Elevated Turbulence Intensity Research Articles

A numerical analysis of the electromagnetic field effect on direct contact membrane distillation performance

Using an advanced CFD model, this study attempts to determine how electromagnetic field (EMF) affects the performance of the direct contact membrane distillation (DCMD) process. The influence of electromagnetic fields (EMF) on the permeate flow, evaporation efficiency (EE), and temperature polarization coefficient (TPC) of the DCMD process are studied using a thorough theoretical model linked with a CFD model. Experiments conducted in-house with and without the EMF influence are used to verify the CFD model. High magnetic potency propagates the turbulence in the feed channel, hence boosting the driving force and transport phenomena for vapor transport to the permeate side. Permeate flux increased by 30%, and EE increased by 15% when the magnetic field strength was raised from 0 to 0.5 T, particularly at a lower intake feed velocity (0.02 m/s) and temperature of 333.05 K. In contrast, with the same magnetic potency (0.5 T) at a higher velocity at an input feed temperature, there is less improvement in EE (5%) and permeate flux (12%) due to the elevated turbulence intensity at high feed velocity. This research contributes to studies aimed at elucidating how electromagnetic fields affect separation processes such as MD, opening the door for more tuning and use of the fascinating phenomenon.

Heat transfer characteristic of a fully cooled turbine vane

To evaluate the heat transfer performance of a fully cooled turbine vane, an experiment was conducted in the low-speed wind tunnel using transient liquid crystal technology. The test vane was fabricated with 18 rows of cylindrical holes, which were supplied by two plenums. In these experiments, three mass flow ratios of 5.5%, 8.4%, and 12.5%, three turbulence intensities of 2%, 9%, and 15%, three Reynolds numbers of 300000, 400000, and 500000, two density ratios of 1.0 and 1.5 were tested. The results show that the heat transfer coefficient in the leading edge is much higher than the pressure and suction side due to the direct impact of the mainstream and the severe mainstream and injection mixing. The increased mass flow ratio significantly increases the heat transfer coefficient over the entire vane surface by 3.6%–71.3%, and the elevated turbulence intensity enhances the heat transfer coefficient by 5%–45% on the suction side and the rear half of the pressure side. The higher Reynolds number causes the thinner boundary layer and the higher velocity near the wall, which results in the Nusselt number increases by 52% in the case of increasing the Reynolds number from 300000 to 500000. Additionally, the heavier coolant leads to a slightly reduced heat transfer coefficient because of the lower injection penetration.

Film Cooling Performance of a Fully Cooled Vane at High Subsonic Conditions

This work focuses on the experimental study of the heat transfer and film cooling characteristics of a fully cooled vane under high subsonic conditions; the transient heat transfer measurements with thermocouples were employed. The inlet Reynolds numbers of the cascade ranged from to and the exit Mach number was 0.8. Two freestream turbulence intensities (1.3% and 14.7%) and three mass flow ratios (5.5%, 8.4%, and 12.5%) were tested. The averaged film cooling effectiveness of the current vane with shaped holes was found to be increased by 15%–43% more than that of the vane only with cylindrical holes, and elevated turbulence intensity reduced the averaged film cooling effectiveness by 3%–26%. The heat transfer augmentation of the current vane was lower than that of the vane only with cylindrical holes. With the mass flow ratio increasing, the heat transfer augmentation gradually increased on the pressure side, whereas it was nearly kept invariable on the suction side. The net heat flux reduction results showed that the vane with shaped holes significantly improved the cooling performance, whereas elevated turbulence intensity significantly deteriorated the cooling performance.

Network topology of turbulent premixed Bunsen flame at elevated pressure and turbulence intensity

For turbulent premixed flame surface, folded regions will be formed mainly depending on the turbulence-flame interaction and are significant structures which can increase flame surface area at large scale and enhance the local displacement speed. The conventional methods, such as PDF distribution of curvature, may not properly label the folded regions of turbulent flame because these regions intrude deeply into the products, resulting in a small curvature probability. This problem may be aggravated at elevated pressure and turbulence intensity conditions when more folded regions appear at the turbulent flame front. To identify the folded regions, Network topology of turbulent premixed flame front is constructed using the “visibility” method. Results show that this method can convert the spatial signal of turbulent flame to network topology, labeling the folded regions. Compared with curvature PDF, node degree distribution of network can reflect the mechanism of turbulence-flame interaction when the non-dimensional turbulence intensity is increased by different ways. The network structure of turbulent flame will transfer from sparse to condense when the dimensionless turbulence intensity is increased by pressure and perforated plate, as these two methods will extend the turbulence-flame interaction time and promote the interaction intensity, respectively. However, although the dimensionless turbulence intensity will increase with the augment of outlet velocity, the node degree distribution of network structure of turbulent flame front keeps almost constant. This is caused by the reduced turbulence-flame interaction time. It suggests that the turbulence-flame interaction time is an factor as important as the dimensionless turbulence intensity in turbulent premixed combustion. For forced-turbulent premixed flame at elevated turbulence intensity, the “bending phenomenon” will be hidden if the outlet velocity is not taken into account, as the outlet velocity is related to the turbulence-flame interaction time.

