Experiments with nonintrusive diagnostic methods on a ... - IEEE Xplore

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EXPERLMENTS WITH NON-INTRUSIVE DIAGNOSTIC METHODS ON A FLAMEPROOF ENCLOSURE MODEL M. Beyer Physikalisch-TechnischeBundesanstalt, Germany

INTRODUCTION The transmission of an explosion through the gap of a flameproof enclosure is a complex combustion and flow process which is influenced by many different parameters. Investigations into this subject therefore require models of the enclosure and the gap, as well as non-intrusive measuring systems.

In addition to the traditional schlieren method the use of laser diagnostic methods such as Rayleigh scattering are well suited to and laser-induced fluorescence (LE) investigate fast combustion and flow processes /1-6/. This paper describes the application of these methods on experiments outside the joint of a flameproof enclosure model. EXPERIMENTAL ARRANGEMENT Laser system. The laser beam of a tuneable ArF excimer laser (Lambda Physik EMC 150 MSC, wavelength 193,4 nm f 0,5 nm, bandwidth 0,005 nm) is formed to a sheet by cylindrical lenses, and passes through the explosion in the second vessel, the heated air blower and the burner flame (fig. la). The blower and the burner flame are used for calibration procedures. With an intensified 12-bit CCD camera the jet is observed at right angles to the laser sheet. The resolution is 0,13 mm. With this camera only one image per explosion can be obtained, but within a short laser pulse time of less than 20 ns, implying that even the most turbulent motion is frozen. The region where observation is possible is limited in height by the laser sheet dimensions (h = 20 mm) and in width by the window (w = 26 mm). This arrangement can be used for measurements with Rayleigh scattering and laser-induced 0 2 fluorescence. The laser must be tuned to an oxygen resonance to measure 0 2 LIF and the wavelength must be off-resonance for optimal measurements of Rayleigh scattering. Both types of light can be well separated by optical filtering because Rayleigh scattering occurs with laser wavelength, and fluorescent light is shifted to a longer wavelength. Enclosure model. The cylindrical enclosure model, the first vessel (fig. Ib), has a length of I = 80 mm and a diameter of d = 60 mm. The enclosure is flanged to the second vessel which has a volume of V = 12 dm3.

Cylindrical joints in symmetry axis with a length of 1 = 25 mm and constant cross section are placed between both chambers. The diameter of the nozzle can be varied in steps of 0,l mm. The explosions in the fmt vessel were ignited by electrical discharges in a still H2/air mixture (28% H2) on the symmetry axis of the vessel at a distance xz from the nozzle (fig. Ib), which could be changed in steps of 8 mm. The trigger pulse for laser and camera is derived from the explosion in the fmt chamber by either light emission from the flame front, or ionisation in the flame front at the point in time when it reaches the joint. The pressure in the fust vessel and the trigger pulse are measured with a 10-bit digital recorder and so the pressure is known when the photograph is taken. Schlieren system. The schlieren technique is employed to obtain a picture of the complete explosion and to get information on the shock waves generated at the nozzle exit, using an intensified high-speed video system (Kodak Ektapro 1000) with a frequency of 3000 frames per second and a gate time of 10 ps. For these experiments the explosion chambers were placed in a schlieren system which works with concave mirrors (f= 1,5 m) and parallel light (fig. 2). A point light source is produced with a continuously running 24 V projector lamp, a condenser and a circular aperture which is in the focus of the concave mirror. The knife edge is parallel to the symmetry axis of the free jet in the focus of the second mirror. EXPLOSION TRANSMISSION THROUGH NOZZLES For safety aspects it is decisive under what conditions the explosion transmission into the outside mixture can be expected. The boundary of the explosion transmission is shown as the experimental safe diameter ds, which is the largest diameter at which an explosion did not transmit in a minimum of ten experiments under the same conditions. The dependence of ds on distance xz for the H2/air mixture used is graphed in fig. 3. It shows increase of the experimental safe diameter ds (0%-graph) with an increasing distance xz. The range of statistical influence on the explosion transmission had been evaluated to be enlarged over two, three or four diameter steps. It increases with distance xz

Electrical Safety in Hazardous Environments, 19-21 April 1994, Conference Publication No. 390, @ /E€, 1994

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(100%-graph, fig. 3). A larger xz corresponds to a higher pressure p I in the fmt vessel at that moment when the reaction zone enters the joint. Fig. 4 describes the dependence of the experimental safe diameter (OYigraph) on pressure p 1 in the fmt vessel which looks the same as the dependence on the ratio pI/p2 of the pressures in the first and second vessel because pressure p2 in the second vessel is constant. Because of the increasing ratiopl/pz, the flow velocity inside the nozzle increases. The sound velocity of the mixture is attained at the = 1,89. A further increase of critical ratio (P1/p2)lrrit p I / p z leads to an underexpanded, supersonic jet and Mach waves occur at the exit of the nozzle. For this reason, both turbulence intensity and expansion cooling also increase 171.

