Reforming by using chemical looping concept is an innovative and recent technology which is used ... more Reforming by using chemical looping concept is an innovative and recent technology which is used to convert the hydrocarbon fuels into hydrogen (H 2). The overall process produces separate streams of H 2 and CO 2. The present study conducts a techno-economic assessment of a three reactor chemical looping reforming plant. The cost data are used for the plant's location to be in the United Arab Emirates. The effects of key operating parameters on the technical performance of the plant are investigated. In addition, the thermodynamic data from the model are also used to perform an economic assessment for calculating the cost of H 2 produced. The cost of H 2 production with CO 2 capture is 1.679 $/ kg which is significantly low when compared to the case of steam methane reforming with CO 2 capture (about 2.39 $/kg). Furthermore, the cost of H 2 production in non-capture scenario is 1.404 $/kg which is comparable with the cost of H 2 production from steam methane reforming technology.
The dynamic performance of the PEM fuel cell is one of the key factors for successful operation o... more The dynamic performance of the PEM fuel cell is one of the key factors for successful operation of a fuel cell-powered vehicle. Maintaining fast time response while keeping stable and high stack performance is of importance, especially during acceleration and deceleration. In this paper, we evaluate the transient response of a PEM fuel cell stack with a dead-end anode during segments of a legislated European driving cycle together with the effect of purging factors. The PEM fuel cell stack comprises of 24 cells with a 300 cm 2 active catalyst area and operates at a low hydrogen and air pressure. Humidified air is supplied to the cathode side and the dry hydrogen is fed to the anode. The liquid coolant is circulated to the stack and the radiator to maintain the thermal envelope throughout the stack. The stack performance deterioration over time is prevented by utilizing the purging, which removes the accumulated water and impurities. The effect of purging period, purging duration, coolant flow rate and cathode stoichio-metry are examined with regard to the fuel cell's transient performance during the driving cycle. The results show that a low purging duration may avoid the undesired deceleration at a high current, and a high purging period may sustain a better performance over time. Moreover, the coolant flow rate is found to be an important parameter, which affects the stack temperature–time response of the cooling control and the stack performance, especially at high operating currents.
Effect of curvature on the performance of a micro-combustor is evaluated. Combustor wall temperat... more Effect of curvature on the performance of a micro-combustor is evaluated. Combustor wall temperature is enhanced by the curvature. Higher energy conversion efficiency is observed for curved combustors. Flame instability is observed in curved combustors. Higher pumping power is required for curved combustors. g r a p h i c a l a b s t r a c t a b s t r a c t Energy conversion efficiency of a thermo photo voltaic (TPV) system strongly depends on the wall temperature and its uniformity. Therefore, improving heat transfer characteristics of these systems has been a focus of many recent studies. This study explores the effect of curvature on heat transfer, overall energy conversion, flame stability and emission levels for a circular micro-combustor with a sudden expansion (backward facing step). Uniformity index and figure of merit (FoM) have been defined to facilitate the analysis of computational results from a three-dimensional turbulent reaction model for curved microcombustor. The results indicate an enhancement of 110 K in the outer wall temperature and a 7.84% maximum increase in overall energy conversion efficiency for curved channels relative to the straight channels. The flame structure in curved channels, however, is found to be more susceptible to instability such as flame flashback. The model indicates a maximum increase of about 2.5 m/s in the lower limit of the inlet mixture velocity for safe combustion operation in curved ducts.
