Flow Visualization Results Research Articles

Impact of a Combustor–Turbine Interface on Turbine Vane Aerodynamics and Film Cooling

Abstract Optimizing the aerothermal performance of the combustor–turbine interface is an important factor in enhancing the efficiency of heavy-duty gas turbines. Also, it is a key requirement to fulfill the lifetime in this hottest area of the gas turbine. Typically transition pieces of can combustors induce a highly nonuniform swirling flow at the turbine inlet. In order to better understand the impact of the nonuniform combustor flow at the first stage vanes, a combined experimental and numerical study was carried out. The experimental facility consisted of a high-speed linear cascade with four vane passages, including an upstream transition piece, which was representative of a heavy-duty gas turbine can combustor–turbine interface geometry. The experiments were conducted at engine representative Mach numbers, and film cooling effectiveness measurements were performed at three different blowing ratios. Computational fluid dynamics (CFD) Reynolds-averaged Navier–Stokes simulations were undertaken using a commercial flow solver. The numerical model was first validated with the experimental data, using inlet traverse five-hole probe measurements, pressure taps along the airfoil perimeter, and oil flow visualization results. The investigation shows that the position of the vane relative to the combustor transition piece has a significant impact on the vane aerodynamics and also film cooling behavior. This understanding was important for a robust first vane aerothermal design of the GT36.

3D Numerical study of the effect of aspect ratio on mixed convection air flow in upward solar air heater

A 3D Numerical study of mixed convection air flow in upward solar air heater with large spanwise aspect ratio (A = 10 to 40) was performed using CFD commercial code Fluent 14.5 (ANSYS). The main objective of this study is to investigate the channel height's effect (aspect ratio) on flow pattern and heat transfer in upward solar air heater in the particular case of low Re and high aspect ratio. The bottom plate (absorber) was submitted to Constant Heat Flux (CHF) in the range of 200 to 1000 W/m2 and Reynolds number was varied from 50 to 1000. Our results are in concordance with most of authors conclusions about Poiseuille–Rayleigh–Benard flows. In mixed convection, increasing heat flux enhances heat transfer unlike forced convection flows. Simulation results of flow visualizations and Nusselt number calculations have shown that depending on Ri*, the velocity and temperature distributions in SAH vary greatly with the channel's height. The obtained results were different from previous studies. Indeed, our investigation of channel's height was achieved for the same heat flux but different Grashof numbers. For low channel's heights (high aspect ratio), increasing heat flux has not a significant effect but for higher channel's heights, an augmentation of heat flux enhances buoyancy effects in the flow and causes high turbulence. Also, increasing Reynolds number in low channel's heights (high A), can enhance substantially heat transfer. For higher channel's heights (low A), increasing Reynolds number decreases Ri* and thus buoyancy forces. Heat transfer is reduced and so Nusselt number. The obtained results may be very useful for engineers in designing and testing solar collectors.

Dynamic mode decomposition of rotorcraft blade tip vortex in hovering state

The flow field structure of a rotorcraft is complex; specifically, the rotor tip vortex structure has a great influence on the rotor performance. Therefore, in this paper, the evolution characteristics of rotor tip vortices and the dynamic mode decomposition (DMD) of rotor tip vortices in a rotor hovering state are studied. Through a time-resolved particle image velocimetry experiment, a comparative study of the blade tip vortex flow field at a fixed rotation speed (1500 rpm) and a collective pitch of 6° and 9° was performed. The method of DMD is used for the reduced-order analysis of the vorticity field of the blade tip vortex in the hovering state. By this method, these important vortex structures are extracted and discussed; meanwhile, the future flow field is also reconstructed. The results of flow visualization indicate that the trajectory of the blade tip vortex is moving down the axis, while moving toward the hub in the radial direction in the hovering state. The results of DMD analysis show that during the evolution of the blade tip vortex, different modes have different contributions to the rotor as a whole. In addition, the larger the collective pitch, the larger the modal coefficient amplitude and the slower the stabilization speed.

