Increasing Momentum Flux Research Articles

Experimental and Numerical Investigations on the Mixing Process of Supercritical Jet Injected into a Supersonic Crossflow

The mixing process and distribution characteristics of a supercritical endothermic hydrocarbon fuel (EHF) jet injected into a supersonic crossflow were investigated by experimental and numerical methods, respectively. The schlieren system and acetone planar laser-induced fluorescence (PLIF) optical system were used to capture the flow-field structural characteristics and instantaneous plume. The mixture and real gas models were employed to calculate the interaction of a transverse jet and supersonic crossflow and reveal a good accuracy with the experimental results. The mixing efficiency and total pressure loss were analyzed based on the numerical results. The results indicate that the supercritical-state EHF directly changes to a gaseous state as it enters the supersonic crossflow from the injector. The EHF jet plume boundary increases with the increasing momentum flux ratio (q). As the streamwise and spanwise distance increases, the traverse heights and expand width increase, and the EHF jet plume presents a semicircle shape in the cross-sectional plane. With the increase in the traverse direction, the concentration distribution shows a fast and then slow power exponential decreasing law; the highest concentration point starts from the near-wall region and rises in the transverse direction with the flow distance increasing. For the same injection condition, the higher the inflow Mach number, the higher the mixing efficiency. For the same Ma, the mixing efficiency is better for the case with low injection pressure and high injection temperature. The total pressure loss is greater in the higher Ma, and high injection pressure conditions cause greater total pressure loss.

Modal Analysis of Breakup Mechanisms for a Liquid Jet in Crossflow

Abstract Liquid fuel jet in crossflow (LJIC) is a vital atomization technique significant to the aviation industry. The hydrodynamic instability mechanisms that drive a primary breakup of a transverse jet are investigated using modal and traveling wavelength analysis. This study highlights the primary breakup mechanisms for aviation fuel Jet-A, utilizing a method that could be applied to any liquid fuel. Mathematical decomposition techniques known as POD (proper orthogonal decomposition) and Robust MrDMD (multiresolution dynamic mode decomposition) are used together to identify dominant instability flow dynamics associated with the primary breakup mechanism. Implementation of the Robust MrDMD method deconstructs the nonlinear dynamical systems into multiresolution time-scaled components to capture the intermittent coherent structures. The Robust MrDMD, in conjunction with the POD method, is applied to data points taken across the entire spray breakup regimes: enhanced capillary breakup, bag breakup, multimode breakup, and shear breakup. The dominant frequencies of breakup mechanisms are extracted and identified. These coherent structures are classified with an associated time scale and Strouhal number. Three primary breakup mechanisms, namely, ligament shedding, bag breakup, and shear breakup, were identified and associated with the four breakup regimes outlined above. Further investigation portrays these breakup mechanisms to occur in conjunction with each other in each breakup regime, excluding the low Weber number enhanced capillary breakup regime. Spectral analysis of the Robust MrDMD modes’ entire temporal window reveals that while multiple breakup mechanisms are convolved, there is a dominant breakup route for each breakup regime. An associated particular traveling wavelength analysis further investigates each breakup mechanism. Lastly, this study explores the effects of an increased momentum flux ratio on each breakup mechanism associated with a breakup regime.

Time-Accurate Evaluation of Film Cooling Jet Characteristics for Plenum and Crossflow Coolant Supplies

