scholarly journals Heat release effects on shear-layer growth and entrainment

AIAA Journal ◽  
1987 ◽  
Vol 25 (4) ◽  
pp. 578-583 ◽  
Author(s):  
J. C. Hermanson ◽  
M. G. Mungal ◽  
P. E. Dimotakis
Keyword(s):  
Heat Release ◽  
Shear Layer ◽  
Layer Growth

scholarly journals Heat release effects on shear-layer growth and entrainment

1985 ◽  
Author(s):  
J. HERMANSON ◽  
M. MUNGAL ◽  
P. DIMOTAKIS
Keyword(s):  
Heat Release ◽  
Shear Layer ◽  
Layer Growth

scholarly journals Effects of heat release in a turbulent, reacting shear layer

1989 ◽  
Vol 199 ◽  
pp. 333-375 ◽  
Author(s):  
J. C. Hermanson ◽  
P. E. Dimotakis
Keyword(s):  
Growth Rate ◽  
Pressure Gradient ◽  
Heat Release ◽  
Shear Layer ◽  
Temperature Rise ◽  
Flame Temperature ◽  
Mixing Layer ◽  
Layer Growth ◽  
Layer Growth Rate ◽  
The Mean

Experiments were conducted to study the effects of heat release in a planar, gas-phase, reacting mixing layer formed between two free streams, one containing hydrogen in an inert diluent, the other, fluorine in an inert diluent. Sufficiently high concentrations of reactants were utilized to produce adiabatic flame temperature rises of up to 940 K (corresponding to 1240 K absolute). The temperature field was measured at eight fixed points across the layer. Flow visualization was accomplished by schlieren spark and motion picture photography. Mean velocity information was extracted from Pitot-probe dynamic pressure measurements. The results showed that the growth rate of the layer, for conditions of zero streamwise pressure gradient, decreased slightly with increasing heat release. The overall entrainment into the layer was substantially reduced as a consequence of heat release. A posteriori calculations suggest that the decrease in layer growth rate is consistent with a corresponding reduction in turbulent shear stress. Large-scale coherent structures were observed at all levels of heat release in this investigation. The mean structure spacing decreased with increasing temperature. This decrease was more than the corresponding decrease in shear-layer growth rate, and suggests that the mechanisms of vortex amalgamation are, in some manner, inhibited by heat release. The mean temperature rise profiles, normalized by the adiabatic flame temperature rise, were not greatly changed in shape over the range of heat release of this investigation. A small decrease in normalized mean temperature rise with heat release was however observed. Imposition of a favourable pressure gradient in a mixing layer with heat release resulted in an additional decrease in layer growth rate, and caused only a very slight increase in the mixing and amount of chemical product formation. The additional decrease in layer growth rate is shown to be accounted for in terms of the change in free-stream velocity ratio induced by the pressure gradient.

The flow over a backward-facing step under controlled perturbation: laminar separation

1992 ◽  
Vol 238 ◽  
pp. 73-96 ◽  
Author(s):  
M. A. Z. Hasan
Keyword(s):  
Shear Layer ◽  
Low Frequency ◽  
Layer Growth ◽  
Stream Velocity ◽  
Backward Facing Step ◽  
Laminar Separation ◽  
Vortex Merging ◽  
Layer Growth Rate ◽  
Merging Process ◽  
Layer Flow

The flow over a backward-facing step with laminar separation was investigated experimentally under controlled perturbation for a Reynolds number of 11000, based on a step height h and a free-stream velocity UO. The reattaching shear layer was found to have two distinct modes of instability: the ‘shear layer mode’ of instability at Stθ ≈ 0.012 (Stθ ≡ fθ/UO, θ being the momentum thickness at separation and f the natural roll-up frequency of the shear layer); and the ‘step mode’ of instability at Sth ≈ 0.185 (Sth ≡ fh/U0). The shear layer instability frequency reduced to the step mode one via one or more stages of a vortex merging process. The perturbation increased the shear layer growth rate and the turbulence intensity and decreased the reattachment length compared to the unperturbed flow. Cross-stream measurements of the amplitudes of the perturbed frequency and its harmonics suggested the splitting of the shear layer. Flow visualization confirmed the shear layer splitting and showed the existence of a low-frequency flapping of the shear layer.

scholarly journals Scaling of separated shear layers: an investigation of mass entrainment

