1. Ph.D Theses

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Effect ff Humpback Whale-Inspired Tubercles on Vertical Axis Wind Turbine Blade
(National Institute Of Technology Karnataka Surathkal, 2023) Joseph, Jeena; A., Sathyabhama
Vertical axis wind turbines (VAWT) are gaining popularity for their potential to be used in urban and off-grid areas and in offshore wind farms. A study on the flow control potential of Humpback whale flipper-inspired tubercles on VAWT blade is the key focus of this thesis. This work studies the aerodynamic effect of leading-edge tubercles (LET) on certain flow phenomena such as Laminar Separation Bubble (LSB), stall, and hysteresis of the VAWT blade. The effect of tubercle shape and geometrical parameters on the steady-state and unsteady-state aerodynamic characteristics has also been studied. Initially, a comparative study on unswept and various swept blades is conducted in order to detect the extent of performance enhancement brought about by incorporating LET on both types of blade. The aerodynamic characteristics of NACA 0021 swept and unswept blades have been studied and compared with their tubercle counterparts of tubercle amplitude and wavelength 6% and 21% of the airfoil chord, respectively. Tubercles along the flow direction have been incorporated on blades swept or inclined at 10◦ , 20◦ , and 30◦ and compared to the respective baselines and to unswept blades, in terms of static aerodynamic forces at a Reynolds number of 1 × 105 . It was seen that tubercles are more beneficial on unswept and blades of low sweep than on blades of high sweep angle, in attaining a smooth stall. Further, a detailed study of the effect of LET on the unswept blade has been done by an- alyzing the static force and surface pressure at varied Reynolds numbers ranging from 2.5 × 105 to 6 × 105 . The LSB begins to appear on the suction surface of unmodified blade at an angle of attack (AOA) of 6◦ , which extends between 24% and 35% of the blade chord. With the increase in AOA, the LSB moves forward towards the blade’s leading edge and decreases in size. However, due to the influence of tubercles, LSB is not present on the tubercle blade. Stall occurs earlier in the tubercle blade as com- pared to the baseline blade. The baseline blade exhibits an abrupt and deep stall that occurs in a single step, whereas the tubercle model has a soft gradual stall that occurs iiin multiple stages for all Reynolds numbers studied. Surface pressure measurements for static models further reveal the mechanism of the stall. Stall initially originates at the midsection of the blade, which progresses towards the tips of the baseline blade. However, with the introduction of tubercles, the stall progression towards the tips is inhibited on the tubercle blade. The baseline blade also exhibits static hysteresis for lift, drag, and moment coefficient curves, and its extent increases with an increase in Reynolds number. However, for the tubercle model, hysteresis was completely absent. The effect of tubercle shape and geometrical parameters on steady-state aerodynamics has been studied at a Reynolds number of 5 × 105 using blades incorporated with sinu- soidal and triangular LET of varying amplitude to wavelength ratio (A/W). The static forces on four blades with sinusoidal LET of A/W 0.25, 0.5, 0.75 and 1, and two blades of triangular LET of A/W 0.5 and 1, are analyzed and compared to that of a baseline. For blades of sinusoidal and triangular LET, the lift coefficient is lower than that of the baseline in the pre-stall region and decreases with an increase in A/W. The drag and pitching moment increases with increase in A/W. However the tubercle blades are better performing in the post-stall region, where they have higher lift than the baseline. The tubercle blades have a stall earlier than the baseline. However, the tubercle blades have a better stall characteristic than baseline in terms of smooth stall. The stall gets smoother with increase in A/W of tubercles. The steady-state aerodynamic character- istics of sinusoidal and triangular blades of corresponding A/W are closely identical to one another. Finally, considering the actual blade movement on VAWT, the effect of the LET on dynamic blades is studied. The effect of tubercles on unsteady flow has been studied by pitch-oscillating the above-mentioned blades with sinusoidal and triangular LET at various frequencies and obtaining the forces acting. The baseline blade has the maxi- mum lift coefficient, CLmax but exhibits deep stall. Large hysteresis loops are seen in the stall region for the baseline blade. The triangular and sinusoidal LET help mitigate the intensity of stall and hysteresis of the blade. The tubercle blades (sinusoidal and triangular LET) with the least tubercle amplitude have the highest CLmax after baseline. The blades of the highest tubercle amplitude has the most desirable stall and hysteresis characteristics. The size of the hysteresis loop decreases with an increase in tubercle amplitude. Blades with triangular LET perform similar to the sinusoidal leading edge except for having a higher coefficient of normal force.
