Slot Exit Research Articles

An experimental investigation on the effects of a slot exit protrusion on film cooling effectiveness

An experimental study was conducted to investigate the effects of a protrusion installed at the slot exit on the film cooling effectiveness of inclined slots. Injection angles of 30 and 45° were considered for the slot, and a protrusion that induces the Coanda effect was installed at the slot exit. Three different geometries and three installation locations for the protrusion were considered – i.e. x/ds = 0, 5, and 10. The experiment was conducted at a mainstream velocity of 10 m/s and corresponding turbulence intensity of 0.5%. Four blowing ratios (0.5, 1.0, 1.5, and 2.0) were tested, and the density ratio was about 1.0. To measure the film cooling effectiveness, the pressure sensitive paint (PSP) technique was applied. Results indicated that the protrusion was able to improve the film cooling effectiveness of the slot, and the improvement was significant for the 45-degree slot at a higher blowing ratio. The film cooling effectiveness of the 45-degree inclined slot was improved by about 6.6% by installing a protrusion immediately after the slot exit. It was also confirmed that the height of the protrusion had a greater effect on the film cooling effectiveness than the width.

Aerodynamic Analysis and Optimization of Embedded Slot for Smooth Morphing Trailing Edge

Smooth morphing trailing edge (SMTE) has been receiving more attention as a new wing design concept in the past few years due to its high potential in reducing cruise drag, structural weight, aerodynamic noise, and fuel consumption. Apart from the necessity of using new flexible materials and compact actuation mechanisms, it is also faced with the challenge of providing sufficient lift as conventional high-lift devices did during low-speed flight phases. Insufficient low-speed aerodynamics could compromise aircraft takeoff and landing performance. In light of this, a concept of the embedded slot is proposed for use with SMTE. To study the full benefits of the embedded slot, constrained single-objective optimization is first conducted on a two-dimensional airfoil using kriging surrogate models built from Reynolds-averaged Navier–Stokes solutions. The final optimal results show that the maximum lift coefficient from SMTE with a slot is only slightly lower than that of a typical three-element Fowler flap. The parametric analysis showed the strong coupling effects on the aerodynamic performance of slot entry and exit geometry dimensions. Based on the optimal two-dimensional slot design, computer-aided design models of three-dimensional SMTE configurations are built and analyzed. Comparisons among High-Lift Common Research Model SMTEs with and without slot show that the embedded slot improved high-lift performance. After introducing the embedded slot, the maximum lift coefficient for SMTE is raised by 7.9% from 2.277 to 2.457, and the stall angle is increased by 2 deg. This is in comparison with the maximum lift coefficient of 2.572 for the conventional Common Research Model Fowler flap configuration. The results demonstrated the benefits of introducing embedded slot into SMTE for future high-lift designs.

A Miniature Four-Channel Ion Trap Array Based on Non-silicon MEMS Technology.

With the increasing application field, a higher requirement is put forward for the mass spectrometer. The reduction in size will inevitably cause a loss of precision; therefore, it is necessary to develop a high-performance miniature mass spectrometer. Based on the researches of rectangular ion trap, the relationship between mass resolution and structural parameters of the ion trap array was analyzed by further simulation. The results indicate that, considering the balance of mass resolution and extraction efficiency, the preferable values for the field radius of exit direction y0 and ion exit slot width s0 are 1.61 mm and 200 μm, respectively. Afterwards, a miniature four-channel ion trap array (MFITA) was fabricated, by using MEMS and laser etching technology, and mass spectrometry experiments were carried out to demonstrate its performance. The mass resolution of butyl diacetate with m/z = 230 can reach 324. In addition, the consistency of four channels is verified within the error tolerance, by analyzing air samples. Our work can prove the correctness of the structural design and the feasibility of MEMS preparation for MFITA, which will bring meaningful guidance for its future development and optimization.

