Effect of pressure on effectiveness of quenching meshes in transmitting hydrogen combustion

Nuclear Engineering and Design 224 (2003) 199–206

Effect of pressure on effectiveness of quenching meshesin transmitting hydrogen combustion

S.Y. Yanga, S.H. Chunga,∗, H.J. Kimb

a School of Mechanical Engineering, Seoul National University, Seoul 151-742, South Koreab Propulsion Performance Test Department, Korea Aerospace Research Institute, P.O. Box 113, Yusung, Daejeon 305-600, South Korea

Received 29 October 2002; received in revised form 14 February 2003; accepted 1 April 2003

Abstract

Applicability of quenching meshes for the control of hydrogen combustion in nuclear power plants under severe accidenthas been investigated experimentally. The effects of initial pressure in hydrogen/air mixtures on quenching distances have beenfirst measured to provide basic data. It has been found that the quenching distance has its minimum near stoichiometry andcan be approximated to be inversely proportional to initial pressure. Small-scale experiments were conducted to identify theapplicability of quenching meshes for the control of hydrogen combustion in a closed system where hydrogen combustionleads to pressure rise. Simplified phenomenological analysis has also been performed to identify parameters that govern theperformance of quenching meshes. It has been experimentally substantiated that quenching meshes could effectively confinehydrogen combustion in a compartment when hot gaseous jet ejection can be mitigated.© 2003 Elsevier Science B.V. All rights reserved.

1. Introduction

Control of hydrogen combustion is one of the keyissues for the safety of nuclear power plants (NPPs),because a large amount of hydrogen generated undera severe accident could cause an explosive reactionwhich influences the containment integrity and equip-ment survivability. Since the TMI-2 accident (NASC,1980), much effort has been directed to control thehydrogen combustion in a containment (Camp et al.,1983). As a result of this effort, igniters and passiveautocatalytic recombiners (PARs) have been consid-ered to prevent damaging burns in NPPs for protectingthe containment integrity under severe accident condi-

∗ Corresponding author. Tel.:+82-2-880-7114;fax: +82-2-889-1842.

E-mail address: [email protected] (S.H. Chung).

tions (Cummings et al., 1983; Thompson et al., 1984;Kumar et al., 1984).

Combustion behavior in sub-compartments is un-certain and depends on geometry, gas concentrations,and availability of ignition sources. Depending ongeometry and gas concentrations, turbulent deflagra-tion, accelerated flames, and transition to detonation(DDT) cannot be excluded.

Installation of quenching meshes between com-partments or around equipments has been suggestedto control flame propagation among compartmentsand to maintain equipment survivability. It has beenfound that quenching meshes could effectively con-fine hydrogen combustion in open vessel tests wherepressure rise is negligible during combustion (Chunget al., 1996; Kim et al., 1999).

In the present study, the characteristics of flamequenching for the control of hydrogen combustionare further examined. Two points are considered. One

0029-5493/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0029-5493(03)00102-X

200 S.Y. Yang et al. / Nuclear Engineering and Design 224 (2003) 199–206

Nomenclature

Cp specific heat at constant pressuredq quenching distanceM̄ mean molecular weightn number of molep pressurepr reduced pressure defined asp0/patm

R̄ universal gas constantSL laminar burning velocityT temperatureV volumeX mole fraction

Greek symbolsδ flame thicknessλ thermal conductivityρ density

Superscript* equilibrium state

Subscripts0 initial1 first chamber2 second chamberatm atmosphericb burnt statee entrained gasH hydrogenj ejected jetu unburned state

is the effect of initial pressure on quenching distance,which is a key parameter that must be characterizedfor the design of quenching meshes. And the otheris the effectiveness of quenching meshes. The effec-tiveness of quenching meshes in preventing flamepropagation to the neighboring compartment withaccompanying pressure rise during combustion hasbeen tested by performing experiments in a closedsmall-scale compartment model. A compartment withhydrogen combustion increases its pressure and thehot burnt gas could be ejected into the neighboringcompartment. Even though quenching meshes mayarrest hydrogen flames, the hot product gaseous jetsejected into the neighboring compartment filled with

fresh combustible mixture can be ignited (Wierman,1979). Parameters that should be considered in eval-uating the effectiveness of quenching meshes for thecontrol of hydrogen combustion are discussed from aphenomenological model and experiments have beenconducted to substantiate the model.

