Experimental Observation of Fast Deflagrations and Transition to Detonations in Hydrogen-Air-Mixtures

Transcription

1 Experimental Observation of Fast Deflagrations and Transition to Detonations in Hydrogen-Air-Mixtures A. Eder, C. Gerlach, F. Mayinger Lehrstuhl A für Thermodynamik Technische Universität München, Germany eder@thermo-a.mw.tum.de; Fax: Keywords: Hydrogen safety, Supersonic combustion, DDT, Optical measurement-techniques ABSTRACT The present paper reports on experimentally obtained results of fast propagating deflagrations, the transition-process from deflagration to detonation, and detonations in hydrogen-air mixtures. These combustion phenomena are investigated by means of various sophisticated optical and conventional measurement-techniques like the Laser-induced Predissociation Fluorescence, which, in particular, gives a new insight into the understanding of these supersonic combustion-modes. Focus is put on the regime of the lean detonation-limit, depending on both, the mixture-composition and the dimension of the test-facility. For this limit criteria of empirical character have been applied for hydrogen-safety considerations until now. This study reports that a transition into a detonation of a fast-propagating flame is possible for leaner mixture-compositions as they are covered in the empirical criterion down to the theoretical detonation limit of the test-facility. It was observed that a detonation is not the most dangerous combustion mode. For mixture-combositions bewtween the theoretical and the empirical detonation limit, it is as well possible that the flame propagates as a supersonic deflagration with the same velocity as the leading shock-wave. The peak-pressure of this combustion mode can be more than two-times higher compared to that of a detonation. 1. INTRODUCTION After a possible failure of any hydrogen infrastructure at industrial or, in future, civil sites, freely propagating flame-fronts are very likely to arise due to both, the low required ignition-energy and the wide ignition-range (4 75 Vol.%) of hydrogen-air mixtures. The pressure-load of the combustion depends strongly on its mode, either deflagrative or detonative. In case of a subsonic deflagration of a hydrogen-air mixture, a structure of a building can withstand the pressure rise of the combustion process. Although a direct ignition of a detonation is very unlikely due to the required high energy-source, an at first slow deflagration can turn into a detonation referred to as DDT, with a peak pressure-rise of more than 100 bar, which can endanger the integrity of a building structure. The transition mechanisms are roughly divided into the following categories: The focusing of a precursor shock-wave, local explosions resulting from re-ignition of partially quenched volumes (hot-jet ignition), and, the exceeding of a critical flame speed due to a turbulent flame-acceleration. The exceeding of a critical flame-speed, which is discussed in the present paper, has been investigated by many authors. Already Urtiev and Oppenheim [1] showed in 1966 by means of the classical Schlieren-Cinematographie that for this transition mode four sub-categories can be distinguished: the onset of a detonation between flame and shock, the onset at the flame-front, the onset at the shock-front, and the onset at a contact discontinuity, where two shock-waves coalesce ahead of the flame. Nevertheless, for the prediction of these transition-modes, only very conservative criteria have been set up, yet. This must be attributed to the fact that for the detailed investigation of this highly transient combustion-phenomenon with propagation-velocities of up to 2000 m/s, sophisticated measurement-techniques have to be applied. For many years this phenomenon has been investigated in detail by means of conventional measurement techniques like pressure-transducers and photo-diodes. But only the combination of conventional measurement-techniques with modern optical methods allows a deep insight into this physical process. Optical measurement methods are very suitable for the investigation of these combustion processes.