Control of jet structure utilizing the change in the outlet velocity distribution with time

Flow control using jets is widely implemented. In particular, the clarification of jet technology is indispensable in the control of the blade stall control of blades and for thermal management, such as refrigeration and air conditioning. As part of the fundamental research into the topics mentioned above, various studies of the flow characteristics of steady jets have long been conducted, and the structure of the jets investigated. However, studies on flow control by synthetic jets that essentially exhibit an unsteady flow have begun only recently. Synthetic jets have a flow wherein the jet structure forms downstream of the slot, although the net flow rate is zero, because of repeated ejection and suction. Attempts have been made to control of the jet’s direction of travel by adjusting only the jet’s frequency without any geometric changes. However, many aspects of the effect of oscillation characteristics on the jet’s structure are still unclear. Moreover, since the flow rate is low, it is not suitable for large-scale fluid control, such as that required for high-power fluid machinery. In this study, excited jets are proposed, wherein the velocity distribution change with time at the slot exit and the time-averaged flow rate are considered. An attempt was made to control the jet structure by means of the oscillation characteristics. The influences of the frequency and the phase of the slot outlet velocity distribution on the maximum jet velocity and the jet width were investigated by numerical calculations and experiments. Our research led us to conclude that the staggered vortex street is generated when the phase of the slot exit velocity distribution is π. Further, we concluded that the jet structure could be controlled by the frequency under a certain specific set of conditions.

Effects of the positive pre-swirl purge flow on endwall aero-thermal performance of a gas turbine blade

The harsh thermal environments and the complicated near-endwall secondary flows pose great challenges to the thermal management of a blade endwall. To improve the endwall aero-thermal performance, a series of inclined guide fins are mounted inside the interface gap, generating a positive pre-swirl purge flow. The effects of the guide fins' inclined angle on aerodynamic losses, film cooling effectiveness and heat transfer performance of an endwall are investigated under three coolant-to-mainstream mass flow ratios (MFR). Results indicate that the guide fins’ inclined angle exerts significant impacts on the flow-thermal interplay between the secondary flow structures and the coolant ejection. A positive pre-swirl purge flow stimulates the boundary layer fluid to weaken the near-endwall cross flows and thereby significantly reduces the blade aerodynamic losses, especially for MFR >1.0%. More importantly, increasing the inclined angle improves both the effectiveness and coverage of endwall film cooling dramatically when MFR <1.0%, although the cooling performance downstream of the slot exit is lowered when MFR = 1.5%. Besides, increasing the inclined angle marginally enhances the endwall heat transfer level when MFR = 0.5%, but significantly decreases that when MFR >1.0%. Overall, a positive pre-swirl purge flow is beneficial to improve the endwall thermal protection level.

Numerical investigation on the flow and heat transfer in swirl chambers with distributed multi exit slots and dimple/protrusion structure

Swirl cooling is an excellent candidate for internal cooling of turbine blade leading edge. In this paper, a novel swirl chamber with distributed multi exit slots is proposed, for which different outlet configurations are considered. Flow and heat transfer characteristics are numerically investigated and compared with those of conventional chamber. Then dimples/protrusions are innovatively introduced into the swirl chamber, with various arrangements. The influence of temperature ratio is further discussed. The results show that, a typical flow structure of “inner vortex + outer vortex” is formed in the jet regions. In novel swirl chambers, the extraction effect of exit slots avoids the formation of crossflow, thus the friction can be effectively reduced while the heat transfer is strengthened, for which a 46.9% improvement of overall thermal performance (TP) is achieved. The dimple/protrusion can realize the local flow control, and produce a 54.8% improvement in TP. The nozzle-side arrangement of dimple/protrusion is better than the slot-side arrangement, and the protrusion is superior to the dimple. With the increase of temperature ratio, the TP shows the variation trend of increasing first and then decreasing. This research will provide important data support for the internal cooling design of turbine blade leading edge.

