ArticlePDF Available

Unexpected Propagation of Ultra-Lean Hydrogen Flames in Narrow Gaps


Abstract and Figures

Very lean hydrogen flames were thought to quench in narrow confined geometries. We show for the first time how flames with very low fuel concentration undergo an unprecedented propagation in narrow gaps: H2-air flames can survive very adverse conditions by breaking the reaction front into isolated flame cells that travel steadily in straight lines or split to perform a fractal-like propagation that resembles the pathway of starving fungi or bacteria. The combined effect of hydrogen mass diffusivity and intense heat losses act as the two main mechanisms that explain the experimental observations.
Content may be subject to copyright.
Unexpected propagation of ultra-lean hydrogen flames in narrow gaps
Fernando Veiga-L´opez1,Mike Kuznetsov2, Daniel Mart´ınez-Ruiz3,
Eduardo Fern´andez-Tarrazo1, Joachim Grune4, and Mario S´anchez-Sanz1
1Dpto. de Ing. ermica y de Fluidos, Universidad Carlos III de Madrid, 28911, Legan´es, Madrid, Espa˜na
2Institut f¨ur Kern- und Energietechnick, Karlsruhe Institut f¨ur Technologie, 76344, Eggenstein-Leopoldshafen, Deutschland
3ETSIAE., Universidad Polit´ecnica de Madrid, Plaza del Cardenal Cisneros 3, 28040, Madrid, Espa˜na and
4Pro-Science GmbH, Parkstrasse 9, 76275, Ettlingen, Deutschland
(Dated: May 2, 2020)
Very lean hydrogen flames were thought to quench in narrow confined geometries. We show for
the first time how flames with very low fuel concentration undergo an unprecedented propagation in
narrow gaps: H2-air flames can survive very adverse conditions by breaking the reaction front into
isolated flame cells that travel steadily in straight lines or split to perform a fractal-like propagation
that resembles the pathway of starving fungi or bacteria. The combined effect of hydrogen mass
diffusivity and intense heat losses act as the two main mechanisms that explain the experimental
Hydrogen is one of the preferred fuel options because
of its high energy density, versatility and null-CO2emis-
sions when it is oxidized to produce energy either in fuel
cells or in combustion systems. One of the main concerns
of hydrogen-based power generation technology in com-
parison to conventional hydrocarbons are the potential
safety issues associated with the storage, use and han-
dling of hydrogen [1–3].
The small size of the H2molecule brings along a higher
permeation of hydrogen through solid walls, especially in
non-metallic containers [4], what significantly increases
the risk of undesired leaks [5]. On top of this, its high
reactivity, with a lean flammability limit around %H2=
4 at Earth’s gravity [3, 6], and ignition energy as low
as 0.02 mJ, ten times lower than other hydrocarbons
[7, 8], makes hydrogen more prone to undesired deflagra-
tions and explosions when the leak takes place in confined
spaces with no ventilation [9]. Furthermore, the dim visi-
ble emissions and weak heat radiated from lean hydrogen
flames makes their detection extremely difficult [10].
Combustion is a complex exothermic chemical pro-
cess formed by a sequence of elementary reactions involv-
ing intermediate species that are created and consumed
during the oxidation of the fuel. Conventional hydrogen
premixed flames propagate ideally as a continuous front
that advances burning the fresh mixture of fuel and ox-
idizer and leaves hot combustion products behind (ide-
ally, only water vapor) [11]. However, premixed flames
are inherently unstable. The viscosity and thermal ex-
pansion gradient across the flame front, the competition
between heat conduction and mass diffusion in the fluid,
the effect of gravity, the interaction with acoustic waves
[12, 13] and the heat losses [14], fold and stretch the flame
altering some of its dynamic and morphological proper-
ties [15].
To further investigate the morphology, stability
and safety issues of ultra-lean confined hydrogen-air
FIG. 1. Schematic of the experimental setup. Z-shape
Schlieren system used for image acquisition. The dimensions
of the cell are 950×200×61 mm (L×W×h). The black ar-
rows at the top end of the chamber represent the unobstructed
release of the combustion products.
