Jet Exit Plane Research Articles

On the structure of buoyant fires with varying levels of fuel-turbulence

This paper employs a novel burner to study the effects of fuel-generated turbulence on the spatial and temporal structure of buoyant turbulent diffusion flames which are representative of large fires. Fuel-turbulence levels are increased using a perforated plate that issues high-velocity jets, enabling shearing of the fuel stream. The perforated plate may be recessed to control the turbulence level at the jet exit plane. It is shown that the exit plane axial velocity fluctuations can be increased from 0.135 m/s to 1.813 m/s. Varying the levels of fuel-turbulence in the burner allows for the control of key processes defining buoyant fires such as the spatial and temporal flame structure and flame instability modes. These processes are characterised by high-speed simultaneous imaging of planar laser-induced fluorescence of the OH radical (OH-PLIF) and Mie scattering from soot particles. Increasing the fuel-turbulence level deforms the flame, which promotes non-radial lateral entrainment into the flame sheet. This results in a sharp increase in the tilting of the near-field flame sheet along the vertical flame axis. Strong angular entrainment forces are shown to overcome the diffusive and thermal expansive forces at the flame neck, which leads to a strained asymmetric sinuous flame pinch-off instability, followed by separation of the flame base. Sinuous pinch-off instabilities occur at a greater frequency than the symmetric varicose pinch-off instabilities observed for flames with low fuel-turbulence. The asymmetric stretching of the flame neck inhibits the formation of the classical puffing instability formed with an axisymmetric plume that defines classically buoyant flames. Probability density functions calculated for the flame front curvature and flame surface area are shown to monotonically broaden in the near-field region of the flame due to lateral entrainment effects. The transition to buoyancy-driven turbulence also shifts to an increasingly more upstream location. This burner, with its well-defined boundary conditions and novel data, forms a platform for advancing capabilities to model complex fire phenomena including turbulence-buoyancy interactions.

Characteristics of ethylene and methane combustion in a range of high temperature and low oxygen environments

New understanding of turbulent jet flames of natural gas, and blends of natural gas with ethylene (C2H4), issuing into a range of preheated coflowing oxidisers with reduced oxygen (O2) concentrations are reported. Comparisons are made for coflow O2 concentrations of 3%, 4%, 5%, 6%, 9% and 11% and coflow temperatures of 1250 K, 1315 K and 1385 K. Instantaneous and simultaneously planar imaging measurements of temperature, hydroxyl radicals (OH) and formaldehyde (CH2O) were taken at eight locations ranging from 9 mm to 75 mm downstream of the jet exit plane. The experimental data is supplemented with chemical reaction modelling. It is shown that increasing the proportion of C2H4 in the fuel supply resulted in an increase in the flame front temperature. At 3% O2, CH2O signal did not very between the three fuels containing C2H4, whilst at 9%, the signal increased with increasing C2H4. OH number density was highest for a blend of 66% NG and 33% C2H4. The fuel composition made little effect on the jet spread rate. Reaction modelling found evidence of changes in the reaction process, both in the suppression of reactions with different fuels, and in the suppression of reactions, and the locations they occur, with reducing O2 concentrations. Reaction modelling also found a significant decrease in ignition delay when C2H4 was present in the fuel mixture, and an increase in ignition delay as all four fuels transition to MILD combustion.

Characteristics of turbulent flames in a confined and pressurised jet-in-hot-coflow combustor

