**8^th^ International Workshop on Premixed Turbulent Flames\
**Tokyo Green Hotel Ochanomizu\
Tokyo, Japan

July 19-20, 2002

**Responses to Questionnaire**

**The responses are summarized below. Answers to each question were
copied without editing. Some respondent did not answer all question.
Parts of hand written responses cannot be transcribed due to difficulty
in deciphering handwriting. This questionnaire will be updated upon
receiving clarifications from the respondents.**

What is the main emphasis of your research?

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  **XXX**                 **XX**                  **XXXXXX**

  theory and modeling     numerical simulation    experiments
  ----------------------- ----------------------- -----------------------

What kind of problems are you interested in solving with your research?
(Please highlight or circle a number)

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  **XXXX**       **XXXXXX**     **X**          **X**          **X**

  1              2              3              4              5

  fundamental                                                 practical
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To what degree do you work directly with your
experimentalist/theoretician counterparts?

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  **X**          **X**          **XXXXXXX**    **X**          **X**

  1              2              3              4              5

  not at all                                                  joint activity
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Will your research benefit by a web-accessible database?

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  **XX**                        **XXX**        **X**          **XXXXX**

  1              2              3              4              5

  no at all                                                   highly
                                                              beneficial
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1.  ***What premixed turbulent flame problem(s) are you currently
    working on? Please provide a short description of the
    objective(s).***

Burning velocity and flame self-acceleration, with emphasis on Pr
dependence (e.g. liquid flames, astrophysical flames)

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## The three-dimensional structure of local flame-fronts in low- and high-turbulence premixed flames.

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A)  2-D Slot bunsen flame (which we argue should be in the database)
    using simultaneous

CH/PIV to

1)  measure the global turbulent burning velocity of this particular
    flame to quantify the "bending" of the curve and influence of mean
    velocity, and

2)  image the vortices crossing the reaction zone to assess the flame
    surface density balance equation, and

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B)  Measure Markstein numbers during flame-vortex interactions and
    inwardly

propagating flames to improve the database of Markstein numbers.

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Recently, I am interested in transition from laminar to turbulent flame.
Especially, I am interested in a non-steady flame motion and a chaotic
description of flame motion in the transition region.

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Direct numerical simulations of turbulent, premixed methane combustion
with additional supporting computations of vortex flame interaction. Our
focus is on study of how chemistry is modified in turbulence flame
interactions

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Modeling and numerical simulations of all terms of unclosed c-tilde
equation in premixed flames. The progress variable c-tilde, the fluc rho
c\"u\"~I~ and the turbulent burnign rate rho omega-tilde-sub-c

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We collected a large amount of experiment data on the velocity and the
scalar fields of v-flames and stagnation point stabilized flame. Our
current emphasis is the use of flame stabilized by low-swirl to access
conditions with very high u\'/S~L~ to investigate the evoluation of
flame front structures, turbulent burning rate and displacement flame
speed.

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Instantaneous flame structure of high pressure lean turbulent premixed
flames; their burning rates and stability conditions and limits

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Burning rate and flame front structure at intense turbulence. Lean
premixed turbulent flames are an attractive means of obtaining a low
polluting combustion configuration but issues of flame stability in this
regime are a problem. This work is directed towards an understanding of
the changes in flame front structure and propagation under these
conditions.

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Study of turbulent burning velocities using aqueous autocatalytic
chemical reactions

Flame propagation in narrow channels and Hele-Shaw cells

Edge flames

Flame instabilities in counterflows

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Quantify the effects of turbulence on the structure of thin reaction
sheets/flamelets. Crossed-plane imaging is being used to quantify sheet
wrinkling and thermal structure (via Rayleigh imaging). Data acquired
can be used to determine sheet surface density, estimate burning rate
and determine for what conditions flamelet based models are useful.
Crossed-plane imaging will to be combined with stereo particle imaging
velocimetry to relate wrinkling to the velocity field.

2.  ***How important is the regime diagram in guiding your research?
    Have recent experimental results (obtianed by you and by others)
    affirmed/altered your views on premixed flame classification? What
    are the open issues? Please explain.***

Recent work has led me to believe that understanding Pr dependencies
will be helpful in establishing connections between liquid flames, in
which clearcut scalings are obtained due to absence of thermal
expansion, and gaseous flames, and will also allow extrapolation of our
knowledge to astrophysical flames. I have devised a two-variable regime
diagram that facilitates study of this issue.

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A concept that leads to modifications of the current existing regime
diagrams has been recently proposed on the basis of comparison between
multiple turbulence scales and multiple flame scales. This is confirmed
by our recent experimental results. The open issue is the further
verification and investigation of the complex-strain flame-front regime.

