1. Ph.D Theses

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Aerodynamic Performance of Leading Edge Protuberances at Low Reynolds Number
(National Institute Of Technology Karnataka, Surathkal, 2024) Chittepu, Jayapal Reddy; A, Sathyabhama
Wind energy plays a pivotal role in the renewable energy sector, offering sustainable and clean electricity, thus mitigating climate change by reducing greenhouse gas emissions and diversifying the energy mix away from fossil fuels. With increasing electricity demand, environmental concerns, and technological advancements, wind energy has gained prominence. The ambition for green energy heavily relies on tapping into the largely untapped potential of Small Horizontal Axia Wind Turbines (SHAWT). These turbines often face challenges such as low wind speed, small rotor size, and laminar separation, resulting in poor performance. Enhancing the aerodynamic efficiency of small wind turbine blades is crucial for increasing power and overall effectiveness. Various active and passive techniques are available to improve turbine blade aerodynamics. One passive method involves adopting leading-edge modifications inspired by Humpback Whale tubercles. These modifications, particularly effective in low Reynolds number flows, enhance aerodynamic performance by elevating the maximum lift coefficient and delaying stall. In the first section of thesis, a detailed investigation on the aerodynamic performance of two distinct airfoils, E216 and SG6043, operating at 100K Reynolds numbers is presented. The emphasis is primarily on the effect of leading-edge protuberances on the aerodynamic properties of these airfoils. Numerical simulations in ANSYS FLUENT using the SST k - 𝜔 turbulent flow model and experimental analyses in a subsonic wind tunnel using a sensitive three-component force balance were carried out. Three protuberance shapes were investigated: sinusoidal, slot, and triangular, with amplitudes (A) of 0.03c, 0.06c, and 0.11c and wavelengths (W) of 0.11c, 0.21c, and 0.43c relative to the chord length (c) of the airfoil. The variation in amplitude and wavelength combinations resulted in nine distinct models. The numerical study examined fifty six models, including baseline and protuberance models of E216 and SG6043 airfoils. Out of these, wind tunnel experiments were conducted on the baseline model as well as one model each of the three protuberance shapes to validate the numerical findings, totalling eight models for validation. The studies covered an angle of attack range of 0° to 20°. Numerical results showed that the sinusoidal protuberances caused a 2° stall delay with lower amplitudes, improving E216 A4.5W64.5 Clmax by 2.88% and (L/D)max by 6.22%. However, for SG6043, Clmax decreased by 10.21%, and (L/D)max dropped by 4.38%. Triangular protuberances also delayed stall by 2° to 4° for E216 A4.5 and A9, enhancing Clmax and (L/D)max. The E216 A4.5W64.5 model exhibited an 11.2% Clmax increase and a 14.43% rise in (L/D)max at stall angle, while SG6043 A4.5W64.5 showed a 4.37% Clmax decrease and a 1.92% (L/D)max drop at stall angle. Slot protuberances also delayed stall by 2° to 4°. The E216 airfoil demonstrated improved Clmax and stall delay, while SG6043 enhanced Clmax but reduced (L/D)max. Slot and triangular A4.5 models excelled in stall delay and post-stall performance, favoring low amplitude and high wavelength configurations. Further, the study was extended to investigate the effects of Reynolds numbers on E216 airfoil experimentally using strain gauges and data acquisition arrangement in subsonic wind tunnel facility. The Reynolds numbers considered for experimental investigation are 50K, 100K, and 150K. The study explored Reynolds number effects on protuberances, revealing minimal impact at 50K Reynolds numbers. However, at 100K and 150K Reynolds numbers, improvements were evident: enhanced post-stall lift coefficients and stall delay compared to the baseline. At 150K Reynolds number, slot, triangular, and sinusoidal protuberances showed notable increases in maximum lift coefficients by 29%, 23%, and 13%, respectively, compared to the baseline. Furthermore, this study was extended to experimentally investigate the aerodynamic behaviour of the E216 airfoil with protuberances under dynamic conditions. This investigation made use of a stepper motor, strain gauges, and a data acquisition system in a subsonic wind tunnel facility. The reduced frequencies (k) considered in this study are 0.025, 0.05, and 0.065. In essence, introducing protuberances to the airfoil not only influences lift coefficient patterns but also affects the hysteresis loop size during dynamic stall. Moreover, protuberance models exhibited smoother post-stall behavior compared to the baseline. Overall, slot and triangular protuberances notably enhanced Clmax compared to both the baseline and sinusoidal protuberance models. The smoke and tuft flow visualization techniques revealed important insights into flow patterns, assisting in understanding the flow physics. The smoke flow study observed increased post-stall lift with increasing angles of attack due to the merging of secondary flow with primary flow. While sinusoidal and triangular profiles exhibited similar behavior, triangular leading edges effectively guided flow to trough regions, resulting in a larger secondary flow volume. Triangular protuberances proved more effective for drag reduction on the SG6043 airfoil, particularly at a Reynolds number of 100K, and contributed to post-stall lift improvement. According to the tuft flow visualization, separation of flow was observed at different distances from the leading edge in protuberance models, around 22% for the sinusoidal model and 24% for the triangular and slot configurations. Significantly, both the slot and triangular protuberance models showcased improved attached flow within the troughs, leading to elevated lift coefficients and stall delay. The investigation was extended further to examine the aerodynamic performance of Small Horizontal Axis Wind Turbines (SHAWT) with protuberance blades, comparing them with the baseline blades. The aerodynamic performance of wind turbines featuring various protuberance blade models – sinusoidal, triangular, and slot designs, were tested and contrasted with the baseline. Based on the results, the enhancement in power coefficient (CP) for protuberance configurations is as follows: 2.7 % for sinusoidal, 5.2 % for triangular, and 7.6 % for slot blades compared to baseline blades. The slot protuberances blade model displayed the highest CP value among the three shapes. This suggests that wind turbines incorporating slot, triangular, and sinusoidal protuberances achieved improved aerodynamic efficiency compared to baseline blades.
