scholarly journals Comparison of Flame Propagation Statistics Extracted from Direct Numerical Simulation Based on Simple and Detailed Chemistry—Part 1: Fundamental Flame Turbulence Interaction

Energies ◽  
2021 ◽  
Vol 14 (17) ◽  
pp. 5548
Author(s):  
Felix Benjamin Keil ◽  
Marvin Amzehnhoff ◽  
Umair Ahmed ◽  
Nilanjan Chakraborty ◽  
Markus Klein
Keyword(s):  
Numerical Simulations ◽  
Flame Propagation ◽  
High Performance ◽  
Three Dimensional ◽  
Quantitative Agreement ◽  
Combustion Regime ◽  
Computing Methods ◽  
Direct Numerical Simulations ◽  
Tangential Strain ◽  
Detailed Chemistry

In the present study, flame propagation statistics from turbulent statistically planar premixed flames obtained from simple and detailed chemistry, three-dimensional Direct Numerical Simulations, were evaluated and compared to each other. To this end, a new database was established encompassing five different conditions on the turbulent premixed combustion regime diagram, using nearly identical numerical methods and the same initial and boundary conditions. A detailed discussion of the advantages and limitations of both approaches is provided, including the difference in carbon footprint for establishing the database. It is shown that displacement speed statistics and their interrelation with curvature and tangential strain rate are in very good qualitative and reasonably good quantitative agreement between simple and detailed chemistry Direct Numerical Simulations. Hence, it is concluded that simple chemistry simulations should retain their importance for future combustion research, and the environmental impact of high-performance computing methods should be carefully chosen in relation to the goals to be achieved.

Instability of strained vortex layers and vortex tube formation in homogeneous turbulence

1995 ◽  
Vol 282 ◽  
pp. 313-338 ◽  
Author(s):  
T. Passot ◽  
H. Politano ◽  
P.L. Sulem ◽  
J.R. Angilella ◽  
M. Meneguzzi
Keyword(s):  
Numerical Simulations ◽  
Vortex Tube ◽  
Three Dimensional ◽  
Quantitative Agreement ◽  
Finite Amplitude ◽  
Tube Formation ◽  
Direct Numerical Simulations ◽  
Homogeneous Turbulence ◽  
Tubular Structures ◽  
Vortex Tubes

A modulational perturbation analysis is presented which shows that when a strained vortex layer becomes unstable, vorticity concentrates into steady tubular structures with finite amplitude, in quantitative agreement with the numerical simulations of Lin & Corcos (1984). Elaborated three-dimensional visualizations suggest that this process, due to a combination of compression and self-induced rotation of the layer, is at the origin of intense and long-lived vortex tubes observed in direct numerical simulations of homogeneous turbulence.

Fuel wall film effects on premixed flame propagation, quenching and emission

2018 ◽  
Vol 21 (6) ◽  
pp. 1055-1066 ◽  
Author(s):  
Mingyuan Tao ◽  
Haiwen Ge ◽  
Brad VanDerWege ◽  
Peng Zhao
Keyword(s):  
Flame Propagation ◽  
Direct Injection ◽  
Flame Structure ◽  
Three Dimensional ◽  
Premixed Flame ◽  
Practical Significance ◽  
Detailed Chemistry ◽  
Rich Mixture ◽  
Wall Film ◽  
Flame Quenching

The formation of fuel wall film is a primary cause for efficiency loss and emissions of unburnt hydrocarbons and particulate matters in direct injection engines, especially during cold start. When a premixed flame propagates toward a wall film of liquid fuel, flame structure and propagation could be fundamentally affected by the vaporization flux and the induced thermal and concentration stratifications. It is, therefore, of both fundamental and practical significance to investigate the consequent effect of a wall film on flame quenching. In this work, the interaction of a laminar premixed flame and a fuel wall film has been studied based on one-dimensional direct numerical simulation with detailed chemistry and transport. The mass and energy balance at the wall film interface have been implemented as boundary condition to resolve vaporization. Parametric studies are further conducted with various initial temperatures of 600–800 K, pressures of 7–15 atm, fuel film and wall temperatures of 300–400 K. By comparing the cases with an isothermal dry wall, it is found that the existence of a wall film always promotes flame quenching and causes more emissions. Although quenching distance can vary significantly among conditions, the local equivalence ratio at quenching is largely constant, suggesting the dominant effects of rich mixture and rich flammability limit. By further comparing constant volume and constant pressure conditions, it is observed that pressure and boiling point variation dominate the vaporization boundary layer development and flame quenching, which further suggests that increased pressure during compression stroke in engines can significantly suppress film vaporization. Emissions of unburnt hydrocarbon, soot precursor and low-temperature products before and after flame quenching are also investigated in detail. The results lead to useful insights on the interaction of flame propagation and wall film in well-controlled simplified configurations and shed light on the development of wall film models in three-dimensional in-cylinder combustion simulation.

scholarly journals Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence

2008 ◽  
Vol 602 ◽  
pp. 175-207 ◽  
Author(s):  
L. E. JONES ◽  
R. D. SANDBERG ◽  
N. D. SANDHAM
Keyword(s):  
Numerical Simulations ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Transition Process ◽  
Direct Numerical Simulations ◽  
Transition To Turbulence ◽  
Absolute Instability ◽  
Laminar Separation ◽  
Velocity Magnitude ◽  
Separation Bubbles

Direct numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Rec=5×104 and incidence 5° are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles.

Direct Numerical Simulations of Transitional Separation–Bubble Development in Swept–Blade Flow Conditions

2012 ◽  
Author(s):  
Joshua R. Brinkerhoff ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Numerical Simulations ◽  
Shear Layer ◽  
Pressure Field ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Direct Numerical Simulations ◽  
Small Scale ◽  
Flow Conditions ◽  
Crossflow Instability

This paper describes numerical simulations of the instability mechanisms in a separation bubble subjected to a three-dimensional freestream pressure distribution. Two direct numerical simulations are performed of a separation bubble with laminar separation and turbulent reattachment under low freestream turbulence at flow Reynolds numbers and streamwise pressure distributions that approximate the conditions encountered on the suction side of typical low-pressure gas-turbine blades with blade sweep angles of 0° and 45°. The three-dimensional pressure field in the swept configuration produces a crossflow-velocity component in the laminar boundary layer upstream of the separation point that is unstable to a crossflow instability mode. The simulation results show that crossflow instability does not play a role in the development of the boundary layer upstream of separation. An increase in the amplification rate and most amplified disturbance frequency is observed in the separated-flow region of the swept configuration, and is attributed to boundary-layer conditions at the point of separation that are modified by the spanwise pressure gradient. This results in a slight upstream movement of the location where the shear layer breaks down to small-scale turbulence and modifies the turbulent mixing of the separated shear layer to yield a downstream shift in the time-averaged reattachment location. The results demonstrate that although crossflow instability does not appear to have a noticeable effect on the development of the transitional separation bubble, the 3D pressure field does indirectly alter the separation-bubble development by modifying the flow conditions at separation.

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