Turbulent Superstructures in Inert Jets and Diffusion Jet Flames
Abstract
:1. Introduction
2. Experimental Setup
3. Results and Discussion
3.1. Inert Jets
3.2. Diffusion Jet Flames
4. Conclusions
- In accordance with the universal scenario of laminar-turbulent transition through intermittency, a two-stage pattern of a subsonic gas jet flow from long tubes in the near region was proposed. This pattern refers to Reynolds numbers, when a laminar-turbulent transition occurs inside the jet source (a long tube) and large-scale puff structures are formed. At the “laminar” stage, there is no puff in the near region of the jet, and at the “turbulent” stage, the puff is available.
- According to the measurements of the intermittency coefficient in an inert jet, the data for different coordinates (x/d = 0.17 and at distance x/d = 20) almost coincide at Re = idem. This indicates that, on the whole, the puff structure is sufficiently stable and conservative in the near field of the jet up to the zone of transition to turbulence.
- Based on the velocity fields measured by the high-speed PIV, the time autocorrelation function on the jet axis in cross-section x/d = 20 was determined. The spatial integral scale in a turbulent flow at Re = 3077 is l/d = 0.04. At the same time, the longitudinal scale of a puff at Re = 2400 is l/d = 9–23, and this justifies the use of the term “superstructure” for turbulent puff formations.
- The results of measuring the flow field using high-speed PIV allowed us to establish that the puff is characterized by complex temporal and spatial dynamics. At the “laminar” stage, in the initial section, there is a vortex-free core near the jet axis and large vorticity in the mixing layer of the jet caused by the Kelvin-Helmholtz instability. At the stage of superstructure propagation, it is possible to diagnose a disturbance extended along the x-axis (20–30d) with a smooth leading and steep trailing edges. In addition, the level of velocity fluctuations increases noticeably, which increases vorticity in the entire puff region. Second, at the moment of a maximal decrease in the velocity on the jet axis to a value of ~0.25 Ucl, a sinusoidal disturbance is observed in the axial region of the puff, which leads to formation of localized vortices. Downstream, the dynamics of these vortices becomes more chaotic. Intense vortex formation leads to a significant increase in the jet in the radial direction up to values of 3–4d.
- The two-stage flow pattern also takes place in the near region of the reacting gas jet flowing out from long tubes. It was found that the presence of a chemical reaction can make a significant addition to this mechanism. It is shown that during methane combustion, the puff movement leads to a significant deformation of the flame front, and at the same time, during the propane flame combustion, the deformation of the front is less significant. For the first time, the P-S diagram [40] was used to describe the interaction of large-scale puff vortices and the flame front of methane and propane. It was found that the location of methane and propane on the diagram corresponds to the “Weak Flame Wrinkling” region. The formation of superstructures can be the determining mechanism for a global change in the flame topology. Thus, the presence of vortices of different sizes in the puff composition can lead to fractal deformation of the flame front. These mechanisms provide a basis for the formation of a new method for controlling diffusion combustion in devices using long tubes and channels.
Supplementary Materials
Author Contributions
Funding
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
Nomenclature
d | tube diameter, mm |
L | tube length, m |
l | integral scale of longitudinal structures, mm |
U | instantaneous velocity, m/s |
Ub | bulk gas velocity in the tube, m/s |
Ucl | axial gas velocity at a given cross-section, m/s |
Um | axial gas velocity at the tube outlet, m/s |
Ur | gas velocity in radial direction, m/s |
W | instantaneous velocity vector, m/s |
u | root-mean-square of velocity fluctuations, m/s |
r | radial coordinate, mm |
t | time, s |
T | temperature, °C |
Tu = u/Um × 100% | turbulence level, % |
x | longitudinal coordinate, mm |
x1 | coordinate of the laminar-turbulent transition in the jet, mm |
P = | linear scale ratio |
R(τ) | Eulerian time-correlation function determined from longitudinal velocity pulsations |
Re = Ubd/ν | Reynolds number |
Recr | critical Reynolds number |
Re1 | lower boundary of the laminar-turbulent transition |
Re2 | top boundary of the laminar-turbulent transition |
Ri = | Richardson number |
S = | characteristic velocity ratio |
t* = | Eulerian integral time scale |
γ | intermittency factor |
δ | thickness of the shear layer, mm |
τ | autocorrelation function time lag, s |
ω | vorticity, 1/s |
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Lemanov, V.; Lukashov, V.; Sharov, K. Turbulent Superstructures in Inert Jets and Diffusion Jet Flames. Fluids 2021, 6, 459. https://doi.org/10.3390/fluids6120459
Lemanov V, Lukashov V, Sharov K. Turbulent Superstructures in Inert Jets and Diffusion Jet Flames. Fluids. 2021; 6(12):459. https://doi.org/10.3390/fluids6120459
Chicago/Turabian StyleLemanov, Vadim, Vladimir Lukashov, and Konstantin Sharov. 2021. "Turbulent Superstructures in Inert Jets and Diffusion Jet Flames" Fluids 6, no. 12: 459. https://doi.org/10.3390/fluids6120459