Dr. Abdelmaged H. Ibrahim Essawey | Cairo University (original) (raw)
Ph.D. and M.Sc. thesis supervised by me by Dr. Abdelmaged H. Ibrahim Essawey
Experimental Investigation of oxy-combustion of CNG flames stabilized over a perforated plate bur... more Experimental Investigation of oxy-combustion of CNG flames stabilized over a perforated plate burner.
This work presents an attempt to improve the natural ventilation in south-facing rooms located in... more This work presents an attempt to improve the natural ventilation in south-facing rooms located in the top floors of buildings located in Cairo, Egypt. According to the city's wind rose in the hot humid months of summer, these rooms suffer from very low incoming air velocity as well as high incident solar radiation, which results in massive air conditioning loads in summer, which can be eliminated or reduced by improving the natural ventilation. The work investigates the effects of increasing the height of the roof of the south-facing room located in the top floor, with respect to the north-facing room, to allow introducing a vent in the other-wise closed north wall. This vent allows admission of high-velocity air stream from the north direction into the room as well as it changes the ventilation from single-sided ventilation to cross-ventilation in the south-facing room. A room model internal thermal load has been modeled inside a computational wind tunnel domain. The effects of changing the window configurations (single/doubled, aligned/staggered, large/small and different heights), room area, vent configuration and furniture arrangement inside the room have been studied. v ACKNOWLEDGEMENT I hereby would like to express my deep gratitude and thanks to Prof. Dr. Essam E. Khalil, Dr. Abd El-Maged H. Ibrahim and Eng. Ahmed Adawy for their support and continuous encouragement. Also, I cannot express; in words; my thanks and gratitude to my family for their great and continuous help and support, they provided me to finish this work in a suitable form. vi
Papers in Aerosol by Dr. Abdelmaged H. Ibrahim Essawey
The motion of 70 µm-diameter stainless steel microspheres, after their detachment from rough, fla... more The motion of 70 µm-diameter stainless steel microspheres, after their detachment from rough, flat surfaces due to turbulent air flow is considered. The microspheres are embedded fully in the viscous sublayer and initially are in static-contact equilibrium. Then, the microspheres are subjected to a slowly accelerating flow. Microvideographic observations of individual microsphere motion after detachment show that these relatively heavy microspheres move along the surface before any possible lift-off. A strobed laser-light sheet technique is used to obtain the microsphere velocities along the surface right after their detachment. The equations of motion of the microspheres are presented, including surfaceroughness effects and dissipative forces and moments. The equations are solved numerically for detachment with and without consideration of a burst-sweep event initiating detachment. The measured microsphere velocities are compared to the numerical solutions. Results reveal that the microspheres undergo pure rolling along the surface before possible entrainment, that the sweep part of the event plays a role in the detachment process and that the dissipative contact forces and moments are negligible compared to the Hertzian and adhesion forces and moments.
The detachment of microparticles from surfaces subjected to various temporal accelerations of air... more The detachment of microparticles from surfaces subjected to various temporal accelerations of air was studied experimentally. Stainless microspheres, approximately 70 m in diameter, deposited as a sparse monolayer on smooth glass substrates, were subjected to temporal accelerations ranging from 0.01 to 2.0 m/s 2 . Higher accelerations up to 23 m/s 2 were investigated without particles on the surface to further characterize the temporally evolving flow. Microvideographic images of the particles on the surfaces were acquired. These were used to determine the detachment fraction of the particles versus the free-stream flow velocity at different temporal flow accelerations. For the relatively slower flow accelerations, approximately 0.3 m/s 2 or less, microparticle detachment was independent of acceleration to within the experimental uncertainty. For the relatively more rapid accelerations, from approximately 0.3 m/s 2 to at least 2.0 m/s 2 , the flow velocity required to detach one half of the microparticles increased with acceleration. Near-wall velocity measurements supported that rapid temporal acceleration delayed the onset of turbulence, thereby affecting the boundary layer characteristics, causing a decline of turbulent bursting in the wall-layer, and, hence, suppressing the detachment process. ᭧
The motion of heavy microspheres after their detachment from at surfaces is considered. The micro... more The motion of heavy microspheres after their detachment from at surfaces is considered. The microspheres initially are in static contact equilibrium, embedded fully in the viscous sublayer and subjected to a slowly accelerating fully developed turbulent ow. The equations of motion of the microspheres are presented, including surface roughness e ects. The equations are solved numerically for detachment with and without consideration of a burst-sweep event initiating detachment. The microsphere velocity along the surface after detachment is measured using a strobed laser-light sheet and compared to the numerical solutions. Results indicate that the microspheres undergo pure rolling along the surface before possible entrainment and that the sweep plays a role in the detachment process. A model for predicting the microsphere velocity along the surface after detachment is presented. ?
This work presents the results of experiments conducted to characterize the detachment of micropa... more This work presents the results of experiments conducted to characterize the detachment of microparticles from surfaces exposed to turbulent air during accelerated free-stream ow. Smooth glass plates used as substrates are scanned with an atomic force microscope to determine their roughness-height distributions. Microparticles of di erent sizes, materials and shapes (mostly microspheres) are deposited as sparse monolayers onto the substrates under controlled clean and dry conditions. The microparticles attach to the substrate in a condition of static equilibrium due to adhesion and reside completely within the viscous sublayer as the ow is accelerated. Microvideographic observations of individual microparticle detachment show that detachment occurs primarily as rolling motion along the surface and not as lift-o . Detachment is not necessarily followed by entrainment in the ow. Results are presented as detachment fractions as function of time.
