- Email: [email protected]
Spatial and radiative properties of an open-flame hydrogen plume
International Journal of Hydrogen Energy 31 (2006) 1332 – 1340 www.elsevier.com/locate/ijhydene
Spatial and radiative properties of an open-flame hydrogen plume R.W. Schefera,∗ , W.G. Houfa , B. Bourneb , J. Coltonb a Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551, USA b SRI International, 333 Ravenwood Ave., Menlo Park, CA 94025, USA
Available online 18 January 2006
Abstract Considerable effort is being directed toward updating safety codes and standards in preparation for production, distribution, and retail of hydrogen as a consumer energy source. In the present study, measurements were performed in large-scale, vertical flames to characterize the dimensional and radiative properties of an ignited hydrogen jet. These data are relevant to the safety scenario of a sudden leak in a high-pressure hydrogen containment vessel. Specifically, the data will provide a technological basis for determining hazardous length scales associated with unintended releases at hydrogen storage and distribution centers. Visible and infrared video and ultraviolet flame luminescence imaging were used to evaluate flame length, diameter and structure. Radiometer measurements allowed determination of the radiant heat flux from the flame. The results show that flame length increases with total jet mass flow rate and jet nozzle diameter. When plotted as a function of Froude number, which measures the relative importance of jet momentum and buoyancy, the measured flame lengths for a range of operating conditions collapse onto the same curve. Good comparison with hydrocarbon jet flame lengths is found, demonstrating that the non-dimensional correlations are valid for a variety of fuel types. The radiative heat flux measurements for hydrogen flames show good agreement with non-dimensional correlations and scaling laws developed for a range of fuels and flame conditions. This result verifies that such correlations can be used to predict radiative heat flux from a wide variety of hydrogen flames and establishes a basis for predicting a priori the characteristics of flames resulting from accidental releases. 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen; Turbulent jet; Combustion; Hydrogen flames
1. Introduction The development of an infrastructure for hydrogen utilization will require new safety codes and standards that establish guidelines for building the components of this infrastructure. Based on a recent workshop on unintended hydrogen releases, one of the most common release scenarios involves leaks from pressurized hydrogen-handling equipment [1]. These leaks range from small-diameter, slow-release leaks originating ∗ Corresponding author. Tel.: +1 925 294 2681; fax: +1 925 294 2595. E-mail address: [email protected] (R.W. Schefer).
from holes in delivery pipes to larger, high-volume releases resulting from accidental breaks in high-pressure storage tanks. In all cases, the resulting hydrogen fuel jet represents a potential fire hazard, and the buildup of a combustible cloud poses a hazard if ignited downstream of the leak. A scenario in which a high-pressure leak of hydrogen is ignited at the source is best described as a classic turbulent-jet flame, shown schematically in Fig. 1. The distances of importance are the radial distance from the geometrical flame centerline, r, and the distance downstream of the jet exit, x. Other variables of interest are the jet exit diameter, dj , and the jet exit velocity and density, uj and j , respectively. The objective of the
0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.11.020
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
present study is to establish a framework for the utilization of turbulent-jet flame data in the literature and to identify additional data needs for hydrogen safety codes and standards. These flames are typically characterized with respect to flame blowout, flame dimensions (length and width), and radiant heat flux. Certainly from a safety standpoint, radiant heat flux from the flame is of significant importance since it is a primary mechanism through which heat is transferred to nearby objects. Previous findings concerning relevant flame characteristics will be summarized in the remainder of this section. In particular, we hope to establish a framework for scaling and similarity of these flames over a range of length scales so that more detailed data (e.g. temperature, species concentration, velocity and radiative flux) obtained in less hostile laboratory environments can be utilized to quantify the larger-scale flames of interest to safety standards.
1333
Flame Envelope
1.1. Flame length An experimentally measured turbulent-jet flame length may be defined as the visible flame length, Lvis . In this case, either visual observation is used or photographs of the visible flame emission are recorded. Becker and Liang [2] used visual observations of the “furthest point at which flaming gas was seen to dwell with an appreciable frequency” to define the flame tip. Kalghatgi [3] determined the flame length as the distance between the tip of the visible flame and the jet 1 exit measured from each of several photographs ( 30 s exposure time) and then averaged over several photographs. As described by Dahm and Dimotakis [4], a stoichiometric flame length, Lst , can be determined as the distance from the jet exit to the axial location where a stoichiometric mixture of fuel from the jet and air entrained from the surrounding ambient fluid has been established along the centerline. That is, Lst is the distance required to molecularly mix every part of jet fluid with entrained air to at least the stoichiometric value. Previous results from for the visible flame length of turbulent hydrogen-jet flames demonstrated that turbulent flame length increases with mass flow rate for a given jet exit diameter and increases with jet diameter for a fixed mass flow rate [3]. The results for other fuels (methane, propane and ethylene) show similar behavior. 1.2. Flame radiation Gaseous flame radiation is the primary heat transfer mechanism from hydrogen and most hydrocarbon flames. As such, radiative heat flux measurements are
x
Jet Exit
r
Hydrogen Flow Fig. 1. Coordinate system for turbulent-jet flame.
integral to the development of safety codes and standards. In fuel-rich hydrocarbon flames where significant amounts of sooty particles are formed, radiation from soot dominates the radiative heat flux, while in flames where sufficient air has mixed with the fuel prior to combustion, little soot is formed and gaseous emission accounts for nearly all the radiative heat flux. In hydrogen flames, the only significant source of radiative emission is excited-state H2 O∗ molecules. For high Reynolds number jet flames burning a variety of fuels, similarity-scaling laws have been shown to apply over a wide range of operating conditions [5,6]. This finding is significant to the present study of large-scale hydrogen flames because experimental findings and empirical laws established for small-scale laboratory flames are directly applicable to larger-scale flames of relevance to the development of safety codes and standards. Verifi-
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
cation that the large-diameter, high-velocity jet flames studied here obey these scaling laws would enable us to quantify the heat-flux characteristics of large-scale jet flames. Following the discussion of Sivathanu and Gore [7] the flame properties of importance to radiant emission are flame length, Lvis , total radiative power emitted from the flame, Srad , and total heat released due to chemical reaction, mfuel Hc where mfuel and Hc are the total fuel mass flow rate and the heat of combustion, respectively. Of particular interest is the radiant fraction, Xrad , which is defined as the fraction of the total chemical heat release that is radiated to the surroundings: Xrad = Srad /mfuel Hc .
C (x/L, r/L) = 4R qrad (x/Lvis , r/Lvis )/Srad , 2
C2H4 C2H4 CH4 CH4 C2H2 C2H2
1.5
11.2 20.2 12.5 6.40 18.1 56.5i
1.0
0.50
0.0
0.0
0.50
1.0
1.5
2.0
2.5
3.0
(1)
(2)
where R is the radial distance from the flame centerline to the location at which the radiant flux is measured and qrad (x, r) is the radiant heat flux measured at a particular axial location, x, and radial location, r. Experimental data further show that C ∗ may be expressed in nondimensionalized form as a function of burner diameter, flow rate and fuel type and, for turbulent-jet flames, is dependent only on the normalized axial distance. Under these conditions Eq. (2) reduces to C ∗ (x/L) = 4R 2 qrad (x/L)/Srad = 4R 2 qrad (x/L)/Xrad mfuel Hc .
