Amjid Khan - Academia.edu (original) (raw)
Papers by Amjid Khan
Energies
In this paper, a thorough 2D unsteady computational fluid dynamic analysis was performed on a pit... more In this paper, a thorough 2D unsteady computational fluid dynamic analysis was performed on a pitching airfoil to properly comprehend the dynamic stall and aerodynamic forces. The computational software ANSYS Fluent was used to solve the unsteady Reynolds-averaged Navier–Stokes equations. Low Reynolds number flows were modeled using the k-ω shear stress transport turbulence model. Aerodynamic forces, fluid flow structures, and flow separation delay angles were explored as a function of the Reynolds number, reduced frequency, oscillation amplitude, and mean angle of attack. The maximum aerodynamic forces, including lift, drag, and the onset of the dynamic stall, were all influenced by these variables. The critical parameters that influenced the optimum aerodynamic forces and ended up causing dynamic stall delay were oscillation amplitude and mean angle of attack. The stall angle was raised by 9° and 6°, respectively, and a large increment in the lift coefficient was also noted in bot...
Scientific Reports
The present study aims to investigate the performance of microchannel heat sink via numerical sim... more The present study aims to investigate the performance of microchannel heat sink via numerical simulations, based on the first and second law of thermodynamics. The heat transfer and flow characteristics of rectangular microchannel heat sinks have been improved by adding six different types of surface enhancers. The cross-sections include rectangular, triangular, and hexagonal-shaped ribs and cones. The cones have been created from the same cross-sections of ribs by drafting them at an angle of 45° orthogonal to the base, which is expected to decrease the pressure drop, dramatically. The performance of ribs and cones has been evaluated using different parameters such as friction factor, wall shear stress, entropy generation rate, augmentation entropy generation number, thermal resistance, and transport efficiency of thermal energy. The results of the present study revealed that the novel effect of coning at an angle of 45° reduces frictional losses (Maximum pressure drop reduced is 8...
2019 3rd International Conference on Energy Conservation and Efficiency (ICECE), 2019
The performance characteristics of vertical axis wind turbines are considerably influenced by dyn... more The performance characteristics of vertical axis wind turbines are considerably influenced by dynamic stall phenomenon which is the processes of delayed flow separation caused by a rapid excursion in the angle of attack of rotating blades. It leads to a sudden drop in lift forces which further reduce the rotor power output and efficiency significantly. The present analysis is to explore the ideas for enhancing the performance and efficiency for a vertical axis wind turbine model by implementing leading-edge slat on blades of the turbine to delay flow separation. In this paper a comprehensive two-dimensional computational fluid dynamics study has been carried out on airfoil under transient pitch oscillating motion, to obtain detailed flow fields for analysis and flow visualization. The Reynolds Averaged Navier-Stokes K-omega shear stress transport model is used to predict turbulence during computations. In addition, a sliding mesh technique along with the user-defined function is used. The outcomes show that the improved design of the turbine with the leading edge slat not only increases the lift coefficient at low Reynolds number but also enhanced the capability of wind energy extraction compared baseline model. The flow separation was delayed by a certain degree due to the leading edge slat. Consequently, the proposed model is appropriate and it can be used various lifting objects, to enhance the output power and efficiency.
Materials, 2021
Downsizing in engine size is pushing the automotive industry to operate compressors at low mass f... more Downsizing in engine size is pushing the automotive industry to operate compressors at low mass flow rate. However, the operation of turbocharger centrifugal compressor at low mass flow rate leads to fluid flow instabilities such as stall. To reduce flow instability, surface roughness is employed as a passive flow control method. This paper evaluates the effect of surface roughness on a turbocharger centrifugal compressor performance. A realistic validation of SRV2-O compressor stage designed and developed by German Aerospace Center (DLR) is achieved from comparison with the experimental data. In the first part, numerical simulations have been performed from stall to choke to study the overall performance variation at design conditions: 2.55 kg/s mass flow rate and rotational speed of 50,000 rpm. In second part, surface roughness of magnitude range 0–200 μm has been applied on the diffuser shroud to control flow instability. It was found that completely rough regime showed effective...