2. Quenching distance

2.1. Experiment

The apparatus for quenching distance measurementconsisted of a combustion chamber, an electrical sparkcircuit for ignition, and a mixing chamber. The com-bustion chamber was a cylindrical closed vessel withthe inner diameter of 50 mm and the depth of 11 mm.It had two ports for filling and purging mixtures andhad a pressure gauge to control initial pressure. Thespark electrodes mounted at the center of the com-bustion chamber were flanged with glass plates withthe diameter of 10 mm, similar to the suggestion byLewis and von Elbe (1987). The glass flanges havethe effect of suppressing ignition when the flangesare within a certain critical distance. This critical dis-tance has been defined as a quenching distance. Thegap between flanges was adjusted by using a built-inmicrometer.

The fuel used was chemically pure grade (>99%)hydrogen and was premixed with dry air in a mixingchamber where the equivalence ratio of mixture wasdetermined based on partial pressures, and supplied tothe combustion chamber. The hydrogen concentrationcan be determined from

XH = pH

p0(1)

whereX is the mole fraction,p is the pressure, andthe subscripts H and 0 indicate the hydrogen and theinitial state, respectively.

When the combustion chamber was filled with atest mixture, the ignition system was activated. Afterignition, the pressure of the combustion chamber wasmonitored by the pressure gauge, which can indicateeither flame quenching (no appreciable pressure rise)or successful ignition with subsequent flame propaga-tion (appreciable pressure rise).

S.Y. Yang et al. / Nuclear Engineering and Design 224 (2003) 199–206 201

Fig. 1. Quenching distance as function of hydrogen concentrationat various initial pressures.

2.2. Results and discussion

Quenching distances for hydrogen/air mixtures at300 K were first determined. The measured quenchingdistances are plotted inFig. 1, demonstrating satis-factory agreement with the previous data at the atmo-spheric pressure represented by the solid line (Lewisand von Elbe, 1987). The quenching distance has itsminimum atXH = 0.3 which is near stoichiometry.The minimum quenching distance is relevant to safetywhen designing quenching meshes for the control ofhydrogen combustion.

The quenching distance decreases with pressure,which can be explained based on the relation of lam-inar burning velocity and flame thickness (Williams,1985). It has been shown that the quenching distance,dq, is proportional to the flame thickness,δ, whichin turn is related to the laminar burning velocity,SL.Thus

dq ∼ δ ∼ λ

CpρuSL∼ λTu

CpM̄

1

p0

1

SL(2)

whereλ is the thermal conductivity,Cp is the specificheat at constant pressure,ρ is the density,T is thetemperature and̄M is the average molecular weight,and the subscript u denotes the unburned state. ThepropertiesCp andλ are less sensitive to temperatureand pressure compared to the sensitivity of the lam-inar burning velocity,SL. The laminar burning veloc-ity normalized by that for the atmospheric pressure,

Fig. 2. Dependence of quenching distance on initial pressure.

SL(patm), can be fitted as (Mauss et al., 1991)

SL(p0)

SL(patm)= 1+0.0069(log10pr)−0.30586(log10pr)

2

− 0.0661(log10pr)3+0.04736(log10pr)

4

(3)

wherepr is the reduced pressure defined asp0/patmand the subscript atm denotes the atmospheric con-dition. Considering only the pressure and laminarburning velocity effects on quenching distance inEq. (2), it can be derived that

dq(p0)

dq(patm)= 1

pr

SL(patm)

SL(p0)(4)

Dependence ofdq (∼δ) on initial pressure is shownin Fig. 2 from the present experimental data shownin Fig. 1. It demonstrates that the quenching distanceprediction fromEq. (4)is in good agreement with theexperimental data. Note that the quenching distancesfor the range of pressure tested can also be approxi-mated to be inversely proportional to initial pressure,as shown in the dotted line.

3. Effectiveness of quenching meshes incombustion control

3.1. Experiment

The previous results demonstrate that the quench-ing distance varies sensitively on pressure. Therefore,


Fig. 3. Schematic of experimental setup for control of flame propagation.

quenching meshes designed based on the data at theatmospheric pressure can be ineffective when a com-partment pressure is increased during combustion. Totest these possibilities, experiments were conducted ina closed small-scale model combustion chamber withtwo compartments to investigate the effectiveness ofquenching meshes for preventing the flame propaga-tion to the neighboring compartment.

