Most previous studies of air-and liquid-gap membrane distillation (AGMD and LGMD) processes using... more Most previous studies of air-and liquid-gap membrane distillation (AGMD and LGMD) processes using a composite membrane have been focused on an experimental approach. In this paper, rigorous theoretical investigations of the AGMD and LGMD processes were performed with a flat sheet type module using a composite membrane comprised of a polytetrafluoroethylene (PTFE) active layer and a polypropylene (PP) support layer. The model predictions were verified by comparing with measured data, where good agreement between the prediction results and experimental data was obtained. It was observed that as the gap size increased the AGMD permeate flux decreased exponentially with increased diffusion resistance. On the other hand, the LGMD permeate flux decreased exponentially and then increased asymptotically after attaining a minimum at a certain liquid-gap size (5 − 7 mm). This phenomenon was due to the onset and enhancement of a natural convection, resulting in an improvement in heat and mass transfer in the liquid gap.
This study provides a comprehensive and systematic overview of the fundamental characteristics of... more This study provides a comprehensive and systematic overview of the fundamental characteristics of heat and mass transfer in the direct contact membrane distillation (DCMD) process that employs different types of spacers on (i.e., adjacent to) one or both surfaces of the membrane. Detailed theoretical investigations were carried out to demonstrate the effects of spacers adjacent to the membrane surface on heat and mass transfer enhancement in the DCMD with a PTFE/PP composite membrane, complemented with experimental data for model validation. Thus, this work aimed to propose and demonstrate the heat transfer correlation for spacerfilled channels to reliably predict the heat and mass transfer improvement by non-woven net spacers in the DCMD process. The results showed that the permeate flux enhancement by the spacers ranged between 7%-19% only for the spacer-filled permeate channels and between 21%-33% only for the spacer-filled feed channels even at higher flow rates, thus indicating lower flux enhancements in the spacer-filled permeate channels. This was because the influence of spacers on flux improvement became more evident at higher temperatures owing to higher temperature polarization. In this study, the maximum flux enhancement of approximately 43% over the empty channels, which was achieved using the thinnest and densest spacer with a hydrodynamic angle of 90°, adjacent to both membrane surfaces.
ABSTRACT This paper presents an optimization of an axial catalyst distribution for the two types ... more ABSTRACT This paper presents an optimization of an axial catalyst distribution for the two types of optimal axial catalyst distributions: continuous exponential and discrete two-zone, to enhance the light-off of a catalytic converter with a fixed amount of a catalyst. The influences of exhaust gas temperature and mass flow rate on the two types of optimal active component distribution profiles are identified and hence an optimum value for each of the operating conditions is quantitatively established. The catalyst surface area of the optimal distribution in the upstream section is greater, and in the downstream is lower than that of the uniform distribution. As an inlet exhaust gas temperature becomes lower than the catalyst light-off temperature, i.e., about 600 K, the catalyst surface area of the optimal distribution at the monolith inlet increases greatly to attain high reaction rates at the monolith entrance. In contrast, at temperatures above 600 K, it decreases slightly and hence its variation becomes negligibly small, as compared to the variation of the inlet gas temperature. The catalyst surface area at the monolith entrance decreases gradually with the increasing mass flow rate at gas temperatures above 550 K.