This work presents the results of experiments conducted to provide quantitative information on pa... more This work presents the results of experiments conducted to provide quantitative information on particle detachment from surfaces. The e ects of certain, controllable factors on the detachment of 70 m-diameter stainless-steel spheres from a glass surface exposed to accelerated air ow in a wind tunnel were studied. Changes in the free-stream velocity required to detach 50% of the particles, the threshold velocity for detachment, were measured for variations in the controlled factors. These factors were air relative humidity, residence time between particle deposition onto the substrate and ow application, mean ow acceleration, deposition density, ÿnal free-stream velocity, and ÿnal ow Reynolds number. Results reveal that deposition density was the most e ective factor that enhanced detachment at all relative humidities. Residence time was found to be the most e ective factor that suppressed detachment at high relative humidity. The threshold velocity increased with increasing relative humidity and was lower for turbulent ow versus laminar ow. Within the uncertainty limits, the mean ow acceleration in the transient period was found not to a ect the threshold velocity in the range from 0.014 to 0:34 m=s 2 . The ÿnal free-stream velocity also did not a ect the threshold velocity, provided it was greater than the threshold. A set of experimental conditions that lead to a relatively small uncertainty is presented. ?
Papers in Natural Ventilation by Dr. Abdelmaged H. Ibrahim Essawey
This work presents an attempt to improve the natural ventilation in south-facing rooms located in... more This work presents an attempt to improve the natural ventilation in south-facing rooms located in the top floors of buildings located in Cairo, Egypt. According to the city's wind rose in the hot humid months of summer, these rooms suffer from very low incoming air velocity as well as high incident solar radiation, which results in massive air conditioning loads in summer, that can be eliminated or reduced by improving the natural ventilation, leading to energy conservation. The work focuses on a south-facing room, where the neighbor's room is facing north with a much higher incoming air velocity and shielding this air stream from the neighbor's room. The work investigates the effects of increasing the height of the roof of the south-facing room located in the top floor, with respect to the northfacing room, to allow introducing a vent in the other-wise closed north wall. This vent allows admission of high-velocity air stream from the north direction into the room as well as it changes the ventilation from single-sided ventilation to cross-ventilation in the south-facing room. The simulation is carried-out numerically using steady, three-dimensional solution of the continuity and Navier-Stokes equations using computational fluid mechanics and the k-ε turbulence model under incompressible flow assumption and utilizing the Boussinesq's approximation. A room model with realistic dimensions and internal thermal loads have been modeled inside a computational wind tunnel domain. The effects of changing the window configurations (single/doubled, aligned/staggered, large/small and different heights) have been studied. These results are contrasted to the case of the north-facing room and the south-facing room without the vent. The results show that introducing the vent in the southfacing room can significantly improve the ventilation by passive means and that this improvement can improve for double windows rather than single window (with the same total surface area) and that using staggered windows enhances the air distribution inside the room more than the aligned windows. Increasing the total window area indeed improves the ventilation and positioning the windows closer to the floor is preferred than closer to the ceiling for the vent configuration used. The numerical simulations show that these improvements can provide air distribution inside the south-facing room even better than in the north-facing room.
Papers in Oxy Combustion by Dr. Abdelmaged H. Ibrahim Essawey
Concerns about global warming have encouraged interest in hydrocarbon combustion techniques that ... more Concerns about global warming have encouraged interest in hydrocarbon combustion techniques that allow easy capture and sequestration of carbon dioxide. One method of achieving this objective is through the use of post-combustion CO 2 capture and sequestration. In this work, Compressed Natural Gas (CNG) flames were burned in confined diffusion situation in oxycombustion environment with different O 2 /CO 2 mixtures. The flames were stabilized on a bluffbody burner and different blockage ratios were studied. The appearance, stability and emissions of these flames are compared to combustion in air and in enriched air. Three different regions were observed, namely like-jet flames, central-jet dominated flames and recirculation zone flames, depending on the ratio between oxidizer and fuel momenta. The flame color changed from yellow in air, to blue with yellow tips in oxy-combustion to bright white in the enriched air. The flame length was the highest in air, then lower in oxy-combustion cases and the lowest in the enriched air cases. CNG flames burned in enriched air have higher flame stability than those burned in air, which in turn have higher stability than those burned in oxy-combustion. The CO emissions in the oxy-combustion cases were much higher than those in air at the same blockage ratio and equivalence ratio.
My Ph.D and My M.Sc. by Dr. Abdelmaged H. Ibrahim Essawey
by Abdelmaged Hafez Ibrahim Essawey This work presents a combined experimental and theoretical in... more by Abdelmaged Hafez Ibrahim Essawey This work presents a combined experimental and theoretical investigation of the conditions under which a fluid flow causes a microparticle to detach from a flat surface. The general approach was to conduct well-controlled experiments, to observe individual microparticle motion and to focus on the basic detachment mechanisms. Microparticles of different sizes, materials and shapes (mostly microspheres) were deposited as monolayers onto the substrates under controlled conditions. The microparticles attached to the substrate in a condition of static equilibrium due to contact adhesion and resided completely within the viscous sublayer. The flow was accelerated during a transient period up to a fixed, constant velocity. Smooth glass plates were used as substrates and scanned with an atomic force microscope to determine their roughness-height distributions. The study was confined to microparticles in the diameter range of approximately 10 µm to 100 µm. They were distributed on the surface as a monolayer in sparse/dense conditions, dry/humid and subjected to laminar or fully developed turbulent flows.
