Flow structures of a lobed mixer and effects of streamwise vortices on mixing enhancement

2019 ◽  
Vol 31 (6) ◽  
pp. 066102 ◽  
Author(s):  
Xin-xin Fang ◽  
Chi-bing Shen ◽  
Ming-bo Sun ◽  
Richard D. Sandberg ◽  
Peng Wang
Keyword(s):  
Flow Structures ◽  
Mixing Enhancement ◽  
Streamwise Vortices

Influence of Micro-Lobed Burner on the Non-Premixed Combustion of Methane and Oxygen

2019 ◽  
Vol 12 (1) ◽  
Author(s):  
Yi Xie ◽  
Chuan-li Yi
Keyword(s):  
Mass Fraction ◽  
Equivalence Ratio ◽  
Premixed Combustion ◽  
Maximum Temperature ◽  
Mixing Enhancement ◽  
Combustion System ◽  
Streamwise Vortices ◽  
Maximum Temperature Difference ◽  
Burner Exit ◽  
Combustion Enhancement

Abstract Non-premixed combustion was implemented in a micro-lobed combustion system, and its influence on combustion was studied using both experiments and simulations. The results show that a micro-lobed burner produces streamwise vortices with intensities that increase with the equivalence ratio of methane to oxygen (Φ). Due to the streamwise vortices and the increment of the contact area between methane and oxygen, the gasses mix well in the micro-lobed burner, giving it a larger OH mass fraction and higher temperatures than the micro-splitter burner. Moreover, the equivalence ratio greatly influences the combustion enhancement from the micro-lobed burner, especially near the burner exit. The maximum temperature difference between the two micro-burners at the Z/D = 0.01 cross section is 171 K, when Φ is 0.6. However, when the mixing enhancement caused by the streamwise vortices disappears, Φ has little influence on the combustion temperature of the micro-lobed burner, especially when Φ ≥ 1. In this case, the maximum temperature variation between the micro-lobed burner and micro-splitter burner remains nearly constant.

The flow characteristics of a confined square jet with mixing tabs

1998 ◽  
Vol 212 (2) ◽  
pp. 63-76 ◽  
Author(s):  
S C M Yu ◽  
Y X Hou ◽  
S C Low
Keyword(s):  
Flow Characteristics ◽  
Side Wall ◽  
Laser Doppler Anemometer ◽  
Mixing Enhancement ◽  
Streamwise Vortices ◽  
Streamwise Vorticity ◽  
Mean Velocity ◽  
Downstream Distance ◽  
Two Component ◽  
Breadth Ratio

The flow characteristics of a confined square jet with mixing tabs have been determined by measurements obtained using a two-component laser Doppler anemometer at a Reynolds number of 1.026 × 105 (based on the exit hydraulic diameter, DH = 60 mm, and bulk mean velocity, Ur, of the stream at 1.71 m/s). Both tabs of rectangular and triangular shapes are considered with the same height-breadth ratio ( h/b = 1.35) and with their apex leaning downstream. Altogether four tabs have been used, with one tab each located at the centre of each side wall at the exit plane. Each tab is found to produce a dominant pair of counter-rotating streamwise vortices. The combined effects of the four tabs bifurcate the jet into four ‘fingers’, resulting in a significant increase in entrainment at the downstream locations. The strength of the streamwise vorticity generated by the rectangular tabs is some 30 per cent higher than the triangular ones and decays faster with downstream distance. This appears due to a larger tab surface area which creates a larger pressure differential across the rectangular tab than the triangular tab. The region of high turbulent kinetic energy is found firstly at the locations where the streamwise vortices stretch the normal vortices and subsequently at locations where the streamwise vortices break down, resulting in significant mixing enhancement. Finally, the effects of the so-called secondary tabs have also been examined and are found to enhance the mixing further. The orientation of the secondary tabs is, however, crucial for the mixing enhancement to occur.

scholarly journals On Unstable Streamwise Vortices as a Means of Mixing Enhancement

1997 ◽  
Vol 63 (605) ◽  
pp. 119-125 ◽  
Author(s):  
Michio NISHIOKA ◽  
Mitsuhiro MATSUOKA ◽  
Takeshi TSUJIMOTO ◽  
Toshihiko HIEJIMA
Keyword(s):  
Mixing Enhancement ◽  
Streamwise Vortices

Enhancement of Mixing Effectiveness in Skewed Streams

2004 ◽  
Author(s):  
B. Deinert ◽  
J. Hourmouziadis
Keyword(s):  
Wake Flow ◽  
Test Facility ◽  
Experimental Investigations ◽  
Mixing Enhancement ◽  
Streamwise Vortices ◽  
Streamwise Vorticity ◽  
Plane Shear ◽  
Jet Mixing ◽  
Mixing Noise ◽  
Rotational Direction