Combustion in hot and low oxygen environments—such as those encountered in practical devices including inter-turbine burners and sequential gas turbines—is not yet fully understood at a fundamental level, particularly in terms of the effects of pressure. To meet this gap in understanding, a confined-and-pressurised jet-in-hot-coflow (CP-JHC) combustor has been developed to facilitate optical diagnostics of turbulent flames in hot and vitiated coflows for the studies of flame stabilisation, structure and soot formation at elevated pressures. The CP-JHC burner has been designed for steady operation at 10 bar with internal temperatures of up to 1975 K, with a water-cooled central jet issuing into a hot oxidant stream of combustion products from a non-premixed natural gas/H2 burner. This work describes the key features and operational capabilities of the CP-JHC burner and presents a selection of experimental results showing characteristics not previously available. Specifically, temperature measurements of the hot coflow are used to estimate the enthalpy deficit of the stream, revealing an increase in thermal efficiency with increasing heat input, and a decrease with increasing pressure. Chemiluminescence imaging of OH∗ and CH∗ is performed for turbulent jet flames to study the flame structure under various operating conditions, and true-colour imaging results are also included to highlight the change in soot formation under elevated pressures. The mean images indicate a change in stabilisation behaviour with changes in pressure and jet Reynolds number (Rejet), which is further investigated by a statistical analysis of the short-exposure CH∗ images. This analysis reveals that an increase in Rejet from 10,000 to 15,000 leads to an increase in the mean lift-off height (from the jet exit plane) from approximately 1.5 to 6 jet diameters at atmospheric pressure, while the flames at elevated pressures show significantly less variation and tend to stabilise at the jet exit for P > 3.5 bar(a). The experimental findings are complemented by numerical simulations of laminar opposed flow flames, providing additional insights into the fundamental chemical kinetics effects which influence these flames. In particular, a monotonic reduction in both the maximum and integrated OH∗ and CH∗ mass fractions is observed with increasing pressure. This reduction is particularly pronounced at lower pressures, with a reduction to 10% of the atmospheric-pressure value at 3 bar(a) for the integrated OH∗ mass fraction. Additionally, this behaviour is shown to be related to the combined effects of a shift in the formation pathways and the increased impact of collisional quenching.

REDIM-PFDF Sub-grid Scale Combustion Modeling for Turbulent Partially-premixed Flame: Assessment of Combustion Modes

ABSTRACT The focus of the present study is to evaluate the capability of reaction-diffusion manifold (REDIM) technique combined with presumed filtered density function (PFDF) method to calculate the important characteristics of turbulent partially-premixed flame with near-homogeneous inlets and inhomogeneous inlets. The REDIM approach reduces detailed chemical kinetics into two-dimensional chemistry table, where mass fractions of CO2 and N2 are used as reduced coordinates. The fluctuation of the scalars within the LES filter volume is modeled by the PFDF method. Higher shear generated turbulent intensity at the mixing layer of the fuel and air streams provides the near-homogeneous inlet conditions. The radial statistics analysis of the mixture fraction, temperature and species mass faction, and their comparison with the experimental data show the good performance of the REDIM-PFDF model. Scatter plots show the better agreement for the inhomogeneous case. The near-homogeneous case exhibits little discrepancy with the measured distribution because of the limited ability of the present model to capture local extinction effects. Flame index parameter identifies the multiple combustion modes present at different locations. This parameter shows the presence of premixed and diffusion dominated types of combustion near the main jet exit plane in the inhomogeneous and near-homogeneous cases, respectively.

Jet-installation noise and near-field characteristics of jet–surface interaction

The link between jet-installation noise and the near-field flow features of the corresponding isolated jet is studied by means of lattice-Boltzmann numerical simulations. The computational set-up consists of a flat plate placed in proximity to a jet, replicating the interaction benchmark study carried out at NASA Glenn. Installation effects cause low-frequency noise increase with respect to the isolated case, mainly occurring in the direction normal to the plate and upstream of the jet's exit plane. It is shown that the Helmholtz number, based on the wavelength of eddies in the mixing layer and their distance to the plate trailing edge, predicts the frequency range where installation noise occurs. Based on the isolated jet near field, scaling laws are also found for the far-field noise produced by different plate geometries. The linear hydrodynamic field of the isolated jet shows an exponential decay of pressure fluctuations in the radial direction; it is shown that the far-field spectrum follows the same trend when moving the plate in this direction. In the axial direction, spectral proper orthogonal decomposition is applied to filter out jet acoustic waves. The resultant hydrodynamic pressure fluctuations display a wavepacket behaviour, which can be fitted with a Gaussian envelope. It is found that installation noise for different plate lengths is proportional to the amplitude of the Gaussian curve at the position of the plate trailing edge. These analyses show that trends of jet-installation noise can be predicted by analysing the near field of the isolated case, reducing the need for extensive parametric investigations.

