Density ratio effects on reacting bluff-body flow field characteristics

2012 ◽  
Vol 706 ◽  
pp. 219-250 ◽  
Author(s):  
Benjamin Emerson ◽  
Jacqueline O’Connor ◽  
Matthew Juniper ◽  
Tim Lieuwen
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Shear Layer ◽  
Parametric Excitation ◽  
Bluff Body ◽  
Density Ratio ◽  
Global Mode ◽  
Measured Velocity ◽  
Number Range ◽  
Flow Field Characteristics

AbstractThe wake characteristics of bluff-body-stabilized flames are a strong function of the density ratio across the flame and the relative offset between the flame and shear layer. This paper describes systematic experimental measurements and stability calculations of the dependence of the flow field characteristics and flame sheet dynamics upon flame density ratio,${\rho }_{u} / {\rho }_{b} $, over the Reynolds number range of 1000–3300. We show that two fundamentally different flame/flow behaviours are observed at high and low${\rho }_{u} / {\rho }_{b} $values: a stable, noise-driven fixed point and limit-cycle oscillations, respectively. These results are interpreted as a transition from convective to global instability and are captured well by stability calculations that used the measured velocity and density profiles as inputs. However, in this high-Reynolds-number flow, the measurements show that no abrupt bifurcation in flow/flame behaviour occurs at a given${\rho }_{u} / {\rho }_{b} $value. Rather, the flow field is highly intermittent in a transitional${\rho }_{u} / {\rho }_{b} $range, with the relative fraction of the two different flow/flame behaviours monotonically varying with${\rho }_{u} / {\rho }_{b} $. This intermittent behaviour is a result of parametric excitation of the global mode growth rate in the vicinity of a supercritical Hopf bifurcation. It is shown that this parametric excitation is due to random fluctuations in relative locations of the flame and shear layer.

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.

scholarly journals Local stability analysis and eigenvalue sensitivity of reacting bluff-body wakes

2016 ◽  
Vol 788 ◽  
pp. 549-575 ◽  
Author(s):  
Benjamin Emerson ◽  
Tim Lieuwen ◽  
Matthew P. Juniper
Keyword(s):  
Stability Analysis ◽  
Acoustic Waves ◽  
Local Stability ◽  
Bluff Body ◽  
Density Ratio ◽  
Global Mode ◽  
Eigenvalue Sensitivity ◽  
Local Stability Analysis ◽  
Measured Time ◽  
The Stability

This paper presents an experimental and theoretical investigation of high-Reynolds-number low-density reacting wakes near a hydrodynamic Hopf bifurcation. This configuration is applicable to the wake flows that are commonly used to stabilize flames in high-velocity flows. First, an experimental study is conducted to measure the limit-cycle oscillation of this reacting bluff-body wake. The experiment is repeated while independently varying the bluff-body lip velocity and the density ratio across the flame. In all cases, the wake exhibits a sinuous oscillation. Linear stability analysis is performed on the measured time-averaged velocity and density fields. In the first stage of this analysis, a local spatiotemporal stability analysis is performed on the measured time-averaged velocity and density fields. The stability analysis results are compared to the experimental measurement and demonstrate that the local stability analysis correctly captures the influence of the lip-velocity and density-ratio parameters on the sinuous mode. In the second stage of the analysis, the linear direct and adjoint global modes are estimated by combining the local results. The sensitivity of the eigenvalue to changes in intrinsic feedback mechanisms is found by combining the direct and adjoint global modes. This is referred to as the eigenvalue sensitivity throughout the paper for reasons of brevity. The predicted global mode frequency is consistently within 10 % of the measured value, and the linear global mode shape closely resembles the measured nonlinear oscillations. The adjoint global mode reveals that the oscillation is strongly sensitive to open-loop forcing in the shear layers. The eigenvalue sensitivity identifies a wavemaker in the recirculation zone of the wake. A parametric study shows that these regions change little when the density ratio and lip velocity change. In the third stage of the analysis, the stability analysis is repeated for the varicose hydrodynamic mode. Although not physically observed in this unforced flow, the varicose mode can lock into longitudinal acoustic waves and cause thermoacoustic oscillations to occur. The paper shows that the local stability analysis successfully predicts the global hydrodynamic stability characteristics of this flow and shows that experimental data can be post-processed with this method in order to identify the wavemaker regions and the regions that are most sensitive to external forcing, for example from acoustic waves.