Blood flow patterns and pressure loss in the ascending aorta: A comparative study on physiological and aneurysmal conditions

An aortic aneurysm is defined as a balloon-shaped bulging of all three histologic components of the aortic vessel walls (intima, media and adventitia). This dilation results from vessel weakening owing to remodeling, i.e. due to cystic degeneration of the Tunica media (Marfan), progression of atherosclerosis or presence of a bicuspid aortic valve. The growth rate of the aortic diameter varies from patient to patient and may progress until the aneurysm ultimately ruptures. The role of hemodynamics, i.e. blood flow patterns, and shear stresses that are supposed to intensify during aneurysm growth are not yet fully understood, but thought to play a key role in the enlargement process. The aim of this study is to characterize the aortic blood flow in a silicone model of a pathological aorta with ascending aneurysm, to analyze the differences in the blood flow pattern compared to a healthy aortic model, and to single out possible blood flow characteristics measurable using phase contrast magnetic resonance imaging (MRI) that could serve as indicators for aneurysm severity. MRI simulations were performed under physiological, pulsatile flow conditions using data obtained from optical three dimensional particle tracking measurements. In comparison to the healthy geometry, elevated turbulence intensity and pressure loss are measured in the diseased aorta, which we propose as a complimentary indicator for assessing the aneurysmal severity. Our results shed a light on the interplay between the blood flow dynamics and their contribution to the pathophysiology and possible role for future risk assessment of ascending aortic aneurysms.

Turbulence Intensity Effects on Laminar Separation Bubbles Formed over an Airfoil

The effects of freestream turbulence intensity on the mean topology and transition characteristics of laminar separation bubbles forming over the suction side of a NACA 0018 airfoil are investigated experimentally for angles of attack between 0 and 20 deg, chord Reynolds numbers between 100,000 and 200,000, and freestream turbulence intensities between 0.09 and 2.03%. The results show that increasing freestream turbulence intensity results in earlier transition and reattachment, contributing to an overall decrease in separation bubble length. At lower angles of attack, this is accompanied by a minor decrease in lift, whereas at prestall angles of attack and higher turbulence intensity levels, lift increases and stall is delayed. Spatial amplification rates of disturbances in the separated shear layer are shown to decrease at elevated levels of turbulence intensity, indicating that the earlier transition is attributed solely to the larger initial amplitude of perturbations. At elevated turbulence intensity levels, a broader range of unstable frequencies is detected in the separated shear layer, with the central frequency of the unstable band showing moderate variations with the level of freestream perturbations. The results indicate a change in the transition process at higher freestream turbulence intensity levels, indicative of bypass transition in the boundary layer. The degree of influence of freestream turbulence intensity on the separation bubble is shown to decrease as the chord Reynolds number is increased.

Local flow field of a surface-mounted finite circular cylinder

The local flow field and near-wake region of a surface-mounted finite circular cylinder were studied experimentally in a low-speed wind tunnel using particle image velocimetry (PIV). The cylinder was mounted normal to a ground plane and was partially immersed in a flat-plate turbulent boundary layer. Four finite circular cylinders of aspect ratios AR=9, 7, 5 and 3 were tested at a Reynolds number of ReD=4.2×104. At the location of the cylinder, the boundary layer thickness relative to the cylinder diameter was δ/D=1.6. PIV velocity field measurements were made in a vertical plane parallel to the mean flow direction on the flow centerline (the symmetry plane), within five diameters upstream and downstream of the cylinder, and also above the free end. Above the free end of the cylinder, flow separation from the leading edge leads to the formation of a mean recirculation zone on the free-end surface. The point of reattachment of the flow onto the free-end surface moves towards the trailing edge as the cylinder aspect ratio is decreased. Large regions of elevated turbulence intensity and Reynolds shear stress are found above the free end. In the near-wake region, the large recirculation zone contains a vortex immediately behind and below the free end; this vortex was found for all four aspect ratios. A second vortex is found behind the cylinder near the cylinder-wall junction; this vortex was not observed for the cylinder of AR=3, indicating a distinct wake structure for this cylinder.

Influence of Wake Passing Frequency and Elevated Turbulence Intensity on Transition.

Experimental results from a study of the effects of passing wakes upon laminar-to-turbulent transition in a low-pressure turbine passage are presented. The test section simulates the effects of unsteady wakes resulting from rotor-stator interaction In turbine blade boundary layers and separated flow regions over suction surfaces. Single-wire thermal anemometry techniques are used to measure time-resolved and phase-averaged wall-normal profiles of streamwise velocity, turbulence intensity, and intermittency at multiple streamwise locations over the turbine airfoil suction surface. The Reynolds number based on suction surface length and stage exit velocity is 5 X 10 4 . This low Reynolds number would apply to small engines flying at high altitude