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Do flames locally extinguish before the reaction zone itself can become
"broadened" by eddies ? Maybe there are only continuous flamelets and
shredded flamelets and nothing else. Is there any real experimental
verification that broadened reaction zones exist ?

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The regime diagram is important from not only fundamental viewpoint but
practical one. My research is in the flamelet regime and no new results
are obtained to alter the regime.

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The regime diagram plays only a modest role in our research at the
present time. This is primarily because we can computationally access
only a small range of the diagram with current computational
capabilties. As available compute resources become more powerful, the
regime diagram will play an increasingly important role.

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a)  the regime diagram is important

b)  yest the first that at increasing u\'/S~L~ takes place blow off long
    before reaching distributed (volume) mecahnisms, the second, that in
    thim along the flame we have variation of the premixed flame type:
    from the flame with increasing v~t~ and δ~t~ (initial stage) that
    with v~t~ = const and increasing δ~t~ (usually oblique flames) and
    finally purtubed convertible flame with v~t~ and δ~t~ constant.

c)  The diagram ?????

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The regime diagram provides a basic guideline for us to assess and
compare different experiments and theories. As discussed in the 1^st^
Workshop at Berkelry, the diagram leaves out many important aspects of
premixed turbulent flames, e.g. boundary conditions, initial T & P,
flame configurations etc. These facts needs to be carefully considered
when using the regime diagram as a means to pose a research problem.
Recognizing that the criteria are hypothesis that has yet to be proven,
we have designed experiments to tests the validity of the
Klimov-Williams criterion. Our recent studies have shown that the
Klimov-Williams criterion represents a statistical probability that the
thin flamelet may be disturbed by small intense turbulence. It should
not be interpreted as a boundary across which the flame structures
suddenly change.

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We need to look at the possible modifications of the regime diagram
under high pressure conditions, through the effects of the pressure both
on the turbulence structure and instantaneous flame structure.

Recent Rayleigh scattering imaging results (2D and normal pressure) of
lean methane-air turbulent premixed flames (open conical or bunsen type)
at the LCSR show that the thickening of the instantaneous flame front
only occurs under the coupled effect of turbulent strain and curvature
effects. Turbulent strain when acts alone has a thinning effect on the
instantaneous flame front (depicted by determining the instantaneous
temperature gradients across the instantaneous flame front-flamelet), as
also shows numericla simulations of laminar strained flames. When the
curvature and strain act together, both the maximum temperature gradient
and those corresponding to the preheating zone are reduced implying a
thickening of the instantaneous flame front compared to its strained
laminar counterpart.

3D measurements under high pressure conditions are needed, also under
intense turbulence conditions, to confirm (or not) such results,
important for turbulent premixed flame regimes and their
characteristics.

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They are important. The theoretical work of Peters and experiment and
analysis at Aachan and LBNL have shown that the corrugated flame regime
when characterized by the reaction zone thickness persists well into the
conventional 'distributed reaction' regime. This finding is significant
because it indicates that the well established analysis tools developed
in low turbulence flames will be applicable over a much wider range of
conditions.

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The regime diagram is very important in my research since I am
interested primarily in how non-Huygens propagation behavior occurs and
what effects it has on turbulent burning velocities.

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Current regime diagram is not very useful in guiding research for at
least two reasons. 1) It focuses attention on the distinction between
flamelets and distributed reaction zones. These concepts are modeling
concepts and the drawing of lines on a diagram to set off these regions
is artificial. 2) The regime diagram focuses attention on a limited set
of independent variables and thereby obscures the potential importance
of other variables, e.g., Markstein number.

3.  ***Which of these parameters: local heat release rate, local flame
    front propagation velocity, turbulent burning speed, flame surface
    density, burning rate integral, and consumption speed are you
    investigating or have investigated? Please support your answer with
    brief description of your*** ***approach and initial conditions.***

I consider mainly the turbulent burning speed though I have done some
previous work related to flame surface density. I use various stochastic
modeling approaches. (AK)

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We have recently measured the mean 3-D flame surface density function in
hydrocarbon/air and lean hydrogen/air Bunsen flames using the two-sheet
two-dimensional Rayleigh scattering technique. (YCC)

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We believe that one useful parameter is the global turbulent burning
velocity -- it is not universal but is associated with a specific
geometry (a 2-D slot bunsen flame in our case). It is the total mass
flow rate of reactants consumed, divided by the area of the
time-averaged location of the flame (properly defined) and divided by
the density of the reactants.