flames, we modified a previously-used experimental setup
(Fig. 1) [16], formed by two parallel flat plates disposed
vertically and separated a small distance hapart. Previ-
ous combustion studies have made use of similar narrow-
channel geometries to investigate the onset and develop-
ment of premixed flame instabilities[17–19]. Here, the
faint emissions of fast hydrogen flames require the uti-
lization of Schlieren techniques and high-speed imaging
to track the reaction front. The path followed by the
flames can be outlined by trailing the condensed water
streaks formed just behind them [20] (Figs. 2 and 3). The
small value of the Reynolds number found in our experi-
ments (Re '33) anticipates a premixed hydrogen flame
that remains in the laminar regime and propagates as a
continuous wrinkled front [21]. The high mass diffusivity
of hydrogen outlines a reactive front characterized by the
formation of small wrinkles related to the development
b h = 2 mm; 10.5% H2
a h = 3 mm; 10.5% H2
Continuous front (A)
Two-headed steady cells (C'')Splitting cells (B)
Flame Flame path (condensed water) Unburned mixture
c h = 2 mm; 10.25% H2
30 mm
FIG. 2. Downward-propagating hydrogen flames and their different propagation modes. Image and scheme of a-a.1 continuous
flame front propagation, b-b.1 splitting cells that propagate forming fractal patterns and c-c.1 several two-headed isolated
steady flame cells. The Supplementary Material includes a video illustrating the three propagation regimes described in the
of thermodiffusive instabilities (Fig. 2aand 3a). In gaps
narrower than h < 6 mm, the expected continuous flame
front breaks into a set of small flame cells separated by
cold, unburned gas, unveiling two unprecedented propa-
gation modes that only emerge in flames with low enough
hydrogen concentration. In the first one, the flame front
breaks into several unstable flame cells (Fig. 2band 3b)
that split continuously and propagate leaving a path that
conforms a fractal-like pattern that reminds of ferns and
tree leaves. This propagation mode evokes the way starv-
ing fungi or bacteria colonies [22, 23] spread, with lack of
nutrients being analogous to fuel scarcity. Also, diffusion-
limited aggregation phenomena reveal similar fractal pat-
terns [24]. In the second regime, the flame front breaks
into a few isolated stable flame cells (Fig. 2cand 3c)
that move steadily delineating an almost straight trajec-
tory that reminds of the fingering patterns found during
smoldering combustion of thin solid materials [25].
From the experimental results it is unclear both how
the flame cells are formed and why hydrogen flames with-
stand more adverse conditions than heavier hydrocarbon
fuels [12]. To investigate the causes that lead to the new
propagation regimes identified experimentally, we mod-
eled the propagation of the H2-air flame using an in-house
finite-element code [14]. Since the flow is laminar and the
flame thickness δT(0.51) mm is comparable to the
gap size h, the propagation problem of the flame can be
treated as quasi two-dimensional, considerably simplify-
ing the simulations.
The computations are carried out using an in-house
finite elements Freefem++ code that uses a self-adaptive
mesh that clusters elements near the reactive front where
the maximum gradients of temperature, reaction rate and
velocity are found. The minimum element size used in
the calculations is about 1 µm. The results (Fig. 4)
show that increasing the heat losses at the plates leads
to a broken reactive front that evolves to form two- or
single-headed flame cells, as observed in the experiments
for decreasing gap thickness h. The characteristic cell
size ranges from 5 to 10 mm for lean hydrogen mixtures
(Fig. 4), very similar to the size found in the experi-
mental results (Figs. 2 and 3). Analogous simulations
performed for heavier hydrocarbons with less mass diffu-
sivity extinguished at relatively low heat losses forming a
continuous reactive front. Our computations, therefore,
identified the intense heat losses at the walls and the high
mass diffusivity of the fuel as the two main mechanisms
controlling the emergence of the newly-discovered prop-
agation modes (see Supplementary Material).
Experimentally, the relative importance of heat losses
is measured using the Peclet number [16] P e =h/δT, a
non-dimensional parameter that compares the character-
istic diffusive and residence times. Lower values of P e in-
dicates more relevant heat conduction and, therefore, the
effect of heat losses can be stimulated by increasing the
wall surface area-to-volume ratio reducing the gap size h
b h = 4 mm; 7% H2
a h = 5 mm; 8% H2
Flame Flame path (condensed water) Unburned mixture
Splitting cells (B)
Continuous front (A)
One-headed steady cells (C')
c h = 3 mm; 7.5% H2
30 mm
FIG. 3. Upward-propagating hydrogen flames and their different propagation modes. Image and scheme of a-a.1 continuous
flame front propagation, b-b.1 fractal-like propagation mode and c-c.1 several one-headed isolated steady flame cells. The
Supplementary Material includes a video illustrating the three propagation regimes described in the figures
or thickening the flame by reducing the concentration of
hydrogen in the mixture [12, 26] (Table S1). For suffi-
ciently narrow chambers (h < 6 mm) and lean mixtures
(%H2<16 -hdependent-), conductive heat losses be-
come decisive [12] and the flame front breaks into pieces.
It is under these conditions when the two unprecedented
propagation regimes appear. Starting with a constant
gap size h, we increase the relative importance of heat
losses by reducing the concentration of hydrogen. In
mixtures leaner than a critical value, the flame evolves
from a continuous front to a group of isolated flame cells
that propagate as a fractal. Such transition arises soon
after ignition and the few cells so formed do propagate
until they reach the end of the chamber. During this
time, the flame cells split and bifurcate cyclically, with
newborn flame cells branching off, almost perpendicu-
larly, from the main path to quench or to later repeat the
splitting cycle (Fig. 2band 3b). The paths followed by
the flame cells are approximately 5-mm wide and form
a unique fractal pattern of fractal dimension df'1.7
(Supplementary Material), that suggests a non-chaotic
behaviour of the propagation, similar to the fractal di-
mension df'1.65 observed in the propagation of starv-
ing bacteria and electrochemical depositions [23, 27].