Analysis of synthetic jet PIV experiment based on dynamic mode decomposition

Synthetic Jet Actuator (SJA) works cyclically with the directionally transportation of fluid near the exit, and the paper presents the performance of a loudspeaker driven SJA in static flow field. Time-Resolved Particle Image Velocimetry (TR-PIV) system is used to measure the flow field characteristics near the SJA slot exit with input voltage changing, and the flow field snapshots obtained by TR-PIV are modality analyzed by Dynamic Mode Decomposition (DMD) method. The PIV experiments show that by varying input voltage at fixed oscillating frequency, the loudspeaker diaphragm vibration displacement is the parameter that affects the jet velocity and the performance of the SJA. The traveling vortex vanishes at high voltage due to the interaction between vortex structures and the synthetic main jet. In DMD method, the first three-order modes can characterize the main information of the original flow field with reverting the flow field snapshot sequence. It indicates that the DMD method is applicable in the SJA flow field research and the reduced order model can effectively simplify the analysis of flow field.

The Acoustic Performance of 3D Printed Multiple Jet Nozzles with Different Configurations

This work investigated multiple jet nozzles with various geometrical shape, number of exits, and material on reducing noise radiated from jet flows. Nozzles are categorized in two groups with few and many exit numbers, each with various exit shapes, slot and circular, and geometry. Firstly, nozzles are designed and then fabricated by a 3D printer, Form Labs, Form2USA, with polymeric resin. Also, the nozzle with the most noise reduction made of stainless steel. Noise and air thrust were measured at three air pressure gauges, 3, 5, 7 BAR and directions from nozzle apex, 30°, 90°, 135°. Nozzles with slot exit shape made of both plastic and stainless steel revealed the most noise reduction among all nozzles with few exit numbers, nearly 11–14 dB(A) and 11.5–15 dB(A), respectively. On average, slotted nozzle noise reduction was nearly 5–6 dB(A) more than finned nozzle. However, nozzles with more exit numbers, finned and finned-central exit, illustrated much more noise reduction than nozzles with few exit numbers, by almost 16–18 dB(A), they represented similar sound. All tested nozzles and open pipe demonstrated equal air thrust at each pressure gauges. The nozzles with slotted exit shape, either plastic or stainless steel, can provide reasonable noise reduction in comparison to other configuration with few exit numbers. In contrast, nozzles with more exit numbers demonstrated the most noise reduction.

Numerical investigation of passive cavitation control using a slot on a three-dimensional hydrofoil

Purpose The purpose of this paper is to study the mechanism and suppression of instabilities induced by cavitating flow around a three-dimensional hydrofoil with a particular focus on cavitation control with a slot. Design/methodology/approach The transient cavitating flow around a Clark-Y hydrofoil was investigated using a transport-equation-based cavitation model and the stress-blended eddy simulation model was used to capture the flow turbulence. A homogeneous Rayleigh–Plesset cavitation model was used to model the transient cavitation process and the results were validated with test data. A slot was applied to the hydrofoil to suppress cavitation instabilities, and various slot widths and exit locations were applied to the blade and the cavitation behavior, as well as drag/lift forces, were simulated and compared to investigate the effects of slot geometries on cavitation suppression. Findings The large eddy simulation based turbulence model was able to capture the interactions between the cavitation and turbulence. Moreover, the simulation revealed that the re-entrant jet was responsible for the periodic shedding of cavities. The results indicated that a slot was able to mitigate or even suppress cavitation-induced instabilities. A jet flow was generated at the slot exit and disturbed the re-entrant jet. If the slot geometry was properly designed, the jet could block the re-entrant jet and suppress the unsteady cavitation behavior. Originality/value This study provides unique insights into the complicated transient cavitation flows around a three-dimensional hydrofoil and introduces an effective passive cavitation control technique useful to researchers and engineers in the areas of fluid dynamics and turbomachinery.

Modified stress grading system for a 13.8 kV inverter-fed rotating machine

Evaluation of thermal and electrical characteristics of a 13.8 kV rotating machine stress grading system shows that a reduced length of conductive armor tape (CAT) from the slot exit reduces the electric field and temperature rise under pulsed voltage. Although, the electric field in stress grading tape (SGT) increased slightly, with a shorter length of CAT during the rise time of the pulse voltage, it decreased during the DC portion of the pulse voltage. The temperature profile of the stress grading system under pulsed voltage at room, elevated room, and near typical operating temperatures are measured and simulated for several CAT lengths; results show lower temperatures for shorter CAT length. The partial discharge inception voltage is unchanged for shorter CAT lengths. A lower temperature rise is desirable as this leads to a longer life in the absence of partial discharges.

