Concentration analysis of extinguishing agents and toxic gases in small cabin aircraft

Transcription

1 2009 Brazilian Symposium on Aerospae Eng. & Appliations 3 rd CTA-DLR Workshop on Data Analysis & Flight Control Conentration analysis of extinguishing agents and toxi gases in small abin airraft Cássio Karassawa Zanoni, assio.zanoni@gmail.om Cláudia Regina de Andrade, laudia@ita.br Edson Luiz Zaparoli, zaparoli@ita.br ITA, Endereço: raça Marehal Eduardo Gomes, 50 - Vila das Aáias CE São José dos Campos S Brasil Abstrat. The objetive of this work was to develop a methodology and a tool to analyze volumetri onentration over time of toxi gases inside small airraft abins during a fire event. These gases ould be generated by the ombustion or by extinguishing agents used to put out the fire. A mathematial model was developed using the perfet stirrer approah sine it has been demonstrated by tests to give very similar data to the real results and this is what is suggested and aepted by the authorities. The inputs of the problem are the mass flow of the extinguishing agent and other toxi gases that ould be generated by the ombustion, the variation of the abin altitude and the variation of the abin air inflow. The outputs are the volumetri onentration of all gases inside the abin over the time. The ontribution of this tool is in the preliminary definition of the abin fire extinguishers and the visualization of the effets of possible abin altitude and air inflow hanges during a fire event. This tool will also be useful in the study of replaements of existing extinguishing agents by others least toxi as well as the reevaluation of the existing fire extinguisher installations with the possible hange at the toxiity levels reommended by the authorities. Even though the results generated by the tool are onservative, they were demonstrated to be onsistent for a simulation ase of permanent regime, for a ase of inflow of more than one gas with the airraft in ruise and onstant altitude, and with the hange in abin altitude. Keywords 1.Extinguishing agent onentration analysis. 2. Fire supression system. 3. MatLab/Runge Kutta 1. INTRODUCTION The ourrene of smoke or fire aboard airraft presents a potential situation of risk, aggravated by the large amount of highly flammable fuel arried and the limited passenger evauation possibilities. Substantial improvement has been made on the flammability requirements on the materials used inside the abin, abin evauation has beome more effiient and the fire fighting on ground has beome better. However, reent data indiates that the probability of passengers experiening an event of smoke in flight is very high. For many years, small ivil airraft were not required to arry portable fire extinguishers inside the abin. Only large airraft ertified under art 25 should be equipped with an approved fire extinguisher. In 1987, through NRM 83-17, art 23 required portable fire extinguishers for ommuter ategory airraft. Small airraft under the ommuter ategory present the agravant of not having to arry a rew member inside the abin. This fat an result in a longer time for reation of a okpit flight rew to extinguish a possible fire in the abin or a regular passenger with no training trying to do so. For this reason, airraft manufatures and operators need to be more areful not only in the hoie of the type of the extinguisher, but also on its loation on the abin, hoosing readily aessible plaes and if neessary using plaards in order to minimize the reation time in an event of fire. The most ommon type of extinguishing agent used in aviation is Halon due to its effetiveness and the low residue left after being used. The trouble with Halon is that it is an ozone-depleting substane, harmful to the Earth's stratospheri ozone layer and also an beome toxi in high onentrations. This last issue is potentialized in a small airraft where the abin volume and air ventilation are limited. The motivation of this work was the fat that the existing reommendations for the hoie of fire extinguishing agents do not take into onsideration some important variables and the fat that new agents are being offered, resulting in hanges in aeptable toxiity levels. The purpose of this work was to develop a simple tool apable of generating more realisti information about the onentration of extinguishing agents inside the abin, inluding abin altitude variation over time, simulating the use of fored outflow of air out of the abin by the pilot or, if provided by the airraft design, the inrease on the air inflow inside the abin. This tool an help in the preliminary seletion of extinguishing agents and their quantities, in the evaluation of existing installations and in other fire studies inside the abin suh as the evaluation of the onentration of gases that are sub produts of the ombustion of materials. 2. EXTINGUISHING AGENTS CONCENTRATION RECOMENDATIONS FAA Advisory irular 20-42C, Hand Fire Extinguishers For Use In Airraft, presents the reommendations for the safe use of Halon 1211, Halon 1301 and Carbon Dioxide fire extinguishers onboard airrafts.

