Slot Exit Research Articles

New specs for ASD motors

The two main issues in pulse-width modulated ASD motors namely: stress concentration at slot exits and at junctions between stress grading and corona protection materials, and winding insulation aging due to partial discharge and/or dielectric heating, often intensified between turns as well as across the main-wall of the stator coils. This paper explores the IEC-60034-18-41 and IEC-60034-18-42 technical specifications for inverter duty motor insulation. These specifications describe the qualification and acceptance tests for both low and medium voltage motor windings in order to ensure reliability

Impact of inverter drives employing fast-switching devices on form-wound AC machine stator coil stress grading

The conventional stress-grading system of form-wound motor coils has been found to perform well under sinusoidal, power-frequency voltage. However, when the motor is fed by inverter-based drives employing fast switching, a significant impact on the electric stress-grading system occurs. Simulations of the stress-grading systems using finite element analysis indicate that, during the fast rise-time, a high electric stress can develop right at the slot exit of the stator if the conductivity of the conductive armor tape (CAT) is not high enough. Increasing the conductivity of the CAT alleviates the problem but increases the eddy current loss in the stator laminations. To address this problem, the conductivity of the area just beyond the slot exit needs to be increased by coating the surface of the CAT with a high-conductive material. The stress-grading at the end of this conductive paint section then is achieved by two coordinated layers, which help to distribute the electric field and heat that is produced

New Generation of Synthetic Jet Actuators

I N the last 20 years, much attention has been given to developing microfluidic systems [1]. Among them, synthetic-jet actuators (SJA) have been intensely studied recently [2]. Synthetic-jet actuators have many potential applications such as jet vectoring [3,4], boundary-layer control [5,6], combustion control [7], and micro-pumps [8], etc. A synthetic-jet actuator is a zero net mass flux device. Figure 1a shows a typical structure of an existing SJA, which is composed of an exit slot (or orifice) and a diaphragm (or double diaphragms [9]) mounted to enclose a volume of fluid in a cavity. Piezoelectric (PZT) diaphragms are used in most minior microSJAs because of their size, rapid time response, and relatively low power consumption. The PZT-driven SJA also fits within the microelectromechanical system (MEMS) discipline and can be coupled with MEMS sensors, control and logic electronics into a single very lightweight and compact device. Unfortunately, the pressure loading limits to obtain a small deflection of the PZTdiaphragm on the existing SJA requires a high power to drive the PZT-diaphragm when the pressure differential between the base flow (to be controlled by the SJA) and the environment is large. This limits applications of the SJA. In addition, the diaphragm of the existing SJA (as shown in Fig. 1a) has one surface exposed to the fluid drawn from the base flow and the other surface exposed to the environment, which indicates that half of the radiation energy of the vibrating diaphragm is wasted in the environment. Keeping the advantages of a PZT-driven SJA,while resolving themajor problems of pressure loading and energy inefficiency, is the main impetus for designing a new generation SJA. A newSJA prototype is schematically shown in Fig. 1b. It consists of two cavities, a PZT-diaphragm, two exit slots, and a slide block (SB). In the new SJA concept, two adjacent jets are established under the two exit slots and are driven by the motion of the same PZTdiaphragm. The slide block regulates the two jets (as shown in Figs. 1b–1d), and then two adjacent jets merge into a single, larger synthetic jet. In this Note, numerical simulations are conducted to study the new SJA. We include a discussion focused on the novelty and merits of the new SJA in comparison with the existing SJAs.

Boundary condition effects on the evolution of a train of vortex pairs in still air