Abstract Gas turbine film cooling creates complicated and highly unsteady flow structures. This study seeks to examine the unsteady characteristics created by different cooling hole inlet geometries using a fast-response pressure-sensitive paint (PSP) technique able to capture time-accurate measurements at 2000 frames per second, resolving frequencies up to 1000 Hz. Time-accurate and time-averaged measurements are used to evaluate the performance of a plenum-style inlet and a crossflow-style inlet in varying turbulence environments over a flat plate. The results of this study are intended to begin the process of breaking down widely accepted time-averaged film effectiveness trends into the cumulative effects of freestream turbulence and momentum flux ratio, with emphasis on the cooling jet motions they cause. Jet behaviors observed in this study include a sweeping oscillation, unsteady attachment and separation from the plate, and time-accurate and time-averaged flow bias. Cooling hole inlet geometry and momentum flux ratio affect the core of the jet, and freestream turbulence affects the periphery of the jet. Crossflow-fed cooling holes show bias to the upstream side of the cooling hole with respect to the internal crossflow direction. Plenum-fed cooling holes outperform crossflow-fed cooling holes, and the difference grows with increasing momentum flux ratio. The frequency of both plenum- and crossflow-fed cooling jet motions is influenced by the freestream turbulent velocity fluctuations. The resulting mode shapes show dominant side-to-side sweeping for higher turbulence environments and a separation and reattachment motion for lower turbulence environments. At higher momentum flux ratio, the jets were seen to increasingly favor separation and reattachment motions. The results of this study are intended to better inform existing predictive tools. With a better understanding of the time-accurate behaviors responsible for creating the commonly accepted time-averaged coolant distributions, simple predictive tools may be better equipped to accurately model film cooling flows.

Experiments on atomization and spray characteristics of an effervescent strut injector

The atomization and the spray characteristics from an effervescent/aerated strut injector were studied experimentally using water as the liquid and nitrogen gas as the secondary fluid. The spray flow features were obtained through various experimental tools such as instantaneous Mie scattering technique, phase Doppler particle analyzer, and time-resolved shadowgraph images for different Weber numbers, momentum flux ratios, and gas to liquid ratios. A proper orthogonal decomposition method was used for an in-depth analysis of the spray structures. The instantaneous Mie scattering images showed the formation of the liquid clusters and the dominant two-phase flow interaction with increasing momentum flux and gas to liquid ratios in addition to the increased spray spread area. The statistical analysis of the Mie images revealed high fluctuating locations in the spray. It was observed that by increasing the momentum flux or gas to liquid ratios, the droplet diameter rapidly decreases due to the flow transition to the annular flow regime. Beyond a certain limit of momentum flux or gas to liquid ratios, no significant change in the diameter was observed. Furthermore, the spatio-temporal fluid dynamic features and the two-phase flow transition inside the spray region were brought out through the proper orthogonal decomposition. The power spectral density revealed the ligament breakup and the droplet formation below ≤500 Hz. Regardless of the momentum flux and gas to liquid ratios, a peak was observed between 2500 Hz and 3000 Hz due to the flow transition where the liquid fragment cluster formation and the flapping were found to be almost constant.

Wake characterization of coupled configurations of vertical axis wind turbines using Large Eddy Simulation

Large Eddy Simulations (LES) of three different coupled configurations of Vertical Axis Wind Turbines (VAWT) are reported for two values of Tip Speed Ratio (TSR). A few studies are currently available on the wake features of isolated VAWTs, while detailed understanding of wake interaction between VAWTs is still limited, both from experiments and computations. A better insight would be very useful, since recent field tests suggested that interaction between VAWTs in array configurations is potentially beneficial, improving performance, compared to isolated turbines. It is shown here that mutual vortex interaction between wakes of coupled VAWTs is weak, while blockage effects play an important role in making available an increased momentum flux downstream. Such effects are more obvious at higher TSRs, corresponding to a larger dynamic solidity of the overall system. This result suggests that arranging turbines in staggered wind farms configurations – relative to the typical wind direction at a particular site – is beneficial in increasing the momentum flux available to rows of turbines located downstream.

