OH*-chemiluminescence during autoignition of hydrogen with air in a pressurised turbulent flow reactor

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OH*-chemiluminescence during autoignition of hydrogen with air in a pressurised turbulent flow reactor Alessandro Scho€nborn a,*, Parisa Sayad a, Alexander A. Konnov b, Jens Klingmann a a b

Division of Thermal Power Engineering, Lund University, Box 118, SE-221 00 Lund, Sweden Division of Combustion Physics, Lund University, Box 118, SE-221 00 Lund, Sweden

article info

abstract

Article history:

Autoignition of hydrogen in air was studied in a turbulent flow reactor using OH*-

Received 31 March 2014

chemiluminescence. High-speed imaging was used to visualise the formation of auto-

Received in revised form

ignition kernels in the flow, and to analyse the conditions under which temporary stabi-

20 May 2014

lisation of the flame kernels occurred. The experiments were carried out at temperatures of

Accepted 25 May 2014

800e850 K, pressures of 0.8e1.2 MPa and an equivalence ratio of 4 ¼ 0.25. Measurements of

Available online 25 June 2014

the autoignition delays yielded values in the range of t ¼ 210e447 ms. The autoignition delay results indicated that, over the range of conditions studied, ignition delays reduced

Keywords:

with decreasing pressure. This observation contradicted homogeneous gas-phase kinetic

Autoignition

calculations, which predicted an increase in autoignition delay with decreasing pressure. If

Hydrogen

the kinetic model was altered to include surface reactions at the reactor walls, the calcu-

OH-chemiluminescence

lations could be qualitatively reconciled with the experimental data, suggesting that wall

Surface reactions

reactions had a significant influence on autoignition delays.

Kinetic modelling

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Flow reactor

Introduction Hydrogen is a potential fuel for gas turbines that has been advanced as potential storage medium for intermittent renewable energy [1]. It has significantly different combustion properties than natural gas, for which gas turbine technology is currently optimised, and can thus cause problems when deployed in current engine technology. Apart from having a higher flame propagation velocity, hydrogen also has a significantly higher autoignition propensity than natural gas

reserved.

[2]. This reduces the time available for premixing of fuel and gas, before autoignition occurs. Combustion in dry-lowemission gas turbine burners occurs under predominantly lean-premixed conditions, in order to control combustion temperatures and thereby the formation of nitrogen oxides (NOx). This requires that fuel and air are premixed above their autoignition temperature, before they are consumed by the flame front, posing a risk for autoignition. It has been shown for propane that the occurrence of autoignition in the fuel-air premixer tube of a gas turbine combustor can result in severe disruption of the combustor flow-field, flame flashback [3]. For

* Corresponding author. Tel.: þ 46 46 222 4771; fax: þ 46 46 222 47 17. € nborn), [email protected] (P. E-mail addresses: [email protected], [email protected] (A. Scho Sayad), [email protected] (A.A. Konnov), [email protected] (J. Klingmann). http://dx.doi.org/10.1016/j.ijhydene.2014.05.157 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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hydrogen, autoignition has been observed to cause flame stabilisation in the premixer tube and even blowout [4]. Accurate knowledge of the autoignition delay times of hydrogenair mixtures under gas turbine conditions is thus necessary, to estimate the allowable premixing times in hydrogen-powered gas turbines, and facilitate reliable turbine operation with hydrogen. In premixing concepts employing micro-mixing tubes with a high ratio of surface to reactants, the catalytic effects of wall reactions may compound the problem of autoignition. It is also of interest to understand the flow conditions under which autoignition does not result in propagation of the flames upstream through the flow, but rather results in a stable autoignition flame. Avoiding autoignition flame propagation upstream into the flow, is of interest in avoiding flashback, but may also serve as an alternative means of flame stabilisation in gas turbine burners. The aims of this investigation are threefold: First, to characterise the autoignition process visually by OH*chemiluminsecence. Second, to determine the flow conditions under which kernels may be stabilised in the flow. Third, to determine the autoignition delay times of hydrogen in air under conditions relevant to gas turbine premixers.

Autoignition studies The fundamental gas-phase reactions of hydrogen with oxygen are well-studied and described in the literature [5e7]. Autoignition delays of hydrogen have been measured at a variety of conditions and using a variety of techniques. Flow reactors, rapid compression machines (RCMs) and shocktubes have been used to experimentally study the autoignition behaviour of hydrogen and air mixtures at different pressures and temperatures. An overview of relevant studies measuring the autoignition delays of hydrogen or syngas in oxygen and nitrogen containing mixtures is shown in Fig. 1(a). The results of the studies are presented as the product of pressure and ignition delay, in order to correlate the data via the assumption of a P1 dependence of ignition delay on pressure [8e10]. Fig. 1(b) shows an overview of the explosion limits of hydrogen in oxygen, and the conditions under which the experimental studies were carried out. In the mild ignition regime, above the second explosion limit, the chain branching reaction

H þ O2 / OH þ O

R1

is moderated by the collisions of H and O2 with a third body in the reaction

H þ O2 þ M / HO2 þ M

Fig. 1 e Product of autoignition delay and reaction pressure, for hydrogen or syngas in air. Black: Flow reactors, Blue: RCMs, Red: Shock-tubes b) Explosion limits and experimental data. First and third explosion limits are for H2eO2 mixtures in a spherical potassium chloride (KCl) coated vessel, as described by Lewis and von Elbe [5]. The rates for calculation of the extended second explosion limit were taken from Ref. [11]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

R2

resulting in a moderate overall reaction rate. A comprehensive description of the kinetics affecting the mild ignition regime can be found in Ref. [12]. Under these conditions of low temperatures but high pressures, autoignition has been reported not to occur simultaneously throughout the mixture, but rather by the

staggered formation of autoignition kernels. The autoignition delay times observed experimentally under these conditions are often significantly shorter than predicted by homogeneous gas-phase chemical kinetic models. A number of relevant autoignition studies shall be discussed in the following sections.