Effects of shear inhomogeneities on the structure of turbulent premixed flames

This paper studies the effects of shear inhomogeneities on the structure and stability characteristics of turbulent piloted premixed flames. The shear is introduced by splitting the fuel-air mixture in two stream and feeding each stream at different velocities through two annular concentric tubes residing within the pilot annulus. The shear is varied by varying the proportion of fluid injected through the streams and also by changing the recess distance of the inner tube with respect to the jet exit plane. It is noted that with increasing rates of shear, the Bunsen-like flames become shorter with the flame tip opening downstream of the pilot stream. Measurements of flow fields reveal useful information about the level and location of the shear layers. Joint imaging of Laser-Induced Fluorescence (LIF) of OH and CH show that, with increasing rates of shear, the degree of flame front fragmentation increases implying the occurrence of local extinction even in regions extremely close to the pilot. The flame surface area, marked by LIF-CH is taken as an indicator of consumption rates of reactants. For all flames examined here, the peak flame surface area increases with increasing shear and subsequently decreases as the flame shortens and approaches blow-off. The peak flame surface density decreases with increasing shear. Regime diagrams show that the flames investigated here bridge the regions of broadened preheat thin reactions as well as the distributed reaction zone. While the thickness of the CH layer remain somewhat uniform, the number of flame fragments, as indicated by breaks in the CH profiles, increases with increasing shear before stabilizing at some peak value. The observed mechanism of local extinction in highly sheared Bunsen-like, turbulent piloted premixed flames complements existing results for bluff-body stabilised flames.

Jet Impingement Heat Transfer Enhancement by Packing High-Porosity Thin Metal Foams Between Jet Exit Plane and Target Surface

High-porosity metal foams are known for providing high heat transfer rates, as they provide a significant increase in wetted surface area as well as highly tortuous flow paths resulting in enhanced mixing. Further, jet impingement offers high convective cooling, particularly at the jet footprint areas on the target surface due to flow stagnation. In this study, high-porosity thin metal foams were subjected to array jet impingement, for a special crossflow scheme. High porosity (92.65%), high pore density (40 pores per inch (ppi)), and thin foams (3 mm) have been used. In order to reduce the pumping power requirements imposed by full metal foam design, two striped metal foam configurations were also investigated. For that, the jets were arranged in 3 × 6 array (x/dj = 3.42, y/dj = 2), such that the crossflow is dominantly sideways. Steady-state heat transfer experiments have been conducted for varying jet-to-target plate distance z/dj = 0.75, 2, and 4 for Reynolds numbers ranging from 3000 to 12,000. The baseline case was jet impingement onto a smooth target surface. Enhancement in heat transfer due to impingement onto thin metal foams has been evaluated against the pumping power penalty. For the case of z/dj = 0.75 with the base surface fully covered with metal foam, an average heat transfer enhancement of 2.42 times was observed for a concomitant pressure drop penalty of 1.67 times over the flow range tested.

Experimental validation of the hybrid scattering model of installed jet noise

Jet installation causes jet noise to be amplified significantly at low frequencies, and its physical mechanism must be understood to develop effective aircraft noise reduction strategies. A hybrid semi-empirical prediction model has recently been developed based on the instability-wave-scattering mechanism. However, its validity and accuracy remain to be tested. To do so, in this paper we carry out a systematic installed jet-noise experiment in the laboratory using a flat plate instead of an aircraft wing. We show that reducing H (the separation distance between the flat plate and jet centerline) causes stronger low-frequency noise enhancement while resulting in little change to the noise shielding and enhancement at high frequencies. Decreasing L (the axial distance between the jet exit plane and the trailing edge of the plate) results in reduced noise amplification at low frequencies and also weakens both the shielding and enhancement at high frequencies. Increasing the jet Mach number abates the installation effects. It is shown that the hybrid model developed in the earlier work agrees with experimental measurements and can capture the effects of varying H, L, and jet Mach number extremely well. It is concluded that the model captures the correct physics and can serve as an accurate and robust prediction tool. This new physical understanding provides insights into innovative strategies for suppressing installed jet noise.