Experimental and Numerical Investigation of Subcooled Flow Boiling Heat Transfer in Conventional Channels
(National Institute of Technology Karnataka, Surathkal., 2024) K., Madan; A., Sathyabhama
Electric vehicles (EVs) and hybrid electric vehicles (HEVs) have quickly become a technological focus to reduce fossil fuel usage and limit negative environmental impacts such as climate change, regional smog, and ground level ozone over the past decade. The battery is a crucial component in the development of EVs and HEVs due to its specific energy, cycling life, and high power. It is crucial to dissipate heat from the battery module and maintain its temperature within the required range, which otherwise, could lead to battery life degradation and reduction in performance. To manage high heat fluxes effectively, it is important to have an efficient thermal management system. One method that shows promise is liquid cooling, which extracts heat from the cold plate. The cold plates will have conventional channels for the coolant to flow through. Using mixtures as a cooling medium have several advantages over using pure fluids as a coolant. These include better adjustment with the intended thermal load, improved system coefficient of performance, and more environmentally friendly and safer fluids. The amount of heat extraction from the cold plate is greater in two phase flow boiling heat transfer than in the single phase convective heat transfer. Hence two phase heat transfer using binary mixtures has attracted wide attention as they provide a wider range of boiling temperature at a given pressure. Further, by geometrical modification of the channels, enhancement in the heat dissipation rate can be achieved. In view of the above facts, the heat extraction from the conventional channels under two phase flow boiling condition with ethanol-water mixture as coolant is investigated in this research work. The flow boiling heat transfer characteristics of the water, ethanol and ethanol-water mixture (25%/75% by volume) were experimentally investigated in conventional rectangular channels. Test sections of channel aspect ratio (AR=w/d) 0.2 and 1.25 were examined for the thermal performance under horizontal flow boiling conditions. The effect of ribs in the flow channels of AR=1.25 on the heat transfer characteristics were studied. Experiments were conducted for different values of mass flux, heat flux and subcooled inlet temperature. The investigation was carried out at atmospheric pressure. i Flow patterns were recorded by employing a Promon-501 high-speed camera, and stages in the bubble cycle were studied. The results show that the AR has a dominant effect on the heat transfer coefficient (HTC). At low heat flux values, higher HTC was observed for the channel of higher AR (AR= 1.25) whereas, at high heat flux conditions, the HTC is higher for the channel of lower AR (AR= 0.2). For all the working fluids, high HTC was observed for the AR=1.25 channel in the forced convective region, whereas in the subcooled boiling region, high HTC was observed for the AR=0.2 channel. The average subcooled HTC obtained for the ethanol-water mixture was 15.75% and 38.85% higher than that of water and ethanol respectively for the AR=0.2 channel. However, it was 18.11% and 41.2% higher than that for water and ethanol for the AR=1.25 channel. With the aid of visualization results, it was found that the bubble waiting and the growth period were minimum for the mixture and maximum for the ethanol, resulting in a higher HTC for the mixture and lower HTC for ethanol in both channels. With an increase in inlet subcooled temperature, the HTC decreased for both channels due to increased thermal boundary layer thickness and reduced bubble formation. Furthermore, the channel of AR=1.25 with ribs performed better than the smooth channel due to the high bubble nucleation rate. Along with the experimentation, numerical investigation was also carried out by using ANSYS Fluent 2022R1 commercial software. The numerical simulation was performed by selecting the Rensselaer Polytechnic Institute (RPI) wall heat flux partitioning approach by employing the Eulerian-Eulerian two-phase model. A three-dimensional computational domain was used for simulation to understand the fluid boiling inside the conventional channel under steady state conditions. The focus of the numerical investigation was to determine the vapor fraction which could not be measured experimentally. In addition to the two test sections considered for experimentation, for numerical work, another two test sections, (AR=0.5 and AR=1) were considered. The simulations were performed for a constant mass flux of 150.46 kg/m2-s with the heat flux value ranging from 10-100 kW/m2 and at the inlet subcooled temperatures of 303K, 313K and 323K. ii The channel surface temperature and the HTC obtained numerically were compared with the experimental results and it was found that the results are in good agreement. The volume of vapor fraction increased with the increase in heat flux for all values of inlet subcooled temperature considered in this study for all the test sections. At low inlet subcooled temperature, the volume of vapor fraction decreased with an increase in AR at all heat fluxes. However, there was no observable trend at higher heat flux and at high inlet subcooled temperature.