152 25.4, 38.1, 50.8 41 38.1 0.05e0.2 0.02e0.05 0.23e0.72 0.14e0.52 Mullins [14] Swigart [15] Peschke [16] Beerer & McDonell [2]

0.03e0.1 0.1 1.21e2.58 0.5e0.7

870e1120 900e1000 633e781 710e810

0.5e50 4e50 38e128 167e451

N2 N2 N2 N2

< 79% 79e87% 51e70% 66e71%

CO2 < 21% none CO, CO2 6e19% none

Thermocouple & observation Observation Thermocouple Thermocouple Photomultiplier tube & thermocouple None N2 < 79% 500e15,000 760e910 0.01e0.7 Not specified

Diluent [mole/mole]

t

[ms]

T [K] P [MPa] 4

Table 1 e Experimental autoignition studies using flow reactors.

Shock-tubes rely on rapidly bringing premixed reactants to their reaction conditions from an unreactive state, by means of shockwave compression of the gases. This experimental method is most suited for measuring short ignition delays at high temperatures (T > 1250 K) and ignition delays shorter than a few milliseconds. At ignition delays longer than several milliseconds, shock tubes cease to act as constant volume and constant internal energy reactors, which must be taken into account when comparing experimental data with modelling results [8,9,29e32]. An overview of relevant experimental studies with low dilution levels is given in Table 3. Further shock tube studies of the ignition delays of hydrogen and oxygen mixtures highly diluted in noble gases have been reported by Schott and Kinsey [47], Asaba et al. [48], Skinner and Ringrose [49], Jachimowski and Houghton [50,51], Gardiner et al. [52], Dean et al. [53], Brabbs and Robertson [54], Yuan et al. [55], Pang et al. [29], Herzler and Naumann [56],  romne  s et al. [11]. These experiments Krejci et al. [57] and Ke were typically performed at high temperatures (typically above 1000 K) and were found in reasonable agreement with kinetic modelling.

Other fuel constituents [mole/mole]

Rapid compression machines (RCM's) compress reactants from a non-reactive state to their reaction pressures and temperature by means of a piston and cylinder arrangement in a short time (10e30 ms). Due to the known volume of the combustion chamber, ignition results in a sensible rise in pressure. Heat transfer from the reactants to the walls limits the typical operating range of RCM's to ignition delays of about 10e100 ms. An overview of relevant experimental studies with nitrogen dilution is given in Table 2. Further RCM studies employing highly argon-diluted mixtures of hydrogen and oxygen were reported by Lee and  romne s Hochgreb [25], Mittal et al. [26], Gersen et al. [27] and Ke et al. [28], and reported good agreement between experiments and gas-phase kinetic modelling.

Coward [13]

Detection

Rapid compression machines

Shock-tubes

Glazed silica

Reactor surface

Flow reactors generally operate under constant pressure and usually rely on turbulent mixing to bring the reactants into contact with each other within a small fraction of the ignition delay. This type of experimental apparatus tends to be particularly suited for measurements of long ignition delay times (100e500 ms) at low temperatures (600e1150 K) and intermediate pressures (0.1e3 MPa). An overview of relevant experimental studies is given in Table 1. In addition to the studies shown in Table 1, Sawyer et al. [17], Yetter et al. [18] and Mueller et al. [19] reported time histories of main species concentration for hydrogen and oxygen mixtures in a flow reactor, but cautioned against deducing absolute ignition delay values from these results due to variations of the equivalence ratio and possible wall catalysis perturbations in the mixing region.

Stainless steel Stainless steel Not specified Stainless steel

Reactor diameter [mm]

Flow reactors

120

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Source

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Table 2 e Experimental autoignition studies using rapid compression machines. 4

Source

t

T [K]

[ms]

Diluent [mole/mole]

Other fuel constituents [mole/mole]

Detection

Reactor surface

Reactor diameter [mm]

Carbon steel Carbon steel Stainless steel, partly chromed & carbon steel Carbon steel, partly chromed Carbon steel, partly chromed

25 110 50.8e101.2

Falk [20] Dixon [21,22] Walton [23]

0.0625e1 0.0625e1 0.1e2.0

3.9e10.3 1.4e25 4.5e27

749e915 740e1500 752e1051

Not measured Not measured 4e24

None None N2 48e76%

None None None

Observation Observation Pressure

Das et al. [24] romne s et al. [11] Ke

1 1

1e7 7

915e1042 914e1010

1e95 1e35

N2 81% N2 81%

None CO 0e11%

Pressure Pressure

50.8 50.8

Table 3 e Experimental autoignition studies using shock-tubes. 4

Source

t

Diluent [mole/mole]

Other fuel constituents [mole/mole]

Detection

Reactor surface

Reactor diameter or width , height [mm]

T [K]

[ms]

1 1

Not specified 0.4e1

400e1000 750e1000

Not specified 0.01e0.380

None None

None None

Photography Photomultiplier

Stainless steel Stainless steel

76.2 50.8 , 50.8

0.5e2.4

0.74e1.65

680e1300

0.45e1.2

N2 39%

None

Not specified

40 , 40

0.5

0.1e0.3

900e1700

10e320

None

None

Not specified

40 , 40

800e1700

0.001e13

None

None

Not specified

Not specified

1110e1240 875e1000 750e1100 760e1160 800e1500 700e1200

0.02e200 0.078e3.4 0.1e0.5 32e8340 0.016e1.2 0.015e8

N2 63e75% None None N2 56% N2 56% N2 67%

None None None None None None

Not specified Stainless steel Carbon steel Not specified Stainless steel Not specified

Not specified 76.2 30 , 21 84 38.1 34 , 50, 54 , 54, 87

850e1440 890e1285 943e1148

0.005e2.6 0.01e10 0.38e3.5

N2 56% N2 65% N2 64%

None CO 20e95% CO 10%, CO2 2%

Pressure & schlieren photography Pressure & schlieren photography Pressure & schlieren photography Photomultiplier Pressure & photomultiplier Schlieren photography Photomultiplier Photomultiplier Shadowgraph, pressure, photomultiplier Pressure & photomultiplier Pressure & photomultiplier Pressure & photomultiplier

Stainless steel Stainless steel Stainless steel

50 76.2 76.2

Fay [33] Steinberg & Kaskan [34] Bazhenova and Soloukhin [35] Saytzev & Soloukhin [36] Voevodsky & Soloukhin [37] Belles & Lauver [38,39] Craig [40] Helm [41] Walker [42] Slack & Grillo [43] Blumenthal et al. [44]

