effect of inhibitor gases on hydrogen flame propagation in a

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Fuel xxx (2015) xxx–xxx

JFUE 9709 No. of Pages 9, Model 5G

16 October 2015

Contents lists available at ScienceDirect

Fuel

journal homepage: www.elsevier .com/locate / fuel

Effect of inhibitor gases on hydrogen flame propagation in a confined teepipe (Part I)

http://dx.doi.org/10.1016/j.fuel.2015.10.0250016-2361/� 2015 Published by Elsevier Ltd.

⇑ Corresponding author.E-mail addresses: [email protected] (R.Md. Kasmani), mahar.diana@

um.edu.my (M.D. Hamid).

Please cite this article in press as: Emami SD et al. Effect of inhibitor gases on hydrogen flame propagation in a confined tee pipe (Part I). Fuel (2015)dx.doi.org/10.1016/j.fuel.2015.10.025

Sina Davazdah Emami a,b, Rafiziana Md. Kasmani b, Mahar Diana Hamid a,⇑, Che Rosmani Che Hassan a,Siti Zubaidah Sulaiman b

aChemical Engineering Department, Faculty of Engineering, University of Malaya, 50603 UM Kuala Lumpur, MalaysiabDepartment of Renewable Energy Engineering, Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

a r t i c l e i n f o

2627282930313233343536

Article history:Received 27 May 2015Received in revised form 17 September 2015Accepted 6 October 2015Available online xxxx

Keywords:HydrogenInhibitorsOverpressureFlame propagation and tee pipe

3738394041

a b s t r a c t

Hydrogen is a promising fuel for the future. In recent years it has been successfully utilised in industries,particularly in refineries and petrochemicals. In previous studies, the effect of inhibitors on hydrogenexplosion behaviour has been investigated in different systems, yet only scarce data are available.Therefore, experimental study is carried out to investigate the effects of argon, nitrogen and carbon diox-ide on hydrogen/air explosion in a branched pipe configuration. The fuel/air mixtures were ignited atthree different ignition positions, A, B and D. The results show that, when ignited at the furthest distance(position A), the tee junction area is most vulnerable to the critical pressure impact of gas explosion.However, no similar trend was observed at the other ignition positions. In addition, mixtures with thecompositions 95% H2–2.5% Ar–2.5% N2/air, 95% H2–5% N2/air, H2/air and 95% H2–5% Ar/air showed higherrisks due to the higher diffusivity ratio and the associated rate of pressure rise. This phenomenon is high-lighted in the discussion part of this paper. The results show that mixtures with CO2 lead to lower sever-ity than other compositions (�50% reduction), as the average recorded maximum flame speed for thisparticular mixture was lower at all of the ignition points. This suggests that the effectiveness of the inhi-bitors should be in the order of Ar < N2 < CO2.

� 2015 Published by Elsevier Ltd.

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1. Introduction

Hydrogen has been promoted for several decades as an energycarrier of the future. As a secondary energy carrier with a lowerheating value of 120 MJ/kg and density of 0.0899 kg/m3, it can beused in many applications, particularly in refineries andpetrochemicals [1]. However, hydrogen explosion hazards haveconstantly been a concern for its storage and transportationthrough pipelines; it is essential to address safety issues relatedto pipeline gas carriers by characterising and adequately quantify-ing their explosion behaviour to protect against the propagation ofunwanted combustion phenomena such as deflagrations anddetonations (including decomposition flames) [2–4]. Previousresearch has reported that hydrogen is susceptible todeflagration-to-detonation transition (DDT) [5]; it is importantthat this parameter can be calculated and predicted with a reason-able degree of accuracy towards the end goal, which is control over

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flame acceleration and the process of DDT. An extensive amount ofwork has therefore been devoted to understanding the phenomenarelated to flame acceleration and DDT in pipes. These previousworks have discussed and illustrated the fact that the changes ofthe flame-front shape and propagation in a tube, depend onnumerous parameters, such as aspect ratio, initial pressure, openor closed tube ends, and equivalence ratio of the mixture [6]. Thedynamics of the flame mechanism during the development of pre-mixed hydrogen–air flames at various equivalence ratios under-goes a complex flame inversion and exhibits more distinctcharacteristics than with other gaseous fuels [7]. Rai et al. [8], intheir report on the high-pressure release of hydrogen gas inside a6-m-long horizontal tube, indicated that the pressure starts tobuild up as the flame propagates closer to the jet velocity, and thatdeflagration that takes place closer to the jet velocity results inhigher pressure peaks [8]. Petukhov et al. [9] furthermore exam-ined shock-wave explosion propagation of hydrogen–oxygen mix-tures. They observed that the instability of flame propagation in anon-stationary combustion front results in the formation of distur-bances, waves and streams prior to the flame front. Subsequently,flame intensification creates secondary combustion or explosions,