Volumetric Velocimetry Measurements of Film Cooling Jets

This paper presents volumetric velocimetry (VV) measurements for a jet in crossflow that is representative of film cooling. VV employs particle tracking to nonintrusively extract all three components of velocity in a three-dimensional volume. This is its first use in a film-cooling context. The primary research objective was to develop this novel measurement technique for turbomachinery applications, while collecting a high-quality data set that can improve the understanding of the flow structure of the cooling jet. A new facility was designed and manufactured for this study with emphasis on optical access and controlled boundary conditions. For a range of momentum flux ratios from 0.65 to 6.5, the measurements clearly show the penetration of the cooling jet into the freestream, the formation of kidney-shaped vortices, and entrainment of main flow into the jet. The results are compared to published studies using different experimental techniques, with good agreement. Further quantitative analysis of the location of the kidney vortices demonstrates their lift off from the wall and increasing lateral separation with increasing momentum flux ratio. The lateral divergence correlates very well with the self-induced velocity created by the wall–vortex interaction. Circulation measurements quantify the initial roll up and decay of the kidney vortices and show that the point of maximum circulation moves downstream with increasing momentum flux ratio. The potential for nonintrusive VV measurements in turbomachinery flow has been clearly demonstrated.

Time-Resolved Flowfield Measurements of Momentum-Driven Pulsed Transient Jets

The unsteady flows produced by consecutively pulsed, transient jets were studied using time-resolved particle image velocimetry, where two successive volumes of fluid were ejected from a nozzle into a quiescent fluid. Previous work has shown that the flows from double-pulsed jets persist to much longer times than the equivalent single-pulsed jets, suggesting powerful fluid dynamic interactions between the two ejection events. The transient jets had an exit Reynolds number of 1500 based on the nozzle diameter. The interactions between the jets was found to change significantly with the time separation between the two ejections. The instantaneous velocity distributions showed that, for shorter time intervals between the ejections, the second starting vortex was not completely formed, which was the primary difference from an individually pulsed jet. The increased momentum flux and turbulent wake of the first ejection was found to stabilize the near the flowfield of the double-pulsed jet at shorter pulse intervals by eliminating the starting vortex structure.

Non-oscillatory response to wind loading dominates movement of Scots pine trees

The response of four trees in a Scots pine (Pinus sylvestris L.) plantation to wind loading was studied using time series decomposition methods. For this purpose, wind speed and stem displacement time series recorded on a windy day were divided into intervals adjusted according to stem displacement. The real wind load acting on the sample trees during the intervals was estimated by the momentum flux at canopy top. To identify components in wind-induced stem displacement that are correlated with the wind, wavelet coherence was calculated. Results from these calculations indicate that the trees mainly responded to wind components with periods longer than their damped fundamental sway periods. Therefore, stem displacement data were decomposed into non-oscillatory and oscillatory components as well as noise using singular spectrum analysis. Results from singular spectrum analysis demonstrate that with increasing momentum flux, the importance of oscillatory components in the stem displacement time series decreases whereas the importance of non-oscillatory displacement components increases. The decreasing importance of the oscillatory components suggests that wind loading in the range of the damped fundamental sway period of the trees is inefficient and insignificant for total tree movement under non-destructive wind conditions. Consequently, there was no evidence of the occurrence of resonance effects between wind and tree response.

Numerical and Experimental Studies on a Syngas-Fired Ultra Low NOx Combustor

Simulations and exhaust measurements of temperature and pollutants in a syngas-fired model trapped vortex combustor for stationary power generation applications are reported. Numerical simulations employing Reynolds-averaged Navier–Stokes (RANS) and large eddy simulations (LES) with presumed probability distribution function (PPDF) model were also carried out. Mixture fraction profiles in the trapped vortex combustor (TVC) cavity for nonreacting conditions show that LES simulations are able to capture the mean mixing field better than the RANS-based approach. This is attributed to the prediction of the jet decay rate and is reflected on the mean velocity magnitude fields, which reinforce this observation at different sections in the cavity. Both RANS and LES simulations show close agreement with the experimentally measured OH concentration; however, the RANS approach does not perform satisfactorily in capturing the trend of velocity magnitude. LES simulations satisfactorily capture the trend observed in exhaust measurements which is primarily attributed to the flame stabilization mechanism. In the exhaust measurements, mixing enhancement struts were employed, and their effect was evaluated. The exhaust temperature pattern factor was found to be poor for baseline cases, but improved with the introduction of struts. NO emissions were steadily below 3 ppm across various flow conditions, whereas CO emissions tended to increase with increasing momentum flux ratios (MFRs) and mainstream fuel addition. Combustion efficiencies ∼96% were observed for all conditions. The performance characteristics were found to be favorable at higher MFRs with low pattern factors and high combustion efficiencies.