A Qualitative Study of the Wind Noise Cavity Inside of a Two Way Radio

The cavity inside microphone of two-way radio can induced noise when involved in outdoor and windy condition. An investigation was performed in Viwika wind tunnel involving a flow visualizations techniques to clarify the flow noise behavior generated by the wind, particularly within the cavity. A smoke wire test was conducted using a cavity model at varying aspect ratio (L/D) between 0.7 and 3 in angle of attack of 0º with Reynolds number of 7831. A two-dimensional Computational Fluid Dynamic (CFD) models based on k-Epsilon turbulence model were conducted to validate the flow visualization result. The results revealed that the deep cavity with an aspect ratio (L/D) of 0.7 produced a single clockwise vortical flow structure and the aspect ratio (L/D) of 3 show the complex flow structure which two recirculation flow vortices were produced inside the cavity. The flow behavior in the CFD simulation was similar to the smoke test. It is shown the flow behaviour inside cavities was control by the aspect ratio (L/D).

Control of Synthetic Hairpin Vortices in Laminar Boundary Layer for Skin-Friction Reduction

The results of flow visualization and hot-film measurement in a water channel are presented in this paper, in which the effectiveness of controlling synthetic hairpin vortices in the laminar boundary layer is examined to reduce skin friction. In this study, hairpin vortices were generated by periodically injecting vortex rings into a cross flow through a hole on a flat plate. To control the hairpin vortices, jets were issued from a nozzle directly onto the head of the hairpins. The results of the flow visualization demonstrated that the jets destroyed the hairpins by disconnecting the heads from their legs, after which the weakened hairpin vortices could not develop. Therefore, the circulation around the legs was reduced, which suggests that the direct intervention on the hairpin heads resulted in the reduction of streamwise stretching. Data obtained by a hot-film sensor showed that the high-speed regions outside the hairpin legs were reduced in speed by this control technique, leading to a decrease in the associated local skin friction.

Study of a Hybrid of Pulsating Heat Pipe and Distributed Jet Array

Pulsating heat pipes (PHPs) are effective devices for transferring a large amount of heat. For the existing PHP design, the U-turn geometry makes it difficult to use on small heat spots, which are very common in electronic cooling and many high heat flux applications. Additionally, the pipe flow in PHPs is not an optimum form for convective heat transfer. So far little research has attempted to resolve these issues. In this study, a hybrid design is introduced incorporating the characteristics of the PHP for its passive operation and the distributed jet array for its efficient convection. Based on the proposed structure, test articles are fabricated and experimentally studied. The design is proved successful. The heat transfer performance is evaluated by parametric studies and compared with the closed-loop PHPs. The flow visualization results are analyzed with different operational stages, flow patterns, and dimensionless numbers. The investigation concludes that the proposed design not only provides a new configuration for passive oscillatory heat transfer but also offers enhanced performance and applicability over the existing PHPs.

Response characteristics of inflow-stimulated Kelvin-Helmholtz vortex in compressible shear layer

By numerically solving the Navier-Stokes equations, the response characteristics of inflow-stimulated Kelvin-Helmholtz vortex in compressible shear layer arestudied. The mixing characteristics and the unique growth mechanism of the vortex structure are clearly revealed. By employing the index of vorticity thickness, the mixing properties are quantitatively analyzed. Based on the flow visualization results, the spatial size and the structure angle of the flow coherent structure are investigated by utilizing spatial correlation analysis. The evolution mechanism of the vortex structure in supersonic mixing layer induced by inlet forcing is revealed by analyzing the dynamical performances of the flow structure under different frequency disturbances. The numerical results show that with low forcing frequency at <i>f</i> = 5 kHz, the mixing efficiency is remarkably increased in the near-field of the flow. Whereas, in the far-field downstream the flow, the size of the structure reaches saturation state and the vortex passage frequency is locked, which causes the vorticity thickness to stabilize from 12mm to 14mm. Meanwhile, in a free mixing layer, the pairing and merging process occur in the flow field to promote the growth of the vortex structure, while in mixing layer with inlet forcing, the growth mechanism is that the vortex core engulfs a string of vortices induced by Kelvin-Helmholtz instability. The process of engulfment contributes much to the growth of the vortex structure. The analysis of spatial correlation distribution shows that in the area where engulfment occurs, the contour line shows the property of long and narrow ellipse instead of full ellipse and the structure in the area possesses the characteristics of intense rotation and inclination. Besides, with high inlet forcing frequency at <i>f</i> = 20 kHz, the size of the vortices becomes full in the near-field, and the vorticity thickness stabilizes between 3mm and 4 mm downstream the flow field. Meanwhile, the size of the vortex in controlled supersonic mixing layer is dominated by the imposed high-frequency forcing. An equation describing the quantitative relationship between the vortex characteristics and the imposed forcing frequency is derived, that is, the size of the uniform distribution vortex is approximately equal to the ratio of the value of convective velocity to inlet forcing frequency.