, http://

http://dx.doi.org/10.1016/j.fuel.2015.10.025

mailto:[email protected]

mailto:mahar.diana@ um.edu.my

mailto:mahar.diana@ um.edu.my


http://www.sciencedirect.com/science/journal/00162361

http://www.elsevier.com/locate/fuel



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16 October 2015

exceeding the value of the Chapman–Jouguet conditions for sta-tionary detonation [9]. Numerical simulation and experimentalstudies on flame propagation in a duct with a 90� curved sectionalso showed good agreement on basic flame-propagation physicalphenomena such as the tulip-shaped flame, flame shedding, pres-sure evolution trends, flame-propagation speed profiles and vortexdevelopment at the bend [4,10]. However, hazard assessments ofthese phenomena are still debatable, even though these concernshave long attracted researchers’ attention. It is essential that reli-able and cost-effective prevention and mitigation methods areavailable to practitioners and researchers to assess hydrogenexplosion hazards in the pipeline in order to ensure that no lossof containment or hydrogen leakage occurs during transportation.One possible means for mitigating the hazards associated withunintended hydrogen release or DDT in the pipe is the use of inhi-bitors [11].

Previous investigations have been more focused on the effect ofthe presence of inhibitors in hydrocarbon and hydrogen fuels onthe laminar flame propagation velocity, laminar combustion veloc-ity, Markstein length, flame stability and maximum combustionpressure [12,13]. These investigations have analysed differentcombustion properties, but there are many unresolved issuesregarding the quantitative relationship between the experimentalscales and real practices. These are due to various assumptionsregarding different concentrations of fuel–inhibitors, tube/vesselsystem applications and condition boundaries in terms of theresponse to the inhibitors’ effects. For example, Qiao et al. [14]studied the flame-stretching effects on the laminar burning veloc-ities of H2–O2-diluent (N2, Ar or He) in a small-scale system (aspherical windowed chamber). They reported that these flameswere very sensitive to flame stretching, exhibiting a ratio ofunstretched-to-stretched laminar burning velocities in the rangeof 0.6 –3.0, with corresponding Markstein numbers in the range±3 to 7. They also indicated that diluents, i.e., N2, Ar, and He, tendto have a significant effect on the preferential-diffusion/stretchinteractions due to their dynamic and kinetic characteristics, i.e.,argon and helium have higher laminar-burning-velocity ratioscompared to nitrogen, based on their higher flame temperaturesand transport rates [14]. A further study on the flame propagationof H2–O2–N2 using an International Thermonuclear ExperimentalReactor (ITER) showed that both overpressure and laminar flamespeed increase as the initial pressure and temperature increase.It was also indicated that this condition was most preferable fora higher nitrogen concentration [15]. On the other hand, DiBenedetto et al. [16] made a different observation on the effective-ness of CO2 in H2–O2–N2–CO2 mixtures. They suggested that theburning velocity of the fuel–inhibitors mixtures is mainly due totheir kinetic mechanism, as the rapid reaction of O, H, CO, HCO,OH, HO2 and CO2 has a negligible effect on the transport rate. How-ever, the explosion behaviour of stoichiometric H2–O2–N2–CO2

mixtures in NTP (normal temperature and pressure) conditionshowed significant stability in the flame; this could suggest thatthere would be no ignition if CO2 presence were higher than 40%.The explosion was suggested to be mainly affected by CO2 andO2 concentration [16]. Moreover, CO2 has a significant radiationand dilution effect on quenching and blow-off limits, as reportedby Shih [17]. It is worth noting that the effectiveness of all of theseinhibitor gases on Markstein numbers has been insignificant inthese studies, due to highly similar dynamic properties[13,14,16]. Despite such extensive studies, the discussion on theinfluence of the inhibitors concerning explosion in the pipe is stillevolving, particularly in relation to the unsteady propagation offlames under branched-pipe conditions.