An economic desalination system with a small scale and footprint for remote areas, which have a l... more An economic desalination system with a small scale and footprint for remote areas, which have a limited and inadequate water supply, insufficient water treatment and low infrastructure, is strongly demanded in the desalination markets. Here, a direct contact membrane distillation (DCMD) process has the simplest configuration and potentially the highest permeate flux among all of the possible MD processes. This process can also be easily instituted in a multi-stage manner for enhanced compactness, productivity, versatility and cost-effectiveness. In this study, an innovative, multi-stage, DCMD module under countercurrent-flow configuration is first designed and then investigate both theoretically and experimentally to identify its feasibility and operability for desalination application. Model predictions and measured data for mean permeate flux are compared and shown to be in good agreement. The effect of the number of module stages on the mean permeate flux, performance ratio and daily water production of the MDCMD system has been theoretically identified at inlet feed and permeate flow rates of 1.5 l/min and inlet feed and permeate temperatures of 70 °C
In this paper, a hybrid desalination system consisting of vacuum membrane distillation (VMD) and ... more In this paper, a hybrid desalination system consisting of vacuum membrane distillation (VMD) and adsorption desalination (AD) units, designated as VMD-AD cycle, is proposed. The synergetic integration of the VMD and AD is demonstrated where a useful effect of the AD cycle is channelled to boost the operation of the VMD process, namely the low vacuum environment to maintain the high pressure gradient across the microporous hydrophobic membrane. A solar-assisted multi-stage VMD-AD hybrid desalination system with temperature modulating unit is first designed, and its performance is then examined with a mathematical model of each component in the system and compared with the VMD-only system with temperature modulating and heat recovery units. The total water production and water recovery ratio of a solar-assisted 24-stage VMD-AD hybrid system are found to be about 21% and 23% higher, respectively, as compared to the VMD-only system. For the solar-assisted 24-stage VMD-AD desalination sy...
This paper presents a solar-assisted direct contact membrane distillation (DCMD) system with nove... more This paper presents a solar-assisted direct contact membrane distillation (DCMD) system with novel energy recovery concepts for a continuous 24-hour-a-day operation. A temperature modulating scheme is introduced to the solar-thermal system that supplies feed seawater to the DCMD modules. This scheme attenuates extreme temperature fluctuations of the feed water by storing the collected energy during solarpeak hours and reutilizing it throughout the day. Thus, the energy savings is realized yet the feed seawater temperature is maintained within the preferred range. Additionally, the system employs heat recovery from the permeate and brine streams to the feed seawater. The simulations for such a system with a shelland-tube type DCMD modules are carried out to examine the spatial property variations and the sensitivity of system performance (i.e., transmembrane pressure, permeate flux and performance ratio) to the operating conditions (inlet temperature and flow rate) and the fiber dimensions (fiber length and packing density). It is found that there are trade-off between mean permeate flux and performance ratio with respect to permeate inlet temperature and flow rate and between total distillate production and performance ratio with respect to packing density. For the solar-assisted DCMD system having evacuated-tube collectors of 3360 m 2 with 160 m 3 seawater storage tanks and 50 DCMD modules, the annual solar fraction and collector efficiency are found to be 77% and 53%, respectively whilst the overall permeate production capacity is 31 m 3 /day. The overall specific thermal energy consumption of DCMD system with heat recovery is found to be 436 kWh/m 3 and it is about 43% lower as compared to the system without heat recovery. It is observed that the specific thermal energy consumption decreases significantly by 55% with increased collector area from 1983 m 2 to 3360 m 2 whereas the specific electrical energy consumption increases slightly by 16%.
Journal of Mechanical Science and Technology, 2009
Experimental and numerical studies were performed by considering convective and radiative heat tr... more Experimental and numerical studies were performed by considering convective and radiative heat transfer to predict the transient thermal behavior of a plate in an indirectly fired continuous heat treatment furnace. The temperature profiles in the plate were determined by solving the transient one-dimensional heat conduction equation in conjunction with appropriate boundary conditions by using a time marching scheme. The results obtained from the transient analysis were substantiated by comparing with experimental results. Additionally, parametric investigations were performed to examine how the thermal behavior of the plate is affected by plate and refractory emissivities, charging temperature and residence time of the plate, gas temperature of the work and drive sides of the heat treatment furnace, and plate thickness.
