Author(s):  
Timothy Tong ◽  
Mohsen Abou-Ellail ◽  
Yuan Li ◽  
Karam R. Beshay
Keyword(s):  
Porous Media ◽  
Reaction Mechanism ◽  
Finite Difference ◽  
Reaction Zone ◽  
Chemical Reactions ◽  
Difference Equations ◽  
Combustion Products ◽  
Reacting Flow ◽  
Finite Difference Equations ◽  
Porous Burner

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 CH4-air mixture, while its radiating characteristics are computed numerically, using a four-flux radiation model. Sixteen species are included, namely CH4, CH3, CH2, CH, CH2O, CHO, CO, CO2, O2, O, OH, H2, H, H2O, HO2, H2O2, 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 H2O, CO2, H2, keep increasing inside the reaction zone staying appreciable in the combustion products. However, unstable products, such as HO2, H2O2 and CH3, 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.

scholarly journals Optimal control of the Sabatier process in microchannel reactors

2021 ◽  
Vol 128 (1) ◽  
Author(s):  
Sebastian Blauth ◽  
Christian Leithäuser ◽  
René Pinnau
Keyword(s):  
Flow Rate ◽  
Wall Temperature ◽  
Optimization Problems ◽  
Product Yield ◽  
Identification Problem ◽  
State Constraint ◽  
Optimization Techniques ◽  
Reacting Flow ◽  
Microchannel Reactor ◽  
Highly Nonlinear

AbstractWe consider the optimization of a chemical microchannel reactor by means of PDE-constrained optimization techniques, using the example of the Sabatier reaction. To model the chemically reacting flow in the microchannels, we introduce a three- and a one-dimensional model. As these are given by strongly coupled and highly nonlinear systems of partial differential equations (PDEs), we present our software package cashocs which implements the adjoint approach and facilitates the numerical solution of the subsequent optimization problems. We solve a parameter identification problem numerically to determine necessary kinetic parameters for the models from experimental data given in the literature. The obtained results show excellent agreement to the measurements. Finally, we present two optimization problems for optimizing the reactor’s product yield. First, we use a tracking-type cost functional to maximize the reactant conversion, keep the flow rate of the reactor fixed, and use its wall temperature as optimization variable. Second, we consider the wall temperature and the inlet gas velocity as optimization variables, use an objective functional for maximizing the flow rate in the reactor, and ensure the quality of the product by means of a state constraint. The results obtained from solving these problems numerically show great potential for improving the design of the microreactor.

The Effect of the Top Wall Temperature on the Laminar Natural Convection in Rectangular Cavities With Different Aspect Ratios

2009 ◽  
Vol 131 (5) ◽  
Author(s):  
Wenjiang Wu ◽  
Chan Y. Ching
Keyword(s):  
Natural Convection ◽  
Aspect Ratio ◽  
Wall Temperature ◽  
Vertical Wall ◽  
Flow Region ◽  
Temperature Profiles ◽  
Aspect Ratios ◽  
Vertical Walls ◽  
C And N ◽  
Laminar Natural Convection

The effect of the top wall temperature on the laminar natural convection in air-filled rectangular cavities driven by a temperature difference across the vertical walls was investigated for three different aspect ratios of 0.5, 1.0, and 2.0. The temperature distributions along the heated vertical wall were measured, and the flow patterns in the cavities were visualized. The experiments were performed for a global Grashof number of approximately 1.8×108 and nondimensional top wall temperatures from 0.52 (insulated) to 1.42. As the top wall was heated, the flow separated from the top wall with an undulating flow region in the corner of the cavity, which resulted in a nonuniformity in the temperature profiles in this region. The location and extent of the undulation in the flow are primarily determined by the top wall temperature and nearly independent of the aspect ratio of the cavity. The local Nusselt number was correlated with the local Rayleigh number for all three cavities in the form of Nu=C⋅Ran, but the values of the constants C and n changed with the aspect ratio.

scholarly journals Fouling in a Steam Cracker Convection Section Part 1: A Hybrid CFD-1D Model to Obtain Accurate Tube Wall Temperature Profiles

2019 ◽  
Vol 41 (2) ◽  
pp. 127-137 ◽  
Author(s):  
Pieter Verhees ◽  
Abdul Rahman Akhras ◽  
Kevin M. Van Geem ◽  
Geraldine J. Heynderickx
Keyword(s):  
Wall Temperature ◽  
Tube Wall ◽  
Temperature Profiles ◽  
Tube Wall Temperature ◽  
1D Model ◽  
Steam Cracker

