Exit Plane Research Articles

Influence of inlet swirl on pattern factor and pressure loss in an aero engine combustor

The effect of compressor exit swirl angle (θsw) at the intake of an aero engine combustor on the exit temperature non-uniformity (pattern factor) and combustor total pressure loss is investigated. Experiments are conducted in the engine test rig, measuring the gas temperature and pressure at the inlet and exit planes of the combustor. These parameters are measured at distinct locations along the circumferential and radial directions in the engine test facility. Simulations are carried out using RANS based turbulence modeling and reacting flow approach with Ansys CFX commercial code. The predicted results are validated with experimental data at 5° swirl angle. The swirl angle at the combustor intake is further varied from 0° to 15° and 4 cases (0°, 5°, 10° and 15°) has been considered to predict the effect on the combustor pattern factor and pressure loss. The changes in the flow structure inside the combustion chamber for all these 4 cases are reported in detail. The pattern factor varies from 0.34 to 0.49 as swirl angle changes from 0° to 15°. The lowest pattern factor of 0.34 occurs at 10° swirl angle. However a linear increase in combustor total pressure loss from 5.85% to 6.53% is predicted with the change in swirl angle from 0° to 15°.

A quasi complex-lamellar solution for a hemispherically bounded cyclonic flowfield

This work describes an inviscid solution for a cyclonic flowfield evolving in a hemispherical chamber configuration. In this context, a vigorously swirling motion is triggered by a gaseous stream that is introduced tangentially to the inner circumference of the chamber's equatorial plane. The resulting updraft spirals around while sweeping the chamber wall, reverses direction while approaching the headend, and then tunnels itself out through the inner core portion of the chamber. Our analysis proceeds from the Bragg–Hawthorne formulation, which proves effective in the treatment of steady, incompressible, and axisymmetric motions. Then using appropriate boundary conditions, we are able to obtain a closed-form expression for the Stokes streamfunction in both spherical and cylindrical coordinates. Other properties of interest are subsequently deduced, and these include the principal velocities and pressure distributions, vorticity, swirl intensity, helicity density, crossflow velocity, and the location of the axial mantle; the latter separates the outer annular updraft from the inner, centralized downdraft. Due in large part to the overarching spherical curvature, the cyclonic motion also exhibits a polar mantle across which the flow becomes entirely radial inward. Along this lower and shorter spherical interface, the polar velocity vanishes while switching angular direction. Interestingly, both polar and axial mantles coincide in the exit plane where the ideal outlet size, prescribed by the mantle position, is found to be approximately 70.7% of the chamber radius. We thus recover the same mantle fraction of the cyclonic flow analog in a right-cylindrical chamber where an essentially complex-lamellar motion is established.

Injector Effects on Hybrid Polymethylmethacrylate Combustion Assessed by Thermochemical Tomography

In this work, two representative hybrid rocket injector designs, a single-port and an axial showerhead design, were examined experimentally to determine the comparative influence on the development of the downstream combustion zone with polymethylmethacrylate (PMMA) fuel. One-dimensional laser absorption tomography was employed to obtain the thermochemical structure of the PMMA–gaseous oxygen reaction layer via quantitative in situ species and temperature measurements. Optical measurements were performed with tunable semiconductor lasers in the midwave infrared to target absorption features in the fundamental vibrational bands of CO, , and . Planar measurements were conducted at the exit plane of cylindrical fuel grains () at axial distances () ranging from 2 to 11 with a constant gaseous oxygen flow for each of the injectors. Assuming azimuthal symmetry, a Tikhonov-regularized Abel inversion technique was used to obtain the radially resolved absorption coefficient, from which temperature and mole fractions were extracted. One-dimensional measurements over all fuel-grain lengths were consolidated to obtain spatially resolved two-dimensional images of the reaction layer thermochemical structure in a hybrid rocket motor geometry. Significant variations in experimental results were observed between the two injectors, suggesting the importance of injection fluid dynamics in hybrid rocket combustion performance. Explanations of the variations are offered from both a theoretical and a computational analysis.