International Journal of Heat and Fluid Flow, 2008
In this study, a numerical investigation was performed to examine the effects of curved surface s... more In this study, a numerical investigation was performed to examine the effects of curved surface shapes, laser power intensity, and the thermophysical properties of a material such as Reynolds and Capillary numbers on the shape, size, and surface topography of the melt pool. This investigation was completed via two-dimensional axisymmetric thermocapillary convection analyses during laser melting processes with deformable free surfaces. In general, a bump has a deep crater at the center and a low peripheral rim, a bowl-like shape. This is caused by the surface temperature gradient-induced thermocapillary flow driving the molten material towards the cooler region, which has higher surface tension. For flat and parabolic free surfaces, surface deformations such as crater depth and rim height increase gradually with decreasing Re and increasing Ca, B f , and especially the curvature of the free surface. In particular, it is noted that the curvature of the free surface has significantly little effect on the crater depth and rim height for a higher Re and lower B f with fixed Ca. Its effect is considerably strengthened at a lower Re and higher B f , indicating stronger convection. In the case of a sinusoidal wavy surface, it is noted that the crater depth and rim height increase monotonically with increasing Ca at fixed Re and B f , while they decrease with increasing Re and B f at fixed Ca due to the inward transfer of relatively large amounts of molten material compared to the surface tension gradient driving force, which causes the recovery of the crater center. Hence, the slope of the crater decreases significantly.
A modeling approach to predict the performance of commercial diesel oxidation catalyst (DOC) is p... more A modeling approach to predict the performance of commercial diesel oxidation catalyst (DOC) is presented in this study. Prior to completing this prediction, the conversion behavior of DOC as previously published was re-evaluated with a verification of the present numerical model. To calibrate and validate the model adopted in this study, steady-state experiments with DOC mounted on a light duty 4-cylinder 2.0 liter turbocharged diesel engine were performed using an enginedynamometer system. The reaction rates for CO, HC, and NO oxidations over a Pt/Al 2 O 3 catalyst were determined in conjunction with a fully transient two-phase 1D+1D monolith channel model with diesel exhaust gas temperature ranges from 150 to 450˚C and space velocity ranges from 10 5 to 5×10 5 h-1. To determine the kinetic parameters which best fit the experimental data, a two-step optimization procedure is introduced. First, the results from the conjugated gradient method (CGM) with individual temperatures for each species are plotted in an Arrhenius plot to generate proper intermediate guesses from initial guesses for all pre-exponential factors and activation energies. Then, kinetic parameters for all species are obtained simultaneously by searching the best fits to experimental data using the CGM from the intermediate guesses for all species. The prediction accuracy of the model was improved by the optimization procedure employed in this study, and the optimized kinetic parameters were validated against experimental data obtained at both 1500 and 2000 rpm.
A one-dimensional monolithic catalyst model was used to evaluate the effect of space between two ... more A one-dimensional monolithic catalyst model was used to evaluate the effect of space between two bricks on the thermal efficiency of a dual-monolithic catalytic converter with a Palladium-only (Pd-only) catalyst and a Palladium=Rhodium (Pd=Rh) catalyst butted together, and which are, in turn, mounted on a commercial vehicle equipped with a 2-L, four-cylinder spark ignition (SI) engine. Prior to the numerical investigation of the converter, tuning of the preexponential factor and activation energy of each reaction for each catalyst was performed to achieve acceptable agreement with experimental data under typical operating condition of automobile application. Two higher cell density 600 cpsi=4 mil substrates were used for faster light-off and improved warm-up performance of the catalytic converter, and the two monoliths have been connected without a space between them. As a result of the constriction of the monoliths, a slight temperature decrease occurred at the substrate interface due to thermal contact resistance. Therefore, to examine the heat transfer mechanism throughout the conforming rough surfaces, the thermal joint conductance between adjacent monoliths was determined using existing theory and correlation. The adequacy of the theory and correlation for thermal joint conductance was elucidated by analyzing heat transfer across the joint. Additionally, parametric investigations were performed to examine how the apparent contact pressure and mass flow rate of the exhaust gas affects the overall temperature drop across the joint. Over a wide range of operating conditions, temperature drop across the interface in the dual-catalyst converter with an air-gap is smaller than that without an air-gap. The relatively large temperature drop across the interface occurs especially during the early warmup period with a lower mass flow rate, that is, under conditions of cold-start and warmup at idle.
