DATA ANALYSIS OF NATURAL VENTILATION IN A FIRE IN TUNNEL.

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1 DATA ANALYSIS OF NATURAL VENTILATION IN A FIRE IN TUNNEL. ABSTRACT. Giuli G., Giorgiantoni G., Zampetti P. ENEA (The Italian Committee for the New Technologies, for Energy and the Environment) It is known that the availability of jet fans in tunnels assures a higher safety degree in the longitudinal ventilation pattern. But these installations have a remarkable cost, especially if the number of tunnels is high. In this paper, we want to explore the behaviour of the natural ventilation in presence of fire in tunnel in those cases in which a favourable slope occurs. Another issue is the emergency management. The study was performed on the basis of the experimental data supplied from Memorial Tunnel Fire Ventilation Program. The analysis is made on the two natural ventilation tests available in this Program. A comparison with the Memorial Tunnel tests which have forced longitudinal ventilation patterns is made to understand how effective the behaviour of natural ventilation is. The analysis has been carried out considering the (P/) ratio between fire power P and air flow. This ratio is a parameter which gives a better information, about the danger of a situation, in comparison with the simple fire power. Moreover, in the ventilated tunnel, the P/ ratio is a function of the power P released by the fire, that is substantially independent of our action, and the air flow which depends on our action. Key words: tunnels, fire in tunnels, data analysis, ventilation, safety 1. MEMORIAL TUNNEL DESCRIPTION The Memorial Tunnel (MT) (MTFVTP, 1995), in fig. 1a, is a two-lanes 853 m long straight motorway tunnel with a medium slope of 3,2 % from the north to the south portal. 4,94 m (a) N o r t h R O O M ,2 m ,35 m ,2 % 615 Forced flow Loop 35 4 pans Loop ,7 m R O O M 7.86 m S o u t h LOOP 35 3,66 m 5 MW 2 LOOP MW 25 1 MW 3 MW LOOP 34 (b) Figure 1. (a) Schematic overview of the Memorial Tunnel. MT is characterized by a slope of 3,2 % and by unusual obstructions on the ends, due to the two rooms. In the natural ventilation tests the 15 Fans are not provided. (b) Map of the pans layout with position of the near instrumentation loops. In each loop, 8 measurements are taken at 8 different vertical levels International Conference Tunnel Safety and Ventilation 24, Graz

2 The full section is 6,4 m 2. On the ends two unusual obstructions (rooms), are present. Fifteen jet-fans are provided inside the tunnel in group of three for the longitudinal ventilation experimental phase. The fire is generated by means of fuel floating on filled pans placed about 24 m far from the south portal. Four pans, shown in fig. 1b, allow to achieve fires from 1 to 1 MW. During the natural ventilation (NV), the jet fan are not provided. 2. TREND OF THE TWO NATURAL VENTILATED TESTS The analysis was undertaken elaborating the experimental data of Memorial Tunnel Fire Ventilation Program (MTFVTP, 1995). At the beginning of the analysis of the longitudinal ventilation on the MT tests, our concern was not to create too many graphs that might ingenerate confusion. We had 17 tests to observe with many measured variables in the time and in the space. Then the intention was to create few graphs with the more possible useful information for our considerations. Thus, to evaluate the situation, we realized to observe the maximum temperature just few meters downstream of the fire exactly at the 35 loop (fig 1) for NV and 34 loop for forced ventilation (FV). The longitudinal forced ventilation tests were already analysed in (Giuli, 23). Here the trend of this maximum temperature versus time, is observed, as reported in figg. 2(a) (2 MW) and 2(b) (5 MW) for the two NV tests. This maximum temperature T max, measured downstream, is every time observed at the highest level on the loop 35. The fire power P (MW) and the air flow (m 3 /s) are reported in the same graph. is taken near the south portal upstream, where density is determined, to avoid the influence of the temperature. Thus, in every instant, has substantially always the same density Observation on the 2 MW NV test In fig. 2(a) we observe that, from the fifth minute to the end, the air flow changes from ~32 m 3 /s to ~9 m 3 /s, while T max decreases (due to the increased flow) of only about 1%. Another observation is that the maximum temperature seems to be low, considering the low air flow rate, respect to the FV tests (Giuli, 23). For comparison, one of the four 2 MW forced ventilation tests is reported in fig. 3(a) Observation on the 5 MW NV test In fig. 2(b) it is observed that the flow rate has no regular increase and has an average value of 65 m 3 /s, that is smaller than the other FV tests with the same power. Here the maximum temperature is very high, for this delivered power, due to the small natural flow. For comparison, one of the five 5 MW forced ventilation tests is reported in fig. 3(b). 3. COMPARISON OF NATURAL AND FORCED VENTILATED TESTS A comparison has been made with the FV tests analysed in (Giuli, 23) to understand better the behaviour of the NV tests. The measured data for natural and forced ventilation are represented in fig. 4. In this figure the P/ [kj/m 3 ] ratio is reported on the X-axis and the maximum T measured on the loop downstream (35 for NV and 34 for FV) is reported on the Y-axis. In each test T max, P and are measured every 3 seconds; is read near the portal where the air is incoming in the tunnel. Instead of representing all the points taken every thirty seconds, the time averaged maximum temperatures T max for each test is reported; the same time averages are made for the values of P/. International Conference Tunnel Safety and Ventilation 24, Graz

