Rajesh Ahluwalia - Academia.edu (original) (raw)
Papers by Rajesh Ahluwalia
Meeting abstracts, Oct 9, 2022
ECS Meeting Abstracts
Fuel cells for heavy-duty vehicles (HDVs) have attracted considerable attention because of their ... more Fuel cells for heavy-duty vehicles (HDVs) have attracted considerable attention because of their unique scalability, better fuel economy, and less demand for hydrogen refilling infrastructure relative to the light-duty vehicle application.1 However, the HDV application requires more stringent fuel cell durability, up to 25,000 h or 1 million miles of operation and increased efficiency. 1-2 Cathodes with relatively high Pt loading (~0.3 mg-Pt/cm2) have drawn extensive attention over initially highly active but unstable Pt-transition metal (Pt-TM) alloy3 catalysts owing to their capability of delivering high performance over long HDV lifetime. To fully exploit the potential of Pt in the HDV cathodes, new insights into the relationship between different drive cycles and operating conditions of HDVs and durability are needed with a particular emphasis on understanding the mechanisms for dissolution of Pt. Several mechanisms for dissolution of platinum have been proposed including direct...
U.S. Department of Energy Office of Scientific and Technical Information - OSTI OAI, Nov 9, 2018
U.S. Department of Energy Office of Scientific and Technical Information - OSTI OAI, Apr 15, 2022
Volume 3: Coal, Biomass and Alternative Fuels; Combustion and Fuels; Oil and Gas Applications; Cycle Innovations, 1988
A mass transfer model is developed that considers diffusive and chemical aspects of sodium sulfat... more A mass transfer model is developed that considers diffusive and chemical aspects of sodium sulfate formation and deposition on cooled blades of coal-fired gas turbines. The roles of gas phase condensation of sodium sulfate and multicomponent diffusion across a chemically frozen thin boundary layer are elaborated. A rational procedure is presented for correlating material wastage with laboratory weight gain data obtained by exposing alloy specimens pre-coated with a thin film of salt to SO2-SO3 in an oxygen environment. The sodium sulfate mass transfer model is used in conjunction with the correlation to project blade corrosion and lifetime as a function of gas turbine inlet temperature, blade cooling, and sodium and sulfur contaminant concentration.
IECEC 96. Proceedings of the 31st Intersociety Energy Conversion Engineering Conference
Analytic& models have been developed to study the dynamic response of steam-reformed, methanol-fu... more Analytic& models have been developed to study the dynamic response of steam-reformed, methanol-fueled, polymer electrolyte fuel cell (PEFC) systems for transportation applications. Attention is focused primarily on the heat transfer effects which are likely to limit rapid response of PEFC systems. Depending on the thermal mass, the heat exchangers and the steam reformer can have time constants of the order of several seconds to many minutes. On the other hand, the characteristic time constants associated with pressddensity disturbances arising from flow rate fluctuations are of the order of milliseconds. In vehicular applications, the response time of the turbomachinery, which is determined by the rotational inertia, can be of the order of seconds or less. The dynamic reformer model has been used to examine the methanol conversion efficiency and the thermal performance during a cold start. Response times are determined to achieve 50-100% of the steady-state methanol conversion for two catalyst tube diameters. The thermal performance is considered in terms of the approach to steady-state temperature., posslbility of catalyst overheating, and the penalty in system efficiency incurred during the start-up time. For the complete reference PEFC system various turn-down scenarios were simulated by varying the relative rates of change of fuel cell loading and system flows. It is shown that depending on the relative rates of cell loading changes to flow rate changes overheating of the catalyst can occur due to excess heat transfer in the reformer preheater. This can be controlled by an additional water quench between the catalyst bed and preheater, but only if the flow rate change is sufficiently fast relative to the load changes.