Heat transfers of helical coiled tube with several cross section profiles are evaluated. Helical ... more Heat transfers of helical coiled tube with several cross section profiles are evaluated. Helical tubes offer higher heat transfer and lower entropy generation. Square cross-section generates the highest entropy, followed by ellipse and circular. Study could serve as a guideline in designing an efficient helical tube heat exchanger. a b s t r a c t This study evaluates heat transfer performance and entropy generation of laminar flow in coiled tubes with various cross-sections geometries i.e. circular, ellipse and square, relatives to the straight tubes of similar cross-sections. A computational fluid dynamics model is developed and validated against empirical correlations. Good agreement is obtained within range of Reynolds and Dean numbers considered. Effect of geometry, wall temperature, Reynolds number and heating/cooling mode were examined. To evaluate the heat transfer performance of the coiled tube configurations, a parameter referred as Figure of Merit (FoM) is defined as the ratio heat transfer rate to the required pumping power. In addition, exergy analysis is carried out to examine the inefficiency of the coiled tube configurations. The results indicate that coiled tubes provide higher heat transfer rate. In addition, it was found to be more efficient as reflected by lower entropy generation as compared to straight tubes. Among the studied cross-section, square cross-section generates the highest entropy, followed by ellipse and circular counterpart. Entropy production from heat transfer contribution is two order-of-magnitude higher than that of entropy contribution from viscous dissipation. Cooling case produces slightly higher entropy than heating counterpart. Finally, this study can provide practical guideline to design more efficient coiled heat exchanger. Crown
Design and operating parameters of an open-cathode PEFC stack is optimized. Optimization is carri... more Design and operating parameters of an open-cathode PEFC stack is optimized. Optimization is carried out with regard to current density and net power. Fuel cell length plays important role on stack performance. Interaction between each parameter is evaluated. a b s t r a c t The design of open-cathode polymer electrolyte fuel cells (PEFC) stacks with forced-convection requires a careful consideration on the geometrical and operating conditions as well as the operating characteristic of PEFC stacks and fan used. This paper evaluates the effect of key geometrical and operating parameters on the stack characteristic and their interactions to the thermal, water and gas managements as well as stack performance. A validated three dimensional model for open-cathode PEFC stack with fan and immediate ambient were solved to evaluate the effect of studied parameters on the stack performance. In tandem, an L 27 orthogonal array (OA) of Taguchi matrix of six factors and three level designs to determine the optimum combination of parameters as well as their interactions for high, medium and low voltage operation. The result indicates that fuel cell length plays important role on determining the fuel cell performance in term of system characteristic, current density and net power. Optimum combination of design and operating parameters were obtained with the objective function of maximizing net power generated by stack by taking into account the parasitic loads. Crown
The three operating parameters – temperature, stoichiometry and the degree of humidification – co... more The three operating parameters – temperature, stoichiometry and the degree of humidification – constitute key factors required to ensure high performance of the solid oxide fuel cell (SOFC). A careful trade-off between performance and parasitic loads is required in order to optimize the output. The present study numerically analyzes the influence of the key operating parameters on the performance of planar type of SOFC and parasitic loads utilizing a validated three dimensional mathematical model which takes into account of the conservation of mass, momentum, species and charge. The numerical results indicate that the cell performance is enhanced at high temperatures and cathode stoichiometry and it declines with increasing cathode relative humidity. Furthermore, cathode stoichiometry is found to have higher influence on the cell performance as compared to the anode stoichiometry. The gain in cell performance however , has to be balanced with the changing parasitic load requirement from pumping, humidification and heating. The results presented herein can assist in the selection of optimum or near-to-optimum operating parameters for high performance planar type SOFC.
High velocity oxygen fuel (HVOF) thermal spray systems are being used to apply coatings to preven... more High velocity oxygen fuel (HVOF) thermal spray systems are being used to apply coatings to prevent surface degradation. The coatings of temperature sensitive materials such as titanium and copper, which have very low melting points, cannot be applied using a single-stage HVOF system. Therefore, a dual-stage HVOF system has been introduced and modeled computationally. The dual-spray system provides an easy control of particle oxidation by introducing a mixing chamber. In addition to the materials being sprayed, the thermal spray coating quality depends to a large extent on flow behavior of reacting gases and the particle dynamics. The present study investigates the influence of various operating parameters on the performance of a dual-stage thermal spray gun. The objective is to develop a predictive understanding of various parameters. The gas flow field and the free jet are modeled by considering the conservation of mass, momentum, and energy with the turbulence and the equilibrium combustion sub models. The particle phase is decoupled from the gas phase due to very low particle volume fractions. The results demonstrate the advantage of a dual-stage system over a single-stage system especially for the deposition of temperature sensitive materials.
Computational study of a dual-stage combustion assisted spray system for coatings. Dual-stage the... more Computational study of a dual-stage combustion assisted spray system for coatings. Dual-stage thermal spray system can be used for temperature sensitive materials. Combustion chamber dimensions affect the particle velocity and temperature. The particle temperature can be controlled by varying the mixing chamber length. a b s t r a c t The high velocity oxygen fuel (HVOF) thermal spray coatings are used to protect the surfaces from deterioration. The base material surface properties can be modified to achieve the longevity of the product. Besides spraying material, the coating quality depends greatly on the gas and particle dynamics. The coating quality is also affected by the particle temperature, particularly for the materials which are temperature sensitive such as titanium and copper. As the gas phase temperature is high, the material gets melted and oxidized before it reaches the substrate. To avoid this problem, a dual-stage thermal spray system, has been developed for coating temperature sensitive materials. This process involves making coatings by high velocity impact of powder particles heated to temperatures below their melting point. The advantages of a dual-stage thermal spray process include an easy control of particle oxidation and production of various coating structures by controlling the particle velocity and temperature. The particle temperature can be controlled by varying the coolant flow rate in the mixing chamber. The present study investigates the effect of various geometric parameters of a dual-stage thermal spray system by developing a comprehensive mathematical model. The objective is to develop a predictive understanding of various design parameters. Conservation of mass, momentum and energy of reacting gases were taken into account in developing the model. Due to low particle loading, the particle phase was decoupled from the gas phase. The results demonstrate the advantage of dual stage system over single stage system especially for the deposition of temperature sensitive materials.
