scholarly journals Role of hydrodynamic shear layer stability in driving combustion instability in a premixed propane-air backward-facing step combustor

2018 ◽  
Vol 3 (6) ◽  
Author(s):  
Santosh Hemchandra ◽  
Santosh Shanbhogue ◽  
Seunghyuck Hong ◽  
Ahmed F. Ghoniem
Keyword(s):  
Shear Layer ◽  
Combustion Instability ◽  
Backward Facing Step ◽  
Hydrodynamic Shear

Investigation on Effect of Inlet Flow Turbulence on the Combustion Instability Using Simultaneous PIV and CH* Chemiluminescence in a Backward Facing Step Combustor

2019 ◽  
Author(s):  
Pankaj Pancharia ◽  
Vikram Ramanan ◽  
Baladandayuthapani Nagarajan ◽  
S. R. Chakravarthy
Keyword(s):  
Turbulence Intensity ◽  
Equivalence Ratio ◽  
Large Scale ◽  
Combustion Instability ◽  
Flow Dynamics ◽  
Airflow Rate ◽  
Backward Facing Step ◽  
Time Resolved ◽  
Flame Holder

Abstract The present study investigates the role of inlet turbulence intensity on the stability characteristics of a lab scale backward facing step combustor (BFS). Turbulence generator placed upstream of the flame holder is used to vary the turbulence levels. The present study utilizes simultaneous chemiluminescence, particle image velocimetry (PIV) and unsteady pressure fluctuation measurement are done in a time-resolved manner to study the role of inlet turbulence intensity on the flame-flow dynamics and identify different modes of combustion instability as a result of the same. The bifurcation plot with airflow rate, in terms of step-based Reynolds number (Re) as the control parameter, indicates a counterintuitive picture, whereby higher turbulence intensity postpones the onset of instability. The finding has been reported in the past by Nagarajan et. al [30], with the present work extending it. It is shown that the flow-flame structures at high (∼1000 Pa) and very high (>4000 Pa), conditions, the dynamics are significantly different across the same turbulence intensity at different equivalence ratio as well as at different turbulence intensities for the same equivalence ratio. Analysis of the flame-flow dynamics reveals the role of the extent of vortex initiated by acoustics and its orientation in forming an unsteady loop, whereby the vortex span and strength aids the flame to propagate upstream of the step, and the flame in-turn being responsible to sustain the large-scale vortex. This phenomenon is distinct from the conventional vortex sustained combustion instability, whereby the vortex is of the lower span and does not influence the upstream flow. The role of inlet turbulence intensity is seen to be more pronounced in the extent of the flame propagating upward, which then completes the fore-mentioned loop.

scholarly journals Unique Slow Crack Growth Behavior of Isotactic Polypropylene: The Role of Shear Layer-Spherulites Layer Alternated Structure

Polymers ◽  
2020 ◽  
Vol 12 (11) ◽  
pp. 2746
Author(s):  
Mingjin Liu ◽  
Jiaxu Luo ◽  
Jin Chen ◽  
Xueqin Gao ◽  
Qiang Fu ◽  
...  
Keyword(s):  
Shear Layer ◽  
Crack Growth ◽  
Isotactic Polypropylene ◽  
Slow Crack Growth ◽  
Crack Growth Behavior ◽  
Polymer Science ◽  
The Past ◽  
Propagation Behavior ◽  
Crack Propagation Behavior

With the development of polymer science, more attention is being paid to the longevity of polymer products. Slow crack growth (SCG), one of the most important factors that reveal the service life of the products, has been investigated widely in the past decades. Here, we manufactured an isotactic polypropylene (iPP) sample with a novel shear layer–spherulites layer alternated structure using multiflow vibration injection molding (MFVIM). However, the effect of the alternated structure on the SCG behavior has never been reported before. Surprisingly, the results showed that the resistivity of polymer to SCG can be enhanced remarkably due to the special alternated structure. Moreover, this sample shows unique slow crack propagation behavior in contrast to the sample with the same thickness of shear layer, presenting multiple microcracks in the spherulites layer, which can explain the reason of the resistivity improvement of polymer to SCG.

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.

Impact of Precessing Vortex Core Dynamics on Shear Layer Response in a Swirling Jet

2018 ◽  
Vol 140 (6) ◽  
Author(s):  
Mark Frederick ◽  
Kiran Manoharan ◽  
Joshua Dudash ◽  
Brian Brubaker ◽  
Santosh Hemchandra ◽  
...  
Keyword(s):  
Stability Analysis ◽  
Shear Layer ◽  
Linear Stability ◽  
Linear Stability Analysis ◽  
Vortex Core ◽  
Combustion Instability ◽  
Forced Response ◽  
Longitudinal Acoustic ◽  
Precessing Vortex Core ◽  
Acoustic Forcing

Combustion instability, the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a nonreacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.

Free surface over a horizontal shear layer: vorticity generation and air entrainment mechanisms

2017 ◽  
Vol 813 ◽  
pp. 1007-1044 ◽  
Author(s):  
Matthieu A. André ◽  
Philippe M. Bardet
Keyword(s):  
Free Surface ◽  
Shear Layer ◽  
Air Entrainment ◽  
Horizontal Shear ◽  
Vortex Instability ◽  
Image Velocimetry ◽  
Planar Laser ◽  
Spatio Temporal ◽  
The Waves

Two air entrainment mechanisms driven by vortex instability are reported in the unstable relaxation of a horizontal shear layer below a free surface. This flow is experimentally investigated by means of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) coupled with surface profilometry. PLIF identifies counter-rotating vortex pairs (CRVP) emanating from the surface following the growth of high steepness two-dimensional millimetre-size waves for Reynolds and Weber numbers based on the momentum thickness of 177 to 222 and 7.59 to 13.9, respectively. High spatio-temporal resolution PIV reveals the role of surface-generated vorticity and flow separation in the highly curved trough of the waves on the injection of a CRVP. Air bubbles are entrapped in the wake of these CRVPs at Reynolds number above 190. PIV data and spanwise PLIF images show two initiation mechanisms: primary vortex instability modulating the spanwise location where the flow separates, resulting in the pinch off of an air ligament, and secondary vortex instability turning a CRVP into$\unicode[STIX]{x1D6FA}$-shaped loops pulling the surface down. Instability wavelengths agree with linear stability analysis, and models for these new air entrainment mechanisms are proposed.

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