and observation path by

with the laser intensity Z, the partial density ni of species i and the Rayleigh cross section oiNnormalized to that of N2 (oiN = oi/oN2) 131. In terms of the total density n with the mole fraction ai = ni/n and with oeg = Z (ai. oiN)the signal S is S

Generally, outside ignition begins at the side of the free jet of exhaust gas when it first appears. Only in a few experiments was a delay of the outside ignition of about 1 ms or double ignition observed. The distance where the ignition could be expected increases and spreads with increasingxZ orp,. Above the critical ratio (P1/p2)lrritMach waves are easily detected in the fresh gas stream. The Mach waves disappear in the exhaust gas because the density gradient which is integrated over the whole jet diameter overlays the relatively weak density gradients of the Mach waves. Because the outside ignition starts at a distance from the nozzle exit where the Mach waves have faded, the shock waves cannot be responsible for an outside ignition in this set-up. LASER DIAGNOSTICS Temperature Measurements using Rayleigh Scattering Theory. The elastic scattering of light quanta from molecules is called Rayleigh scattering. The Rayleigh scattering signal S is a function of density weighted with the corresponding Rayleigh cross section for each species in the flow and is given for constant wavelength

PI

I.n.oeff

On the assumption of homogeneity in the small direction of the laser sheet, the total density distribution is

z

S(XJ)

131

4x8) =

OBSERVATIONS WITH SCHLIEREN METHOD Fig. 5a shows an outside ignition with d = 0,9 mm and xz = 8 mm starting at the nozzle exit. This is a typical image for xz = 8 mm and xz = 16 mm with a diameter above ds. The ignition occurs immediately the hot jet appears. In fig. 5b (d = 1,0 mm, xz = 32 mm) a hot turbulent jet of exhaust gas leaves the nozzle. The outside ignition starts at a distance of about 30 mm and a turbulent flame front moves back to the nozzle. For x - 56 mm the jet shows stronger turbulence and z :. ignition occurs at a distance of about 50 mm.

-

'

% Y )' oe&J)

where S(XJ) is the camera image and k, is a constant. For an ideal gas at constant pressure the temperature distribution is given by

[41

TABLE 1 - Rayleigh cross sections.

I

gas

measurement

calculation

i

OiN,meas

oeff,cak

02

134 E1 0,23 0,88

N2 H2 H20 Air fresh gas exhaust gas

1,14

1,11 0,89 0,96

Rayleigh cross section. To evaluate the temperature distribution qxy) from the camera image S(XJ) k,, I(xJ) and oe&8), which depends on the gas composition, must be known. For the majority species of the H2/air combustion oiNwere measured at constant and known pressure and temperature. The oegof fresh gas and exhaust gas were calculated (Table 1). The measured oiNof dry air shows good agreement with the data calculated from 0 2 and N2 Rayleigh cross sections. The small difference between the oegof fresh gas and exhaust gas is only completely effective if the

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combustion is complete. The fact that the flame is quenched inside the nozzle and ignition occurs at the head of the emerged reaction products leads to the assumption that the combustion could not be complete and, furthermore, the free jet contains a mixture of fresh and burnt gas. So the partial density of burnt gas in the turbulent free jet is successively reduced by additional fresh gas from the environment. This results in a variation range of oedxy),which is much below the difference of oeffof fresh and exhaust gas. A constant oeE=0,89 is therefore used. Procedure. Because the absorption and scattering are very weak compared with the laser intensity, the variation along the beam path, which is defmed as the x direction, is negligible. The intensity can thus be reduced to a one-dimensional distribution I@) and Eq. [4]is simplified to

PI The camera image S(XJ) is normalized by a vertical profile taken from S(xy) from a region outside the jet which consists of fresh gas at ambient temperature. This profile contains the I(y) and oeffinformation and leads to a digital value of 1 outside the jet. Inversion and multiplication by the ambient temperature, which is obtained from a thermocouple measurement, fmally results in the quantitative temperature distribution T ( x y ) . The overall relative uncertainty of the temperature measurement is estimated to be better than 10%. Results. In the underexpanded, supersonic jet which can be perceived at ignition distances greater than xz = 40 mm, the pressure distribution is unknown inside the Mach waves and quantitative measurements are very difficult. For small values of xz the outside ignition starts directly at the nozzle exit and a comparison of the temperature distribution in the jet for explosion transmission and non-transmission is not possible. For this reason ignition distances between xz = 24 mm and xz = 40 mm are considered in this chapter. The diameter of the nozzle was changed between the boundaries for 0% and 100% transmission (fig. 3). Images were taken at different points in time and also at various distances from the nozzle, if it is necessary according to the place where outside ignition occurs. It can be seen that for an explosion transmission the temperature on the jet axis increases with the distance from the nozzle until independent flame propagation is possible. With a decreasing diameter of the nozzle, the temperature on the axis falls and independent flame propagation is prevented. In this case combustion can be seen in the jet to some extent, but cooling due to