Mechanistic models have been developed for particle and vapor deposition on the blades of coal-fi... more Mechanistic models have been developed for particle and vapor deposition on the blades of coal-fired gas turbines. The particle deposition models include the simultaneous contribution of Brownian and turbulent diffusion, thermophoresis, eddy impaction, and inertial impingement. The diffusive mechanisms have been validated against experimental data for low-speed cascade flow and particle-laden flow through pipes. The inertial deposition treatment is shown to collapse to the well-known expression for particle capture in a flow turning around a bend. A method is presented for calculating Na/sub 2/SO/sub 4/ and K/sub 2/SO/sub 4/ vapor deposition on cooled blades. Scaling laws are formulated for estimating the contribution of boundary layer homogeneous and heterogeneous nucleation mechanism for highly cooled turbine blades.
ECS Meeting Abstracts, 2021
ECS Meeting Abstracts, 2020
Polymer electrolyte membrane fuel cell (PEMFC) systems for heavy-duty vehicles (HDV) are attracti... more Polymer electrolyte membrane fuel cell (PEMFC) systems for heavy-duty vehicles (HDV) are attracting attention for several reasons including tighter emission regulations and, compared with light duty vehicles (LDVs), possibility of a gradual build-up of hydrogen infrastructure during the market introduction phase, and higher allowable cost (60 /kWevs.30/kWe vs. 30 /kWevs.30/kWe) and Pt loadings.1 However, competing with the incumbent diesel technology requires longer stack lifetimes than for LDVs (30,000 h vs. 8,000 h) and higher efficiencies to compensate for the likely price differential between diesel and hydrogen.2 Heat rejection in fuel cells for HDVs can also be an issue. We conducted simulations to compare radiator heat loads in two propulsion systems for a Class-8 HDV: a 450-hp turbocharged diesel engine, and a hybrid 275-kWe FCS with a 35-kWh battery. The simulations showed that the radiator heat load at rated power is higher for the hybrid propulsion system even though it is 20% more effic...
ABSTRACT A comprehensive study is conducted into the analysis of a coal-fired Component Developme... more ABSTRACT A comprehensive study is conducted into the analysis of a coal-fired Component Development and Integration Facility (CDIF) 1-B channel performance to simulate the characteristics of the reference Engineering Test Facility (ETF) channel. The performance criteria are established by careful review of the role played by the gas dynamic and electrical variables in determining channel behavior. On the basis of this review, a constant-velocity subsonic channel operating at nominal combustor pressure of 5.4 atm (0.54 MPa) is selected. The channel loading is selected to obtain a maximum Hall field of 2.4 kV/m and maximum transverse current density of 1.1 A/sq cm.
ECS Meeting Abstracts, 2019
We have investigated the mechanism of increase in oxygen transport resistance (Rm) due to aging o... more We have investigated the mechanism of increase in oxygen transport resistance (Rm) due to aging of electrodes with low-loaded de-alloyed PtCo cathode catalysts supported on high surface area carbon (d-PtCo/C). Commercially available membrane electrode assemblies with 0.05, 0.1 and 0.15 mg/cm2 Pt loadings in d-PtCo/C cathode catalysts were assembled into cells and subjected to accelerated stress tests (ASTs) that consisted of 0.6-0.95 V square wave potentials with 3-s hold at upper and lower potential limits. Catalyst degradation was characterized by measuring mass activity for the oxygen reduction reaction, electrochemically active surface area (ECSA) and polarization performance in H2/air at beginning of test (BOT) and after 15k and 30k (EOT) potential cycles in H2/N2. Concurrently, limiting current densities (iL) were measured for different pressures (1.3, 1.8, 2.3, 2.8 and 3.2 atm) and oxygen concentrations (0.01, 0.02 and 0.04 O2 mole fraction in dry air) at 80oC and 90% relativ...