2 2009 Brazilian Symposium on Aerospae Eng. & Appliations 3 rd CTA-DLR Workshop on Data Analysis & Flight Control The minimum onfined volume spae reommended by AC 20-42C is 312 ubi feet for the standard 2.5 pounds Halon 1211 used onboard airraft and 132 ubi feet for the Halon 1301, These volumes were determined before the ardiotoxi test beame the standard for industries in general. The volume for Halon 1211 is based in a volumetri onentration allowed of 2% maximum where the rate of air hange inside the abin is unknown. For airraft where the exhange air time is known, the reommendations are 4%.min for Halon 1211 and 10%.min for Halon This means that a volumetri onentration of 4% of Halon 1211 an be kept during a period of 1 minute without ausing any harm to the oupants. The minimum safe ompartment volumes are alulated using the perfet stirrer approah, in whih the ompartment volume and the air exhange time are used to obtain the maximum amount of agent that an be used. The perfet stirrer approah assumes that all extinguishing agent disharged immediately mixes homogeneously with the air. In addition, the new air injeted at the abin mixes with the existing gas mixture dereasing the onentration of the agent over time while the mixture is ventilated out the abin. The alulations on AC 20-42C assume a abin pressure based on an altitude of 8000 feet and a temperature of 70 o F. In addition, the alulations assume that the extinguisher is disharged instantaneously. Based on the urrent reommendations, an airraft with a ventilation rate of one air exhange per minute would need to have a abin volume of 197,5 ubi feet in order to be able to use a 2.5 pounds Halon 1211 safely. AC 20-42C was last revised in the 80 s. Sine new agents are being developed to replae Halon 1211 whih is an ozone depleting substane, a ommittee was reated to update this irular. Several disussions were made by this group regarding the appropriate methodology that should be used to determine the minimum volume of the required ompartment for the safe use of Halon 1211 and its substitutes onboard airraft. A possible approah would be to use the BK (hysiologially Based harmookineti) method to provide a first approah. When air ontaining a haloarbon agent is inhaled, part of the haloarbon is introdued in the blood stream through the lungs. The onentration of the haloarbon dissolved in the blood is related to the onentration in the air inhaled and the exposing time. If the exposing time is high enough, the heart an be sensitized to adrenalin. In the presene of a high level of adrenalin, the heart an experiene arrhythmia. The BK methodology onsiders this dynami and was implemented in this work. 3. MATHEMATICAL MODEL In order to obtain the mathematial equations that represents the disharge of extinguishing agents or toxi gas generation resulted by the ombustion of materials inside an airraft abin, the ontrol volume represented by Fig. 1 an be used: ṁ air ṁ h ṁ x a m = m + m + m air = f ( ρ, M ) h x T V ṁout Where: m Air mass aumulated inside the abin air m Halon mass aumulated inside the abin h m Additional gas mass aumulated inside the abin x Figure 1. Control volume used m Combination of the masses aumulated inside the abin ṁ air Air inflow ṁ h Halon inflow ṁ x Additional gas inflow ṁ out Mixture outflow (air + halon + x) a External ressure Cabin ressure (ombined partial pressures)

3 2009 Brazilian Symposium on Aerospae Eng. & Appliations 3 rd CTA-DLR Workshop on Data Analysis & Flight Control ρ Mixture density M Moleular mass of the mixture V Total abin Volume T Cabin temperature The differential equations of the mass variation of eah omponent over time an be obtained by the mass onservation priniple and the perfet stirrer approah in whih all omponents of the ontrolled volume are homogeneously distributed in every part of the abin: dm... = mair + mh + m x m. out (1) dt dm.. air mair = mair mout (2) dt m dm.. h mh = mh mout (3) dt m dm.. x mx = m x mout (4) dt m As desribed by (Zaparoli, E.L; KUROKAWA, F.Y.; ANDRADE, C.R, 2004), the mass flow rate leaving the abin an be determined by the following equations: 2 γ + 1. γ γ γ mout = CA 2 ρ ( rp) ( rp) γ 1 if rp rp 1 (5) or γ γ 1. m out = CA ρ γ if rp < rp (6) γ + 1 Where: rp = a a if if < a a > C Orifie oeffiient A Equivalent leaking area γ Relation between speifi heat in onstant pressure and onstant volume ρ Density of the mixture of gases Total pressure in the abin a External atmospheri pressure rp ressure relation rp Critial pressure relation The abin pressure is obtained by the equation below:

4 2009 Brazilian Symposium on Aerospae Eng. & Appliations 3 rd CTA-DLR Workshop on Data Analysis & Flight Control m V = RT (7) M Where: M h m = (8) mh mair mx + + M M M air 4. SOLUTION METHODOLOGY x In order to obtain the gases onentrations over time, omputational tools an be used. In this partiular work, Matlab was used to solve the ordinary system of differential equations represented by equations (1), (2), (3) and (4). The fourth order Runge Kutta method was used using the fluxogram represented by Fig RESULTS Figure 2 Solver Fluxogram Simulations were arried representing an airraft with a abin volume of 10.2 ubi meters, kept in a temperature of 24 o C, initial abin altitude of 6700 feet and flying at ruise at feet. Initially, the only existing mass inside the abin is the air. In other words the total mass of the ompartment is the air mass. Two other gases were introdued, one being Halon 1211 and the other a fititious one that was alled X representing a ompound resulted by the ombustion or a seond extinguishing agent. The first simulated ase represents a fire situation where a 2.5 pounds Halon 1211 bottle would be enough to extinguish it and the resulting gases from ombustion ould be ignored. In addition, having the fire extinguished and the risk situation ontrolled, the pilot would not perform any desend maneuver. The simulation was run during 1000 seonds with the fire extinguisher being disharged at the instant 50 seonds. Table 1 shows the input data used. Table 1 Input data for first simulation Total simulation time Temperature inside the abin Disharge time for Halon 1211 Disharged Halon 1211 mass Variable Value 1500 s 297 K 8 s 2.5 lb

5 2009 Brazilian Symposium on Aerospae Eng. & Appliations 3 rd CTA-DLR Workshop on Data Analysis & Flight Control Cabin Volume 10.2 m 3 Initial flight altitude Moleular mass of Halon 1211 Moleular mass of air Universal onstant of gas feet kg/mol kg/mol kg/mol Gravity aeleration 9.8 m/s 2 Relation between speifi heat of onstant pressure and volume 1.33 Absorption oeffiient rate of Halon Elimination oeffiient rate for Halon Area oeffiient 0.01 Leak area for the outflow valve m 2 Time instant for the disharge of Halon Figure 3 shows the Halon 1211 onentration inrease starting at 0 seonds. Eight seonds after the disharge, whih is the period of time that the extinguisher was used in this example, the Halon onentration dereases until it reahes null levels Volumetri onentration X Time 0 s Volumetri Conentration X: Y: Time (s) Figure 3 Halon 1211 onentration over time If the area under the urve is determined, the value Volumetri Conentration x Time as referred in AC 20-42C an be determined. For this simulation, the value was 4.38 %.min. For this simulation, the value was 4.38 %.min found by using the trapezoidal method. If the maximum allowed onentration of 4%.min of Halon 1211 is onsidered as reommended by AC 20-42C and onsidering that the alulated onentration is valid for all points of the abin, it an be onluded that the extinguisher onsidered ould not be used with the risk of resulting in health issues to the airraft oupants.

6 2009 Brazilian Symposium on Aerospae Eng. & Appliations 3 rd CTA-DLR Workshop on Data Analysis & Flight Control The BK method was also applied for this simulation resulting in a peak blood onentration of twie the allowed by the reommendations whih is 21mg/l. 100 Blood Conentration X Time B (mg/l) Time (s) Figure 4 Halon 1211 onentration on the blood stream If a similar ase is run with a abin altitude hange, the onentration an ahieve aeptable levels. This would represent the use of the airraft funtion dump where the outflow valve is open and the abin altitude is inreased. The inputs of this simulation and the onentration results are presented in Tab. 2 and Fig. 5 respetively. Table 2 Input data for seond simulation Total simulation time Temperature inside the abin Disharge time for Halon 1211 Disharged Halon 1211 mass Variable Value 1500 s 297 K 10 s 2.5 lb Cabin Volume 10.2 m 3 Moleular mass of Halon 1211 Moleular mass of air Universal onstant of gas kg/mol kg/mol kg/mol Gravity aeleration 9.8 m/s 2 Relation between speifi heat of onstant pressure and volume 1.33 Area oeffiient 0.1 Flight altitude. A desent was simulated 10 seonds after the disharge of the Halon The desent rate was 3000 feet/min. The airraft stabilizes at (50(T-20)) feet. Leak area for the outflow valve. The outflow ontrols the abin pressure for a abin altitude inrease of 2000 ft/min up to feet. Funtion

7 2009 Brazilian Symposium on Aerospae Eng. & Appliations 3 rd CTA-DLR Workshop on Data Analysis & Flight Control 5 Volumetri onentration X Time 4.5 Volumetri onentration X: Y: Time (s) Figure 5 Halon 1211 onentration over time Even though the rate of abin altitude limb is limited in order to preserve the human health, the result of this proedure is the faster evauation of toxi gases from the abin. The onentration on this simulation is 4%.min whih is the limit aepted level for AC 20-42C. This kind of proedure is reommended by AC 20-42C but there is no alulation method presented. A third ase was simulated to represent the disharge of two gases. One being the extinguishing agent and the other a gas resulted by the ombustion. Table 3 presents the input data for this ase. Table 2 Input data for third simulation Total simulation time Temperature inside the abin Disharge time for Halon 1211 Disharged Halon 1211 mass Variable Value 1000 s 297 K 8 s 2.5 lb Disharge time for X 5 Disharged X mass 1 lb Cabin Volume 10.2 m 3 Initial flight altitude pés Moleular mass of Halon kg/mol Moleular mass of air kg/mol Moleular mass of X kg/mol Universal onstant of gas kg/mol Gravity aeleration 9.8 m/s 2 Relation between speifi heat of onstant pressure and volume 1.33 Area oeffiient 0.1 Leak area for the outflow valve m 2