AbstractEffects of boundary conditions on the performance of compact oscillatory momentum and vorticity generators, commonly known as ‘synthetic jet’, ejecting a train of vortex-pairs into still air, were studied experimentally. The different boundary conditions altered the near-device entrainment process of the zero-net-mass-flux actuator. The measurements included hot-wire and Particle Image Velocimetry, cavity pressures and temperatures.When the actuator operates in still air, a quasi-2D vortex pair is generated due to the extreme shear at the edges of the fluid slug ejected during the blowing stage of each excitation cycle. The vorticity flux exiting the slot determines the resulting vortex-pair circulation. The threshold slot exit velocity, for the current configuration and operating conditions, determines if the vortices will be sucked into the actuator’s cavity or be released. Once released, the vortex convection speed approximately scales with the peak velocity at the slot exit. However, the normalised convection velocity increases with the slot Reynolds number.When even a very short extension is attached to one ‘lip’ of the actuator exit, the jet is deflected in the direction opposite the extended lip, due to the restriction on the entrainment process. When long, one ‘lip’ extension is attached, such that the vortex pair is ejected parallel to a plate, the coherence of the vortices improve, their phase speed and magnitude decrease.The effects of high-frequency excitation, ejected perpendicular to a wall into still air, were also investigated. It was found that the presence of the plate does not have a measurable effect on the wall normal excitation, indicating that the majority of the entrainment is taking place from the forward 180° of the actuator exit plane. When the slot is inclined to the surface at a shallow angle of 30 degrees, an unsteady wall jet is formed, transferring momentum along the wall. This is a direct result of the symmetry break, altering the relative magnitudes of the vortex pair.

Cooling of a Finned Cylinder by a Jet Flow of Air

A submerged slot jet of air is used to cool an externally finned cylinder, heated by electric current. The cylinder ensemble is made of a stainless steel finned tube and a Teflon bar core inside. Five thermocouples, pressed inside the steel tube by the Teflon bar, measure the wall temperature to determine local and mean convective heat transfer coefficients. The local Nusselt number has the maximum on the impinging point and the minimum on the rear point. The variation of local and mean Nusselt numbers with the distance from the slot exit is investigated. Empirical expressions are proposed to correlate the experimental data. The cooling of a finned cylinder with a jet flow realizes a higher heat transfer as compared to a smooth cylinder.

A New Class of Synthetic Jet Actuators—Part I: Design, Fabrication and Bench Top Characterization

Although the potential of synthetic jets as flow separation control actuators has been demonstrated in the existing literature, there is a large gap between the synthetic jet actuators (SJA) used in laboratory demonstrations and the SJAs needed in realistic, full-scale applications, in terms of compactness, weight, efficiency, control authority and power density. In most cases, the SJAs used in demonstrations are either too large or too weak for realistic applications. In this work, we present the development of a new class of high-power synthetic jet actuators for realistic flow control applications. The operating principle of the actuator is the same as that of crankshaft driven piston engines, which makes a significant part of the technology necessary for the actuator development available off-the-shelf. The design of the actuator is modular and scalable. Several “building block” units can be stacked in series to create the actuator of the desired size. Moreover, active exit slot reconfiguration, in the form of variable exit slot width, decouples the actuator frequency from the actuator jet momentum coefficient and allows the user to set the two independently (within limits). Part I of this paper presents the design, fabrication and bench top characterization of the actuator. Several versions of the actuator were designed, built and tested, leading up to the development of a six-piston compact actuator that has a maximum power consumption of 1200 W (1.6 hp) and can produce (for the tested conditions) peak exit velocities as high as 124 m/s. In Part II, the actuator was housed in the interior of a NACA0015 profiled wing with a chord of 0.375 m (14.75 inches). The assembly’s performance in controlling flow separation was studied in the wind tunnel.

Modeling Pulsed-Blowing Systems for Flow Control

A linear model for analyzing the performance of pulsed-blowing actuator systems commonly used in many flow-control applications is presented and compared with experiments. The pulsed-blowing system consists of a regulated air supply, an oscillatory valve, transmission tubing, and an actuator. The elements of the actuator are a cavity and slot at the location in the flow to be controlled. The typical design objective is to achieve the largest possible velocity-fluctuation levels at the actuator-slot exit for a given fluctuating pressure input. However, the performance of the system is strongly dependent on component geometry, flow rate, and frequency of pulsation. Lumped-element models are useful for estimating the performance of the actuator cavity and slot, but fail to account for the effects of the transmission tubing. A distributed model for the tubing combined with a lumped-element model for the actuator provides estimates of the system resonant frequencies and amplitudes. Comparisons with experiments are made for a wide range of tubing lengths, slot widths, mean flow velocities, and forcing frequencies. The resonances of the pulsed blowing system and the changes in resonant behavior are characterized by considering the reflection coefficient of the system. Unexpected behavior such as reversed flow at the slot exit and reduced fluctuation levels with increasing pressure are demonstrated and explained with the system model.