Spray Characteristics of Standard and Alternative Aviation Fuels at Cold-start Conditions

The spray characteristics of standard and alternative aviation fuels created by a hybrid pressure-swirl airblast (HPSA) atomizer were investigated at conditions corresponding to engine cold start for an aviation gas turbine. Drop size and drop velocity of sprays from three different fuels, Jet-A (A-2), JP-5 (A-3), and a JP-5/farnesane blend (C-3) were investigated using a phase Doppler anemometry. The spatially resolved drop measurements were obtained at three axial locations downstream of the swirler exit plane and up to 30 mm in the radial direction from the spray centerline. Smaller , , and were observed when increasing the pressure drop across the atomizer swirler (). Increasing also increased the mean axial velocity. No definitive trend was observed with viscosity on drop size, while liquid–gas surface tension showed a weak correlation with drop size due to its small variation in value among fuels. The spray unsteadiness was examined using interparticle arrival time distribution functions. A semi-empirical model was also developed using a phenomenological three-step model for the atomization process of the HPSA atomizer and was used to predict drop-size values at engine cold-start conditions in this work and near lean blowout conditions presented in earlier work.

Bubble optics: Leonardo's cross revisited-Part 2, paraxial analytical methods.

Around 1508, Leonardo da Vinci first noted and sketched a cross-shaped caustic produced by a plane wave diagonally incident on the meniscus surrounding a bubble on the surface of a water-filled container. This caustic was analyzed in a companion paper using a numerical ray theory spot diagram approach and is analyzed here using analytic ray theory approaches appropriate to the paraxial regime. We demonstrate that the caustic assumes the standard astroid form when the maximum meniscus slope is small, and the shape starts to distort as the maximum meniscus slope increases. We show that competition between generalized coma and generalized astigmatism, with generalized coma dominating, is responsible for the cusp points of the astroid in the plane of the incident light being reversed with respect to their images in the exit plane of the optical system. We also show that Leonardo's cross is the result of the twofold symmetry of the astigmatic focusing of the diagonally incident plane wave by the circularly symmetric meniscus.

Shock dynamics and expansion characteristics of an aerospike nozzle and its interaction with the rotating detonation combustor

The flow of products through the aft-end of a detonation combustor is intrinsically unsteady, ultimately resulting in periodic and nonuniform flow parameters at the exit plane of a rotating detonation combustor. Therefore, the integrated design of the nozzle and combustion chamber is an important challenge in rotating detonation engines (RDEs). This paper describes a thorough three-dimensional numerical simulation of a hydrogen–air rotating detonation combustor with/without an aerospike nozzle and investigates the nozzle flow without a combustor. The simulation results allow us to analyze the coupling characteristics of the rotating detonation combustor and the aerospike nozzle. The new features of the shock wave dynamics and expansion characteristics in the nozzle with/without an integrated RDE combustor are compared. For the same rotating detonation combustor exit, the propagation mode of the detonation wave and backpressure in the RDE combustor exit are affected by the aerospike nozzle. The intensity of the shock wave and the angle with respect to the direction of the air flow are lower with the aerospike nozzle than in the nozzleless model, which is consistent with the effect of backpressure increase on the shock dynamics. Regardless of whether the nozzle is connected to the combustor, the nozzle affects the dynamic alternation of the design point state to over-expansion and under-expansion along the circumferential direction. The shock wave intensity and total pressure recovery inside the nozzle with the combustor are slightly stronger than those in the nozzle without the combustor. Consequently, the thrust and total pressure recovery of the nozzle integrated with the RDE combustor are enhanced by 7% and 5.6%, respectively. Thus, decoupling the nozzle from the rotating detonation combustor is feasible for testing the design performance of the unsteady nozzle.