2019 Sixth International Conference on Aerospace Science and Engineering (ICASE), 2019
One of the prime contributors to entropy generation and performance loss in centrifugal compresso... more One of the prime contributors to entropy generation and performance loss in centrifugal compressors is the presence of tip-clearance gap and associated tip-leakage flow. This paper aims to study the fundamental performance variation of a high-pressure transonic centrifugal compressor at different tip clearances using three-dimensional (3D) numerical simulations. In this study the test case used is the transonic SRV2-O compressor designed and developed by DLR (German Aerospace Center). Reynolds Averaged Navier-Stokes (RANS) based mathrmk−mu\mathrm{k}-\mumathrmk−mu model has been used for numerical simulations. The performance of centrifugal compressor is significantly affected by tip-leakage flow, which combines with the secondary flow and results in a considerable entropy generation accumulated at the shroud suction-side of the impeller blade. For understanding of flow mechanism, eleven different tip clearances have been simulated along with comprehensive evaluation of the tip-leakage flow. At constant tip-clearance ranging 0.01 to 1mm, a loss of about 17.7% occurred in the stage pressure ratio of compressor and isentropic efficiency was dropped by 5.5%, at design conditions of 2.55 kg/s of mass flow rate and 50,000 1/min. The compressor isentropic efficiency and pressure ratio is observed optimum at constant tip-clearance in range 0.09 mm and 0.18 mm. While at variable tip-clearance between leading-edge (LE) and trailing-edge (TE), almost 11 percent loss has been noted in the stage pressure ratio, while isentropic efficiency was reduced by 3.3 percent as tip clearance increased from 0.3 to 1.1mm at the leading edge and 0.1 to 0.9 mm at the blade exit of the impeller. This paper provides the quantitative estimate of losses in compressor pressure ratio and efficiency due to tip-clearance.
International Journal of Engineering Works, 2019
This paper carries out a comprehensive numerical investigation of turbocharger high-pressure rati... more This paper carries out a comprehensive numerical investigation of turbocharger high-pressure ratio centrifugal compressor impeller. The aim is to study the effect of varying mass flow rate on the pressure ratio and efficiency from stall to choke using (3D) numerical simulations. The transonic SRV2 compressor developed by DLR (German Aerospace Center) has been used as the test case in this study. Numerical simulations have been performed using Reynolds Averaged Navier-Stokes (RANS) based k-ɛ model to predict turbulence. Y-plus is kept 35 for the structured mesh near the boundaries. In first part, calculations were carried out for design speed of 50,000 1/min to study the suitability of ANSYS CFX in the design procedure and compared the results with experimental data and four other (3D) solvers. The numerical simulations showed that ANSYS CFX over predicts the experimental data by 9% in this compressor. The second part describes the effect of vaneless diffuser exit width on performance parameters of centrifugal compressor at design high rotational speed, which shows that decreasing vaneless diffuser exit width increases pressure ratio, isentropic efficiency and operating range from stall to choke.
Journal of Non-Equilibrium Thermodynamics, 2021
In the present study, conjugate heat transfer and fluid flow performance of microchannel heat sin... more In the present study, conjugate heat transfer and fluid flow performance of microchannel heat sink has been investigated using dimensionless parameters. Novel ribs of four different types are introduced on the side walls of channel, which include trapezoidal ribs, rectangular ribs, hydrofoil ribs, and elliptical ribs. The performance evaluation has been conducted by comparing friction factor (f), Nusselt number ( N u Nu ), fluid bulk temperature ( T f {T_{f}} ), wall shear stress (τ), field synergy number ( F c Fc ), irreversible heat loss ( Q d {Q_{d}} ), and Bejan number ( B e Be ) in a Reynolds number, ranging from Re = 100 \mathit{Re}=100 to Re = 1000 \mathit{Re}=1000 . The results revealed that the addition of these novel ribs are helpful in improving the overall thermal and hydraulic performance of microchannel heat sink. From the results of Bejan number, it has been revealed that more than 96 % of losses are because of heat transfer. However, at low Reynolds number, the frict...
IOP Conference Series: Materials Science and Engineering, 2020
The focus of this research is to numerically investigate the effect of blade hub line variation o... more The focus of this research is to numerically investigate the effect of blade hub line variation on the performance (Total pressure-ratio and Isentropic-Efficiency) of centrifugal compressor from stall to choke to study the operating range and stall margin. An optimization technique is carried out in which the meridional profile hub line is modified and compared with the high Mach number SRV2 compressor designed and fabricated by DLR (German Aerospace Center). Numerical simulations showed significant increase in the stall margin and operating range by bargaining on pressure ratio and isentropic efficiency. Reynolds Averaged Navier Stokes (RANS) based k-ε model is used to predict turbulence using numerical simulations. The value of Y plus for the structured mesh near the boundaries is kept 35. Blade hub line being the important parameter, has substantial performance improvement of centrifugal compressor. The novel design improved the stall margin by 44 percent while operating range fr...
