ML17319B197

Study of Hydrogen Combustion Near Lower Flammability Limits.

ML17319B197
Person / Time
Site:	Cook
Issue date:	12/31/1981
From:	Hudson F, Shiu K, Wilder J AMERICAN ELECTRIC POWER CO., INC., DUKE POWER CO., TENNESSEE VALLEY AUTHORITY
To:
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ML17319B196	List: ML17319B195 ML17319B197 ML17319B198 ML17319B199 ML17334A364 ML20041B520
References
NUDOCS 8202240179
Download: ML17319B197 (50)

Text

STUDY OF HYDROGEN COMBUSTION NEAR LOWER FLAMMABILITYLIMITS INTERIM PROJECT REPORT DECEMBER, 1981 Prepared by:

K. K. Shiu, American Electric Power F. G. Hudson, Duke Power Company J. J. Wilder, Tennessee Valley Authority Pr object Conducted at:

Whiteshell Nuclear Research Establishment Pro)cot Sponsors:

American Electric Power Service Corporation Atomic Energy of Canada Limited Duke Power Company Electric Power Research Institute Tennessee Valley Authority Ontar io Hydro

'8202240179 8202i7 PDR ADOCK 05'0003i5 PDR

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h CONTENTS 1 ~ INTRODUCTION 2~ DESCRIPTION AND INSTRUMENTATZON OF TEST FACILITY 2.1 Description of Test Facility 2.2 Instrumentation 3~ EXPERIMENTAL PROCEDURE 3.1 Preparation of the Mixture 3.2 Sampling 3.3 Turbulent Combustion Experiments 4~ RESULTS AND DXSCUSSZON 4.1 Combustion at Low Concentrations 4.2 Combustion at Relatively High H2 Concentrations 4.3 Effect of Turbulence 4.4 Effect of Turbulence with Steam Addition 4.5 Effect of Hydrogen Concentrations with Turbulence 4.6 Effect of Steam Addition and Turbulence on Combustion with Central Ignition 4.7 Effect=of Ignitor Location 4.8 Temperature Effects on Flammability Limits

5. CONCLUSIONS TABLES AND FIGURES

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INTRODUCTION This report provides preliminary information on the Mhiteshell combustion study currently underway at the Contaiment Test Facility at Mhiteshell Nuclear Research Establishment. Included in this report are static and turbulent test results obtained for varying pre-mixed hydrogen mixtures. Data on the effects of turbulence are also presented.

DESCRIPTION AND INSTRUMENTATION OF THE TEST FACILITY

2. 1 DESCRIPTION OF THE TEST FACILITY The test configuration consists of two test volumes: a sphere and a pipe which may be interconnected. The dimensions of the two vessels are shown in Table 1.

For the series of experiments reported here only the sphere is used, Figure 1. The pipe/sphere geometry experiments are being conducted at the test facility.

The sphere has three large openings and several small ones. The smaller openings are used for mounting instruments and measurement probes. The sphere is insulated and trace heated with steam. The temperature of the spher8 can be maintained at any desired value up to about 275 F (135 C). Steam is infected into the sphere through one of the ports when it is required. A view of the sphere is shown in Figure 1. Two fans driven by air motors are mounted diametrically opposite to each other as shown in Figure 2. Some of the fan characteristics are shown in Table 2.

2.2 INSTRUMENTATION A schematic of the instrumentation within the sphere is shown in Figure 3. The transient pressures during combustion were measured by three piezoelectric type transducers with a rise time of 2 microseconds and a Rosemount capacitance transducer with a response time of 0.2 seconds. The piezo-transducers were mounted flush with the inner surface of the flanges. A resistance temperature detector was employed to monitor the steady state temperature of the gases. It was not intended for fast transient measurements.

A spark ignition source was used for all the tests performed in this facility. The use'f a spark made it easier to instrument these experiments.

The passage of the flame front was detected by two seven point ion probes mounted approximately in the radial direction opposite to each other as shown in Figure 3. 'he departure from the radial direction was

slight, so for all practical purposes the orientation of the probes can be assumed radial. Each of the seven points consisted of two electrodes of .04 inches (1 mm) diameter bare wires separated by .08 inches (2 mm) gap. The ion probes for these experiments were developed by Liu et. al. and details are presented in reference (1) ~

The signals from the piezoelectric transducers and ion probes were processed by an analog to digital, convertor with a scan time of 1.5 millisecond per scan. For low hydrogen concentration experiments this was considered adequate. A two channel transient recorder was available for any selected two channels, if required.

All the transducer and probe amplifiers were mounted as close to the vessel as practically possible in purge boxes and explosion proof casing so that the cables connecting the transducers and the amplifiers were not excessively long.

