Browsing by Subject "Crossflow"

Now showing 1 - 6 of 6

CFD analysis of a Coanda jet in a crossflow over a NACA 0012 airfoil
(2020-12-04) Janecka, Matthew Ryan; Bisetti, Fabrizio
A Coanda jet exposed to a crossflow is examined via RANS simulations with a particular focus on rolling moments. The wing employed in the study is a NACA 0012 profile with an aspect ratio of 1. The Coanda surface is formed by a revolution of the NACA 0012 profile at the tip of the wing. The Reynolds number of the crossflow is 5 × 10⁵ and angle of attack is 2 degrees. The rolling moments generated by the Coanda jet are compared to those estimated from a theoretical aileron that spans half the length of the wing with a 25 percent chord. Theory indicates that a blowing factor, (Cᵤ /α) [superscript (2/3)], of 1.13 corresponds to a control surface deflection of approximately 10.7 degrees. Furthermore, scaling laws apply to the rolling moments and lift forces for low blowing factors. At higher values of the blowing factor, secondary vortices form in the wake of the Coanda jet which cause the scaling laws to fail. In addition, a qualitative analysis is conducted which shows the formation of wing tip vortices and their evolution in the streamwise direction. A secondary vortex forms as a result of the Coanda jet detaching and reattaching to the Coanda surface. Lastly, it is seen that the Wray-Agarwal-RCM turbulence model exhibits much better convergence properties than the Spalart-Allmaras-RCM, and based on comparisons with previous studies, is thought to be the more accurate model.
Diffused-exit film cooling holes fed by an internal crossflow
(2017-05-02) McClintic, John W.; Bogard, David G.; Moser, Robert D; Clemens, Noel T; Bahadur, Vaibhav; Dyson, Thomas E; Davidson, Frederick T
Film cooling is an essential technology to the operation of modern gas turbine engines, allowing for greater efficiency and part durability. Due to film cooling’s complexity, laboratory studies of film cooling isolate various effects by intentionally simplifying or neglecting various aspects of the film cooling problem. One such aspect that had been consistently neglected by film cooling studies is how the internal flow within the turbine blade affects film cooling performance. Studies have found that feeding the holes with an internal crossflow, directed perpendicular to the mainstream flow, can cause up to a 50% reduction in film cooling effectiveness. This result is of concern because internal crossflow is a common internal flow condition in gas turbine engines. However, none of the former studies have made a concerted effort to examine the important scaling parameters governing this effect. Nor have they provided experimental evidence showing the cause of this reduction in effectiveness due to internal crossflow. In this study, a wide range of flow conditions was studied for two common film cooling hole geometry types: axial and compound angle diffused-exit film cooling holes. Internal crossflow-to-mainstream velocity ratios of VR [subscript c] = 0.2-0.6 were tested along with jet-to-mainstream velocity ratios of VR = 0.2-1.7. Film cooling effectiveness and discharge coefficients were measured for this full range of flow conditions for both geometries in order to produce a sufficiently large data set to observe important trends in the data. It was found that the discharge coefficients, centerline effectiveness, and centerline location all scaled with the crossflow-to-jet velocity ratio, VR [subscript i] for the axial holes. Temperature and velocity fields showed that VR [subscript i] also scaled the in-hole temperature and velocity fields. A swirling flow within the hole was shown to cause ingestion of mainstream into the diffused exit of the hole and biasing of the issuing jet in the outlet diffuser, which reduced film cooling effectiveness. The direction of bias at the exit resulted from the direction of the internal crossflow and was critical for compound angle holes. Crossflow directed counter to the lateral direction of coolant injection caused improved film cooling effectiveness relative to the in-line crossflow direction.