Forced mixers are used to improve the performance (thrust and SFC) and to reduce jet-mixing noise of turbofan engines. Therefore every effort has been made to enhance the mixing from forced exhaust mixers. A lobed type of forced mixer induces rapid mixing by enhancing the streamwise vorticity and increasing the interfacial area. Lobed mixer effectiveness can be further enhanced through the introduction of smaller scale mixing devices. For the design of these mixing devices it is important to keep in mind, that the devices have to produce smaller scale vorticity, but with an acceptable pressure loss. Several investigations have been reported on mixing enhancement in a plane shear layer and in wake flows using tabs. One important result was that each tab produces a dominant pair of counter-rotating streamwise vortices with a rotational direction opposite to that of the vortices generated by the induced upstream stagnation field (horseshoe vortex). Due to the high relevance for many engineering applications such as for the mixing processes behind lobed exhaust mixers in turbojet engines, the present investigation also considers skewed streams with delta tabs. In an attempt to explain the basic flow behaviour downstream of a mixing device in a constant skewed stream, qualitative and quantitative measurements have been carried out. The experiments are carried out in a low-speed test facility, which generates a two-stream skewed flow at different angles. This investigation considers the evolution of coherent structures and the characteristic flow field of parallel and skewed streams generated by mixing devices in the form of delta tabs (triangular-shaped tabs with their apex leaning downstream). Later on the results of this study are to be applied to the design of mixing devices introduced on a lobed mixer. One significant result of these experimental investigations in skewed streams is that each tab generates a pair of counter-rotating streamwise vortices with the same rotational direction as in the unskewed stream, but both vortices are not equally strong. This is caused by the fact that the pressure field ceases to be symmetric upstream of the tab. Farther downstream, the wake flow appears to be wrapped around the stronger vortex.

Direct numerical simulation of high-speed transition due to roughness elements

2019 ◽  
Vol 868 ◽  
pp. 762-788 ◽  
Author(s):  
Prakash Shrestha ◽  
Graham V. Candler
Keyword(s):  
Spectral Density ◽  
Power Spectral Density ◽  
High Speed ◽  
Shear Layers ◽  
Dynamic Mode Decomposition ◽  
Flow Structures ◽  
Dynamic Mode ◽  
Streamwise Vortices ◽  
Vortex System ◽  
Power Spectral

We study and compare instability mechanisms of a Mach 5.65 laminar boundary layer tripped by an isolated diamond-shaped trip and by an array of diamond-shaped trips using direct numerical simulations. A low-Reynolds-number experiment, consisting of the trip array (Semper & Bowersox, AIAA J., vol. 55 (3), 2017, pp. 808–817), is used to validate our simulations. Three dynamically prominent flow structures are observed in both trip configurations. These flow structures are the upstream vortex system, the shock system, and the downstream shear layers/counter-rotating streamwise vortices that originate from the top and sides of the trips. Analysis of the power spectral density of pressure reveals the source of instability to be an interaction between the shear layers and the counter-rotating streamwise vortices downstream of both trip configurations. The interaction leads to the formation of hairpin-like structures that eventually break down to turbulent flow. This finding contrasts with that of an isolated cylindrical trip (Subbareddy et al., J. Fluid Mech., vol. 748, 2014, pp. 848–878) where the upstream vortex system is found to be the source of instability. Therefore, the shape of a trip plays an important role in the instability mechanism. Furthermore, dynamic mode decomposition (Rowley et al., J. Fluid Mech., vol. 641, 2009, pp. 115–127; Schmid, J. Fluid Mech., vol. 656, 2010, pp. 5–28) of three-dimensional snapshots of pressure fluctuations unveil globally dominant modes consistent with the power spectral density analysis in both diamond-shaped trip configurations.

Numerical Study of Combinations of Strut and Cavity in a Round Supersonic Combustor

2019 ◽  
Vol 36 (3) ◽  
pp. 219-231
Author(s):  
Wenyan Song ◽  
Dongqing Zhang ◽  
Deyong Shi
Keyword(s):  
Fuel Injection ◽  
Numerical Study ◽  
Velocity Curve ◽  
Flow Structures ◽  
Streamwise Vortices ◽  
Nozzle Outlet ◽  
Engine Thrust ◽  
Upward Transport ◽  
Center Ring

Abstract Different strut-cavity configurations and fuel injection patterns employed in a round supersonic combustor are numerically investigated. A new parameter defined as an ideal nozzle outlet velocity is used to evaluate performances of different strut-cavity configurations and fuel injection patterns. This parameter is a reflection of the contribution of the combustor performance to the engine thrust. The peak of the velocity curve stands for the best performance of the combustor. Then the flowfields are discussed in detail. The flow structures indicate that the injection from the wall behind a strut achieves a great penetration because of the strong upward transport that results from the streamwise vortices behind the strut. Results show that the combustor performance and the flowfields are greatly influenced by the fuel injection patterns. The fuel injections from the strut sides and from the center ring achieve the better performances than that from the wall behind the struts.

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