The apparatus consisted of a model combustionchamber, a mixing chamber, a visualization setup,and an ignition system, as schematically shown inFig. 3. The model combustion chamber consisted oftwo compartments with 30 mm× 20 mm× 20 mm,which are connected by a 10 mm inner diameter pas-sage with 40 mm in length. Windows, on the frontand rear of each compartment, are made of quartz foroptical access. The visualization setup was composedof a Schlieren system and a high-speed camera. Toinitiate flame propagation, an electrical spark igniteris installed on one side of the first compartment.

Attention was focused on the control of flame prop-agation from one compartment to the other. To demon-strate the usefulness and validity of quenching meshesin flame control, the size of metallic meshes installedbetween the compartments should be smaller than theminimum quenching distance. Since the quenchingdistance has its minimum value near stoichiometry,the two compartments are initially filled with stoichio-metric hydrogen/air mixture.

It should be considered that the combustion in aclosed chamber results in the increase of chamberpressure. Considering this pressure rise, it is knownthat the size of meshes for the control of flame

propagation should be smaller than the maximumexperimental safe gap (MESG;British StandardsInstitution, 1957), which is approximately a half ofthe minimum quenching distance (Phillips, 1963).Accounting the minimum quenching distance at theatmospheric pressure and the MESG, a woven wiremesh was installed between the compartments, whichwas made of stainless steel having the wire diameter0.2 mm and the opening size 0.3 mm.

3.2. Results and discussion

A simplified phenomenological analysis is first pre-sented to identify parameters that influence the effec-tiveness of quenching meshes. A schematic diagramof a simplified model is shown inFig. 4.

It is assumed that the whole chamber is adiabaticand the first compartment is initially filled with hotburnt gas with the volumeV1 (Fig. 4a). The hotburnt gas starts to eject into the second compartment,which is filled with H2/air mixture with the volumeV2. The temperature of this burnt gas jet,Tj , will belower than the burnt gas temperature,Tb, because ofheat loss through quenching meshes. When the hotgas is ejected, the cold H2/air mixture in the sec-ond compartment will be entrained, and therefore thetemperature of jet,T, decreases (Fig. 4b). As the jetejection proceeds, the composition of jet can changeto a combustible condition, while the temperaturekeeps decreasing. Therefore, at a certain appropriatecondition, ignition in the second compartment couldoccur. However, ignition can be prevented when theignition criteria cannot be satisfied until the pressure


Fig. 4. Schematic of simplified phenomenological model.

in both compartments becomes equal (Fig. 4c), mean-ing that the jet ejection is complete. Note that theeffect of inertia which can induce flow back and forthbetween the compartments is neglected since ignitionmost probably occurs at the initial jet ejection stage.

The thermodynamic relations and the equation ofstate for ideal gas dictate:

Initial state:

pbV1 = n1R̄Tb (5)

puV2 = n2R̄Tu (6)

Transient state:

n = ne + nj (7)

njC̄p,j(Tj − T) = neC̄p,e(T − Tu) (8)

p1V1 = (n1 − nj)R̄Tb (9)

p(V2 − Vj) = (n2 − ne)R̄Tu (10)

pVj = nR̄T (11)

Equilibrium state:

p∗V ∗j = n∗R̄T ∗ (12)

wheren is the number of mole, the subscripts 1, 2, u, b,j, and e indicate the first chamber, the second chamber,the unburned state, the burnt state, the ejected jet, and

the entrained gas, respectively, and the superscript *indicates the equilibrium state.