0.5

0.05e0.4

0.125e2.75 1 1 1 1 0.42

0.021e0.046 0.1e0.4 0.3e0.6 0.1e1.1 0.02e0.20 0.3e5

Martynenko et al. [45] Kalitan et al. [46] Petersen et al. [10]

1 0.5 0.5

0.26e2.07 0.1 1.67e3.31

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P [MPa]

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P [MPa]

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Early ignition under mild conditions The literature on autoignition of hydrogen and air mixtures indicates that agreement between experiments and gas-phase chemical kinetic modelling generally exist at high temperatures, but that experiments tend to disagree with gas-phase chemical kinetic models at the low temperature and high pressure conditions of the mild ignition regime. This disagreement has been observed in various types of experimental apparatus, and has generally been attributed to perturbations such as variations in equivalence ratio or catalytic effects from reactor vessel walls or particles. It has been argued [8,9] that ignition delays in the mild ignition regime are highly sensitive to perturbations. Perturbations are inherent to most experimental arrangements, but are often not modelled in homogeneous gas-phase kinetic models. They may comprise the presence of contaminants, compressible fluid dynamic effects, inhomogeneity of the reactants (e.g. during mixing), and catalysis on particles or reactor surfaces. A number of studies [29e32] reported that shock-tube measurements involving ignition delays longer than a few milliseconds display measurable increases in reaction pressure prior to ignition. Such pressure rises represent a departure from homogeneous kinetic predictions assuming constant volume and constant internal energy. These may occur as a result of boundary layer effects on the sidewalls and attenuation of the incident shock wave, as well as chemical energy release [29]. Reasonable agreement between chemical kinetic modelling and experiments may be achieved if these experimentally determined increases in pressure were incorporated into the kinetic modelling by isentropic compression or expansion [58]. Dryer and Chaos [8,9] suggested that catalytic reactions on surfaces of the experimental apparatus may further be responsible for significantly reducing autoignition times by effectively accelerating reactions H2 þ HO2 ¼ H2O2 þ H, and H2O2 þ M ¼ OH þ OH. Beerer and McDonell [2] noted that autoignition events were not always repeatable, and that ignition manifested itself in various forms. In some cases only a small temperature rise was observed, and in others a large temperature rise of several hundred Kelvin was observed. The authors observed that for ignition events showing only a small rise in temperature, the increase in temperature was up to 20 K larger close to the reactor walls, than it was at the central axis of the reactor. The authors argued that this may be owed to the longer residence time or to catalytic activity along the reactor walls. Medvedev et al. [59,60] proposed that the ignition delays of hydrogen and air mixture may alternatively be described by a characteristic deflagration time tB. tB describes the ratio of a characteristic length, such as the radius of the reactor tube, to the displacement velocity of the flame. This approach of describing autoignition via deflagration has been shown [59] to correctly predict the results reported by Martynenko et al. [45], Petersen et al. [10] and Blumenthal et al. [44]. The characteristic deflagration time tB does not provide a reason as to why autoignition occurs before the expected chemical autoignition delay, but it shows that if autoignition occurs early

somewhere in the mixture, the remaining reactants will be consumed by deflagration rather than autoignition. Mixing in axial direction occurring as a result of turbulence has also been reported to influence the ignition process. Computational fluid dynamic modelling has shown that turbulent flow conditions, in conjunction with inhomogeneous initial conditions, in terms of fluctuations in temperature and mixture composition can somewhat shift the location of autoignition from plug-flow conditions [61,62]. This has been shown to result in a larger stochastic spread of autoignition delays, and in a more gradual rise in reactant temperature during ignition. Unless large variations in initial temperature (~75 K) are present in the flow, the average time of 50% temperature rise was shown to remain relatively similar to those of plug-flow conditions, especially at high Reynolds numbers [62]. The current study is aimed at visualising autoignition in a turbulent flow reactor under mild ignition conditions. This is to further investigate the hypotheses on early ignition in the mild ignition regime, and to determine the conditions under which autoignition would result in flame stabilisation rather than flashback.

Material and methods Experimental apparatus The experiments were carried out in a pressurised, turbulent and optically accessible flow reactor of circular cross-section. An overview of the reactor can be seen in Fig. 2. The experimental apparatus consisted of an air supply system supplying air to the flow reactor at variable temperature and pressure, a fuel supply system supplying pressurised hydrogen at room temperature, the pipe of circular crosssection representing the test-section of the flow reactor, and an exhaust system in which the gas mixture was cooled by water injection and subsequently expanded to atmospheric pressure by means of an exhaust throttling valve. A detailed description of the test apparatus can be found in an earlier paper [3]. A significant change implemented in the current work, was that the water jet used to quench the reactants was located ahead of the thermocouple used to measure the exit temperature of the reactor, in order to avoid any recirculation zones in the reacting flow. The water jet was temporarily interrupted when the reactant temperature was measured. The overall length of the test-section was 2.905 m. A centrally located axial fuel jet represented the first point of fuel injection and was defined as the start of the reactor testsection. The point of water injection was defined as the end of the test-section. Hydrogen was added to the air using one centrally located axial fuel jet ahead of a Venturi tube and four radially inward pointing jets (∅ ¼ 1 mm) located in the converging section of the Venturi tube. The Venturi tube constricted the diameter of the tube from 23.7 mm to 11.85 mm. A detailed schematic of the fuel injection arrangement can be found in Ref. [3]. The straight part of the circular test-section consisted in a pipe having an internal diameter of 23.7 mm. Thermocouples

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Fig. 2 e Flow reactor schematic. 1) Air inlet from compressor 2) Safety burst disc 3) Electric air heater 4) Thermocouples 5) Fuel injector 6) Ignition delay section 7) Pressure transducer 8) Cooling air for outer quartz windows 9) Outer quartz windows 10) Inner quartz tube 11) Water injector 12) Air dilution 13) Throttling valve 14) Exhaust duct.

