High-Pressure Combustion Phenomena

2007 ◽  
Author(s):  
Hideaki Kobayashi
Keyword(s):  
High Pressure ◽  
High Temperature ◽  
Flame Spread ◽  
Burning Velocity ◽  
Premixed Flames ◽  
Spread Rate ◽  
Flame Spread Rate ◽  
Droplet Array ◽  
Turbulent Burning Velocity ◽  
Turbulent Premixed

Two high-pressure combustion phenomena recently observed by the author’s group are reviewed. The first one is the flame spread of a droplet array in the supercritical pressure range of the fuel in microgravity. Microgravity experiments are essential for research on droplet combustion, especially at high pressure, because of the large Grashof number in normal gravity. The flame spread rate for an n-decane droplet array was measured at high pressure, and a fuel-vapor jet was found to be generated due to an imbalance of surface tension of the droplet surface, leading to a higher flame spread rate. The second phenomenon is turbulent premixed combustion at high pressure and high temperature, environmental conditions of which are very close to those in SI engines and premixed-type gas turbine combustors. Information on the flame characteristics in such conditions has been very limited. A high-pressure combustion test facility with a large high-pressure combustion chamber enabled us to stabilize turbulent premixed flames with a high turbulence Reynolds number and to perform flame observations and measurements for extended period using lasers. Turbulent burning velocity was successfully measured and significant effects of intrinsic flame instability on flame structure and turbulent burning velocity at high pressure were revealed. Effects of CO2 dilution on high-pressure and high-temperature premixed flames were also investigated to evaluate the fundamental effects of exhaust gas recirculation (EGR) in practical high-load high-pressure combustors.

Flame-Spread Behavior of Fuel-Droplet Arrays With Uneven Inter-Droplet Distance at High Temperature in Microgravity

2007 ◽  
Author(s):  
Kazuki Yagi ◽  
Hisashi Shigeno ◽  
Hiroshi Oyagi ◽  
Mikami Masato ◽  
Naoya Kojima
Keyword(s):  
High Temperature ◽  
Combustion Chamber ◽  
Waiting Time ◽  
Flame Spread ◽  
Room Temperature ◽  
Fuel Droplet ◽  
Hot Wire ◽  
Spread Rate ◽  
Flame Spread Rate ◽  
Droplet Array

This research experimentally studied flame-spread behavior of fuel-droplet arrays with uneven inter-droplet distance at high temperature in microgravity. In experiments, first, n-decane droplets were generated in room-temperature air. Next, the droplet array was inserted into the combustion chamber with 600 K air. The droplet array was ignited at one end by a hot wire igniter after a certain waiting time to initiate the flame spread. Local inter-droplet distances were varied to investigate the effect of droplet interaction on local flame-spread-limit distance. Waiting time for ignition was varied to investigate the prevaporization effect on flame spread. Experimental results showed that as the inter-droplet spacing became smaller, local flame-spread-limit distance to the next droplet became larger. The local flame-spread-limit distance at high temperature was larger than that at room temperature. Flame-spread rate from the interactive droplets to the next droplet was greater with longer waiting time for ignition.

COMPUTATIONAL FLUID DYNAMIC ANALYSIS OF COUNTERFLOW TURBULENT PREMIXED FLAMES BY THE COHERENT FLAMELET MODEL

2000 ◽  
Vol 24 (1A) ◽  
pp. 33-44
Author(s):  
E. Lee ◽  
K.Y. Huh
Keyword(s):  
Source Term ◽  
Fluid Dynamic ◽  
Burning Velocity ◽  
Premixed Flames ◽  
Turbulent Premixed Flames ◽  
Computational Fluid Dynamic Analysis ◽  
Flamelet Model ◽  
Turbulent Burning Velocity ◽  
The Relationship ◽  
Turbulent Premixed

The Coherent Flamelet Model (CFM) is applied to symmetric counterflow turbulent premixed flames studied by Kostiuk et al. The flame source term is set proportional to the sum of the mean and turbulent rate of strain while flame quenching is modeled by an additional multiplication factor to the flame source term. The turbulent rate of strain is set proportional to the turbulent intensity to match the correlation for the turbulent burning velocity investigated by Abdel-Gayed et al. The predicted flame position and turbulent flow field coincide well with the experimental observations. The relationship between the Reynolds averaged reaction progress variable and flame density seems to show a wrong trend due to inappropriate modeling of the sink and source term in the transport equation.

Iso-surface mass flow density and its implications for turbulent mixing and combustion

2007 ◽  
Vol 590 ◽  
pp. 381-409 ◽  
Author(s):  
SEUNG HYUN KIM ◽  
ROBERT W. BILGER
Keyword(s):  
Mass Flow ◽  
Burning Velocity ◽  
Premixed Flames ◽  
Flow Density ◽  
Scalar Mixing ◽  
Surface Mass ◽  
Entrainment Velocity ◽  
The Mean ◽  
Turbulent Burning Velocity ◽  
Turbulent Premixed

A new result is derived for the mass flow rate per unit volume through a scalar iso-surface – called here the ‘iso-surface mass flow density’. The relationship of the surface mass flow density to the local entrainment rate per unit volume in scalar mixing and to the local reaction rate in turbulent premixed combustion is considered. In inhomogeneous flows, integration of the surface mass flow density across the layer in the direction of the mean scalar inhomogeneity yields the mean entrainment velocity in scalar mixing and the turbulent burning velocity in premixed combustion. For non-premixed turbulent reacting flow, this new result is shown to be consistent with the classical result of Bilger (Combust. Sci. Technol. vol. 13, 1976, p. 155) for fast one-step irreversible chemical reactions. Direct numerical simulation data for conserved scalar mixing, isothermal reaction front propagation and turbulent premixed flames are analysed. It is found that the entrainment velocity in the conserved scalar mixing case is sensitive to a threshold value. This suggests that the entrainment velocity is not a well-defined concept in temporally developing mixing layers and that scaling laws for the viscous superlayer warrant further investigation. In the isothermal reaction fronts problem, the characteristics of iso-surface propagation in a low Damköhler number regime are investigated. In premixed flames, the effects of non-stationarity on the turbulent burning velocity are addressed. The difference from the existing methods for determining turbulent burning velocity, and the implications of the present results for flames with multi-dimensional complex geometry are discussed. It is also shown that the surface mass flow density is related to the turbulent scalar flux in statistically stationary one-dimensional premixed flames. Variations of the local propagation characteristics due to departure from an unstretched laminar flame structure are shown to decrease the tendency to counter-gradient transport in turbulent premixed flames.

An Analysis of Local Quantities of Turbulent Premixed Flames Using DNS Databases

2007 ◽  
Author(s):  
Kazuya Tsuboi ◽  
Shinnosuke Nishiki ◽  
Tatsuya Hasegawa
Keyword(s):  
Reaction Rate ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Premixed Flames ◽  
Three Dimensional ◽  
Flame Surface ◽  
Turbulent Premixed Flames ◽  
Density Ratios ◽  
Turbulent Burning Velocity ◽  
Turbulent Premixed

An analysis of local flame area was performed using DNS (Direct Numerical Simulation) databases of turbulent premixed flames with different density ratios and with different Lewis numbers. Firstly, a local flame surface at a prescribed progress variable was identified as a local three-dimensional polygon. And then the polygon was divided into some triangles and local flame area was evaluated. The turbulent burning velocity was evaluated using the ratio of the area of turbulent flame to that of planar flame and compared with the turbulent burning velocity obtained by the reaction rate.

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