2017 ◽  
Vol 826 ◽  
pp. 851-887 ◽  
Author(s):  
Francesco Stella ◽  
Nicolas Mazellier ◽  
Azeddine Kourta
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Separated Flow ◽  
Layer Growth ◽  
Upper And Lower Bounds ◽  
Stream Velocity ◽  
Physical Parameters ◽  
Separated Shear Layer ◽  
Mass Entrainment ◽  
Recirculation Length

We report an experimental investigation of the separating/reattaching flow over a descending ramp with a $25^{\circ }$ expansion angle. Emphasis is given to mass entrainment through the boundaries of the separated shear layer emanating from the upper edge of the ramp. For this purpose, the turbulent/non-turbulent interface and the separation line inferred from image-based analysis are used respectively to mark the upper and lower bounds of the separated shear layer. The main objective of this study is to identify the physical parameters that scale the development of the separated shear layer, by giving a specific emphasis to the investigation of mass entrainment. Our results emphasise the multiscale nature of mass entrainment through the separated shear layer. The recirculation length $L_{R}$, step height $h$ and free-stream velocity $U_{\infty }$ are the dominant scales that organise the separated flow (and related large-scale quantities as pressure distribution or shear layer growth rate) and set mean mass fluxes. However, local viscous mechanisms seem to be responsible for most of local mass entrainment. Furthermore, it is shown that large-scale mass entrainment is driven by incoming boundary layer properties, since $L_{R}$ scales with $Re_{\unicode[STIX]{x1D703}}$, and in particular by its turbulent state. Surprisingly, the relationships evidenced in this study suggest that these dependencies are established over a large distance upstream of separation and that they might also extend to small scales, at which viscous entrainment is dominant. If confirmed by additional studies, our findings would open new perspectives for designing effective separation control systems.

Velocity Field Response of a Forced Swirl Stabilized Premixed Flame

2017 ◽  
Author(s):  
Kiran Manoharan ◽  
Travis Smith ◽  
Benjamin Emerson ◽  
Christopher M. Douglas ◽  
Tim Lieuwen ◽  
...  
Keyword(s):  
Heat Release ◽  
Shear Layer ◽  
Recirculation Zone ◽  
Stokes Equations ◽  
Quantitative Agreement ◽  
Forced Response ◽  
Response Analysis ◽  
Annular Jet ◽  
Experimental Response ◽  
Prediction Approach

This study is motivated by the necessity to develop a low order prediction approach for unsteady heat release response characteristics in lean premixed gas turbine combustors. This in turn requires an accurate description of the coherent hydrodynamic oscillations induced in the combustor flow by acoustic forcing. Time resolved velocity and flame position fields are obtained using sPIV and OH-PLIF measurements on a single nozzle, swirl-stabilized, premixed, methane-air flame in a model “unwrapped” annular combustor rig. A natural acoustic oscillation in the rig at 115 Hz results in a coherent flow oscillation that is concentrated primarily within the shear layer between the annular jet flow and the central recirculation zone. A linear stability analysis performed about time averaged base flow fields shows that the flow does not have any self-excited hydrodynamic modes. We then compare predictions from a forced response analysis at a forcing frequency of 115 Hz, based on the linearized Navier-Stokes equations for this coherent response. Good qualitative agreement between linear forced response analysis predictions and experimental response results, is seen for the spatial variation of velocity oscillation amplitude fields, away from the burner centerline. Further, good quantitative agreement between predictions and the experimental response is seen for the phase speed of velocity oscillations along the shear layer between the annular jet and the central recirculation zone. This phase velocity is an important flow field characteristic that has a significant impact on the heat release response that results from these coherent velocity oscillations. Present methods for forced response analysis assume uniform forcing amplitude along the radial direction at the forcing location, as well as, open flows along the streamwise direction. Both these assumptions are not strictly true for the present burner which has a center body on its axis. This maybe the reason for somewhat poor qualitative and quantitative agreement between experiments and predictions at the centerline.