Fuel S (kW)
x/L vis
For turbulent-jet flames, the radiative power can be expressed in terms of a non-dimensional radiant power, C ∗ , given by the general expression ∗
2.0
C*
1334
(3)
The form of Eq. (3) was verified by Sivathanu and Gore using a heat flux transducer, or radiometer, to measure the total heat flux in turbulent-jet flames over a range of conditions. Subsequent integration of the measured radiative flux over the cylindrical flame surface yielded the total radiant power, Srad . Fig. 2 shows typical profiles of C ∗ measured in six different turbulent-jet flames using CH4 , C2 H2 and C2 H4 as the fuel. It can be seen that the profiles collapse onto a single curve consistent with Eq. (3). In fact, the collapse of the function C ∗ is within 15% in the region x/Lvis = 0.5–0.7, indicating that a point in this region is well suited for the measurement of radiant heat flux, from which total radiant output can be determined from Eq. (3). The remainder of this paper highlights the experimental system, measurement techniques, and results for flame tests using a 7.94-mm-diameter hydrogen (H2 ) jet. This jet and the operating conditions were selected
Fig. 2. Axial variation of normalized radiative heat flux (Sivathanu and Gore [7]).
to provide data in the high-flow rate, momentumdominated regime where many unintended release scenarios are expected to occur. Comparisons are made to similar measurements obtained in smaller, laboratoryscale hydrogen flames and to jet flames for a variety of fuels found in the literature. These comparisons will highlight the applicability of jet scaling laws and similarity variables to the present H2 jets and identify any differences between hydrocarbon and hydrogen-fuel jets. Finally, a procedure is described in which, given a limited set of operating variables (release pressure and leak diameter, for example), the radiant heat flux can be a priori predicted. 2. Experimental system 2.1. Flow system The hydrogen gas fuel was released from a “sixpack” of high-pressure cylinders, each connected to a central manifold with a common outlet. Typical pressure in the full cylinders was 2000–2500 psia. A total of three tests were run with the flame oriented in the vertical direction. Two cylinders were used for each of the three open-flame tests, while the remaining four cylinders in each pack were closed to the manifold. Gas was released remotely with a pneumatic valve that allowed flow through the 3.175-mm manifold orifice, through a 7.6-m straight section of 7.94-mm diameter stainless steel tubing, and into the ambient air through the open end of the tube. With an initial cylinder pressure of approximately 2250 psia for a two-cylinder test, the pressure decreased to atmospheric pressure at an exponential decay rate over a period of approximately 100 s.
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
2.2. Flame length measurements Visible, infrared (IR) and ultraviolet (UV) digital images of the flame were obtained to characterize the flame structure, length and width. The IR images reveal the high temperature regions of the flame since IR emis-
1.00
0.80
P/Pinit
Isothermal Tank Prediction (Nozzle Dia. = 7.94 mm)
0.60
Adlabatic Tank Prediction (Nozzle Dia. = 7.94 mm) Isothermal Tank Prediction (Nozzle Dia. = 3.175 mm)
0.40
Adlabatic Tank Prediction (Nozzle Dia. = 3.175 mm)
0.20
0.00
0
20
40 60 Time (sec)
80
100
20
40
80
100
(a) 80 70 60 m (gm/sec)
Additional cylinders increased the test duration, but did not impact the maximum pressure or initial mass flow rate. Ignition of the hydrogen jet was achieved using two electrically heated nichrome wires strung across the width of the jet and placed at 0.6-m axial increments from the jet exit. Typically the flame was ignited by the second wire, located farthest from the jet exit. This is likely due to the lower flow velocity farther downstream. A tank blowdown model was used to predict the flow through the piping leading up to the jet exit [8–10]. The model assumes either adiabatic or isothermal flow from the stagnation conditions in the piping, and was used to predict the jet exit conditions. These jet exit conditions were then used with the flame length and radiant fraction correlations described below to predict the hydrogen jet flame characteristics. Comparisons of the measured and predicted pressure history curves were used to validate the model. The pressure-history curve in Fig. 3a shows the normalized predicted and measured blowdown behavior for the two-cylinder tests. In general, it was found that the flow was choked at the manifold exit orifice while the tank pressure was still high early in the test, then recovered while flowing though the length of tubing leading up to the jet exit, and reached choked flow conditions again at the jet exit. After about 40 s the tank pressure had dropped sufficiently that the flow was no longer sonic at the jet exit. With an assumed choked flow through the 3.175-mm-diameter manifold orifice near the cylinder outlets, we found that the predicted adiabatic tank flow provided an excellent fit to the data. From the model, the resulting mass flow rate during the tank blow down is shown in Fig. 3b. The predicted gas temperature at the jet exit varied from about 258 K near the start of the test to 284 K near the end. Knowledge of these conditions and the mass flow rate at the jet exit allowed the jet exit velocity and density to be calculated. Results of the present 7.94-mm-diameter jet flame tests were compared with measurements obtained in a laboratory-scale, 1.91-mm-diameter H2 jet flame in which measurements were obtained as part of the present study. Shown in Table 1 are the properties associated with each of these flames. The times shown in the table for the 7.94-mm-diameter flame correspond to different times in the cylinder blow down tests.
1335
50 40 30 20 10 0.0
(b)
0
60
Time (sec)
Fig. 3. (a) Tank blowdown history for three vertically oriented two-cylinder flame tests, with calculated isothermal and isentropic pressure history predictions. (b) Total mass flow rate history for isentropic expansion through a 3.175-mm-diameter orifice.
sion results primarily from vibrationally excited H2 O∗ molecules that exist in high temperature combustion products. The UV images are primarily due to emission from excited-state OH∗ molecules and are thus expected to indicate the location and structure of the primary reaction zones where the fuel is being oxidized. The visible and IR wavelength images were obtained using a Sony Model DCR TRV27 IR-sensitive video camera. An X-Nite 1000B filter with a 50% lower cutoff at 1000 nm and a 1300 nm passband (transmission > 90%) was used to remove both the UV and visible emission from the flame in the IR images. The images were stored at a standard 30 fps video frame rate, which is not of sufficient temporal resolution to follow the flame movement. In addition, the individual frame exposure time of 33 ms is insufficient to capture the instantaneous flame structure, which is averaged over
1336
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
Table 1 Flame conditions Flame
dJ (mm)
Lab flame
1.91 “ “ “ “
SRI flame (s) t =5 t = 20 t = 40 t = 60 t = 70
7.94 “ “ “ “ “
uJ (m/s) 87.7 116.3 174.5 261.6 349.0
15 20 30 45 60
1233 1231 1078 644 446
a h = 119, 962 kJ/kg for hydrogen heat of combustion. c
Re
Q (slm)
41,026 16,589 4954 1482 810
1569 2081 3123 4686 6247 9.8 × 105 3.9 × 105 1.2 × 105 4.2 × 105 1.9 × 105
m (g/s) 0.021 0.028 0.042 0.062 0.083 57.3 23.17 6.92 2.07 1.13
m h c a (kW) 2.64 3.34 5.01 7.52 10.0 6874 2779 830 248 135
(f Wf2 Lvis fs )/(3o d 2 uJ [11].