Further reduction in hydrogen concentration triggers a
second transition that stops the flame cell splitting mech-
anism. After ignition, now only a few stable isolated cells
travel steadily at an almost constant velocity of around
30 cm/s –case dependent–, keeping their size also con-
stant (Supplementary Material) until they reach the end
wall of the chamber or extinguish as they collide with the
water trail left by other flame cell, where no more fuel
is available (Fig. 2cand 3c). Conductive heat losses to
the walls of the chamber offset heat production to keep
the size of the flame cell constant. The unbalance be-
tween heat production and heat losses would explain the
splitting mechanism observed in richer flames. An anal-
ogous behavior is observed in unconfined environments,
when radiative heat losses were found to be key for the
stabilization of three-dimensional hydrogen flames balls
at micro-gravity [28–30].
The relative importance of heat losses was also mod-
ified keeping a constant hydrogen concentration and
changing the gap size. Qualitatively, the results are sim-
ilar regarding the emergence of the two above-described
regimes and are used to delineate a stability map in the
h–%H2parametric space (Fig. 4). The criteria used to
define the different regions traced in Fig. 4 is based on
the fractal dimension of the condensed water path formed
during the flame propagation, that evolves from df'2
(continuous front Fig. 2aand 3a), df'1.7 (splitting
cells Fig. 2band 3b) to df'1 (steady traveling cells
Fig. 2cand 3c). Also, the fractal dimension provides in-
formation about the degree of utilization of the available
fuel, with df= 2 indicating total fuel consumption and
df= 1 showing that most of the fresh mixture remains
unburned (Supplementary Material).
The effect of mass diffusivity was explored by test-
A. Continuous front
B. Splitting cells
C'. One-headed steady cells
C''. Two-headed steady cells
D. No ignition
Representative experiments
shown in Figs. 1 and 2
Simulated one-headed cells
Simulated two-headed cells
h h
FIG. 4. Flame propagation modes in the h-%H2parametric space for both adownward- and bupward-propagating flames. The
solid lines separate the different propagation modes and the symbols represent the cases actually measured. Every experiment
is repeated at least 3 times to reduce the uncertainty of the measurements. The mixture composition was obtained with an
approximate error of ±0.04 %H2and the gap thickness hhas an average error of ±0.114 mm. cand ddetail the single and
double-headed flame cells Ω and the dimensionless temperature field T= (T0Tu)/(TbTu) obtained from our non-buoyant
numerical computations in the limit of very narrow channels. T0is the local temperature and Tbrepresents the adiabatic
(equilibrium) flame temperature of the simulated mixtures (Supplementary Material) with Tu= 298 K being the ambient
ing experimentally methane and dymethilether (DME)
flames, fuels with much lower mass diffusivity. The rel-
evant parameter here is the Lewis number Le, a non-
dimensional number defined as the thermal-to-mass dif-
fusivity ratio that takes the value Le 0.3,1,1.75 in
lean hydrogen, methane and DME flames, respectively.
For methane and DME (Le 1), the flame does not
withstand the heat losses that increase as the gap size
his reduced and extinguishes from a continuous reac-
tive front. This result then identifies mass diffusivity
as the mechanism that counteracts heat losses [14] and
thermodiffusive instabilities, triggered by the high diffu-
sivity of hydrogen, become the survival mechanism that
enables local flame quenching under non-adiabatic con-
ditions and gives birth to flame cells within which the
temperature is high enough to sustain combustion [31].
The instantaneous high concentration gradient across the
front triggers the fast diffusion of hydrogen from the un-
burned region towards the surroundings of the flame, in-
creasing the local availability of H2and keeping the gas
above the crossover temperature 1000 K, temperature
below which the chemical reaction cannot proceed [32–
34]. The additional energy released by the burning of this
extra fuel is used to counteract conductive heat losses,
extending hydrogen combustion towards ultra-lean mix-
tures below %H2<5.
Considering safety-related real-life scenarios, Gravity
also plays an important role in stabilizing or destabi-
lizing flames in vertical channels [15]. To analyse to
a greater extent how gravity intervenes in the develop-
ment of the described propagation modes, we tested ex-
perimentally both upward- and downward-propagating
flames to expand our stability map (Fig. 4). Similarly to
methane and DME flames, in the downwards propagating
case, extinction takes place with the H2flames forming
a continuous reactive front for fuel concentrations below
%H2<9 in channels wider than 6 mm, approximately.
Surprisingly, leaner mixtures can be found in narrower
gaps once the reactive front of the H2flame is broken
into small separated cells that show, mainly, a double-
headed structure similar to that obtained numerically in
our computations (Fig. 4c). Simultaneously, the reduc-
tion in the gap size increases the relative importance of
the gas viscosity, reducing the buoyant velocity induced
by gravity and facilitating the propagation of the flame.