h i g h l i g h t s Experimental and computational study of PEM fuel cell stack with a dead-end a... more h i g h l i g h t s Experimental and computational study of PEM fuel cell stack with a dead-end anode. Investigation of the fuel cell performance, gas and water management. Gas and water management is better under low operating current conditions. Lower cathode stoichiometry is preferred to minimize nitrogen crossover. Anode purging is recommended to clear out the impurities for longer operations. a b s t r a c t The operation of polymer electrolyte membrane fuel cell (PEMFC) stack with a dead-end anode requires careful consideration on the gas and water management. Water accumulation at the anode and the nitrogen crossover from cathode to anode lead to performance deterioration over time. The accumulated water and nitrogen need to be removed properly by purging method to ensure good and stable stack performance. Thus, the careful selection of the operating parameters – inlet humidification, stoichiometry, and operating current – is the key factor for ensuring efficient water and gas management. This study aims at the experimental and numerical evaluation of the effect of the key operating parameters on the transient performance of a dead-end anode fuel cell stack. The experiments were carried out on a stack with 24 cells and a catalyst active area of 300 cm 2. By employing a validated transient two-phase mathematical model of a PEMFC with a dead-end anode, numerical simulations were performed which yield a better and deeper understanding of local distribution of water and species, i.e., hydrogen, oxygen, water vapor and nitrogen. The results suggest that the performance deterioration over time is closely related to the choice of the operating conditions. The study reveals that the anode and cathode inlet conditions become a limiting factor for the stack performance. Liquid accumulation at the anode is found to be strongly related to the inlet humidification as well as water transport across the membrane, whereas the cathode stoichiom-etry affects the nitrogen crossover.
Modeling can assist in achieving better understanding of various complex physicochemical processe... more Modeling can assist in achieving better understanding of various complex physicochemical processes occurring in fuel cells, which is critical in improving the fuel cell performance and making them more cost effective. Modeling efforts in PEM fuel cell area have been focused on developing both single and multidimensional (2D and 3D) PEM fuel cell models. The higher dimensional models include more realistic and accurate descriptions of the fuel cell processes; however, they also involve more complexity and require considerably extensive computational resources. Hence, despite the availability of higher dimensional fuel cell models, the lower dimensional models still retain their relevance and are being extensively used. Past studies commented on the effect of multidimensionality by comparing the results of higher and lower dimensional models which had differences in fuel cell geometry, operating conditions , modeling assumptions and properties. Owing to these differences between models, the difference in their results could not be solely attributed to the effect of multidimensionality. The present study was motivated by recognizing this gap in literature. The multidimensional effect is analyzed by developing two similar steady state 2D and 3D models in COMSOL. Both of these models have similar geometry and are simulated under similar operating conditions. The effect of multidimensionality on species concentration is investigated at various inlet stoichiometries, membrane conductivities and relative humidity values. The multidimensional effect, as represented by difference in species distribution in the two models, is found to be significant at lower operating voltages and more prominent at the cathode side. The inlet stoichiometry at the cathode and membrane conductivity values, are also found to influence the multidimensionality effects.
Automotive catalytic converters, which are employed to reduce engine exhaust emissions, are subje... more Automotive catalytic converters, which are employed to reduce engine exhaust emissions, are subjected to highly transient conditions during a typical driving cycle. These transient conditions arise from changes in driving mode, the hysteresis and flow lags of the feedback control system, and result in fluctuations of air-fuel ratio, exhaust gas flow rates and temperatures. The catalyst performance is also strongly influenced by the oxygen storage capacity. This paper presents a computational investigation of the effect of oxygen storage capacity on the dynamic behavior of an automotive catalytic converter subjected to modulations in exhaust gases. The modulations are generated by forcing the temporal variations in exhaust gases air-fuel ratio, gas flow rates and temperatures. The study employs a single-channel based, one-dimensional, non-adiabatic model. The results show that the imposed modulations cause a significant departure in the catalyst behavior from its steady behavior, and the oxygen storage capacity plays an important role in determining the catalyst's response to the imposed modulations. Modulations and oxygen storage capacity are found to have relatively greater influence on the catalyst's performance near stoichiometric conditions.