The point at which ignition takes place is an important factorduring the initial stages of combustion [18]. When the ignitionsource is further from a sealing flange or vessel wall, the flame will

Please cite this article in press as: Emami SD et al. Effect of inhibitor gases on hydx.doi.org/10.1016/j.fuel.2015.10.025

have a longer period to spherically propagate, leading to initiallyhigher overall flame speed and rate of pressure rise. Changes inthese initial conditions will also affect the continuation of combus-tion further along the pipe, particularly when the pipe has differentconfigurations, i.e., obstructed, bending and branched. Investiga-tions of fuel transfer in pipes have shown that 36.5% v/v hydrogenin air has the potential to develop DDT [19,20]. Blanchard et al. [21]have also reported that the ignition position has a significant effecton DDT development in hydrogen/air explosion. In their study, theshortest run-up distance to DDT relative to the end pipe wasrecorded when the ignition source was placed 0.7 m from the pipeends. From the discussion above, it can be postulated that there is adirect relationship between the changes of ignition positions andthe presence of inhibitors on the pressure-development mecha-nism and flame dynamics during gas explosion inside the branchedpipe – this is the focus of the current research.

The present paper reports the explosion pressures at differentignition positions, the maximum rate of pressure rise and severitymagnitude of explosion of hydrogen–air mixtures. The effect ofinhibitors in this context is also explored; this is achieved by theclose examination of pressure waves at various points in thebranched pipe configuration.

2. Methodology

In this study, a tee-pipe rig configuration was used. The pipelength was 4.32 m with a diameter of 0.1 m. The junction pipe seg-ment was 1.375 m long (refer to Fig. 1). The pipe was made up of anumber of segments, ranging from 0.5 to 1 m in length, boltedtogether with a gasket seal in between the connections and blindflanges at both ends.

Measurements of flame speed were made using an array ofexposed-junction, mineral-insulated, type-K thermocouples CHALOMEGA (accuracy ±0.001 s) along the centre-line of the entire pipe(indicated by T1–T8 in Fig. 1). The flame-speed data were gener-ated from the thermocouple flame arrival times, the time of travelbetween two adjacent thermocouples, and the distance betweenthem. The flame speeds were plotted at the position midwaybetween the thermocouples, or in the case of the first flame speedat the time between the spark and arrival at the first thermocouple.This technique does not measure the flame temperature as thethermocouple junction was too large (�0.5 mm); however, thereis no dead time and the flame is detected as a sudden increase intemperature from a near-ambient base temperature. The thermo-couple flame arrival time in the pipe was taken to be the first pointat which the reading started to rise. For the thermocouples in thepipe, the arrival time was delayed by a pre-compression waveahead of the flame (and the associated high flow velocity aroundthe thermocouple), which gave rise to two distinct gradients onthe thermocouple trace. In this case, the point at which the second(steeper) gradient became apparent was taken as the flame arrivaltime. Pressure measurements were taken using seven piezoresis-tive pressure transducers (P1–P7 in Fig. 1) to continually measurethe pressure development and rates of pressure rise inside the testpipe. The pressure transducers were Keller Series 11, with accuracyof ±0.001 s. The data generated were collected using a 34-channeltransient data recorder manufactured by NI Compact DAQ. Infor-mation about the accuracy and reproducibility of detected param-eters can be found in Wang et al. [22]. The ignition positions andeach location of the sensors are presented in Table 1.

The mixtures of H2–Ar/air, H2–CO2/air, H2–N2/air, H2–Ar–CO2/air, H2–Ar–N2/air and H2–N2–CO2/air with a constant ratio(95:5 = H2: inhibitors) were applied at a stoichiometric equivalentratio (/ = 1) by considering hydrogen as the primary gas. The par-tial pressure method of mixture preparation adds the flammablegas to a vacuum (VACUUBRAND RE 2.5) and then adds air to

drogen flame propagation in a confined tee pipe (Part I). Fuel (2015), http://



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Fig. 1. Scheme of tee pipe.

Table 1Position of each pressure transducer and thermocouple from ignition points.