Adiabatic and Overall Effectiveness for the Showerhead Film Cooling of a Turbine Vane

There have been a number of previous studies of the adiabatic film effectiveness for the showerhead region of a turbine vane, but no previous studies of the overall cooling effectiveness. The overall cooling effectiveness is a measure of the external surface temperature relative to the mainstream temperature and the inlet coolant temperature, and consequently is a direct measure of how effectively the surface is cooled. This can be determined experimentally when the model is constructed so that the Biot number is similar to that of engine components, and the internal cooling is designed so that the ratio of the external to internal heat transfer coefficient is matched to that of the engine. In this study, the overall effectiveness was experimentally measured on a model turbine vane constructed of a material to match Bi for engine conditions. The model incorporated an internal impingement cooling configuration. The cooling design consisted of a showerhead composed of five rows of holes with one additional row on both pressure and suction sides of the vane. An identical model was also constructed out of low conductivity foam to measure adiabatic film effectiveness. Of particular interest in this study was to use the overall cooling effectiveness measurements to identify local hot spots which might lead to failure of the vane. Furthermore, the experimental measurements provided an important database for evaluation of computational fluid dynamics simulations of the conjugate heat transfer effects that occur in the showerhead region. Continuous improvement in both measures of performance was demonstrated with increasing momentum flux ratio.

Separation of Different Sized Nanoparticles with Time Using a Rotational Flow

In this paper, we describe the development of a microfluidic centrifuge with two inlets and two outlets potentially capable of rapidly separating nanoparticles and nanovesicles. Compared with the microfluidic centrifuge with a single inlet and outlet, the 2 ×2 microfluidic centrifuge gives improved centrifugation performance by increasing momentum flux transfer, angular velocity, and centrifugal acceleration. The center of flow rotation and the symmetry of the horizontal velocity in the microchamber were examined numerically. On the basis of the determined maximum velocity, the angular velocity and centrifugal acceleration were also evaluated. The centrifugation time of three different nanoparticles was examined by calculating the time when the nanoparticles left the microchamber for the first time. For visual observation and quantitative measurement of nanoparticle centrifugation, a 2 ×2 microfluidic centrifuge was fabricated and the experimental results demonstrate similar physical behavior to those of a mechanical centrifuge. On the basis of a comparison of the centrifugation time of two different nanoparticle populations of 300 and 700 nm in diameter, we propose that nanoparticles of different sizes can be physically separated by time under a range of inlet volume flow rates.

Effect of orifice inner lip radius on synthetic jet efficiency

A synthetic jet is formed by periodic oscillation of a fluid through an orifice. The oscillatory motion is driven by a diaphragm or other driver. Previous studies have demonstrated that synthetic jet formation and time-averaged cavity pressure are a function of the orifice shape. Traditionally, the performance of the jet is evaluated with varying configurations of fixed driver input voltage or fixed driver displacement. Neither of these measures accurately reflect the efficiency of the actuator. Defining efficiency as “desired output divided by required input,” these traditional measures may not account for increase in required driving current or force. A sharp inside edge of a thin synthetic jet orifice can result in separated flow and increased momentum flux (due to the decreased flow area) for a fixed driver displacement. This can lead one to believe that efficiency has been improved, when, in reality, much more power was required for the driver. Acoustic power, which is the time-average of volume flow rate through the orifice multiplied by the driving pressure, accurately accounts for the power required to drive the actuator. For any synthetic jet actuator, the power to the driver is the power to the fluid (acoustic power) divided by the driver efficiency. If we assume that the driver efficiency is not a strong function of the load, any change to the acoustic power will result in the same proportional change in the driver input power. This study investigates the efficiency of a round (axisymmetric) synthetic jet actuator as a function of the radius of curvature of the interior edge of the orifice. Simultaneous particle image velocimetry measurements at the jet exit and cavity pressure measurements are used to measure the acoustic power required to generate the jet. The resultant momentum flux of the jet is used as a measure of output of the jet. Results are obtained for a range of displacement amplitudes (or stroke lengths) and radii of curvature, while Reynolds number is held fixed.

