Fundamental combustion properties of oxygen enriched hydrogen/air mixtures relevant to safety analysis: experimental and simulation study.

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1 Fundamental combustion properties of oxygen enriched hydrogen/air mixtures relevant to safety analysis: experimental and simulation study. R. Mével 2, J. Sabard 1, J. Lei 1, N. Chaumeix 1 1 ICARE-CNRS 2 California Institute of Technology mevel@caltech.edu ABSTRACT In order to face the coming shortage of fossil energies, a number of alternative methods of energy production are being considered. One promising approach is nuclear fusion as investigated within the framework the ITER project, International Thermonuclear Experimental Reactor. However, the operation of ITER may rise safety problems including the formation of a flammable dust/hydrogen/air atmosphere. A first towards the accurate assessment of accidental explosion in ITER consist in better characterizing the risk of explosion in gaseous hydrogen-containing mixtures. In the present study, the laminar burning speed, the ignition delay-time behind reflected shock wave, and the detonation cell size were measured over wide ranges of composition and equivalence ratios. The performance of five detailed reaction models were evaluated with respect to the present data. 1. INTRODUCTION For the last decades, our societies are faced with an energy problem due to the depletion of the natural resources of coal, natural gas and oil. Therefore, there is a strong need to find new sources of abundant and affordable energy. One of the foreseen solutions resides in the fusion energy which led the international community to design the international scientific program ITER, for International Thermonuclear Experimental Reactor, to help in resolving this problem by producing large amounts of energy using nuclear fusion. However, ITER operation may rise safety problems. Indeed, during the operation, plasma contained in the Vacuum Vessel (VV) erodes the surfaces of the walls composed of tungsten, beryllium and graphite. This plasma-wall interaction can produce several hundred of kilograms of metallic dust and graphite particles. In case of water or air ingress into the VV, when reaching high temperatures, steam may react with metallic dust and materials (beryllium, tungsten and carbon) on hot surfaces and produces hydrogen and carbon monoxide. After steam condensation, air ingress from the ex-vessel to the in-vessel break forms a flammable mixture, which can lead to high pressure loads in case of an explosion. The aim of the present work is to acquire data concerning the combustion of dust-hydrogen-air mixtures in a closed vessel, the first step being the study of gaseous H 2 -O 2 -N 2 mixtures. Several studies can be found in the literature concerning hydrogen based mixtures combustion and parameters such as: laminar flame speeds [1 20] auto-ignition delay times [21 34] and detonation parameters [35 51]. However, no data could be found in the literature concerning the particular mixtures of interest for ITER application. In order to be able to have a good assessment of the combustion parameters important in safety analysis and to provide these fundamental data to the CFD codes, it is important to acquire a detailed experimental database covering the different phenomena: auto-ignition, laminar flame speeds and detonation parameters. This work is a first step towards these goals.

2 2. MATERIALS AND METHODS 2.1. Flame speed experiments The constant volume vessel is a stainless steel sphere (i.d. 476 mm) equipped with two opposite windows (97 mm diameter, 30 mm thick). The inner surface is black polished. Two metallic electrodes located along a diameter of the sphere are linked to a high voltage source. Ignition was produced by as spark located at the center of the sphere. The voltage and current of the discharge were measured with a high voltage probe and a current probe as seen in Figure 1. The spherical bomb is equipped with a Kistler pressure sensor (601A equipped with a flame arrestor model 6505) in order to measure the evolution of the pressure as the flame propagates. Thermocouple Voltage probe Electrodes Current probe Pressure Transducer Figure 1: Schematic of the spherical vessel and the ignition setup. The visualization of the flame is obtained via a Schlieren system. It consists of two concave spherical mirrors (focal length 1 m), the light source was a white continuous lamp and it was made as a point source via one bi-convex lens (focal length 20 mm). A numerical high speed camera, PHOTRON APX, with an acquisition frequency ranging between 2,000 and 120,000 images per second, is used to record the Schlieren images of the growing flames. These images allow the measurement of the radius of the flame as a function of time (see Figure 2) using an automatic in-house MATLAB program or a manual process with the 5.2 VISILOG Software as discribed in Dubois et al. [52]. Figure 2: Schlieren images of the flame evolution in the spherical bomb. Initial conditions are: X H2 =0.35; X O2 =0.275; X N2 =0.375; P 1 = kpa; T 1 =305 K. Framing rate: 30,000 images/s. To measure flame speed, the spherical configuration has the advantage that the flame is well characterized through the experimental assessment of the stretch rate that undergoes the flame. The flame propagation is visualized via the recording of the flame front evolution as a function of time. When the observation is limited to the initial part of the flame expansion, where the pressure does not vary yet as seen in Figure 3, a simple relationship links the spatial flame velocity, VS 0, or flame speed, to the fundamental one, S 0 L, or burning speed, S 0 L = V0 S ρ b ρ u = V0 S σ. (1) 2