were positioned in the reactor walls at minimum intervals of 0.845 m to ensure constant heating of the test-section. The optically accessible section of the flow reactor was located at the end of this test-section and featured a transparent quartz tube of 135 mm length and wall-thickness of 2.5 mm. The quartz tube was located within a pressurised chamber that had three quartz windows of 100 mm length, 30 mm height and 20 mm thickness, through which the inner quartz tube could be observed. Apart from the quartz tube and windows, the material of the test-section was austenitic alloyed steel (AISI 253 MA, EN 1.4835). The test-section was instrumented with Chromel-Alumel (K-type) thermocouples for temperature measurements, and was heated using feedback-controlled electric resistance heaters. A diaphragm pressure sensor (Tecsis E113) was used to measure static pressure within the tube at a distance of 69 mm upstream of the quartz tube. The inlet temperatures of hydrogen and air into the reactor were measured using K-type thermocouples. The mass flow rate of air was measured using a Coriolis mass flow meter (Micromotion) and the mass flow rate of hydrogen was measured using a thermal mass flow meter (Bronckhorst). The air used was compressed using a screw compressor (Ceccato CSB 25-13), cooled and filtered to keep impurities below 0.1 ppm. The hydrogen was supplied from bottles at a purity of 99.9%. A photoelectric cell (Hamamatsu UVtron R9454) was used to sense the emission of ultraviolet (UV) light at wavelengths of 185e260 nm, through the top window of the optically accessible section of the flow reactor. A high-speed CMOS camera (Vision Research Phantom V 7.1) was used in conjunction with an image intensifier (Hamamatsu C4598), a band-pass filter (Acton Research 310.5 ± 5.75 nm) and a phosphate glass lens (UV-Nikkor 105 mm, f/4.5) to photograph

OH*-chemiluminescence of the flame around 306 nm. OH*chemiluminescence images were recorded at a resolution of 512  512 pixels, at a frame rate of 4000 frames per second, each frame having an exposure time of 100 ms.

Experimental method The experimentally determined ignition delay time t of the mixture at a specific temperature, pressure and equivalence ratio was defined as the critical plug-flow residence time in the reactor necessary to initiate autoignition. The experimental procedure for determining this critical residence time was as follows: the mass flow rate of the reactants in the testsection was gradually decreased in small steps, as the mixture pressure, equivalence ratio, and temperature were kept constant. The limiting residence time, at which UV light was detected by the photoelectric cell, was recorded as the critical autoignition delay. The residence time of the reactants was calculated by assuming plug-flow conditions according to equation (1), where L is the length of the reactor, and Ui is the initial plug-flow velocity through the reactor. This is similar to the procedure reported in the earlier autoignition studies carried out in flow reactors [2,3,10,16,63e67]. t¼

L Ui

(1)

The reaction temperature was assumed to correspond to the mixing temperature of hydrogen and air. This mixing temperature was calculated using the inlet temperature and mass flow rates of fuel and air respectively, assuming adiabatic conditions. The electric heaters along the test-section were set so that the exit temperature of the flow reactor was equal to the calculated mixing temperature, when reactions

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were omitted by replacing the hydrogen flow by a nitrogen flow of the same total heat capacity. This made the reactor essentially adiabatic. The water injector used to quench the reactants just upstream of the thermocouple measuring the reactor exit temperature was temporarily interrupted during the measurement of exit temperature. The conditions for modelling the experiments with a plugflow reactor are perfect radial mixing and no axial mixing of the reactants. Molecular transport via diffusion may be charclet number (Pe) of the flow reactor, as acterised by the Pe defined by equation (2): Pe ¼

LUi D

(2)

clet numbers for the where D is the molecular diffusivity. Pe experiments of this study are much larger than unity (Pe >> 1), indicating that the rate of transport by the main convective flow velocity is fast in comparison to the rate of transport by diffusion. This suggests that diffusional transport is slow in axial and radial directions. Transport in the experiments is instead dominated by turbulence. Turbulent transport in radial direction is, on its largest scale, a fast process compared to the overall ignition delay, as a result of the high turbulence levels in the reactor (14,000 < Re < 34,000). For all conditions tested in this study, the turnover time of an eddy of the radius of the reactor can be shown to be shorter than the autoignition time, by more than an order of magnitude. This is given in equation (3): taxial ¼ teddy ¼

p$D ≪t 2$Ui

(3)

Turbulent transport in axial direction on the contrary, is a slower process than the ignition delay taxial ¼ teddy

L >t D

Within limitations, this warrants an initial approximation of the experimental apparatus using a plug-flow reactor model. A discussion of various fluid dynamic limitations in a similar experimental apparatus can be found in Ref. [62]. Further discussion of the effect of local mixing by turbulent diffusion is provided in Section 4.2.2.

Image processing

et al.'s [68] recent comparison of published hydrogen mechanisms.

Uncertainty analysis The uncertainty in reaction temperature and pressure, equivalence ratio, and of the ignition delay, occurred due to measurement errors and deviation from plug-flow conditions. The mixing temperature of the reactants was determined using an energy balance of the mixing flows, and was estimated to have an uncertainty of 7%, or 56 K, based on temperature and mass flow measurements. The reaction pressure bore an uncertainty and variation of 2%, based on pressure measurements, limitations in controlling the pressure in the reactor and pressure drop. The equivalence ratio was accurate to 3%, based on mass flow stability. The ignition delay was estimated to have an uncertainty of 4%, by assuming plug-flow conditions through the reactor. This uncertainty in ignition delay was calculated by adding the variance in measurement used to calculate the plug-flow residence time, to the uncertainty in autoignition delay resulting from using a finite mixing time (15.3 ms) of the reactants during fuel injection. The variance in plug-flow residence time was calculated on the basis of temperature, pressure, mass flow, and spatial dimension measurements. The uncertainty in autoignition delay resulting from using a finite mixing time was estimated using a transient gas-phase kinetic model. This model assumed that during a finite mixing time, fuel mixed with air causing the equivalence ratio to progress from infinity to that of the final mixture of reactants, and air mixed with fuel causing the equivalence ratio to progress from zero to that of the final mixture. A detailed description of this model can be found in Ref. [3].

Results The results comprise OH*-chemiluminescence images and measurements of autoignition delays for hydrogen and air at temperatures of 800e850 K, pressures of 0.8e1.2 MPa and an equivalence ratio of 4 ¼ 0.25. Ignition was recorded under turbulent conditions of ReD ¼ 15,000e34,000 and delays were in the range of t ¼ 210e447 ms.

Visualisation of autoignition The OH*-chemiluminescence of the reactants was recorded in 12-bit grayscale images with a resolution of 512  512 pixels. The images were corrected for background noise, calibrated spatially to the dimensions of the reactor tube, and false coloured for improved clarity.