This paper presents a computational investigation of the effect of composition modulations on an ... more This paper presents a computational investigation of the effect of composition modulations on an automotive catalytic converter. The objective is to develop a better fundamental understanding of the converter's performance under actual driving conditions. Such an understanding will be beneficial in devising improved emission control methodologies by exploiting the catalyst transient behavior. The study employs a single-channel based, one-dimensional, non-adiabatic model. Two types of imposed transients (sinusoidal and step changes) are considered. The results show that composition modulations cause a significant departure in the catalyst behavior from its steady behavior, and modulations have both favorable and harmful effects on pollutant conversion.
This paper numerically investigates the role of heat and mass transfer coefficients on the perfor... more This paper numerically investigates the role of heat and mass transfer coefficients on the performance of automotive catalytic converters, which are employed to reduce engine exhaust emissions. The pollutant conversion performance of a converter is influenced by a number of physical and chemical processes that take place in gaseous and solid phases as the exhaust gases flow through the catalyst. A quantitative predictive understanding of these complex catalyst processes involving flow dynamics, heterogeneous surface reactions, and heat and mass transport mechanisms is important in improving the converter design. The role of convective transport phenomena becomes important at high temperature when the mass transfer becomes rate limiting to an increasing extent. The objective of the present study is to elucidate the influence of convective heat and mass transfer mechanisms. The mathematical model considers the conservation of mass, momentum and energy in both gaseous and solid phases. In addition to the heterogeneous surface reactions, the 2 model also takes into account the adsorption/desorption of oxygen in the catalyst during non-stoichiometric composition of air-fuel mixtures. The governing equations are solved by an implicit scheme using a successive line under relaxation method. The converter performance under both the steady state and the transient conditions is analyzed. Key Words engine emissions, engine exhaust after-treatment, heat and mass transfer, numerical simulations. Nomenclature C g j gas phase concentration of species j, mol/m 3 C s j surface concentration of species j, mol/m 3 c pg specific heat of gas, J/kg·K c ps specific heat of substrate, J/kg·K D h hydraulic diameter, m D j diffusion coefficient of species j, m 2 /s G a geometric surface area, m 2 /m 3 ∆H k heat of reaction of species k, J/mol h g heat transfer coefficient between flow and substrate, J/m 2 ·s·K h ∞ heat transfer coefficient between substrate and atmosphere, J/m 2 ·s·K k m j mass transfer coefficient for species j, m/s L catalyst length, m Nu Nusselt number Pr Prandtl number
Journal of Engineering for Gas Turbines and Power-transactions of The Asme, 2002
... Pattas et al. used a simple five-step reaction scheme and a one-dimensional model to predict ... more ... Pattas et al. used a simple five-step reaction scheme and a one-dimensional model to predict the catalytic converter performance during a regulated European driving cycle, ECE-15. ... However, they did not report comparison of time-resolved emissions for HC and NO. ...
The combustion assisted thermal spray systems are being used to apply coatings to prevent surface... more The combustion assisted thermal spray systems are being used to apply coatings to prevent surface degradation. They offer a highly attractive way to modify the surface properties of the substrate to extend the product life. In addition to the materials being sprayed, the quality of combustion assisted thermal spray coating depends greatly on the flow behavior of reacting gases and particle dynamics. The present study investigates the effect of gas phase and its interaction with particles through the nozzle of a thermal spray gun by developing a comprehensive mathematical model. The objective is to develop a predictive understanding of various design parameters of combustion assisted thermal spray systems. The model was developed by considering the conservation of mass, momentum and energy of reacting gases.
This paper presents a computational investigation of the effect of exhaust temperature modulation... more This paper presents a computational investigation of the effect of exhaust temperature modulations on an automotive catalytic converter. The objective is to develop a better fundamental understanding of the converter's performance under transient driving conditions. Such an understanding will be beneficial in devising improved emission control methodologies. The study employs a single-channel based, one-dimensional, non-adiabatic model. The transient conditions are imposed by varying the exhaust gas temperature sinusoidally. The results show that temperature modulations cause a significant departure in the catalyst behavior from its steady behavior, and modulations have both favorable and harmful effects on pollutant conversion. The operating conditions and the modulating gas composition and flow rates (space velocity) have substantial influence on catalyst behavior.