Drilling induced delamination assessment of nanoparticles reinforced polymer matrix composites

The knowledge of polymer matrix composites machinability has increased with its rise in multi-field structural applications. Drilling of polymer matrix composites is required to achieve structural integration with greater accuracies. Delamination due to drilling affects the material strength and it occurs at both the entry and hole exit planes. The delamination on the exit plane is critical. The effect of nano-fillers on drilling-induced delamination of reinforced plastics has not been documented extensively and in this article, an attempt is made to explore the application potential of montmorillonite clay and graphene with amine functionalization in minimizing delamination through toughening the epoxy matrix. Experimentation was based on Taguchi’s L16 approach. The delamination factor and circularity ratio were analyzed using an image processing technique in Matlab and Image J. To evaluate the effects of various parameters and parameters interaction effect on the delamination factor, ANOVA was employed and Signal/noise ratios were calculated. The mathematical models were developed for delamination. It was observed that the reduction in the delamination factor was observed in nanoclay and graphene reinforced polymer matrix toughened composites in comparison to the base composites with a reduction in thrust force, specific cutting energy, and increase in circularity ratio.

Efficiency of pulsed-jet propulsion via thrust-drag decomposition

By using an axisymmetric immersed-boundary model, fluid dynamics of a cephalopod-inspired propeller undergoing periodic inflation–deflation deformation in background flow is numerically studied in a low Reynolds number regime. A thrust-drag decoupling method based on physical analysis is proposed, in which the jet-related thrust is obtained as the summation of three parts: the jet momentum flux, the normal stress at the exit plane, and the flow acceleration inside the body. This method enables the calculation of the propulsive efficiency, especially the efficiency at the steady-swimming state. Systematic simulations are then conducted to study the effects of the Reynolds number and stroke ratio on force generation and efficiency. Two Reynolds numbers, the incoming-flow Reynolds number Re∞ and the jet-flow Reynolds number Rej, are involved. When Re∞ is fixed, the thrust generation is found to depend mostly on jet-flow velocity at high Rej, while the effect of incoming-flow velocity is pronounced at relatively low Rej, mostly through its influence on the excessive pressure at the nozzle. Within the range of incoming-flow Reynolds number considered in this study (40–150), our results show that the whole-cycle propulsive efficiency of the propeller lies in the range of 11%–30%.

Flow and performance analysis of a natural gas-air rotating detonation engine with high-speed velocimetry

An experimental investigation of a high pressure, natural gas-air rotating detonation engine is presented. Simultaneous high-speed measurements of combustion chamber pressure, axially-integrated chamber chemiluminescence, and particle image velocimetry at the combustor exit plane were used to qualitatively and quantitatively analyze the exhaust flow of two test conditions with distinct wave dynamics. A single detonation wave with an intermittent counter-rotating wave was characterized in the first test case, while a steady counter-rotating mode was studied in the second. Analysis using the continuous wavelet transform informs the coupled shift in the frequency and amplitude of pressure and velocity fluctuations observed in the raw timeseries. The high-speed measurements are used to reconstruct the wave structure for each case over the course of a cycle, revealing the canonical rotating detonation engine exhaust flow structure with a trailing oblique shock sweeping through the products for the single wave case. Interactions between counter-propagating detonation fronts result in a flowfield that varies with cycle phase angle, in contrast to the self-similar field of the single propagating wave. The axial velocity profile around the circumference of the combustor is independent of wave location, while azimuthal velocity fluctuations from the trailing oblique shocks are observed to track the detonation fronts and destructively interfere at wave intersections. The total pressure and temperature of flow exiting the combustor are computed using the phase-resolved velocity measurements along with the measured reactant flowrate and thrust to close the global balance of mass and momentum. The case with counter-rotating waves delivers 5% higher performance than the single-wave case. The application of simultaneous high-speed diagnostics helps elucidate the flow structure in a rotating detonation engine and provides a connection between the local flow structure and global combustor performance.