The minimum fuel concentration %H2= 8.5 was found
for h= 4 mm (Fig. 4a).
In upward-propagating flames, buoyancy accelerates
the flames and dilates the flammability limits to mix-
tures significantly leaner than in downward-propagating
flames (Fig. 4). As in downward-propagating flames, ex-
tinction takes place for richer mixtures in channels wider
than h > 5 mm. The minimum fuel concentration was
found for h= 5 mm when a fuel concentration as lean
as %H2'4.5 was capable of sustaining a solitary one-
headed flame cell (Fig. 4d) steadily traveling along the
combustion chamber. In narrower channels, conductive
heat losses shift the extinction limit towards richer H2
mixtures (Fig. 4). The influence of gravity diminishes in
very narrow channels, with both upward and downward
propagating flames presenting an almost coincident ex-
tinction limit when h= 1 mm (Fig. 4b). From the ex-
periments, one can conclude that one and two-headed
flames mainly appear when propagating up and down-
wards, respectively. However, we found both structures
for gravity-free conditions in our simplified numerical
model [14], result that leaves the influence of gravity on
the shape of these isolated traveling flames as an open
The discovery of the propagation regimes described
above opens new research lines regarding near-limit hy-
drogen combustion in narrow geometries. Based on the
experimental results and in the mathematical modelling
of the problem carried out in this paper, we appoint the
conductive heat losses to the surrounding walls and the
high diffusivity of hydrogen flames as the two physical
mechanisms governing the onset of the two propagation
regimes unveiled in our paper. As the use of hydrogen
in the near future is expected to increase [1, 3], we an-
ticipate a raising concern about the safety of hydrogen-
powered devices [16] that will motivate the exploration
of interactions between different phenomena. That inter-
est may uncover unknown flame behaviours relevant in
the development of safety measures for intentional release
or unintentional leakage of hydrogen in narrow gaps and
confined environments.
[1] I. Staffell, D. Scamman, A. V. Abad, P. Balcombe, P. E.
Dodds, P. Ekins, N. Shah, and K. R. Ward, Energy &
Environmental Science 12, 463 (2019).
[2] B. Salvi and K. Subramanian, Renewable and Sustain-
able Energy Reviews 51, 1132 (2015).
[3] A. L. S´anchez and F. A. Williams, Progress in Energy
and Combustion Science 41, 1 (2014).
[4] A. Friedrich, J. Grune, T. Jordan, N. Kotchourko,
M. Kuznetsov, K. Sempert, and G. Stern, Safety of Hy-
drogen as an Energy Carrier (HySafe Project Contract
NSES6-CT-2004-502630, 2009).
[5] V. P. Utgikar and T. Thiesen, Technology in Society 27,
315 (2005).
[6] H. F. Coward and F. Brinsley, Journal of the Chemical
Society, Transactions 105, 1859 (1914).
[7] R. Ono, M. Nifuku, S. Fujiwara, S. Horiguchi, and
T. Oda, Journal of Electrostatics 65, 87 (2007).
[8] W. Zhang, X. Gou, and Z. Chen, Fuel 187, 111 (2017).
[9] D. A. Crowl and Y. Jo, Journal of Loss Prevention in the
Process Industries 20, 158 (2007).
[10] R. W. Schefer, W. D. Kulatilaka, B. D. Patterson,
and T. B. Settersten, Combustion and flame 156, 1234
[11] A. Li˜an and F. A. Williams, Fundamental aspects of
combustion (Oxford University Press, 1993).
[12] F. Veiga-L´opez, D. Mart´ınez-Ruiz, E. Fern´andez-
Tarrazo, and M. S´anchez-Sanz, Combustion and Flame
201, 1 (2019).
[13] D. Mart´ınez-Ruiz, F. Veiga-L´opez, and M. S´anchez-
Sanz, Physical Review Fluids 4, 100503 (2019).
[14] D. Mart´ınez-Ruiz, F. Veiga-L´opez, D. Fern´andez-
Galisteo, V. N. Kurdyumov, and M. S´anchez-Sanz, Com-
bustion and Flame 209, 187 (2019).
[15] P. Clavin and G. Searby, Combustion waves and fronts
in flows: flames, shocks, detonations, ablation fronts and
explosion of stars (Cambridge University Press, 2016).
[16] M. Kuznetsov and J. Grune, International Journal of Hy-
drogen Energy 44, 8727 (2019).
[17] J. Sharif, M. Abid, and P. Ronney, NASA Technical
Report (1999).
[18] S. Shen, J. Wongwiwat, and P. Ronney, in AIAA Scitech
2019 Forum (2019) p. 2365.
[19] E. Al Sarraf, C. Almarcha, J. Quinard, B. Radisson, and
B. Denet, Flow, Turbulence and Combustion 101, 851
[20] B. Bregeon, A. S. Gordon, and F. A. Williams, Combus-
tion and Flame 33, 33 (1978).