mixing with cold fresh gas from the environment cannot compensate for the heat release of the combustion. The images of xz = 32 mm are chosen as examples to illustrate the results. Fig. 6 shows the temperature on the jet axis evaluated as the average of 9 pixels (0,017 mm2) around the maximum intensity of the vortices. For the diameter d = 1,0 mm, transmission was obtained in all experiments and for d = 0,9 mm, 80% of the experiments resulted in non-transmission. The figure contains data from several experiments under the same conditions and shows a significant difference between transmission and non-transmission. The temperature distribution of single experiments are graphed in fig. 7 in false colour representation. In the right-hand part of the figure the geometry relation is sketched. The jet shows a turbulent structure as in the schlieren images. Considerable temperature fluctuations can therefore be seen in the 3D representation. Nevertheless, a falling tendency in the jet for diameter d = 0,9 mm (fig. 7a) was perceived in contrast to a steady or increasing temperature for d = 1,0 mm (fig. 7b). Laser-induced Fluorescence of 0 2 Molecules Procedure. In contrast to Rayleigh scattering, LIF is based on spectroscopy. The emission of fluorescent light depends on particular features of the excitation (wavelength, bandwidth, etc.). The LIF signal is therefore a function of density and temperature. Moreover, LIF is affected by interactions of the excited molecules. For this reason, quantitative measurements are only possible with strong restrictions. An exception is shown in the use of the predissociation states of molecules. Because of the rather short time constants of predissociation compared with those of the collision processes, the influence of fluorescence quenching is negligible /8/. In the tuning range of the ArF laser different predissociation states of 0 2 are attainable. The experiments were carried out with excitation of the B-X transition (v"=2, j"=17, wavelength 193,417 nm) of 0 2 where molecules are traceable at temperatures above 500 K /9, IO/. Calibration can be performed by means of a heated air blower via comparison with thermocouple measurements and temperature evaluation is possible if the exact concentration of 0 2 is known. Results. Because only hot 0 2 can be detected, the high intensity in fig. 8 indicates both the presence of 0 2 and a temperature greater than 500 K. The detection limit is about 10l6 molecules/cm3 /ll/. 0 2 can only be recognized at a certain distance from the nozzle, which can be explained by the fact that inside the conical core of the jet at the exit of the nozzle only exhaust gas is

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present. 0 2 from the environment reaches the jet axis fust at the head of the conical core /12, 13/. For the core length xc and diameter d of the nozzle a ratio x,ld = 4...6 is known /14/. Due to a lack of fresh gas, combustion cannot therefore occur inside the core, despite the high temperature. According to the turbulent mixing process, the gas transportation in the jet increases proportionally to the distance from the nozzle. This leads to a continuous decrease of the temperature in the jet for the mixing of non-reactive components. The results achieved with Rayleigh scattering clearly show that combustion occurs in the jet. The increase in 0 2 is overlaid by its decrease due to the reaction, and only little intensity is obtained in regions of high temperature. Considering the reduction in 0 2 and the length of the jet core, good agreement with the temperature distribution in fig. 7 can be seen if the extension of the regions above 500 K is considered. In the case of nontransmission, the mixing process and the quantity of 0 2 in the jet are similar. For this reason the image of the diameter d = 0,smm (fig. Sa) again shows a significant decrease in temperature compared with d = 0,9 mm (fig. Sb), and for diameter d = 1,0 mm (fig. 8c) the maxima of the intensity and the temperature are not at the same place because combustion inside the jet increases with the diameter of the nozzle. CONCLUSIONS The experiments described here in a flameproof enclosure model clearly show that explosion transmission depends on the pressure p 1 prevailing in the fust chamber when the flame enters the nozzle. The pressure p , at the point in time when the flame enters the joint is caused by the explosion and therefore strongly depends on the distance between the ignition source in the frst chamber and the joint. It can be seen from the schlieren images that independent flame propagation normally starts at the head of the ermerged jet within 1 ms after the fust appearance of the exhaust gas. The distance at which independent flame propagation is possible again increases with the pressure in the frst chamber, and Mach waves at the nozzle exit which accompany a supersonic jet cannot be responsible for an outside ignition. Temperature distributions evaluated from Rayleigh scattering show an increase of the temperature on the jet axis if the diameter of the nozzle is large enough. If explosion transmission is prevented, the temperature on the axis falls. These fust results using Rayleigh scattering in

combination with 0 2 LIF and time-resolved schlieren methods demonstrate that the measuring system is well suited for investigations using a flameproof enclosure model. Future investigationswill deal with the sequence of the explosion transmission, various joint geometries and improving the uncertainty of the temperature measurements. The author wishes to thank the Bundesministerium fih Wirtschafi, Bonn, for fmancially assisting this research work. REFERENCES Stepowski, D., Labacci, K., Borghi, R., 1986, (Int.) on Comb, 1561-1568

-.