Experimental Investigation of Coffee Husk Biodiesel as A Renewable Fuel In Compression Ignition Engine
(National Institute Of Technology Karnataka Surathkal, 2023) Emma, Addisu Frinjo; A, Sathyabhama; Yadav, Ajay Kumar
In the present study, coffee husk (CH) and spent coffee ground (SCG) are used for the production of biodiesel. The CH is a by-product of the coffee processing industry, and SCG is obtained after the coffee is brewed. Field Emission Gun Scanning Electron Microscope (FEG-SEM) is used to investigate the elemental composition of the CH and the SCG samples and identify the presence of different elements with their distribution and concentration. The compositional analysis indicates that the CH comprised 49.84% of carbon and 48.06% of oxygen by weight. On the other hand, it is found that the SCG had 67.72% of carbon and 26.18% of oxygen by weight. The CH is selected for further study for the production of oil due to its higher oxygen distribution than SCG. From 1Kg of CH, 250g of oil is produced. By using the transesterification process, the produced oil is converted into biodiesel. Subsequently, 700 mL of coffee husk oil methyl ester (CHOME) biodiesel was produced from 1000 mL of coffee husk oil. After characterization of obtained biodiesel, the experiments are conducted in a single-cylinder direct injection diesel engine at a constant speed by varying the loads (0%, 25%, 50%, 75%, and 100%) for different biodiesel-diesel blends (B10, B20, B30, B40, B50, B80, and B100), and the results are compared with the baseline diesel. The brake thermal efficiency (BTE) of the blends, B10, B20, B30, and B50, is reduced by 0.6, 0.7, 1.29, and 3%, respectively, compared to the regular diesel. Similarly, the brake specific energy consumption (BSEC) is increased by 0.1, 0.3, 0.44, and 0.77% for B10, B20, B30, and B50, respectively. Exhaust gas emissions are reduced for all biodiesel-diesel blends with a marginal increase in NOx emission. Compared to regular diesel, at full load, CO, HC, and smoke opacity of B30 are reduced by 13.2%, 4%, and 12%, respectively. Whereas NOx and CO2 of B30 at full load are increased by 3.8% and 8.63% respectively The viscosity of CHOME biodiesel is found to be higher than diesel; hence a preheating mechanism is set to reduce the viscosity and density of the fuel before injecting it into the combustion chamber. Preheating the neat CHOME biodiesel (B100) to 95 °C decreased its viscosity and density by 49.5% and 3.7%, respectively. Running the engine with preheated B100 reduces CO, HC, and smoke opacity by 34%, 34%, and 35%, respectively, compared to unheated regular diesel. The percentage of CO2 in the exhaust gas is increased by 45.5% for preheated B100 compared with unheated B0 at 100% load. Furthermore, the injection timing of the engine is altered to find the optimum injection timing of the biodiesel-diesel blend. The BSEC is increased by 0.53 kg/kWh and reduced by 1.4 kg/kWh for advanced and retarded injection timing, respectively. By advancing injection timing, the HC, CO, and smoke opacity were reduced by 7.4%, 36%, 5.7%, and 7%, respectively, compared to the B30 at standard injection timing.
Investigation on Subcooled Flow Boiling Heat Transfer to Water-Ethanol Mixture in Conventional Channel
(National Institute of Technology Karnataka, Surathkal, 2018) B. G, Suhas; A, Sathyabhama