We also believe that local parameters such as burning integral also are
important, so long as one realizes that they too are geometry-specific.

The global burning velocity is analogous to the length of a nonpremixed
jet flame, while the local burning integral is analogous to profiles at
different positions. Both are important for assessing numerical
simulations.

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I have estimated consumption speed in a periodic variation in the
approaching speed of a premixed mixture.

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For the numerical simulations we have done we have investigated all of
the above parameters except the local flame propagation velocity. Local
flame propagation velocity would be useful but is more difficult to
define accurately.

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1)  local flame front propagation velocity and thicknesses

2)  Thickened flamelet sheet area

3)  Turbulent burning rate (local averaged combustion (heat release)
    intensity

4)  turbulent burning speed

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We have measured all of the above.

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All these parameters are important and investigated; however they are
also all inter-related. Therefore unified appraoches are needed in order
to clarify the present situation.

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All

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Turbulent burning speed -- in Taylor-Couette flow and Hele-shaw cells

Also using autocatalytic chemical reactions

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We have used crossed-plane imaging to study surface density, burning
rate integral and local mean heat release rate. We also are looking at
instantaneous thermal structure via crossed-plane Rayleigh imaging.
Measurements to date have been in V-flames. They will be extended to
Bunsen and swirl-stabilized flames.

4.  ***Which premixed turbulent flame configuration do you work with?
    Does your configuration (or configurations) involve fully developed
    turbulence or simplified flame vortex interactions? How does your
    configuration (configurations) serve your scientific goals?***

I consider fully developed turbulence in order to investigate the
scaling laws for this regime.

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## • hydrocarbon/air and lean hydrogen/air Bunsen flames,• hydrocarbon/air stagnation-type flames 

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2-D Slot Bunsen Burner with turbulence producing grids.

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The flame is formed in non-steady state flow conditions.

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Most of our work focuses on direct numerical simulation of the
interaction of decaying isotropic turbulence with an initially laminar
flame. This is not an ideal configuration because it is not
experimentally realizable; however, it is the simplest configuration to
study computationally. (JBB)

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Mainly turbulent premixed combustion at hith Re~t~ numbers (large scale
high velocity combustion) large-scale gasturbine combustors.

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All our configurations have fully developed turbulence and close to
isotropy. This allows us to compare the turbulence length scales with
flame wrinkle length scales directly.

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Essentially open conical (Bunsen type) turbulent premixed flames with
grid generated turbulence, under various turbulence, mean flow,
equivalence ratio and pressure conditions.

High flow rate, high air temperature and high pressure turbulent
premixed flame studies are required for further progress.

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Burner stabilized flames with quasi-isotropic turbulence. The use of the
low swirl stabilizer on a slot burner has allowed a wide Ka and Da space
to be investigated.

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Taylor-Couette, Hele-Shaw, vibrating grid and capillary wave flows (the
last for aqueous autocatalytic chemical reactions only, obviously)

It has always amazed me that practically all turbulent combustion
studies are done using flows that are very difficult to characterize and
are inhomogeneous and anisotropic. This is especially surprising
considering that the fluid mechanics community has characterized the
aforementioned flows to death in thousands of papers. Moreover,
Taylor-Couette and vibrating grid flows in particular provide very
homogeneous and isotropic turbulence over many integral scales and so
quasi-steady burning velocities (if such a thing exists) can be
measured.

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V-flames in grid turbulence have been studied. This configuration has
good optical access and is easy to use. Measurements show that flamelet
wrinkling and burning rate are functions of distance from the flame
stabilizer rod. It is likely that wrinkling and the burning rate will
vary with position in different ways in different flame configurations.

5.  ***In your opnion, what type of experimental data are lacking? How
    will these measurements contribute to
    theoretical/modeling/simulation development?***

A freely propagating flame in planar geometry would contribute to
answering the question I pose in question 12 below. I don\'t know how to
create such a flame.

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Measurements of the local flame propagation velocity.

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We need to identify several "canonical" geometries and discuss them with
the DNS/LES people. Can they be modeled in the forseeable future ?

Role of pressure gradients may cause problems in the simulations, and
may be more important in all of our experiments than we have thought.

The degree of wrinkling typically changes from point to point in many
flames -- we should develop parameters we can measure to quantify this,
and make sure the simulations predict the correct degree of flame
wrinkling.

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The turbulence Reynolds number (of experiments) is too low !!