3 where ρ b is the burned gas density and ρ u, the unburned gas one. The expansion factor, σ, is evaluated using the equilibrium code of COSILAB [53]. a) Pressure signal b) Schlieren sequence Figure 3: Pressure signal recorded during the test: (a): total time recorded. (b): zoom around the observation time. Initial conditions are: X H2 =0.35; X O2 =0.275; X N2 =0.375; P 1 = kpa; T 1 =305 K. Framing rate: 30,000 images/s. However, since the flame has a spherical shape, it undergoes stretch which depends on both the flame speed (dr f /dt) and the flame radius itself (R f ). Following the analysis of Ronney and Sivashinsky [54] who showed that a non-linear relationship exists between the stretched laminar flame speed and the stretch rate according to: V S V 0 S 2 ln V S V 0 S 2 = 2 L b κ VS 0, (2) where V 0 S is the unstretched spatial flame speed, κ the strech rate and L b the Markstein length. L b is a parameter characterizing the effect of the stretch on the flame propagation. Due to the very fast flame propagation of these mixtures and due to the capabilities of the camera used in these studies, the extraction of the unstretched flame speed relies on a lower number of images than what was used in our previous studies [55]. All the experiments were performed at kpa and around 303 K. The equivalence ratio ranged between 0.49 and Ignition delay-time experiments Ignition delay time of mixtures of H 2 -O 2 -Air-Ar with various hydrogen contents in the temperature range of K and pressure of KPa were experimentally studied in a stainless steel shock tube (52 mm i.d. and 5.15 m long) that has been previously described [56]. The driver gas used is Helium (Air Liquide 99.99%). Both sections are evacuated using two primary vacuum pumps to reach pressure around 1 Pa for a leak-outgasing rate of 0.1 Pa/min. The last part of the driven section (1.4 m long) has been blackened by anodic oxidation to prevent multiple reflections of light near the measurement section. This surface treatment has no effect on the chemistry taking place during the experiments. The shock velocity was measured via four piezo-electric pressure transducers equally spaced by 150 mm, the last one being 10 mm away from the shock tube end wall. At the same location as the last pressure transducer, a fused silica window (9 mm optical diameter and 6 mm thickness) is mounted across a photomultiplier tube HAMAMATSU 1P28 equipped with a 306 nm centered narrow-band filter, characteristic of OH* radicals emission. A kistler pressure transducer (603B) is mounted at the 3