Performance of Two-Fluid Atomization of Gel Propellant

Two-fluid atomization of highly viscous and shear thinning non-Newtonian fluids must overcome the viscous resistance. Conventional air-blast atomizers address this requirement by operating at higher gas–liquid mass ratios (GLRs) at the cost of deteriorated efficiency. In this investigation, the atomization efficiency of a two-fluid atomizer is characterized. Fluidic design of the atomizer reduces the viscosity-induced dampening of disruptive inertia force and speeds up the breakup by providing kinetic energy of strategically positioned microjets of air. Three fluids of different viscosities and surface tensions were tested to examine the effects of properties on atomization efficiency. The presence of droplets and ligaments near the exit plane of the atomizer revealed that the atomizer enables prompt breakup of fluids without appreciable delay by viscosity. Consequently, it could break up complex non-Newtonian Jet A1 gel propellant with efficiency in the range of 0.015–0.1. Moreover, it is also capable of atomizing Newtonian fluids like water (high surface tension) with efficiency of 0.06–0.1 and Jet A1 (low surface tension) with efficiency of 0.02–0.08. Note that varied as ( to ) for the liquids tested in this work. The results of this investigation show that the deterioration in the performance of internally impinging atomizers at higher air and liquid throughputs is not determined by the viscosity of liquid but is predominantly due to the loss in kinetic energy of the excess air.

An exact irrotational solution for a hemispherically bounded cyclonic flowfield

This study focuses on the development of an internal potential flow solution in the context of a hemispherically bounded cyclonic chamber. The analysis proceeds from the Bragg–Hawthorne equation, which is quite effective in the treatment of steady, inviscid, and axisymmetric flows when expressed in terms of the streamfunction. Once the streamfunction is obtained, other flow properties are readily deduced; these include the principal velocity and pressure distributions, swirl intensity, crossflow velocity, and mantle location. Furthermore, given the overarching spherical geometry, two different types of mantles are identified and related to the coexistence of axially bidirectional and circularly bipolar regions. The first, axial mantle, which is traditionally used in the analysis of cylindrical and conical cyclone separators, consists of a rotating, non-translating interfacial layer along which the axial velocity vanishes. It thus separates the outer, vertical updraft, from the inner, swirling downdraft. The second, polar mantle, which arises in the context of a hemispherical flow configuration, coincides with the spherical interface along which the polar velocity vanishes. It hence partitions the flow domain into a much larger outer region, where the flow direction remains strictly counterclockwise, and a proportionally smaller inner region, where the outflow becomes clockwise. Despite their dissimilar structures, both axial and polar mantles meet in the exit plane at a fractional radius of 1/e2 or 13.53%. In this study, the unique characteristics of the resulting irrotational motion, which reduces to a continuously looping, hemispherically cyclonic potential vortex, are evaluated and discussed.

Investigation of the Operating Principle of Diffuser Augmented Hydrokinetic Turbines

Alternative sustainable technologies for energy production are currently receiving increased attention, one of them being hydrokinetic turbines. For some applications an augmentation of the turbine rotor through a diffuser is a reasonable option offering some advantages. In order to get a better understanding of diffuser augmented turbines an extensive literature study is performed also including relevant publications on ducted wind turbines. The published geometry and performance data are collected and reevaluated applying a new procedure, which references the turbine power on the diffuser outlet area instead of the rotor area. Additionally, geometries are generated, manually optimized and compared to the data gathered in literature. Dominant diffuser attributes are identified independent from the exact diffuser concept such as the shape of the diffuser close to the exit plane and the diffuser area ratio. By using the new evaluation approach a strong correlation between the area ratio and the overall turbine power coefficient is observed. Observations suggest that an increase of the exit area is only reasonable up to a certain limit. To a some extent, this constraint can be overcome through boundary layer control, e.g. by applying multi-stage diffuser concepts. The presented work gives a guide line for effective design of diffusers for hydrokinetic turbines and generally improves the understanding of the operating principle of diffuser augmented turbines.

Design and characterization of a multifunctional low-speed anechoic wind tunnel at HKUST

