We studied the propagation and the dynamics of combustion waves, i.e., deflagrations and detonations, in hydrogen-air mixtures with transverse concentration gradients. Flame acceleration and transition from deflagration to detonation were phenomena of particular interest. Experimental results were published online in the "GraVent DDT Database". The interpretation of experimental data showed that concentration gradients can lead to stronger explosions with higher peak pressures, higher flame speeds, and a higher susceptibility for transition to detonation, compared to homogeneous mixtures at the same average hydrogen concentration.
Research Item (37)
- Jul 2018
- 5th International Conference on Experimental Fluid Mechanics
Experimental and numerical analyses were performed to resolve the complex 3D deformation and periodic shape oscillations of deformable air bubbles in water (~ 4−5 mm diameter). Characterizing the temporal variation of the surface-to-volume ratio was the primary interest of this investigation. It is shown that the surface-to-volume ratio oscillates with two dominant frequencies (or modes) called fR and fS modes in the following. In the literature, the oscillation modes are derived from 2D data by evaluating the temporal variation of the elliptical axes of the deformed bubbles. Significant differences can be detected between the literature values and our 3D data, suggesting that, for bigger bubbles, a 3D evaluation becomes necessary to avoid bias errors.
- May 2018
- 11th International Conference on Computational Heat and Mass Transfer
In this study, Direct Numerical Simulations (DNS) have been conducted to analyse unsteady three-dimensional Rayleigh-Bénard convection of yield stress fluids obeying a Bingham model in cubic domains (i.e. differentially heated horizontal walls heated from below where other walls are adiabatic) at high values of nominal Rayleigh number. The simulations have been carried out for a nominal Rayleigh number 10^7 and 10^8 for a representative value of nominal Prandtl number (i.e. Pr = 320 which corresponds to a 0.05 % Carbopol-water solution).
- Aug 2017
In the framework of a research project funded by the German Federal Ministry of Economic Affairs and Energy (BMWi), a CFD combustion solver has been extended for the purpose of nuclear safety analysis. The methodology aims at the prediction of industry-scale hydrogen explosions like in the Fukushima-Daiichi core meltdown accident, with a particular focus on the hazardous Deflagration-to-Detonation Transition (DDT) phenomenon. Independent of the specific nuclear safety motivation of this work, the methodology can equally be applied to other potentially hazardous situations involving accidental release of hydrogen in air, e.g. in chemical and process industry. Contrary to the state-of-the-art in large-scale explosion modeling, the entire combustion process is computed within a single solver framework. The usage of empirical combustion regime transition criteria is deliberately avoided. The multi-physics multi-scale problem poses several challenges which are met by special numerical techniques. As a key element, the developed hybrid flame-tracking shock-capturing scheme reduces grid dependency by treating the flame as a reactive discontinuity which is propagated by a geometrical Volume-of-Fluid method. At the same time, gas-dynamic discontinuities, especially shocks, are calculated by an approximate Riemann solver. Adaptive mesh refinement is additionally implemented to reduce overall computational cost. Spatial resolution is locally adapted according to the highly unsteady evolution of explosions. For the validation of the numerical model, the largest ever conducted indoor DDT experiments in the RUT facility (Kurchatov Institute, Moscow region) are chosen because of their industry-scale geometrical dimensions. Investigated DDT mixtures are close to the safety-relevant lower detonation limit which was measured at 12.5% of hydrogen in air. As the simulations show, the methodology is generally able to capture the essential phenomena behind flame acceleration and DDT in obstructed channels, even on necessarily under-resolved meshes. The quality of DDT predictions itself depends on the underlying mechanism. In contrast to successful simulations of DDT by shock focusing, prediction of DDT in the vicinity of the turbulent flame brush is less reliable. The code accurately reproduces key safety characteristics such as the detonation propagation velocity and associated pressure loads. Following the motivation of this work, the developed solver is finally applied to a full-scale Konvoi-type light water reactor, i.e. a standardized German pressurized water reactor design. Massively parallelized computations are executed on the SuperMUC high performance cluster. Hypothetical DDT scenarios in globally lean hydrogen-air mixtures demonstrate a highly three-dimensional behavior of flame propagation in the containment. As expected, a strong sensitivity of the explosion process on mixture composition is observed. Further analysis concerns the characteristics of typical transient pressure loads at different positions in the containment. A step towards deterministic DDT simulations on full reactor scale has been made. Finally, several ideas are presented to further enhance the solvers capabilities (with respect to nuclear safety analysis): extending the hydrogen-air mixture by steam and carbon monoxide, including the effect of unresolvable small obstacles via the distributed porosity concept, improving the sub-grid modeling of intrinsic flame instabilities and increasing the solver's parallel efficiency by dynamic load balancing.