Experimental Study of Advanced Convective Cooling Techniques for Combustor Liners

2008 ◽  
Author(s):  
Michael Maurer ◽  
Uwe Ruedel ◽  
Michael Gritsch ◽  
Jens von Wolfersdorf
Keyword(s):  
Heat Transfer ◽  
Experimental Study ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Flow Field ◽  
Transfer Coefficient ◽  
Heat Transfer Enhancement ◽  
Transfer Enhancement ◽  
Number Range ◽  
Convective Cooling

An experimental study was conducted to determine the heat transfer performance of advanced convective cooling techniques at the typical conditions found in a backside cooled combustion chamber. For these internal cooling channels, the Reynolds number is usually found to be above the Reynolds number range covered by available databases in the open literature. As possible candidates for an improved convective cooling configuration in terms of heat transfer augmentation and acceptable pressure drops, W-shaped and WW-shaped ribs were considered for channels with a rectangular cross section. Additionally, uniformly distributed hemispheres were investigated. Here, four different roughness spacings were studied to identify the influence on friction factors and the heat transfer enhancement. The ribs and the hemispheres were placed on one channel wall only. Pressure losses and heat transfer enhancement data for all test cases are reported. To resolve the heat transfer coefficient, a transient thermocromic liquid crystal technique was applied. Additionally, the area-averaged heat transfer coefficient on the W-shaped rib itself was observed using the so-called lumped-heat capacitance method. To gain insight into the flow field and to reveal the important flow field structures, numerical computations were conducted with the commercial code FLUENT™.

Convective and Absolute Instabilities in Reacting Bluff Body Wakes

2011 ◽  
Author(s):  
Benjamin Emerson ◽  
Julia Lundrigan ◽  
Jacqueline O’Connor ◽  
David Noble ◽  
Tim Lieuwen
Keyword(s):  
Reynolds Number ◽  
Bluff Body ◽  
Flow Behavior ◽  
Density Ratio ◽  
Intermittent Flow ◽  
Flow Problems ◽  
High Reynolds ◽  
Bluff Body Flow ◽  
Density Ratios ◽  
Parameter Values

This paper describes the variation of bluff body wake structure with flame density ratio. It is known that the bluff body flow structure at “high” and “low” flame density ratios is fundamentally different, being dominated by the convectively unstable shear layers or absolutely unstable Von Karman vortex street, respectively. This paper characterizes the aforementioned transition and shows that the bifurcation in flow behavior does not occur abruptly at some ρu/ρb value. Rather, there exists a range of transitional density ratios at which the flow exists intermittently in both flow states, abruptly shifting back and forth between the two. The fraction of time that the flow spends in either state is a monotonic function of ρu/ρb. This behavior is to be contrasted with lower Reynolds number, laminar flow problems where the convective/absolute instability transition occurs at a well defined value of bifurcation parameter. With this distinction in mind, however, this paper also shows that local parallel stability analyses developed for laminar base wake flows can capture many of the observed flow dependencies. These results have important implications on the dynamics of high Reynolds number, vitiated flows, where typical parameter values fall into the highly intermittent flow regime characterized in this study. This suggests that such flows exhibit two co-existing dynamical states, intermittently jumping between the two.

Experimental and Numerical Investigation of Convective Heat Transfer in a Gas Turbine Can Combustor

2009 ◽  
Author(s):  
Sunil Patil ◽  
Santosh Abraham ◽  
Danesh Tafti ◽  
Srinath Ekkad ◽  
Yong Kim ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Convective Heat Transfer ◽  
Gas Turbine ◽  
Convective Heat ◽  
Swirl Number ◽  
Number Range ◽  
Heat Transfer Augmentation ◽  
Peak Location

Experiments and numerical computations are performed to investigate the convective heat transfer characteristics of a gas turbine can combustor under cold flow conditions in a Reynolds number range between 50,000 and 500,000 with a characteristic swirl number of 0.7. It is observed that the flow field in the combustor is characterized by an expanding swirling flow which impinges on the liner wall close to the inlet of the combustor. The impinging shear layer is responsible for the peak location of heat transfer augmentation. It is observed that as Reynolds number increases from 50,000 to 500,000, the peak heat transfer augmentation ratio (compared to fully-developed pipe flow) reduces from 10.5 to 2.75. This is attributed to the reduction in normalized turbulent kinetic energy in the impinging shear layer which is strongly dependent on the swirl number that remains constant at 0.7 with Reynolds number. Additionally, the peak location does not change with Reynolds number since the flow structure in the combustor is also a function of the swirl number. The size of the corner recirculation zone near the combustor liner remains the same for all Reynolds numbers and hence the location of shear layer impingement and peak augmentation does not change.