I am doing LES of premixed combustion for gas turbine operational
conditions for which only limited data (some LDV but mostly pressure,
i.e., combustion dynamics, and emission, primarily extracted) is
available ONLY from industry (academia is non-existent). The flame
structure and regime covers the ENTIRE Borghi diagram (including partial
premixing since there is variation in premixing at the inflow). Also,
the flow is highly turbulent (Re\_\\lambda \>\> 1000 in some regions,
u\'/S_L \>\> 20) in the flame region, the inflow is highly swirling
(Swirl number \> 0.7), the inflow is heated (600-800K), the pressure is
relatively high (10-20 atm), AND acoustic effects (i.e.,
compressibility) are NOT negligible since there is significant
combustion dynamics (downstream boundary is nearly choked or choked). If
there is any data in this area I would love to see it.

My subgrid model is designed for high Re flows and for flows where
acoustics play a major role in the combustion dynamics. However, most
lab data is for relatively low Re, low speed (zero-Mach) flows and in
nearly all cases, not very well defined inflow conditions.

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My recent experience indicates that some chaotic analysis will be useful
to predict near future motion of the flame condition (flame motion etc.)
in a flamelet regime.

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Our work would benefit substantially from performing small-scale
turbulent flame interactions in extremely simple geometries. It should
be possible to design an experiment that is accessible to direct
numerical simulation. Ideally for such an experiment the measurements
would include not only flame morphology but also detailed chemical
measurements of the flame.

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Most if not all current theories involving RANS are based on Favre
averaging. Data involving Reynolds averaging is of little use in
assessing such theories. The need for Favre averages in assessing the
results of DNS and LES calculations is unclear so I focus on RANS
assessments for which experimentalists must make the extra effort
required to measure the velocity and state variables at the same point
in space and time so that Favre averages are provided. I know that this
requirement poses problems for experimentalists but if Bray-Moss
thermochemistry is assumed to apply, a good assumption for many premixed
turbulent flows, then there are relatively simple techniques for
providing the required experimental conincident scalar and velocity
data. I am configdent your colleagues know the various strategies which
can be used to this end; I urge here that they bring them to bear so
that their results will be of some use other than keeping librarians
busy.

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May be investigations of local extinctions of the flamelet sheets at
strong turbulence. In general investigation of the combustion mechanism
at regimes near flow-off boundary. What is the reason of the ??? effect?
Local extinctions only responsible for the blow-off?

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Premixed turbulent flame propagation in very high speed flows.

Blow-off mechanisms

Flames under high pressured

Changes in flamelet structures with highly preheated reactants.

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Data on the distributions of instantaneous flame front thicknesses (or
temperature gradients) coupled with flame curvature and strain rate
obtained with 3D visualisation techniques are needed, again mainly under
high pressure and intense turbulence conditions.

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LES calculations of turbulent premixed flames should be further
developed

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Instantaneous flame front structure in highly turbulent flows. 3D
measurements would provide a data base directly to test models (e,g,
DNS)

PTF at high pressures.

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Measurements of quenching limits as a function of pressure, ignition
energy and radiative properties (Shy has made a step in the direction of
the last of these). See 12 for its contributions.

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V-flames in grid turbulence have been studied. This configuration has
good optical access and is easy to use. Measurements show that flamelet
wrinkling and burning rate are functions of distance from the flame
stabilizer rod. It is likely that wrinkling and the burning rate will
vary with position in different ways in different flame configurations.

6.  ***What types of experimental/theoretical/modeling/simulation
    results have you obtained? Have you compared your results with
    experiments or calculations? How will you use the
    experimental/theoretical data obtained by others if they become
    accessible on the web?***

Not very active in this area recently.

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The three-dimensional structure of local flame-fronts in hydrocarbon/air
and lean hydrogen/air Bunsen flames, and hydrocarbon/air stagnation-type
flames have been measured. These data should be useful for comparison
with numerical predictions.

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My results are some chaotic characteristics of Bunsen type flame.

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none

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We have obtained DNS data for a lean, premixed methane flame interacting
with decaying isotropic turbulence using DRM19 which is a reduced form
of GRIMech with 19 reacting species. We have done some limited
comparisons with flamelet theory. We would be particularly interested in
experimental data available on the web.

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Numerical simulations of open and impinging flames. The main attention
we pay to correct prdcition of the combustion rate rho omega sub c and
the transport rho u\"c\" overbar (prediction of the counter-gradient
transport phenomenon.

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None

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See Question 6 above. A unique format to present both experimental and
numerical results to stimulate comparisons would indeed be enhanced by
sharing them on the web.

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Where possible comparisons have been made with models. Such comparisons
wiuld be facilitated by use of the web.