The pressure and temperature of the ejected jet canbe derived as follows:

p − pu

pb − p1= V1

V2

[C̄p,j

C̄p,e

Tj − T

Tb+ T

Tb

](13)

When the pressure of each compartment becomesequal, i.e.p1 = p = p∗, the jet ejection ceases. Therelation of pressure and temperature of jet at thiscondition,p∗ andT∗, can be expressed as follows:

p∗ = pu

1 + {(pb/pu)(V1/V2)[(C̄p,j/C̄p,e)

× (Tj − T ∗)/Tb + (T ∗/Tb)]}1 + {(V1/V2)[(C̄p,j/C̄p,e)

× (Tj − T ∗)/Tb + (T ∗/Tb)]}

(14)

Since the pressure of jet increases frompu to p∗during the ejection, the possibility of ignition be-comes low when the equilibrium pressure,p∗, and theequilibrium temperature,T∗, are low. The lower thepressure at the unburned state,pu, and the temperatureof jet, Tj , the lower is the equilibrium pressure,p∗. Inthe same manner, the smaller the pressure rise due tocombustion in the first compartment with respect tothe pressure of the second compartment,pb/pu, and thesmaller the volume ratio of two compartments,V1/V2(in other words, the larger the volume of compart-ment toward which the flame propagates), the loweris the equilibrium pressure. Therefore, the preventionof flame propagation by using quenching meshes inthe second compartment can be effectively achievedby lowering p∗ through the variation of above men-tioned parameters, which can be easily understoodfrom Eq. (14).

To test these parameter effects, various experimentshave been conducted. First,Fig. 5 shows the effect ofinitial pressure (related topu) on flame propagationand ignition. The sequence of Schlieren images forthe initial pressurep0 = 25 kPa, taken with 0.4 msinterval (Fig. 5a) shows that the flame propagation canbe effectively confined to the first compartment. Whilethe sequence of Schlieren images forp0 = 50 kPa,taken with 0.2 ms interval (Fig. 5b) demonstrates thatthe ignition occurs in the second compartment by thehot gaseous jet from the first compartment. This result


Fig. 5. Schlieren photographs showing the effect of initial pressureon flame propagation between compartments with initial pressureof (a) 25 kPa (�t = 0.4 ms) and (b) 50 kPa (�t = 0.2 ms).

demonstrates the importance of initial pressure on theeffectiveness of quenching meshes.

The effect of hot gas temperature is investigatedby comparing two experiments: one with only thequenching meshes and the other with the quenchingmeshes and steel balls. Supplementary steel balls of3 mm diameter are installed in the quenching mesh re-gion to enhance heat loss and subsequently to reducethe hot gas temperature. Experiments were conductedwith the initial pressure of 50 kPa and the visualiza-tions are shown inFig. 6. With only the quenchingmeshes installed, ignition occurs in the second com-partment by the hot gaseous jet (Fig. 6a). On the otherhand, with the installation of additional steel balls, ig-nition of the mixture in the second compartment didnot occur by the hot gaseous ejection (Fig. 6b) due tothe decrease in hot gas temperature by heat loss to thesteel balls.

To test the effect of pressure rise due to combustionin the first compartment with respect to the pressureof the second compartment,pb/pu, a pre-combustionvolume is attached to the first compartment. Thepre-combustion chamber with 10 mm diameter and20 mm depth is made in the inner wall of the firstcompartment. The initial pressure of the compart-ment,p0, equals 30 kPa, and only quenching meshesare installed. When the igniter is located on the sur-

Fig. 6. Schlieren photographs showing the effect of hot gas temper-ature on flame propagation between compartments (p0 = 50 kPa,�t = 0.2 ms): (a) quenching meshes only and (b) meshes andsteel balls.

face of the first compartment in the absence of thepre-combustion chamber, the laminar flame propa-gates only in the first compartment without propagat-ing to the second compartment (Fig. 7a).

Fig. 7. Schlieren photographs showing the effect of pres-sure increase ratio by varying igniter location (p0 = 30 kPa,�t = 0.2 ms): (a) on the wall and (b) in the pre-combustion vol-ume.


Fig. 8. Pressure histories for flame propagation to small and largecompartments.

When the ignition occurs in the pre-combustionvolume, the flame in the first compartment can beaccelerated compared to the case with the ignition oc-curring on the surface of the first compartment. Con-sequently, the pressure in the first compartment maybe increased before the hot gaseous jet ejection to thesecond compartment. In this situation, the experimentshows that the flame surface in the first compartmentis somewhat wrinkled and is propagated at a rela-tively high speed compared to the case (Fig. 7a). Insuch a situation, the second compartment was ignited(Fig. 7b), meaning that the quenching meshes areineffective.