Reforming by using chemical looping concept is an innovative and recent technology which is used ... more Reforming by using chemical looping concept is an innovative and recent technology which is used to convert the hydrocarbon fuels into hydrogen (H 2). The overall process produces separate streams of H 2 and CO 2. The present study conducts a techno-economic assessment of a three reactor chemical looping reforming plant. The cost data are used for the plant's location to be in the United Arab Emirates. The effects of key operating parameters on the technical performance of the plant are investigated. In addition, the thermodynamic data from the model are also used to perform an economic assessment for calculating the cost of H 2 produced. The cost of H 2 production with CO 2 capture is 1.679 $/ kg which is significantly low when compared to the case of steam methane reforming with CO 2 capture (about 2.39 $/kg). Furthermore, the cost of H 2 production in non-capture scenario is 1.404 $/kg which is comparable with the cost of H 2 production from steam methane reforming technology.
The dynamic performance of the PEM fuel cell is one of the key factors for successful operation o... more The dynamic performance of the PEM fuel cell is one of the key factors for successful operation of a fuel cell-powered vehicle. Maintaining fast time response while keeping stable and high stack performance is of importance, especially during acceleration and deceleration. In this paper, we evaluate the transient response of a PEM fuel cell stack with a dead-end anode during segments of a legislated European driving cycle together with the effect of purging factors. The PEM fuel cell stack comprises of 24 cells with a 300 cm 2 active catalyst area and operates at a low hydrogen and air pressure. Humidified air is supplied to the cathode side and the dry hydrogen is fed to the anode. The liquid coolant is circulated to the stack and the radiator to maintain the thermal envelope throughout the stack. The stack performance deterioration over time is prevented by utilizing the purging, which removes the accumulated water and impurities. The effect of purging period, purging duration, coolant flow rate and cathode stoichio-metry are examined with regard to the fuel cell's transient performance during the driving cycle. The results show that a low purging duration may avoid the undesired deceleration at a high current, and a high purging period may sustain a better performance over time. Moreover, the coolant flow rate is found to be an important parameter, which affects the stack temperature–time response of the cooling control and the stack performance, especially at high operating currents.
Effect of curvature on the performance of a micro-combustor is evaluated. Combustor wall temperat... more Effect of curvature on the performance of a micro-combustor is evaluated. Combustor wall temperature is enhanced by the curvature. Higher energy conversion efficiency is observed for curved combustors. Flame instability is observed in curved combustors. Higher pumping power is required for curved combustors. g r a p h i c a l a b s t r a c t a b s t r a c t Energy conversion efficiency of a thermo photo voltaic (TPV) system strongly depends on the wall temperature and its uniformity. Therefore, improving heat transfer characteristics of these systems has been a focus of many recent studies. This study explores the effect of curvature on heat transfer, overall energy conversion, flame stability and emission levels for a circular micro-combustor with a sudden expansion (backward facing step). Uniformity index and figure of merit (FoM) have been defined to facilitate the analysis of computational results from a three-dimensional turbulent reaction model for curved microcombustor. The results indicate an enhancement of 110 K in the outer wall temperature and a 7.84% maximum increase in overall energy conversion efficiency for curved channels relative to the straight channels. The flame structure in curved channels, however, is found to be more susceptible to instability such as flame flashback. The model indicates a maximum increase of about 2.5 m/s in the lower limit of the inlet mixture velocity for safe combustion operation in curved ducts.