Energy or heat recovery schemes are keys for the performance improvement of any heat-activated cy... more Energy or heat recovery schemes are keys for the performance improvement of any heat-activated cycles such as the absorption and adsorption cycles. We present two innovative heat recovery schemes between the condensing and evaporating units of an adsorption desalination (AD) cycle. By recovering the latent heat of condenser and dumping it into the evaporative process of the evaporator, it elevates
Most previous studies of air-and liquid-gap membrane distillation (AGMD and LGMD) processes using... more Most previous studies of air-and liquid-gap membrane distillation (AGMD and LGMD) processes using a composite membrane have been focused on an experimental approach. In this paper, rigorous theoretical investigations of the AGMD and LGMD processes were performed with a flat sheet type module using a composite membrane comprised of a polytetrafluoroethylene (PTFE) active layer and a polypropylene (PP) support layer. The model predictions were verified by comparing with measured data, where good agreement between the prediction results and experimental data was obtained. It was observed that as the gap size increased the AGMD permeate flux decreased exponentially with increased diffusion resistance. On the other hand, the LGMD permeate flux decreased exponentially and then increased asymptotically after attaining a minimum at a certain liquid-gap size (5 − 7 mm). This phenomenon was due to the onset and enhancement of a natural convection, resulting in an improvement in heat and mass transfer in the liquid gap.
This study provides a comprehensive and systematic overview of the fundamental characteristics of... more This study provides a comprehensive and systematic overview of the fundamental characteristics of heat and mass transfer in the direct contact membrane distillation (DCMD) process that employs different types of spacers on (i.e., adjacent to) one or both surfaces of the membrane. Detailed theoretical investigations were carried out to demonstrate the effects of spacers adjacent to the membrane surface on heat and mass transfer enhancement in the DCMD with a PTFE/PP composite membrane, complemented with experimental data for model validation. Thus, this work aimed to propose and demonstrate the heat transfer correlation for spacerfilled channels to reliably predict the heat and mass transfer improvement by non-woven net spacers in the DCMD process. The results showed that the permeate flux enhancement by the spacers ranged between 7%-19% only for the spacer-filled permeate channels and between 21%-33% only for the spacer-filled feed channels even at higher flow rates, thus indicating lower flux enhancements in the spacer-filled permeate channels. This was because the influence of spacers on flux improvement became more evident at higher temperatures owing to higher temperature polarization. In this study, the maximum flux enhancement of approximately 43% over the empty channels, which was achieved using the thinnest and densest spacer with a hydrodynamic angle of 90°, adjacent to both membrane surfaces.
ABSTRACT This paper presents an optimization of an axial catalyst distribution for the two types ... more ABSTRACT This paper presents an optimization of an axial catalyst distribution for the two types of optimal axial catalyst distributions: continuous exponential and discrete two-zone, to enhance the light-off of a catalytic converter with a fixed amount of a catalyst. The influences of exhaust gas temperature and mass flow rate on the two types of optimal active component distribution profiles are identified and hence an optimum value for each of the operating conditions is quantitatively established. The catalyst surface area of the optimal distribution in the upstream section is greater, and in the downstream is lower than that of the uniform distribution. As an inlet exhaust gas temperature becomes lower than the catalyst light-off temperature, i.e., about 600 K, the catalyst surface area of the optimal distribution at the monolith inlet increases greatly to attain high reaction rates at the monolith entrance. In contrast, at temperatures above 600 K, it decreases slightly and hence its variation becomes negligibly small, as compared to the variation of the inlet gas temperature. The catalyst surface area at the monolith entrance decreases gradually with the increasing mass flow rate at gas temperatures above 550 K.