Papers in ThermoAcoustics by Dr. Abdelmaged H. Ibrahim Essawey
Concentrated Solar Power (CSP) is a key energy production technology of the 21 st century. A key ... more Concentrated Solar Power (CSP) is a key energy production technology of the 21 st century. A key component in a concentrated solar power system is the power block. Thermoacoustic heat engines offer an innovative solution to and improve the performance of CSP systems. They operate by converting heat into acoustic power. One of the main advantages of these engines is that they can easily be driven by solar energy or waste heat. The efficient and economical operation of these engines requires their hot side to be at the highest possible temperature allowed by the heat source. However, different technologies of solar energy provide heat sources at different temperatures and different operational conditional may also affect the temperature of the heat source. Therefore, it is essential to investigate the operation of thermoacoustic engines at different operating temperatures. This work discusses some of the technical problems associated with the operating temperature of the engine and its onset temperature, suggests some potential solutions and presents experimental results on the onset temperature, operating frequency, harmonic content in the resulting pressure wave, as well as the dependence of the frequency of the fundamental mode on the operating temperature for different operating temperatures and different gases
Thermoacoustic refrigerators create a refrigeration effect using a pressure wave confined in a re... more Thermoacoustic refrigerators create a refrigeration effect using a pressure wave confined in a resonator filled with air or inert gases. The pressure wave is supplied by a loudspeaker and can also be supplied by a thermoacoustic heat engine. However, such devices have not been widely-commercialized yet because of their low efficiencies and power densities, compared to vapor compression cycles. This work analyzes all components of the loud-speaker driven thermoacoustic engine in order to highlight the constraints and the challenges associated with each component. This is critical to further develop these devices. This work analyzes the constraints and the challenges in the loud speaker (dimensions, power, efficiency, frequency response, position, frequency matching, and enclosure volume); amplifier (power, fidelity, impedance matching); cone (length and cone angle); resonator (shape, length, and cross sectional area); stack (material, dimensions, porosity, geometry and position) and hot and cold heat exchanger design (blockage fraction, porosity, length, material, heat transfer resistances and heat transfer fluid). A prototype of a thermoacoustic refrigerator is currently being built at the American University in Cairo and some of the experiences gained are shared.
Experimental Investigation of oxy-combustion of CNG flames stabilized over a perforated plate bur... more Experimental Investigation of oxy-combustion of CNG flames stabilized over a perforated plate burner.
This work presents an attempt to improve the natural ventilation in south-facing rooms located in... more This work presents an attempt to improve the natural ventilation in south-facing rooms located in the top floors of buildings located in Cairo, Egypt. According to the city's wind rose in the hot humid months of summer, these rooms suffer from very low incoming air velocity as well as high incident solar radiation, which results in massive air conditioning loads in summer, which can be eliminated or reduced by improving the natural ventilation. The work investigates the effects of increasing the height of the roof of the south-facing room located in the top floor, with respect to the north-facing room, to allow introducing a vent in the other-wise closed north wall. This vent allows admission of high-velocity air stream from the north direction into the room as well as it changes the ventilation from single-sided ventilation to cross-ventilation in the south-facing room. A room model internal thermal load has been modeled inside a computational wind tunnel domain. The effects of changing the window configurations (single/doubled, aligned/staggered, large/small and different heights), room area, vent configuration and furniture arrangement inside the room have been studied. v ACKNOWLEDGEMENT I hereby would like to express my deep gratitude and thanks to Prof. Dr. Essam E. Khalil, Dr. Abd El-Maged H. Ibrahim and Eng. Ahmed Adawy for their support and continuous encouragement. Also, I cannot express; in words; my thanks and gratitude to my family for their great and continuous help and support, they provided me to finish this work in a suitable form. vi
The motion of 70 µm-diameter stainless steel microspheres, after their detachment from rough, fla... more The motion of 70 µm-diameter stainless steel microspheres, after their detachment from rough, flat surfaces due to turbulent air flow is considered. The microspheres are embedded fully in the viscous sublayer and initially are in static-contact equilibrium. Then, the microspheres are subjected to a slowly accelerating flow. Microvideographic observations of individual microsphere motion after detachment show that these relatively heavy microspheres move along the surface before any possible lift-off. A strobed laser-light sheet technique is used to obtain the microsphere velocities along the surface right after their detachment. The equations of motion of the microspheres are presented, including surfaceroughness effects and dissipative forces and moments. The equations are solved numerically for detachment with and without consideration of a burst-sweep event initiating detachment. The measured microsphere velocities are compared to the numerical solutions. Results reveal that the microspheres undergo pure rolling along the surface before possible entrainment, that the sweep part of the event plays a role in the detachment process and that the dissipative contact forces and moments are negligible compared to the Hertzian and adhesion forces and moments.