A Mechanism for the Effect of Tropospheric Jet Structure on the Annular Mode–Like Response to Stratospheric Forcing

Abstract For many climate forcings the dominant response of the extratropical circulation is a latitudinal shift of the tropospheric midlatitude jets. The magnitude of this response appears to depend on climatological jet latitude in general circulation models (GCMs): lower-latitude jets exhibit a larger shift. The reason for this latitude dependence is investigated for a particular forcing, heating of the equatorial stratosphere, which shifts the jet poleward. Spinup ensembles with a simplified GCM are used to examine the evolution of the response for five different jet structures. These differ in the latitude of the eddy-driven jet but have similar subtropical zonal winds. It is found that lower-latitude jets exhibit a larger response due to stronger tropospheric eddy–mean flow feedbacks. A dominant feedback responsible for enhancing the poleward shift is an enhanced equatorward refraction of the eddies, resulting in an increased momentum flux, poleward of the low-latitude critical line. The sensitivity of feedback strength to jet structure is associated with differences in the coherence of this behavior across the spectrum of eddy phase speeds. In the configurations used, the higher-latitude jets have a wider range of critical latitude locations. This reduces the coherence of the momentum flux anomalies associated with different phase speeds, with low phase speeds opposing the effect of high phase speeds. This suggests that, for a given subtropical zonal wind strength, the latitude of the eddy-driven jet affects the feedback through its influence on the width of the region of westerly winds and the range of critical latitudes on the equatorward flank of the jet.

Comparative Study of Spray Characteristics of Gas-Centered and Liquid-Centered Swirl Coaxial Injectors

This study investigated the spray and Sauter mean diameter characteristics of gas-liquid swirl coaxial injectors by measuring and analyzing spray angles and mean drop sizes. Two different types of gas-liquid swirl coaxial injectors were designed and tested for this experiment: 1) a gas-centered swirl coaxial injector and 2) a liquid-centered swirl coaxial injector. The spray patterns were obtained for both types of injectors. The spray angles of these liquidcentered swirl coaxial injectors showed a decreasing trend as a function of the momentum flux ratio over the conditions studied. Good agreement was generally achieved between the predicted and measured spray angles. On the other hand, with increasing momentum flux ratio, the spray angle of the gas-centered swirl coaxial injector initially decreased when the momentum flux ratios were relatively low, but the trend reversed over the high momentum flux ratio range studied. The Sauter mean diameter data of these two types of injectors were also obtained using an image-processing method. Results showed that the Sauter mean diameter distribution of the liquid-centered swirl coaxial injector had a solid cone shape, whereas the gas-centered swirl coaxial injector exhibited two distinct Sauter mean diameter distribution regimes. The gas-centered swirl coaxial injector had a solid cone shape along the centerline, and it had a hollow cone shape at the periphery.