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Turbulent burning velocity, quenching limits, flame shapes

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As stated in 5 above: Quantify the effects of turbulence on the
structure of thin reaction sheets/flamelets. Crossed-plane imaging is
being used to quantify sheet wrinkling and thermal structure (via
Rayleigh imaging). Data acquired can be used to determine sheet surface
density, estimate burning rate and determine for what condition flamelet
based models are useful. Crossed-plane imaging to be combined with
stereo particle imaging velocimetry to relate wrinkling to the velocity
field.

A web data base will make it much easier for me to track the work of
others and make it easy for all researchers to compare results from
different groups and data from different flame configurations. Given the
large amount of work on premixed turbulent combustion, a web database
will help to focus efforts and accelerate progress.

7.  **Are you willing to share you results on the web?**

Yes x 7

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We have done so. Please check the following website:
<http://www.itm.RWTH-Aachen.DE/Downloadarea/Flammendaten/flammendaten.html>

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We would be happy to share results of our computations on the web.
Furthermore, if DNS results for tractable geometries and mechanisms are
needed by the community we could perform the computations and make data
available over the web. (One caveat in this is that the datasets for
three-dimensional time-dependent flows are quite large (terabytes) and
often difficult to manipulate.)

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Yes. The LCSR resluts will also be available soon on the LCSR web site.

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8.  **In you opinion, what other open issues need to be addressed by the
    research community? Are these issues common or different for the
    different flame configurations, initial and boundary conditions? How
    can these issues be resolved experimentally, numerically and
    theoretically?**

Is there an intrinsic turbulent burning velocity, or is it inherently
configuration dependent? I don\'t have any great ideas about how to
study this.

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Dependency of the turbulent flame speed on boundary conditions.

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We need to get away from (in my opinion) the common formulas that imply
that there is a universal relation between burning velocity to turbulent
velocity fluctuations. For example, it is erroneous to use bunsen flame
data to explain the propagation of the base of lifted nonpremixed flame
(the 'edge' flame), as people have done. The boundary conditions lead to
very different degrees of wrinkling in the two cases. Spherical flames
in engines get more wrinkled as time goes on, even though u' may be
constant -- so the common formulas are not applicable. (JFD)

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As mentioned above, chaotic viewpoint will open new method to predict
near future of the premixed flame.

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A key open issue is the quantitative assessment of how well turbulence
chemistry models capture flame dynamics and their degree of chemical
fidelity. For example, to what degree can we accurately predict
emissions from a turbulent premixed flame.

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I think that several experimental ??? must be fixed as the standard date
for testing of premixed combustion model to check correct prediction of
trends of the models (dependence of phi, T, turbulence, type of fuel).
What set of experimentla data should be chosen is a food topic for
discussion.

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Whether or not the knowledge we gained from studying steady-state
stretched laminar premixed flame can actually be use in modeling
premixed turbulent flames (i.e. the foundation of the stretched lamianr
flamelet library concept). In a truly turbulent flow, the strain field
changes so rapidly that the flamelets may not have time to respond. It
is almost impossible (given the tools we have right now) to investigate
this experimentally. DNS can be very useful for looking into this issue.

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Premixed Turbulent Flame investigations under high pressure, high
premixing air temperature and high mass flow rate conditions are
necessary for meaningful results transferable to real situations. A link
should also be established between turbulent premixed flame studies and
prevaporised, premixed flames, essentially under lean conditions.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

See above. More development of the results obtained by the community for
application to practical systems. Buoyancy and pressure field effects.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

What are the mechanisms of flame quenching by turbulence? That is, why
doesn't increasing turbulence intensity indefinitely increase turbulent
burning velocity indefinitely?

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There are several. I would like to raise one. How useful are old flame
speed/burning velocity data and correlations based on these data? Since
recent findings suggests that these quantities depend on the flame
configuration and on space/time, I am not sure there is much of value in
these data.

9.  **What important insights have you gained from your recent studies
    of premixed turbulent flames? Have these new insights been helpful
    for you to define your research direction and goal? Please
    elaborate**

Nothing remarkable recently.

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Turbulent premixed combustion involves multiple scales of turbulence as
well as of flame structure. They can not be simplified to comparison
between one turbulent scale and one flame scale.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

The bending of the burning velocity curve does occur if you keep mean
velocity constant and vary u' independently -- this appears to be due to
a residence time effect.