Turbulent spray flames of intermediate density: Stability and near-field structure

This paper presents an experimental study of turbulent spray jets and flames where the liquid loading is intermediate between the two extremes of dense and dilute; a region of significant importance in practical applications. A new piloted burner is introduced for this purpose featuring air-blast atomization, with liquid injection from a needle that can be translated within a co-flowing air stream. The stability characteristics are presented for three recess lengths using acetone as fuel. High-speed shadowgraph imaging is performed in the atomization region of jets and flames having different liquid loading and recess distances. Three types of fluid fragments are used to map the evolution of these sprays: droplets, ligaments and ‘irregular’ shapes. Statistics for each class of these fluid shapes are presented to map the boundary conditions at the jet exit plane and to track their evolution with downstream distance along the jet. These show clearly how ligaments and irregular shapes break down to supply more droplets further downstream. Measurements of liquid surface area show a peak in the mean area and its rms of fluctuations around two jet diameters from the point of injection marking the region of maximum atomization. It is also evident that the process of primary and secondary atomization is completed by about seven jet diameters from the liquid injection point. This burner provides a versatile platform for studying these flows and the resulting information is already proving to be extremely useful in the development and validation of related models for turbulent spray jets and flames.

Flow structures and noise from a supersonic impinging jet

Purpose The purpose of this paper is to carry out an extensive numerical study in order to understand the flow structures and the resulting noise generated by a supersonic impinging jet on a flat plate. One of the parameters varied in this study is the distance between the jet exit plane and the flat plate. Design/methodology/approach Because of the unsteady nature of the problem a time-dependent computation is carried out using the detached eddy simulation turbulence model. The OVERFLOW 2 CFD code was used with a highly resolved grid and small time steps. Findings The authors found that as the separation distance increases, the dominant frequencies in the noise spectrum decrease. In addition, the relative strength of the various frequencies to each other changes with changing distance, indicating the changing modes of the jet. The CFD results indicate a strong interaction between the acoustic waves emanating from the impingement plate and the jet plume. This feedback mechanism is responsible for destabilizing the jet shear layer leading to the jet changing modes. The computed near field spectra, convection velocities of the jet vortical structures and mean jet centerline velocity profile are in good agreement with experimental measurements. The results also show very high sound pressure levels all over the impingement plate but especially near the impingement point. These levels, if sustained, are detrimental to both human operators as well as the surrounding structures. Research limitations/implications Given the large-scale nature of the computations carried out, it is very costly to run the computations long enough to collect a good, statistically steady time sample to achieve a low frequency bandwidth resolution. Such a long time sample could actually improve the results in terms of frequency resolution and obtained an even better agreement with experiments. Off course there is always the issue of grid resolution as well, but given the good agreement with experiments that the authors obtained, the authors are confident in their results. Practical implications The practical implications of the results the authors obtained are significant in that, the authors now know that hybrid RANS-large eddy simulation methods can be used for this complex, unsteady engineering problems. In addition, the results also show the high noise level both on the impingement surface and in the surroundings of the jet. This could have a negative impact on the structural integrity of the flat surface. Social implications Noisy environments are never desirable anywhere especially in places where human operations take place. Therefore, given the high noise levels obtained in the simulations and confirmed by experiments, any human presence around the jet will be harmful to hearing and precautions need to be taken. Originality/value This is a physics-based study; i.e. understanding the physical phenomena involved in supersonic jet impingement. Of particular interest is the interaction of the jet shear layer with the acoustic waves emanating from the impingement area. This feedback loop is found to be responsible for intensifying the instability of the jet shear layer.

Effect of the mixing fields on the stability and structure of turbulent partially premixed flames in a concentric flow conical nozzle burner

The mixing field is known to be one of the key parameters that affect the stability and structure of partially premixed flames. Data in these flames are now available covering the effects of turbulence, combustion system geometry, level of partially premixing and fuel type. However, quantitative analyses of the flame structure based on the mixing field are not yet available. The aim of this work is to present a comprehensive study of the effects of the mixing fields on the structure and stability of partially premixed methane flames. The mixing field in a concentric flow conical nozzle (CFCN) burner with well-controlled mechanism of the mixing is investigated using Rayleigh scattering technique. The flame stability, structure and flow field of some selected cases are presented using LIF of OH and PIV. The experimental data of the mixing field cover wide ranges of Reynolds number, equivalence ratio and mixing length.The data show that the mixing field is significantly affected by the mixing length and the ratio of the air-to-fuel velocities. The Reynolds number has a minimum effect on the mixing field in high turbulent flow regime and the stability is significantly affected by the turbulence level. The temporal fluctuations of the range of mixture fraction within the mixing field correlate with the flame stability. The highest point of stability occurs at recess distances where fluid mixtures near the jet exit plane are mostly within the flammability limits. This paper provides some correlations between the stability range in mixture fraction space and the turbulence level for different equivalence ratios.