Liquid-Infused Surfaces for Boiling Heat Transfer Enhancement and Mitigation of Corrosion and Inorganic Scaling
(National Institute Of Technology Karnataka, Surathkal, 2024) A.V.V.R., PRASAD Y; A., Sathyabhama
One of the key sustainable development objectives of the United Nations is to ensure a reliable and consistent supply of fresh water. However, the growing population, industrialization, and urbanization have made it increasingly difficult to access an adequate amount of potable water. Consequently, desalination of brackish and seawater, which accounts for 97% of the global water resources, has emerged as an effective solution to provide potable water. In recent years, advancements in desalination research have led to increased capacity in large-scale commercial desalination plants using traditional technologies such as membrane and thermal desalination. However, these methods consume significant amounts of energy, typically obtained by burning fossil fuels and coal, leading to inevitable environmental concerns. Therefore, desalination methods those work on alternative/renewable energy sources, such as solar still, holds great potential for replacing traditional methods on a large scale. Conventional solar still desalination works by evaporating fresh water from brackish sources, a process that is extremely slow. Research has been focused on enhancing the efficiency of solar stills. Historically, efforts were directed at maximizing the evaporation process to increase distillate output. A recent study suggested that instead of relying on evaporation, inducing nucleate boiling in the solar still basin by integrating it with tools that achieve concentrated solar power could significantly enhance distillate output. To maximize distillate output when nucleate boiling is induced, modifying the morphology of the solar still basin to enhance boiling heat transfer is necessary. Additionally, continuous and enhanced fresh water recovery from brackish sources poses challenges such as inorganic scaling and corrosion. In recent years, novel methods such as superhydrophobic surfaces and slippery liquid-infused surfaces have demonstrated excellent capabilities in inhibiting corrosion and inorganic scaling. However, concerns have been raised about their heat transfer capabilities due to insufficient liquid surface contact on these surfaces. In view of the above facts, the aim of this study was to demonstrate the effectiveness of a type of liquid-infused surface called binary surface (BiS) to inhibit scaling and corrosion without compromising the heat exchange efficiency. To this end, a highly-wetting Ultra-Omniphilic Surface (UOS) was prepared from a plain aluminum alloy surface (PS) using a bulk micro manufacturing approach. Later, the sub-surface micro/nanocavities of UOS were infused with a liquid lubricant to get BiS, which has two distinct superficial phases — solid phase as islands and liquid phase as puddles. Saturated boiling heat transfer experiments were conducted on the BiS and the critical heat flux (CHF) and the boiling heat transfer coefficient (HTC) were measured. The results were compared with the UOS and PS. In addition, high-speed visualization was employed for capturing the bubble dynamics at different heat fluxes and parameters such as bubble departure diameter (Dd), bubble departure frequency (f), and nucleation site density (NSD) were measured. The results revealed that the boiling heat transfer performance of water on the BiS surpasses both the PS and the UOS. The HTC on the BiS was 1.33 times and two times larger than those on UOS and the PS, respectively. The CHF obtained on the BiS was comparable to that on the UOS and 1.47 times larger than that on the PS even though a considerable portion of the BiS surface area was covered with the liquid lubricant and unavailable for boiling. Remarkably, an inspection of the high-speed videos has suggested the presence of the same liquid lubricant as the reason for the better boiling heat transfer performance of the BiS. The liquid lubricant that was spread over the BiS as puddles was found to prevent the growth of large vapor bubbles and extend the isolated bubble regime by delaying the lateral coalescence of adjacent bubbles. Lab-scale corrosion and scaling experiments were conducted on BiS in a simulated brackish water environment. Results indicated that BiS significantly outperformed PS and UOS in hindering scaling and corrosion. BiS exhibited nearly 50% less mass gain due to mineral deposition than on PS and UOS. Moreover, corrosion rates obtained from electrochemical and immersion tests showcased a notably slower metal degradation on BiS than on PS and UOS. This enhancement is attributed to well-distributed liquid puddles on BiS, promoting a smooth, defect-free surface that reduces foulants adhesion and shields the underlying metal from corrosion.