The Hong Kong University of Science and Technology (HKUST) has installed and commissioned an Ultra-quiet Noise Injection Test and Evaluation Device (UNITED). This facility is the first closed-circuit anechoic wind tunnel in Hong Kong. Calibrations and improvements have been made to improve the aerodynamic and aeroacoustic performance since its commissioning in 2016, and various experiments have been conducted in it. The test section is enclosed within a 3.2 m × 3.1 m × 2 m anechoic chamber, and it can be configured as either open-jet or closed test section. The exit plane of the contraction section has a dimension of 0.4 m × 0.4 m, and the flow speed of the test section can be varied from 10 m/s up to 70 m/s. In this paper, the major design constraints and design principles of key wind tunnel sections are described. To achieve low background noise, a silencer network is carefully designed, and a low-noise axial fan is specially designed and manufactured for the wind tunnel circuit. It is shown that the metrics of aerodynamic and aeroacoustic performances are comparable to similar facilities across the world. Representative research activities are also briefly summarized in this paper. Particularly, the measurement of trailing edge noise of a flat plate is demonstrated. The measured trailing edge noise collapses well under the classical fifth power-law scaling, suggesting the good performance of the entire measurement system.

Pressure drop of a Boger fluid in a converging channel

• Pressure drop measurements of a viscoelastic fluid in a converging channel match those of Newtonian fluids. • No apparent effect of elasticity in converging flows up to a Deborah number of 4. • Predictions of pressure drop for Newtonian fluids in a slowly converging channel, by analysis and Fluent, agree with experiments. • Die swell at the exit of the converging channel was measured and analyzed. • The Oldroyd model is shown not to be appropriate for N1 data of Boger fluids. The pressure to drive a Boger fluid through a long converging channel was measured as a function of flow rate. The channel geometry and flow rates were designed to avoid an upstream vortex and to maximize extensional effects, specifically, to achieve Deborah numbers up to 3 along with a minimum strain of 3. The first measurements of pressure drop were made with two Newtonian fluids, and predictions were made by analysis and Fluent. The latter two agreed to within 1%, and the measurements deviated from these by 5% at most. Four Boger fluids were prepared – three PIB/PB solutions and a solution of PAA in corn syrup. Only one produced reliable pressure-drop measurements, and the data exactly matched that for an equivalent Newtonian fluid, in contrast to the literature where non-Newtonian pressure drops were up to several times higher. Although elasticity apparently had no effect, significant die swell was observed at the channel exit. The explanation for the matching data appears to lie with the radial presssure distribution, which caused migration of the fluid inward, thus lowering shear rates and shear stresses at the wall. Measurements of die swell were made and used to calculate N 1 , from which the axial stress due to shearing on the exit plane was estimated. The extensional stress at the centreline was also estimated, and found to be only a small fraction of the shearing axial stress.

Effect of tabs on transverse jet instabilities, structure, vorticity dynamics and mixing

Abstract

Self-electromagnetic emission from a thruster with anode layer operating with krypton and xenon

This paper considers the study of self electromagnetic emission from the experimental model of low-power thruster with anode layer (TAL) developed by the Bauman Moscow State Technical University. The measurements were carried out during the thruster operation on krypton and xenon for three operating modes: 600 W, 800 W and 1000 W at a discharge voltage of 300 V. Parameters of the generated electromagnetic fields were measured in frequency range from 1 GHz to 12 GHz in the filter bandwidth of 1 MHz. The receiving antenna was installed at a distance of 1 m from the thruster exit plane, perpendicularly to its axis. The results of analysis and comparison of TAL emission spectral characteristics during its operation on krypton and xenon are presented.

Alteration in Structure of Underexpanded Freejet Through Gas-Dynamic Perspective

Studying the underexpanded jet has been essential for many fields of research because it might occur intentionally or spontaneously wherever a high-speed gas flow is possible. Therefore, an in-house compressible flow solver is employed to simulate an underexpanded sonic freejet. The behavior of the jet is examined in the near-field area, immediately downstream of the nozzle exit plane, to determine the parameters influencing the characteristics of the jet. The Mach reflection phenomenon is thoroughly studied, and a criterion is defined for the transformation from regular reflection to Mach reflection. Furthermore, the location and height of the Mach disk are found to be governed by the exit pressure ratio, the stagnation temperature, and the type of gas used. Although the temperature does not have a significant impact, the pressure ratio and specific heat ratio are found to affect the jet structure. The results obtained from the solver are compared with existing experimental results, highlighting the utility of the solver in probing the compressible flowfields.