Partial premixing and stratification in turbulent flames

This paper reviews recent advances in understanding the structure of turbulent partially premixed and stratified flames. The term “partially premixed” refers here to compositionally inhomogeneous mixtures that include flammable and non-flammable fluid while “stratified” combustion refers to a reacting front propagating through a range of compositions within the flammable limits. An overview of relevant laminar flame concepts is first introduced. In laminar partially premixed flames, the interaction between rich and lean mixtures is significant leading to improvement in the flame’s resistance to extinction by straining. In lean back-supported laminar stratified flames, the flux of excess heat and radicals into the lean fluid results in higher flame speeds, broader reaction zones, and extended flammability limits compared to homogeneous counterparts. Rich stratified flames are more complex due to the combined fluxes of heat as well as reactive species such as H2 and CO.Recent research in turbulent partially premixed as well as stratified flames is reviewed. Detailed measurements in burners representative of those found in gas turbine combustors show that partial premixing at the lifted flame base increases with instability. Well-characterised laboratory burners where different fuel concentration gradients may be imposed at the jet exit plane show improved flame stability due to mixed-mode combustion. Maximum stability is reached at some optimum level of compositional inhomogeneity. Highly resolved measurements in turbulent stratified flames show that the mass fractions of CO and H2 increase with stratification; a result that is consistent with laminar flame studies. Such experiments are, however, very difficult and require multi-level conditioning of the data. The paper concludes with a brief review of potential numerical approaches employed in the calculations of turbulent flames with inhomogeneous inlet conditions. A key challenge here is to reproduce the effects of increasing levels of stratification and/or inhomogeneity on the compositional structure of turbulent flames.

Influence of Stokes number on the velocity and concentration distributions in particle-laden jets

AbstractThe first measurement of the influence of the Stokes number on the distributions of particle concentration and velocity at the exit of a long pipe are reported, together with the subsequent influence on the downstream evolution of these distributions through a particle-laden jet in co-flow. The data were obtained by simultaneous particle image velocimetry (PIV) and planar nephelometry (PN), using four cameras to provide high resolution through the first 30 jet diameters and also correction for optical attenuation. These data provide much more detailed information than is available from previous measurements. From them, a new understanding is obtained of how the Stokes number influences the flow at the jet exit plane and how this influence propagates throughout the jet.

Stabilization of piloted turbulent flames with inhomogeneous inlets

This paper investigates the stabilization mechanism of turbulent jet flames with highly inhomogeneous inlet conditions. A modification of the standard piloted burner is employed here with the addition of a central tube (inner) carrying fuel that can slide within the existing (outer) tube carrying air. Both tubes are located within the pilot annulus and inhomogeneity is varied by translating the inner tube upstream of the jet exit plane. Two flames with identical overall air/fuel ratios, bulk jet velocities, and pilot conditions but different levels of homogeneity in the fuel/air mixture are selected for detailed investigations. Measurements are performed using Sandia’s Raman–Rayleigh–LIF line facility, and Large Eddy Simulation (LES) using the stochastic fields approach are also conducted for the same flames. Results reported here focus on the early stabilization region.The flame with inhomogeneous inlet conditions is more stable being at 57% of blow-off compared to the homogeneous counterpart, which is at 78% of blow-off. It is found that, very close to the jet exit plane, premixed combustion dominates the flame with an inhomogeneous profile. This is in contrast to the homogeneous case, which behaves like a diffusion flame. Further downstream, but still within the pilot region, partial mixing starts to occur between richer samples and hot combustion products. A comparison of the relative conditional scalar dissipation rates, χr shows that in the upstream region, and within the reactive limits, the homogeneous case has higher values of χr. Premixed combustion with higher rates of heat release and lower scalar dissipation rates in the near field are therefore key reasons for the improved stability of the flames with inhomogeneous inlets. These findings are corroborated by results from LES.