The detachment of microparticles from surfaces subjected to various temporal accelerations of air... more The detachment of microparticles from surfaces subjected to various temporal accelerations of air was studied experimentally. Stainless microspheres, approximately 70 m in diameter, deposited as a sparse monolayer on smooth glass substrates, were subjected to temporal accelerations ranging from 0.01 to 2.0 m/s 2 . Higher accelerations up to 23 m/s 2 were investigated without particles on the surface to further characterize the temporally evolving flow. Microvideographic images of the particles on the surfaces were acquired. These were used to determine the detachment fraction of the particles versus the free-stream flow velocity at different temporal flow accelerations. For the relatively slower flow accelerations, approximately 0.3 m/s 2 or less, microparticle detachment was independent of acceleration to within the experimental uncertainty. For the relatively more rapid accelerations, from approximately 0.3 m/s 2 to at least 2.0 m/s 2 , the flow velocity required to detach one half of the microparticles increased with acceleration. Near-wall velocity measurements supported that rapid temporal acceleration delayed the onset of turbulence, thereby affecting the boundary layer characteristics, causing a decline of turbulent bursting in the wall-layer, and, hence, suppressing the detachment process. ᭧
The motion of heavy microspheres after their detachment from at surfaces is considered. The micro... more The motion of heavy microspheres after their detachment from at surfaces is considered. The microspheres initially are in static contact equilibrium, embedded fully in the viscous sublayer and subjected to a slowly accelerating fully developed turbulent ow. The equations of motion of the microspheres are presented, including surface roughness e ects. The equations are solved numerically for detachment with and without consideration of a burst-sweep event initiating detachment. The microsphere velocity along the surface after detachment is measured using a strobed laser-light sheet and compared to the numerical solutions. Results indicate that the microspheres undergo pure rolling along the surface before possible entrainment and that the sweep plays a role in the detachment process. A model for predicting the microsphere velocity along the surface after detachment is presented. ?
This work presents the results of experiments conducted to characterize the detachment of micropa... more This work presents the results of experiments conducted to characterize the detachment of microparticles from surfaces exposed to turbulent air during accelerated free-stream ow. Smooth glass plates used as substrates are scanned with an atomic force microscope to determine their roughness-height distributions. Microparticles of di erent sizes, materials and shapes (mostly microspheres) are deposited as sparse monolayers onto the substrates under controlled clean and dry conditions. The microparticles attach to the substrate in a condition of static equilibrium due to adhesion and reside completely within the viscous sublayer as the ow is accelerated. Microvideographic observations of individual microparticle detachment show that detachment occurs primarily as rolling motion along the surface and not as lift-o . Detachment is not necessarily followed by entrainment in the ow. Results are presented as detachment fractions as function of time.
This work presents the results of experiments conducted to provide quantitative information on pa... more This work presents the results of experiments conducted to provide quantitative information on particle detachment from surfaces. The e ects of certain, controllable factors on the detachment of 70 m-diameter stainless-steel spheres from a glass surface exposed to accelerated air ow in a wind tunnel were studied. Changes in the free-stream velocity required to detach 50% of the particles, the threshold velocity for detachment, were measured for variations in the controlled factors. These factors were air relative humidity, residence time between particle deposition onto the substrate and ow application, mean ow acceleration, deposition density, ÿnal free-stream velocity, and ÿnal ow Reynolds number. Results reveal that deposition density was the most e ective factor that enhanced detachment at all relative humidities. Residence time was found to be the most e ective factor that suppressed detachment at high relative humidity. The threshold velocity increased with increasing relative humidity and was lower for turbulent ow versus laminar ow. Within the uncertainty limits, the mean ow acceleration in the transient period was found not to a ect the threshold velocity in the range from 0.014 to 0:34 m=s 2 . The ÿnal free-stream velocity also did not a ect the threshold velocity, provided it was greater than the threshold. A set of experimental conditions that lead to a relatively small uncertainty is presented. ?
This work presents an attempt to improve the natural ventilation in south-facing rooms located in... more This work presents an attempt to improve the natural ventilation in south-facing rooms located in the top floors of buildings located in Cairo, Egypt. According to the city's wind rose in the hot humid months of summer, these rooms suffer from very low incoming air velocity as well as high incident solar radiation, which results in massive air conditioning loads in summer, that can be eliminated or reduced by improving the natural ventilation, leading to energy conservation. The work focuses on a south-facing room, where the neighbor's room is facing north with a much higher incoming air velocity and shielding this air stream from the neighbor's room. The work investigates the effects of increasing the height of the roof of the south-facing room located in the top floor, with respect to the northfacing room, to allow introducing a vent in the other-wise closed north wall. This vent allows admission of high-velocity air stream from the north direction into the room as well as it changes the ventilation from single-sided ventilation to cross-ventilation in the south-facing room. The simulation is carried-out numerically using steady, three-dimensional solution of the continuity and Navier-Stokes equations using computational fluid mechanics and the k-ε turbulence model under incompressible flow assumption and utilizing the Boussinesq's approximation. A room model with realistic dimensions and internal thermal loads have been modeled inside a computational wind tunnel domain. The effects of changing the window configurations (single/doubled, aligned/staggered, large/small and different heights) have been studied. These results are contrasted to the case of the north-facing room and the south-facing room without the vent. The results show that introducing the vent in the southfacing room can significantly improve the ventilation by passive means and that this improvement can improve for double windows rather than single window (with the same total surface area) and that using staggered windows enhances the air distribution inside the room more than the aligned windows. Increasing the total window area indeed improves the ventilation and positioning the windows closer to the floor is preferred than closer to the ceiling for the vent configuration used. The numerical simulations show that these improvements can provide air distribution inside the south-facing room even better than in the north-facing room.