Viscoelastic liquid curtains: experimental results on the flow of a falling sheet of polymer solution

We experimentally investigate the extensional flow of a sheet – or curtain – of viscoelastic liquid falling freely from a slot at constant flow rate under gravity. Extruded liquids are aqueous solutions of flexible polyethylene oxide (PEO) and of semi-rigid partially hydrolysed polyacrylamide (HPAM) with low shear viscosities. Velocimetry measurements reveal that the mean velocity field$U(z)$(where$z$is the distance from the slot exit) does not reduce to a free fall. More precisely, we show that the liquid falls initially with sub-gravitational accelerations up to a distance from the slot which scales as$g\unicode[STIX]{x1D70F}_{fil}^{2}$(where$g$is gravity and$\unicode[STIX]{x1D70F}_{fil}$is the extensional relaxation time of the liquid) due to the stretching of polymer molecules. Beyond this elastic length, inertia dominates and the local acceleration reaches the asymptotic free-fall value$g$. The length of the sub-gravitational part of the curtain is shown to be much larger than the equivalent viscous length$((4\unicode[STIX]{x1D702}/\unicode[STIX]{x1D70C})^{2}/g)^{1/3}$for Newtonian liquids of density$\unicode[STIX]{x1D70C}$and dynamic viscosity$\unicode[STIX]{x1D702}$which is usually small compared to the curtain length. By analogy with Newtonian curtains, we show that the velocity field$U(z)$rescales on a master curve. Besides, the flow is shown to be only weakly affected by the history of polymer deformations in the die upstream of the curtain. Furthermore, investigations on the curtain stability reveal that polymer addition reduces the minimum flow rate required to maintain a continuous sheet of liquid.

Numerical analysis of pulsatile flows in a slot-die manifold

We analyze the effects of inlet periodic pulsatile flows on the flow inside a slot-die manifold for different locations of the inlet feed port. This type of flow rate disturbance can be caused by a coating liquid pumping mechanism that may lead to coating thickness nonuniformity. In this study, we analyze the transient behavior of the three-dimensional flows inside the slot-die manifold using parallel finite element computations. For a small magnitude of inlet flow rate disturbance with a given frequency, the local flow rate per unit width and the velocity profile at the feed slot exit become periodic. Therefore, the amplification factor can be used to quantify the effect of the inlet pulsatile flows. We found that the flow inside the feed slot, which is the confined channel with a high aspect ratio, dominates the dynamics of the outlet flow. However, the overall trend of the amplification factor along the cross-web direction resembles the steady-state pressure profile inside the cavity, and hence the steady-state profile of the outlet flow rate per unit width, which is strongly affected by the choice of the inlet port location.

Optimization of the Coanda bump to improve the film cooling effectiveness of an inclined slot

The present study attempted to improve film cooling effectiveness by installing a Coanda effect-inducing protrusion, called a Coanda bump, at the exit of the 45-degree inclined slot. The inclined slot used in this study was rectangular in shape with dimensions of 4 mm by 50 mm. For optimization, three geometric Coanda bump parameters – the width, height, and streamwise position of the peak point – were selected. The Box-Behnken design (BBD) method was employed to calculate the cases, and the response surface method (RSM) was applied to optimize the Coanda bump. The objective function (i.e., the area averaged film cooling effectiveness) was numerically investigated via a Reynolds-averaged Navier-Stokes (RANS) analysis. The results showed that when the optimized Coanda bump was applied to the slot exit, secondary flows such as the kidney vortex decreased and the coolant lift-off diminished compared to the reference design point without the Coanda bump. The area averaged film cooling effectiveness of the optimized design points improved by about 1.3, 2.4, and 7.6% at blowing ratios of 1.0, 1.5, and 2.0, respectively, compared to that of the reference design point.