The CH reaction zones seem to retain their laminar-flamelet identity
even for fairly large turbulence levels, and there is not any local
extinction since premixed flames are much harder to locally extinguish
than nonpremixed flames.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Perhaps the most important thing we have learned from our recent work is
not an insight but an anti-insight. In particular, we have found that
the mechanisms by which flame chemistry is modulated by turbulence are
extremely complex. It is difficult to relate observations about the
flame to the behavior that caused them. This has led us to pursue a
number of mathematical analysis tools aimed at diagnosing the behavior
of a turbulent flame.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

We understand that the conunter-gradient transport phenomenon is not a
turbulent but a gas dynamical phenomenon that there are two mechanisms
(turbulent and gas dynamical) that control rho u\"c\" and the balance
between them control transition from gradient to counter-gradient
transport ofr c-tild but the ??? of a ???? passive concentration in
premixed flame is gradient

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Our study of low-swirl burner has clearly shown that the bending effect
of S~T~ does not occur in this configuration. Otherwise, the flame would
have blown-off when velocity is increased. Therefore, we need to look at
S~T~ not as a \"universal\" property but one that is highly dependent on
flame configuration and perhaps initial conditions. This observation has
prompted us to investigate premxied flame propagation in high speed
flows.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Structure of instantaneous flame fronts

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

The structure of instantaneous flame fronts and turbulent flame
propagation.

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All flames in confined geometries (thus most practical flames) propagate
at about 3 SL due to wrinkling induced by Darreius-Landau instability,
but most experiments are conducted in open geometries where thermal
expansion is relaxed in the third dimension. It's not clear to me to
what extent such experiments will apply to real, practical flames.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Our measurements in SI engine flames and V-flames of isoscalar surface
normal vectors suggest that the probability density function of the
vectors has a universal form and is defined by a single parameter. These
measurements have been made with crossed-plane laser tomography,
crossed-plane acetone LIF and crossed-plane Rayleigh imaging.

10. In your opinion what is the single most important development in the
    last four years?

Hard question

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Modification of the combustion regime diagram based on the discussions
in \"Simultaneous 2-D imaging measurements of reaction progress variable
and OH radical concentration in turbulent premixed flames: instantaneous
flame-front structure,\" Combust. Sci. Tech. 167: 187-222 ( 2001 ).

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

The realization that we should treat most of the previous burning
velocity data as qualitative only and now repeat the entire data set
using modern diagnostics and a few canonical burners. Diagnostics
recommened include tomography, PIV, stereo PIV, Cinema PIV, CH/PIV,
OH/PIV.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Probably not single most important but . . .Algorithm and compute power
have evolved to the point that we can now perform DNS simulations for
simple fuels in simple (and small) configurations with detailed
kinetics.

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Experimental data that confirmed more than 20 years old theoretical
prediction that in spite of coutinuous spectrum of turbulence at y \<
δ~L~ combustion takes place in thin (through more thick in comparison
with δ~L~) and strongly wrinkled flamelet sheet, i.e. in spite of
continuing ε(k) thickening has a limit (Y.Chen, M.S. Mansour, 27^th^
Symp.) In another work, direct contrication existstence of thickened
flamelets.

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Can\'t name a single development but there are several noteworthy new
insights:

The acckonwledgement that S~T~ is not universal.

The regime diagram needs to be modified.

Counter gradient only occurs at low turbulence

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Various experimental and numerical studies on the structure of
instantaneous flame fronts

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Experimental and theoretical investigations of flame front structure.

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Hmmm...

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I could not pick one single one.

**Additional comment of Prof. V. Zimont on Regime Diagram**

Dear Professor Cheng,

From you questionnaire I could understand that there is interest to the
regime diagram. In this connection I would like to let you know about
the prehistory of this question in

Russia that influenced on the West interpretation of this point.

1.  Damköhler and . Shchelkin ( Damköhler, G., NACA Tech. Memo. 1112
    **(**1947**)** and Shchelkin, K.I., NACA Tech. Memo. 1110
    **(**1947**))** discussed the case of strong turbulence
    $u^{'}\text{>>}s_{L}$ and proposed that $U_{t} \approx u^{'}$, i. e.
    the flame speed does not depend directly on chemistry . (From my
    point of view this result is physically correct but it refer only to
    practically unachievable 1D stationary flames, in this case
    $U_{t}$is equal to the leading points, Zel\'dovich term, velocity
    controlling by hydrodynamics and in fact is equal to the speed of
    turbulent diffusion front velocity that also $u^{'}$. Real flames
    are transient with increasing brush width and their combustion
    velocity $U_{t}$(integral reactants consumption rate per the area
    unit) depends both on chemistry and turbulence and strictly speaking
    does not correlate with the flame front edge speed. This remark
    refers also to the impinging flames that only look like 1D
    stationary flames but are also in fact the transient flames but this
    point for serious discussion.)