2 Figure 1: Schematic sketch of the test facility Since they work non-intrusively and inertia-less they do not influence the combustion-process. This aspect is very important, especially for the investigation of the interaction of a shock-wave and its following flame-front. The experimentally determined data with a very high resolution in time and in space are very important for the validation of computational tools, simulating these phenomena. The detailed modelling of the transition-process is not a primary goal for hydrogen-safety considerations. Furthermore, the fast deflagrations have to be simulated with a high accuracy and, if the conditions for a detonation are reached, a detonation code can be used for further calculations. Therefore, it is very important to investigate the fast-propagating deflagrations up to the transition to the detonation in detail for the understanding and the determination of characteristic criteria for this combustion-phenomenon. 2. Experimental apparatus The experiments are carried out in an explosion tube which facilitates the application of modern laser-optical measurement-techniques as well as conventional measurement-techniques. This section describes the geometry and the set-up of the test facility and shows the application of various measurement-techniques for the investigation of the deflagration to detonation transition process. Explosion Tube The explosion-tube has a length l of 6.5 m, an inner diameter d of 66 mm, and a design-pressure of 200 bar (see Fig. 1). It consists of four segments with a length of 1.5 m each and a window section which allows an optical access to the explosion tube over a length of 300 mm. The window-section can be placed at any position in-between the segments or at the tube-ends so that all stages of the combustion-process can be investigated. The gas-composition is determined by the method of partial pressures. The tube has a coaxial shape with oil between the inner and outer tube-walls. Heating cable allow a variation of the temperature of the initial gas-mixture of up to 200 o C and, therefore, facilitate the investigation of the influence of steam on the flame-propagation. The ignition is provided by means of a spark-plug. Turbulence promoting obstacles facilitate an acceleration of the flame. The acceleration itself can be varied by both, using obstacles of different blockage ratios BR (in this case 30-90%) as well as varying

3 the spacing between the obstacles l SP and the total length of the obstacle path L OP. As the focus for the present study is put on the transition process in an unobstructed area, the window-section was positioned in the middle of the tube and a maximum length of 3 m for the obstacle path was used. All applied obstacle-configurations are summarised in Tab. 1. Table 1: Applied obstacle-configurations for the turbulent flame-acceleration Configuration BR l SP [mm] L OP [m] Configuration BR l SP [mm] L OP [m] 1 30% % % % % % % % % without obstacles Optical Measurement Techniques Schlieren Technique. The classical Schlieren Technique, which was first described by Toepler [2] in 1867 is used to record both, the global flame propagation process by means of visualising the density gradients between the unburned mixture and the exhaust-gas, as well as the propagation of precursor shock- and pressure-waves. The quantitative evaluation of the Schlieren-images is almost impossible as only integral images through the whole depth of the combustion chamber can be recorded. But in order to get an idea of the strength of the density-gradients, the Schlieren-wedge can be replaced by a colour or a grey-scale slide. Depending on the strength of the gradient, each gradient can be assigned to an unique colour/grey-scale on the resulting image, which is recorded by means of a fast-shutter, colour-ccd Camera (Resolution pixel). Therefore, it is possible to get a good imagination of the density-gradient field of a detonation-front or of the space between the leading shock-wave and the flame-front with this technique. Nevertheless, the Schlieren-technique allows no distinction between the flame front and the exhaust gas. Therefore, the Laser-induced Predissociation Fluorescence is applied to the combustion process, too. Laser-induced Predissociation Fluorescence. The Laser-induced Predissociation-Fluorescence (LIPF) is a very accurate measurement method in order to visualise the location of the reaction zone with a very high spatial resolution. Combustion-radicals are an intermediate product of the fuel-air reaction. In the case of hydrogen-combustion, OH-radicals indicate the exact position of the flame-front. By choosing an appropriate laser-wavelength, these radicals are excited to a higher electronic energy state. The fluorescence can be observed by the transition from an excited electronic state to a lower state. The radicals are excited within a lightsheet with a thickness of less than 1 mm in order to visualise thin layers of the flame [3,4]. By using an excimer laser running with KrF as laser medium and emitting light with a wavelength of 248 nm, the excitation of the OH-radicals appears. The pulse duration of the laser is 17 ns, the lifetime of the OH-radicals in the excited state ranges between and 10 5 sec [5], which facilitates to visualize the combustion process in a frozen state. The fluorescence appears frequency-shifted at a wavelength of nm. Both, additional fluorescence signals and Rayleigh scattering are tuned out by means of appropriate filters. The emitted fluorescence signal of the excited radicals can be observed by an intensified CCD camera. The obtained images contain information about the shape of the flame and the local radical concentration.