Measurements versus Numerical Simulations for Slotted Synthetic Jet Actuator

In many studies concerning synthetic jet flow fields the analysis is usually restricted to simple configurations, such as a single diaphragm oscillating in a cylindrical cavity, which is linked to the external environment with only one orifice/slot. Nonetheless, in many applications the requirement of small sizes and weights leads to many implementation issues, such as asymmetric actuator geometries, presence of several slots and diaphragms and irregular cavity shapes. Therefore, the design of a synthetic jet actuator for a specific flow control problem requires a dedicated study in order to characterize its behavior even in quiescent conditions. The aim of this work is to investigate the behavior of a novel synthetic jet actuator, composed of three independent diaphragms, acting on a single cavity, and linked to the external environment through four slots per diaphragm. The device has been studied in quiescent conditions, both numerically and experimentally. The experimental investigation has been carried out by means of hot-wire measurements. In particular, the distribution of the phase-averaged streamwise velocity along the slot spanwise direction has been detected near the slot exit plane. From the computational side, incompressible direct numerical simulations have been carried out using the open-source OpenFOAM code. The diaphragm motion is mimicked by a inhomogeneous inlet boundary condition, whose amplitude is chosen to match the experimental velocity at the exit plane. A fair agreement between the numerical and the experimental results is achieved for both the velocity field at the slot exit and the main non-dimensional parameters of the synthetic jet. After the validation, the numerical results are finally processed, to obtain information about the vortex motion in the external environment.

Particle Image Velocimetry Measurements and Performance Analysis of Rotors with Slotted Blades

Particle image velocimetry measurements were taken to document the initial formation of the tip vortices trailed from a rotor operating in ground effect with pumping rotor blades. The pumping blade design used an internal duct connecting the intake slot near the root of the blade with exit slots at the blade tip. The baseline nonpumping and pumping blades were tested in a hovering state at a blade loading coefficient of 0.08. Performance measurements were taken both out of ground effect and in ground effect to quantify the additional power requirements associated with the pumping blades. A first-order analysis was performed assuming one-dimensional, incompressible, fully developed, steady flow through the internal duct. The method yielded a simple equation that was dependent on the assumed conditions of the internal flow through the duct (laminar, turbulent, etc.). The flow field measurements showed that the spanwise blowing of the pumping blades initially generated a vortex core that was significantly more distorted than the baseline blade, but this initial distortion was overcome by the rotation of the vortex. Furthermore, the pumping blades generated significantly more diffused tip vortices, producing peak swirl velocities approximately half that of the vortices from the baseline blades.

Turbine Blade Leading Edge Cooling With One Row of Normal or Tangential Impinging Jets

Jet impingement cooling has been extensively used in the leading edge region of a gas turbine blade. This study focuses on the effect of jet impinging position on leading edge heat transfer. The test model is composed of a semicylindrical target plate, side exit slots, and an impingement jet plate. A row of cylindrical injection holes is located along the axis (normal jet) or the edge (tangential jet) of the semicylinder, on the jet plate. The jet-to-target-plate distance to jet diameter ratio (z/d) is 5 and the ratio of jet-to-jet spacing to jet diameter (s/d) is 4. The jet Reynolds number is varied from 10,000 to 30,000. Detailed impingement heat transfer coefficient distributions were experimentally measured by using the transient liquid crystal (TLC) technique. To understand the thermal flow physics, numerical simulations were performed using Reynolds-averaged Navier–Stokes (RANS) with two turbulence models: realizable k–ε (RKE) and shear stress transport k–ω model (SST). Comparisons between the experimental and the numerical results are presented. The results indicate that the local Nusselt numbers on the test surface increase with the increasing jet Reynolds number. The tangential jets provide more uniform heat transfer distributions as compared with the normal jets. For the normal jet impingement and the tangential jet impingement, the RKE model provides better prediction than the SST model. The results can be useful for selecting a jet impinging position in order to provide the proper cooling distribution inside a turbine blade leading edge region.