Experimental Analysis of Microchannel Entrance Length Characteristics Using Microparticle Image Velocimetry

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Tariq Ahmad ◽  
Ibrahim Hassan
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Entrance Region ◽  
Working Fluid ◽  
Parallel Plates ◽  
Entrance Length ◽  
Number Range ◽  
Micro Piv ◽  
Image Velocimetry ◽  
Microparticle Image Velocimetry

The study of the entrance region of microchannels and microdevices is limited, yet important, since the effect on the flow field and heat transfer mechanisms is significant. An experimental study has been carried out to explore the laminar hydrodynamic development length in the entrance region of adiabatic square microchannels. Flow field measurements are acquired through the use of microparticle image velocimetry (micro-PIV), a nonintrusive particle tracking and flow observation technique. With the application of micro-PIV, entrance length flow field data are obtained for three different microchannel hydraulic diameters of 500 μm, 200 μm, and 100 μm, all of which have cross-sectional aspect ratios of 1. The working fluid is distilled water, and velocity profile data are acquired over a laminar Reynolds number range from 0.5 to 200. The test-sections were designed as to provide a sharp-edged microchannel inlet from a very large reservoir at least 100 times wider and higher than the microchannel hydraulic diameter. Also, all microchannels have a length-to-diameter ratio of at least 100 to assure fully developed flow at the channel exit. The micro-PIV procedure is validated in the fully developed region with comparison to Navier–Stokes momentum equations. Good agreement was found with comparison to conventional entrance length correlations for ducts or parallel plates, depending on the Reynolds range, and minimal influence of dimensional scaling between the investigated microchannels was observed. New entrance length correlations are proposed, which account for both creeping and high laminar Reynolds number flows. These correlations are unique in predicting the entrance length in microchannels and will aid in the design of future microfluidic devices.

scholarly journals Numerical Investigation on the Effects of Vortex Shedding on Boundary Layer Unsteadiness

1970 ◽  
Vol 37 ◽  
pp. 33-39
Author(s):  
ABM Toufique Hasan ◽  
Dipak Kanti Das
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Finite Element ◽  
Flow Field ◽  
Flat Plate ◽  
Vortex Shedding ◽  
Stokes Equations ◽  
Number Range ◽  
Rotating Speed ◽  
Rotating Circular Cylinder

The interaction between an initially laminar boundary layer developed spatially on a flat plate under the influence of vortex shedding induced from a rotating circular cylinder has been simulated numerically. The rotational speed of the cylinder is varied to generate the vortex shedding of different intensities. Also the flat plate is kept at different positions from the cylinder. Due to asymmetry in the flow field, the present problem is governed by unsteady Navier-Stokes equations which are simulated numerically by finite element method. Computations are carried out for low Reynolds number range up to 1000. Instantaneous development of the flow field, unsteady boundary layer integral parameters, and wall skin friction are presented on different streamwise locations over the plate. From the computation, it is observed that the vortex shedding substantially affects the boundary layer development. The disturbed displacement and momentum thicknesses of the plate increase up to 1.6 times and 2.6 times of the undisturbed flow, respectively. Also the plate shape factor approaches a value of 1.5 which is typical for turbulent flow. This interaction strongly depends on the rotating speed of the cylinder, the relative positions of the cylinder and the plate and also on Reynolds number of the flow. Keywords: Vortex shedding, finite element, boundary layer, wall skin friction.doi:10.3329/jme.v37i0.817Journal of Mechanical Engineering Vol.37 June 2007, pp.33-39

The Effect of Rotary Arms on Corotating Disk Flow

1995 ◽  
Vol 117 (2) ◽  
pp. 259-262 ◽  
Author(s):  
John Girard ◽  
Scott Abrahamson ◽  
Kevin Uznanski
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Radial Flow ◽  
Disk Drives ◽  
Number Range ◽  
Reynolds Number Range ◽  
Through Flow ◽  
Dominant Feature ◽  
Flow Visualizations ◽  
The Impact

This investigation studied the impact of rotary style arms on the flow between corotating disks contained by a stationary cylindrical enclosure. Both ventilated and nonventilated hub configurations were considered. The particular geometry used represents a simplified model for common disk drives. Flow visualizations were performed over the Reynolds number range of 3.4 × 104 to 3.4 × 105. The arms were observed to dramatically alter the flow field and to produce an azimuthal pressure gradient throughout the flow field. The dominant feature of the flow between two disks was the arm wake. Moreover, an exchange of fluid across the shroud opening, which provided arm access, was observed. Arm effects became stronger as the arm tips were positioned closer to the hub. The combination of arms and radial through flow was studied over a similar Reynolds number range. In this case, the flow field remained dominated by arm effects, although some effects arising from the radial flow were observed.

Local Flow and Heat Transfer Behavior in Convex-Louver Fin Arrays

1999 ◽  
Vol 121 (1) ◽  
pp. 136-141 ◽  
Author(s):  
N. C. DeJong ◽  
A. M. Jacobi
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Drop ◽  
Shear Layer ◽  
Local Flow ◽  
Number Range ◽  
Flow And Heat Transfer ◽  
Transfer Behavior ◽  
Strip Geometry

Local and surface-averaged measurements of convection coefficients and core pressure-drop data are provided for an array of convex-louver fins. For a Reynolds number range from 200 to 5400, these data are complemented with a flow visualization study and contrasted with new measurements from a similar offset-strip geometry. The results clarify the effects of boundary layer restarting, shear-layer unsteadiness, spanwise vortices, and separation, reattachment, and recirculation on heat transfer in the convex-louver geometry.

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