4 Figure 2: Schlieren images of a Fast-Deflagration (A), the Transition from Deflagration to Detonation (B), and a Detonation (C) [6,7] Conventional Measurement Techniques The explosion tube is equipped with a standard conventional instrumentation of 16 photodiodes which are sensitive to ultraviolet light for a precise detection of the position of the flame-front. The pressure is recorded by means of 6 piezoelectric pressure-transducers (five along the tube-axis side-on and one headon in the end-flange). In addition, a transducer measuring the heat-flux to the side-walls is positioned right in front of the window-section and opposite to a pressure-transducer. The measurement-principle of this transducer is based on the thermo-electrical effect of oblique superconductor-layers. For a detailed description of this measurement principle refer to the specialised literature of Langfellner et al. [8] or to the producing company Results and discussion For the different geometrical conditions, which are listed in Tab. 1, the DDT-spot was adjusted to the optical section of the explosion tube by varying the composition of the initial-mixture. The mode of combustion is determined by applying the Schlieren-Technique. In Fig. 2, a comparison of a fast-deflagration just prior to the DDT, the moment of the transition, and a detonation is shown. In case of a fast-deflagration, the flame-front is propagating into a system of transversal and reflected shock-waves, which are generated by the leading main shock-wave. For this propagation mode it was possible to accelerate the flame up to a velocity of about 1000 m/s right behind the obstacle-path. Depending on the mixture-composition the flame decelerated to a subsonic speed and decoupled from the shock-wave or propagated further with that constant velocity. In this study as well as in the work of Chan et al. [9] and Brehm [10] it was found that the flame-front and the shock-front propagate with a constant distance up to spot at which the transition occurs, as shown in Fig. 2B. The flame-front is closing up in the middle of the tube to the shock-front and ignites it so that it propagates further as a detonation wave (Fig. 2C). The typical traces of the flame- and the shock-velocity along the tube-length for this process are shown in Fig. 3A. The shock-wave and the flame-front propagate with an identical velocity and constant distance for a length of 1 m behind the obstacle-path before the transition to the detonation occurs. The velocity-data are derived by the time-dependency of the location of the shock/flame-front, which are shown in Fig. 3, too. The maximum propagation velocity of a stable detonation is the Chapman-Joguet (CJ) Detonation-Velocity. In this case the flow-velocity behind the detonation-front is equal to the local velocity of sound and, therefore, no disturbance can reach the reaction-front. With this information, it is possible to calculate the CJ-velocity for the frictionless propagation. In Fig. 4, the CJ-Detonationvelocity in dependence on the mixture composition, calculated with the chemical equilibrium solver Stanjan [11] is shown. It is of interest to note that after the transition in Fig. 3A the detonation-front propagates approximately with the Chapman-Jouguet (CJ) Detonation velocity although many authors observed that the 1 FORTECH HTS Ltd., Germany