this exposure time. The UV images were captured with a Xybion, intensified CCD video camera system. The camera was gated (typical gate widths were around 160 s) to effectively freeze the flame motion and thus record the instantaneous reaction zone structure, albeit with some blurring at the start of a test when the flow velocities were highest. Multiple images were averaged together to provide information on the time-averaged flame properties and provide quantitative data on the relevant flame length scales. A narrow-band interference filter (310.5 nm center wavelength, 10 nm bandwidth) combined with a 358 nm low-pass filter was placed in front of the Xybion camera lens to eliminate flame emission and background light in the visible and infrared wavelength ranges. 2.3. Radiative flux measurements Heat flux measurements were obtained using a Medtherm Model 64P-1-22 Schmidt–Boelter thermopile detector with a 150◦ view angle. A zinc selenide (ZnSe) window on the face of the radiometer has 70% transmission between 0.7 and 17 m. Our procedure followed the method of Sivathanu and Gore [7] in which the thermopile detector was placed at a radial distance, R, of half the estimated visible flame length, Lvis /2. Two approaches were used to measure the heat flux along the length of the flame and at several radial distances in a plane normal to the flame axis in the jet exit plane. In the 7.94-mmdiameter jet flame, separate radiometers were placed at six axial locations along the length of the flame and spaced to include axial distances up to three times the visible flame length. Since the flame length varied with
time during each tank blowdown, matching R to half the flame length required an adjustment of the radial distance between tests so that R equaled Lvis /2 at selected times during the tank blowdown. Thus, during the three tests, the distance R was adjusted to equal one half the visible flame length at times of 5, 10 and 20 s into the blowdown cycle. This data was then used to verify the applicability of the functional dependence of radiative heat flux seen in Fig. 2 for hydrocarbon flames to the present hydrogen flames. Measurements along the radial line located within the jet inlet plane were obtained using four radiometers with a 0.3-m spacing. In the smaller 1.90-mm-diameter laboratory H2 -jet flame, a single detector was traversed along the flame length and normal to the flame axis in the jet inlet plane to record the radiative heat flux at various positions. Each measurement was then integrated over the flame surface to yield the total flame radiant flux, Srad (Eqs. (1)–(3)). When properly normalized, a direct comparison of the present flame measurements with those presented in the literature can be made.
3. Experimental results 3.1. Flame length Fig. 4 shows typical visible, IR and UV images of the 7.94-mm jet at approximately the same instant in time. In each image, the flame is vertically oriented with jet flow from bottom to top, and the jet exit is located near the bottom center of the image. These video images were taken early in the tests (5 s) when the flame length was near maximum. The field-of-view
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
1337
6.0 Lir (m) Lvis (m) Luv (m)
5.0
Flame Length (m)
4.0
3.0
2.0
1.0
0.0 0
Fig. 4. Visible, IR and UV images of turbulent, hydrogen-jet flame. djet = 7.94 mm.
in the visible and IR images is about 5.5 m in the vertical direction, and about 3.5 m in the UV image. The irregularities in the outer edges of the flame reflect the unsteady turbulent mixing of the fuel with ambient air. The UV camera exposure was gated for 160 s using the intensifier. This exposure time is sufficiently short to nearly freeze the flow motion and reveal many features of the instantaneous flame structure. The chemiluminescence intensity recorded within each flame image is spatially irregular and also varies from image to image, which reflects the temporal and spatial variations found in the instantaneous structure of turbulent flames. Flame lengths based on all three images were used to determine the time-average flame length (Fig. 5). The average flame length was then taken as the flame length averaged over five successive frames around the indicated time for each point. The flame length decreases with time due to the decrease in mass flow rate as tank pressure is reduced. It can be seen that the shortest flame lengths are based on the UV flame emission, while the longest flame lengths are based on IR emission. The average values for Lvis /LIR and Luv /LIR are about 0.88 and 0.78, respectively. As discussed previously, it is expected that LUV should indicate the location of the primary reaction zone where the fuel is being oxidized while LIR should be more indicative of the high temperature combustion products. The measured flame length ratios are consistent with this proposed flame behavior. Based on an analysis of the transition from momentum-controlled to buoyancy-controlled turbu-
20
40
60
80
100
Time (sec)
Fig. 5. Flame length history using visible, infrared and ultraviolet flame emission.
lent jet flame dynamics, Delichatsios [6] developed a useful correlation for turbulent flame lengths. The correlation is based on a non-dimensional Froude number that measures the ratio of buoyancy to momentum forces in jet flames. Using the nomenclature of Turns [5] the Froude number is defined as 3/2
F rf =
ue fs
(e /∞ )1/4 [(Tf /T∞ )gd j ]1/2
,
(4)
where ue is the jet exit velocity, fs is the mass fraction of fuel at stoichiometric conditions, (e /∞ ) is the ratio of jet gas density to ambient gas density, dj is the jet exit diameter, and Tf is the peak flame temperature rise due to combustion heat release. Small values of F r f correspond to buoyancy-dominated flames while large values of F r f correspond to momentumdominated flames. Note that the parameters known to control turbulent flame length such as jet diameter and flow rate, stoichiometry, and (e /∞ ) are included in F r f . Further, a non-dimensional flame length, L∗ , can be defined as L∗ =
Lvis fs dj (e /∞ )1/2
=
Lvis fs , d∗
(5)
where Lvis is the visible flame length and d ∗ is the jet momentum diameter (=dj (e /∞ )1/2 ). Fig. 6 shows the resulting correlation of flame length data from Ref. [3] for a range of fuels (H2 , C3 H8 and CH4 ) and
1338
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
10 2
0.4
0.35
0.3
L*=23
W VIS / LVIS
L*
0.25
10
0.2 0.17 0.15
2/5
2 1/5
L*=13.5Fr /1+0.07Fr )
0.1
H2 choked (d=7.94 mm) H2 unchoked (d=7.94 mm) H2 (d=1.91 mm) CH4 (Ref. [3]) C3H8 " H2 "
0.05
0
1.0
0
0.1
1.0
10.0
20
40
60
100.0
80
100
120
Time (sec)
Fr
Fig. 6. Variation of visible flame length with Froude number. Data are for vertical jet orientation. Solid lines indicate correlations for buoyancy- and momentum-dominated regimes as described by Eqs. (6a) and (6b).
Fig. 7. Ratio of maximum flame width to flame length determined from visible flame emission.
t=5 sec t=20 sec t=40 sec t=60 sec t=70 sec
8.0
inlet flow conditions. In the buoyancy-dominated regime, L∗ is correlated by the expression 2/5
13.5F r f
(1 + 0.07F r 2f )1/5
for F r f < 5
(6a)
and in the momentum-dominated regime by the expression L∗ = 23 for F r > 5.
Q (kW/m2 )
L∗ =
6.0
4.0
2.0
(6b) 0.0
It can be seen that turbulent flame lengths are well correlated over a large range of flow conditions using these non-dimensional parameters. The present data for the 7.94-mm H2 jet are shown as solid symbols in Fig. 6 over the range of 2 < F r f < 20. Flame length data from the 1.91-mm-diameter jets of the present study are also shown. The data are seen to collapse well onto the correlations given by Eqs. (6a) and (6b). Fig. 7 shows the ratio of the flame width to the flame length based on the visible flame images. The measured width, Wvis , is the maximum flame width as determined from the images at each time during the tank blowdown. Within experimental uncertainty, the value of Wvis /Lvis agrees well with the value in the literature of 0.17 [11].
0.0
0.50
1.0
1.5
2.0
2.5
x/L VIS
Fig. 8. Profiles of radiative heat flux along the centerline of a turbulent, hydrogen-jet flame. Jet diameter is 7.94 mm. Jet orientation is vertical.