Hanson, R. K., 1986,21st S y p . (1nt.). on

Comb.. 1677-1691 Koch, A., Voges, H., Andresen, P., Wolff, D., Hentschel, W., Oppermann, W., Rothe, E., 1993, 0 1 . Phys. B. 56, 177-184 Kim, G.-S., Hitchcock, L. M., Siegler, F., Rothe, E. W., Tung, C. C., Reck, G. P., 1993, &pL Phvs. B. 56.139-145 Reckers, W., Hliwel, L., Griinefeld, G., Andresen, P., 1993, ADD].Opt.. 32, 907-9 18 Kampmann, S., Leipertz, A., Dtibbeling, K., Haumann, J., Sattelmayer,T., 1993, Appl. Opt, 2 6167-6172 Beyer, M., 1993, Experimentelle Untersuchungen zum Ziinddurchschlagverhalten elektrischer Betriebsmittel an Modellanordnungen, PTB-Bericht W-54, Braunschweig Andresen, P., Meijer, G., Schliiter, H., Voges, H., Koch, A., Hentschel, W., Oppemann, W., Rothe, E., 1990, Appl. Opt.. 29. 2392-2404 Kim, G.-S.,Hitchcock, L. M., Rothe, E. W., Reck, G. P., 1991, A o o l . 5 . B. 53.180-186 Lalo, C., Masanet, J., Deson, J., Ben-Aim, R., Rostas, J., 1990, Appl. Spectrosc.. 44.442-447 Voges, H., 1992, Application of W-LFP to Physical Ignition Phenomena, critical design report, Fa. LaVision GmbH, Gtittingen Kuethe, A. M., 1935, J. of ADD1. Mech.. 2. A-8795 Wagenknecht, U., Untersuchungen der Strtimungsverhaltnisse in GaslFeststoffInjektoren, 1981, Diss. TU Braunschweig Ebrahimi, I., 1976, Forsch. he.-Wes.. 1,33-35

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explosion vessel

Gd

PC

ArF excimer laser

Fig. la: Laser system

Fig. I b Optical access to the jet inside the second vessel for laser beam and camera light source

aperture

concove mirror

f = 1500 mm d

camera

Fig. 2:

= 150

mm

knife edge

Schlieren system

1 3

1 ::: mm

1,1

~

b c

E ._ XJ

1,o

0.9 0.8

0,7

0,6 0.5

0

10

Fig. 3:

--

20 30 40 50

distance x z

60 mm 60

0% and 100% outside ignition in relation to the location of the ignition source for H,/air mixture

1.0

1.2 1,4 1.6 pressure p

Fig. 4:

1.6 2.0

2,2 b a r 2,6

1-

0% and 100% outside ignition in relation to the pressure inside the first vessel for H2/air mixture

45

0 ms 1 Fig. 5a: Schlieren photograph of an outside ignition (d = 0,7 mm, sz = 8 mm)

0 ms 1 Fig. 5b: Schlieren photograph of an outside ignition ( d = 1,0 mm, sz = 32 mm) 800

K

4

700

I +

: :

600

&

500

6

&

0 ms 1 Fig. 5c: Schlieren photograph of an outside ignition (d = 1,0 mm, sz = 56 mm)

X/d

-

Fig. 6: Temperature on jet axis for d = 0,9 mm and rl = 1,0 nim (sZ = 32 mm)

nozzle

I L 0

0 700 600

c

F Q

500

10

E

3

400

mm

300 20

-10

0 X

mm

-

10

Fig. 7a: Temperature distribution evaluated from Raylcigh scattering in talse colour representation (d = 0,9 mm; sz = 32 mm)

46

nozzle 900 7 1

K 0 700 a,

5

600

c

z

0

Q

500

10

E

2

400

mm

300

Fig. 7b: Temperature distribution evaluated from Rayleigh scattering in false colour representation (d = 1,0 mm; xz = 32 mm)

nozzle

0

Fig. 8d: O2 LIF signal of the jet without outside ignition (d

=

0,s mni; sz = 32 mm)

47

nozzle

0

Fig. 8b: 0, LIF signal of the jet without outside ignition (d

=

0,9 mm; .vz

=

32 mm)

nozzle

0

-10

Fig. 8c: 0, LIF signal of the jet with outside ignition ( d

=

1,0 mm; sz = 32 mm)

o

mm

IO