Ignition position

A B D

Pressure transducersP1 0.26 3.99 4.45P2 1.18 3.07 3.53P3 2.53 1.72 2.18P4 3.42 0.83 1.29P5 3.9 0.35 1.77P6 4.03 1.44 0.6.P7 4.52 1.93 0.19ThermocouplesT1 0.45 3.84 4.25T2 1.26 3.03 3.44T3 1.65 2.64 3.05T4 2.75 1.54 1.95T5 3.42 0.87 1.23T6 3.99 0.26 1.85T7 4.12 1.54 0.56T8 4.38 1.8 0.3

S.D. Emami et al. / Fuel xxx (2015) xxx–xxx 3


16 October 2015

approx. 101,325 Pa. The explosion was carried out after a delay ofabout 10 min. This method of mixture preparation ensures com-plete mixing, as the initial vacuum condition rapidly dispersesthe fuel added and subsequent addition of air takes place understill very low pressure; together with the turbulence from the airinjection, this ensures rapid mixing. The mixture compositionwas controlled to an accuracy of 10 Pa (0.01% of composition).The flammable mixture was initiated by an electrical spark, giving16 J energy for the explosion tests. For this study, three ignitionpositions were used, denoted as A, B and D, as shown in Fig. 1.The ignition position at C is not considered, as it will be discussedin our next paper. Repeatability and accuracy were confirmed byconducting at least three tests at each concentration.

3. Results and discussion

Previous studies indicate that pipe configurations and initialconditions have a significant impact on flame reactivity and theacceleration of hydrogen/air explosions [23–26]. However, theeffect of inhibitors on the flame reactivity of hydrogen/air mixturesin a pipe has not been fully explored. The discussion here willtherefore focus on the pressure and flame-propagation mecha-nisms of hydrogen-inhibitors/air mixtures explosions in a tee-pipe configuration with the effect of different ignition positions.

3.1. Effect of inhibitors on pressure and flame speed during explosiondevelopment

Sets of recorded maximum overpressure along the tee pipe aregiven in Fig. 2 at three different ignition positions, denoted as A, B


and D (see Table 1). These are in reference to the stoichiometrichydrogen/air mixtures, which were diluted with various amountsof CO2, Ar and N2. Flame speed and rate of pressure rise (dP/dt)are plotted in Figs. 3 and 4 as a function of pressure transducerpoints located along the pipe. Considering hydrogen as a primefuel, a significant pressure drop of up to �50% and flame linearityin the presence of CO2 were observed, particularly with the mix-tures 95% H2–5% CO2/air, 95% H2–2.5% CO2–2.5% N2/air and 95%H2–2.5% Ar–2.5% CO2/air. As reported by Di Benedetto et al. [16],the presence of free radicals, including O, H, CO, HCO, OH, HO2

and CO2, causes the rate of overpressure, flame speed and rate ofpressure rise to decrease significantly, as illustrated in Figs. 2–4.Among the inhibitors, argon in the hydrogen/air mixtures gave ahigher overpressure than the others. Movileanu et al. [27] showedthat the presence of additives changes the thermo-physical proper-ties of the unburned mixtures in terms of thermal conductivity andheat capacity, and this condition will directly affect the overallflame reaction rate within the reaction zone. Kwon and Faeth[13] also claimed that, even though nitrogen and argon have verysimilar transport properties, the addition of argon into hydrogencould lead to a higher laminar burning velocity due to increasedflame temperature. This mechanism suggests that the effectivenessof inhibitors should be in the order of Ar < N2 < CO2.

On the other hand, we can postulate from Fig. 3 that there was asignificant change in the flame speed of fuel–inhibitors/air mix-tures along the centreline of the pipe and at the end points at allignition positions. It can be said that fuels’ reactivity plays a majorrole within this context [28]. Fig. 3 shows that the highest flamespeed was at an average of 736, 719, 706 and 703 m/s for 95%H2–2.5% Ar–2.5% N2/air, 95% H2–5% N2/air, H2/air and 95% H2–5%Ar/air mixtures, respectively, for all ignition positions. Theseresults are in good agreement with a previous study carried outby Kwon and Faeth [13,14]. Comparing the flame speed of thesemixtures with 95% H2–5% CO2/air under similar conditions, the lin-earity of the latter mixtures was higher, as the average recordedmaximum flame speed was 599 m/s for all ignition positions. Thisshows that the addition of CO2 into hydrogen mixtures caused theheat release rate to decrease, consequently lowering the averagevalue of the flame speed. This present observation is in similaragreement with previous investigations in terms of the effects ofCO2 on reducing the flame propagation, thus decreasing the overalloverpressure inside the pipe [16].