- Aug 2017
- 26th International Colloquium on the Dynamics of Explosions and Reactive Systems
The promoting effect of intrinsic flame instabilities on explosive combustion is generally known. Most detailed computational studies are limited to generic configurations however. In the present numerical investigation, direct qualitative and quantitative comparison with experimental data from a laboratory-scale explosion channel is provided. Characterized by Lewis numbers clearly smaller than unity, lean hydrogen-air mixtures are particularly prone to intrinsic flame instabilities. An imbalance of heat conduction and species fluxes leads to the development of a cellular flame structure, known as the thermal-diffusive instability. Increased flame surface area and interconnected flame stretch effects significantly affect the flame speed. Experimental evidence suggests a strong influence of pressure on the phenomenon. However, a thorough quantification is currently missing. One difficulty arises from the fact that the thermal-diffusive instability is superimposed by the hydrodynamic Landau-Darrieus instability. Additional insight is thus gained by manipulation of the mathematical model to separate different effects. It is shown that pressure variation does not only affect the burning velocity but also the cellular structure of the flame in sufficiently lean hydrogen-air mixtures. The higher the pressure, the smaller the cell size. Flame surface area increases accordingly. The diminishing effect of elevated pressure on (laminar) burning velocity is a standard in modeling whereas the promoting effect of enhanced flame wrinkling via intrinsic instabilities is usually neglected. Burning laws without such correction might lead to an underestimation of flame speed for under-resolved explosion simulations.
- Aug 2017
- 26th International Colloquium on the Dynamics of Explosions and Reactive Systems
This work aims to quantify the effect of pressure on flame front wrinkling caused by intrinsic flame instabilities in lean hydrogen-air explosions. Work concerning pressure dependency was conducted, however only for fuels other than pure hydrogen, like methane, ethylene, propane or methane/hydrogen blends. Furthermore, these flames were investigated for a stationary case under conditions with high turbulence intensity compared to this case, where the turbulence level is assumed to be negligible. The goal of this work is to improve hydrogen-air combustion CFD modeling on under-resolved grids in the context of Unsteady Reynolds-Averaged Navier-Stokes simulations (URANS). In this approach, an effect like the observed flame front wrinkling cannot be resolved and must be modeled. The dependency of flame front wrinkling caused by laminar instabilities under pressure variation was investigated in a complementary approach. DNS simulations (Part I) and high-speed OH-PLIF measurements (Part II) of propagating lean hydrogen-air flames were conducted. A reduction of the thermal-diffusive cell size was observed with increasing initial pressure. A quantitative evaluation of the data showed an increase of the wrinkling factor under increasing normalized pressure to the power of 0.18. The dependency identified leads to the conclusion that the pressure influence on flame front wrinkling of a propagating lean hydrogen-air flame should not be neglected in an under-resolved CFD subgrid model.
The present study concerns the three-dimensional CFD analysis of hydrogen explosions in APR1400 containment. Initial conditions for combustion simulations are obtained by a cost-effective coupling with a lumped parameter code for preceding mixture distribution analysis. Three severe accident scenarios are examined: Small-Break Loss-Of-Coolant Accident (SB-LOCA) and Station Black-Out (SBO) with activated as well as deactivated three-way valve. Only in the last case, thermal ignition of the hydrogen-air-steam mixture could be triggered. Flame propagation in the containment and particularly in the In-containment Refueling Water Storage Tank (IRWST) is investigated in detail. Additional generic scenarios demonstrate the methods ability to reproduce strong Flame Acceleration (FA) and even the hazardous Deflagration-to-Detonation Transition (DDT).
For the purpose of nuclear safety analysis, a reactive flow solver has been developed to determine the hazardous potential of large-scale hydrogen explosions. Without using empirical transition criteria, the whole combustion process including deflagration-to-detonation transition (DDT) is computed within a single solver framework. In this paper, we present massively parallelized three-dimensional explosion simulations in a full-scale pressurized water reactor (PWR) of the Konvoi type. Several generic DDT scenarios in globally lean hydrogen–air mixtures are examined to assess the importance of different input parameters. It is demonstrated that the explosion process is highly sensitive to mixture composition, ignition location, and thermodynamic initial conditions. Pressure loads on the confining structure show a profoundly dynamic behavior depending on the position in the containment. Computational cost can effectively be reduced through adaptive mesh refinement (AMR).