ECS Meeting Abstracts, 2018
The primary PEMFC catalyst layer architecture used since the early 1990’s is a porous dispersed P... more The primary PEMFC catalyst layer architecture used since the early 1990’s is a porous dispersed Pt-supported carbon catalyst with recast solid ionomer component forming a continuous (percolating) network through the electrode for proton transport; the carbon facilitating the electrical conduction; the active electrochemical reactions conducted on the metallic Pt sites [1,2]. The thin catalyst layer consists of microscale carbon particles each supporting nanoscale platinum catalyst particles all loosely embedded in a matrix of ionomer creating a porous structure. The fuel cell catalyst layer is a complex structure that facilitates the electrochemical conversion of hydrogen and oxygen; it provides pathways for reactant transport, provides both electrical and proton conductivity pathways. This has been traditionally referred to as the 3-phase boundary of an electrolyte, an electrode, and a gaseous fuel. In addition to the gas-reactant transport to the active site, the result product wa...
ECS Meeting Abstracts, 2017
FC-PAD (Fuel Cell – Performance and Durability) consortium coordinates U.S. national laboratory a... more FC-PAD (Fuel Cell – Performance and Durability) consortium coordinates U.S. national laboratory activities related to PEM fuel cell performance and durability. The cost and durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are two major barriers to the commercialization of these systems for transportation power applications. Transportation conditions include operation in the presence of fuel and air impurities, start/stop, freeze/thaw, and humidity and load cycling that result in mechanical and chemical stresses. MEA durability decreases with decreasing catalyst loading, making cost reduction even more difficult. While there has been significant progress in improving PEM Fuel Cell durability with lower cost materials, further improvements are needed to meet the commercialization targets. Degradation in the cathode catalyst layer is the primary cause of performance loss, with both recoverable and irreversible losses occurring. One recoverable loss for PEMFCs includes cat...
ECS Meeting Abstracts, 2015
Corrosion of catalyst support-carbon during unprotected start-up and shut-down of H2-air polymer ... more Corrosion of catalyst support-carbon during unprotected start-up and shut-down of H2-air polymer electrolyte fuel cells (PEFC) is a serious concern. Many studies have investigated the underlying mechanisms of current reversals that occur as H2 sweeps the air in the anode compartment during start-up and, conversely, as air sweeps the H2 in anode during shut-down. Carbon corrodes during these transients as high interfacial potentials exceeding 1.5 V develop in the air/air portion of the cell. Protective measures have been proposed to limit these high interfacial potentials and to limit the exposure time to high potentials. Material-based mitigation strategies have also been proposed that include suppressing the oxygen reduction reaction (ORR) on anode and promoting the oxygen evolution reaction (OER) on cathode. Catalyst support-carbon can also corrode below the open-circuit potentials typical of normal operation of automotive fuel cells. Figure 1a is a typical trace of the measured c...
![Research paper thumbnail of Erratum: Carbon Corrosion in PEM Fuel Cells and the Development of Accelerated Stress Tests [J. Electrochem. Soc., 165, F3148 (2018)]](https://attachments.academia-assets.com/97882412/thumbnails/1.jpg)
](Journal of The Electrochemical Society, 2018
Journal of Power Sources, 2018
• Summarizes components and manufacturing assumptions for an 80 kW net PEMFC system.
ECS Transactions, 2013
An important step in achieving fundamental understanding of fuel cell failure mechanisms and deve... more An important step in achieving fundamental understanding of fuel cell failure mechanisms and development of technology to mitigate these failures is accomplished by analysis of directed lifetime and failure test results. Several lifetime, accelerated stress, and drive cycle test protocols have been developed and carried out. The two major ASTs that have been developed to evaluate membrane degradation are 1) Open circuit voltage tests, which are designed to accelerate chemical degradation, and 2) Relative humidity cycling tests, which are designed to accelerate mechanical degradation. The results from these tests have been compared to field tests. The ultimate goal is to use the laboratory tests to predict data in the field. An overall predictive decay model is being developed through a combination of specific modeling and tests.