Heat transfers of helical coiled tube with several cross section profiles are evaluated. Helical ... more Heat transfers of helical coiled tube with several cross section profiles are evaluated. Helical tubes offer higher heat transfer and lower entropy generation. Square cross-section generates the highest entropy, followed by ellipse and circular. Study could serve as a guideline in designing an efficient helical tube heat exchanger. a b s t r a c t This study evaluates heat transfer performance and entropy generation of laminar flow in coiled tubes with various cross-sections geometries i.e. circular, ellipse and square, relatives to the straight tubes of similar cross-sections. A computational fluid dynamics model is developed and validated against empirical correlations. Good agreement is obtained within range of Reynolds and Dean numbers considered. Effect of geometry, wall temperature, Reynolds number and heating/cooling mode were examined. To evaluate the heat transfer performance of the coiled tube configurations, a parameter referred as Figure of Merit (FoM) is defined as the ratio heat transfer rate to the required pumping power. In addition, exergy analysis is carried out to examine the inefficiency of the coiled tube configurations. The results indicate that coiled tubes provide higher heat transfer rate. In addition, it was found to be more efficient as reflected by lower entropy generation as compared to straight tubes. Among the studied cross-section, square cross-section generates the highest entropy, followed by ellipse and circular counterpart. Entropy production from heat transfer contribution is two order-of-magnitude higher than that of entropy contribution from viscous dissipation. Cooling case produces slightly higher entropy than heating counterpart. Finally, this study can provide practical guideline to design more efficient coiled heat exchanger. Crown
Design and operating parameters of an open-cathode PEFC stack is optimized. Optimization is carri... more Design and operating parameters of an open-cathode PEFC stack is optimized. Optimization is carried out with regard to current density and net power. Fuel cell length plays important role on stack performance. Interaction between each parameter is evaluated. a b s t r a c t The design of open-cathode polymer electrolyte fuel cells (PEFC) stacks with forced-convection requires a careful consideration on the geometrical and operating conditions as well as the operating characteristic of PEFC stacks and fan used. This paper evaluates the effect of key geometrical and operating parameters on the stack characteristic and their interactions to the thermal, water and gas managements as well as stack performance. A validated three dimensional model for open-cathode PEFC stack with fan and immediate ambient were solved to evaluate the effect of studied parameters on the stack performance. In tandem, an L 27 orthogonal array (OA) of Taguchi matrix of six factors and three level designs to determine the optimum combination of parameters as well as their interactions for high, medium and low voltage operation. The result indicates that fuel cell length plays important role on determining the fuel cell performance in term of system characteristic, current density and net power. Optimum combination of design and operating parameters were obtained with the objective function of maximizing net power generated by stack by taking into account the parasitic loads. Crown
The three operating parameters – temperature, stoichiometry and the degree of humidification – co... more The three operating parameters – temperature, stoichiometry and the degree of humidification – constitute key factors required to ensure high performance of the solid oxide fuel cell (SOFC). A careful trade-off between performance and parasitic loads is required in order to optimize the output. The present study numerically analyzes the influence of the key operating parameters on the performance of planar type of SOFC and parasitic loads utilizing a validated three dimensional mathematical model which takes into account of the conservation of mass, momentum, species and charge. The numerical results indicate that the cell performance is enhanced at high temperatures and cathode stoichiometry and it declines with increasing cathode relative humidity. Furthermore, cathode stoichiometry is found to have higher influence on the cell performance as compared to the anode stoichiometry. The gain in cell performance however , has to be balanced with the changing parasitic load requirement from pumping, humidification and heating. The results presented herein can assist in the selection of optimum or near-to-optimum operating parameters for high performance planar type SOFC.
High velocity oxygen fuel (HVOF) thermal spray systems are being used to apply coatings to preven... more High velocity oxygen fuel (HVOF) thermal spray systems are being used to apply coatings to prevent surface degradation. The coatings of temperature sensitive materials such as titanium and copper, which have very low melting points, cannot be applied using a single-stage HVOF system. Therefore, a dual-stage HVOF system has been introduced and modeled computationally. The dual-spray system provides an easy control of particle oxidation by introducing a mixing chamber. In addition to the materials being sprayed, the thermal spray coating quality depends to a large extent on flow behavior of reacting gases and the particle dynamics. The present study investigates the influence of various operating parameters on the performance of a dual-stage thermal spray gun. The objective is to develop a predictive understanding of various parameters. The gas flow field and the free jet are modeled by considering the conservation of mass, momentum, and energy with the turbulence and the equilibrium combustion sub models. The particle phase is decoupled from the gas phase due to very low particle volume fractions. The results demonstrate the advantage of a dual-stage system over a single-stage system especially for the deposition of temperature sensitive materials.
Computational study of a dual-stage combustion assisted spray system for coatings. Dual-stage the... more Computational study of a dual-stage combustion assisted spray system for coatings. Dual-stage thermal spray system can be used for temperature sensitive materials. Combustion chamber dimensions affect the particle velocity and temperature. The particle temperature can be controlled by varying the mixing chamber length. a b s t r a c t The high velocity oxygen fuel (HVOF) thermal spray coatings are used to protect the surfaces from deterioration. The base material surface properties can be modified to achieve the longevity of the product. Besides spraying material, the coating quality depends greatly on the gas and particle dynamics. The coating quality is also affected by the particle temperature, particularly for the materials which are temperature sensitive such as titanium and copper. As the gas phase temperature is high, the material gets melted and oxidized before it reaches the substrate. To avoid this problem, a dual-stage thermal spray system, has been developed for coating temperature sensitive materials. This process involves making coatings by high velocity impact of powder particles heated to temperatures below their melting point. The advantages of a dual-stage thermal spray process include an easy control of particle oxidation and production of various coating structures by controlling the particle velocity and temperature. The particle temperature can be controlled by varying the coolant flow rate in the mixing chamber. The present study investigates the effect of various geometric parameters of a dual-stage thermal spray system by developing a comprehensive mathematical model. The objective is to develop a predictive understanding of various design parameters. Conservation of mass, momentum and energy of reacting gases were taken into account in developing the model. Due to low particle loading, the particle phase was decoupled from the gas phase. The results demonstrate the advantage of dual stage system over single stage system especially for the deposition of temperature sensitive materials.