3.2. Radiative flux Profiles of the radiative heat flux, qrad , for the 7.94-mm-diameter H2 flame are shown in Fig. 8. The axial distance has been normalized by the visible flame length, Lvis , to better compare the profiles for the different tank blowdown times. In agreement with the data of Sivathanu and Gore [7], the maximum heat
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
0.80 0.60
0.25 Radiant Fraction
C2H4 11.2 C2H4 20.2 CH4 12.5 CH4 6.4 C2H2 18.1 C2H2 56.5 Fit to data of Ref. [7] Present H2 data: d=7.94 mm t=5 sec t=10sec t=20sec
1.0
C*
0.3
Fuel S (kW)
1.2
1339
0.2
CO/H2 CH4 C3H8 C2H4 Present Data H2 (d=1.91 mm) H2 (d=7.94 mm)
0.15 0.1 0.05
0.40
0 1
0.20
10
100
1000
Flame Residence Time (ms)
0.0 0.0
0.50
1.0
1.5
2.0
2.5
3.0
Fig. 10. Radiant fraction as a function of flame residence time.
x/L vis Fig. 9. Profiles of normalized radiative heat flux along the centerline of a turbulent, hydrogen-jet flame. Jet diameter is 7.94 mm. Jet orientation is vertical.
fluxes are measured near x/Lvis = 0.5.0.7 for all times shown. As expected, the higher flow-rate flames at shorter blow down times produce larger radiative heat fluxes since qrad is proportional to the total combustion heat release rate in the flame. As described previously, the radiative heat flux measurements were integrated to obtain the total radiative heat flux, Srad , which allowed the non-dimensional radiant power, C ∗ , to be calculated from Eq. (3). The distribution of C ∗ for the 7.94-mm-diameter H2 -jet flame is shown in Fig. 9 for different tank blow down times. It can be seen that the data collapse onto a single curve and show excellent agreement with the data for hydrocarbon flames from Ref. [7] that is also shown in the figure. Thus the functional dependence for radiative heat flux established previously for a range of hydrocarbon flames is also applicable to hydrogen-jet flames. Turns and Myhr [11] measured the total radiant heat flux from turbulent-jet flames using four fuels with a wide variety of sooting tendencies. These fuels included methane, ethylene, propane and a 57% CO/43% H2 mixture. Radiometer measurements were taken at a single radial location of 667 mm from the jet axis and an axial location approximately half the visible flame height. Following the approach of Sivathau and Gore [7], the total radiant heat flux Srad and radiant fraction Xrad were then derived. These results, plotted in Fig. 10, illustrate an important relationship between the radiant fraction and the global flame properties, in this case the flame residence time. The flame residence time has been defined by Turns and Myhr to be given by the expression 2 f = (f Wvis Lf fs )/(3o dJ2 uJ ),
(7)
where f , Wvis , and Lvis are the flame density, width and length. This definition of residence time takes into account the actual flame density and models the flame as a cone. For turbulent-jet flames the flame width, Wvis , is approximately equal to 0.17 Lvis (see Fig. 7). Fig. 10 suggests that for flames with a lower sooting tendency, there is a well-defined linear relationship between radiant fraction and global flame residence time. At larger residence times, the radiant fraction for fuels that have a high sooting tendency (propane and ethylene) becomes strongly dependent on residence time and behaves in a highly nonlinear fashion. In contrast, both methane and the CO/H2 mixture continue to show a well behaved, nearly linear dependence on residence time and nearly collapse onto the same curve over the range of conditions studied. This behavior is again consistent with the scaling laws described above for these flames. Values of Xrad and f calculated for the present flames are plotted with the results of Turns and Myhr [11] in Fig. 10. Similar to other low sooting tendency flames (CH4 and CO/H2 ), the present H2 flame data follow a well-defined linear dependence between radiative fraction and residence time. However, for a given residence time, the radiative fraction for H2 flames is nearly a factor of two lower than non-sooting hydrocarbon flames. 3.3. Relevance of background literature to safety codes and standards The above discussion provides much of the framework for a method by which most quantities relevant to flame safety and, in particular, the radiant heat flux from hydrogen-fueled flames can be determined. Specifically, in most potential accident scenarios the probable size of a high-pressure tank rupture and the initial pressure and temperature of the hydrogen inside the tank will be known or can be estimated. Assuming the tank
1340
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
rupture behaves similar to a high pressure stagnation source with a small-diameter outlet nozzle, then knowledge of the effective hole diameter and the (stagnation) pressure and temperature within the tank is sufficient to calculate the temporal history of the tank blowdown (i.e. the gas properties at the jet exit, the jet velocity and the mass flow rate of fuel as a function of time) using isentropic expansion laws. Using Fig. 6 or the correlations given by Eqs. (4)–(6), the flame length can then be determined. Substitution of the flame length and other known quantities into Eq. (7) then yields the global flame residence time, f . Finally, the radiant fraction Xrad can be determined from the correlations implied in Fig. 10 and, knowing the fuel mass flow rate and heat of combustion, the total radiative power from the flame, Srad , can be determined. This approach effectively provides a method to a priori predict the radiative heat flux hazard from turbulent-jet flames originating from a ruptured, high-pressure hydrogen gas storage source or delivery pipeline. The data presented here for larger-scale, turbulent-jet hydrogen flames confirms the data trends and correlations in Figs. 6–10 for this class of flows.
linear dependence of radiative fraction on flame residence time was found, in agreement with non-sooting hydrocarbon flames, but the radiative fraction for the H2 flames at a fixed residence time is nearly a factor of two lower. The results verify that such correlations can be used to predict radiative heat flux from a wide variety of hydrogen flames and establishes a basis for predicting a priori the characteristics of flames resulting from accidental releases.
4. Summary and conclusions
References
Measurements were performed in large-scale, vertical flames to characterize the dimensional, thermal, and radiative properties of an ignited hydrogen jet. This data is relevant to the safety scenario of a sudden leak in a high-pressure hydrogen containment vessel. The results show that flame length increases with total jet mass flow rate and jet nozzle diameter. When plotted as a function of Froude number, which measures the relative importance of jet momentum and buoyancy, the measured flame lengths for a range of operating conditions collapse onto the same curve. This shows that turbulent flame lengths are well correlated over a large range of flow conditions. Good comparison with hydrocarbon jet flame lengths is found, demonstrating that the non-dimensional correlations are valid for a variety of fuel types. The radiative heat flux measurements for hydrogen flames also show good agreement with nondimensional correlations and scaling laws developed for a range of fuels and flame conditions. A well-behaved
Acknowledgements This research was supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cells and Infrastructure Technologies Program. The large-scale hydrogen-jet experiments were conducted at the SRI International Corral Hollow Experimental Site. Laboratory-scale flame experiments were conducted at the Sandia Combustion Research Facility in laboratories supported by the US Department of Energy, Office of Basic Energy Sciences, Chemical Sciences.
[1] Schefer RW, Houf WG, Moen CD, Chan JP, Maness MA, Keller JO, et al. Hydrogen codes and standards unintended release workshop: workshop analysis. Workshop held December 12, 2003 at Sandia National Laboratories, Livermore, CA; 2004. [2] Becker HA, Liang D. Combust Flame 1978;32:115–37. [3] Kalghatgi GT. Combust Sci Technol 1984;41:17–29. [4] Dahm WJA, Dimotakis PE. AIAA J 1987;25:1216–23. [5] Turns SR. An introduction to combustion. 2nd ed., New York: Mcgraw-Hill; 2000. [6] Delichatsios MA. Combust Flame 1993;92:349–64. [7] Sivathanu YR, Gore JP. Combust Flame 1993;94:265–70. [8] Winters WS. TOPAZ—a computer code for modeling heat transfer and fluid flow in arbitrary networks of pipes, flow branches, and vessels. SAND83-8253, Sandia National Laboratories, Livermore, CA; January 1984. [9] Winters WS. TOPAZ—the transient one-dimensional pipe flow analyzer: user’s manual. SAND85-8215, Sandia National Laboratories, Livermore, CA; July 1985. [10] Winters WS. TOPAZ—the transient one-dimensional pipe flow analyzer: an update on code improvements and increased capabilities. SAND87-8225, Sandia National Laboratories, Livermore, CA; September 1987. [11] Turns SR, Myhr FH. Combust Flame 1991;87:319–35.