Modelling Chemical kinetic modelling of autoignition in the reactor was carried out using homogeneous gas-phase reactions as well as heterogeneous surface reactions at constant pressure. All calculations were implemented using various chemical reactor models in Chemkin. The gas-phase reaction mechanism used for the results presented herein was that reported romne  s et al. [11], and was chosen according to Zse  ly by Ke

Kernel formation, deflagration and flashback The appearance of one or several autoignition kernels is a stochastic process. Fig. 3 shows the staggered formation of several autoignition kernels in the flow reactor at conditions of T ¼ 850 K, P ¼ 0.8 MPa and a flow velocity of Ui ¼ 13.8 m/s. Distance x describes the flow-wise direction in the reactor, and y describes the vertical distance from the central axis of the reactor. The reactor walls were located at y ¼ ±11.85 mm. The OH*-chemiluminescence images show that ignition kernels formed at a number of locations along the central axis of the reactor and subsequently merged with each other by deflagration. The occurrence of ignition via ignition centres and their subsequent propagation via deflagration has previously been described for mild ignition in shock-tubes [37].

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Fig. 3 e OH*-chemiluminescence sequence of hydrogen autoignition in air at T ¼ 850 K, P ¼ 0.8 MPa, 4 ¼ 0.25, t ¼ 211 ms, and Ui ¼ 13.8 m/s, showing the nucleation of multiple kernels followed by deflagration and flashback.

Fig. 3 illustrates how autoignition and the ensuing deflagration cause flame flashback in the flow reactor. The reaction conditions in the short optical part of the testsection differed from the remaining test-section in that the wall material consisted of quartz rather than austenitic alloyed steel, and in that heat-losses existed, due to absence of electrical heating of the quartz tube. This may allow to explain why the autoignition kernels formed along the central axis of the reactor rather than at the walls. The optical test-section represented only a small fraction of the flow reactor (<3.5%), and it is assumed that the effects of heat transfer and change of material on total reaction times were relatively small. If the ignition kernels initiated in another part of the reactor, deflagration of the reactants would still be visible in the optical section, and allow detection of the autoignition event. In addition to this, thermocouples were installed flush with the wall at intervals of 0.845 m to allow thermal detection of autoignition throughout the reactor.

Turbulent flame speed Fig. 3 illustrates how autoignition and the ensuing deflagration cause flame flashback in the flow reactor. An earlier optical study [3] has shown that upstream flame propagation from the initial autoignition site can occur, even if the initial velocity of the flow Ui is higher than the turbulent flame speed ST. This observation can be verified for the conditions shown in Fig. 3, by determining the turbulent flame speed from OH*chemiluminescence images, and comparing it to the flow velocity. Fig. 4 shows an autoignition kernel of approximately spherical dimensions, which can be used to determine the turbulent flame speed. This kernel was recorded in another

sequence of images, at the same experimental conditions as Fig. 3. The turbulent flame speed may be calculated from the expansion of the approximately spherical ignition kernel presented in Fig. 4 by equation (4): ST ¼ ur

  rb ru

(4)

where ST is the turbulent flame speed through the unburned mixture, ur is the propagation velocity of the flame kernel radius, rb is the burned gas density, and ru is the unburned gas density. Under the conditions shown in Fig. 4, an average turbulent flame speed of 7.4 m/s may be calculated, which is substantially lower than the initial plug-flow velocity Ui ¼ 13.8 m/s, and confirms that flame flashback may occur under autoignition conditions, even when the initial flow velocity is above that of the turbulent flame speed. Flashback can have adverse effects on combustor operation, when it occurs in gas turbine premixer tubes. Knowledge of the critical flow conditions under which flame stabilisation will occur under autoigniting conditions is thus important, in order to avoid autoignition flashback.

Flame stabilisation during autoignition At sufficiently high flow velocities, flashback may be avoided, and temporary stabilisation of the autoignition flame front can be observed. This may occur when the velocity of the flow remains above that of the turbulent flame speed, even once the flow has slowed down, due to thermal expansion of the gases. Under simplified plug-flow, this condition may be expressed by the criterion in equation (3):

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Fig. 5 e Flame front stabilisation of autoignition kernel in plug-flow.

n_ ¼ 2ST

  ru 1 A rb

Where ST is the turbulent flame speed, A is the cross-section area of the reactor. Assuming equilibrium and constant pressure throughout the reactor, the discharge velocity of the reactants may be assumed to remain constant at time t and Dt, i.e. equal to the initial flow velocity Ui. This is because changes in pressure may be assumed to be small during the autoignition event, as was verified experimentally by the pressure sensor, and the valve controlling the discharge velocity still operates on unburned gases of the same density during this transient event. The final velocity of the incoming flow Uf, during propagation of the autoignition kernel in upstream and downstream directions thus becomes:

Fig. 4 e OH*-chemiluminescence sequence of hydrogen autoignition in air at T ¼ 850 K, P ¼ 0.8 MPa, 4 ¼ 0.25, t ¼ 211 ms, and Ui ¼ 13.8 m/s, used to determine flame propagation velocity ST.

Uf ¼ Ui  2ST

Uf ¼ ST

(5)

where Uf is the plug-flow velocity of the reactants after autoignition has occurred, and ST is the turbulent flame speed through the unburned mixture. If the flame kernel forms at the very end of the flow reactor, the final plug-flow velocity depends on the initial plug-flow velocity through the reactor Ui, as expressed in equation (6): Uf ¼ Ui

  rb ru

(7)

(6)

where Ui is the plug-flow velocity of the reactants before autoignition has occurred. If the kernel forms in the middle of the flow, where it is surrounded by reactants in the upstream and downstream directions, flame propagation will occur in upstream and downstream directions (see Fig. 5). Under such conditions, as illustrated by the hatched area in Fig. 5, the volume increase n_ of the reacting gases as a result of upstream and downstream expansion of the kernel after a time step Dt, may be described by equation (7):

  ru 1 rb

(8)

When Uf matches the turbulent flame speed, the upstream flame front may be observed to stabilise. The initial plug-flow velocity Ui, for which flame stabilisation is likely to occur, may thus be described by equation (9):   r Ui ¼ ST 2 u  1 rb

(9)