h i g h l i g h t s Experimental and computational study of PEM fuel cell stack with a dead-end a... more h i g h l i g h t s Experimental and computational study of PEM fuel cell stack with a dead-end anode. Investigation of the fuel cell performance, gas and water management. Gas and water management is better under low operating current conditions. Lower cathode stoichiometry is preferred to minimize nitrogen crossover. Anode purging is recommended to clear out the impurities for longer operations. a b s t r a c t The operation of polymer electrolyte membrane fuel cell (PEMFC) stack with a dead-end anode requires careful consideration on the gas and water management. Water accumulation at the anode and the nitrogen crossover from cathode to anode lead to performance deterioration over time. The accumulated water and nitrogen need to be removed properly by purging method to ensure good and stable stack performance. Thus, the careful selection of the operating parameters – inlet humidification, stoichiometry, and operating current – is the key factor for ensuring efficient water and gas management. This study aims at the experimental and numerical evaluation of the effect of the key operating parameters on the transient performance of a dead-end anode fuel cell stack. The experiments were carried out on a stack with 24 cells and a catalyst active area of 300 cm 2. By employing a validated transient two-phase mathematical model of a PEMFC with a dead-end anode, numerical simulations were performed which yield a better and deeper understanding of local distribution of water and species, i.e., hydrogen, oxygen, water vapor and nitrogen. The results suggest that the performance deterioration over time is closely related to the choice of the operating conditions. The study reveals that the anode and cathode inlet conditions become a limiting factor for the stack performance. Liquid accumulation at the anode is found to be strongly related to the inlet humidification as well as water transport across the membrane, whereas the cathode stoichiom-etry affects the nitrogen crossover.
Modeling can assist in achieving better understanding of various complex physicochemical processe... more Modeling can assist in achieving better understanding of various complex physicochemical processes occurring in fuel cells, which is critical in improving the fuel cell performance and making them more cost effective. Modeling efforts in PEM fuel cell area have been focused on developing both single and multidimensional (2D and 3D) PEM fuel cell models. The higher dimensional models include more realistic and accurate descriptions of the fuel cell processes; however, they also involve more complexity and require considerably extensive computational resources. Hence, despite the availability of higher dimensional fuel cell models, the lower dimensional models still retain their relevance and are being extensively used. Past studies commented on the effect of multidimensionality by comparing the results of higher and lower dimensional models which had differences in fuel cell geometry, operating conditions , modeling assumptions and properties. Owing to these differences between models, the difference in their results could not be solely attributed to the effect of multidimensionality. The present study was motivated by recognizing this gap in literature. The multidimensional effect is analyzed by developing two similar steady state 2D and 3D models in COMSOL. Both of these models have similar geometry and are simulated under similar operating conditions. The effect of multidimensionality on species concentration is investigated at various inlet stoichiometries, membrane conductivities and relative humidity values. The multidimensional effect, as represented by difference in species distribution in the two models, is found to be significant at lower operating voltages and more prominent at the cathode side. The inlet stoichiometry at the cathode and membrane conductivity values, are also found to influence the multidimensionality effects.
Automotive catalytic converters, which are employed to reduce engine exhaust emissions, are subje... more Automotive catalytic converters, which are employed to reduce engine exhaust emissions, are subjected to highly transient conditions during a typical driving cycle. These transient conditions arise from changes in driving mode, the hysteresis and flow lags of the feedback control system, and result in fluctuations of air-fuel ratio, exhaust gas flow rates and temperatures. The catalyst performance is also strongly influenced by the oxygen storage capacity. This paper presents a computational investigation of the effect of oxygen storage capacity on the dynamic behavior of an automotive catalytic converter subjected to modulations in exhaust gases. The modulations are generated by forcing the temporal variations in exhaust gases air-fuel ratio, gas flow rates and temperatures. The study employs a single-channel based, one-dimensional, non-adiabatic model. The results show that the imposed modulations cause a significant departure in the catalyst behavior from its steady behavior, and the oxygen storage capacity plays an important role in determining the catalyst's response to the imposed modulations. Modulations and oxygen storage capacity are found to have relatively greater influence on the catalyst's performance near stoichiometric conditions.