Thermal analysis of airflow inside a refrigerated container

A numerical study of conjugated heat transfer in ceiling-slot refrigerated containers is carried out to analyze the temperature distribution effectiveness and to determine the ventilation characteristics. The effect of slot size on thermal characteristics is studied by considering half-span and full-span injection. The container walls are defined as conductive opaque and are interacting with outside environment. The outer surface heat transfer coefficients of conductive walls are computed by studying the flow around the refrigerated truck. The Reynolds number at the slot exit varied between 2 × 104 ≤ Re ≤ 2 × 105. The gravity effect is taken into account, and the coupled mass, momentum, and energy equations are discretized in finite volumes. The heat transfer coefficients of inner flow are presented as plots of the mean Nusselt number versus the modified Reynolds number. The maximum dispersion in the numerical data being at 14.54-percent, the mean Nusselt number, the modified Reynolds number, and the aspect ratio of the container are correlated.

Investigation of interfacial instabilities with a two-layer slide coating process

Organic electronics have been thoroughly investigated due to their broad application potential, ranging from light-emitting diodes to photovoltaics. The processing of organic electronics is trending from vacuum toward wet chemical deposition, which allows fast low-cost mass production of devices with scalable dimension. One of the current challenges of wet film processing is the redissolution of already dried active materials when applying a liquid top layer. Further, increasing overall process efficiency by coating multiple liquid layers in one step raises such challenges as liquid–liquid mixing or dewetting. This article describes the experimental investigation of these instabilities for two-layer flows with organic solvents. A modified slide coating device was chosen where an extended plate after the slot exit allows prolonged observation of the flow while it travels down the plate. During experimentation, stable and unstable two-layer flows as well as different types of instabilities were detected. The key finding is a correlation of flow stability with the spreading coefficient, a combined measure of surface and interfacial tensions. Focusing on fluid properties, this paper succeeds in defining a three-dimensional stability window for a dual-layer flow.

Design study of Coanda devices for transonic circulation control

ABSTRACTCirculation control via blowing over Coanda surfaces at transonic freestream Mach numbers is investigated using numerical simulations. The performance and sensitivity of several circulation control devices applied to a supercritical aerofoil are assessed. Different Coanda devices were studied to assess the effect of Coanda radius-to-slot height ratio, nozzle shape and Coanda surfaces with a step. The range of operating conditions for which a supersonic Coanda jet remained attached at transonic freestream conditions were extended by increasing the radius of curvature at the slot exit for Coanda devices with a converging nozzle. Additional improvements were found by reducing the strength of shock boundary-layer interactions on the Coanda surface by expanding the jet flow using a converging-diverging nozzle and also by introducing a step between the Coanda surface and the nozzle exit. The performance when using a converging-diverging nozzle can be matched using a simple stepped Coanda device. It is shown that circulation control has the potential to match the performance of traditional control surfaces during regimes of attached flow at transonic speeds, up to an equivalent aileron deflection angle of 10°. In addition, lift augmentation ratios ΔCl/Cμof over 100 were achieved.

Experimental Investigation on the Effects of DBD Plasma on the Film Cooling Effectiveness of a 30-Degree Slot

The effects of dielectric barrier discharge (DBD) plasma on the film cooling effectiveness of a 30-degree slot was experimentally investigated in a low-speed wind tunnel. The pressure sensitive paint (PSP) technique was used to measure the film cooling effectiveness, and two blowing ratios (0.5 and 1.0) were tested. A sinusoidal waveform with a 1-kHz frequency was supplied to the exposed electrode. Two input voltages (6 and 7 kV) and two exposed electrode locations were considered. The results showed that the film cooling effectiveness of the slot was higher for the blowing ratio of the 1.0 case than that for the blowing ratio of the 0.5 case regardless of plasma operation. The higher input voltage case (7 kV) showed higher film cooling effectiveness than the lower input voltage case (6 kV). The improvement in film cooling effectiveness facilitated by the DBD plasma was more significant when the coolant had less momentum. The maximum improvement of the area averaged film cooling effectiveness was 2.3% for the case with the exposed electrode located at the slot exit and a blowing ratio of 0.5.