In order to further justify the effect of inhibitors on the explo-sion development of stoichiometric hydrogen/air mixtures, Lewisnumbers were calculated as a basic analysis of the diffusivity rateof involved species in the unburnt gases, corresponding to themass burning rate. This is the most important factor that affectsthe burning velocity and heat release rate during flame propaga-tion [27]. Lewis number, Le, was plotted as a function of pressuretransducers’ points along the pipe as shown in Fig. 5. From the




Fig. 2. Maximum overpressure vs. the recorded points in a tee pipe at differentignition positions.

Fig. 3. Maximum flame speeds vs. the pressure transducer points in a tee pipe atdifferent ignition positions.



16 October 2015

Please cite this article in press as: Emami SD et al. Effect of inhibitor gases on hydrogen flame propagation in a confined tee pipe (Part I). Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.10.025



Fig. 5. Rate of pressure rise vs. the recorded points in a tee pipe at different ignitionpositions.

Fig. 4. Lewis number vs. the recorded points in a tee pipe at different ignitionpoints.



16 October 2015




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16 October 2015

figures, the value of the Lewis number for each of the fuel–inhibitors/air mixtures was lower than unity, which is in fairagreement with previous research data at NTP condition [29–31].Analysis of the results presented in Fig. 5 clearly implies that allof the mixtures were affected by mass diffusivity, and thatcondition was more apparent when argon was added. The factthat the diffusion coefficient under normal conditions for Ar(2 � 10�5 cm2 s�1) was higher than both N2 (1.88 � 10�5 cm2 s�1)and CO2 (1.92 � 10�5 cm2 s�1) contributes to this condition; thisgives a higher ratio of the flame temperature and diffusion coeffi-cient [32]. It can be said that H2 diffusivity is at its highest whenadding argon in the hydrogen/air mixtures, and this could furtheraccelerate the flame propagation inside the pipe. This explainsthe higher overpressure and flame speed shown in the H2/air mix-tures explosion when argon is present.

From the overall observations, it is worth noting that the over-pressure plot of the 95% H2–5% N2/air mixture poses a differenttrend at all ignition positions. The maximum peak overpressure,flame speed and rate of pressure rise were recorded at differentpositions along the pipe. A possible explanation for this phe-nomenon is as follows: the momentum movement of the N2O,NNH and NO radicals resulted in variation on the explosion pres-sure development, and this directly affected the rate of pressurerise and flame speed ratio. The contribution of the N2O intermedi-ate route rises much faster than that of the other routes involved inthe reaction [33]. Besides, it should be noted that the differentflame paths greatly depend on the initial ignition position, as thisvaries the flame propagation mechanism. Another interestingobservation worth mentioning is that the mixture 95% H2–2.5%Ar–2.5% N2/air gave higher values for overall overpressure, flamespeed and rate of pressure rise; indeed, the values were approxi-mately twice those of the 95% H2–2.5% Ar–2.5% CO2 composition,as illustrated in Fig. 5. As mentioned earlier, with regard to theeffect of N2 on the inhibitors–fuel mixtures, the unique dynamicproperties (flame diffusivity and thermal conductivity) of argonalso play an important role in intensifying the combustion of themixtures.

3.2. The influence of ignition positions on overall explosiondevelopment

The location of the maximum pressure was observed for allignition positions. It was found that the highest overpressure forthe pure hydrogen/air mixture was observed at the tee junction(P7) for the ignition positions of A and B, while P1 gave the highestpressure at the ignition position D. Further attention should bepaid to the overpressure profile of the hydrogen/air flames as plot-ted in Fig. 2. The propagated flames when ignited at A, B and Dexperienced a greater acceleration after the tee position, leadingto a higher pressure at the end pipe.

To further clarify this condition, the blast wave plots of all mix-tures at P1, tee point (P4), P5 and P7 at t = 1 ms are presented inFig. 6. From the figures, it is apparent that the pressure profile ateach point, depending on the mixtures and ignition position, hasmore or less fluctuation within a 1 ms period. The kinetic reactionof each mixture was suspected to contribute to this condition.However, this and the Lewis number effect, which increases flameacceleration due to the flame wrinkling mechanism [34], are themost likely causes of the pressure variation. For instance, it tooka shorter time to reach the maximum overpressure for the hydro-gen/air, 5% Ar–95% H2/air, 5% N2–95% H2/air and 2.5% Ar–2.5%N2–95% H2/air mixtures at P1, P5 and P7, compared to othermixtures. Furthermore, the hydrogen/air, 5% Ar–95% H2/air, 5%N2–95% H2/air and 2.5% Ar–2.5% N2–95% H2/air mixtures travelledmore quickly to the end pipe, giving a net result of a higher rate ofpressure rise and flame speed. It is noticeable that the influence of