Spatial and radiative properties of an open-flame hydrogen plume R.W. Schefera,∗ , W.G. Houfa , B. Bourneb , J. Coltonb a Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551, USA b SRI International, 333 Ravenwood Ave., Menlo Park, CA 94025, USA
Available online 18 January 2006
Abstract Considerable effort is being directed toward updating safety codes and standards in preparation for production, distribution, and retail of hydrogen as a consumer energy source. In the present study, measurements were performed in large-scale, vertical flames to characterize the dimensional and radiative properties of an ignited hydrogen jet. These data are relevant to the safety scenario of a sudden leak in a high-pressure hydrogen containment vessel. Specifically, the data will provide a technological basis for determining hazardous length scales associated with unintended releases at hydrogen storage and distribution centers. Visible and infrared video and ultraviolet flame luminescence imaging were used to evaluate flame length, diameter and structure. Radiometer measurements allowed determination of the radiant heat flux from the flame. The results show that flame length increases with total jet mass flow rate and jet nozzle diameter. When plotted as a function of Froude number, which measures the relative importance of jet momentum and buoyancy, the measured flame lengths for a range of operating conditions collapse onto the same curve. Good comparison with hydrocarbon jet flame lengths is found, demonstrating that the non-dimensional correlations are valid for a variety of fuel types. The radiative heat flux measurements for hydrogen flames show good agreement with non-dimensional correlations and scaling laws developed for a range of fuels and flame conditions. This result verifies that such correlations can be used to predict radiative heat flux from a wide variety of hydrogen flames and establishes a basis for predicting a priori the characteristics of flames resulting from accidental releases. 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen; Turbulent jet; Combustion; Hydrogen flames
1. Introduction The development of an infrastructure for hydrogen utilization will require new safety codes and standards that establish guidelines for building the components of this infrastructure. Based on a recent workshop on unintended hydrogen releases, one of the most common release scenarios involves leaks from pressurized hydrogen-handling equipment [1]. These leaks range from small-diameter, slow-release leaks originating ∗ Corresponding author. Tel.: +1 925 294 2681; fax: +1 925 294 2595. E-mail address: [email protected] (R.W. Schefer).
from holes in delivery pipes to larger, high-volume releases resulting from accidental breaks in high-pressure storage tanks. In all cases, the resulting hydrogen fuel jet represents a potential fire hazard, and the buildup of a combustible cloud poses a hazard if ignited downstream of the leak. A scenario in which a high-pressure leak of hydrogen is ignited at the source is best described as a classic turbulent-jet flame, shown schematically in Fig. 1. The distances of importance are the radial distance from the geometrical flame centerline, r, and the distance downstream of the jet exit, x. Other variables of interest are the jet exit diameter, dj , and the jet exit velocity and density, uj and j , respectively. The objective of the
0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.11.020
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
present study is to establish a framework for the utilization of turbulent-jet flame data in the literature and to identify additional data needs for hydrogen safety codes and standards. These flames are typically characterized with respect to flame blowout, flame dimensions (length and width), and radiant heat flux. Certainly from a safety standpoint, radiant heat flux from the flame is of significant importance since it is a primary mechanism through which heat is transferred to nearby objects. Previous findings concerning relevant flame characteristics will be summarized in the remainder of this section. In particular, we hope to establish a framework for scaling and similarity of these flames over a range of length scales so that more detailed data (e.g. temperature, species concentration, velocity and radiative flux) obtained in less hostile laboratory environments can be utilized to quantify the larger-scale flames of interest to safety standards.
1333
Flame Envelope
1.1. Flame length An experimentally measured turbulent-jet flame length may be defined as the visible flame length, Lvis . In this case, either visual observation is used or photographs of the visible flame emission are recorded. Becker and Liang [2] used visual observations of the “furthest point at which flaming gas was seen to dwell with an appreciable frequency” to define the flame tip. Kalghatgi [3] determined the flame length as the distance between the tip of the visible flame and the jet 1 exit measured from each of several photographs ( 30 s exposure time) and then averaged over several photographs. As described by Dahm and Dimotakis [4], a stoichiometric flame length, Lst , can be determined as the distance from the jet exit to the axial location where a stoichiometric mixture of fuel from the jet and air entrained from the surrounding ambient fluid has been established along the centerline. That is, Lst is the distance required to molecularly mix every part of jet fluid with entrained air to at least the stoichiometric value. Previous results from for the visible flame length of turbulent hydrogen-jet flames demonstrated that turbulent flame length increases with mass flow rate for a given jet exit diameter and increases with jet diameter for a fixed mass flow rate [3]. The results for other fuels (methane, propane and ethylene) show similar behavior. 1.2. Flame radiation Gaseous flame radiation is the primary heat transfer mechanism from hydrogen and most hydrocarbon flames. As such, radiative heat flux measurements are
x
Jet Exit
r
Hydrogen Flow Fig. 1. Coordinate system for turbulent-jet flame.
integral to the development of safety codes and standards. In fuel-rich hydrocarbon flames where significant amounts of sooty particles are formed, radiation from soot dominates the radiative heat flux, while in flames where sufficient air has mixed with the fuel prior to combustion, little soot is formed and gaseous emission accounts for nearly all the radiative heat flux. In hydrogen flames, the only significant source of radiative emission is excited-state H2 O∗ molecules. For high Reynolds number jet flames burning a variety of fuels, similarity-scaling laws have been shown to apply over a wide range of operating conditions [5,6]. This finding is significant to the present study of large-scale hydrogen flames because experimental findings and empirical laws established for small-scale laboratory flames are directly applicable to larger-scale flames of relevance to the development of safety codes and standards. Verifi-
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
cation that the large-diameter, high-velocity jet flames studied here obey these scaling laws would enable us to quantify the heat-flux characteristics of large-scale jet flames. Following the discussion of Sivathanu and Gore [7] the flame properties of importance to radiant emission are flame length, Lvis , total radiative power emitted from the flame, Srad , and total heat released due to chemical reaction, mfuel Hc where mfuel and Hc are the total fuel mass flow rate and the heat of combustion, respectively. Of particular interest is the radiant fraction, Xrad , which is defined as the fraction of the total chemical heat release that is radiated to the surroundings: Xrad = Srad /mfuel Hc .
C (x/L, r/L) = 4R qrad (x/Lvis , r/Lvis )/Srad , 2
C2H4 C2H4 CH4 CH4 C2H2 C2H2
1.5
11.2 20.2 12.5 6.40 18.1 56.5i
1.0
0.50
0.0
0.0
0.50
1.0
1.5
2.0
2.5
3.0
(1)
(2)
where R is the radial distance from the flame centerline to the location at which the radiant flux is measured and qrad (x, r) is the radiant heat flux measured at a particular axial location, x, and radial location, r. Experimental data further show that C ∗ may be expressed in nondimensionalized form as a function of burner diameter, flow rate and fuel type and, for turbulent-jet flames, is dependent only on the normalized axial distance. Under these conditions Eq. (2) reduces to C ∗ (x/L) = 4R 2 qrad (x/L)/Srad = 4R 2 qrad (x/L)/Xrad mfuel Hc .