An economic desalination system with a small scale and footprint for remote areas, which have a l... more An economic desalination system with a small scale and footprint for remote areas, which have a limited and inadequate water supply, insufficient water treatment and low infrastructure, is strongly demanded in the desalination markets. Here, a direct contact membrane distillation (DCMD) process has the simplest configuration and potentially the highest permeate flux among all of the possible MD processes. This process can also be easily instituted in a multi-stage manner for enhanced compactness, productivity, versatility and cost-effectiveness. In this study, an innovative, multi-stage, DCMD module under countercurrent-flow configuration is first designed and then investigate both theoretically and experimentally to identify its feasibility and operability for desalination application. Model predictions and measured data for mean permeate flux are compared and shown to be in good agreement. The effect of the number of module stages on the mean permeate flux, performance ratio and daily water production of the MDCMD system has been theoretically identified at inlet feed and permeate flow rates of 1.5 l/min and inlet feed and permeate temperatures of 70 °C
In this paper, a hybrid desalination system consisting of vacuum membrane distillation (VMD) and ... more In this paper, a hybrid desalination system consisting of vacuum membrane distillation (VMD) and adsorption desalination (AD) units, designated as VMD-AD cycle, is proposed. The synergetic integration of the VMD and AD is demonstrated where a useful effect of the AD cycle is channelled to boost the operation of the VMD process, namely the low vacuum environment to maintain the high pressure gradient across the microporous hydrophobic membrane. A solar-assisted multi-stage VMD-AD hybrid desalination system with temperature modulating unit is first designed, and its performance is then examined with a mathematical model of each component in the system and compared with the VMD-only system with temperature modulating and heat recovery units. The total water production and water recovery ratio of a solar-assisted 24-stage VMD-AD hybrid system are found to be about 21% and 23% higher, respectively, as compared to the VMD-only system. For the solar-assisted 24-stage VMD-AD desalination sy...
This paper presents a solar-assisted direct contact membrane distillation (DCMD) system with nove... more This paper presents a solar-assisted direct contact membrane distillation (DCMD) system with novel energy recovery concepts for a continuous 24-hour-a-day operation. A temperature modulating scheme is introduced to the solar-thermal system that supplies feed seawater to the DCMD modules. This scheme attenuates extreme temperature fluctuations of the feed water by storing the collected energy during solarpeak hours and reutilizing it throughout the day. Thus, the energy savings is realized yet the feed seawater temperature is maintained within the preferred range. Additionally, the system employs heat recovery from the permeate and brine streams to the feed seawater. The simulations for such a system with a shelland-tube type DCMD modules are carried out to examine the spatial property variations and the sensitivity of system performance (i.e., transmembrane pressure, permeate flux and performance ratio) to the operating conditions (inlet temperature and flow rate) and the fiber dimensions (fiber length and packing density). It is found that there are trade-off between mean permeate flux and performance ratio with respect to permeate inlet temperature and flow rate and between total distillate production and performance ratio with respect to packing density. For the solar-assisted DCMD system having evacuated-tube collectors of 3360 m 2 with 160 m 3 seawater storage tanks and 50 DCMD modules, the annual solar fraction and collector efficiency are found to be 77% and 53%, respectively whilst the overall permeate production capacity is 31 m 3 /day. The overall specific thermal energy consumption of DCMD system with heat recovery is found to be 436 kWh/m 3 and it is about 43% lower as compared to the system without heat recovery. It is observed that the specific thermal energy consumption decreases significantly by 55% with increased collector area from 1983 m 2 to 3360 m 2 whereas the specific electrical energy consumption increases slightly by 16%.
Journal of Mechanical Science and Technology, 2009
Experimental and numerical studies were performed by considering convective and radiative heat tr... more Experimental and numerical studies were performed by considering convective and radiative heat transfer to predict the transient thermal behavior of a plate in an indirectly fired continuous heat treatment furnace. The temperature profiles in the plate were determined by solving the transient one-dimensional heat conduction equation in conjunction with appropriate boundary conditions by using a time marching scheme. The results obtained from the transient analysis were substantiated by comparing with experimental results. Additionally, parametric investigations were performed to examine how the thermal behavior of the plate is affected by plate and refractory emissivities, charging temperature and residence time of the plate, gas temperature of the work and drive sides of the heat treatment furnace, and plate thickness.
International Journal of Heat and Fluid Flow, 2008
In this study, a numerical investigation was performed to examine the effects of curved surface s... more In this study, a numerical investigation was performed to examine the effects of curved surface shapes, laser power intensity, and the thermophysical properties of a material such as Reynolds and Capillary numbers on the shape, size, and surface topography of the melt pool. This investigation was completed via two-dimensional axisymmetric thermocapillary convection analyses during laser melting processes with deformable free surfaces. In general, a bump has a deep crater at the center and a low peripheral rim, a bowl-like shape. This is caused by the surface temperature gradient-induced thermocapillary flow driving the molten material towards the cooler region, which has higher surface tension. For flat and parabolic free surfaces, surface deformations such as crater depth and rim height increase gradually with decreasing Re and increasing Ca, B f , and especially the curvature of the free surface. In particular, it is noted that the curvature of the free surface has significantly little effect on the crater depth and rim height for a higher Re and lower B f with fixed Ca. Its effect is considerably strengthened at a lower Re and higher B f , indicating stronger convection. In the case of a sinusoidal wavy surface, it is noted that the crater depth and rim height increase monotonically with increasing Ca at fixed Re and B f , while they decrease with increasing Re and B f at fixed Ca due to the inward transfer of relatively large amounts of molten material compared to the surface tension gradient driving force, which causes the recovery of the crater center. Hence, the slope of the crater decreases significantly.