Temporary flame stabilisation was observed experimentally at an equivalence ratio of 4 ¼ 0.25, a pressure of P ¼ 1.0 MPa and a temperatures of T ¼ 850 K and Ui ¼ 12.88 m/s. Visualisation of such a flame using OH*-chemiluminescence is shown in Fig. 6. Fig. 6 shows that the upstream flame front of the ignition kernel remained static in the flow. This autoignition kernel did not form at the end of the reactor, but some distance upstream, in the free flow. The kernel was thus surrounded by reactants upstream and downstream, and flame propagation took place in both upstream and downstream directions. Stabilisation of the upstream flame front of the kernel implies that the turbulent flame speed of the kernel matched that of the flow velocity in the reactor during expansion of the kernel. The turbulent flame speed ST may thus be calculated based on the initial plug-flow velocity Ui, and the densities of the unburned and burned gases ru and rb using equation (7). The

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procedure delineated in Section 3.2. Delays in the range

t ¼ 210e447 ms were determined for hydrogen in air, at temperatures of 800e850 K, pressures of 0.8e1.2 MPa and at a constant equivalence ratio of 4 ¼ 0.25. In the following sections, the ignition delays are compared with homogeneous gas-phase chemical kinetic calculations, deflagration times and reaction times for heterogeneous gas-phase and surface reaction combinations.

Homogeneous gas-phase chemical kinetic modelling The autoignition delays were compared with literature results at similar equivalence ratios (0.2 < 4 < 0.3), and with chemical kinetic calculations obtained using homogeneous gas-phase reactions in a plug-flow reactor model. Fig. 7 shows the autoignition delays of this study alongside literature data and chemical kinetic calculations. A full list of the experiments conducted to determine these ignition delays is available as supplementary material to this article. Three main observations can be made when comparing the experimental results of this study to the chemical kinetic calculations: First, the ignition delays measured in this study are shorter than those predicted by chemical kinetic modelling. Second, the ignition delays recorded in the current study are shorter for lower pressures than for higher pressures. This contradicts the results of the chemical kinetic calculations, which predict longer ignition delays for lower pressures than for higher pressures. Third, the magnitude of the slope of the ignition delay data, which represents the activation energy of the reaction, was lower for the experimental data (73 kJ/mol) than that for the chemical kinetic calculations (185 kJ/mol). Fig. 6 e OH*-chemiluminescence sequence of hydrogen autoignition in air at T ¼ 850 K, P ¼ 1.0 MPa, 4 ¼ 0.25, Ui ¼ 12.88 m/s, Uf ¼ 5.18 m/s, showing temporary kernel stabilisation when Uf ¼ ST.

densities of burned and unburned gases can be determined by chemical kinetic calculations using a closed homogeneous batch reactor model at constant pressure. For the conditions shown in Fig. 6, the densities were calculated as ru ¼ 3.72 kg/ m3, and rb ¼ 2.13 kg/m3. The turbulent flame speed for the kernel shown in Fig. 6, may thus be estimated as ST ¼ 5.18 m/s, for a Reynolds number based on the initial flow velocity of Re ¼ 30,000. This also corresponds to the flow velocity Uf to which the flow is slowed down after the onset of autoignition. Stabilisation of the flame at the initial autoignition location was observed to only be short-lived, as longer sequences of images revealed that new autoignition sites formed upstream of the initial autoignition site. The occurrence of new autoignition sites further upstream, was likely to be a result of increased residence time of the reactants in the reactor, due to the reduction in flow velocity from Ui to Uf upstream in the reactor.

Experimental autoignition delays The autoignition delays of the reactants were measured under varying conditions of temperature and pressure, using the

Fig. 7 e Experimental results of this study, modelling results for homogeneous gas-phase plug-flow reactor and experimental data of 0.2 < 4 < 0.3 reported in the literature. Symbols: experiments, solid lines show experimental : 0.8 MPa C and uncertainties; lines: modelling. : and : 1.0 MPa - and : 1.2 MPa B Beerer et al. [2],  romne  s et al. [11] RCM data. Walton et al. [23], Ke

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The first observation, of the experimental data displaying shorter ignition delays than the chemical kinetic modelling, is consistent with the previous experimental studies of autoignition delay times in the mild ignition regime. Voevodsky and Soloukhin [37], Blumenthal et al. [44], and Petersen et al. [10] reported such behaviour for autoignition delays in shocktubes, Walton et al. [23] indicated similar behaviour in a rapid compression machine, and Swigart [15], Peschke [16], Beerer et al. [2], and Petersen et al. [10] reported similar results from flow reactors. The second observation, that the ignition delays recorded in the current study are shorter for lower pressures than for higher pressures, does not seem to have been reported in the literature. Yet, the results obtained by Swigart [15] are similar, in that they demonstrated that increasing the surface area with respect to the amount of reactants reduced the ignition delays by increased catalytic activity. In the present study, decreasing pressure but keeping temperature and surface area of the reactor constant, resulted in an analogous increase in the surface area with respect to the amount of reactants. The observed reduction of ignition delays could thus be indicative of increased catalytic reactivity. The third observation, that experimental results display a significantly lower activation energy than predicted by gasphase kinetic calculations, is consistent with the results obtained by Swigart [15], Peschke [16], Beerer et al. [2], and Petersen et al. [10] from flow reactors. Swigart [15] observed a systematic reduction in activation energy with decreasing reactor diameter, and ascribed this to the catalytic activity of the walls. Different approaches to explain and to model the effects described above were tested in the present work. First, the time scales of chemical kinetic autoignition time and deflagration time are evaluated, and factors such as turbulent diffusion time scale, Reynolds number and diffusivity considered. Second, causes for early autoignition such as temperature and equivalence ratio inhomogeneities and surface reactions are discussed. Simplified surface reaction modelling on the reactor walls is conducted with the simplified assumption of fast radial transport.