This paper presents a computational investigation of the effect of composition modulations on an ... more This paper presents a computational investigation of the effect of composition modulations on an automotive catalytic converter. The objective is to develop a better fundamental understanding of the converter's performance under actual driving conditions. Such an understanding will be beneficial in devising improved emission control methodologies by exploiting the catalyst transient behavior. The study employs a single-channel based, one-dimensional, non-adiabatic model. Two types of imposed transients (sinusoidal and step changes) are considered. The results show that composition modulations cause a significant departure in the catalyst behavior from its steady behavior, and modulations have both favorable and harmful effects on pollutant conversion.
This paper numerically investigates the role of heat and mass transfer coefficients on the perfor... more This paper numerically investigates the role of heat and mass transfer coefficients on the performance of automotive catalytic converters, which are employed to reduce engine exhaust emissions. The pollutant conversion performance of a converter is influenced by a number of physical and chemical processes that take place in gaseous and solid phases as the exhaust gases flow through the catalyst. A quantitative predictive understanding of these complex catalyst processes involving flow dynamics, heterogeneous surface reactions, and heat and mass transport mechanisms is important in improving the converter design. The role of convective transport phenomena becomes important at high temperature when the mass transfer becomes rate limiting to an increasing extent. The objective of the present study is to elucidate the influence of convective heat and mass transfer mechanisms. The mathematical model considers the conservation of mass, momentum and energy in both gaseous and solid phases. In addition to the heterogeneous surface reactions, the 2 model also takes into account the adsorption/desorption of oxygen in the catalyst during non-stoichiometric composition of air-fuel mixtures. The governing equations are solved by an implicit scheme using a successive line under relaxation method. The converter performance under both the steady state and the transient conditions is analyzed. Key Words engine emissions, engine exhaust after-treatment, heat and mass transfer, numerical simulations. Nomenclature C g j gas phase concentration of species j, mol/m 3 C s j surface concentration of species j, mol/m 3 c pg specific heat of gas, J/kg·K c ps specific heat of substrate, J/kg·K D h hydraulic diameter, m D j diffusion coefficient of species j, m 2 /s G a geometric surface area, m 2 /m 3 ∆H k heat of reaction of species k, J/mol h g heat transfer coefficient between flow and substrate, J/m 2 ·s·K h ∞ heat transfer coefficient between substrate and atmosphere, J/m 2 ·s·K k m j mass transfer coefficient for species j, m/s L catalyst length, m Nu Nusselt number Pr Prandtl number
Journal of Engineering for Gas Turbines and Power-transactions of The Asme, 2002
... Pattas et al. used a simple five-step reaction scheme and a one-dimensional model to predict ... more ... Pattas et al. used a simple five-step reaction scheme and a one-dimensional model to predict the catalytic converter performance during a regulated European driving cycle, ECE-15. ... However, they did not report comparison of time-resolved emissions for HC and NO. ...
The combustion assisted thermal spray systems are being used to apply coatings to prevent surface... more The combustion assisted thermal spray systems are being used to apply coatings to prevent surface degradation. They offer a highly attractive way to modify the surface properties of the substrate to extend the product life. In addition to the materials being sprayed, the quality of combustion assisted thermal spray coating depends greatly on the flow behavior of reacting gases and particle dynamics. The present study investigates the effect of gas phase and its interaction with particles through the nozzle of a thermal spray gun by developing a comprehensive mathematical model. The objective is to develop a predictive understanding of various design parameters of combustion assisted thermal spray systems. The model was developed by considering the conservation of mass, momentum and energy of reacting gases.
This paper presents a computational investigation of the effect of exhaust temperature modulation... more This paper presents a computational investigation of the effect of exhaust temperature modulations on an automotive catalytic converter. The objective is to develop a better fundamental understanding of the converter's performance under transient driving conditions. Such an understanding will be beneficial in devising improved emission control methodologies. The study employs a single-channel based, one-dimensional, non-adiabatic model. The transient conditions are imposed by varying the exhaust gas temperature sinusoidally. The results show that temperature modulations cause a significant departure in the catalyst behavior from its steady behavior, and modulations have both favorable and harmful effects on pollutant conversion. The operating conditions and the modulating gas composition and flow rates (space velocity) have substantial influence on catalyst behavior.
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Papers by Tariq Shamim