3D laminar flow simulations were conducted using OpenFOAM to resolve the temperature, concentration, velocity, and pressure field for two hollow fiber vacuum membrane distillation configurations with feed solution flowing either inside or outside a single hollow fiber. The fiber has a circular cross-section, a fixed length of 120mm, and an inner diameter of 2 mm. The wall thickness was varied from 150 to 450μm, and the pore diameter was varied from 0.1 to 0.3μm based on commercial fibers. The feed solution is an aqueous solution of water and NaCl. The feed flow was simulated at a Reynolds number of 500 and vacuum pressure of 5,000 Pascals. It was found that there was a 75% increase in flux, from 9.58 to 41.41 kg/m2h, between the worst and the best case in membrane properties. Increasing the pore diameter or wall thickness while the other value was fixed resulted in a 45–57% flux increase depending on the fixed value. The module with the feed solution flowing outside a hollow fiber yields 24% higher flux than the module with the feed solution flowing inside the hollow fiber at the same conditions.
2D Large Eddy Simulations (LES) were conducted to study the effect of channel geometry on the Vacuum Membrane Distillation process. The geometry was altered by imposing a sinusoidal (“wiggly”) wall profile. The results of the study show at a critical Reynolds number between 750 and 1,000, transient vortices appear in the channel. As the Reynolds number is increased from 1,000 to 1,500, the origin of the vortices moves further upstream, and the frequency and intensity of the vortex activity increase. Areas, where vortex shedding provides mixing, serve to enhance the performance of membrane distillation profoundly. The mixing reduces temperature and concentration polarization along the membrane surface. With better mixing provide at Reynolds number 1,500 versus 1,000, the difference in performance versus the corresponding flat sheet membrane case is increased. The vapor flux in the wiggly channel module increases 6% at Reynolds number 1,000 and 22% at Reynolds number 1,500 compared to the flat channel module. The change in flux is from 61.6 to 65.7 kg/m2/h at Reynolds number 1,000 and 66.5 to 81.4 kg/m2/h at Reynolds number 1,500. Temperature polarization was also mitigated at Reynolds number 1,500 even though the flux was increased.
CFD simulations were run to determine the impact of buoyancy-driven natural convection on Membrane Distillation, a thermally driven filtration process. A computational domain was created with a high Grashoff number, and vertical and horizontal modules were considered. Convection cells were dominant in the channel at low Reynolds numbers and effectively increased the performance of Membrane Distillation modules by up to 66%. This was done by generating convection cells that reduced the temperature/concentration polarization inside that module, typically forming due to the low Reynolds numbers and correspondingly high thermal and concentration boundary layers. It is shown here that natural convection enhances the flux performance of membrane distillation immensely.
The open-source C++ toolbox OpenFOAM is used to perform the Computational Fluid Dynamics (CFD) simulations in two-dimensional microfluidic devices to characterize the viscoelastic flow. The Oldroyd-B constitutive equation is coupled with continuity and momentum equations. Multiple stabilization methods are applied to the numerical simulation to simulate High Weissenberg Number Problem (HWNP) in the microchannel. We applied the Log Conformation Reformulation (LCR) method to guarantee the positive definiteness of the stress tensor. The CUBISTA scheme and the improved Both Side Diffusion (iBSD) method are applied to predict the flow behavior at high elasticity regions without numerical oscillations. Various microstructures, including circles and flat plates, are placed in the center of the channel as the confinement. Our previous work demonstrated that the polyhedral mesh with hexahedral inflation layers effectively meshes complex microstructures in microchannels. A viscoelastic fluid is injected from the inlet at varying flow rates, corresponding to the local Weissenberg number up to 25. A parametric study is conducted on the first normal stress difference (N1) in specific regions with an accurate prediction of the viscoelastic flow field near the microstructures.
Design and optimization using computational fluid dynamics to enhance the hydro turbine’s performance are becoming gradually more common because of its flexibility, minor detailed flow description, and cost-effectiveness. These features are not easily achievable in model testing. k–ω simulations conducted in OpenFOAM 7 characterize the flow structure inside an industrial-sized Kaplan turbine module operating at the peak design flowrate. The power signal, velocity, vorticity, and pressure field are presented over the blades and throughout the draft tube. Additionally, pressure fluctuations were probed along the draft tube wall. The simulation shows a tip vortex rope in the narrow gap between the blade tip and turbine casing. The strong influence of the swirl leaving the runner had a negative impact on the flow pressure fluctuation. Also, high vortical activity was presented near the draft tube wall, leading to turbine instability. It was demonstrated that the turbine generates 14.923 MW of average power. The power signal showed minor fluctuations induced by the vortical activity close to the runner region and the corresponding pressure fluctuations. The Fast Fourier Transform showed the system is dominated by low frequency, high amplitude fluctuations.
The performance of a pump-turbine under partial flow rates, 85%, 75%, and 65%, is studied using the LES model. The power signal, velocity, vorticity, and pressure field is presented over the blades and throughout the draft tube. Pressure fluctuations are probed at various locations over the wall of the draft tube. Examining the flow field in the blade region can provide further insights into the system performance. Flow-induced pressure fluctuations can disrupt system stability. For this turbine, a strong swirling region is observed around the draft tube walls, causing pressure fluctuations. The size and intensity of this region decrease with the flow rate. A vortex rope is present in all cases. At the design point, the strength is constant throughout the draft tube. However, at partial load, the rope is weakened along the draft tube. Between the region dominated by the vortex rope and the wall, there is a swirling shear layer, which moves closer to the wall as the flow rate decreases. Both the magnitude of pressure fluctuations at the wall and the pressure difference over the blade decrease with the flow rate. The decreased pressure differences over the blade represent less power produced, and the decline in fluctuation magnitude at the wall represents more system stability. For this turbine, there appears to be a trade-off between power and strength of pressure fluctuations.
The fluid flow and heat transfer inside a concrete thermal energy storage module is simulated for various heat transfer fluid flow rates and inlet temperatures. The storage performance of the module is characterized based on the volume-averaged temperature and normalized energy distribution through the block versus time. In the turbulent flow regime, induced mixing in the pipe strongly enhanced the performance of the module compared to the laminar regime. The block was able to fully charge and discharge in a turbulent flow regime, whereas that behavior was not present in the laminar flow regime. Varying the heat transfer temperature had an effect on the time rate of change of temperature as well as the charge times. As the thermal gradient increased, the initial time rate of temperature in the block increased as well as the charge time. Since the block has higher theoretical energy at a larger gradient, power over a longer duration is necessary to reach a saturation point. By characterizing the thermal performance of the module, the effect of material properties and operational parameters can be studied in order to design a module that can meet the needs of a power generation plant.
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