Examination of microalgal biofouling in a constant-flux crossflow filtration system comparing polymeric and carbon-nanotube membranes
(2020-08) Gol, Reuben Deane; Manning, Schonna Rachelle; King, Carey Wayne, 1974-
The commercial microalgal industry consists of three major stages in the development of bioproducts, biomaterials, and biofuels: the growth of microalgae in open ponds or closed photobioreactors, the various harvesting and dewatering processes, and the final product preparation, extraction and/or conversion. Harvesting and dewatering of microalgae accounts for ~30% of the production costs in the industry and there is no economy-of-scale, limiting the impact of this industry to high-value, low-volume products. There are several methods utilized across the industry for the harvesting and dewatering of microalgae, with no universal method identified for all microalgal strains and products. This research focused on crossflow filtration, a relatively new harvesting method with the potential to reduce costs and energy inputs at scales suitable for high-volume products, e.g., animal feed, human food, biofuels, etc. The major fouling effects algae have on membranes are the primary limitations of this method, incurring high maintenance and operating costs. Polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes were compared with novel, non-woven carbon nanotube (CNT) membranes to assess the anti-biofouling effects of these materials. Membranes were characterized through contact-angle measurements and permeability assessments in a novel constant-flux crossflow filtration system. The fouling effects on the membranes were assessed using cultures of Chlorella vulgaris and a wild strain of Nannochloropsis through threshold flux (TF) calculations and scanning electron microscopy. In all experiments, CNT membranes had less pore blocking and were more resistant to adhesion by bacteria, microalgae, extracellular organic material, salts, and minerals with less cake layering, suggesting improved fouling resistance when compared to PVDF and PES membranes. TF and permeability experiments demonstrated that CNT membranes exhibited improvements in fouling resistance by 1.6-fold and 1.9-fold for Chlorella and Nannochloropsis cultures, respectively, compared to PVDF membranes. CNT membranes also resulted in 8.7-fold and 7.9-fold improvements in fouling resistance compared to PES membranes for the Chlorella and Nannochloropsis cultures, respectively. Lastly, a techno-economic analysis (TEA) was performed to estimate the production costs for harvesting microalgal biomass using an industrial spiral-wound filtration system. Using this model, TEA evaluations supported that CNT membranes with increased permeability have the potential to reduce production costs.
Experimental investigation of viscous forces during surfactant flooding of fractured carbonate cores
(2016-08) Parra Perez, Jose Ernesto; Pope, G. A.; Balhoff, Matthew T.
The objective of this research was to investigate the effects of viscous forces on the oil recovery during surfactant flooding of fractured carbonate cores, specifically, to test the effects of using surfactants that form viscous microemulsions in-situ. The hypothesis was that a viscous microemulsion flowing inside a fracture can induce transverse pressure gradients that increase fluid crossflow between the fracture and the matrix, thus, enhancing the rate of surfactant imbibition and thereby the oil recovery. Previous experimentalists assumed the small viscous forces were not important for oil recovery from naturally fractured reservoirs (NFRs) since the pressure gradients that can be established are very modest due to the presence of the highly conductive fractures. Hence, the most common approach for studying surfactants for oil recovery from NFRs is to perform static imbibition experiments that do not provide data on the very important viscous and pressure forces. This is the first experimental study of the effect of viscous forces on the performance of surfactant floods of fractured carbonate cores under dynamic conditions. The effects of viscous forces on the oil recovery during surfactant flooding of fractured carbonate cores were tested by conducting a series of ultralow interfacial tension (IFT) surfactant floods using fractured Silurian Dolomite and Texas Cream Limestone cores. The viscosity of the surfactant solution was increased by adding polymer to the surfactant solution or by changing the salinity of the aqueous surfactant solution, which affects the in-situ microemulsion viscosity. The fractured cores had an extreme permeability contrast between the fracture and the matrix (ranging from 2500 to 90,000) so as to represent typical conditions encountered in most naturally fractured reservoirs. Also, non-fractured corefloods were performed in cores of each rock type for comparison with the results from the fractured corefloods. In all the experiments, the more viscous surfactants solutions achieved the greater oil recovery from the fractured carbonate cores which contradicts conventional wisdom. A new approach for surfactant flooding of naturally fractured reservoirs is presented. The new approach consists of using a surfactant solution that achieves ultralow IFT and that forms a viscous microemulsion. A viscous microemulsion can serve as a mobility control agent analogous to mobility control with foams or polymer but with far less complexity and cost. The oil recovery from the fractured carbonate cores was greater for the surfactant floods with the higher microemulsions, thus, it is expected that using viscous microemulsion can enhance the oil recovery from naturally fractured reservoirs.