On cumulative nonlinear acoustic waveform distortions from high-speed jets

AbstractA model is proposed for predicting the presence of cumulative nonlinear distortions in the acoustic waveforms produced by high-speed jet flows. The model relies on the conventional definition of the acoustic shock formation distance and employs an effective Gol’dberg number$\Lambda $for diverging acoustic waves. The latter properly accounts for spherical spreading, whereas the classical Gol’dberg number$\Gamma $is restricted to plane wave applications. Scaling laws are then derived to account for the effects imposed by jet exit conditions of practical interest and includes Mach number, temperature ratio, Strouhal number and an absolute observer distance relative to a broadband Gaussian source. Surveys of the acoustic pressure produced by a laboratory-scale, shock-free and unheated Mach 3 jet are used to support findings of the model. Acoustic waveforms are acquired on a two-dimensional grid extending out to 145 nozzle diameters from the jet exit plane. Various statistical metrics are employed to examine the degree of local and cumulative nonlinearity in the measured waveforms and their temporal derivatives. This includes a wave steepening factor (WSF), skewness, kurtosis and the normalized quadrature spectral density. The analysed data are shown to collapse reasonably well along rays emanating from the post-potential-core region of the jet. An application of the generalized Burgers equation is used to demonstrate the effect of cumulative nonlinear distortion on an arbitrary acoustic waveform produced by a high-convective-Mach-number supersonic jet. It is advocated that cumulative nonlinear distortion effects during far-field sound propagation are too subtle in this range-restricted environment and over the region covered, which may be true for other laboratory-scale jet noise facilities.

Influence of the choice of the inlet turbulence intensity on the performance of numerically simulated moderate Reynolds jet flows – Part 1 – the near exit region of the jet

A real problem when trying to develop a numerical model reproducing the flow through an orifice is the choice of a correct value for the turbulence intensity at the inlet of the numerical domain in order to obtain at the exit plane of the jet the same values of the turbulence intensity as in the experimental evaluation. There are few indications in the literature concerning this issue, and the imposed boundary conditions are usually taken into consideration by usage without any physical fundament. In this article we tried to check the influence of the variation of the inlet turbulence intensity on the jet flow behavior. This article is focusing only on the near exit region of the jet. Five values of the inlet turbulence intensity Tu were imposed at the inlet of the computational domain, from 1.5% to 30%. One of these values, Tu= 2% was the one measured with a hot wire anemometer at the jet exit plane, and another one Tu= 8.8% was issued from the recommendation of Jaramillo (1). The choice of the mesh-grid and of the turbulence model which was the SST k-ω model were previously established (2). We found that in the initial region of the jet flow, the mean streamwise velocity profiles and the volumetric flow rate do not seem to be sensitive at all at the variation of the inlet turbulence intensity. On the opposite, for the vorticity and the turbulent kinetic energy (TKE) distributions we found a difference between the maximum values as high as 30%. The closest values to the experimental case were found for the lowest value of Tu, on the same order of magnitude as the measurement at the exit plane of the jet flow. Mean streamwise velocity is not affected by these differences of the TKE distributions. Contrary, the transverse field is modified as it was displayed by the vorticity distributions. This observation allows us to predict a possible modification of the entire mean flow field in the far region of the jet flow.

A modified piloted burner for stabilizing turbulent flames of inhomogeneous mixtures

A modification of the well-known jet piloted burner is introduced to enable the stabilization of partially premixed flames with varying degrees of inhomogeneity in mixture fraction or equivalence ratio. A second tube is added within the pilot annulus which now surrounds two concentric pipes, one carrying fuel and the other air. The central pipe can also be recessed within the annulus upstream of the burner’s exit plane. Two flow configurations are tested: FJ which refers to fuel issuing from the central pipe while air issues from the annulus, and FA where the reverse is true. A key feature of the FJ configuration is that when the central tube is slightly recessed, the fuel partially premixes with air from the annulus inducing inhomogeneity, the extent of which depends on the recession distance.It is found that flame stability is significantly improved due to this inhomogeneity such that, for intermediate recession distances in the range 50–100mm, and for the same air/fuel ratio, the blow-off limits for the FJ cases are more than 50% higher than those of the FA counterparts where fuel is injected in the annulus. Detailed stability limits for both the FJ and FA configurations are presented here along with measurements of velocity and mixing fields at the jet exit plane. Rayleigh scattering is used to image mixture fraction in non-reacting jets while measurements of velocity and turbulence fields are made using standard Laser Doppler Velocimetry. It is shown that, at intermediate recession distances, significant differences in the mean and rms fluctuations of the velocity and mixture fraction profiles exist between the FA and FJ cases. An indicator of stratification, extracted from the mixture fraction images at the exit plane of non-reacting jets, confirms that a high degree of inhomogeneity correlates well with improved flame stability.