CFD computation of flow in the flow path of a torque flow pump

The results are presented of numerical and experimental research of fluid flow in the flow path of a torque flow pump with specific speed ns ;: 55. The 3D methods of CFD have been shown to allow for predicting energy characteristics of this type of pumps with a sufficient accuracy. According to the results of flow visualization the work process has been analysed and conclusions drawn to enhance TFP efficiency.

Low-Speed Aerodynamic Characteristics of Supercritical Airfoil with Small High-Lift Devices from Flow Pattern Measurements

In this paper, the lift coefficients of SC-0414 airfoil are estimated by applying modified Yamana’s method to the flow visualization results, which are obtained by utilizing the smoke tunnel. The application of the modified Yamana’s method is evaluated with two calculation methods. Additionally, the lift estimation, wake measurements, and numerical simulations are performed to clarify the low-speed aerodynamic characteristics of the SC airfoil with flaps. The angle of attack was varied from −5° to 8°. The flow velocity was 12 m/s and the Reynolds number was 1.6 × 105. As a result, the estimated lift coefficients show a good agreement with the results from reference data and numerical simulations. In clean condition, the lift coefficients calculated from the two methods show quantitative agreement, and no significant difference could be confirmed. However, the slope of the lifts calculated from ys is higher and closer to the reference data than those obtained from sc, where ys denotes the height where the distance from the streamline to the reference line is the largest, and sc denotes the displacement of the center of pressure from the origin of the coordinate, respectively. In the case of flaps, the GFs have an observable effect on the aerodynamic performance of the SC-0414 airfoil. When the height of the flap was increased, the lift and drag coefficients increased. The installation of a GF with a height equal to 1% of the chord length of the airfoil significantly improved the low-speed aerodynamic performance of SC airfoils.

Fluidic oscillator actuated by a cavity at high frequencies

In this study, a flat device for generating the small jets flapping at several tens of kHz (i.e., high-frequency fluidic oscillator) is proposed. The device involves a thin flow passage (2.0 mm thickness) with a cavity whose length is adjustable. The jets are discharged through two slots located at both ends of the cavity. The widths of the upstream and downstream slots are 1.0 mm and 4.0 mm, respectively, and the cavity width is 9.0 mm. The flapping frequencies and motions of the jets are experimentally investigated varying the ratio of the upstream stagnation pressure to backpressure from 2.0 to 3.0. In the experiments, the jets are visualized using the schlieren technique and the time-dependent pressure fluctuations in the cavity are measured with a semiconductor-type pressure sensor. The experimental results demonstrate that a flapping jet whose width is ∼1 mm is generated successfully from the device and the flapping frequency is selectable just by adjusting the cavity length between 15 and 29 kHz keeping a supply pressure constant, i.e., the flapping frequency is controllable independently of the mass flow rate in the device. In addition, the flapping frequencies are evaluated by modeling the oscillation mechanism on the basis of the flow visualization results. The flapping frequencies measured in the experiments are successfully reproduced using the model.