Fuel S (kW)
x/L vis
For turbulent-jet flames, the radiative power can be expressed in terms of a non-dimensional radiant power, C ∗ , given by the general expression ∗
2.0
C*
1334
(3)
The form of Eq. (3) was verified by Sivathanu and Gore using a heat flux transducer, or radiometer, to measure the total heat flux in turbulent-jet flames over a range of conditions. Subsequent integration of the measured radiative flux over the cylindrical flame surface yielded the total radiant power, Srad . Fig. 2 shows typical profiles of C ∗ measured in six different turbulent-jet flames using CH4 , C2 H2 and C2 H4 as the fuel. It can be seen that the profiles collapse onto a single curve consistent with Eq. (3). In fact, the collapse of the function C ∗ is within 15% in the region x/Lvis = 0.5–0.7, indicating that a point in this region is well suited for the measurement of radiant heat flux, from which total radiant output can be determined from Eq. (3). The remainder of this paper highlights the experimental system, measurement techniques, and results for flame tests using a 7.94-mm-diameter hydrogen (H2 ) jet. This jet and the operating conditions were selected
Fig. 2. Axial variation of normalized radiative heat flux (Sivathanu and Gore [7]).
to provide data in the high-flow rate, momentumdominated regime where many unintended release scenarios are expected to occur. Comparisons are made to similar measurements obtained in smaller, laboratoryscale hydrogen flames and to jet flames for a variety of fuels found in the literature. These comparisons will highlight the applicability of jet scaling laws and similarity variables to the present H2 jets and identify any differences between hydrocarbon and hydrogen-fuel jets. Finally, a procedure is described in which, given a limited set of operating variables (release pressure and leak diameter, for example), the radiant heat flux can be a priori predicted. 2. Experimental system 2.1. Flow system The hydrogen gas fuel was released from a “sixpack” of high-pressure cylinders, each connected to a central manifold with a common outlet. Typical pressure in the full cylinders was 2000–2500 psia. A total of three tests were run with the flame oriented in the vertical direction. Two cylinders were used for each of the three open-flame tests, while the remaining four cylinders in each pack were closed to the manifold. Gas was released remotely with a pneumatic valve that allowed flow through the 3.175-mm manifold orifice, through a 7.6-m straight section of 7.94-mm diameter stainless steel tubing, and into the ambient air through the open end of the tube. With an initial cylinder pressure of approximately 2250 psia for a two-cylinder test, the pressure decreased to atmospheric pressure at an exponential decay rate over a period of approximately 100 s.
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
2.2. Flame length measurements Visible, infrared (IR) and ultraviolet (UV) digital images of the flame were obtained to characterize the flame structure, length and width. The IR images reveal the high temperature regions of the flame since IR emis-
1.00
0.80
P/Pinit
Isothermal Tank Prediction (Nozzle Dia. = 7.94 mm)
0.60
Adlabatic Tank Prediction (Nozzle Dia. = 7.94 mm) Isothermal Tank Prediction (Nozzle Dia. = 3.175 mm)
0.40
Adlabatic Tank Prediction (Nozzle Dia. = 3.175 mm)
0.20
0.00
0
20
40 60 Time (sec)
80
100
20
40
80
100
(a) 80 70 60 m (gm/sec)
Additional cylinders increased the test duration, but did not impact the maximum pressure or initial mass flow rate. Ignition of the hydrogen jet was achieved using two electrically heated nichrome wires strung across the width of the jet and placed at 0.6-m axial increments from the jet exit. Typically the flame was ignited by the second wire, located farthest from the jet exit. This is likely due to the lower flow velocity farther downstream. A tank blowdown model was used to predict the flow through the piping leading up to the jet exit [8–10]. The model assumes either adiabatic or isothermal flow from the stagnation conditions in the piping, and was used to predict the jet exit conditions. These jet exit conditions were then used with the flame length and radiant fraction correlations described below to predict the hydrogen jet flame characteristics. Comparisons of the measured and predicted pressure history curves were used to validate the model. The pressure-history curve in Fig. 3a shows the normalized predicted and measured blowdown behavior for the two-cylinder tests. In general, it was found that the flow was choked at the manifold exit orifice while the tank pressure was still high early in the test, then recovered while flowing though the length of tubing leading up to the jet exit, and reached choked flow conditions again at the jet exit. After about 40 s the tank pressure had dropped sufficiently that the flow was no longer sonic at the jet exit. With an assumed choked flow through the 3.175-mm-diameter manifold orifice near the cylinder outlets, we found that the predicted adiabatic tank flow provided an excellent fit to the data. From the model, the resulting mass flow rate during the tank blow down is shown in Fig. 3b. The predicted gas temperature at the jet exit varied from about 258 K near the start of the test to 284 K near the end. Knowledge of these conditions and the mass flow rate at the jet exit allowed the jet exit velocity and density to be calculated. Results of the present 7.94-mm-diameter jet flame tests were compared with measurements obtained in a laboratory-scale, 1.91-mm-diameter H2 jet flame in which measurements were obtained as part of the present study. Shown in Table 1 are the properties associated with each of these flames. The times shown in the table for the 7.94-mm-diameter flame correspond to different times in the cylinder blow down tests.
1335
50 40 30 20 10 0.0
(b)
0
60
Time (sec)
Fig. 3. (a) Tank blowdown history for three vertically oriented two-cylinder flame tests, with calculated isothermal and isentropic pressure history predictions. (b) Total mass flow rate history for isentropic expansion through a 3.175-mm-diameter orifice.
sion results primarily from vibrationally excited H2 O∗ molecules that exist in high temperature combustion products. The UV images are primarily due to emission from excited-state OH∗ molecules and are thus expected to indicate the location and structure of the primary reaction zones where the fuel is being oxidized. The visible and IR wavelength images were obtained using a Sony Model DCR TRV27 IR-sensitive video camera. An X-Nite 1000B filter with a 50% lower cutoff at 1000 nm and a 1300 nm passband (transmission > 90%) was used to remove both the UV and visible emission from the flame in the IR images. The images were stored at a standard 30 fps video frame rate, which is not of sufficient temporal resolution to follow the flame movement. In addition, the individual frame exposure time of 33 ms is insufficient to capture the instantaneous flame structure, which is averaged over
1336
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
Table 1 Flame conditions Flame
dJ (mm)
Lab flame
1.91 “ “ “ “
SRI flame (s) t =5 t = 20 t = 40 t = 60 t = 70
7.94 “ “ “ “ “
uJ (m/s) 87.7 116.3 174.5 261.6 349.0
15 20 30 45 60
1233 1231 1078 644 446
a h = 119, 962 kJ/kg for hydrogen heat of combustion. c
Re
Q (slm)
41,026 16,589 4954 1482 810
1569 2081 3123 4686 6247 9.8 × 105 3.9 × 105 1.2 × 105 4.2 × 105 1.9 × 105
m (g/s) 0.021 0.028 0.042 0.062 0.083 57.3 23.17 6.92 2.07 1.13
m h c a (kW) 2.64 3.34 5.01 7.52 10.0 6874 2779 830 248 135
(f Wf2 Lvis fs )/(3o d 2 uJ [11].