4 end-wall. The pressure and chemiluminescence signals are transferred and recorded by two numerical oscilloscopes (Tektronix TDS5054B). Incident and reflected shock conditions (temperature, pressure and density) were computed from the conservation equations assuming thermal equilibrium and no reaction before ignition. The method used for the calculations assumes that γ (ratio between heat capacity at constant pressure and heat capacity at constant volume, C p /C v ) varies with temperature [57]. This method allows determining the pressure and the temperature with a good accuracy as the uncertainties on temperature and pressure are estimated to be ±1% and ±1.5%, respectively. Figure 4 shows typical OH* radical chemiluminescence signal recorded by the numerical oscilloscope as well as the pressure measured at the end-wall. From those signals it is possible to calculate the ignition delay time which is defined as the time interval between the reflected shock wave passage, determined from the pressure trace, and 50% of the maximum OH* radicals emission. Figure 4: Example of recorded signals for a mixture containing 8%H 2 /4.00%O 2 /8.00%Air/80%Ar. The reflected shock temperature and pressure are respectively 6 K and 306 kpa Detonation experiments Mixtures were prepared in a 50 L stainless steel cylinder from H 2, O 2 and N 2 gas cylinders supplied by Air Liquide using the partial pressure method. Once the appropriate composition is obtained, the gases are allowed to mix through diffusion for at least 4 hours. The detonation tube is a 4.6 m long stainless steel tube with 78 mm inner diameter. Prior to each experiment, the tube is evacuated to a pressure below 15 Pa. A 1 m long Shchelkin spiral with a blockage ratio around 0.5 is attached at one end of the tube. In order to measure the detonation velocity, the other end of the tube is equipped with 7 pressure transducers located 15 or 30 cm from each other. The uncertainty on the detonation velocity is on the order of 1%. A high voltage electric spark is used to initiate a flame in the mixture and the spiral allows the transition to detonation of the flame. For each experiment, a soot foil is placed at the opposite end of the tube Detailed reaction models The following five reaction models have been employed for comparison with the present experimental results: (i) the GRI 3.0 [58], 325 reactions and 53 species, (ii) the model of Hong [59] (referred to as Hanson s model), composed of 20 reactions and 10 species, (iii) the JetSurf mechanism [60], 2163 reactions and 348 species, (iv) Konnov s model [61], 1200 reactions and 127 species, and (v) the model 4

5 of Mével [62 68], composed of 920 reactions and 115 species. It is to note that Hanson s model and JetSurf do not include nitrogen chemistry so that the impact of NOx formation is not accounted for. Because a number of studies [69, 70, 34, 71] have underlined the importance of including excited OH* radical chemistry to reproduce ignition delay-time based on UV emission (around 306 nm), a sub-model for OH*, based on [69, 70, 72, 28], was added in the different reaction models. 3. RESULTS AND DISCUSSION 3.1. Flame speed results The laminar flame speeds of H O 2 +Air have been measured at ambient temperature (in this case 303±1 K) and for an initial pressure of kpa. The results are summarized in Figure 5 a). As expected, an increase of the equivalence ratio induces a strong increase of the laminar flame speed for these mixtures until we reach the limit of the stoichiometric H 2 -O 2 mixture. The flame speed reaches in this case a value above 10 m/s. In the same figure, are plotted the simulated laminar flame speeds predicted by the five selected reaction models. a) Pressure signal b) Schlieren sequence Figure 5: Laminar flame speeds as a function of equivalence ratio for H O 2 +Air mixtures initially at 303±1 K and kpa. These simulations have been carried out using COSILAB including the thermal diffusion. The grad and curv parameters were set to 10 5 and the maximum number of grid points to 1500 to ensure that a final, grid independent solution has been reached. In this case, all the mechanisms reproduced fairly well the experimental results, the deviation between the experiments and the different models decreases with equivalence ratio. However, it is to note that the experimental errors also increase with equivalence ratio since less data points can be used to derive the laminar flame speed. To have a better view of the performance of the different mechanisms, the deviation of the models from the experimental values is plotted as a function of the equivalence ratio (Figure 5 b)). The most suitable mechanisms seems to be that of Mével and GRI 3.0, for which the deviation is below 20 %. 5