Concerns about global warming have encouraged interest in hydrocarbon combustion techniques that ... more Concerns about global warming have encouraged interest in hydrocarbon combustion techniques that allow easy capture and sequestration of carbon dioxide. One method of achieving this objective is through the use of post-combustion CO 2 capture and sequestration. In this work, Compressed Natural Gas (CNG) flames were burned in confined diffusion situation in oxycombustion environment with different O 2 /CO 2 mixtures. The flames were stabilized on a bluffbody burner and different blockage ratios were studied. The appearance, stability and emissions of these flames are compared to combustion in air and in enriched air. Three different regions were observed, namely like-jet flames, central-jet dominated flames and recirculation zone flames, depending on the ratio between oxidizer and fuel momenta. The flame color changed from yellow in air, to blue with yellow tips in oxy-combustion to bright white in the enriched air. The flame length was the highest in air, then lower in oxy-combustion cases and the lowest in the enriched air cases. CNG flames burned in enriched air have higher flame stability than those burned in air, which in turn have higher stability than those burned in oxy-combustion. The CO emissions in the oxy-combustion cases were much higher than those in air at the same blockage ratio and equivalence ratio.
by Abdelmaged Hafez Ibrahim Essawey This work presents a combined experimental and theoretical in... more by Abdelmaged Hafez Ibrahim Essawey This work presents a combined experimental and theoretical investigation of the conditions under which a fluid flow causes a microparticle to detach from a flat surface. The general approach was to conduct well-controlled experiments, to observe individual microparticle motion and to focus on the basic detachment mechanisms. Microparticles of different sizes, materials and shapes (mostly microspheres) were deposited as monolayers onto the substrates under controlled conditions. The microparticles attached to the substrate in a condition of static equilibrium due to contact adhesion and resided completely within the viscous sublayer. The flow was accelerated during a transient period up to a fixed, constant velocity. Smooth glass plates were used as substrates and scanned with an atomic force microscope to determine their roughness-height distributions. The study was confined to microparticles in the diameter range of approximately 10 µm to 100 µm. They were distributed on the surface as a monolayer in sparse/dense conditions, dry/humid and subjected to laminar or fully developed turbulent flows.
Concentrated Solar Power (CSP) is a key energy production technology of the 21 st century. A key ... more Concentrated Solar Power (CSP) is a key energy production technology of the 21 st century. A key component in a concentrated solar power system is the power block. Thermoacoustic heat engines offer an innovative solution to and improve the performance of CSP systems. They operate by converting heat into acoustic power. One of the main advantages of these engines is that they can easily be driven by solar energy or waste heat. The efficient and economical operation of these engines requires their hot side to be at the highest possible temperature allowed by the heat source. However, different technologies of solar energy provide heat sources at different temperatures and different operational conditional may also affect the temperature of the heat source. Therefore, it is essential to investigate the operation of thermoacoustic engines at different operating temperatures. This work discusses some of the technical problems associated with the operating temperature of the engine and its onset temperature, suggests some potential solutions and presents experimental results on the onset temperature, operating frequency, harmonic content in the resulting pressure wave, as well as the dependence of the frequency of the fundamental mode on the operating temperature for different operating temperatures and different gases
Thermoacoustic refrigerators create a refrigeration effect using a pressure wave confined in a re... more Thermoacoustic refrigerators create a refrigeration effect using a pressure wave confined in a resonator filled with air or inert gases. The pressure wave is supplied by a loudspeaker and can also be supplied by a thermoacoustic heat engine. However, such devices have not been widely-commercialized yet because of their low efficiencies and power densities, compared to vapor compression cycles. This work analyzes all components of the loud-speaker driven thermoacoustic engine in order to highlight the constraints and the challenges associated with each component. This is critical to further develop these devices. This work analyzes the constraints and the challenges in the loud speaker (dimensions, power, efficiency, frequency response, position, frequency matching, and enclosure volume); amplifier (power, fidelity, impedance matching); cone (length and cone angle); resonator (shape, length, and cross sectional area); stack (material, dimensions, porosity, geometry and position) and hot and cold heat exchanger design (blockage fraction, porosity, length, material, heat transfer resistances and heat transfer fluid). A prototype of a thermoacoustic refrigerator is currently being built at the American University in Cairo and some of the experiences gained are shared.
Oscillating flows differ significantly from steady flows in many terms including onset of turbule... more Oscillating flows differ significantly from steady flows in many terms including onset of turbulence, flow morphology, amount of turbulence and heat transfer characteristics during sudden expansion and contraction. This work uses Particle Image Velocimetry measurements to capture the vortex generation and flow morphology at the edges of a set of parallel plates in an oscillating flow driven by an electrodynamic loudspeaker inside a thermoacoustic resonator. The current work investigates the temporal development of the vortex at different points along the acoustic cycle, examines the effects of plate thickness, plate separation and drive ratio on the flow morphology around the stack edge. This is of particular interest to thermoacoustic engines and refrigerators where the vortex structures generated at the inlets and exits of thermoacoustic stacks play a role in the non-linear losses that dissipate flow energy into heat and thus degrade the performance. The work presents measured velocity field for different cases. Results visualize and quantify how the vortex size and the size of the disturbance zone increase with the plate thickness and how the size of vortex-vortex interaction along the same plate or between two different plates decreases. Increasing the plate separation does not seem to affect the vortex size nor the size of the disturbance zone but it reduces the vortex-vortex interaction along the same plate. Increasing the drive ratio increases the vortex size, the vortex-vortex interaction and the size of the disturbance zone and greatly affects the dissipation of flow energy into heat. This work visualizes and quantifies these effects for a range of values of dimensionless numbers.