ABSTRACT A steady state, two-dimensional MHD generator code, GEN, is presented. The code solves t... more ABSTRACT A steady state, two-dimensional MHD generator code, GEN, is presented. The code solves the equations of conservation of mass, momentum, and energy, using a Von Mises transformation and a local linearization of the equations. By splitting the source terms into a part proportional to the axial pressure gradient and a part independent of the gradient, the pressure distribution along the channel is easily obtained to satisfy various criteria. Thus, the code can run effectively in both design modes, where the channel geometry is determined, and analysis modes, where the geometry is previously known. The code also employs a mixing length concept for turbulent flows, Cebeci and Chang's wall roughness model, and an extension of that model to the effective thermal diffusities. Results on code validation, as well as comparisons of skin friction and Stanton number calculations with experimental results, are presented.
ABSTRACT Flow, heat and mass transfer processes in the radiant boilers of magnetohydynamic power ... more ABSTRACT Flow, heat and mass transfer processes in the radiant boilers of magnetohydynamic power plants are analyzed. Flow field in the radiant boiler is simulated by assuming a uniformly mixed zone to exist in the entrance region, followed by a developing boundary layer region. Heat transfer is presumed to occur by both turbulent convection and radiation. The radiation model includes the simultaneous contribution to heat transfer of carbon dioxide, water vapor, potassium atoms, and slag particles which are allowed to absorb as well as scatter thermal radiation. The absorption coefficients of gaseous species are determined from band correlations and experimental data. The extinction and scattering coefficients of slag particles are computed directly from Mie theory. For computing heat transfer, radiation transport equation is solved on a spectral basis. The complete flow, heat and mass transfer model is used to study the thermal characteristics of the radiant boilers. Impacts of slag-refractory interface temperature on corrosion and erosion of refractory, and of slag layer thickness on plant start-up time are discussed. By presenting the scale-up of heat transfer with refractory thickness and boiler diameter, the factors involved in designing an experimental facility for simulating base load boilers are highlighted. The temperature history computed from the heat transfer model is used in an extant chemical kinetics code to determine the practical levels to which NO/sub x/ can be decomposed in MHD radiant boilers. Calculations indicate that the EPA specified limit on NO/sub x/ levels can be met in properly designed radiant boilers operating with sub-stoichiometric products of combustion. The potential of the complete model to serve as a strong analytical tool in selecting an optimum geometry for radiant boilers is stressed by proposing an optimization procedure. (WHK)
Meeting abstracts, Oct 9, 2022
ECS Meeting Abstracts
Fuel cells for heavy-duty vehicles (HDVs) have attracted considerable attention because of their ... more Fuel cells for heavy-duty vehicles (HDVs) have attracted considerable attention because of their unique scalability, better fuel economy, and less demand for hydrogen refilling infrastructure relative to the light-duty vehicle application.1 However, the HDV application requires more stringent fuel cell durability, up to 25,000 h or 1 million miles of operation and increased efficiency. 1-2 Cathodes with relatively high Pt loading (~0.3 mg-Pt/cm2) have drawn extensive attention over initially highly active but unstable Pt-transition metal (Pt-TM) alloy3 catalysts owing to their capability of delivering high performance over long HDV lifetime. To fully exploit the potential of Pt in the HDV cathodes, new insights into the relationship between different drive cycles and operating conditions of HDVs and durability are needed with a particular emphasis on understanding the mechanisms for dissolution of Pt. Several mechanisms for dissolution of platinum have been proposed including direct...
U.S. Department of Energy Office of Scientific and Technical Information - OSTI OAI, Nov 9, 2018
U.S. Department of Energy Office of Scientific and Technical Information - OSTI OAI, Apr 15, 2022
Volume 3: Coal, Biomass and Alternative Fuels; Combustion and Fuels; Oil and Gas Applications; Cycle Innovations, 1988
A mass transfer model is developed that considers diffusive and chemical aspects of sodium sulfat... more A mass transfer model is developed that considers diffusive and chemical aspects of sodium sulfate formation and deposition on cooled blades of coal-fired gas turbines. The roles of gas phase condensation of sodium sulfate and multicomponent diffusion across a chemically frozen thin boundary layer are elaborated. A rational procedure is presented for correlating material wastage with laboratory weight gain data obtained by exposing alloy specimens pre-coated with a thin film of salt to SO2-SO3 in an oxygen environment. The sodium sulfate mass transfer model is used in conjunction with the correlation to project blade corrosion and lifetime as a function of gas turbine inlet temperature, blade cooling, and sodium and sulfur contaminant concentration.