8 2009 Brazilian Symposium on Aerospae Eng. & Appliations 3 rd CTA-DLR Workshop on Data Analysis & Flight Control 5 Volumetri onentration X Time 4.5 X: Y: Volumetri onentration Halon 1211 Gas X Figure 6 Volumetri onentration over time for the disharge of Halon and the gas represented by X The same method an be used to simulate as many gases as neessary, just by adding basi data and similar differential equations to the other gases presented. 5. CONCLUSION In this work, it was possible to reate a tool using the perfet stirrer approah to analyze the onsequene of disharging extinguishing agents inside an airraft abin. Even though the results are onservative, these were shown to be more realisti then the ones presented in the urrent reommendations. In addition, the methodology allows more flexibility on the inputs refleting the evolution on the tehnology of ontrolling the abin altitude that affets diretly the onentration analysis. The tool allows that the effet of maneuvers performed be analyzed in the sense of dereasing the onentration of toxi gases inside the abin. Based on the simulation, the inrease on the outflow by dereasing the abin pressure an help dereasing the toxi gas onentration, but not in a signifiant way. The methodology presented an be very useful in the pre evaluation of fire extinguisher installation in airraft abins. 6. REFERENCES Time (s) Abramowitz, A.;Neese, W.; Slusher, G., SMOKE AND EXTINGUISHER AGENT DISSIATION IN A SMALL RESSURIZED FUSELAGE, New Jersey, 1990, 45p. Bejan, Adrian. ADVANCED ENGINEERING THERMODYNAMICS, John Wiley & Sons, New York, NY, Colton, Bradford A.; Nay, Angela, EVALUATION OF CALCULATION METHODOLOGIES FOR THE SAFE USE OF CLEAN AGENTS ONBOARD AIRCRAFT, Nevada, 14p. Doolittle, Jesse S.; Hale, Franis J. THERMODINAMYSCS FOR ENGINEERS, Department of mehanial engineering North Carolina State University, John Wiley & Sons, New York, NY, Eklund, Thor I.. Analysis of Dissipation of Gaseous Extinguisher Agents in Ventilated Compartments, New Jersey, 1983, 28 p. FAA, Hand fire extinguisher for use in airraft, Washington, D.C., (AC20-42C).

9 2009 Brazilian Symposium on Aerospae Eng. & Appliations 3 rd CTA-DLR Workshop on Data Analysis & Flight Control FAA Fire Safety. Handheld Fire Extinguishers. resents data and results of the latest disussions of the Fire rotetion Working Group. Available at: < Aessed in:jun.12th International Airraft Systems Fire rotetion Working Group, OTIONS TO THE USE OF HALONS FOR AIRCRAFT FIRE SURESSION SYSTEMS, Washington, D.C., 2002, 89p. Krasner, L. M., Study of Hand-held Fire Extinguishers Aboard Civil Aviation Airraft, Fatory Mutual Researh Corporation, Slusher, G.; Wright, J.; Demaree J, Neese, W, EXTINGUISHER AGENT BEHAVIOR IN A VENTILATED SMALL AIRCRAFT, New Jersey, 1984, 57p. Slusher, G.; Wright, J.; Demaree J., HALON EXTINGHUISHER AGENT BEHAVIOR IN A VENTILATED SMALL AIRCRAFT, New Jersey 1986, 60p. Vinegar, Allen., Modeling Cardia Sensitization otential of Humans Exposed to Halon 1211 and Halon 1301 Aboard Airraft, Aviation, Spae, and Environmental Mediine. Vol. 72, No. 10, Zaparoli, E.L; KUROKAWA, F.Y.; ANDRADE, C.R.;. Numerial Simulation of theconentration of Fire Extinguishing Agent in Airraft Cargo Compartment. In: Brazilian Congress of Thermal Sienes and Engineering ENCIT, 10., 29-3 dez. 2004, Rio de Janeiro, RJ. Braz. So. Of Mehanial Sienes and Engineering ABCM, 2004 ZANONI, Cassio. Conentration Analysis Of Extinguishing Agents And Toxi Gases In Small Cabin Airraft, p 7. RESONSIBILITY NOTICE The authors are the only responsible for the material inluded in this paper.