Finally, the effect of the volume ratio betweenthe two compartments is considered. To test theeffect, a new hexagonal second compartment withthe dimensions of 250 mm× 250 mm × 300 mm(V2/V1 = 1562.5) was prepared. The initial pressureof the chamber was maintained at 70 kPa. When thetwo compartment sizes are the same, the mixture inthe second compartment can be ignited, similar to theprevious cases. When the volume of the second com-partment is large enough, the flame can be effectivelyconfined, thus ignition by hot gaseous jet can be pre-vented. This effect is exhibited in terms of pressurehistory in Fig. 8 due to the geometrical limitation inthe visualization setup. As is shown inFig. 8, whenthe second compartment is large, the total pressureremains nearly the same as the initial pressure, mean-ing that the flame propagation is confined in the firstcompartment.

4. Concluding remarks

In order to provide database for the design ofquenching meshes, quenching distances were mea-sured by varying the initial pressure for hydrogen/airmixtures. It has been shown that the quenching dis-tances decrease inversely proportional to the initialpressure.

The effectiveness of the quenching meshes forthe control of hydrogen combustion has been exper-imentally investigated in closed small-scale modelcompartments to identify controlling parameters,which govern the effectiveness of the quenchingmeshes. The lower the initial pressure of the com-bustion chamber, the lower the hot ejected gas tem-perature due to heat loss through quenching meshes,the smaller the pressure rise due to combustion inthe first compartment, and the larger the volume ofthe second compartment, the more effective is thecontrol of hydrogen combustion using quenchingmeshes.

Acknowledgements

This work has been supported by the ElectricalEngineering & Science Research Institute, SeoulNational University.

References

British Standards Institution, 1957. Flame proof enclosure ofelectrical apparatus. British Standard 229, London.

Camp, A.L., Cummings, J.C., Sherman, M.P., Kupiec, C.F., Healy,R.J., Caplan, J.S., Sandhop, J.R., Saunders, J.H., 1983. Lightwater reactor hydrogen manual. SAND Report, Sandia NationalLaboratories, SAND82-1137, NUREG/CR-2726.

Chung, S.H., Sohn, C.H., Kim, H.J., Yoo, C.H., 1996.The development of hydrogen combustion analysis model.KAERI Report, Korea Atomic Energy Research Institute,KAERI/CM-114/96.

Cummings, J.C., Camp, A.L., Sherman, M.P., Wester, M.J.,Tomasko, D., Byers, R.K., Burnham, B.W., 1983. Review ofthe grand gulf hydrogen igniter system. SAND Report, SandiaNational Laboratories, SAND82-0218, NUREG/CR-2530.

Kim, H.J., Sohn, C.H., Chung, S.H., Kim, H.D., 1999. LaserRayleigh Measurement of mixing processes and control ofhydrogen combustion using quenching meshes. Nucl. Eng.Design 187, 291–302.


Kumar, R.K., Tamm, H., Harrison, W.C., 1984. Intermediatescale combustion studies of hydrogen–air–steam mixtures. EPRIReport, NP-2955.

Lewis, B., von Elbe, G., 1987. Combustion, Flames and Explosionof Gases, 3rd ed. Academic Press, Orlando.

Mauss, F., Peters, N., Rogg, B., Williams, F.A., 1991. Reducedkinetic mechanisms for premixed hydrogen flames. In: ReducedKinetic Mechanisms for Applications in Combustion Systems,Vol. 15. Lecture Notes in Physics. Springer-Verlag, Berlin,pp. 29–43.

NASC, 1980. Three mile island—Unit 2 accident. Nuclear SafetyAnalysis Center Report.

Phillips, H., 1963. On the transmission of explosion through agap smaller than the quenching distance. Combust. Flame 7,129–135.

Thompson, L.B., Haugh, J.J., Sehgal, B.R., 1984. Large scalehydrogen combustion experiments. In: Proceedings of ANSInternational Conference on Containment Design, Toronto,Canada, pp. 120–125.

Wierman, R.W., 1979. Experimental study of hydrogen jet ignitionand jet extinguishment. HEDL-TME 78-80.

Williams, F.A., 1985. Combustion Theory, 3rd ed. Addison-Wesley,Menlo Park, CA.

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Effect of pressure on effectiveness of quenching meshes in transmitting hydrogen combustion