[21] J. Yanez, M. Kuznetsov, and J. Grune, Combustion and
Flame 162, 2830 (2015).
[22] R. Halvorsrud and G. Wagner, Physical Review E 57,
941 (1998).
[23] E. Ben-Jacob, O. Schochet, A. Tenenbaum, I. Cohen,
A. Czirok, and T. Vicsek, Nature 368, 46 (1994).
[24] T. Witten Jr and L. M. Sander, Physical review letters
47, 1400 (1981).
[25] O. Zik, Z. Olami, and E. Moses, Physical review letters
81, 3868 (1998).
[26] M. S´anchez-Sanz, Combustion and Flame 159, 3158
[27] V. Fleury, J. Kaufman, and D. Hibbert, Nature 367,
435 (1994).
[28] Y. B. Zeldovich, Theory of Combustion and Detonation
in Gases (USSR Academy of Science, 1944).
[29] P. Ronney, K. Whaling, A. Abbud-Madrid, J. Gatto, and
V. Pisowicz, AIAA journal 32, 569 (1994).
[30] P. D. Ronney, M.-S. Wu, H. G. Pearlman, and K. J.
Weiland, AIAA journal 36, 1361 (1998).
[31] G. Joulin and P. Clavin, Combustion and Flame 35, 139
[32] E. Fern´andez-Tarrazo, A. L. S´anchez, A. Li˜an, and
F. Williams, Proceedings of the Combustion Institute 33,
1203 (2011).
[33] E. Fern´andez-Tarrazo, A. L. S´anchez, A. Li˜an, and
F. A. Williams, International journal of hydrogen energy
37, 1813 (2012).
[34] E. Fern´andez-Tarrazo, A. L. S´anchez, and F. A.
Williams, Combustion and flame 160, 1981 (2013).
... In this light, Hele-Shaw cells, consisting of two parallel plates separated by a narrow gap, are very instrumental for the experimental observation of premixed flame dynamics and instabilities. [4][5][6][7][8][9][10][11] Hele-Shaw cells are widely used in experimental studies of fingering instability in two-phase flows. [12][13][14][15][16][17] The flow motion between the two plates can be treated as a quasi-two-dimensional problem, since the length scale in the normal direction is much smaller than the one along the plates. ...
... Previous experiments 11,18,19 on the hydrodynamic instability of a premixed flame in Hele-Shaw cells have shown that non-negligible viscous and heat losses at cell walls affect the flame stability. The effect of momentum and heat losses on the hydrodynamic stability of premixed flames propagating in Hele-Shaw cells was first studied theoretically by Joulin and Sivashinsky. ...
... where h is the heat loss rate [not to be confused with wall heat loss coefficient H in Eq. (11)]. In the adiabatic case (H ¼ 0), the nondimensional burning flux tends to unity (C ! 1). ...
The linear stage of hydrodynamic instability of a laminar premixed flame propagating in a Hele–Shaw cell is investigated. Our theoretical model takes into account momentum and heat losses, temperature-dependent transport coefficients, and the continuous internal structure of the flame front. The dispersion relation is obtained numerically as a solution to an eigenvalue problem for the linearized governing equations. The obtained results are in good qualitative and quantitative agreement with previous studies. It is shown that the wall heat losses tend to weaken the hydrodynamic flame instability. On the contrary, momentum losses enhance the flame instability. It is demonstrated that for the adiabatic walls, an increase in the Hele–Shaw cell width results in a reduction of the instability growth rate. For the non-adiabatic walls, there is a competition between momentum and heat losses in narrow channels that may result in a non-monotonic dependence of the instability growth rate on the Hele–Shaw cell width. It is shown that the effects of the Prandtl number and the thermal expansion vary with the wall heat loss coefficient. A possibility of non-monotonic dependence of the maximum instability growth rate on the thermal expansion has been demonstrated.
... Of definite interest from the fire and explosion safety point of view are the processes of flame propagation or quenching in narrow gaps, and flame penetration through holes in fire barriers. For example, in [27], combustion of ultralean hydrogen-air mixtures in the narrow space between parallel plates was studied, both experimentally, and numerically. It was shown that, contrary to expected flame quenching, propagation of several compact flame kernels was observed. ...
Full-text available
Flame interaction with obstacles can affect significantly its behavior due to flame front wrinkling, changes in the flame front surface area, and momentum and heat losses. Experimental and theoretical studies in this area are primarily connected with flame acceleration and deflagration to detonation transition. This work is devoted to studying laminar flames propagating in narrow gaps between closely spaced parallel plates (Hele–Shaw cell) in the presence of internal obstacles separating the rectangular channel in two parts (closed and open to the atmosphere) connected by a small hole. The focus of the research is on the penetration of flames through the hole to the adjacent channel part. Experiments are performed for fuel-rich propane–air mixtures; combustion is initiated by spark ignition near the far end of the closed volume. Additionally, numerical simulations are carried out to demonstrate the details of flame behavior prior to and after penetration into the adjacent space. The results obtained may be applicable to various microcombustors; they are also relevant to fire and explosion safety where flame propagation through leakages may promote fast fire spread.