Thermal Transport of Viscoelastic Fluids Within Rotating Couette Flows

The remarkable ability of viscoelastic fluids to augment local and global surface heat transfer characteristics is demonstrated by new experimental results, which are provided for a rotating Couette flow (RCF) environment (also referred to as von Karman swirling flow) with convective heat transfer. Included are Nusselt number variations, flow visualization results, and spectral analysis of flow static temperature fluctuations. Augmented surface Nusselt numbers are measured for sucrose-based, viscoelastic solutions with polyacrylamide (as they are subject to different magnitudes of flow strain), relative to Newtonian flows and relative to fluids with zero shear rate. The resulting Nusselt number enhancements are related to the experimental conditions that are believed to be associated with the onset and development of elastic instabilities. Resulting comparisons show that the Weissenberg number is the parameter that best correlates and characterizes Nusselt number augmentations, which are strongly correlated with pronounced redistributions of fluorescein FWT red tracer dye fronts, obtained from flow visualizations. As such, the present investigation provides new insight into thermal transport of viscoelastic fluids, and new experimental data that illustrate the utility of different analytic and numerical models for predicting experimental conditions associated with significant Nusselt number augmentations.

Investigation and visualization of surfactant effect on flow pattern and performance of pulsating heat pipe

Pulsating heat pipes (PHPs) are one of the new devices used for cooling in several applications such as electronic and aerospace systems. Their low cost, effectiveness at various conditions, being equipped for passive energy conversion, and well distribution of temperature compared to conventional heat pipes are among the reasons of their popularity. To investigate the effect of surface tension of the working fluid on the behavior of PHPs, a copper heat pipe is fabricated with inner and outer diameters of 2 mm and 4 mm, respectively. Five different concentrations of cetrimonium bromide (C-Tab) surfactant are dissolved in water and are tested with a filling ratio of 50% (± 1%). A piece of glass is placed in the adiabatic section to make the flow visualizations possible. Thermal resistance and flow visualization results are compared. Visualization of the flow shows that by increasing the surfactant concentration, annular and semi-annular regimes can be observed at lower powers. It is also detected that by increasing the surfactant concentration, thermal resistance will decrease, while the maximum heat flux is reduced. This can be explained by thinner film thickness in the evaporator. The lowest thermal resistance was detected to be 0.44 K/W for the 0.25 g L−1 C-Tab concentration at 25 W heat input power which shows a significant improve of 77.5% compared to pure water at the same power.

Experimental and numerical analysis of heat transfer enhancement and flow characteristics in grooved channel for pulsatile flow

Heat transfer enhancement and flow characteristics in grooved channel for pulsatile flow are investigated experimentally and numerically in the present work. The amplitude of the pulsatile flow is focused at different Reynolds numbers and oscillatory fractions by measuring time-varying flow rate through the electromagnetic flowmeter. In addition, the pulsatile flow patterns are visualized through aluminum dust method. The experimental results showed that the oscillatory fraction decreases with frequency of pulsatile flow, which is also proved by the flow visualization results. It is further shown that the amplitude of pulsatile flow can approach the setting value only when the frequency is lower than the critical frequency fc. It is found that the steady and unstable flow states exist in a pulsatile period. The unstable flow enhances fluid mixing between mainstream and recirculation vortex, and the flow mixing increases with the frequency increment, which is the main reason of heat transfer enhancement. Two-dimensional numerical simulations are further carried out to study this problem. The numerical results showed that heat transfer performance is better at high frequency. Moreover, the heat transfer efficiency decreases with oscillatory fraction when frequency is low. Phase shift is found to exist between the pulsatile flow rate, the outlet temperature and the area-averaged wall Nusselt number. Based on the experimental and numerical results, it is found that higher frequency and small oscillatory fraction could cause a better heat transfer performance in the grooved channel.

Investigation of a serpentine micro-pin fin heat and mass exchanger for absorption systems

An investigation of a heat and mass exchanger with micro-pin fins in serpentine passages for vapor absorption systems is presented. A serpentine micro-pin fin absorber sheet with a pin fin diameter of 350 µm and height of 406 µm is used. Visualization of the two-phase mixture flowing through the micro-feature component, and an investigation of the heat and mass transfer mechanisms over a range of temperatures, pressures, concentrations and flow rates are conducted. A model for the absorption of ammonia into a dilute ammonia-water solution in the geometry is developed based on insights from flow visualization results, and validated by comparing its predictions with heat transfer measurements. A computational fluid dynamics study of flow through the serpentine micro-pin fin passages is also used for additional insights. Liquid-phase mass transfer resistance is found to be the bottleneck to improved rates of vapor absorption in these absorbers.