IECEC 96. Proceedings of the 31st Intersociety Energy Conversion Engineering Conference
Analytic& models have been developed to study the dynamic response of steam-reformed, methanol-fu... more Analytic& models have been developed to study the dynamic response of steam-reformed, methanol-fueled, polymer electrolyte fuel cell (PEFC) systems for transportation applications. Attention is focused primarily on the heat transfer effects which are likely to limit rapid response of PEFC systems. Depending on the thermal mass, the heat exchangers and the steam reformer can have time constants of the order of several seconds to many minutes. On the other hand, the characteristic time constants associated with pressddensity disturbances arising from flow rate fluctuations are of the order of milliseconds. In vehicular applications, the response time of the turbomachinery, which is determined by the rotational inertia, can be of the order of seconds or less. The dynamic reformer model has been used to examine the methanol conversion efficiency and the thermal performance during a cold start. Response times are determined to achieve 50-100% of the steady-state methanol conversion for two catalyst tube diameters. The thermal performance is considered in terms of the approach to steady-state temperature., posslbility of catalyst overheating, and the penalty in system efficiency incurred during the start-up time. For the complete reference PEFC system various turn-down scenarios were simulated by varying the relative rates of change of fuel cell loading and system flows. It is shown that depending on the relative rates of cell loading changes to flow rate changes overheating of the catalyst can occur due to excess heat transfer in the reformer preheater. This can be controlled by an additional water quench between the catalyst bed and preheater, but only if the flow rate change is sufficiently fast relative to the load changes.
Mechanistic models have been developed for particle and vapor deposition on the blades of coal-fi... more Mechanistic models have been developed for particle and vapor deposition on the blades of coal-fired gas turbines. The particle deposition models include the simultaneous contribution of Brownian and turbulent diffusion, thermophoresis, eddy impaction, and inertial impingement. The diffusive mechanisms have been validated against experimental data for low-speed cascade flow and particle-laden flow through pipes. The inertial deposition treatment is shown to collapse to the well-known expression for particle capture in a flow turning around a bend. A method is presented for calculating Na/sub 2/SO/sub 4/ and K/sub 2/SO/sub 4/ vapor deposition on cooled blades. Scaling laws are formulated for estimating the contribution of boundary layer homogeneous and heterogeneous nucleation mechanism for highly cooled turbine blades.
ECS Meeting Abstracts, 2021
ECS Meeting Abstracts, 2020
Polymer electrolyte membrane fuel cell (PEMFC) systems for heavy-duty vehicles (HDV) are attracti... more Polymer electrolyte membrane fuel cell (PEMFC) systems for heavy-duty vehicles (HDV) are attracting attention for several reasons including tighter emission regulations and, compared with light duty vehicles (LDVs), possibility of a gradual build-up of hydrogen infrastructure during the market introduction phase, and higher allowable cost (60 /kWevs.30/kWe vs. 30 /kWevs.30/kWe) and Pt loadings.1 However, competing with the incumbent diesel technology requires longer stack lifetimes than for LDVs (30,000 h vs. 8,000 h) and higher efficiencies to compensate for the likely price differential between diesel and hydrogen.2 Heat rejection in fuel cells for HDVs can also be an issue. We conducted simulations to compare radiator heat loads in two propulsion systems for a Class-8 HDV: a 450-hp turbocharged diesel engine, and a hybrid 275-kWe FCS with a 35-kWh battery. The simulations showed that the radiator heat load at rated power is higher for the hybrid propulsion system even though it is 20% more effic...