The parametric forcing of premixed flames is observed here in the geometry of a Hele-Shaw burner. Using a vibroacoustic coupling, we are able to study the thresholds of the parametric flattening (where the flame wrinkles are suppressed) and the parametric instability (where the flame wrinkles oscillate at twice the acoustic period) and compare the measured values to a low-frequency theory. It is shown that the dependency of the parametric threshold with equivalence ratio is lower than predicted, suggesting that the Markstein lengths are frequency dependent and that the flattening threshold is independent of the Markstein number and of the nondimensional forcing frequency, in agreement with the Bychkov limit.
Full-text available
We present a numerical analysis to calculate the minimum ignition energy of hydrogen–ammonia blends in air at both under and over atmospheric pressure. Unlike previous calculations, we used the full compressible and reactive Navier–Stokes equations coupled with detailed chemical kinetics (San Diego mechanism for hydrogen, complemented with the San Diego chemistry for nitrogen). The effect of the size of the energy deposition region and the deposition time are considered to determine the most efficient method to ignite the mixture. Our calculations also evaluate the impact of the gas compressibility on the minimum ignition energy after a sudden energy deposition. The results are validated first by comparing the minimum ignition energy of pure hydrogen–air mixtures as a function of the equivalence ratio ϕ with available experimental data and previous numerical results. Then, fuel blends made of mixtures of hydrogen and ammonia (NH3) are considered to calculate the minimum ignition energy as a function of fuel composition, equivalence ratio and pressure. The full range of ammonia volumetric content in the blend is varied between the extreme cases of pure hydrogen and pure ammonia. For each fuel blend, we computed a wide range of equivalence ratios ϕ that, in the case of pure hydrogen at atmospheric conditions, ranged from ϕ=7 to ϕ≃0.07, near the lean flammability limit, to theoretically explain the experimental evidence of ultra-lean flames reported in the literature.
Full-text available
Dendritic combustion in Hele-Shaw cells is investigated qualitatively using a simplified one-dimensional thermo-diffusive model. Formulae for the velocity, size and temperature of the flamelets are derived. The temperature and velocity of the flames increase for small radii to allow for their survival regardless the activation energy. In addition, the results obtained with very large activation energy were compared with experimental results, finding that additional tests are required due to the strong influence of gravity on the velocity and size estimations. Conditions for the existence of this anomalous propagation are investigated, confirming analytically that it can only happen for low Lewis numbers.
Full-text available
The propagation of an isobaric premixed flame into a quiescent gas mixture of fuel and oxidizer contained between two parallel plates is investigated numerically. The plates are separated by a small distance h and considered as adiabatic. The mixture is assumed to be lean in fuel and the combustion model includes a single-step Arrhenius-type reaction, constant heat capacity and unity fuel Lewis number. Transport properties are considered to be temperature dependent or constant, which allows us to decouple two different instability mechanisms of hydrodynamic nature: (i) Darrieus-Landau (associated with the density change due to thermal expansion) and (ii) Saffman-Taylor (associated with the viscosity contrast). By performing three-dimensional (3D) simulations, the propagation rate and the flame front shape is analyzed as a function of the dimensionless parameter a=h/δT, where δT is the thermal thickness of the planar flame. The parameter a ranges from very small values to large enough ones so that flame curvature between the plates manifests itself. Results show that, as the distance between the plates decreases, loss of momentum enhances the hydrodynamic instability in comparison with that of a freely (unconfined) propagating flame. Likewise, viscosity contrast across the flame brings about an additional destabilizing mechanism. When distance between the plates increases, flame curvature can become important and contribute significantly to the overall propagation rate. Finally, by comparison with the 3D simulations, we show that confinement effects can be effectively described by a two-dimensional formulation written in the limit a→0, in which momentum conservation is reduced to a linear equation for the velocity similar to Darcy’s law.
This chapter sheds light on the historical development of the flameless combustion based on various numerical and experimental studies. The first section presents state of the art on discovering flameless combustion and preliminary investigations in industrial furnaces and burners. It is discussed that how a nonconventional combustion regime, achieved through specific flow and temperature conditions, helped lower the NOx emissions while maintaining the overall system efficiency. Modeling approaches for the flameless regime are discussed in detail using computational fluid dynamics. It is observed that both turbulence and chemistry equally drive the flameless regime, and hence, models accounting for better turbulence-chemistry interaction capture the flameless regime well. It is concluded based on the state-of-the-art survey that a standard/modified κ-ɛ model for turbulence works satisfactorily. For combustion modeling, the eddy dissipation concept with detailed chemical kinetics is required to capture the flameless combustion characteristics. This chapter also highlights the importance of the selection of combustor geometry and how it generates increased recirculation levels required to sustain flameless combustion conditions. Among the presented investigations, geometries based on a cyclonic flow field provide promising results regarding pollutant emissions. In the later sections of this chapter, flameless combustion's potential to burn low graded dirty fuels is discussed. It is presented how creating a heated and diluted environment can lead to clean burning of low calorific value fuels such as syngas, biogas, ammonia, coke oven gas, etc. It is observed that fuel flexibility is an inherent talent of this combustion regime, which is needed to be explored further in future studies. In the final section of this chapter, studies based on the applicability of flameless combustion in gas turbine engines are presented. The issues observed are high-pressure loss, narrow operational range, etc. Solutions in terms of fuel/air staging two-stage combustors are discussed.