A Study of the Flow Patterns Between Two Corrugated Plates With an Egg-Carton Configuration

Enhancing mixing in heat exchangers for low Re regimes is vital. A better mixing may be achieved by using corrugated plates. In this work, the flow patterns between corrugated plates with a novel egg-carton geometry were studied. Three-dimensional (3D) numerical models were developed for the steady laminar flow between two corrugated plates having 180 deg or 0 deg phase angles. The Reynolds number (Re ≤ 600) was defined as a function of the average distance between the corrugated plates. The numerical models were strictly developed and corroborated to achieve global convergence, local convergence, and grid-size independence. For both phase angles, it was determined that “close recirculations” decrease in size downstream and finally disappear becoming “open recirculations” due to the flow developing characteristics; the secondary flow regions were found to grow downstream; interestingly, increments on the Reynolds number favor recirculation growth and early flow detachment; the behavior and geometry of the recirculation were in line with previous flow visualization results. The recirculations were determined to be z-symmetric with respect to the channel center only for the 180 deg model. The recirculations in the 0 deg model were smaller and became “open recirculations” earlier than in the 180 deg model. Convex geometries on the transversal direction were found to favor detachment, while concave geometries inhibit it. The capability of the numerical methods to track flow paths in any direction showed a complex three-dimensional flow causing 3D-interaction among secondary flows and the main flow not reported before for these channels and just hinted by previous flow visualization studies.

Flow hydrodynamics of immiscible liquids with low viscosity ratio in a rectangular microchannel with T-junction

We present an experimental study of the liquid-liquid system with extremely low viscosity ratio (10−3) in a T-shaped microchannel with 120 × 120 μm inlets and a 240 × 120 μm outlet channel. Six different flow patterns have been observed: plug, droplet, slug, throat-annular and parallel flow. A specific plug flow pattern was found where micron-sized droplets or even a jet breaks off from the rear meniscus of a plug. In addition to typical Taylor-shaped plugs the dumbbell-like plugs were observed. Flow visualization data were summarized in the flow pattern maps. Plug length and velocity were measured based on flow visualization results. It was found that the plug velocity can be fitted by power function rather than the linear function, which disagrees with experimental data at a low plug velocity. Front and tail plug surface curvature was scaled using dimensionless parameter, the flow rate ratio multiplied by Capillary number based on bulk velocity (Qd/Qc * Cabulk). Instantaneous velocity vector fields inside water plugs were measured by means of PTV technique. Different flow structures were found and discussed. As a result, it is proposed to use the values of Qd/Qc * Cabulk for distinguishing different plug shapes and circulation patterns inside the plugs.

Experimental Measurement of Vortex Ring Screen Interaction Using Flow Visualization and Molecular Tagging Velocimetry

The interaction of vortex rings with thin wire mesh screens is investigated using laser-induced fluorescence (LIF) and molecular tagging velocimetry (MTV). The existence of vortex shedding from individual wires of the porous screens, suggested by prior works, is shown and compared to flow visualization results. A range of interaction Reynolds numbers and screen porosities are studied to determine the conditions affecting the interaction. Transmitted vortex (TV) ring formation is shown to be a function of vortex shedding and the shedding Reynolds number, but not a function of porosity. Screen porosity is shown to affect the TV convective speed but did not impact the formation behaviors. Three major flow regimes existed for the interaction: TV formation with no vortex shedding, TV formation with visible vortex shedding, and no downstream formation with strong shed vortices.

Experimental study of square riblets effects on delta wing using smoke visualization and force measurement

Riblets are effective approach to improve the aerodynamic performance of low speed wing configurations. For a delta type wing, it is important to alleviate the flow features such as large rollup and tip vortices in low Reynolds number. In this paper, for first time, the effect of the square riblets on delta wings is studied. In the first step, flow visualization is accomplished to find out the effects of riblets on the flow pattern. For this purpose, two slender delta wings with and without riblet are examined in a vertical wind tunnel in which smoke stream lines are passed over the delta wings to visualize the flow pattern. The visualization tests are performed at five angles of attack; i.e., 0, 10, 20, 30 and 35 degrees. The results of flow visualization indicate that considerable improvements are obtained because of delaying flow separation at high attack angles. In the second step, the aerodynamic performance of the two delta wings are quantitatively evaluated by measuring the lift and drag forces using two load cells in another wind tunnel. The force tests are performed at attack angles of 5–65 degrees with 5-degree step sizes. The measured forces show that the riblets decreases the drag coefficient for all attack angles and increase the lift to drag ratio more than 12 percent for 5–20 degrees attack angles.