A modeling approach to predict the performance of commercial diesel oxidation catalyst (DOC) is p... more A modeling approach to predict the performance of commercial diesel oxidation catalyst (DOC) is presented in this study. Prior to completing this prediction, the conversion behavior of DOC as previously published was re-evaluated with a verification of the present numerical model. To calibrate and validate the model adopted in this study, steady-state experiments with DOC mounted on a light duty 4-cylinder 2.0 liter turbocharged diesel engine were performed using an enginedynamometer system. The reaction rates for CO, HC, and NO oxidations over a Pt/Al 2 O 3 catalyst were determined in conjunction with a fully transient two-phase 1D+1D monolith channel model with diesel exhaust gas temperature ranges from 150 to 450˚C and space velocity ranges from 10 5 to 5×10 5 h-1. To determine the kinetic parameters which best fit the experimental data, a two-step optimization procedure is introduced. First, the results from the conjugated gradient method (CGM) with individual temperatures for each species are plotted in an Arrhenius plot to generate proper intermediate guesses from initial guesses for all pre-exponential factors and activation energies. Then, kinetic parameters for all species are obtained simultaneously by searching the best fits to experimental data using the CGM from the intermediate guesses for all species. The prediction accuracy of the model was improved by the optimization procedure employed in this study, and the optimized kinetic parameters were validated against experimental data obtained at both 1500 and 2000 rpm.
A one-dimensional monolithic catalyst model was used to evaluate the effect of space between two ... more A one-dimensional monolithic catalyst model was used to evaluate the effect of space between two bricks on the thermal efficiency of a dual-monolithic catalytic converter with a Palladium-only (Pd-only) catalyst and a Palladium=Rhodium (Pd=Rh) catalyst butted together, and which are, in turn, mounted on a commercial vehicle equipped with a 2-L, four-cylinder spark ignition (SI) engine. Prior to the numerical investigation of the converter, tuning of the preexponential factor and activation energy of each reaction for each catalyst was performed to achieve acceptable agreement with experimental data under typical operating condition of automobile application. Two higher cell density 600 cpsi=4 mil substrates were used for faster light-off and improved warm-up performance of the catalytic converter, and the two monoliths have been connected without a space between them. As a result of the constriction of the monoliths, a slight temperature decrease occurred at the substrate interface due to thermal contact resistance. Therefore, to examine the heat transfer mechanism throughout the conforming rough surfaces, the thermal joint conductance between adjacent monoliths was determined using existing theory and correlation. The adequacy of the theory and correlation for thermal joint conductance was elucidated by analyzing heat transfer across the joint. Additionally, parametric investigations were performed to examine how the apparent contact pressure and mass flow rate of the exhaust gas affects the overall temperature drop across the joint. Over a wide range of operating conditions, temperature drop across the interface in the dual-catalyst converter with an air-gap is smaller than that without an air-gap. The relatively large temperature drop across the interface occurs especially during the early warmup period with a lower mass flow rate, that is, under conditions of cold-start and warmup at idle.
Energy or heat recovery schemes are keys for the performance improvement of any heat-activated cy... more Energy or heat recovery schemes are keys for the performance improvement of any heat-activated cycles such as the absorption and adsorption cycles. We present two innovative heat recovery schemes between the condensing and evaporating units of an adsorption desalination (AD) cycle. By recovering the latent heat of condenser and dumping it into the evaporative process of the evaporator, it elevates
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Papers by Young-Deuk Kim