6 3.2. Ignition delay-time results Table 1 presents the mixture compositions and experimental conditions employed to investigate the autoignition delay-time of air diluted hydrogen-oxygen mixtures. Two main experimental trends can be underlined. First, the ignition delay-time increases significantly as the air mole fraction is increased, which also corresponds to a decrease of the equivalence ratio. For examples, (i) at 4 K, the delay-time is 454 µs for X Air =0.40 against 673 µs for X Air =0.80; (ii) at 927 K, the delay-time is 2760 µs for X Air =0.40 against 3400 µs for X Air =0.60. Second, a strong change of activation energy is observed around 0 K. Above the 0 K, E a =73 kj/mol whereas it increases up to 190 kj/mol below 0 K. Figure 6 shows the comparison between the experimental and the predicted ignition delay-times for the five mixtures presently studied. Both constant volume, CV, and volume as a function of time, VTIM [73], reactor models were employed for the modeling study. Figure 6 a) and Figure 6 b) compare the prediction of the reaction models for a CV and a VTIM reactor, respectively. In the low temperature range, the VTIM reactor model enables much better predictions than the CV reactor model. This is due to the non-ideal pressure variations that are observed behind reflected shock waves [73, 74]. In the present experiments, these pressure changes were measured to be between 2 and 4%/ms. In the VTIM simulations, we considered a change of specific volume through an isentropic compression induced by a change of pressure (1/P) (dp/dt)=3%/ms. In Figure 6 b) to f), the experimental delay-times are compared with the simulated ones considering a VTIM reactor. The model of Konnov predicts the highest reactivity, shortest delay, both at high and low temperature whereas the GRI predicts the lowest reactivity, longest delay. The JetSurf s predictions are close to those of Konnov s model. The results from Mével and Hanson s model are consistent with each other and lie in between those from Konnov and GRI models. The best predictions are obtained with the JetSurf with an average error around 35%. Except the GRI, all the models presently tested reproduce the delay-time within 50%. Mix Φ X Air X H2 X O2 X N2 X Ar P 5 (kpa) T 5 (K) Table 1: Composition of the H 2 -O 2 -N 2 -Ar mixtures and initial conditions used to study the ignition delay-time behind reflected shock wave Detonation results The compositions of the studied H 2 -O 2 -Air mixtures and the initial conditions before ignition are presented in Table 2. The outcome of the experiment is also indicated in Table 2. It is seen that the initiation of detonation has been possible for air mole fraction up to Above this value, accelerated deflagrations were obtained. Figure 7 compares two soot foils resulting from a successful and an un-successful detonation initiation. In Figure 7 a), the typical diamond-shape pattern resulting from the propagation of a cellular detonation can be seen whereas in Figure 7 b), the soot has been simply blown off the foil, which indicates that on detonation was initiated. Figure 8 presents the evolution of the detonation velocity for H 2 -O 2 -Air mixtures as a function of the air mole fraction and of equivalence ratio. As expected, the measured velocity decreases as the air mole fraction, and equivalence ratio increases. The red lines in Figure 8 corresponds to the theoretical Chapman-Jouguet velocity for a planar detonation wave. In each experiment, quasi-chapman-jouguet 6

7 detonation were obtained, with a mean velocity deficit of 1%. At the high air mole fraction limit (lean limit), the velocity deficit reaches 3%. This is consistent with velocity deficit previously reported [75]. It can be concluded that self-sustained detonations were obtained in the present study, ensuring a reliable detonation cell width measurement. a) Mix 1: CV b) Mix 1: VTIM c) Mix 2: VTIM d) Mix 3: VTIM e) Mix 4: VTIM f) Mix 5: VTIM Figure 6: Experimental and predicted ignition delay-times for H 2 -O 2 -N 2 -Ar mixtures. Initial conditions given in Table 1. CV: constant volume. VTIM: specific volume changes with time as calculated by imposing a (1/P) (dp/dt)=3%/ms. As seen in Figure 7 a), the cell width is quite small and the detonation structure appears somehow irregular. Figure 9 presents the evolution of the measured cell size as a function of the air mole fraction 7