The gases in the sphere before and after combustion were measured using a gas chromatograph employing a Hydrogen Transfer System (HTS). The details of the chromatograph, its calibration, and sampling technique are given in reference (2). A schematic of the sampling loop is shown in Figure 4.

EXPERIMENTAL PROCEDURE

3. 1 PREPARATION OF THE MIXTURE First, the vessel was evacuated to a sufficiently low pressure, 0.73 lb/in (5 kPa absolute). Next, hydrogen was introduced to the appropriate partial pressure, followed by the introduction of steam and air to their partial pressures. The gases were further mixed by turning on the fans for about 30 seconds prior to the initiation of any test.

3.2 SAMPLING Before any sampling was initiated at least two calibration mixtures were run through the gas chromatograph (GC) several times to ensure the proper performance of'he GC. Only when the GC measurements were repeatable, was the sampling loop activated. The sampling lines run from the sphere to the control room and are steam trace heated all the way to the infection port.

The sampling line was thoroughly flushed by the mixture in the sphere for at least 5 minutes to ensure that the sample passing through the GC was the same as that in the sphere. Two samples were normally taken and, if the two GC measurements agreed with each other and also with the amount of hydrogen introduced by the partial pressure method within the limits of accuracy, it was assumed that the constituents were in the right amount to carry out the test. The same procedure was repeated for sampling the combustion products. The products were mixed by turning on the fans before sampling.

Table 3 shows the precision of the gas chromatograph. The accuracy shown represent the upper limits. However, the measurements tended to be much more accurate than what the numbers indicate. For example, a calibration mixture with 9.62$ hydrogen was measured by the partial pressure method and by the GC within +0.2$ .

3 +3 TURBULENT COMBUSTION EXPERIMENTS In this case the fans were turned on for a short time (~ 1 minute) before ignition and were kept operating during the test. Though the fan speed can be varied, the present series of experiments have been done at a constant fan speed of about 1500 rpm.

Measurements of the turbulence created by the fan have been made in the open atmosphere simulating conditions prototypic of those in the sphere. The results are shown in Figure 8. The turbulent intensities, which can be represented by the root mean square of the difference between the local velocity squared and the mean velocity squared, indicate the degree of local fluctuations in the velocity components at that particular location.

For instance, at a location ten inches away from the fan in the axial direction and zero inches away from the central axis, the RMS velocity is about 9 .5 feet per second, whereas at the same axial distance, but at a radial location four inches away, the RMS velocity increases to 10.5 feet per second, which indicates an increase in the degree of turbulence of the second location.

RESULTS AND DISCUSSION 4.1 COMBUSTION AT LOW CONCENTRATIONS Combustion of hydrogen at low-- concentrations of around 5'5 hydrogen by volume is characterized by low burning velocities and a low degree of burn completeness., Only about 20$ of hydrogen is burned. Figure 5 shows the pressure time history for a 5$ H in air and steam-air mixtures. In In this cake, it appears that combustion is dominated by buoyancy effects.. The fireball initiated at the bottom moves upward at a speed greater than the burning velocities of the mixture and no downward propagation is possible. Thus for low initial hydrogen concentrations only a small fraction is burned. Larger concentrations result in correspondingly increased amounts of hydrogen burned and higher peak pressures. Once the fireball reaches the top, it is quenched and the pressure in the system decays.

The middle curve of Figure 5 shows the pressure time history of 5$ hydrogen with 15$ steam added.

The, behavior seems to be similar to the dry case except that the peak pressure in the system is lower. The extent of ccmbustion is virtually the same and the reduced pressure can be attributed to the increased heat capacity of the mixture due to the presence of steam.'his results in reduced flame temperatures and thus in reduced peak pressures. The behavior of a 30$ steam case is similar to that of a 15$ .

4.2 COMBUSTION AT RELATIVELY HIGH H CONCENTRATIONS The extent of combustion at higher hydrogen concentrations, around 8$ by volume, is characterized by 100$ burning. The pressure peaks are much higher than the 5$ tests depicted in Figure 5 as can be expected. Figure 6 shows combustion at 8$ H2. Addition of 15$ steam has not altered the shape of the curve very much. The reduction in peak pressure, as explained earlier, is due to increased heat capacity of the mixture yielding lower flame temperatures. Combustion in both dry and 15$ steam case resulted in 100$

hydrogen consumption.

The bottom curve of Figure 6 is for 30$ steam addition. In this case only about 38$ H was burned and the peak pressure is about 25) that of the fully burned case. Larger quantities of steam appear to reduce the burning velocity of the mixture by decreasing the flame temperature and increasing (he radiation loss from flame to steam.

As the burning velocity is reduced, the combustion is again governed by buoyancy effects and downward flame propagation is negligible.

These findings agree with the findings of Liu et al (3) that moderate (0-15$ ) steam additions do not affect significantly the degree of combustion for bottom ignition.