Internal crossflow effects on turbine airfoil film cooling adiabatic effectiveness with compound angle round holes
(2014-05) Klavetter, Sean Robert; Bogard, David G.
Internal crossflow is an important element to actual gas turbine blade cooling; however, there are very few studies in open literature that have documented its effects on turbine blade film cooling. Experiments measuring adiabatic effectiveness were conducted to investigate the effects of perpendicular crossflow on a row of 45 degree compound angle, cylindrical film cooling holes. Tests included a standard plenum condition, a baseline crossflow case consisting of a smooth-walled channel, and various crossflow configurations with ribs. The ribs were angled to the direction of prevailing internal crossflow at 45 and 135 degrees and were positioned at different locations. Experiments were conducted at a density ratio of DR=1.5 for a range of blowing ratios including M=0.5, 0.75, 1.0, 1.5, and 2.0. Results showed that internal crossflow can significantly influence adiabatic effectiveness when compared to the standard plenum condition. The implementation of ribs generally decreased the adiabatic effectiveness when compared to the smooth-walled crossflow case. The highest adiabatic effectiveness measurements were recorded for the smooth-walled case in which crossflow was directed against the spanwise hole orientation angle. Tests indicated that the direction of perpendicular crossflow in relation to the hole orientation can significantly influence the adiabatic effectiveness. Among the rib crossflow tests, rib configurations that directed the coolant forward in the direction of the mainstream resulted in higher adiabatic effectiveness measurements. However, no other parameters could consistently be identified correlating to increased film cooling performance. It is likely that a combination of factors are responsible for influencing performance, including internal local pressure caused by the ribs, the internal channel flow field, jet exit velocity profiles, and in-hole vortices.
Membrane fouling : mechanisms, modeling, and mitigation
(2019-08-14) Kirschner, Alon Yeshayahu; Freeman, B. D. (Benny D.); Paul, Donald R.; Field, Robert W; Sharma, Mukul M; Lynd, Nathaniel A
Membrane systems are used for water treatment in many industries due to their small footprint, low chemical and energy use, and ease of operation. However, membrane fouling remains a challenge, especially for highly concentrated feeds. Fouling increases hydraulic resistance, lowers water permeance and increases energy consumption. Fouled membranes require expensive cleaning or replacement, increasing operating costs. This study focuses on understanding fouling mechanisms in constant flux crossflow operation, commonly used in industry, and on development of novel fouling-resistant membrane coatings. A model combining two accepted fouling mechanisms, intermediate pore blocking and cake filtration, was developed to describe fouling in constant flux crossflow ultrafiltration (UF). The model was fit to experimental fouling results using rigid and deformable particles. Observations of the model’s accuracy at different fluxes shed light on the physical meaning of the threshold flux: the threshold flux is the flux below which cake buildup is negligible and above which cake filtration becomes the dominant fouling mechanism. Further development of the model may enable fouling prediction. To mitigate fouling in oil-water separations, two novel membrane coatings were developed. The first coating was based on polydopamine (PDA), a well-established fouling-resistant coating material. Poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), a polymer zwitterion, was co-deposited with PDA to form a composite coating on polysulfone (PS) UF membranes. Fouling experiments showed that addition of PMPC to PDA significantly improved fouling resistance. The difference in fouling performance is likely due to the strongly hydrophilic surface properties contributed by PMPC. The co-deposition method opens opportunities for expansion of the concept in which PDA acts as a robust platform for the integration of non-fouling co-adsorbates. The second coating addresses a weakness of PDA coatings – their sensitivity to aqueous chlorine. Chlorine is widely used as a disinfectant in water purification processes. Chlorine oxidation results in rapid removal of PDA coatings from membrane surfaces, rendering them vulnerable to fouling. Poly(N-methylaniline) (PNMA) is a polyaniline derivative which contains a tertiary amine, rather than a secondary amine as in PDA, making PNMA less vulnerable to chlorine oxidation. PNMA-modified membranes were more stable and had higher fouling resistance than PDA-modified membranes after chlorine exposure.