Interactions of an Array of Finite Span Synthetic Jets and a Crossflow

The formation of secondary flow structures due to the interaction of an array of three finite span synthetic jets with a crossflow was investigated experimentally over a finite swept-back wing (NACA 4421, , sweep angle of 30 deg, Reynolds number of ). Stereoscopic particle image velocimetry data were collected in the midspan section of the wing, where the effects of the jets’ combinations and their blowing ratios were analyzed based on the three-dimensional flowfield using time- and phase-averaged statistics. The focus of the present work was to capture the formation and advection of secondary flow structures in the vicinity of the synthetic jets for different combinations of jets, including 1) only the center jet activated, 2) two synthetic jets off the middle jet activated, and 3) all three synthetic jets actuated. An angle of attack of 13.5 deg was selected, in which the boundary layer was either attached (in the vicinity of the middle jet) or partially separated (in the vicinity of the jet closer to the wing tip). The time-averaged total velocity fields exhibited secondary streamwise structures that were formed along the jets orifice span. Furthermore, the effect of the synthetic jet(s) was felt at spanwise locations (toward the wing tip) away from the point of activation. From the phase-averaged flowfield, it was shown that, during the blowing portion of the synthetic jet cycle, quasi-two-dimensional pairs of counter-rotating spanwise rollers were formed at the jet exit plane. Each pair was advected downstream as time progressed, bent and tilted toward the tip, and eventually broke down into smaller structures farther downstream, which resulted in the formation of secondary streamwise structures.

Internal wall-jet film cooling with compound angle cylindrical holes

The flow field associated with cylindrical coolant jets inclined in tangential and azimuthal direction employed inside a circular pipe has been studied. Numerical results are compared with in-house experimental data for a row of circumferential film cooling holes with two distinct geometric configurations. Results provide insight into the film cooling performance and the heat transfer characteristics associated with this type of film-cooling jets. Secondary flow recirculation zones are found near the jet exit in the regimes close to the wall and the center. Its occurrence and length is found to be controlled by the geometric configuration of the coolant hole. This secondary flow structure and the jet impingement on the wall are found responsible for developing a maximum local Nusselt number downstream of coolant injection. Reverse heat transfer regimes are noted for conjugate walls with higher thermal conductivity. The spreading of the coolant around the circumference is mainly due to the asymmetric vorticity levels present at the jet exit plane. Higher tangential orientation of the coolant jet with the mainstream has resulted in lower injectant concentration near the test section wall and lower effectiveness throughout the test section. The results indicate that through the use of optimal coolant injector configurations, reduction in coolant requirement can be achieved.

The Structure of the Auto-Ignition Region of Turbulent Dilute Methanol Sprays Issuing in a Vitiated Co-flow

This paper presents planar imaging of laser induced fluorescence (LIF) from key reactive species in the auto-ignition region of dilute turbulent spray flames of methanol. High-speed (5 kHz) LIF-OH imaging as well as low speed (10 Hz) imaging of joint LIF-OH-CH2O is performed. The product of the OH and CH2O signals is used as a qualitative indicator of local heat release. The burner is kept intentionally simple to facilitate computations and the spray is formed upstream of the jet exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The studied flames are all lifted but differ in the shape of their leading edge and heat release zones. Similarities with auto-ignition of gaseous fuels, as well as differences, are noted here. Formaldehyde is detected earlier than OH implying that the former is a key precursor in the initiation of auto-ignition. Growing kernels of OH that are advected from upstream, close in on the jet centreline and ignite the main flame. The existence of double reaction zones in some flames may be due to ignitable mixtures formed subsequent to local evaporation of droplets and subsequent mixing. When air is used as spray carrier, reaction zones broaden with distance, possibly due to increased partial premixing and regions of intense heat release occur near the flame centreline further downstream. With nitrogen as carrier, the flame maintains a nominal diffusion-like structure with reaction zones of uniform width and substantially less concentration of heat release on the flame centreline.