2.  So the main feature of actual flames at strong turbulence
    ($u^{'}\text{>>}U_{f})$ is nearly constant $U_{t}$ and increasing
    brush width. A. G. Prudnikov probably was the first who thoroughly
    investigate the end of 50th\[5\] such flames and he experimentally
    showed that increasing of $\delta_{t}$ took place in accordance with
    the turbulent diffusion law and is controlled by turbulent diffusion
    coefficient of reactants flow (in spite of the counter-gradient
    transport revealed later), there is his publication in the 7th
    International Combustion Symposium but the main results were
    published in Russian. It is in fact transient, intermediate flames
    between initial stage of forming of developed turbulent flame and
    final stage of stationary flame (the last is a matter for many
    theoretical enquiries). We called these flames Intermediate Steady
    Propagation (ISP) flames and relevant regime the ISP combustion
    regime\[4\]. There are strong grounds (theoretical and experimental)
    for believing that at these flames constitute well-defined class of
    flames and that proper intermediate asymptotic theory could be
    adequate tool for their description, our combustion model is in fact
    an attempt to develop this tool. We explain this paradoxical
    peculiarities of premixed flames in simple aerodynamic situation by
    the fact that in actual transient flames the large-scale wrinkles
    are nonequilibrium so the brush width is growing while the
    small-scale wrinkles giving the main contribution in the sheet area
    and hence in $U_{t}$ is quasi-equilibrium.

3.  Damköhler analyzed two extreme cases: turbulence contains only large
    eddies with the size $l\text{>>}\delta_{L}$, in this case
    $U_{t} \approx u^{'}$, and only small eddies with the size
    $l\text{<<}\delta_{L}$, in this case
    $U_{t} \approx (\frac{D_{t}}{\tau_{\text{ch}}})^{\frac{1}{2}}$. In
    Russia discussed the question about a combustion mechanism at strong
    developed turbulence when the integral scale $L$, equal to the
    characteristic size of large eddies, is $L\text{>>}\delta_{L}$ and
    at the same time the Kolmogorov microscale of turbulence
    $\eta = L \cdot \text{Re}_{t}^{\frac{- 3}{4}}\text{,Re}_{t} = u^{'}\frac{L}{\nu}$
    equal to the characteristic size of minimal eddies, is
    $\eta\text{<<}\delta_{L}$. There were two point of view:

    a.  The instantaneous flame (a flamelet in modern terminology) with
        increase his width and finally combustion would be distributed,
        the volume or stirred reactor type combustion mechanism.

    b.  The flame thickening has a limit and combustion takes place in
        thickened and strongly wrinkled flame surfaces; A. Prudnikov
        believed in this mechanism and Shchelkin share this point of
        view. He wrote in his book (K.I.Shchelkin and Ya. K. Troshin,
        Gasdynamics of Combustion, Moscow, 1963) \"By returning to
        fine-scale turbulence, we recall the discussion found in the
        literature which is transform into a good example of sophistry
        when it is not accompanied by quantitative estimates. The
        discussion is as following. Fine-scale turbulence, which is
        always present in the spectrum of scales, extends the normal
        flame combustion zone. Coarser scales become small relative to
        the expanded zone. By falling into the combustion zone, they
        extend it still more, etc. All the scales gradually are drawn
        into the combustion zone. Hence, there is no sense in comparing
        turbulence scales to the width of the normal combustion zone,
        which is not and cannot be in turbulent flame.\"

4.  As far as I know I was the first (and Prof. Borghi recently
    confirmed it) who made these quantitative estimations and published
    it in Russian, the English translation published in USA was: V. L.
    Zimont \"Theory of turbulent combustion of a homogeneous fuel
    mixture at high Reynolds numbers\", Combustion, Explosion, and Shock
    Waves, vol. 15, no.3, Nov. 1979, pp. 305-311 ( Many results of this
    paper are rewritten in Zimont, V.L., Experimental Thermal and Fluent
    Science 21: 179-186, 2000). This paper was based on the general
    Kolmogorov method: unresolved small-scale processes could be
    expressed in terms of large-scale parameters by assumption of
    equilibriums of small-scale structures (in our case they where the
    equilibrium structure of the thickened flamelet and the equilibrium
    structure of the flamelet sheet small-scale wrinkled giving the main
    contribution in the flamelet sheet area and hence in $U_{t}$. I
    showed that

