Mahbub Ahmed

Wind tunnel testing and numerical simulation on aerodynamic performance of a three-bladed Savonius wind turbine

The purpose of this research work is to investigate experimentally and computationally the feasibility of improving the performance of the vertical-axis Savonius wind turbine. The authors first performed a series of wind tunnel investigations on semi-cylindrical three-bladed Savonius rotor scale models with different overlap ratios and without overlap. These experiments were conducted in front of a low-speed subsonic wind tunnel at different Reynolds numbers. Pressures around the concave and convex surfaces of each blade, as well as the static torque for the rotor models, were measured. Using these experimental data, the authors calculated aerodynamic characteristics such as drag coefficients, static torque coefficients, and power coefficients. The authors then performed computational fluid dynamics (CFD) simulations using the commercial CFD software FLUENT and GAMBIT to analyze the static rotor aerodynamics of those models. The experimental and computational results were then compared for verification. Three different models with different overlap ratios were designed and fabricated for the current study to find the effect of overlap ratios. The results from the experimental part of the research show a significant effect of overlap ratio and Reynolds number on the improvement of aerodynamic performance of the Savonius wind turbine. At higher Reynolds number, the turbine model without overlap ratio gives better aerodynamic coefficients, and at lower Reynolds number, the model with moderate overlap ratio gives better results.

Mixing Dynamics of A Channel Type Microcombustor

The paper presents a numerical investigation on the mixing behavior of hydrogen and air inside a 5 × 5 × 40 mm channel type microcombustor. The compressible Navier-Stokes equations are solved numerically over a 3-D computational domain to understand the isothermal mixing of fuel and air. For a constant equivalence ratio, the mass flow rates of fuel and air are continuously increased to understand the effects of the power throughput on the combustor stability. At low flow rates the combustor yields adequate mixing and the airfuel ratio stays within the flammable limit throughout the second half of the combustion chamber. Also in compared to the local flow velocity, the laminar flame velocity remains higher along the second half of the chamber which indicates a possibility of stable combustor operation without the velocity blowout. However, at higher mass flow rates (m> 8.0×10 7 kg/s, P > 96W), local flow velocities become higher and tend to surpass the local flame propagation velocity which marks the threshold limit of the combustor stability in terms of power output and velocity blowout.

An Investigation of Methane Combustion in a Rectangular Shaped Meso Chamber

Hydrocarbon-based miniature power generators are promising any many application since hydrocarbon based fuels have higher power densities compared to conventional lithium batteries. A 40mm long meso-combustor of two different configurations, two-inlet and three-inlet, were used to investigate the combustion of methane in the meso-chamber. A non-premixed combustion of methane and oxygen was simulated numerically using a steady laminar flamelet model. The mesh generation and the CFD simulation were performed using ANSYS FLUENT software. A a finite volume approach was used for the simulation. The fuel-oxidizer mixing, thermal behavior and fuel burning efficiency were studied. An adequate mixing that supports the combustion was observed in certain locations. The exhaust gas was analyzed experimentally. The temperature distributions were also observed to predict the flame locations. According to the numerical analysis it was apparent that the flame would be anchored in the well mixed regions of the chamber the flames were found to be attached in two distinct locations. One in the upstream zone and the other one in the downstream zone. Another important finding was that the fuel lean condition produced higher efficiency than the fuel rich condition.Copyright

An Investigation of Lean Limits of Hydrogen Flame at Meso-Scale

The demands for hydrogen-based power generators are increasing in the field of very small scale power generation due to their very high power density. Combustion is very crucial to these scaled down versions of combustors, as the flame stability range is narrowed down as their size decreases. The current research is focused on the stability of hydrogen flames in rectangular channel meso-combustors. The combustion of hydrogen fuel with air as oxidizer has been investigated at different size meso-combustors and different flow configurations. The different size combustors are 5mυ5mυ40mm, 2.75mυ5mυ50mm, and 5mυ5mυ20mm, and the flow configurations for the 40 mm long combustor are 3-inlet and 5inlet. The experimental investigation of the hydrogen flame in the meso-combustor reveals that the 5-inlet combustor has higher flame stability than the 3-inlet configuration for the same size combustor and same mass flow rates. Thus, by splitting the axial air mass to an extra pair of vertical inlets, the stability will be enhanced. The width of the combustor is more crucial to the stability of a flame than the length of the combustor. By increasing the length of the meso-combustor, the stability can be increased slightly. The experiment also shows that a slight decrease in the combustor width can reduce the flame stability remarkably. Numerical simulations and experimental flame visualizations were also performed to investigate the mixing and flame behavior in case of 40 mm combustors.

Cross Flow Mixing in a Meso-chamber

The flow-field of a generic cross-flow mixing configuration in a millimeter scale channel was computed and experimentally measured. The characteristics Reynolds number of the mixing configuration was less than 1000 and the jet to cross velocity ratios were maintained at 2 and 3.95. The mean velocity in the jet symmetry plane and in planes normal to the jet axis was measured using a micro LDV. In order to understand the scaling behavior of fuel-air mixing in mesoscale combustors, the jet trajectory and entrainment behavior are compared with existing macroscale correlations. It was found that the computed origin of the jet trajectory was offset from the jet injection point due to interactions of the thick boundary layer of the jet to the cross flow. The study also shows a strong dependency of the trajectory path with the jet to cross velocity ratio (R) at millimeter scale combustors. At higher R, the wall boundary affects the growth of the jet and the prediction of jet behavior through existing correlations becomes uncertain. At velocity ratio, R=3.95, the entrainment coefficient in the millimeter scale channel has been found higher compared to values reported in literature.