Recent achievements in development of ultra-lean combustion technology and important historical milestones concerning excess-enthalpy burning (i.e., superadiabatic combustion) of near-limit mixtures are summarized within this chapter. Fundamental knowledge and practical findings obtained through relevant experimental and computational studies are reviewed. Scientific progress in studying ultra-lean (i) hydrogen, (ii) methane, and (iii) dimethyl ether flames is highlighted in the given context. Specific issues concerning combustion chemistry and physics as well as interactions of various phenomena involved within these systems are particularly discussed.
Full-text available
In this work, a computational study on nearly two-dimensional reacting fronts under canonical configurations is proposed. Specifically, a quasi-2D description of the conservation equations with a one-step irreversible Arrhenius chemical model is used to simulate ultra-lean hydrogen-air premixed flames that slowly propagate over streams of reactants that are confined between plates and are subject to conductive heat losses. Under these conditions, recent previous experiments identified two unprecedented stable flame configurations: two-headed and circular isolated flames. This study identified buoyancy and the relative importance of conductive heat losses as the two parameters controlling the formation of these isolated flames. The numerical evaluation of the different terms ascertains the physical mechanisms that drive the formation of two-headed or circular flames. Additionally, the systematic variation of the controlling parameters depicts the maps of stable solutions that determine the capability of the two flame configurations to emerge. In particular, the existence of a range of parametric values in which both flame configurations are stable proves, also, that initial conditions are crucial to determine which of the two configurations prevails.
The paper presents an experimental study of the processes of ignition and combustion for a stoichiometrical propane-oxygen mixture in a closed volume (a single cylinder of internal combustion engine). The object of study was the dynamics of gas ignition and combustion in a cylinder of diameter D = 72 mm and height h = 4 mm as a function of location of N spark points for ignition (N = 1÷9) under the condition of simultaneous ignitions. The big number of ignition points reduces the interval for generating the force impact on the engine’s piston and increases the maximum amplitude of the force.
Full-text available
This paper is associated with a video winner of a 2018 APS/DFD Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion,
Full-text available
The propagation of low-Lewis-number premixed flames is analyzed in a partially confined Hele–Shaw chamber formed by two parallel plates separated a distance h apart. An asymptotic-numerical study can be performed for small gaps compared to the flame thickness δT . In this narrow-channel limit, the problem formulation simplifies to a quasi-2D description in which the velocity field is controlled by dominant viscous effects. After accounting for conductive heat losses through the plates in our formulation, we found that the reaction front breaks into one or several isolated flame cells where the temperature is large enough to sustain the reaction, both in absence and in presence of buoyancy effects. Under these near-limit conditions, the isolated flame cells either travel steadily or undergo a slow random walk over the chamber in which the reacting front splits successively to form a tree-like pathway, burning only a small fraction of the fuel before reaching the end of the chamber. The production of quasi-2D circular or comet-like flames under specific favorable conditions is demonstrated in this paper, with convection, conductive heat losses and differential diffusion playing an essential role in the formation of the isolated one and two-headed flame cells.
Full-text available
An experimental study of methane, propane and dimethyl ether (DME) pre-mixed flames propagating in a quasi-two-dimensional Hele-Shaw cell placed horizontally is presented in this paper. The flames are ignited at the open end of the combustion chamber and propagate towards the closed end. Our experiments revealed two distinct propagation regimes depending on the equivalence ratio of the mixture as a consequence of the coupling between the heat-release rate and the acoustic waves. The primary acoustic instability induces a small-amplitude, of around 8 mm, oscillatory motion across the chamber that is observed for lean propane, lean DME, and rich methane flames. Eventually, a secondary acoustic instability emerges for sufficiently rich (lean) propane and DME (methane) flames, inducing large-amplitude oscillations in the direction of propagation of the flame. The amplitude of these oscillations can be as large as 30 mm and drastically changes the outline of the flame. The front then forms pulsating finger-shaped structures that characterize the flame propagation under the secondary acoustic instability. The experimental setup allows the recording of the flame propagation from two different points of view. The top view is used to obtain accurate quantitative information about the flame propagation, while the lateral view offered a novel three dimensional perspective of the flame that gives relevant information on the transition between the two oscillatory regimes. The influence of the geometry of the Hele-Shaw cell and of the equivalence ratio on the transition between the two acoustic-instability regimes is analyzed. In particular, we find that the transition to the secondary instability occurs for values of the equivalence ratio φ above (below) a critical value φ c for propane and DME (methane) flames. In all the tested fuels, the transition to the secondary instability emerges for values of the Markstein number M below a critical value M c. The critical Markstein number varies with the gap size h formed by the two horizontal plates that bound the Hele-Shaw cell. As h is reduced, the critical Markstein number is shifted towards larger values.