5 Figure 3: Propagation of a flame/shock-system with DDT for a 17.5 Vol.% H 2 in Air mixture (left) and without DDT for a 17 Vol.% H 2 in Air mixture (right), Obstacle-Configuration #2. P1-P5: pressuretransducers side-on, P6: pressure-transducer head-on. CJ-velocity can only be reached in tubes with a greater diameter (about 5 times greater compared to the diameter of this test-facility) due to the friction loss in the smaller tubes. The pressure-records in Fig. 3A are in a range as they are expected to be for a detonation. Nevertheless, the pressure-load obtained by a detonation is not the maximum load of the investigated hydrogen-air combustion modes. It has as well been observed that the flame accelerates up to a velocity which is high enough for the onset of a detonation, but no transition occurs, as shown in Fig. 3B. Furthermore, the shock- and the flame-front propagate as described above with constant velocity and distance up to the tube-end. The head-on pressure is in this case twice as high compared to that of the detonation shown in the example of Fig. 3A. In order to determine criteria for the occurrence of this very dangerous combustion mode, both, the maximum head-on pressure as well as the respective averaged flame-speed in the last section of the tube (x tube = 5 6.5m) were evaluated for experiments with the obstacle-configurations #1-9 (see Tab. 1). The detonation-cellwidth λ is often used for the determination of conservative criteria for the decision whether a detonative combustion mode is possible or not. The detonation cell-width is a characteristic Figure 4: Chapman-Jouguet Detonation Velocity for dry H 2 -Air mixtures (p = 1 bar, T = 293 K), calculated with the Chem. Equilibrium Solver Stanjan Figure 5: Detonation cell-width for dry H 2 -Air mixtures (p = 1 bar, T = 293 K), experimental data from Guirano et al. [12] (square symbol) and Tieszen et al. [13] (round symbol)

6 measure of the reactivity of the mixture. It can only be determined by means of experiments where the trajactories generated by the triple-point (Mach stem) of the leading and transversal shock-waves are printed on a smoked foil within the explosion tube. Selected data found in literature [12,13] for the cell-size for hydrogen-air mixtures are shown in Fig. 5. The theoretical detonation limit for a circular tube is that the tube-diameter is greater than the detonation-cellwidth of the initial-mixture divided by π [14]. Very often, the criterion d > λ is applied as a conservative criterion. This criterion is of empirical character and was obtained by means of various measurements in tubes of 5 30 cm in diameter. In Fig. 6, the dependency of the maximum Figure 6: Dependency of the head-on pressure on the mixture-composition as well as the mean flame speed in the last tube section, obstacle-configurations #1-9. head-on pressure on the mixture-composition as well as the mean flame speed in the last tube section for the obstacle-configurations # 1-9 is shown. It can be seen that a detonative combustion mode is, as well as the deflagrative mode, possible for initial hydrogen-air mixtures within the range d λ d π, which is in this case Vol.% H 2 in Air. The correlation of the head-on pressure, mixture-composition, and flame-speed in Fig. 6 shows in addition that the occurrence of experiments with maximum head-on pressures of more than 200 bar can be limited to this mixture-range. For these experiments the flame-front propagates like in the example shown in Fig. 3 with a velocity of about 1000 m/s, which is considerably lower than the propagationspeed of a detonation. The application of the heat-flux transducer facilitates the assignment of each experiment to the specific combustion phenomenon. A comparison of the heat-fluxes to the side-walls for a detonation and a fast deflagration which is coupled to the leading shock wave is shown in Fig. 7. It can be seen that the shock-induced ignition of a detonation initiates a heat-flux, which is about 7-times higher compared to that of a deflagration. Furthermore, it is possible to detect the distance between the leading shock-wave and the reaction-zone. The comparison shows that in the case of a fast deflagration the shock-wave is Table 2: Classification of combustion modes Condition Combustion Mode max. pressure d < λ/π Deflagration 25 bar d λ d π Deflagration 25 bar Detonation 150 bar Fast Deflagration > 200 bar d > λ Detonation 100 bar