Use of Fuel Assembly/Backfill Gas Effective Thermal Conductivity Models to Predict Basket and Fuel Cladding Temperatures Within a Rail Package During Normal Transport

2006 ◽  
Author(s):  
Miles Greiner ◽  
Kishore Kumar Gangadharan ◽  
Mithun Gudipati
Keyword(s):  
Thermal Conductivity ◽  
Fuel Assembly ◽  
Effective Thermal Conductivity ◽  
Wall Temperature ◽  
Experimental Studies ◽  
Temperature Profiles ◽  
Fuel Cladding ◽  
Dimensional Finite Element ◽  
Normal Transport ◽  
Conductivity Models

Two-dimensional finite element thermal simulations of a generic rail package designed to transport twenty-one spent PWR assemblies were performed for normal transport conditions. Effective thermal conductivity models were employed within the fuel assembly/backfill gas region. Those conductivity models were developed by other investigators assuming the basket wall temperature is uniform. They are typically used to predict the maximum fuel cladding temperature near the package center. The cladding temperature must not exceed specified limits during normal transport. This condition limits the number and heat generation rate of fuel assembles that can transported. The current work shows the support basket wall temperatures in the periphery of the package are highly non-uniform. Moreover the thermal resistance of those regions significantly affects the maximum fuel clad temperature near the package center. This brings the validity of the fuel/backfill gas thermal conductivity models into question. The non-uniform basket wall temperature profiles quantified in this work will be used in future numerical and experimental studies to develop new thermal models of the fuel assembly/backfill gas regions. This will be an iterative process, since the assembly/backfill model affects the predicted basket wall temperature profiles.

Effect of Constant Heat Flux Boundary Condition on Wall Temperature Fluctuations

2000 ◽  
Vol 123 (2) ◽  
pp. 213-218 ◽  
Author(s):  
A. Mosyak ◽  
E. Pogrebnyak ◽  
G. Hetsroni
Keyword(s):  
Boundary Conditions ◽  
Wall Temperature ◽  
Solid Wall ◽  
Rectangular Channel ◽  
Temperature Fluctuations ◽  
Temperature Fields ◽  
Flux Boundary Condition ◽  
Wall Boundary ◽  
Wall Boundary Conditions ◽  
Thermal Entrance

An experimental study of the wall temperature fluctuations under different thermal-wall boundary conditions was carried out. Statistics obtained from the experiments are compared with existing experimental and numerical data. The wall temperature fields are also examined in terms of the coherent thermal structures. In addition the effect of the thermal entrance region on the wall temperature distribution is also studied. For water flow in a flume and in a rectangular channel, the mean spacing of the thermal streaks does not depend on the thermal entrance length and on the type of thermal-wall boundary conditions. The wall temperature fluctuations depend strongly on the type of wall thermal boundary conditions. Overall, the picture that emerges from this investigation confirms the hypothesis that moderate-Prandtl-number heat transfer at a solid wall is governed by the large-scale coherent flow structures.

Molecular Dynamics Simulations of Coupling between Flow and Heat Transfer in a Nanochannel

2012 ◽  
Vol 455-456 ◽  
pp. 155-160
Author(s):  
Zhi Hai Kou ◽  
Min Li Bai
Keyword(s):  
Heat Transfer ◽  
Molecular Dynamics ◽  
Molecular Dynamics Simulations ◽  
Wall Temperature ◽  
Solid Wall ◽  
Slip Length ◽  
Three Dimensional ◽  
Flow And Heat Transfer ◽  
Kapitza Length ◽  
Dynamics Simulations

Simulation of microscale thermo-fluidic transport has attracted considerable attention in recent years owing to rapid advances in nanoscience and nanotechnology. The three-dimensional molecular dynamics simulations are performed for coupling between flow and heat transfer in a nanochannel. Effects of interface wettability, shear rate and wall temperature are discussed. It is found that there exist the relatively immobile solid-like layers adjacent to each solid wall with higher number density. Both slip length and Kapitza length at the solid-liquid interface increase linearly with the increasing wall temperature. The Kapitza length decreases monotonously with the increasing shear rates. The slip length is found to be overestimated by 5.10% to 10.27%, while Kapitza length is overestimated by 8.92% to 19.09% for the solid-solid interaction modeled by the Lennard-Jones potential.

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