Browsing by Author "Xu, Ben"
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Item Investigating the Effects of Helical-shaped Blades on the Wake Characteristics and Performance of Vertical Axis Wind Turbines using Large Eddy Simulations(2023-08) Gharaati, Masoumeh; Yang, Di; Liu, Dong; Alba, Kamran; Floryan, Daniel; Xu, BenTurbulent wake flows behind helical- and straight-bladed vertical-axis wind tur bines (VAWTs) are studied numerically using the large eddy simulation method com bined with the actuator line model. The effects of blade geometry on turbine wake characteristics are explored at both lab- and field-scale VAWTs differed by their oper ational tip-speed ratios (TSRs). For the lab-scale VAWT, a small-size 5-blade VAWT design is considered, which operates at relatively low TSRs of 0.4-0.6 at the wind speed of 13m/s. For the field-scale VAWT, a commercial 3-blade VAWT is consid ered, which operates at a higher TSR of 1.19 at the wind speed of about 11 − 12m/s. In both cases, the wake flows behind straight- and helical-bladed VAWTs were sim ulated, and the flow characteristics were quantified and compared. At low TSRs, the simulation results show that the wake behind the straight-bladed VAWT expands considerably in the spanwise direction due to the quasi-2D vortex shedding effect from the straight blades. In contrast, the helical-bladed VAWT generates highly 3D wake flow structures to produce a relatively narrow wake with faster decay of turbulent intensity. At high TSRs, the helical-bladed VAWTs generate a screwdriver effect to induce mean vertical flow motions at the spanwise edges of the wake flow, which are balanced by a mean vertical counter-flow (i.e., with reversed direction) near the center of the wake. As a result, the wake flows behind helical VAWTs exhibit vertical tilting that affects the turbulent intensity and the Reynolds transport of momentum in the shear layers around the VAWT wake region, leading to faster wind speed recovery than the wake behind straight-bladed VAWTs. Suppose the different VAWT designs are used in wind farm applications. In that case, the field-scale helical-bladed VAWTs may improve the mean power production rate for the fully developed region of the wind farm by up to about 7.33% compared with the corresponding straight-bladed VAWT. Using the helical-bladed VAWTs also reduces the fatigue load on the structure by significantly reducing the spanwise bending moment (relative to the bottom base), which may improve the longevity of the VAWT system to reduce the long-term maintenance cost.Item INVESTIGATION OF PHASE CHANGE HEAT TRANSFER ON NANOSTRUCTURED SURFACES(2023-05-11) Talari, Vishal; Liu, Dong; Varghese, Oomman K.; Yang, Di; Zhao, Bo; Xu, BenPhase change heat transfer, such as evaporation, boiling, and condensation, is critical to various applications in thermal management, power generation, and water harvesting. Enhancing phase change heat transfer through micro/nanoengineered surfaces has been effective but suffers from manufacturing complexity and long-term performance degradation. This study focuses on utilizing a cost-effective and mature nano-tubular structure, titanium dioxide (TiO2) nanotube, to enhance two major modes of phase change heat transfer - evaporation and condensation. The unique hollow morphology of TiO2 nanotubes results in significant changes in surface roughness, solid fraction, and porosity, which influence the capillary pressure and permeability and accelerates liquid spreading over the surface. A comprehensive theoretical model is developed and validated by experiments, which will serve as a predictive tool for the design of future nanotube-based wetting surfaces. Subsequently, this work proves that TiO2 nanotube-enabled fast liquid spreading raises the Leidenfrost point, delaying the formation of continuous vapor film and, therefore, increasing heat transfer efficiency. Finally, a condensation apparatus is constructed to explore if and how TiO2 nanotube surface enhances condensation heat transfer. The data suggest that filmwise condensation persists on the TiO2 nanotube surface, but the ultra-thin thickness of the liquid film guarantees heat transfer performance comparable to those achieved in jumping droplet condensation on superhydrophobic micro/nanostructured surfaces. The interfacial slip model is hypothesized to explain the observed condensation enhancement on TiO2 nanotube surface. However, more research is necessary to fully elucidate the underlying mechanisms. In summary, this study reveals the unique liquid wetting and spreading properties enabled by TiO2 nanotube structure and their potentials in enhancing phase change heat transfer. The findings provide a promising direction for creating efficient, robust, and low-cost functional surfaces to improve the performance and efficiency of various energy systems that involve phase change heat transfer.Item Shale Gas Production Forecasting using Reservoir Simulation with Hydraulic Fracture Mapping(2019-05) Xu, Ben; Qin, Guan; Thakur, Ganesh C.; Ehlig-Economides, Christine; Nikolaou, Michael; Lee, Kyung JaeThis dissertation presented a hybrid Embedded Fracture Dual Porosity (EF-DP) model for shale gas production forecasting. We presented a new correlation between the microseismic magnitude and the shape factor coefficients in the classic dual-porosity model accompany a procedure to calibrate this coefficient with real production data. The novel contribution was the employment of the shape factor in the dual-porosity model to characterize a relatively complex small scale fracture network, which numerically integrates with the large scale fractures geometry in the EDFM model to forecast shale gas well production. A constrained result of using microseismic data to measure the length and direction of the large scale fractures was put into the embedded discrete fracture model (EDFM) to integrate the large scale fracture into a corner point grid. The EF-DP model considered stimulation data such as total fluid volume for hydraulic fracturing, flow rate, wellhead pressure, sand concentration, and proppant size. To verify the credibility of this model, we performed two sets of parameter sensitivity analyses for the large scale fractures and the small scale fractures. We used two sets of real-world shale gas production data for history matching and successfully used the EFDP model to quantify analysis the impact of frac-hit on a shale gas producing well. Parameter sensitivity analysis confirmed that enhanced small scale fracture permeability could effectively increase production, mainly by strengthening far-field reservoir drainage volume. According to the application results, we found that the EFDP model was effectively and accurately predict shale gas production, and quantitatively evaluate the impact of frachit between multiple wells. The refracturing candidate selection results guided further well completion strategy improvement. Microseismic-based approaches provide a robust fracture network model, further reservoir modeling calibration and simulation studies can reveal invaluable information about the active stimulated zone.