계단형 슬롯출구의 높낮이 변화에 따른 2차원 막냉각 특성

Film cooling characteristics has been examined numerically for the height variation of a stepped slot exit. In this study, the upstream wall height of the stepped slot exit from -2d (d=slot width) to 3d, blowing ratio ranges from 0.5 to 3, and injection angles are 15˚, 30˚, and 45˚. The results showed that film cooling performance was mainly subjected to the magnitude of recirculation region near the downstream-side slot exit as well as the mangitude and the distribution region of turbulent kinitic energy due to the local velocity and momentum defferences between the coolant and the main flow near the slot exit. The up-ld type slot at higher blowing ratios over 2 and the flat type slot at lower blowing ratios below 1 have the best film cooling performances, in case of the injection angles of 30˚ and 45˚, respectively. Compared with the other injection angles, in case of the injection angles of 15˚, the best film cooling performances was shown in even a higher upstream wall (up-3d) at higher blowing ratio like 3 by the gradual reduction of the coolant velocity which minimized the local velocity differences between the coolant and the main flow near the slot exit.

Effects of Synthetic Jet Actuation on a Ramping NACA 0015 Airfoil

An experimental investigation was undertaken to evaluate the effectiveness of a synthetic jet actuator (SJA) for flow control on a pitching airfoil. A NACA 0015 profiled airfoil was ramped up at various rates with concomitant surface-pressure acquisition. A self-contained synthetic jet actuator was positioned in the interior of the airfoil. Synthetic-jet-actuation parameters included the jet-momentum coefficient and the slot exit width. The behavior of the surface pressures is investigated as well as their integrated properties. The data suggest that the effect of the SJA is to delay the onset of the dynamic-stall vortex formation to higher incidence and in some cases, for the parameters and incidence range investigated, suppress it totally. However, in the instances when formed, the synthetic jet appears to increase the loading induced by the stall vortex. At low ramp rates, the synthetic jet forcing causes the formation of a stall vortex where the unforced data showed a psuedostatic stall-type behavior

Flow field of an unconfined low-Reynolds-number binary-mixture slot jet

The flow field of a slot jet at low Reynolds numbers ( Re=39.4 and 78.8) was examined numerically and experimentally. The slot exit velocities are small, so the gravitational force has to be taken into consideration in numerical calculation. Experimentally, the fluid containing oxygen and nitrogen was issued from a slot jet into the quiescent atmosphere. The concentration field was measured by using a gas chromatograph and a gas-extracting quartz probe. Furthermore, smoke particles were used to visualize the flow field. Due to the interaction among momentum transfer, mass transfer and gravitational force, the flow and concentration fields at low Reynolds numbers with considerable mass transfer are found very different from those at high Reynolds numbers. The flow field is greatly influenced by the gravitational force due to concentration-induced density change; and the flow field in turn affects the concentration distribution. For an upward jet of density higher than air, at Re=39.4, one vortex in the far region exists. However, at Re=78.8, two vortices appear; one attached at the slot exit and one in the far region. The calculated concentration field is in fairly good agreement with the experimental result.

A numerical study of hypersonic laminar film cooling

AbstractA computational study has been performed to investigate the effectiveness of film cooling in hypersonic laminar flows. Both the primary and the coolant flow are air. First, three different primary flow conditions are used for validation. A uniform boundary condition at the slot exit is found to give unrealistic predictions of the heat transfer rate, whilst a boundary condition involving extended coolant inlet gives improvement. Then five coolant injection rates and three slot heights are examined. It is confirmed that increasing the coolant injection rate can increase the film cooling effectiveness for laminar cases. But slot height does not play an important role under the flow conditions in this study. The computational results are compared with the experimental results and good agreement is achieved.

Influence of aspect-ratio on the mixing and acoustic characteristics of plain and modified elliptic slot jets

This paper presents the results of an experimental program that was conducted to examine the effect of aspect-ratio on the flow development and the related acoustic properties of jets issuing from elliptic-slots. The aspect-ratios investigated are 2:1, 3:1 and 4:1. Fully expanded cases show a decrease in shear layer growth with increasing aspect-ratio resulting in a downstream shift in axis-switching location. Further the small aspect-ratio jet shows a dominance of large-scale structures close to slot exit relative to 3:1 and 4:1 jets. In the underexpanded condition the shock-structure development shows appreciable changes with aspect-ratio. At the end of first cell, oblique shock cross-over point in 2:1 jet is gradually replaced by a normal shock in 3:1 and 4:1 jets. The frequency content of jet noise also shows a corresponding increase in amplitude of fundamental screech frequency with increase in aspect-ratio. Introduction of notches along the minor-axis sides further modifies the shock development process which significantly alters the screech amplitudes for notched 3:1 and 4:1 jets indicating the weakening of the feedback process.