An advantage of thermoacoustic engines is the utilization of environmentally friendly gases. In t... more An advantage of thermoacoustic engines is the utilization of environmentally friendly gases. In this work, guidelines for the selection of a working gas are presented and discussed. Subsequently, the effect of using different gases as working fluid and different stack porosities on the engine's performance was evaluated. In each case, the onset temperature difference, the generated acoustic power, working frequency and the excitation of higher harmonics are quantified and used as performance indicators. These working gases scan a range of sonic speeds from 374 m/s to 1101 m/s, a range of Prandtl number from 0.40 to 0.67, a range of density of 0.48 kg/m 3 to 0.99 kg/m 3 and a range of thermal conductivity from 0.030 W/m K to 0.091 W/m K. It is shown that it is not possible to achieve the best values for the performance indicators simultaneously, such as low onset temperature and reduced viscous losses. The results indicate that when operating at low mean pressures, the mixture density plays a role as important as the sonic speed or the Prandtl number. It is shown that for low pressure ratios, the square of the dynamic pressure amplitude of the fundamental is proportional to the input heat power. The proportionality constant depends on the sonic speed, the mixture density and the Prandtl number. On the other hand, the dynamic pressure amplitude of the first harmonic is proportional to the square of that of the fundamental mode throughout the full studied pressure range. Remarkably, the proportionality constant is quite the same for all gas mixtures and stack porosities used. Analysis of the transient profiles shows that the transient overshoot in the dynamic pressure as well as the size of the dynamic pressure-temperature hysteresis loop may be correlated to the thermal conductivity of the gas.
Thermoacoustic engines are devices that rely on the interaction between thermodynamics and acoust... more Thermoacoustic engines are devices that rely on the interaction between thermodynamics and acoustics to convert thermal energy into acoustic energy in the form of a pressure wave (sound wave). This sound wave is one form of mechanical work. The pressure wave is not generated and sustained unless the temperature of the hot side of the stack exceeds a certain critical value, known as the onset temperature. This work considers an existing optimized thermoacoustic engine design and analyzes the sensitivity of the effects of main engine factors (mean gas pressure, stack position, stack length, stack porosity and plate spacing) on the engine performance, evaluated by the onset temperature and the first and second-law efficiencies. The results indicate the relative importance of the different engine factors on the engine performance. This relative importance is evaluated for variations within ± ± ± ± 10% of the nominal values of the engine design. The results indicate that the most important engine parameters that affect the engine performance at the presented design conditions are the mean pressure, the plate spacing and the stack length for the onset temperature; the stack porosity and position for the first-law efficiency and the stack porosity and position for the second-law efficiency.
Thermoacoustic engines convert heat to acoustic power using the rich interactions between thermod... more Thermoacoustic engines convert heat to acoustic power using the rich interactions between thermodynamics and acoustics. These engines operate with inert gases with no moving parts and thus enjoy high reliability and low cost. Operating these engines at high temperatures is essential for high conversion efficiency but non-linearities have to be controlled. One of the main sources of non-linearity is the generation of harmonics, which extract acoustic power from the fundamental wave. This work studies the use of specificallydesigned inserts to suppress the generation of harmonics. Inserts of different shapes, porosities, thicknesses are positioned along the standing-wave in an otherwise iso-diameter resonator. The results indicate that largest suppression occurs when the insert is positioned at the velocity anti-node of the first harmonic. Among different shapes considered, the insert of porosity of 50% open area distributed over several circles is found to suppress harmonics more than other studied porosities or shapes. Results on the harmonic content, acoustic power in the fundamental and first harmonics, pressure and temperature distributions are presented for cases without inserts and with different insert shapes. Results comparing the engine performance with and without inserts for the identified best insert configuration are presented when the engine is operated with a gas mixture made of 80% and 20% argon at mean atmospheric pressure.
Thermoacoustic heat engines (TAHE) convert heat into mechanical work in the form of an acoustic w... more Thermoacoustic heat engines (TAHE) convert heat into mechanical work in the form of an acoustic wave. TAHE are further advantageous if they exploit waste heat and/or solar energy. For the acoustic wave to be generated, the heat must be added to TAHE at a temperature higher than a certain value, known as the onset temperature. In order to expand the use of thermoacoustic engines to more sources of waste heat and/or renewable energy, an optimization process is required to minimize the onset temperature while not severely compromising the first and second law efficiencies. Such optimization needs to address the effects of several TAHE's parameters on the onset temperature and efficiencies.
Thermoacoustic heat engines are intrinsically simple, reliable, environmentally friendly and reas... more Thermoacoustic heat engines are intrinsically simple, reliable, environmentally friendly and reasonably efficient. In this work, a thermoacoustic prototype was designed, built and operated and its performance indices quantified. The second law of thermodynamics dictates that higher conversion efficiencies necessitate high temperatures at the hot side of the engine's stack. This work investigates one of the limits associated with the hot side temperature, which is the excitation of higher harmonics and corresponding limitations on the dynamic pressure amplitude and conversion efficiency. It is observed that operating the engine at a temperature just above the onset temperature generates an acoustic wave at the fundamental mode with no harmonics observed but with relatively low dynamic pressure amplitude. As the hot-side temperature increases, higher harmonics are excited causing distortions in the resulting acoustic wave. The excitation of the harmonics at increasing temperatures is quantified and analyzed. For example, as the temperature difference across the stack increases by 140 % from 235 o C to 567 o C, the ratio of the pressure amplitude of the first harmonic to that of the fundamental mode increases by 950 %, from 0.04 to 0.38. At the temperature difference of 567 o C across the stack, approximately 30 % of the generated acoustic power is contained in the first mode. Furthermore, analysis of the measured ACcoupled dynamic pressure waves shows that the distorted waves can be numerically re-constructed by considering only the fundamental mode and the first three harmonics, indicating that at the operation range considered the harmonics were the main source of non-linearity.