Smart Passive Control of Thermoacoustic Instability in a Bluff-Body Stabilized Combustor: A Lagrangian Analysis of Critical Structures

2020 ◽  
Author(s):  
C. P. Premchand ◽  
Manikandan Raghunathan ◽  
Midhun Raghunath ◽  
K. V. Reeja ◽  
R. I. Sujith ◽  
...  
Keyword(s):  
Wavelet Transform ◽  
Flow Field ◽  
Heat Release ◽  
Shear Layer ◽  
Heat Release Rate ◽  
Release Rate ◽  
Sound Production ◽  
Bluff Body ◽  
Passive Control ◽  
Thermoacoustic Instability

Abstract The tonal sound production during thermoacoustic instability is detrimental to the components of gas turbine and rocket engines. Identifying the root cause and controlling this oscillatory instability would enable manufacturers to save in costs of power outages and maintenance. An optimal method is to identify the structures in the flow-field that are critical to tonal sound production and perform control measures to disrupt those “critical structures”. Passive control experiments were performed by injecting a secondary micro-jet of air onto the identified regions with critical structures in the flow-field of a bluff-body stabilized, dump, turbulent combustor. Simultaneous measurements such as unsteady pressure, velocity, local and global heat release rate fluctuations are acquired in the regime of thermoacoustic instability before and after control action. The tonal sound production in this combustor is accompanied by a periodic flapping of the shear layer present in the region between the dump plane (backward-facing step) and the leading edge of the bluff-body. We obtain the trajectory of Lagrangian saddle points that dictate the flow and flame dynamics in the shear layer during thermoacoustic instability accurately by computing Lagrangian Coherent Structures. Upon injecting a secondary micro-jet with a mass flow rate of only 4% of the primary flow, nearly 90% suppression in the amplitude of pressure fluctuations are observed. The suppression thus results in sound pressure levels comparable to those obtained during stable operation of the combustor. Using Morlet wavelet transform, we see that the coherence in the dominant frequency of pressure and heat release rate oscillations during thermoacoustic instability is affected by secondary injection. The disruption of saddle point trajectories breaks the positive feedback loop between pressure and heat release rate fluctuations resulting in the observed break of coherence. Wavelet transform of global heat release rate shows a redistribution of energy content from the dominant instability frequency (acoustic time scale) to other time scales.

Passive control of deep cavity shear layer flow at subsonic speed

2017 ◽  
Vol 95 (10) ◽  
pp. 894-899
Author(s):  
Mouhammad El Hassan ◽  
Laurent Keirsbulck
Keyword(s):  
Shear Layer ◽  
Passive Control ◽  
Control Method ◽  
Leading Edge ◽  
Layer Growth ◽  
Cavity Resonance ◽  
Passive Flow Control ◽  
Acoustic Coupling ◽  
Deep Cavity ◽  
Layer Flow

Passive control of the flow over a deep cavity at low subsonic velocity is considered in the present paper. The cavity length-to-depth aspect ratio is L/H = 0.2. particle image velocimetry (PIV) measurements characterized the flow over the cavity and show the influence of the control method on the cavity shear layer development. It is found that both the “cylinder” and the “shaped cylinder”, placed upstream from the cavity leading edge, result in the suppression of the aero-acoustic coupling and highly reduce the cavity noise. It should be noted that the vortical structures impinge at almost the same location near the cavity downstream corner with and without passive control. The present study allows to identify an innovative passive flow control method of cavity resonance. Indeed, the use of a “shaped cylinder” presents similar suppression of the cavity resonance as with the “cylinder” but with less impact on the cavity flow. The “shaped cylinder” results in a smaller shear layer growth than the cylinder. Velocity deficiency and turbulence levels are less pronounced using the “shaped cylinder”. The “cylinder” tends to diffuse the vorticity in the cavity shear layer and thus the location of the maximum vorticity is more affected as compared to the “shaped cylinder” control. The fact that the “shaped cylinder” is capable of suppressing the cavity resonance, despite the vortex shedding and the high frequency forcing being suppressed, is of high interest from fundamental and applied points of view.

scholarly journals Turbulent shear-layer mixing: growth-rate compressibility scaling

2000 ◽  
Vol 414 ◽  
pp. 35-45 ◽  
Author(s):  
M. D. SLESSOR ◽  
M. ZHUANG ◽  
P. E. DIMOTAKIS
Keyword(s):  
Growth Rate ◽  
Shear Layer ◽  
Free Stream ◽  
Layer Growth ◽  
Turbulent Shear ◽  
Turbulent Shear Layer ◽  
Thermal Energy Conversion ◽  
Layer Growth Rate ◽  
Stream Density ◽  
Convective Mach Number

A new shear-layer growth-rate compressibility-scaling parameter is proposed as an alternative to the total convective Mach number, Mc. This parameter derives from considerations of compressibility as a means of kinetic-to-thermal-energy conversion and can be significantly different from Mc for flows with far-from-unity free-stream-density and speed-of-sound ratios. Experimentally observed growth rates are well-represented by the new scaling.

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