Energies
In this paper, a thorough 2D unsteady computational fluid dynamic analysis was performed on a pit... more In this paper, a thorough 2D unsteady computational fluid dynamic analysis was performed on a pitching airfoil to properly comprehend the dynamic stall and aerodynamic forces. The computational software ANSYS Fluent was used to solve the unsteady Reynolds-averaged Navier–Stokes equations. Low Reynolds number flows were modeled using the k-ω shear stress transport turbulence model. Aerodynamic forces, fluid flow structures, and flow separation delay angles were explored as a function of the Reynolds number, reduced frequency, oscillation amplitude, and mean angle of attack. The maximum aerodynamic forces, including lift, drag, and the onset of the dynamic stall, were all influenced by these variables. The critical parameters that influenced the optimum aerodynamic forces and ended up causing dynamic stall delay were oscillation amplitude and mean angle of attack. The stall angle was raised by 9° and 6°, respectively, and a large increment in the lift coefficient was also noted in bot...
Scientific Reports
The present study aims to investigate the performance of microchannel heat sink via numerical sim... more The present study aims to investigate the performance of microchannel heat sink via numerical simulations, based on the first and second law of thermodynamics. The heat transfer and flow characteristics of rectangular microchannel heat sinks have been improved by adding six different types of surface enhancers. The cross-sections include rectangular, triangular, and hexagonal-shaped ribs and cones. The cones have been created from the same cross-sections of ribs by drafting them at an angle of 45° orthogonal to the base, which is expected to decrease the pressure drop, dramatically. The performance of ribs and cones has been evaluated using different parameters such as friction factor, wall shear stress, entropy generation rate, augmentation entropy generation number, thermal resistance, and transport efficiency of thermal energy. The results of the present study revealed that the novel effect of coning at an angle of 45° reduces frictional losses (Maximum pressure drop reduced is 8...
2019 3rd International Conference on Energy Conservation and Efficiency (ICECE), 2019
The performance characteristics of vertical axis wind turbines are considerably influenced by dyn... more The performance characteristics of vertical axis wind turbines are considerably influenced by dynamic stall phenomenon which is the processes of delayed flow separation caused by a rapid excursion in the angle of attack of rotating blades. It leads to a sudden drop in lift forces which further reduce the rotor power output and efficiency significantly. The present analysis is to explore the ideas for enhancing the performance and efficiency for a vertical axis wind turbine model by implementing leading-edge slat on blades of the turbine to delay flow separation. In this paper a comprehensive two-dimensional computational fluid dynamics study has been carried out on airfoil under transient pitch oscillating motion, to obtain detailed flow fields for analysis and flow visualization. The Reynolds Averaged Navier-Stokes K-omega shear stress transport model is used to predict turbulence during computations. In addition, a sliding mesh technique along with the user-defined function is used. The outcomes show that the improved design of the turbine with the leading edge slat not only increases the lift coefficient at low Reynolds number but also enhanced the capability of wind energy extraction compared baseline model. The flow separation was delayed by a certain degree due to the leading edge slat. Consequently, the proposed model is appropriate and it can be used various lifting objects, to enhance the output power and efficiency.
Materials, 2021
Downsizing in engine size is pushing the automotive industry to operate compressors at low mass f... more Downsizing in engine size is pushing the automotive industry to operate compressors at low mass flow rate. However, the operation of turbocharger centrifugal compressor at low mass flow rate leads to fluid flow instabilities such as stall. To reduce flow instability, surface roughness is employed as a passive flow control method. This paper evaluates the effect of surface roughness on a turbocharger centrifugal compressor performance. A realistic validation of SRV2-O compressor stage designed and developed by German Aerospace Center (DLR) is achieved from comparison with the experimental data. In the first part, numerical simulations have been performed from stall to choke to study the overall performance variation at design conditions: 2.55 kg/s mass flow rate and rotational speed of 50,000 rpm. In second part, surface roughness of magnitude range 0–200 μm has been applied on the diffuser shroud to control flow instability. It was found that completely rough regime showed effective...