7 Figure 7: Heat-flux to the side-walls for a detonation (left) and a fast-deflagration (right). Obstacle- Configuration #9, t = 0 s corresponds to ignition. directly, but with a discrete distance followed by the reaction-zone. The classification of combustion modes in dependence of tube-diameter and detonation cell-width for flames which accelerate up to their specific maximum flame speed by means of adequate turbulence promoting obstacles is summarised in Tab. 2. By means of the Laser-induced Predissociation Fluorescence it is possible to get a detailed understand- Figure 8: Shock-/flame-structure of a Fast-Deflagration, Obstacle-Configuration #6, 17.2 Vol.% H 2 in Air. Left figure taken with Schlieren, right figure taken with LIPF ing for the observed combustion-phenomena. In Fig. 8, both, the visualisation of the shock-structure by means of the Schlieren-Technique as well as the distribution of the OH-radicals taken by means of LIPF behind the obstacle path (obstacle configuration #6) are shown. Right behind the shock-wave, the formation of OH-radicals starts. The flame is, therefore, propagating into a preconditioned mixture of elevated pressure and temperature. Due to the high flow-velocities and the resulting high turbulencefluctuations behind the shock-wave, the flame itself has a wrinkled structure and is distributed into separated but coherent reacting zones. Jordan et al. [15] showed that the existence of OH-radicals has an enormous influence on the burning-velocity of a hydrogen-air flame. This could be one reason for the occurrence of the high head-on pressure-peaks [16]. The leading shock wave is reflected at the tube-end and due to the high temperature and pressure after the shock-reflection, the whole volume of preconditioned mixture starts simultaneously to react. A comparison of a fast-propagating deflagration, a near-limit detonation with an initial gas composition of d < λ and a detonation with d > λ is shown in Fig. 9. The flame-structure of the fast propagating flame is like in the example shown above wrinkled which leads to an increased surface of the flame. Therefore, more hydrogen per unit time is consumed which facilitates a flame-propagation of 650 m/s. The total flame-thickness is about 2.5 cm. The structure of a propagating detonation is often described with the one-dimensional model of Zeldovich, Döring and von Neumann (ZND-model). The ZND-model describes the detonation wave as a

8 Figure 9: OH-Radical-Distribution of a deflagration (A), a near-limit detonation with λ > d (B), and a detonation with λ < d (C), taken with LIPF. shock wave, immediately followed by a reaction zone (i.e., the flame). The thickness of this zone is given by the reaction rate. By linking this model to a chemical equilibrium-solver like Chemkin-II [17], it is possible to calculate the OH-radical distribution of a detonation-wave and, therefore, to compare the LIPF-measurement with this calculations. The LIPF-measurements show that the application of the ZND-model are only valid for detonations with an initial mixture with the condition λ < d. As shown in Fig. 9C, the mixture starts simultaneously to react in a parallel line to the leading shock-wave. Due to the high temperatures after the detonation, the mole-fraction of OH in the oxygen-nitrogen-steam mixture is still very high (about 40% of the maximum OH-concentration in the main reaction zone according to ZND-calculations for a 20 Vol.% hydrogen-air detonation). In case of a detonation in a mixture with the condition d λ d π, the one-dimensional model is no longer valid. Although the flame propagates like in the richer mixture with the specific CJ-Detonation- Velocity, the reaction-zone has an unsymmetrical, three-dimensional shape. The reaction starts at single spots behind the shock-wave rather than simultaneously in a parellel line behind the shock-wave, but the heat-release of the reaction-zone is still high enough and close enough to the shock wave, so that it can propagate as a detonation wave with constant velocity. 4. Conlusions The combination of optical and conventional measurement-techniques facilitates the understanding of the highly transient combustion-phenomena of fast-deflagrations and detonations. In order to determine quantitative criteria for the transition process from the deflagrative to the detonative combustion-mode or the occurrence of abnormal high pressure-peaks for fast deflagrations, the results shown in this study have to be put in their proper place among all studies which have been carried out in this field yet. The results determined by various authors in facilities of different geometry and scale deviate sometimes to a very high extend. The goal for the future is, therefore, to develop a tool for the numerical simulation of these phenomena, which is not depending on the geometrical boundary-conditions as the dominating input-parameter. One dominant parameter especially for the simulation of combustion-phenomena is the correct modelling of the heat-flux to the confining walls of the combustion process, which has an essential influence on the propagation-velocity of the flame and, therefore, the pressure rise. The qualitative traces of the heat-flux have been shown in this paper for the described combustion modes. These data will be evaluated in a quantitative way, too, which shall serve as an essential input-parameter for numerical codes simulating these phenomena. Acknowledgement It is gratefully acknowledged that the work presented in this paper has been supported by the German Ministry of Economics and Technology BMWi.

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