A numerical study of hypersonic laminar film cooling

AbstractA computational study has been performed to investigate the effectiveness of film cooling in hypersonic laminar flows. Both the primary and the coolant flow are air. First, three different primary flow conditions are used for validation. A uniform boundary condition at the slot exit is found to give unrealistic predictions of the heat transfer rate, whilst a boundary condition involving extended coolant inlet gives improvement. Then five coolant injection rates and three slot heights are examined. It is confirmed that increasing the coolant injection rate can increase the film cooling effectiveness for laminar cases. But slot height does not play an important role under the flow conditions in this study. The computational results are compared with the experimental results and good agreement is achieved.

Optimal slot height in the jet cooling of a circular cylinder

Experimental results are presented for heat transfer cooling of an electrically heated circular cylinder, of diameter D=10 mm, by a slot jet, of height S=2.5 mm, i.e. with D/ S=4. Mean and local Nusselt numbers are presented versus the Reynolds numbers, the distance of the cylinder from the slot exit, H, and the angle from the impinging point. Mean Nusselt numbers, defined according to the cylinder diameter, D, are higher than the values in an infinite flow. The maximum mean Nusselt number is measured when the cylinder is set at a distance H, from the slot exit, such that H/ S=8. The minimum local Nusselt number is measured at an angle of 180° from the impinging point, i.e. on the rear of the cylinder. Comparisons have been done among the mean Nusselt numbers of this work and previous experimental results of the same authors, carried out with diameter to slot height ratios equal to D/ S=1 and 2. The conclusion is that the mean Nusselt number, defined as Nu m D =h m D/k , is higher for D/ S=4, versus the Reynolds number, defined as Re D= WD/ ν. On the contrary, the mean Nusselt number, defined on the basis of the slot height, Nu m S =h m S/k , has similar values, for the different slot heights, versus the Reynolds number, defined as Re S= WS/ ν, with the trend of higher results for the slot with D/ S=2.

Experience with development and evaluation of corona-suppression systems for HV rotating machines

This paper presents the results of a project aimed at the development of reliable corona-suppression systems for high voltage rotating machines. These systems are based on both conducting and semi-conducting dry and B-stage tapes as well as paints of different resistivity. Three groups of anti-corona systems were developed; namely, paint (on the slot portion) - paint (on the slot exit) system, paint (on the slot portion) - tape (on the slot exit) system, and tape (on the slot portion) - tape (on the slot exit) system. Sample results of standard evaluation and acceptance tests are presented. The accelerated life-aging test was run on test coil samples at the National Electric Coil HV testing facility. A numerical 2-dimensional finite difference model for the electric potential and field analysis along the end-turn zone was developed and the model results are used as guidelines through the design process of the grading system. Laboratory test results on randomly selected sample coils were confirmed by an independent testing facility. The proposed corona-suppression systems were applied on thousands of production coils.

Streak-Line Defect Minimization in Multi-Layer Slide Coating Systems

The results from complementary computational and experimental investigations of streak-line defect formation in industrial slide/cascade coating systems are presented. Under typical operating conditions the propensity for eddies to reside within the flow, in the vicinity of feed slot exits and the bead forming region, is shown to be directly linked to deposit growth on adjacent surfaces, and hence, the occurrence of streak-lines. Single- and double-layer slide coating systems are examined both in relation to geometrical factors which lead to eddy formation under equilibrium flow conditions and static contact line effects. The consequence of back wetting is also explored as is the influence of the internal interfacial layers present in multi-layer slide coating systems. It is shown that minor geometrical adjustments and/or changes to wetting characteristics, particularly in relation to static contact angles, can eliminate the occurrence of eddies, reduce signiflcantly associated surface deposit growth, and hence, avoid the formation of streak-line defects by widening the operating window available to coating practitioners.