ABSTRACT A comprehensive study is conducted into the analysis of a coal-fired Component Developme... more ABSTRACT A comprehensive study is conducted into the analysis of a coal-fired Component Development and Integration Facility (CDIF) 1-B channel performance to simulate the characteristics of the reference Engineering Test Facility (ETF) channel. The performance criteria are established by careful review of the role played by the gas dynamic and electrical variables in determining channel behavior. On the basis of this review, a constant-velocity subsonic channel operating at nominal combustor pressure of 5.4 atm (0.54 MPa) is selected. The channel loading is selected to obtain a maximum Hall field of 2.4 kV/m and maximum transverse current density of 1.1 A/sq cm.
ECS Meeting Abstracts, 2019
We have investigated the mechanism of increase in oxygen transport resistance (Rm) due to aging o... more We have investigated the mechanism of increase in oxygen transport resistance (Rm) due to aging of electrodes with low-loaded de-alloyed PtCo cathode catalysts supported on high surface area carbon (d-PtCo/C). Commercially available membrane electrode assemblies with 0.05, 0.1 and 0.15 mg/cm2 Pt loadings in d-PtCo/C cathode catalysts were assembled into cells and subjected to accelerated stress tests (ASTs) that consisted of 0.6-0.95 V square wave potentials with 3-s hold at upper and lower potential limits. Catalyst degradation was characterized by measuring mass activity for the oxygen reduction reaction, electrochemically active surface area (ECSA) and polarization performance in H2/air at beginning of test (BOT) and after 15k and 30k (EOT) potential cycles in H2/N2. Concurrently, limiting current densities (iL) were measured for different pressures (1.3, 1.8, 2.3, 2.8 and 3.2 atm) and oxygen concentrations (0.01, 0.02 and 0.04 O2 mole fraction in dry air) at 80oC and 90% relativ...
ECS Meeting Abstracts, 2018
The primary PEMFC catalyst layer architecture used since the early 1990’s is a porous dispersed P... more The primary PEMFC catalyst layer architecture used since the early 1990’s is a porous dispersed Pt-supported carbon catalyst with recast solid ionomer component forming a continuous (percolating) network through the electrode for proton transport; the carbon facilitating the electrical conduction; the active electrochemical reactions conducted on the metallic Pt sites [1,2]. The thin catalyst layer consists of microscale carbon particles each supporting nanoscale platinum catalyst particles all loosely embedded in a matrix of ionomer creating a porous structure. The fuel cell catalyst layer is a complex structure that facilitates the electrochemical conversion of hydrogen and oxygen; it provides pathways for reactant transport, provides both electrical and proton conductivity pathways. This has been traditionally referred to as the 3-phase boundary of an electrolyte, an electrode, and a gaseous fuel. In addition to the gas-reactant transport to the active site, the result product wa...
ECS Meeting Abstracts, 2017
FC-PAD (Fuel Cell – Performance and Durability) consortium coordinates U.S. national laboratory a... more FC-PAD (Fuel Cell – Performance and Durability) consortium coordinates U.S. national laboratory activities related to PEM fuel cell performance and durability. The cost and durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are two major barriers to the commercialization of these systems for transportation power applications. Transportation conditions include operation in the presence of fuel and air impurities, start/stop, freeze/thaw, and humidity and load cycling that result in mechanical and chemical stresses. MEA durability decreases with decreasing catalyst loading, making cost reduction even more difficult. While there has been significant progress in improving PEM Fuel Cell durability with lower cost materials, further improvements are needed to meet the commercialization targets. Degradation in the cathode catalyst layer is the primary cause of performance loss, with both recoverable and irreversible losses occurring. One recoverable loss for PEMFCs includes cat...