Full-text available
Hydrogen technologies have experienced cycles of excessive expectations followed by disillusion. Nonetheless, a growing body of evidence suggests these technologies form an attractive option for the deep decarbonisation of global energy systems, and that recent improvements in their cost and performance point towards economic viability as well. This paper is a comprehensive review of the potential role that hydrogen could play in the provision of electricity, heat, industry, transport and energy storage in a low-carbon energy system, and an assessment of the status of hydrogen in being able to fulfil that potential. The picture that emerges is one of qualified promise: hydrogen is well established in certain niches such as forklift trucks, while mainstream applications are now forthcoming. Hydrogen vehicles are available commercially in several countries, and 225 000 fuel cell home heating systems have been sold. This represents a step change from the situation of only five years ago. This review shows that challenges around cost and performance remain, and considerable improvements are still required for hydrogen to become truly competitive. But such competitiveness in the medium-term future no longer seems an unrealistic prospect, which fully justifies the growing interest and policy support for these technologies around the world.
Full-text available
We show in this paper that a Hele-Shaw burner can be used for studying the development of premixed flame instabilities in a quasi-two dimensional configuration. It is possible to ignite a plane flame at the top of the cell, and to measure quantitatively the growth rates of the instability by image analysis. Experiments are performed with propane and methane-air mixtures. It is found that the most unstable wavelength, and the maximum linear growth rate of perturbations, directly measured in the present experiments, have the same order of magnitude as those previously measured on flames propagating freely downwards in wide tubes.
Full-text available
Combustion is a fascinating phenomenon coupling complex chemistry to transport mechanisms and nonlinear fluid dynamics. This book provides an up-to-date and comprehensive presentation of the nonlinear dynamics of combustion waves and other non-equilibrium energetic systems. The major advances in this field have resulted from analytical studies of simplified models performed in close relation with carefully controlled laboratory experiments. The key to understanding the complex phenomena is a systematic reduction of the complexity of the basic equations. Focusing on this fundamental approach, the book is split into three parts. Part I provides physical insights for physics-oriented readers, Part II presents detailed technical analysis using perturbation methods for theoreticians, and Part III recalls the necessary background knowledge in physics, chemistry and fluid dynamics. This structure makes the content accessible to newcomers to the physics of unstable fronts in flows, whilst also offering advanced material for scientists who wish to improve their knowledge.
A series of experiments in a thin layer geometry performed at the HYKA test site of the KIT. Experiments on different combustion regimes for lean and stoichiometric H2/air mixtures were performed in a rectangular chamber with dimensions of 200 × 900 x h mm³, where h is the thickness of the layer (h = 1, 2, 4, 6, 8, 10 mm). To model a gap between a fuel cell assembly and a metal housing, three different layer geometries were investigated: (1) a smooth channel without obstructions; (2) a channel with a metal grid filled 25% of chamber length and (3) a metal grid filled 100% of chamber length. The blockage ratio of metal grid has changed from 10 to 60% of cross-section. Detail measurements of H2/air combustion behavior including flame acceleration (FA) and DDT in closed rectangular channel have been done. Five categories of flame propagation regimes were classified. Special attention was paid to analysis of critical condition for different regimes of flame propagation as function of layer thickness and roughness of the channel. It was found that thinner layer suppresses the detonation onset and even with a roughness, the flame may quench or, in thicker layer, is available to accelerate to speed of sound. The detonation may occur only in a channel thicker than 4 mm.
Water vapor dilution has great impact on fundamental combustion processes such as ignition, flame propagation and extinction. In the literature, there are many studies on how water vapor addition affects flame propagation and extinction limit. However, the influence of water vapor addition on ignition receives little attention. In this study, numerical simulations considering detailed chemical mechanisms are conducted for the ignition of methane, n-butane and n-decane/air/water vapor mixtures. The emphasis is spent on examining the effects of water vapor dilution on the ignition of these fuels at normal and reduced pressures. The minimum ignition energies (MIE) at different dilution ratios and initial pressures are obtained. It is found that at normal and reduced pressures, the MIE is proportional to the inverse of pressure and it increases exponentially with water vapor dilution ratio. A general correlation among the MIE, pressure and dilution ratio is proposed for each fuel. Furthermore, for stoichiometric methane/air/water vapor mixtures, the chemical and radiation effects of water vapor dilution are isolated and quantified. It is found that the three-body recombination reaction greatly increases the MIE and reduces the dilution limit.