ignition positions on the Lewis number changed for each mixturewith a value less than unity, as shown in Fig. 5. However, a peculiarobservation was recorded for the Lewis number when 2.5%Ar–2.5% N2–95% H2 ignited at B. We can interpret that flame per-turbation occurred at an early stage of flame propagation in thismixture, illustrated by a lower Lewis number of 0.038. It is possiblethat flame wrinkling, due to a substantial increase in mass burningrate, triggers mass diffusivity instability and subsequently insti-gates the flame speed and overall pressure system as mentionedearlier. The flame instability was more apparent when the flamepropagated towards the end pipe, i.e., P1 and P2. Sharp rises ofLewis number of up to 0.057 are displayed in Fig. 5, indicating thatflame instabilities are pronounced. In this condition, it can be pre-sumed that reflective wave or rarefaction propagations combinedwith flame wrinkling were the major factors leading to the increaseof maximum pressure to �4 � 105 Pa. The Lewis number is calcu-lated as follows:

Le ¼ aD

ð1Þ

where a is thermal diffusivity and D is mass diffusivity. Thermal dif-fusivity is the thermal conductivity divided by density and specificheat capacity at constant pressure:

a ¼ kqCP

ð2Þ

where k is thermal conductivity (W/(m K)), q is density (kg/m3) andCP is specific heat capacity (J/(kg K)). The mass diffusivity depen-dence of the diffusion coefficient on temperature for gases for thecurrent system can be expressed using:

D ¼1:858� 10�3T3=2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1M1

þ 1M2

q

Pð3Þ

Subscript 1 and 2 indicate the two different species present in thegaseous mixture; T is the absolute temperature (K), M is the molar

mass (g/mol) and P is the relative pressure referring to PxPatmosphere

� �

where Px is the pressure on each measured point in this work [35].The Lewis number effect can be further discussed based on the

following observation: the flame propagation of the mixtures, inparticular of the 5% N2–95% H2/air and 2.5% Ar–2.5% N2–95%H2/air mixtures along the pipe, was inconsistent with that of othermixtures when the mixtures ignited at position B, as illustrated inFigs. 3 and 5. The appearance of the multiple peak pressures (dis-played in Fig. 6) indicates that, at a shorter obstacle distance, theflame experienced a strong interaction of transverse pressurewaves from the tee junction and induced more turbulence. Arecorded Lewis number of �0.03 at the tee junction (P4) for thosemixtures mentioned above suggests that the wrinkling of the flamebecame so great that the pockets of trapped unburned gas alongthe Tee junction area preheated. The net effect of the flame wrin-kling and rapid turbulence is that the mass-burning rate of theflame increases, causing the flame to accelerate rapidly. This, sub-sequently, gives a higher overpressure. A similar situation wasobserved by Kim et al. [34], who demonstrated that the flame fora Le < 1 mixture was wrinkled by diffusional–thermal instability,accelerating the flame speed and consequently increasing the over-pressure with time.

To justify the findings, the unburnt gas velocity ahead of theflame (Sg) was calculated and plotted for all the mixture composi-tions, as given in Fig. 7. As mentioned earlier, the flame reactivity of5% Ar–95% H2/air, and partially 5% N2–95% H2/air and 2.5% Ar–2.5%N2–95% H2/air, were mostly affected due to the dynamic effect,i.e., mass diffusivity and rapid mixing of the induced turbulenceand faster flame downstream in the pipe. Since the induced maxi-mum value of Sg was calculated to be more than 340 m/s, the




Fig. 6. Blast wave in P1, tee point, P5 and P7.



16 October 2015




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Fig. 7. Unburnt gas flame speed vs. the recorded points in a tee pipe at differentignition points.