tdefl. < tchem. < tdiff. Since the current experiments were conducted under conditions of varying turbulence levels. Turbulence may aid in enhancing mixing, and reducing initial variations in temperature and equivalence ratio variation. The influence of Reynolds number on autoignition on a flow reactor has been studied numerically by Wu and Ihme [62], who showed that for a transition from lower Reynolds numbers (Re ¼ 104) to higher Reynolds numbers (Re ¼ 105), the approximation of autoignition length to plug-flow conditions improves with increasing turbulence. Fig. 8 shows a plot of Reynolds numbers of the experiments versus ignition delays at varying temperatures and pressures. An experimental study systematically studying the influence of turbulence levels on autoignition delays at constant temperature and pressure would be necessary to isolate the effect of Reynolds numbers on autoignition delays in the reactor. In the absence of such a study, Fig. 8 illustrates that Reynolds number is not the dominating effect in determining the autoignition delays in a flow reactor. Diffusivity may also affect ignition delays, if reaction rates are limited by diffusive transport, for instance near the walls. Since diffusivity decreases at higher pressures, this could contribute to an increase in ignition delays at higher pressures. Diffusivity could be of particular importance in combination with catalytic reactions occurring at the walls. Under such conditions, higher diffusivity may lead to improved transport to the walls and may lead to a stronger shortening of ignition delays at lower pressures. Since diffusivity is dependent on temperature and pressure, the effect of diffusivity cannot readily be isolated from that of temperature and pressure, as Reynolds number can. Fig. 9 illustrates that increased diffusivity at lower pressures could to some extent contribute to decreasing ignition delays at lower pressures. Medvedev et al. [57,60] showed that the transition from the sharp ignition regime to the mild ignition regime can often be characterised by the deflagration time scale becoming shorter than the chemical reaction time scale, thus allowing the bulk

Characteristic time scales The chemical kinetics of the reactions may be affected by perturbations, which alter the reaction rates and change the ignition delay. Such perturbations may consist of local deviations of temperature, pressure, equivalence ratio, or of locally accelerated reaction rates due to catalytic surface activity. The characteristic time scales of chemical reactions, deflagration and turbulent diffusion determine whether such perturbations remain localised phenomena, or whether they are able to affect the remaining reactants. Although turbulent transport in radial direction is, on its largest scale, a fast process compared to the overall ignition delay, turbulent transport near the walls may be slow. Wu and Ihme [62] showed by numerical simulation of similar conditions, that the boundary layer based turbulent diffusion time scale in a flow reactor may be significantly longer than the chemical reaction and deflagration time scales. For the conditions of the presented experiments the following order of time scales may be calculated:

Fig. 8 e Influence of Reynolds number on ignition delays.

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autoignition delays and characteristic laminar and turbulent deflagration times is shown in Fig. 10. Fig. 10 shows that the characteristic deflagration time tB correlated with the autoignition delays measured behind reflected shocks by Blumenthal et al. [44], Martynenko et al. [45], and Petersen et al. [10] for temperatures below 1000 K, but disagreed with the ignition delays measured in the current study. The characteristic deflagration time tB calculated for the current experimental conditions was more than an order of magnitude shorter than the experimentally measured ignition delays if the laminar flame speed is used to calculate tB, and two orders of magnitude if the turbulent flame speed is used to calculate tB. The characteristic deflagration time tB is significantly shorter than the autoignition delay times, illustrating that mild ignition comprising consumption of the majority of the reactants via deflagration would be expected under the current experimental conditions. The characteristic deflagration time only shows that deflagration would occur once early ignition occurred somewhere in the mixture. It does not provide a reason for early autoignition. Fig. 9 e Effect of diffusivity on autoignition delays.

Causes for early ignition

of the reactants to be affected by early ignition caused by local perturbations of the reactants. They defined a parameter tB, to predict experimental autoignition delays in the mild ignition regime. The characteristic deflagration time tB represents the time taken for a deflagrative process to consume the reactants, if a single source of ignition is provided at the beginning of the ignition delay period. The authors showed that tB correlated with the autoignition delays reported by Martynenko et al. [45], Petersen et al. [10] and Blumenthal et al. [44]. The characteristic deflagration time may be written as equation (10): tB ¼

r$rb SL $ru

The causes for localised autoignition kernels forming prior to the bulk of the mixture undergoing autoignition, may be due to localised deviations in temperature, pressure or equivalence ratios, or as a result of accelerated reaction rates due to catalytic effects.

(10)

where, r is the reactor radius, SL is the laminar flame speed. The characteristic deflagration time tB was calculated for the conditions of this study, as well as for the experiments reported by Blumenthal et al. [44], Martynenko et al. [45], and Petersen et al. [10]. The characteristic deflagration time tB was calculated for each set of experimental conditions, by performing laminar flame speed calculations, using the chemical  romne  s et al. [11]. The kinetic mechanism presented by Ke flame conditions used to calculate tB for the current experimental study were T ¼ 825 K, and P ¼ 1 MPa. The autoignition delay t was also calculated for each study, and is shown alongside the experimental data for comparison. Since the current study differs from the shock-tube studies of Blumenthal et al. [44], Martynenko et al. [45], and Petersen et al. [10] in that the reactants were stirred by turbulence, the characteristic deflagration time tB needs to employ the turbulent flame speed ST rather than the laminar flame speed SL. The turbulent flame speed, may be determined from the analysis in Section 4.1.2, and if a constant level of turbulence is assumed, this may be extrapolated to other temperatures following changes in laminar flame speed. A plot of the

Fig. 10 e Experimentally determined autoignition delays with chemical kinetic modelling of autoignition delay and the characteristic time tB. Symbols experiment, lines modelling C 0.8 MPa : 1.0 MPa - 1.2 MPa - - - 0.8 MPa d 1.0 MPa e , e 1.2 MPa B tb using SL (1 MPa, 825 K) x tb using ST (1 MPa, 825 K) Petersen et al. [10] Petersen tb Petersen et al. [10] Blumenthal et al. [44] et al. [10] Blumenthal et al. [44] tb for Blumenthal et al. [44] Martynenko et al. [45] Martynenko et al. [45] tb Martynenko et al. [45].