2019 Sixth International Conference on Aerospace Science and Engineering (ICASE), 2019
One of the prime contributors to entropy generation and performance loss in centrifugal compresso... more One of the prime contributors to entropy generation and performance loss in centrifugal compressors is the presence of tip-clearance gap and associated tip-leakage flow. This paper aims to study the fundamental performance variation of a high-pressure transonic centrifugal compressor at different tip clearances using three-dimensional (3D) numerical simulations. In this study the test case used is the transonic SRV2-O compressor designed and developed by DLR (German Aerospace Center). Reynolds Averaged Navier-Stokes (RANS) based mathrmk−mu\mathrm{k}-\mumathrmk−mu model has been used for numerical simulations. The performance of centrifugal compressor is significantly affected by tip-leakage flow, which combines with the secondary flow and results in a considerable entropy generation accumulated at the shroud suction-side of the impeller blade. For understanding of flow mechanism, eleven different tip clearances have been simulated along with comprehensive evaluation of the tip-leakage flow. At constant tip-clearance ranging 0.01 to 1mm, a loss of about 17.7% occurred in the stage pressure ratio of compressor and isentropic efficiency was dropped by 5.5%, at design conditions of 2.55 kg/s of mass flow rate and 50,000 1/min. The compressor isentropic efficiency and pressure ratio is observed optimum at constant tip-clearance in range 0.09 mm and 0.18 mm. While at variable tip-clearance between leading-edge (LE) and trailing-edge (TE), almost 11 percent loss has been noted in the stage pressure ratio, while isentropic efficiency was reduced by 3.3 percent as tip clearance increased from 0.3 to 1.1mm at the leading edge and 0.1 to 0.9 mm at the blade exit of the impeller. This paper provides the quantitative estimate of losses in compressor pressure ratio and efficiency due to tip-clearance.
International Journal of Engineering Works, 2019
This paper carries out a comprehensive numerical investigation of turbocharger high-pressure rati... more This paper carries out a comprehensive numerical investigation of turbocharger high-pressure ratio centrifugal compressor impeller. The aim is to study the effect of varying mass flow rate on the pressure ratio and efficiency from stall to choke using (3D) numerical simulations. The transonic SRV2 compressor developed by DLR (German Aerospace Center) has been used as the test case in this study. Numerical simulations have been performed using Reynolds Averaged Navier-Stokes (RANS) based k-ɛ model to predict turbulence. Y-plus is kept 35 for the structured mesh near the boundaries. In first part, calculations were carried out for design speed of 50,000 1/min to study the suitability of ANSYS CFX in the design procedure and compared the results with experimental data and four other (3D) solvers. The numerical simulations showed that ANSYS CFX over predicts the experimental data by 9% in this compressor. The second part describes the effect of vaneless diffuser exit width on performance parameters of centrifugal compressor at design high rotational speed, which shows that decreasing vaneless diffuser exit width increases pressure ratio, isentropic efficiency and operating range from stall to choke.
Journal of Non-Equilibrium Thermodynamics, 2021
In the present study, conjugate heat transfer and fluid flow performance of microchannel heat sin... more In the present study, conjugate heat transfer and fluid flow performance of microchannel heat sink has been investigated using dimensionless parameters. Novel ribs of four different types are introduced on the side walls of channel, which include trapezoidal ribs, rectangular ribs, hydrofoil ribs, and elliptical ribs. The performance evaluation has been conducted by comparing friction factor (f), Nusselt number ( N u Nu ), fluid bulk temperature ( T f {T_{f}} ), wall shear stress (τ), field synergy number ( F c Fc ), irreversible heat loss ( Q d {Q_{d}} ), and Bejan number ( B e Be ) in a Reynolds number, ranging from Re = 100 \mathit{Re}=100 to Re = 1000 \mathit{Re}=1000 . The results revealed that the addition of these novel ribs are helpful in improving the overall thermal and hydraulic performance of microchannel heat sink. From the results of Bejan number, it has been revealed that more than 96 % of losses are because of heat transfer. However, at low Reynolds number, the frict...
IOP Conference Series: Materials Science and Engineering, 2020
The focus of this research is to numerically investigate the effect of blade hub line variation o... more The focus of this research is to numerically investigate the effect of blade hub line variation on the performance (Total pressure-ratio and Isentropic-Efficiency) of centrifugal compressor from stall to choke to study the operating range and stall margin. An optimization technique is carried out in which the meridional profile hub line is modified and compared with the high Mach number SRV2 compressor designed and fabricated by DLR (German Aerospace Center). Numerical simulations showed significant increase in the stall margin and operating range by bargaining on pressure ratio and isentropic efficiency. Reynolds Averaged Navier Stokes (RANS) based k-ε model is used to predict turbulence using numerical simulations. The value of Y plus for the structured mesh near the boundaries is kept 35. Blade hub line being the important parameter, has substantial performance improvement of centrifugal compressor. The novel design improved the stall margin by 44 percent while operating range fr...