this exposure time. The UV images were captured with a Xybion, intensified CCD video camera system. The camera was gated (typical gate widths were around 160 s) to effectively freeze the flame motion and thus record the instantaneous reaction zone structure, albeit with some blurring at the start of a test when the flow velocities were highest. Multiple images were averaged together to provide information on the time-averaged flame properties and provide quantitative data on the relevant flame length scales. A narrow-band interference filter (310.5 nm center wavelength, 10 nm bandwidth) combined with a 358 nm low-pass filter was placed in front of the Xybion camera lens to eliminate flame emission and background light in the visible and infrared wavelength ranges. 2.3. Radiative flux measurements Heat flux measurements were obtained using a Medtherm Model 64P-1-22 Schmidt–Boelter thermopile detector with a 150◦ view angle. A zinc selenide (ZnSe) window on the face of the radiometer has 70% transmission between 0.7 and 17 m. Our procedure followed the method of Sivathanu and Gore [7] in which the thermopile detector was placed at a radial distance, R, of half the estimated visible flame length, Lvis /2. Two approaches were used to measure the heat flux along the length of the flame and at several radial distances in a plane normal to the flame axis in the jet exit plane. In the 7.94-mmdiameter jet flame, separate radiometers were placed at six axial locations along the length of the flame and spaced to include axial distances up to three times the visible flame length. Since the flame length varied with
time during each tank blowdown, matching R to half the flame length required an adjustment of the radial distance between tests so that R equaled Lvis /2 at selected times during the tank blowdown. Thus, during the three tests, the distance R was adjusted to equal one half the visible flame length at times of 5, 10 and 20 s into the blowdown cycle. This data was then used to verify the applicability of the functional dependence of radiative heat flux seen in Fig. 2 for hydrocarbon flames to the present hydrogen flames. Measurements along the radial line located within the jet inlet plane were obtained using four radiometers with a 0.3-m spacing. In the smaller 1.90-mm-diameter laboratory H2 -jet flame, a single detector was traversed along the flame length and normal to the flame axis in the jet inlet plane to record the radiative heat flux at various positions. Each measurement was then integrated over the flame surface to yield the total flame radiant flux, Srad (Eqs. (1)–(3)). When properly normalized, a direct comparison of the present flame measurements with those presented in the literature can be made.
3. Experimental results 3.1. Flame length Fig. 4 shows typical visible, IR and UV images of the 7.94-mm jet at approximately the same instant in time. In each image, the flame is vertically oriented with jet flow from bottom to top, and the jet exit is located near the bottom center of the image. These video images were taken early in the tests (5 s) when the flame length was near maximum. The field-of-view
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
1337
6.0 Lir (m) Lvis (m) Luv (m)
5.0
Flame Length (m)
4.0
3.0
2.0
1.0
0.0 0
Fig. 4. Visible, IR and UV images of turbulent, hydrogen-jet flame. djet = 7.94 mm.
in the visible and IR images is about 5.5 m in the vertical direction, and about 3.5 m in the UV image. The irregularities in the outer edges of the flame reflect the unsteady turbulent mixing of the fuel with ambient air. The UV camera exposure was gated for 160 s using the intensifier. This exposure time is sufficiently short to nearly freeze the flow motion and reveal many features of the instantaneous flame structure. The chemiluminescence intensity recorded within each flame image is spatially irregular and also varies from image to image, which reflects the temporal and spatial variations found in the instantaneous structure of turbulent flames. Flame lengths based on all three images were used to determine the time-average flame length (Fig. 5). The average flame length was then taken as the flame length averaged over five successive frames around the indicated time for each point. The flame length decreases with time due to the decrease in mass flow rate as tank pressure is reduced. It can be seen that the shortest flame lengths are based on the UV flame emission, while the longest flame lengths are based on IR emission. The average values for Lvis /LIR and Luv /LIR are about 0.88 and 0.78, respectively. As discussed previously, it is expected that LUV should indicate the location of the primary reaction zone where the fuel is being oxidized while LIR should be more indicative of the high temperature combustion products. The measured flame length ratios are consistent with this proposed flame behavior. Based on an analysis of the transition from momentum-controlled to buoyancy-controlled turbu-
20
40
60
80
100
Time (sec)
Fig. 5. Flame length history using visible, infrared and ultraviolet flame emission.
lent jet flame dynamics, Delichatsios [6] developed a useful correlation for turbulent flame lengths. The correlation is based on a non-dimensional Froude number that measures the ratio of buoyancy to momentum forces in jet flames. Using the nomenclature of Turns [5] the Froude number is defined as 3/2
F rf =
ue fs
(e /∞ )1/4 [(Tf /T∞ )gd j ]1/2
,
(4)
where ue is the jet exit velocity, fs is the mass fraction of fuel at stoichiometric conditions, (e /∞ ) is the ratio of jet gas density to ambient gas density, dj is the jet exit diameter, and Tf is the peak flame temperature rise due to combustion heat release. Small values of F r f correspond to buoyancy-dominated flames while large values of F r f correspond to momentumdominated flames. Note that the parameters known to control turbulent flame length such as jet diameter and flow rate, stoichiometry, and (e /∞ ) are included in F r f . Further, a non-dimensional flame length, L∗ , can be defined as L∗ =
Lvis fs dj (e /∞ )1/2
=
Lvis fs , d∗
(5)
where Lvis is the visible flame length and d ∗ is the jet momentum diameter (=dj (e /∞ )1/2 ). Fig. 6 shows the resulting correlation of flame length data from Ref. [3] for a range of fuels (H2 , C3 H8 and CH4 ) and
1338
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
10 2
0.4
0.35
0.3
L*=23
W VIS / LVIS
L*
0.25
10
0.2 0.17 0.15
2/5
2 1/5
L*=13.5Fr /1+0.07Fr )
0.1
H2 choked (d=7.94 mm) H2 unchoked (d=7.94 mm) H2 (d=1.91 mm) CH4 (Ref. [3]) C3H8 " H2 "
0.05
0
1.0
0
0.1
1.0
10.0
20
40
60
100.0
80
100
120
Time (sec)
Fr
Fig. 6. Variation of visible flame length with Froude number. Data are for vertical jet orientation. Solid lines indicate correlations for buoyancy- and momentum-dominated regimes as described by Eqs. (6a) and (6b).
Fig. 7. Ratio of maximum flame width to flame length determined from visible flame emission.
t=5 sec t=20 sec t=40 sec t=60 sec t=70 sec
8.0
inlet flow conditions. In the buoyancy-dominated regime, L∗ is correlated by the expression 2/5
13.5F r f
(1 + 0.07F r 2f )1/5
for F r f < 5
(6a)
and in the momentum-dominated regime by the expression L∗ = 23 for F r > 5.
Q (kW/m2 )
L∗ =
6.0
4.0
2.0
(6b) 0.0
It can be seen that turbulent flame lengths are well correlated over a large range of flow conditions using these non-dimensional parameters. The present data for the 7.94-mm H2 jet are shown as solid symbols in Fig. 6 over the range of 2 < F r f < 20. Flame length data from the 1.91-mm-diameter jets of the present study are also shown. The data are seen to collapse well onto the correlations given by Eqs. (6a) and (6b). Fig. 7 shows the ratio of the flame width to the flame length based on the visible flame images. The measured width, Wvis , is the maximum flame width as determined from the images at each time during the tank blowdown. Within experimental uncertainty, the value of Wvis /Lvis agrees well with the value in the literature of 0.17 [11].
0.0
0.50
1.0
1.5
2.0
2.5
x/L VIS
Fig. 8. Profiles of radiative heat flux along the centerline of a turbulent, hydrogen-jet flame. Jet diameter is 7.94 mm. Jet orientation is vertical.