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Wu and Ihme [62] showed by extensive numerical simulations that variations in temperature, and to a lesser extent the variation of equivalence ratio may be responsible for reducing the autoignition delays under the turbulent conditions of flow reactors. Dryer and Chaos [8,9] demonstrated through chemical kinetic modelling that the short ignition delays reported in a number of studies carried out under mild ignition conditions [10,16,23,44], can be explained by catalytic activity accelerating the two reactions R3 and R4. H2O2 (þM) 4 OH þ OH (þM)

R3

H2O2 þ H 4 H2 þ HO2

R4

This approach was applied to the current study by  romne s increasing the reaction rates of R3 and R4, in the Ke et al. [11] mechanism, until the autoignition delays at (T ¼ 825 K, P ¼ 1.0 MPa) would agree with the experimental ignition delay. Both reaction rates were thus multiplied by a factor of 11.14 and further multiplied by the ratio of surface area to reactants applicable to these pressures (0.83, 1 and 1.25, for P ¼ {0.8, 1.0, 1.2} MPa respectively). Fig. 11 shows that such an approach was able to reconcile the magnitude of the ignition delays, as well as reducing the difference between autoignition delays predicted at high and low pressures. It did not however, result in a full reversal of the pressure dependence as observed in the experiments of this study, nor was it able to reproduce the activation energy observed in the experiments. The activation energy determined from the modelling was 173 kJ/mol compared to 73 kJ/mol from the experiments. In order to study the influence of surface reactions further, the experiments were modelled using a kinetic model that simulated the effect of surface kinetics. This was done by adapting the surface reaction model of Deutschmann et al. [69] to the current experiments. Fig. 12 shows the experimentally determined autoignition delays together with calculations from a gas-phase kinetic model, and a model combining gas-phase and surface reactions for platinum and steel respectively. The mechanism of the surface reactions was developed for platinum, which is clearly a much more efficient catalyst than the austenitic alloyed steel used in this study. Direct implementation of the reaction set for platinum gives very short ignition delays at temperatures below about 1200 K as shown in Fig. 12(a). A brute-force sensitivity analysis showed that ignition delays were most sensitive to the adsorption reaction of hydrogen R5. H2 þ 2Pt(s) / 2H(s)

R5

The adsorption rate in the surface mechanism was modified so that the combined kinetic model using gas-phase and surface kinetics would correctly reproduce the autoigntion delay at (P ¼ 1 MPa, T ¼ 825 K). The pre-exponential factor A of the dissociative adsorption rate of diatomic hydrogen on the surface was reduced from 4.46,1010 from the original platinum model [69] to 1.49,108 for the current experiments to adjust the model to the lower catalytic activity of the steel.

Fig. 11 e Effect of changing rate coefficients for reactions R3 and R4 by a factor proportional to the ratio of reactant concentration and reactor wall surface area. Symbols: experiment, black lines: gas-phase reactions modelling, red lines: gas-phase reactions modelling with modified reaction rates R3 and R4. : and ,,, : 0.8 MPa C and d: 1.0 MPa - and e , e : 1.2 MPa. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 12 shows that addition of the surface reactions to the kinetic calculations improved the predictions in three ways: First, the ignition delays were reduced to that of the experiments. Second, the model was able to qualitatively predict the relationship between pressure and ignition delays under these conditions, in that autoignition delays increase with pressure. Third, the activation energy (91 kJ/mol) was significantly closer to that suggested by the experimental data (73 kJ/ mol) than for other modelling approaches. Fig. 12(a) illustrates that catalytic activity of the walls was calculated to have no influence on ignition delays at temperatures above 1000 K, but to have an increasing effect in shortening ignition delays at lower temperatures. This agrees with reports in the literature of larger deviations from chemical kinetic calculations at lower temperatures. Fig. 12(b) which is an enlargement of Fig. 12(a) shows that surface reactions may be able to explain anomalously short ignition delays, pressure dependence and low activation energy at temperatures below 1000 K. As discussed previously, this catalytic model uses the simplified assumption of fast transport in radial direction, and may thus be influenced by transport phenomena such as turbulence and diffusion, particularly in the boundary layer. The development of more accurate models for the surface reactions of austenitic steel surfaces, as well as on materials commonly used in gas turbine premixers would thus be of

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Fig. 12 e Experimentally determined autoignition delays and chemical kinetic modelling using a combination of surface and gas-phase reactions. Symbols experiment, lines modelling : 0.8 MPa C 1.0 MPa - 1.2 MPa - - 0.8 MPa d 1.0 MPa e , e 1.2 MPa.

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autoignition process visually by OH*-chemiluminsecence, determine the flow conditions under which flame flashback from autoignition could be avoided, and measuring autoignition delay times under conditions relevant to gas turbine premixers. The ignition process was visualised using high-speed OH*chemiluminescence imaging. It was observed that autoignition kernels formed in the central regions of the tube and subsequently expanded and merged through deflagration. Flashback occurred although the turbulent flame speed was measured to be lower than the initial flow velocity in the reactor. Analysis of the flow showed that this was because expansion of the gases from autoignition could slow-down the flow to below the turbulent flame speed. The conditions for flame stabilisation were established in equation (9). This condition describes the initial flow velocity necessary to ensure that the flow velocity through the reactor remained above the turbulent flame speed of the reactants, even during the nucleation and deflagration of autoignition kernels. Temporary flame stabilisation was verified experimentally, and was illustrated in Fig. 6. Stabilisation of the flame at the initial autoignition location was observed to only be shortlived, as longer sequences of images revealed that new autoignition sites formed upstream of the initial autoignition site as a result of increased residence time. Autoignition delays of t ¼ 210e447 ms were measured at temperatures of 800e850 K and pressures of 0.8e1.2 MPa and an equivalence ratio of 4 ¼ 0.25 in an austenitic stainless steel turbulent flow reactor. Three main observations with respect to the ignition delays were made: First, the ignition delays measured in this study were shorter than those predicted by recent chemical kinetic models. Second, the ignition delays recorded in the current study were shorter for lower pressures than for higher pressures, which contradicted chemical kinetic calculations. Third, the magnitude of the slope of the ignition delay data, which represents the activation energy of the reaction, was lower than that predicted by the chemical kinetic model. It was shown through simplified chemical kinetic modelling, that these observations may be explained when surface reactions occurring on the austenitic stainless steel reactor walls are taken into account. This was done using chemical kinetic modelling, which combined gas-phase and surface reactions, under the assumption of fast radial transport in radial direction. Transport phenomena such as turbulence and diffusion, particularly in the boundary layer may further influence the catalytic wall effects. The influence of surface reactions on autoignition delays may have implications for the design and choice of surface materials in gas turbine premixer tubes.

Acknowledgement interest to the understanding of ignition delays under practical conditions.

This work was financially supported by the Swedish Research Council (Vetenskapsrådet) under contract A0088001.

Summary and conclusions Appendix A. Supplementary data In the current study, autoignition of hydrogen in air was investigated in a pressurised flow reactor, operating under mild ignition conditions. The aim of the study was to characterise the

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.05.157.

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