ECS Meeting Abstracts, 2015
Corrosion of catalyst support-carbon during unprotected start-up and shut-down of H2-air polymer ... more Corrosion of catalyst support-carbon during unprotected start-up and shut-down of H2-air polymer electrolyte fuel cells (PEFC) is a serious concern. Many studies have investigated the underlying mechanisms of current reversals that occur as H2 sweeps the air in the anode compartment during start-up and, conversely, as air sweeps the H2 in anode during shut-down. Carbon corrodes during these transients as high interfacial potentials exceeding 1.5 V develop in the air/air portion of the cell. Protective measures have been proposed to limit these high interfacial potentials and to limit the exposure time to high potentials. Material-based mitigation strategies have also been proposed that include suppressing the oxygen reduction reaction (ORR) on anode and promoting the oxygen evolution reaction (OER) on cathode. Catalyst support-carbon can also corrode below the open-circuit potentials typical of normal operation of automotive fuel cells. Figure 1a is a typical trace of the measured c...
Journal of The Electrochemical Society, 2018
Journal of Power Sources, 2018
• Summarizes components and manufacturing assumptions for an 80 kW net PEMFC system.
ECS Transactions, 2013
An important step in achieving fundamental understanding of fuel cell failure mechanisms and deve... more An important step in achieving fundamental understanding of fuel cell failure mechanisms and development of technology to mitigate these failures is accomplished by analysis of directed lifetime and failure test results. Several lifetime, accelerated stress, and drive cycle test protocols have been developed and carried out. The two major ASTs that have been developed to evaluate membrane degradation are 1) Open circuit voltage tests, which are designed to accelerate chemical degradation, and 2) Relative humidity cycling tests, which are designed to accelerate mechanical degradation. The results from these tests have been compared to field tests. The ultimate goal is to use the laboratory tests to predict data in the field. An overall predictive decay model is being developed through a combination of specific modeling and tests.
ABSTRACT A steady state, two-dimensional MHD generator code, GEN, is presented. The code solves t... more ABSTRACT A steady state, two-dimensional MHD generator code, GEN, is presented. The code solves the equations of conservation of mass, momentum, and energy, using a Von Mises transformation and a local linearization of the equations. By splitting the source terms into a part proportional to the axial pressure gradient and a part independent of the gradient, the pressure distribution along the channel is easily obtained to satisfy various criteria. Thus, the code can run effectively in both design modes, where the channel geometry is determined, and analysis modes, where the geometry is previously known. The code also employs a mixing length concept for turbulent flows, Cebeci and Chang's wall roughness model, and an extension of that model to the effective thermal diffusities. Results on code validation, as well as comparisons of skin friction and Stanton number calculations with experimental results, are presented.
ABSTRACT Flow, heat and mass transfer processes in the radiant boilers of magnetohydynamic power ... more ABSTRACT Flow, heat and mass transfer processes in the radiant boilers of magnetohydynamic power plants are analyzed. Flow field in the radiant boiler is simulated by assuming a uniformly mixed zone to exist in the entrance region, followed by a developing boundary layer region. Heat transfer is presumed to occur by both turbulent convection and radiation. The radiation model includes the simultaneous contribution to heat transfer of carbon dioxide, water vapor, potassium atoms, and slag particles which are allowed to absorb as well as scatter thermal radiation. The absorption coefficients of gaseous species are determined from band correlations and experimental data. The extinction and scattering coefficients of slag particles are computed directly from Mie theory. For computing heat transfer, radiation transport equation is solved on a spectral basis. The complete flow, heat and mass transfer model is used to study the thermal characteristics of the radiant boilers. Impacts of slag-refractory interface temperature on corrosion and erosion of refractory, and of slag layer thickness on plant start-up time are discussed. By presenting the scale-up of heat transfer with refractory thickness and boiler diameter, the factors involved in designing an experimental facility for simulating base load boilers are highlighted. The temperature history computed from the heat transfer model is used in an extant chemical kinetics code to determine the practical levels to which NO/sub x/ can be decomposed in MHD radiant boilers. Calculations indicate that the EPA specified limit on NO/sub x/ levels can be met in properly designed radiant boilers operating with sub-stoichiometric products of combustion. The potential of the complete model to serve as a strong analytical tool in selecting an optimum geometry for radiant boilers is stressed by proposing an optimization procedure. (WHK)