3 MW m 3 /s NV test 51nat 51 2 (2 MW MW) - P : P, : T, Tmax C 33, 3 T max MW m 3 /s NV test 52nat 52 5 (5 MW MW) C - P : P, : T, Tmax 22, 8 T max 22, P 2 11, 1 (a),, 1, 2, minuti 11, 4 (b) P,, 1, minuti Figure 2. NATURAL Ventilated tests. Trend vs time of: air flow, power P, and T max at few meters downstream. (a) 2 MW test. The is reported in absolute value. It goes in direction of the stack effect except for the first 2 initial minutes where it goes from north to south. (b) 5 MW test. This test is stopped after only 15 minutes.. MW m 3 /s FV test (2 MW MW) - P : : P, T, C Tmax 2525, , T max , , , 1, 4, 4 14, , min m No. JF N JF 6 P (a) (b) MW m 3 /s C FV test 61 (5 MW) P,, Tmax 22, 61 5 MW - P : : T 8 T max 1, P, 4, 4 14, 24,min No.JF N min m min m Figure 3. FORCED Ventilated tests. Trend vs time of: air flow, power P, and T max at few meters downstream. (a) 2 MW test with the No. of JF in operation. (b) 5 MW test with the No. of JF in operation The time average is calculated until the shut-off, having discarded the initial transient (usually the initial two-four minutes of the time interval). In the graph the dashed straight line represents the temperature of an air flow rate [m 3 /s], at the initial temperature T, which absorbs the total power P [W] reaching an uniform temperature T [ C] according to the following well known law: 1 P T = T + (1) c ρ where: ρ [kg/m 3 ] = air density at T ; c p [J/(kg C)] = air specific heat. From fig. 4 we can see that FV tests points are distributed in an approximately linear mode, parallel at the dashed line which represents eq. (1). The two triangular symbols ( ) related to NV are not aligned with the forced ventilation series. p International Conference Tunnel Safety and Ventilation 24, Graz

4 Analysing the position of 5 MW NV point, it is recognized that this is the one at the highest P/ value among the 5 MW cases due to the low value. Moreover we see that, while the FV points are shifted from the straight line of about 2 C, the NV point is shifted of about 4 C. This large shift is caused by a strong stratification. Using the same considerations, the -point of 2 MW NV test is expected at about 5 C instead of 3 C observed in fig. 4. This different behaviour will be analysed in the next section. C C T max 1 8 Naturally Ventilated MW 5 MW 2 MW 1 MW P/ [kj/m 3 ] kj/m3 Figure 4. Time averaged downstream maximum temperature T max, vs P/ ratio for all tests. The two NV tests are shown with a big D. The dashed line represents eq. (1) 4. ANALYSIS OF THE TEMPERATURE FIELD AND VELOCITY PROFILE In this section, the temperature fields and the velocity profiles for the two NV tests are analysed and compared with the FV tests. It will be seen how much the velocity profile is important for the heat transport Analysis of 2 MW test The fig 5, directly extracted from (MTFVTP, 1995), shows the instantaneous situation at 1 minutes from the fire start for the 2 MW NV test. The flow rate is 53,3 m 3 /s and the fire power is 13,6 MW. In fig. 6 the situation at 22 minutes is shown for the same test, with a flow rate of 86,3 m 3 /s and a fire power of 1,5 MW. In the two situations the fields of temperature are similar, with a maximum value of about 3 C, notwithstanding that in the first case the flow rate is less and the power is larger than in the second case. This behaviour is due to the different velocity profiles of the flow. Comparing the two marked profiles (figg. 5 and 6) we can see, in first case, that the negative velocity near the floor, concentrates the positive mass flow in the zone at the highest temperatures. So this positive mass flow has a larger energy density respect to the second case. So the lower mass flow of fig. 5 is enough to transport an even larger power respect to the situation of fig 6. In order to better understand the phenomena, a forced test of nominal 2 MW is shown in fig. 7 at the time of 9 5 from the start. The maximum temperature is about 3 C, as in the NV test and the power is 12,4 MW, about the same of fig. 5. The velocity profile is flatter than the profile of fig 6 and the flow rate is larger than the one in the NV test. International Conference Tunnel Safety and Ventilation 24, Graz