Effects of free-stream turbulence on rough surface turbulent boundary layers

Several effects of nearly isotropic free-stream turbulence in transitionally rough turbulent boundary layers are studied using data obtained from laser Doppler anemometry measurements. The free-stream turbulence is generated with the use of an active grid, resulting in free-stream turbulence levels of up to 6.2%. The rough surface is characterized by a roughness parameterk+≈ 53, and measurements are performed at Reynolds numbers of up toReθ= 11300. It is confirmed that the free-stream turbulence significantly alters the mean velocity deficit profiles in the outer region of the boundary layer. Consequently, the previously observed ability of the Zagarola & Smits (J. Fluid Mech., vol. 373, 1998, p. 33) velocity scaleU∞δ*/δ to collapse results from both smooth and rough surface boundary layers, no longer applies in this boundary layer subjected to high free-stream turbulence. In inner variables, the wake region is significantly reduced with increasing free-stream turbulence, leading to decreased mean velocity gradient and production of Reynolds stress components. The effects of free-stream turbulence are clearly identifiable and significant augmentation of the streamwise Reynolds stress profiles throughout the entire boundary layer are observed, all the way down to the inner region. In contrast, the Reynolds wall-normal and shear stress profiles increase due to free-stream turbulence only in the outer part of the boundary layer due to the blocking effect of the wall. As a consequence, there is a significant portion of the boundary layer in which the addition of nearly isotropic turbulence in the free-stream, results in significant increases in anisotropy of the turbulence. To quantify which turbulence length scales contribute to this trend, second-order structure functions are examined at various distances from the wall. Results show that the anisotropy created by adding nearly isotropic turbulence in the free-stream resides mostly in the larger scales of the flow. Furthermore, by analysing the streamwise Reynolds stress equation, it can be predicted that it is the wall-normal gradient of 〈u2v〉 term that is responsible for the increase in 〈u2〉 profiles throughout the boundary layer (i.e. an efficient turbulent transport of turbulence away from the wall). Furthermore, a noticeable difference between the triple correlations for smooth and rough surfaces exists in the inner region, but no significant differences are seen due to free-stream turbulence. In addition, the boundary layer parameters δ*/δ95,Handcfare also evaluated from the experimental data. The flow parameters δ*/δ95andHare found to increase due to roughness, but decrease due to free-stream turbulence, which has significance for flow control, particularly in delaying separation. Increases incfdue to high free-stream turbulence are also observed, associated with increased momentum flux towards the wall.

An experimental study of an axisymmetric turbulent pulsed air jet

An experimental study is carried out to elucidate the structure of a high Reynolds number (~105) turbulent pulsed jet. Particle image velocimetry measurements showed that the near flow field is dominated by a series of vortex rings with jet-like flows in between. The data show that the vortex rings convect at nearly constant speed of 0.6Uj(Uj: mean jet exit velocity) and the spacing between the rings assumes a value of about 0.6/St(St: Strouhal number=fd/Uj, wherefis the pulsing frequency anddis the nozzle exit diameter). With increasing Strouhal number, the rings are closely spaced and the flow tends to assume a steady jet character at five diameters downstream of the nozzle exit. At lower Strouhal numbers there is a distinct region of jet flow in between the rings. Many of the global characteristics, entrainment, mass and momentum flux are essentially determined by the strength and spacing of the rings which, in turn, depend onSt. We show that the increase in momentum is due to both increased momentum flux and overpressure at the exit in accordance with Krueger & Gharib (AIAA J., vol. 43 (4), 2005, p. 792). This increase in momentum comes at the expense of higher energy required to produce the jet. We also present results of organized and random components of the fluctuations and production of the random turbulence in a pulsed jet. The two regions of dominant turbulence production are identified with the ring and the trailing jet shear layers.

Influences of Hole Shape on Film Cooling Characteristics with CO2 Injection

This article presents the data about heat transfer coefficient ratios, film cooling effectiveness and heat loads for the injection through cylindrical holes, 3-in-1 holes and fanned holes in order to characterize the film cooling performance downstream of a row of holes with 45° inclination and 3 hole spacing apart. The trip wire is placed upstream at a distance of 10 times diameter of the cooling hole from the hole center to keep mainstream fully turbulent. Both inlet and outlet of 3-in-1 holes have a 15° lateral expansion. The outlet of fanned holes has a lateral expansion. CO2 is applied for secondary injection to obtain a density ratio of 1.5. Momentum flux ratio varies from 1 to 4. The results indicate that the increased momentum flux ratio significantly increases heat transfer coefficient and slightly improve film cooling effectiveness for the injection through cylindrical holes. A weak dependence of heat transfer coefficient and film cooling effectiveness, respectively, on momentum flux ratio has been identified for the injection through 3-in-1 holes. The increase of the momentum flux ratio decreases heat transfer coefficient and significantly increases film cooling effectiveness for the injection through fanned holes. In terms of the film cooling performance, the fanned holes are the best while the cylindrical holes are the worst among the three hole shapes under study.