4.3 EFFECT OF TURBULENCE It has been well established that turbulence enhances the rate and degree of combustion.

Recent investigations of Abdel-Gayed et al (4) have shown that turbulence effects are large even for hydrogen. Preliminary results from these tests appear to confirm their conclusion. This is illustrated in Figure 7 by plotting pressure as a function of time for the combustion of 5.5$

hydrogen-air mixture with and without fan. The dashed curve is for the quiescent mixture. The degree of burn is only 26$ , showing the dominance of buoyancy. The corresponding pressure rise is only a small fraction of the calculated adiabatic pressure rise. Mhen turbulence is produced by turning on the fans, the rate of combustion is increased drastically and nearly 83~< of the hydrogen initially present is burned. The peak pressure observed is close to the adiabatic pressure expected for 83$ burn. The peak pressure measured is 15.2 psi ( 105 kPa) and the pressure calculated for adiabatic burning is 18.6 psi (128 kPa). The adiabatic pressure rise is 6.1 psi (42 kPa) with 26$ burn and the measured is about 3.5 psi (24 kPa).

A further comparison of combustion with and without turbulence is shown in Figure 9 which is for 7$ hydrogen. Data for both 7$ and 8$ (figure 9A) hydrogen cases show complete combustion with or without fan turbulence. For bottom ignition seven percent hydrogen with quiescent burning

appears to be the minimum concentration for complete combustion at 212 F (100 C). All other concentrations above 7$ result in complete burning. This value is lower than the 8.5$

hydrogen limit for complete combustion at 86 F 0

(30 C) ~ Increase in initial temperature results in an apparent shift in the downward propagation limit, thus allowing more complete combustion.

Figure 10 is a summary of several tests with and without turbulence. The mixtures were ignited either at the top or at the bottom as shown.

For cases with the fan in operation, data consistenty revealed more complete combustion and a noteable increase in combustion peak pressure.

The lowest concentration at which the downward propagation could be achieved in a quiescent; mixture was observed to be 8 .5$ hydrogen for top ignition.

4.4 EFFECT OF TURBULENCE WITH STEAM ADDITION Figure 11 shows the effect of added steam when turbulence is present. Normally, 30$ steam would exhibit limiting effects on combustion of the mixture when no turbulence is present (see Figures 5 and 6). But as can be seen from Figure 11, its effect on combustion in the presence of turbulence is minimal. The peak pressure with steam is reduced due to the increased heat capacity of the mixture. '

Steam and turbulence have competing effects on combustion; whereas the addition of steam tends to reduce the rate and degree of combustion, turbulence promotes rapid and more complete combustion.

4 ~5 EFFECT OF HYDROGEN CONCENTRATIONS WITH TURBULENCE As the hydrogen concentration is increased from 5.5$ to 8$ , combustion progresses from a partially burned to a fully burned situation.'he rate of combustion also appears to increase with concentration.

Figure 12 presents the resulting pressure rise as a function of'ime for various hydrogen concentrations with turbulence. It can be noted that as the concentration is increased, the time to reach maximum pressure is shortened.

Though most of the results presented here are for bottom ignition, it is expected that similar arguments hold true for central ignition.

4.6 EFFECT OF STEAM, ADDITION AND TURBULENCE ON COMBUSTION WITH CENTRAL IGNITION Figure 13 shows the relative effects of steam and turbulence on the rate of burning. At 7'$

hydrogen in air the pressure rise at first is slow indicating that the fireball is moving upwards.

When it reaches the top, the downward propagation starts and the entire combustion occurs in about 12 seconds. Hydrogen was nearly fully burned.

Addition of 15$ steam to such a mixture almost completely suppressed combustion. The pressure rise was trivial, indicating that only very little hydrogen was burned. Gas chromatography measurements showed that less than 0.5$ hydrogen was consumed. The apparent difference in combustion behavior between what is shown on Figure 13 and Figure 5 can be ascribed to principally a hydrogen concentration difference and possibly the difference in igniter location.

The effect of steam disappeared once the fans were turned on indicative of the ability to promote combustion by turbulence.

EFFECT OF IGNITOR LOCATION Ignitor location affects the degree and rate of burn significantly in lean quiescent mixtures.

This is illustrated in Figures 14 and 15. Figure 14 shows the difference between central and bottom ignition with 8.5$ hydrogen at room t8mperature and Figure 15 with 7$ hydrogen at 100 C. It is clear from the figures that bottom ignition results in faster combustion at these concentrations. This is in contrast to combustion at hydrogen concentrations in excess of 10$ where central ignition will exhibit faster combustion than bottom ignition due to the diminishing effects of buoyancy. For a spherical vessel, with central ignition the flame will propogate in all directions over 'a distance of the radius of the shere; whereas for bottom ignition-at 10$ hydrogen the flame will propogate. over twice the distance of the radius.