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flame travels at a greater speed, giving a massive compressionimpact at the end pipe and thus, promoting a strong interactionbetween the fast flame and reflective wave. This phenomenon isknown as retonation, and was significant when the ignition initi-ated at position A and D. Previous studies [15,16] provide similarobservations on this phenomenon. However, the ignition positionB showed the opposite impact on both pressure developmentand flame propagation in the tee pipe configuration, and retonationwas barely observed. It can be interpreted that the blended inhibi-tors–fuel/air mixtures did not guarantee that the explosion riskposed on the obstacles would be minimised. Instead, in this study,when inerting was considered and ignition occurred at B, a ‘vulner-able condition’ resulted. As shown in Fig. 2, the maximum over-pressures for 2.5% CO2–2.5% N2–95% H2/air, 2.5% Ar–2.5% N2–95%H2/air and 2.5% CO2–2.5% Ar–95% H2/air were observed at P2 whenignited at B. Previous research has shown that, with a wider fuelvariation, i.e., composition of the blended mixtures, the Lewisnumber is directly proportional to the wider Markstein numbers[36]. This implies that the flame acceleration, with ignition at posi-tion B, was mainly affected by mass diffusivity (refer to Fig. 5), asmentioned in the previous section. It further suggests that thetransition of the flame deformation into detonation and a stronginteraction of reflective waves and fast flames downstream of thepipe could be possible causes of the maximum overpressure atP2. Furthermore, at ignition position B, the mixtures consisting of5% N2–95% H2/air and 2.5% Ar–2.5% N2–95% H2/air gave the highestoverpressure, recorded at P5 and P7 in the former and P5 for thelatter. However, it can be postulated that one of the dominantcauses of the fast flame propagation was mass diffusivity, particu-larly in the presence of Ar and N2, as explained in the previous sec-tion. This phenomenon is also discussed by Qiao et al. [14].

3.3. Explosion severity

Based on the rate of pressure rise, it can be depicted that theseverity of all of the mixtures at ignition point B was higher thanthe others. Moreover, the maximum rate of pressure rise valueswere measured at 78.08 � 105, 74.64 � 105, 73.82 � 105 and58 � 105 Pa s�1 at all of the ignition points for 95% H2–2.5%Ar–2.5% N2/air, 95% H2–5% N2/air, H2/air and 95% H2–5% Ar/air,respectively (refer to Fig. 4). From the data, it is found that flamemixtures with CO2 had lower severity. This suggests that severityin the pipeline not only depends on the vessel size [37], initialcondition and mixtures [10], but also on points of ignition.

4. Conclusion

Based on the importance of hydrogen fuel for future transporta-tion, this research was conducted to investigate the effectiveness ofargon, nitrogen and carbon dioxide on hydrogen/air flame acceler-ation in a tee pipe configuration.

The findings can be summarized as follows:

1. The ignition position and initial fuel composition had a stronginfluence on the overpressure, flame speed and rate of pressurerise. The blending of the inhibitors gave a lower severity in allthe studied compositions of hydrogen/inhibitors/air mixtures.From the study, CO2 is the most efficient inhibitor, followedby N2 and argon.

2. The explosion severity recorded at ignition position B washigher than at A or D. Flame acceleration, with ignition at posi-tion B, was mainly affected by mass diffusivity; consequently, itcan be said to promote the transition of flame wrinkling, whichincreases the mass burning rate in the system. However, flame




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acceleration at ignition positions A and D was affected by thestrong interaction of a reflective wave at the end pipe and a fastinduced flame, highlighting the retonation-like obstaclesphenomenon.

3. Of the studied mixtures, 95% H2–2.5% Ar–2.5% N2/air, 95%H2–5% N2/air, H2/air and 95% H2–5% Ar/air showed the highestseverity risks in terms of rate of pressure rise. This is due totheir dynamic properties. However, it was found that themixtures in the presence of CO2 had lower severity, since theaverage recorded maximum flame speed was lower at all ofthe ignition points (at an average of 599 m/s).

Most important to clarify are the methodologies to classify thepropagating flames on a tee pipe configuration, taking into accountthe distance position of the tee pipe junction as a function of theignition position. This phenomenon will form the focus of our nextpaper.

Acknowledgements

The authors would like to express the gratefulness and appreci-ation to PPP Grant (Project No: PG105-2013A), FRGS Grant (ProjectNo: FP013-2014B) and UMRG Grant (Project No: RP015-2012H)from University of Malaya (UM). We also would like to show ourgratitude to Universiti Teknologi Malaysia (UTM) for the researchGrant (No; QJ130000 2542 03H41) that have developed the explo-sion test facility.

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