Abdelmaged Hafez Ibrahim Essawey This work presents a combined experimental and theoretical inves... more Abdelmaged Hafez Ibrahim Essawey This work presents a combined experimental and theoretical investigation of the conditions under which a fluid flow causes a microparticle to detach from a flat surface. The general approach was to conduct well-controlled experiments, to observe individual microparticle motion and to focus on the basic detachment mechanisms. Microparticles of different sizes, materials and shapes (mostly microspheres) were deposited as monolayers onto the substrates under controlled conditions. The microparticles attached to the substrate in a condition of static equilibrium due to contact adhesion and resided completely within the viscous sublayer. The flow was accelerated during a transient period up to a fixed, constant velocity. Smooth glass plates were used as substrates and scanned with an atomic force microscope to determine their roughness-height distributions. The study was confined to microparticles in the diameter range of approximately 10 µm to 100 µm. They were distributed on the surface as a monolayer in sparse/dense conditions, dry/humid and subjected to laminar or fully developed turbulent flows. Quantitative information on the increase of flow velocity at which 50 % of the microparticles detached (U th) at different controlled conditions are presented. The variability in the U th was found to be in the range of 10 % to 20 %. The techniques used to obtain such a low variability are described. The measured values then were compared to a model based on force/moment balance approach. The model accounts for the surface roughness effects and is applicable to dry/humid conditions and laminar or fully developed turbulent flows. The results of the model agreed with the measurements, to within the uncertainties, for the cases studied. The sensitivity of the U th to five factors contained in the experiments and the Abdelmaged Hafez Ibrahim Essawey model is analyzed. The motion of microparticles after their detachment is considered. The microparticle velocities along the surface after detachment are measured using a strobed laserlight sheet and compared to the numerical solutions. Results indicate that the microparticles undergo pure rolling along the surface before possible entrainment, that the sweep part of a burst-sweep event plays a role in the detachment process and that the dissipative forces and moments are negligible.
Le Centre pour la Communication Scientifique Directe - HAL - Inria, Sep 11, 2011
10th International Energy Conversion Engineering Conference, 2012
This work presents an attempt to improve the natural ventilation in south-facing rooms located in... more This work presents an attempt to improve the natural ventilation in south-facing rooms located in the top floor in crowded unplanned residential areas in Cairo, Egypt. These rooms suffer from very low incoming air velocity and high solar radiation, which calls for the use of massive air conditioning loads in summer, which can be eliminated or reduced by improving the natural ventilation, leading to energy conservation. The work focuses on a south-facing room, where the neighbor’s room is facing north with a much higher incoming air velocity. The work investigates the effects of increasing the height of the roof of the south-facing room located in the top floor, with respect to the northfacing room, to allow introducing a vent in the other-wise closed north wall. This vent changes the ventilation from single-sided ventilation to cross-ventilation in the south-facing room .The numerical methodology is based on the finite volume numerical solution of the Navier–Stokes equations, using the CFD commercial code FLUENT. Steady, k-� turbulence model and incompressible flow of a constant property fluid under the Boussinesq’s approximation have been considered. A room model with actual dimensions and internal and external thermal loads have been used. The effects of changing the window dimensions and position, the vent dimensions and the room dimensions on the air velocity inside the south-facing room with and without the vent are presented and compared to the north-facing room. The results show that introducing the vent can significantly improve the ventilation inside the south-facing room, mainly because of the use of cross ventilation. Good results is achieved through adding the vent to the south-facing room in addition to the effect of The angle of the vent which plays a major role since it governs the recirculation zone inside the room, also the window configuration has a vital effect on the distribution of the air flow inside the room. The effects made improvements to the natural ventilation inside the south-facing room that make it exceed the north facing case ventilation in some places of the room.
Abdelmaged Hafez Ibrahim Essawey This work presents a combined experimental and theoretical inves... more Abdelmaged Hafez Ibrahim Essawey This work presents a combined experimental and theoretical investigation of the conditions under which a fluid flow causes a microparticle to detach from a flat surface. The general approach was to conduct well-controlled experiments, to observe individual microparticle motion and to focus on the basic detachment mechanisms. Microparticles of different sizes, materials and shapes (mostly microspheres) were deposited as monolayers onto the substrates under controlled conditions. The microparticles attached to the substrate in a condition of static equilibrium due to contact adhesion and resided completely within the viscous sublayer. The flow was accelerated during a transient period up to a fixed, constant velocity. Smooth glass plates were used as substrates and scanned with an atomic force microscope to determine their roughness-height distributions. The study was confined to microparticles in the diameter range of approximately 10 µm to 100 µm. They were distributed on the surface as a monolayer in sparse/dense conditions, dry/humid and subjected to laminar or fully developed turbulent flows. Quantitative information on the increase of flow velocity at which 50 % of the microparticles detached (U th) at different controlled conditions are presented. The variability in the U th was found to be in the range of 10 % to 20 %. The techniques used to obtain such a low variability are described. The measured values then were compared to a model based on force/moment balance approach. The model accounts for the surface roughness effects and is applicable to dry/humid conditions and laminar or fully developed turbulent flows. The results of the model agreed with the measurements, to within the uncertainties, for the cases studied. The sensitivity of the U th to five factors contained in the experiments and the Abdelmaged Hafez Ibrahim Essawey model is analyzed. The motion of microparticles after their detachment is considered. The microparticle velocities along the surface after detachment are measured using a strobed laserlight sheet and compared to the numerical solutions. Results indicate that the microparticles undergo pure rolling along the surface before possible entrainment, that the sweep part of a burst-sweep event plays a role in the detachment process and that the dissipative forces and moments are negligible.