Experimental investigation of the hypersonic boundary layer transition on a 7° straight cone

In this paper, the experiments about the boundary layer transition on a 7° half-angle straight cone are carried out in a Mach 6 low-noise wind tunnel. The wall fluctuation pressure is measured by the transducer with megahertz response frequency, and the development process of the disturbance in the hypersonic boundary layer is investigated. The peaks in power spectrum density of the fluctuation pressure are related to the second mode wave, which is indicated through verifying the existence of the longitudinal acoustic second mode waves reflected between the relative sonic line and the solid wall by the flow visualization result. The wavelength and the characteristic frequency of the second mode wave in the hypersonic boundary layer are found to be greatly influenced by Reynolds number. The characteristic frequency of the second mode wave changes from 55 kHz to about 226 kHz when the Reynolds number increases from 2×106 m-1 to 8×106 m-1. The second mode wave appears at the position closer to the upstream with a higher disturbance growth speed under higher unit Reynolds number. As the second mode wave propagates downstream, its characteristic frequency gradually decreases. The freestream noise level also has a great influence on the development of the disturbance wave. The characteristic frequency of the second mode wave decreases significantly in a relatively quiet environment. The cross-correlation analysis results show that the propagation velocity of the second mode wave in the boundary layer is about 0.8-0.9 times the local mainstream velocity. The wavelength of the second mode wave is about 5.01 mm at the location from X=380 mm to X=440 mm when the unit Reynolds number is 5×106 m-1. At 1° angle of attack, the development of the boundary layer on the windward side and the leeward side of the cone are significantly different. The characteristic frequency of the second mode wave in the leeward surface is almost the same as the result at zero angle of attack under the same unit Reynolds number. However, the position of the second mode wave is greatly advanced. Results show that the disturbance development in the boundary layer of the leeward surface is accelerated, and the second mode wave appears at the position closer to the upstream. The velocity of the second mode wave in the leeward surface rapidly increases when it propagates downstream. While on the windward side, the disturbance development is inhibited and the second mode wave has a higher characteristic frequency. The wavelength of second mode wave also decreases obviously.

Experimental studies of surface modified oscillating heat pipes

Oscillating heat pipe (OHP) is a two-phase heat transfer device which has the characteristics of simple construction, high heat flux capability and no need of wicking structures for liquid transport. There are many studies in finding the ways how to improve the system performance OHP. In this paper, studies of the effects of contact angle (θc) on the inner wall of OHP system have been conducted first. Glass OHP systems with unmodified (θc = 26.74°), superhydrophobic (θc = 156.2°), superhydrophilic (θc < 10°) and hybrid (superhydrophilic within evaporator region and superhydrophobic within condensation region) surfaces, are studied. The research results indicated that thermal resistance of these four OHP systems can be significantly affected by different surface modification approaches. Although superhydrophobic OHP system can still work, the thermal resistance (Rth) is the highest one of the four OHP systems, Rth = 0.36 °C/W at 200 W. Unmodified pure glass and superhydrophilic OHP systems have similar performance. Thermal resistances are 0.28 and 0.27 °C/W at 200 W respectively. The hybrid OHP achieves the lowest thermal resistance, Rth = 0.23 °C/W at 200 W in this study. The exact mechanism and effects of contact angle on OHP systems are investigated with the help of flow visualization. By comparing the flow visualization results of OHP systems before and after surface modification, one tries to find the mechanism how the surface modified inner wall surface affects the OHP system performance. In additional to the reason that the superhydrophobic dropwise condensation surface inside the hybrid OHP system, hybrid OHP system shows more stable and energetic circulation flow. It is found that instead of stratified flow, vapor slug flows are identified within the evaporator section of the hybrid OHP system that can effectively generate higher pressure force for two phase interfacial flow. This effect is attributed to be the main mechanism for better performance of the hybrid OHP system.