Effects of coflow on turbulent flame puffs
01 Jan 2002-AIAA Journal (American Institute of Aeronautics and Astronautics (AIAA))-Vol. 40, Iss: 7, pp 1355-1362
TL;DR: In this paper, the authors examined pulsed turbulent jet diffusion flames in an air coflow of variable strength and found that the result was that the mean flame length of these puffs was at least 51% less than that of a steady-state, that is, nonpulsed, flame for the same injection Reynolds number.
Abstract: Pulsed, turbulent jet diffusion flames in an air coflow of variable strength were examined experimentally. In all cases, the flames were fully modulated, that is, the fuel flow was completely shut off between pulses. Isolated puffs of unheated ethylene fuel were injected using a 2-mm-diam-nozzle into a combustor with an air coflow at 1-atm pressure. For short injection times (τ<50 ms), compact, pufflike structures were generated. The mean flame length of these puffs was at least 51% less than that of a steady-state, that is, nonpulsed, flame for the same injection Reynolds number. More elongated flame structures, with a flame length closer to that of steady-state flames, occurred for longer injection times of up to 300 ms. The addition of coflow generally causes an increase in the mean flame length. For short injection times (r < 50 ms), this resulted in an increase in flame length of up to 27% for a coflow strength of U cof /U jet = 0.02. The fractional increase in the flame length due to coflow of pulsed flames with longer injection times, as well as steady flames, was significantly less. The mean flame length for the flame with the coflow duct generally exceeded that of the corresponding free flame, even for the case of zero coflow. The amount of coflow required to achieve a given increase in mean flame length is quantitatively consistent with a scaling argument developed as part of this investigation.
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01 Jun 1986
TL;DR: In this paper, the entrainment of axisymmetric air jets flowing into surrounding air has been made for a range of orifice sizes, Strouhal numbers, and excitation powers.
Abstract: Direct measurements of entrainment by acoustically pulsed axisymmetric air jets flowing into surrounding air have been made for a range of orifice sizes, Strouhal numbers, and excitation powers. The entrainment was considerably increased, by up to 5.8 times at distances greater than 15 diameters axially downstream of the orifice exit plane. The entrainment of the excited jet varied linearly with downstream distance. The jet response varied nonlinearly with excitation strength, indicating that there may be a practical upper limit to the acoustic augmentation of entrainment. The response depends on Strouhal number and appears to be optimum at about 0.25.
61 citations
TL;DR: In this article, the authors present the results of experimental investigations into the instabilities of turbulent swirling non-premixed flames using high-speed imaging of laser Mie scattering and shadowgraphs.
38 citations
TL;DR: In this article, a single fuel nozzle with diameter d = 2 mm is centered in a combustor 20 20 cm in cross section and 67 cm in height, and the gaseous fuel flow (ethylene or a 50/50 ethylene/nitrogen mixture by volume) is fully-modulated by a fast-response solenoid valve with injection times from tau = 4 to tau < 300 ms.
23 citations
TL;DR: In this paper, the CO and unburned hydrocarbon (UHC) emissions of pulsed turbulent diffusion flames were examined by injecting unheated ethylene into a combustor with an air coflow at atmospheric pressure.
Abstract: The CO and unburned hydrocarbon (UHC) emissions of pulsed turbulent diffusion flames were examined by injecting unheated ethylene into a combustor with an air coflow at atmospheric pressure. In all cases the flames were fully modulated (fuel flow fully shut off between injection intervals). Video imaging was performed and time-averaged emissions were measured on the combustor centerline. For short injection times ( ≤ 46 ms), compact, puff-like structures were generated. Longer injection times produced elongated flame structures with flame lengths closer to that of steady flames. The highest emission indices of CO and UHC were found for compact, isolated puffs. The emissions for all flames approached the steady-flame levels for a duty cycle (jet-on fraction) of approximately 0.4. This suggests that there are combinations of injection time and duty cycle for fully modulated, turbulent diffusion flames that produce emissions comparable to the steady flame but with a significantly shorter flame length.
10 citations
Cites background from "Effects of coflow on turbulent flam..."
...The velocity of the oxidizer coflow was 0.5% of the jet exit velocity; this coflow strength has been shown not to have a significant impact on the characteristics of fully modulated flames for these fuel injection conditions (Hermanson et al., 2002)....
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01 Jan 2010
TL;DR: The effects of co-flow air velocity on the flickering behavior and stabilisation mechanism of a laminar flickering methane diffusion flame are investigated in this paper, where it has been observed that the flame dynamics and combustion characteristics are strongly affected by the co flow air velocity.
Abstract: The effects of co-flow air velocity on the flickering behaviour and stabilisation mechanism of a laminar flickering methane diffusion flame are investigated. Photomultipliers, high speed photography accompanied with digital image processing techniques have been used to study the change in global flame shape, the instability initiation point, the frequency and magnitude of the flame oscillation. It has been observed that the flame dynamics and combustion characteristics of co-flow diffusion flame are strongly affected by the co-flow air velocity. The oscillation frequency was observed to increase linearly with the co-flow velocity, whilst, the frequency amplitude was observed to continuously decrease. When the co-flow velocity has reached a certain value the buoyancy driven flame oscillation was completely suppressed. The high speed imaging has revealed that the co-flow of air is able to push the location of instability initiation point beyond the visible flame to create a very steady laminar flow region in the reaction zone. It is observed that the oscillation magnitude and wavelength decrease continuously as the co-flow air increases. The average oscillating flame height behaviour, however, was observed to be bimodal. It was initially enhanced by the co-flow air then starts to decrease towards the stabilised level. This height was observed to remain almost constant after stabilisation, despite further increase at air flow rate. It has been confirmed that, the flickering frequency is not a function of fuel flow rate but more co-flow rates are needed in order to suppress the flickering of the flames at higher fuel flow rates.