3.2. Radiative flux Profiles of the radiative heat flux, qrad , for the 7.94-mm-diameter H2 flame are shown in Fig. 8. The axial distance has been normalized by the visible flame length, Lvis , to better compare the profiles for the different tank blowdown times. In agreement with the data of Sivathanu and Gore [7], the maximum heat
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
0.80 0.60
0.25 Radiant Fraction
C2H4 11.2 C2H4 20.2 CH4 12.5 CH4 6.4 C2H2 18.1 C2H2 56.5 Fit to data of Ref. [7] Present H2 data: d=7.94 mm t=5 sec t=10sec t=20sec
1.0
C*
0.3
Fuel S (kW)
1.2
1339
0.2
CO/H2 CH4 C3H8 C2H4 Present Data H2 (d=1.91 mm) H2 (d=7.94 mm)
0.15 0.1 0.05
0.40
0 1
0.20
10
100
1000
Flame Residence Time (ms)
0.0 0.0
0.50
1.0
1.5
2.0
2.5
3.0
Fig. 10. Radiant fraction as a function of flame residence time.
x/L vis Fig. 9. Profiles of normalized radiative heat flux along the centerline of a turbulent, hydrogen-jet flame. Jet diameter is 7.94 mm. Jet orientation is vertical.
fluxes are measured near x/Lvis = 0.5.0.7 for all times shown. As expected, the higher flow-rate flames at shorter blow down times produce larger radiative heat fluxes since qrad is proportional to the total combustion heat release rate in the flame. As described previously, the radiative heat flux measurements were integrated to obtain the total radiative heat flux, Srad , which allowed the non-dimensional radiant power, C ∗ , to be calculated from Eq. (3). The distribution of C ∗ for the 7.94-mm-diameter H2 -jet flame is shown in Fig. 9 for different tank blow down times. It can be seen that the data collapse onto a single curve and show excellent agreement with the data for hydrocarbon flames from Ref. [7] that is also shown in the figure. Thus the functional dependence for radiative heat flux established previously for a range of hydrocarbon flames is also applicable to hydrogen-jet flames. Turns and Myhr [11] measured the total radiant heat flux from turbulent-jet flames using four fuels with a wide variety of sooting tendencies. These fuels included methane, ethylene, propane and a 57% CO/43% H2 mixture. Radiometer measurements were taken at a single radial location of 667 mm from the jet axis and an axial location approximately half the visible flame height. Following the approach of Sivathau and Gore [7], the total radiant heat flux Srad and radiant fraction Xrad were then derived. These results, plotted in Fig. 10, illustrate an important relationship between the radiant fraction and the global flame properties, in this case the flame residence time. The flame residence time has been defined by Turns and Myhr to be given by the expression 2 f = (f Wvis Lf fs )/(3o dJ2 uJ ),
(7)
where f , Wvis , and Lvis are the flame density, width and length. This definition of residence time takes into account the actual flame density and models the flame as a cone. For turbulent-jet flames the flame width, Wvis , is approximately equal to 0.17 Lvis (see Fig. 7). Fig. 10 suggests that for flames with a lower sooting tendency, there is a well-defined linear relationship between radiant fraction and global flame residence time. At larger residence times, the radiant fraction for fuels that have a high sooting tendency (propane and ethylene) becomes strongly dependent on residence time and behaves in a highly nonlinear fashion. In contrast, both methane and the CO/H2 mixture continue to show a well behaved, nearly linear dependence on residence time and nearly collapse onto the same curve over the range of conditions studied. This behavior is again consistent with the scaling laws described above for these flames. Values of Xrad and f calculated for the present flames are plotted with the results of Turns and Myhr [11] in Fig. 10. Similar to other low sooting tendency flames (CH4 and CO/H2 ), the present H2 flame data follow a well-defined linear dependence between radiative fraction and residence time. However, for a given residence time, the radiative fraction for H2 flames is nearly a factor of two lower than non-sooting hydrocarbon flames. 3.3. Relevance of background literature to safety codes and standards The above discussion provides much of the framework for a method by which most quantities relevant to flame safety and, in particular, the radiant heat flux from hydrogen-fueled flames can be determined. Specifically, in most potential accident scenarios the probable size of a high-pressure tank rupture and the initial pressure and temperature of the hydrogen inside the tank will be known or can be estimated. Assuming the tank
1340
R.W. Schefer et al. / International Journal of Hydrogen Energy 31 (2006) 1332 – 1340
rupture behaves similar to a high pressure stagnation source with a small-diameter outlet nozzle, then knowledge of the effective hole diameter and the (stagnation) pressure and temperature within the tank is sufficient to calculate the temporal history of the tank blowdown (i.e. the gas properties at the jet exit, the jet velocity and the mass flow rate of fuel as a function of time) using isentropic expansion laws. Using Fig. 6 or the correlations given by Eqs. (4)–(6), the flame length can then be determined. Substitution of the flame length and other known quantities into Eq. (7) then yields the global flame residence time, f . Finally, the radiant fraction Xrad can be determined from the correlations implied in Fig. 10 and, knowing the fuel mass flow rate and heat of combustion, the total radiative power from the flame, Srad , can be determined. This approach effectively provides a method to a priori predict the radiative heat flux hazard from turbulent-jet flames originating from a ruptured, high-pressure hydrogen gas storage source or delivery pipeline. The data presented here for larger-scale, turbulent-jet hydrogen flames confirms the data trends and correlations in Figs. 6–10 for this class of flows.
linear dependence of radiative fraction on flame residence time was found, in agreement with non-sooting hydrocarbon flames, but the radiative fraction for the H2 flames at a fixed residence time is nearly a factor of two lower. The results verify that such correlations can be used to predict radiative heat flux from a wide variety of hydrogen flames and establishes a basis for predicting a priori the characteristics of flames resulting from accidental releases.
4. Summary and conclusions
References
Measurements were performed in large-scale, vertical flames to characterize the dimensional, thermal, and radiative properties of an ignited hydrogen jet. This data is relevant to the safety scenario of a sudden leak in a high-pressure hydrogen containment vessel. The results show that flame length increases with total jet mass flow rate and jet nozzle diameter. When plotted as a function of Froude number, which measures the relative importance of jet momentum and buoyancy, the measured flame lengths for a range of operating conditions collapse onto the same curve. This shows that turbulent flame lengths are well correlated over a large range of flow conditions. Good comparison with hydrocarbon jet flame lengths is found, demonstrating that the non-dimensional correlations are valid for a variety of fuel types. The radiative heat flux measurements for hydrogen flames also show good agreement with nondimensional correlations and scaling laws developed for a range of fuels and flame conditions. A well-behaved
Acknowledgements This research was supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cells and Infrastructure Technologies Program. The large-scale hydrogen-jet experiments were conducted at the SRI International Corral Hollow Experimental Site. Laboratory-scale flame experiments were conducted at the Sandia Combustion Research Facility in laboratories supported by the US Department of Energy, Office of Basic Energy Sciences, Chemical Sciences.
[1] Schefer RW, Houf WG, Moen CD, Chan JP, Maness MA, Keller JO, et al. Hydrogen codes and standards unintended release workshop: workshop analysis. Workshop held December 12, 2003 at Sandia National Laboratories, Livermore, CA; 2004. [2] Becker HA, Liang D. Combust Flame 1978;32:115–37. [3] Kalghatgi GT. Combust Sci Technol 1984;41:17–29. [4] Dahm WJA, Dimotakis PE. AIAA J 1987;25:1216–23. [5] Turns SR. An introduction to combustion. 2nd ed., New York: Mcgraw-Hill; 2000. [6] Delichatsios MA. Combust Flame 1993;92:349–64. [7] Sivathanu YR, Gore JP. Combust Flame 1993;94:265–70. [8] Winters WS. TOPAZ—a computer code for modeling heat transfer and fluid flow in arbitrary networks of pipes, flow branches, and vessels. SAND83-8253, Sandia National Laboratories, Livermore, CA; January 1984. [9] Winters WS. TOPAZ—the transient one-dimensional pipe flow analyzer: user’s manual. SAND85-8215, Sandia National Laboratories, Livermore, CA; July 1985. [10] Winters WS. TOPAZ—the transient one-dimensional pipe flow analyzer: an update on code improvements and increased capabilities. SAND87-8225, Sandia National Laboratories, Livermore, CA; September 1987. [11] Turns SR, Myhr FH. Combust Flame 1991;87:319–35.