Influence of Injection Type and Feed Arrangement on Flow and Heat Transfer in an Injection Slot

An experimental investigation is conducted to improve a slot film cooling system used for the cooling of a gas turbine combustor liner. The tangential slots are constructed of discrete holes with different injection types which are the parallel, vertical, and combined to the slot lip. The investigation is focused on the coolant supply systems of normal, inline, and counter-flow paths to the mainstream flow direction. A naphthalene sublimation technique has been employed to measure the local heat/mass transfer coefficients in a slot wall with various injection types and coolant feeding directions. A numerical simulation is also conducted to help understand the flow patterns inside the slot for different injection types. The velocity distributions at the exit of slot lip for the parallel and vertical injection types are fairly uniform with mild periodical patterns with respect to the injection hole positions. However, the combined injection type increases the nonuniformity of flow distribution with the period equaling twice that of hole-to-hole pitch due to splitting and merging of the ejected flows. The dimensionless temperature distributions at the slot exits differ little with blowing rates, injection types, and secondary flow conditions. In the results of heat/mass transfer measurements, the best cooling performance inside the slot is obtained with the vertical injection type among the three different injection types due to the effects of jet impingement. The lateral distributions of heat/mass transfer coefficients with the inline and counter-flow paths are more uniform than the normal-flow path. The average heat/mass transfer coefficients with the injection holes are about two to five times higher than that of a smooth two-dimensional slot path.

Spatially resolved absolute concentration and fluorescence-lifetime determination of H2CO in atmospheric-pressure CH4/air flames

Using laser-induced fluorescence (LIF), spatially resolved concentration profiles of formaldehyde (H2CO) were obtained in the preheating zone of atmospheric-pressure premixed CH4/air flames stabilized on the central slot of a multiple-slot burner similar in construction to domestic boilers. The isolated pQ1(6) rotational line (339.23 nm) in the 21 041 0 vibronic combination transition in the A1A2- \tilde{X} 1A1 electronic band system around 339 nm was excited in the linear LIF intensity regime. For a quantification of quenching effects on the measured LIF signal intensities, relative fluorescence quantum yields were determined from direct fluorescence lifetime as a function of height above the slot exit. Absolute H2CO number densities in the flames were evaluated from a calibration of measured LIF signal intensities versus those obtained in a low-pressure sample with a known H2CO vapor pressure. Peak concentrations in the slightly lean and rich flames reached (994±298) and (174±52) ppm, respectively.

Jet mixing in a slot

Jet mixing inside a slot has the potential to be applied for cooling in various components in gas turbines. An experimental program was conducted to focus on investigating the flow mixing behavior inside the slots. Various parameters including orientation angle, inclination angle, slot width, effect of primary flow and slot depth were systematically examined. An array of seven jets with a jet-to-jet spacing of five diameters and velocity ratio of unity was used in the experiment. The results indicated that the flow distribution at the slot exit becomes more uniform as the orientation angle was increased from 0° to 60°. Wider slot width brought about high non-uniformity and was clearly undesirable. The optimum slot depth was found to range from 2 to 2.8 times the jet diameter. The flow field was the most uniform where the shear layers of the two adjacent jets met. Regions of low velocity and high pressure caused non-uniform velocity distribution downstream where the jets met, so deeper slots do not necessarily render better uniformity. When the orientation angle increased from 0 to 30° and 60°, the jets bent toward the wall in the inclined direction caused by the Coanda effect. The presence of the primary flow forced the jet to spread faster in the lateral direction and caused some non-uniform flow pockets at the slot exit. The compound angle configuration (60° jet orientation and 30° slot inclination angles) was discovered as the best choice.

Numerical investigation of the aerodynamics of the near-slot film cooling

Fluid injection from slot or holes into cross-flow produces highly complicated flow fields. Physical situations encountering the above problem range from turbine blade cooling to waste discharge into rivers. The flow field created by a two-dimensional slot cooling geometry is examined using the finite volume approach with a second-order upwind differencing scheme. The time-averaged Navier-Stokes equations were solved on a collocated Cartesian grid with a two-equation model of turbulence. Attempting to solve the flow field by assuming a uniform velocity profile at the slot exit leads to inaccurate results, while extending the solution domain improves significantly the results, but proves to be costly, both in memory and in computing time (particularly in the case of multiple holes). A pressure-type boundary condition, based on uniform total pressure, is developed for the slot exit (easily applied to a three-dimensional geometry), which yields more accurate results than the widely used uniform velocity assumption