I I

4.8 TEMPERATURE EFFECTS ON FLAMMABILITYLIMITS Though the purpose of the present ser ies of experiments has not been one of establishing the limits of propagation, it does in a way shed some light on the process that must be taking place.

Figure 16 shows the combustion peak pressures plotted against hydrogen concentration. For quiescent mixtures, the pressure rise is abrupt above about 8$ hydrogen suggesting, that the nature of flame propagation or combustion has changed.

Agreement between the present data and the data of Furno et al (5) is good.

Figure 17 is s&ilar to Figure 16 except that it is drawn for bottom ignition. Here also the threshold concentration is around 8$ hydrogen.

This can be compared with Figure 10. The shift to the left of threshold concentration at elevated temperature is obvious. A test was conducted to establish if indeed the downward propagation limit has shifted to 7$ . A mixture containing 7$

hydrogen was ignited at the top. The mixture failed to ignite even after several attempts under quiescent conditions. However, the mixture could be ignited down to 5.5$ when fans were turned on.

Using the information provided in references (6) and (7) and assuming that the flammability limit for downward propagation as 9$ , the estimated value for the downward limit at 100 C is about 8.5$ hydrogen, agreeing with present work. The downward propagation in the sphere experiments, when ignited at the center or bottom, may not necessarily be related to the propagation limit.

The propagation under these conditions may be due to some turbulence or circulation currents set up by the moving flame.

Figure 18 shows that combustion is possible even at 5.7$ when turbulence is present. Ft om these it could be inferred that the absolute limit of propagation is the limit for upward propagation.

CONCLUSIONS The following conclusions can be made from the present'nvestigations.

For small quantities of steam addition, the nature and degree of combustion is not affected very much with bottom ignition. Only at higher steam concentr ations, around 30$ or above would steam

begin to have significant effects on combustion.

2. Turbulence increases the rate and degree of burn in almost all cases.

3. Bottom ignition results in the highest degree of burn and is more effective in establishing a flame even at very low hydrogen concentrations, with or without fans.

4. Central ignition combustion is more susceptible to the influence of steam in lean mixtures than bottom ignition combustion.

5. For low steam concentrations, around 15$ , steam effects if any are not significant in the presence of turbulence.

REFERENCES Ltu, D.,D. S. et al, "Development of Instrument for Combustion Studies: Part I, Test of Ionization Probes for CTF Applications",

WNRE-512-1, (1981) .

Howe, P. T. and Myers, M. E., "On-Line Gas Analysis for the Containment Test Facility", WNRE Report, WNRE-217.

3 ~ Liu, D. D. S. et al, ~'Canadian Hydrogen Combustion Studies Related'o Nuclear Reactor Safety Assessment", paper 80-33, Western States Section/The Combustion Institute, 1980, 1980 Fall Meeting at Los Angeles, Calif, Oct. 20-21 (1980).

4. Abdel-Gayed, R. G. and Bradley, D., Sixteenth Combustion (International) Symposium pp. 1725-1735, (1976) ~

5. Furno, A. L. et al, "Some Observations of Near Limit Flames", 13th Combustion Symposium, pp. 593-599, (1971) ~

6. Shapiro, Z. M. et al, "Hydrogen Flammability Data and Application to,PWR Loss-of-Coolant Accident", Bottis Plant, Pittsburgh, WAPD-SC-545, 1957.

7. Edmundson, H and Heap, M. P., "The Burning Velocity of Hydrogen-Air Flames", Combustion and Flame, 16, 161 (1971).

E71351.29

TABLE 1 r

CTF VESSEL DIMENSIONS Sphere Interconnecting Pipe Internal diameter (ft) 7.5 0.95 Length (ft) 19.7 Wall thickness (inch) 2.14 .67 Volume (ft ) 223 17 7 Design pressure (psi) 1450 1450

TABLE 2 FAN DETAILS Air Motor Speed 1850 RPM (Max)

Motor Horse Power 0.22 Fan Tip Diameter 16 inches No. of Blades on the Fan Fan Capacity 1500 CFM at 1100 RPM Continuously Variable speed Motor

TABLE 3 Comnonent Concentration Ran e Precision (2c)

Volume~+ Volume 5 H2 0.5 - 2.0 0' H2 0 - 30 1.2 02 0-21 1e6 N2 70 - 90 2.1 H20 3 - 8 0.7 E71351,29

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Attachment No. 6 to AEP:NBC:00500G Donald C. Cook Nuclear Plant Unit Nos. l and 2 Information on Hydrogen Ifitigation and Control

'dditional Report on Hydrogen Mixing and Distribution Studies

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