Thermoacoustic heat engines are intrinsically simple, reliable, environmentally friendly and reas... more Thermoacoustic heat engines are intrinsically simple, reliable, environmentally friendly and reasonably efficient. In this work, a thermoacoustic prototype was designed, built and operated and its performance indices quantified. The second law of thermodynamics dictates that higher conversion efficiencies necessitate high temperatures at the hot side of the engine’s stack. This work investigates one of the limits associated with the hot side temperature, which is the excitation of higher harmonics and corresponding limitations on the dynamic pressure amplitude and conversion efficiency. It is observed that operating the engine at a temperature just above the onset temperature generates an acoustic wave at the fundamental mode with no harmonics observed but with relatively low dynamic pressure amplitude. As the hot-side temperature increases, higher harmonics are excited causing distortions in the resulting acoustic wave. The excitation of the harmonics at increasing temperatures is ...
An advantage of thermoacoustic engines is the utilization of environmentally friendly gases. In t... more An advantage of thermoacoustic engines is the utilization of environmentally friendly gases. In this work, guidelines for the selection of a working gas are presented and discussed. Subsequently, the effect of using different gases as working fluid and different stack porosities on the engine’s performance was evaluated. In each case, the onset temperature difference, the generated acoustic power, working frequency and the excitation of higher harmonics are quantified and used as performance indicators. These working gases scan a range of sonic speeds from 374 m/s to 1101 m/s, a range of Prandtl number from 0.40 to 0.67, a range of density of 0.48 kg/m3 to 0.99 kg/m3 and a range of thermal conductivity from 0.030 W/m K to 0.091 W/m K. It is shown that it is not possible to achieve the best values for the performance indicators simultaneously, such as low onset temperature and reduced viscous losses. The results indicate that when operating at low mean pressures, the mixture density ...
Thermoacoustic engines convert heat to acoustic power using the rich interactions be-tween thermo... more Thermoacoustic engines convert heat to acoustic power using the rich interactions be-tween thermodynamics and acoustics. These engines operate with inert gases with no mov-ing part and thus enjoy high reliability and low cost. Operating these engines at high tem-peratures is essential for high conversion efficiency but nonlinearities have to be controlled. One of the main sources of non-linearity is the generation of harmonics, which extract acoustic power from the fundamental wave. This work studies the use of specifically-designed inserts to suppress the generation of harmonics. Inserts of different shapes, porosities, thick-nesses are positioned along the standing-wave in an otherwise iso-diameter resonator. The results indicate that largest suppression occurs when the insert is positioned at the velocity anti-node of the first harmonic. Among different shapes considered, the insert of poros-ity of 50% open area distributed over several circles is found to suppress harmonics more...
An advantage of thermoacoustic engines is the utilization of environmentally friendly gases. In t... more An advantage of thermoacoustic engines is the utilization of environmentally friendly gases. In this work, guidelines for the selection of a working gas are presented and discussed. Subsequently, the effect of using different gases as working fluid and different stack porosities on the engine’s performance was evaluated. In each case, the onset temperature difference, the generated acoustic power, working frequency and the excitation of higher harmonics are quantified and used as performance indicators. These working gases scan a range of sonic speeds from 374 m/s to 1101 m/s, a range of Prandtl number from 0.40 to 0.67, a range of density of 0.48 kg/m3 to 0.99 kg/m3 and a range of thermal conductivity from 0.030 W/m K to 0.091 W/m K. It is shown that it is not possible to achieve the best values for the performance indicators simultaneously, such as low onset temperature and reduced viscous losses. The results indicate that when operating at low mean pressures, the mixture density ...
Thermoacoustic heat engines (TAHE) convert heat into mechanical work in the form of an acoustic w... more Thermoacoustic heat engines (TAHE) convert heat into mechanical work in the form of an acoustic wave. TAHE are further advantageous if they exploit waste heat and/or solar energy. For the acoustic wave to be generated, the heat must be added to TAHE at a temperature higher than a certain value, known as the onset temperature. In order to expand the use of thermoacoustic engines to more sources of waste heat and/or renewable energy, an optimization process is required to minimize the onset temperature while not severely compromising the first and second law efficiencies. Such optimization needs to address the effects of several TAHE's parameters on the onset temperature and efficiencies. In this work, an optimization process was carried-out to develop a TAHE driven by a waste/renewable heat source at 590 K or higher. Such a TAHE utilizes helium at 1 bar as a working fluid and delivers an acoustic power of 119 W at first-law and second-law efficiencies of 11.9 % and 26%, respectiv...