Interactive momentum flux forcing over sea ice in a global ocean GCM

The sensitivity of Southern Ocean sea ice to the strength of the atmospheric momentum forcing is investigated in the framework of a global ocean general circulation model. In contrast to the usual approach of having the momentum flux just depend on the wind speed and a constant drag coefficient, the newly introduced momentum flux driving sea ice considers the local stratification and roughness over ice in one case, and the flux‐aggregated stratification and roughness using the blending‐height concept in the other case. While both cases thus allow for an interactive feedback, only the latter case accounts for the subgrid‐scale heterogeneity of the sea‐ice pack. In particular, the sea‐ice feedback is in the former case only provided by the simulated ice thickness, affecting the surface temperature and local stratification, while in the latter case it is also determined by the ice concentration. Both parameterizations yield predominantly statically stable, but dynamically unstable conditions at any instant over the wintertime sea‐ice pack. In the winter mean, statically and dynamically unstable conditions prevail over coastal polynyas, and lead to a positive feedback with increased momentum flux. The larger momentum flux enhances the along and offshore ice drift, leading to corresponding changes in the winter‐mean ice‐thickness distribution, a reduction in coastal ice concentration, and an increase of heat loss due to sensible heat flux. In the case where surface heterogeneity is accounted for, the impact of the lower coastal ice concentration leads to a larger momentum flux than in the homogeneous case. The long‐term deep‐ocean properties are only affected when in the heterogeneous case the form drag is raised by increasing the ice freeboard and decreasing the maximum ice concentration. Only the combination of both yields a significant increase of Antarctic Bottom Water formation, as reflected by a long‐term cooling and freshening of the global deep‐ocean properties.

Tangentially directed acoustic jet controlling boundary layer

The nozzle of an acoustic jet directs high momentum flux gas particles essentially tangentially into the boundary layer of the flow in a diffuser, an engine air inlet, a jet engine gas flow path or on the suction surface of an airfoil, the gas particles in the chamber of the tangential acoustic jet being replenished with approaching low momentum flux particles drawn from the gas flow in a direction normal to the surface, thereby to provide a net time-averaged flow of increased momentum flux particles to defer the onset of boundary layer separation and/or reduce the thickness of the boundary layer. The acoustic jet is driven by a gas pressure oscillation generator which may be a loudspeaker, a resonant solenoid piston, a cranked piston, or the like.

The effect of flame radiation on the scaling of nitrogen oxide emissions in turbulent hydrogen non-premixed flames

The relationship among the flame radiation, buoyancy, and NOx emissions is examined for turbulentnon-premixed jet flames with wide variations in coaxial air conditions. The major parameters used to modify the fuel/air mixing are fuel jet velocity, coaxial air velocity, and helium dilution. Measurements of NOx emission, flame geometry, and flame radiation were made to explain the observed NOx emission scaling. The overall 1/2-power scaling is observed in coaxial air flames, irrespective of coaxial air conditions, but for pure hydrogen flames, the degree of deviation from the 1/2-slope curve in each case is different. From the comparison between the results of pure hydrogen flames and those of helium-diluted hydrogen flames, it is observed that the deviation from 1/2-power scaling may be explained in two ways: the differences in the flame radiation and flame buoyancy in coaxial air flames. Helium dilution reduces deviation from the 1/2-power scaling in two ways: it reduces the effect of flame radiation and it reduces the effect of flame buoyancy through reduced flame length and increased momentum flux of fuel jet. From the radiation measurements, EINOx is observed to be proportional to the radiant fraction to the two-thirds power for the momentum-dominated turbulent hydrogen jet flames. This observed tendency seems to be related to the 1/2-power NOx emission scaling.