10 citations
References
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Book•
23 Feb 1973
TL;DR: In this article, the authors introduce linear internal waves and herar flows in a stratified fluid and double-diffusive convection in stably stratified fluids, and show that the shear flows can produce turbulence.
Abstract: Preface 1. Introduction and preliminaries 2. Linear internal waves 3. Finite amplitude motions in stably stratified fluids 4. Instability and the production of turbulence 5. Turbulent shear flows in a stratified fluid 6. Buoyant convection from isolated sources 7. Convection from heated surfaces 8. Double-diffusive convection 9. Mixing across density interfaces 10. Internal mixing processes Bibliography and author index Recent publications Subject index.
2,722 citations
TL;DR: In this paper, the authors show that a large-scale orderly pattern may exist in the noiseproducing region of a round subsonic jet by observing the evolution of orderly flow with advancing Reynolds number.
Abstract: Past evidence suggests that a large-scale orderly pattern may exist in the noiseproducing region of a jet. Using several methods to visualize the flow of round subsonic jets, we watched the evolution of orderly flow with advancing Reynolds number. As the Reynolds number increases from order 102 to 103, the instability of the jet evolves from a sinusoid to a helix, and finally to a train of axisymmetric waves. At a Reynolds number around 104, the boundary layer of the jet is thin, and two kinds of axisymmetric structure can be discerned: surface ripples on the jet column, thoroughly studied by previous workers, and a more tenuous train of large-scale vortex puffs. The surface ripples scale on the boundary-layer thickness and shorten as the Reynolds number increases toward 105. The structure of the puffs, by contrast, remains much the same: they form at an average Strouhal number of about 0·3 based on frequency, exit speed, and diameter.To isolate the large-scale pattern at Reynolds numbers around 105, we destroyed the surface ripples by tripping the boundary layer inside the nozzle. We imposed a periodic surging of controllable frequency and amplitude at the jet exit, and studied the response downstream by hot-wire anemometry and schlieren photography. The forcing generates a fundamental wave, whose phase velocity accords with the linear theory of temporally growing instabilities. The fundamental grows in amplitude downstream until non-linearity generates a harmonic. The harmonic retards the growth of the fundamental, and the two attain saturation intensities roughly independent of forcing amplitude. The saturation amplitude depends on the Strouhal number of the imposed surging and reaches a maximum at a Strouhal number of 0·30. A root-mean-square sinusoidal surging only 2% of the mean exit speed brings the preferred mode to saturation four diameters downstream from the nozzle, at which point the entrained volume flow has increased 32% over the unforced case. When forced at a Strouhal number of 0·60, the jet seems to act as a compound amplifier, forming a violent 0·30 subharmonic and suffering a large increase of spreading angle. We conclude with the conjecture that the preferred mode having a Strouhal number of 0·30 is in some sense the most dispersive wave on a jet column, the wave least capable of generating a harmonic, and therefore the wave most capable of reaching a large amplitude before saturating.
2,108 citations
TL;DR: The formation of vortex rings generated through impulsively started jets is studied experimentally in this paper, where the velocity and vorticity field of the leading vortex ring formed is disconnected from that of the trailing jet.
Abstract: The formation of vortex rings generated through impulsively started jets is studied experimentally. Utilizing a piston/cylinder arrangement in a water tank, the velocity and vorticity fields of vortex rings are obtained using digital particle image velocimetry (DPIV) for a wide range of piston stroke to diameter (L/D) ratios. The results indicate that the flow field generated by large L/D consists of a leading vortex ring followed by a trailing jet. The vorticity field of the leading vortex ring formed is disconnected from that of the trailing jet. On the other hand, flow fields generated by small stroke ratios show only a single vortex ring. The transition between these two distinct states is observed to occur at a stroke ratio of approximately 4, which, in this paper, is referred to as the ‘formation number’. In all cases, the maximum circulation that a vortex ring can attain during its formation is reached at this non-dimensional time or formation number. The universality of this number was tested by generating vortex rings with different jet exit diameters and boundaries, as well as with various non-impulsive piston velocities. It is shown that the ‘formation number’ lies in the range of 3.6–4.5 for a broad range of flow conditions. An explanation is provided for the existence of the formation number based on the Kelvin–Benjamin variational principle for steady axis-touching vortex rings. It is shown that based on the measured impulse, circulation and energy of the observed vortex rings, the Kelvin–Benjamin principle correctly predicts the range of observed formation numbers.
1,042 citations
TL;DR: In this paper, the authors measured the width and distance travelled of buoyant fluid in a water tank and found that the velocity and the distance travelled follow roughly the laws of the Froude number.
Abstract: Isolated masses of buoyant fluid were released in a water tank. Their width, 2r , and the distance travelled, z , were measured as functions of time and were found to follow roughly the laws
$r = nz,\;\;\;\;\; w = C(g \overline {B}r)^ \frac{1}{2},$ where w is the vertical velocity, $\overline {B}$ the mean buoyancy, and n and C are constants. These equations are predicted by dimensional analysis, assuming viscosity to be negligible, and the constants appear to be independent of the Reynolds number. It is found that C [eDot ] 1·2 and n is in the neighbourhood of 4. Since the Froude number relating the buoyancy and inertia forces is the same as for isolated masses of buoyant air in the atmosphere, it is concluded that the constants will have the same value in this latter case. This is confirmed roughly by observation of cumulus cloud towers. Some of the characteristics of the motion observed in the experiments are described and comparison is made with vortex rings.
275 citations