    a.  The flamelet thickening has a limit in spite of continuous
        turbulence spectrum$$and that at $\text{Da}\text{>>}1$thickened
        flamelet width is $\delta_{f}\text{<<}L$, and I showed the
        physical reason of it: in a coordinate system connected with the
        thickened flamelet the heat fluxs due to microturbulence and and
        convection, and heat release because of chemical reaction have
        the same order of magnitude. I derived formulas for the flamelet
        speed, width, the scale, pulsation and transport coefficient
        inside the flamelet
        $U_{f}$$u^{'} \cdot \text{Da}^{\frac{- 1}{2}},$$\delta_{f}$$L_{f} \approx L \cdot \text{Da}^{\frac{- 3}{2}}$,$u_{f}^{'} \approx u^{'} \cdot \text{Da}^{\frac{- 1}{2}}$,$\chi_{f}$$D_{t}(\text{Da})^{- 2}$,
        and showed boundaries for this mechanism:
        $\text{Da}^{\frac{3}{2}}\text{Re}_{t}\text{<<}1\text{<<}\text{Da}^{\frac{1}{2}}$,
        for fixed $\text{Re}_{t}$ at increasing of $\text{Da}$we have
        wrinkled laminar flamelet, at decreasing of $\text{Da}$we have
        decreasing of wrinkling and finally distributed combustion.
        Borghi gave these boundaries in terms of $\frac{u^{'}}{s_{L}}$
        and $\frac{L}{\delta_{L}}$his diagram introduced in 1985 in:
        Recent Advances in Aerospace Sciences, Pergamon, Oxford, pp.
        117-138. Borghi was corrected and he cited my 1979 year paper.
        But after this the origin of this diagram was forbidden, though
        Borghi never analyzed thickened and wrinkled flamelet and never
        used this concept for modeling.

    b.  It should be stressed that my analysis was not complete, it
        contained an error as well as hence the Borghi diagram: in
        reality at increasing of $u^{'}$ long before formation of
        distributed stirred reactor type mechanism usually takes place
        the flamelet extinction and blow-off, in the mentioned paper
        2000 I added this boundary.

    c.  I am sure that Peters thin reaction combustion regime is
        identical to our thickened wrinkled flamelet regime, at least
        his expression for $\delta_{f}$ is identical to our formula (as
        far as know Peters did not analyzed $U_{f},u_{f}^{'},\chi_{f}$
        so we cannot compare results).

    d.  Very interesting that the Kolmogorov method gives an opportunity
        to derive expression not only for this parameters, but for the
        model of ISP flames ($\delta_{t}$ is growing in accordance with
        the turbulent diffusion law while $U_{t} = \text{const}$)
        expression for the flamelet sheet area and hence for $U_{t}$:
        $(\overline{S}/S_{0})$$\left( u^{'}/U_{f} \right)^{\frac{3}{2}} \approx \left( \text{Da} \right)^{\frac{3}{4}}$,
        $U_{t} = U_{f}(\overline{S}/S_{0}) = A \cdot u' \cdot \text{Da}^{\frac{1}{4}} = A \cdot u'^{\frac{3}{4}}U_{l}^{\frac{1}{2}}$$\chi^{\frac{- 1}{4}}L^{\frac{1}{4}}$.
        Very significant that all mentioned formulas are not
        approximations but they follows from some physical assumptions
        that results theoretical meanings of all exponents (similar to
        Kolmogorov formulas for the dissipation rate
        $\varepsilon \approx \frac{u^{'3}}{L}$ and
        $E(k) \approx \varepsilon^{\frac{2}{3}}k^{\frac{- 5}{3}}$ .

5.  From my point of view the diagram must contain also the time (or
    coordinate) axis that shows time or space distance from the premixed
    flame beginning that would added two additional boundaries for both
    laminar or thickened flamelet combustion mechanisms that separate

    a.  Initial stage of combustion: nonequilibrium both large-scale and
        small-scale wrinkles resulting fast increasing of $U_{t}$ and
        $\delta_{t}$. This regime is significant only for spark ignition
        engines;

    b.  Intermediate asymptotic stage: nonequilibrium large-scale and
        quasi-equilibrium small-scale wrinkles resulting increasing
        brush width and at the same time nearly constant $U_{t}$. It is
        observed premixed flames. We believe that at these flames (in
        fact moving turbulent mixing layers) constitute well-defined
        class of flames

    c.  Final stage when both large-scale and small-scale wrinkles are
        equilibrium, 1D stationary flames. This flames practically
        unattainable.

6.  I have impression (may be wrong, I have no relevant literature here)
    that pure stirred reactor regime is not attainable and at real
    stirred reactor experiment in fact took place flamelet combustion
    with very wrinkled flamelet sheet.