8 and equivalence ratio. For all experiments, a minimum of 50 cell widths have been measured. The procedure described in [76] has been employed. For the successful detonation initiation, the cell widths is always below 10 mm. For the mixture with Φ=0.97 and XAir =0.05, the mean cell size is around 1.2 mm. This value is in good agreement with the literature results which report a cell size of 1 mm for the stoichiometric H2 -O2 mixture at ambient initial temperature and P1 = kpa [45]. For XAir =0.05, that is XN2 =0.04, the effect of dilution on the cell width is negligible. As the air mole fraction increases, and equivalence ratio decreases, the measured cell size exponentially increases. Such an evolution is usually observed with respect to dilution and equivalence ratio. N Φ XAir XH2 XO2 XN2 P1 (kpa) T1 (K) Result NO NO Table 2: Composition of the H2 -O2 -Air mixtures and initial conditions before ignition used to study the detonation cell size. : successful initiation. NO : un-successful initiation. a) Successful initiation b) Un-successful initiation Figure 7: Examples of soot foils obtained for H2 -O2 -Air mixtures. Initial conditions: a): Φ=0.97; XAir =0.05; P1 =98 kpa; T1 = K. b): Φ=0.35; XAir =0.75; P1 = kpa; T1 = K. It is admitted that the detonation cell size is related to the induction distance via a proportionality factor [46, 77] which is a function of the chemical system and initial conditions. This factor can be obtained in the framework of the ZND [47, 78] model using experimental cell size data and an appropriate kinetic 8

9 scheme. In the present study, we assumed a value of 27.5 for the proportionality factor. It corresponds to the average between the value reported for H 2 -Air mixtures and the value reported for H 2 -O 2 [47]. The computed induction distance, i, was defined as the distance to peak thermicity. Figure 9 compares the experimental cell widths and the prediction of the kinetic model using the following relationship: λ=27.5 i. The five models predict cell size within 35% of the experimental value. The best agreement is obtained with the model of Mével with a mean error of 7%. For X Air =0.75, large discrepancies between the predictions of the different models are observed. A detonation cell size between 63 mm, for Konnov, and 668 mm, for GRI, is predicted at P 1 = kpa. At higher pressure, P 1 =176 kpa, the induction length increases due to the increased reaction rate of H+O 2 +M=HO 2 +M as compared to H+O 2 =OH+O [79]. Assuming a critical tube diameter for detonation propagation, d cp, equal to d cp = π d [77, 80], with d the tube diameter, it could be concluded that Konnov s and Mével s models fail in predicting the impossibility of the detonation initiation under these experimental conditions. Nevertheless, they effectively predict the dramatic decrease of the sensitivity to detonation of these mixtures. Figure 8: Experimental and theoretical detonation velocity of stoichiometric H 2 -O 2 mixtures with different dilution levels in air. Initial conditions given in Table CONCLUSION In the framework of accidental explosion risk assessment related to ITER, we have measured a number of explosion combustion parameters of oxygen enriched hydrogen-air mixtures. The laminar flame speed, the ignition delay-time and the detonation cell size have been measured over wide ranges of temperature, equivalence ratio and air mole fraction added. It was shown that even for air mole fraction as high as 0.6, the laminar flame speed and the sensitivity to detonation remain very high. In addition to the experimental study, five detailed reaction models from the literature have been tested against the present data. The best compromise seems to be obtained with the JetSurf and the model of Mével. The GRI 3.0 predicts a too low reactivity under auto-ignition and detonation conditions whereas the Konnov s model predicts a too high reactivity. The model of Hanson tends to predicts too low flame speeds. ACKNOWLEDGEMENTS This work has been performed at the institute ICARE-CNRS Orléans. It has been partly funded by Commissariat l énergie atomique et aux énergies alternatives under the contract number CNRS/CEA 9

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