Numerical study of R134a liquid-vapor flow in a vertical header for phase separation with low inlet quality

• CFD simulation is conducted in an intermediate header for flow separation in a microchannel condenser. • The 3-D CFD model is validated by experiments and compared to a 1-D model. • The liquid separation efficiency decreases with higher vapor separation efficiency. • The inlet tube intrusions make the other half of the header the “free-flow” region. • Void fraction and pressure profiles are also presented in the header. The separation circuitry has been proven in the past to improve the performance of microchannel condensers. In the vertical second header of the condenser, liquid separates from vapor mainly due to gravity. However, separation is usually not perfect, expressed through the separation efficiency. This study presents the phase separation result in the second header calculated by the Euler-Euler method of Computational Fluid Dynamics (CFD). Simulations are conducted for two-phase refrigerant R134a flow in the second header with 21 microchannel tubes in the 1st pass. The inlet mass flux to the second header (through the microchannels of the 1st pass) in the simulation is 166 kg m − 2 s − 1 , 207 kg m − 2 s − 1 , and 311 kg m − 2 s − 1 . The inlet quality is 0.13 to 0.21. The results agree well with the experimental results with flow visualization and the results of a simpler 1-D numerical model. Results show that the liquid separation efficiency decreases as the vapor separation efficiency increases, following a linear trend in the experimental range. The void fraction result shows liquid mainly flows in the half of the header without microchannel tube intrusions. The velocity profile in the header is presented and reverse flow is identified on the exit planes of the inlet section connecting to the 2nd-upper pass and the 2nd-lower pass. The pressure profile in the header is also revealed and it indicates that the 1-D pressure assumption may still apply to two-phase flow in a header.

Tunneling-induced Talbot effect

We investigate the reforming of a plane wave into a periodic waveform in its propagation through a structural asymmetry four-level quantum dot molecule (QDM) system that is induced by an inter-dot tunneling process and present the resulting tunneling-induced Talbot effect. The tunneling process between two neighborhood dots is provided with the aid of a gate voltage. Using a periodic coupling field the response of the medium to the propagating plane probe beam becomes periodic. The needed periodic coupling field is generated with the interference of two coherent plane waves having a small angle and propagating almost parallel to the probe beam direction. In the presence of the tunneling effect of an electron between two adjacent QDs, for the probe beam propagating through the QDM system, the medium becomes transparent where the coupling fields interfere constructively. As a result, the spatial periodicity of the coupling field modulates the passing plane probe beam. We determine the minimum length of the QDM system to generate a periodic intensity profile with a visibility value equal to 1 for the probe field at the exit plane of the medium. It is also shown that by increasing the propagation length of the probe beam through the QDM medium, the profile of the maximum intensity areas becomes sharper. This feature is quantified by considering a sharpness factor for the intensity profile of the probe beam at the transverse plane. Finally, we investigate free space propagation of the induced periodic field and present the Talbot images of the tunneling-induced periodic patterns at different propagation distances for different values of the QDM medium lengths. The presented dynamically designing method of the periodic coherent intensity patterns might find applications in science and technology. For instance, in optical lithography, the need to use micro/nanofabricated physical transmission diffraction gratings, in which preparation of them is expensive and time-consuming, can be eliminated.

Simulation of Pole Erosion in Magnetically Shielded and Unshielded Hall Thrusters

Prior experiments have demonstrated that the pole in a magnetically shielded Hall thruster sputters and erodes visibly, in contrast with that in an unshielded thruster. To determine the reason for this, a particle-in-cell simulation was conducted, and the difference between shielded and unshielded thrusters in terms of plasma behavior near the exit plane was studied. The results show that in the case of the magnetically shielded thruster, the ion flux peak is on the magnetic pole face with higher ion energy, indicating severer erosion at the pole. By contrast, the flux peak of the unshielded thruster is on the head face of the channel wall, indicating severer bombardment at the wall and milder erosion at the pole. Therefore, pole erosion occurs in the magnetically shielded thruster, and channel head face erosion can be observed in the unshielded thruster. In addition, the simulation results indicate that the greater the outward shift of the magnetic field, the severer the pole erosion.