5 ,3= m 3 /s Extracted from (MTFVTP, 1995) Figure 5. 2 MW - NATURAL ventilated test after 1 minutes. The marked velocity profile on the 35 loop, just few meters downstream the fire, shows a peak near the ceiling 86,3= m 3 /s Extracted from (MTFVTP, 1995) Figure 6. 2 MW - NATURAL ventilated test after 22 minutes. The marked velocity profile on the 35 loop, just few meters downstream the fire, tends to a flat profile due to the higher flow rate International Conference Tunnel Safety and Ventilation 24, Graz

6 =135,5 m 3 /s Extracted from (MTFVTP, 1995) Figure 7. 2 MW - FORCED ventilated test. The marked velocity profile on the 34 loop, just few meters downstream the fire. This is the typical profile of all forced ventilated tests observed at this loop To show the importance of the velocity profile geometry, the counterflow profile marked in fig. 5 is represented in fig. 8(a) and is compared with a supposed flat velocity profile with the same total mass flow, represented in fig 8(b). Let us divide the total mass flow rate crossing 35-section in 3 subsections: M B, +M B, M 35 (fig. 8a). M B (backwards flow) and +M B have the same absolute value, so the remaining flow section M 35 is the total mass flow rate. experimental velocity profile theoretical flat velocity profile M C 275 C 229 C 33 C 275 C 229 C 178 C M C + M B 38 C 87 C velocity profile 87 C 38 C 3 C 28 C - M B (a) (b) 3 C 28 C Figure 8. Comparison between different velocity profiles. (a) Experimental counterflow profile. (b) A supposed flat profile with the same mass flow rate M 35 [kg/s] of the experimental test. Notwithstanding the mass flow rates are equal, in the case (a) more than 3 times the heat power of the case (b) is transported. International Conference Tunnel Safety and Ventilation 24, Graz

7 Calculating, in both cases, the heat that crosses the section 35, we see that in the (a) case the transported heat is more than 3 times respect to the (b) case. In fact the average temperature of the flux of the (b) case is about 9 C. Instead from the calculation of the only grey flow M 35 in case (a) the average temperature is about 25 C. Taking into account that the reference environment temperature at the inlet portal is 1 C the M 35 flux in the case (a) carries exactly 3 times the power of the case (b). Moreover, in case (a), the two fluxes M B and +M B, have to be considered. Even if they do not carry any mass, they carry some power due to the different temperatures of the two fluxes. If we impose. in the case (b), the same carried power as in (a), instead of the same mass flow rate, we need a mass flow about 3 times that in (a) case. This last consideration may explain the different flow rate between the experimental tests of fig. 7 and fig ,1= m 3 /s Extracted from (MTFVTP, 1995) Figure 9. 5 MW - NATURAL ventilated test. The marked velocity profile on the 35 loop, just few meters downstream the fire, has a peak near the ceiling 4.2. Analysis of 5 MW test As we have observed in the previous fig. 4, T max for the NV test of 5 MW is higher than the corresponding FV test of 5 MW. In fig 9 the instantaneous situation at 1 minutes from the fire start for the 5 MW NV test is shown. The velocity profile is quite flat, unlike the 2 MW NV test (fig. 5-6) and is similar to the profiles of the FV tests. The T max difference between the NV and FV tests of 5 Mw is due to the low NV flow rate (~7 m 3 /s against ~15 m 3 /s for FV) which causes a strong temperature stratification. This stratification with a large T max (~85 C) is observable in fig. 9. International Conference Tunnel Safety and Ventilation 24, Graz

8 CONCLUSIONS As far as the maximum temperature is concerned, the 2 MW NV test has a better behaviour (T max is a bit lower than the corresponding FV test) than the 5 MW NV test (T max is much higher than the corresponding FV test). This better behaviour is due to the counterflow velocity profile downstream the fire, at loop 35, and it seems induced, by a small flow rate. A fire with large power, generating a large natural flow, tends to have a flat velocity profile. Analysing all the data of the two NV tests, we observe that the counterflow velocity profile is present below a flow rate 6 65 m 3 /s. It should be interesting to evaluate other experimental results on natural ventilation, to confirm these observation. REFERENCES MTFVTP, Memorial Tunnel Fire Ventilation Test Program (1995). Interactive CD-ROM and Test Report, Massachusetts Highway Department & Bechtel/Parsons Brinckerhoff, Massachusetts G.Giuli, G.Giorgiantoni, P.Zampetti (23) Analysis of Fire Test Measurement in an Experimental Tunnel. Tunnel Management International, Vol. 6, issue 3 International Conference Tunnel Safety and Ventilation 24, Graz