The presented work aims to improve CFD explosion modeling for lean hydrogen-air mixtures on under-resolved grids. Validation data is obtained from an entirely closed laboratory scale explosion channel (GraVent facility). Investigated hydrogen-air concentrations range from 6 to 19 vol.-%. Initial conditions are p = 1 atm and T = 293 K. Two highly time-resolved optical measurement techniques are applied simultaneously: (1) 10 kHz shadowgraphy captures line-of-sight integrated macroscopic flame propagation; and (2) 20 kHz OH-PLIF (planar laser-induced fluorescence of the OH radical) resolves microscopic flame topology without line-of-sight integration. This paper presents the experiment, measurement techniques, data evaluation methods and simulation results. The evaluation methods encompass the determination of flame tip velocity over distance and a detailed time-resolved quantification of flame topology based on OH-PLIF images. One parameter is the length of wrinkled flame fronts in the OH-PLIF plane obtained through automated post-processing. It reveals the expected enlargement of flame surface area by instabilities on microscopic level. A strong effect of mixture composition is observed. Simulations based on the new model formulation, incorporating the microscopic enlargement of the flame front, show a promising behavior, where the impact of the augmented flame front on the observed flame front velocities can be detected.
The presented work aims to improve CFD explosion modeling for lean hydrogen-air mixtures on under-resolved grids. Validation data is obtained from an entirely closed laboratory scale explosion channel (GraVent facility). Investigated hydrogen-air concentrations range from 6 to 19 vol.-%. Initial conditions are p = 1 atm and T = 293 K. Two highly time-resolved optical measurement techniques are applied simultaneously: (1) 10 kHz shadowgraphy captures line-of-sight integrated macroscopic flame propagation; and (2) 20 kHz OH-PLIF (planar laser-induced fluorescence of the OH radical) resolves microscopic flame topology without line-of-sight integration. This paper presents the experiment, measurement techniques, data evaluation methods and initial results. The evaluation methods encompass the determination of flame tip velocity over distance and a detailed time-resolved quantification of flame topology based on OH-PLIF images. One parameter is the length of wrinkled flame fronts in the OH-PLIF plane obtained through automated post-processing. It reveals the expected enlargement of flame surface area by instabilities on microscopic level. A strong effect of mixture composition is observed.
For the purpose of nuclear safety analysis, a reactive flow solver has been developed to determine the hazard potential of large-scale hydrogen explosions. Without using empirical transition criteria, the whole combustion process (including DDT) is computed within a single solver framework. In this paper, we present massively parallelized three-dimensional explosion simulations in a full-scale pressurized water reactor of the Konvoi type. Several generic DDT scenarios in globally lean hydrogen/air mixtures are examined to assess the importance of different input parameters. It is demonstrated that the explosion process is highly sensitive to mixture composition, ignition location and thermodynamic initial conditions. Pressure loads on the confining structure show a profoundly dynamic behavior depending on the position in the containment.
- Jun 2016
Planar laser-induced fluorescence (PLIF) is considered a standard experimental technique in combustion diagnostics. However, it has only been occasionally applied to explosion experiments with fast combustion regimes. It has been shown that single-shot OH-PLIF with high pulse energies yields clear fluorescence images of fast deflagrations and also detonations. This paper presents the first application of high-speed OH-PLIF at 20 kHz repetition rate to a deflagration-to-detonation transition experiment. Hydrogen–air mixtures at initial atmospheric pressure and ambient temperature are investigated. Satisfactory results are obtained for flame speeds up to about 500 m/s. Flame instabilities and turbulence–flame interactions are observed. Two factors limit the applicability of HS OH-PLIF toward higher flame speeds: excessive flame luminescence masking the HS OH-PLIF signal and strong absorption of laser light by the flame. The variation in OH-PLIF signal-to-background ratio across a DDT process is studied using a 1D laminar premixed flame simulation extended by spectroscopic models.
- Mar 2016
An open-access online platform containing data from experiments on deflagration-to-detonation transition conducted at the Institute of Thermodynamics, Technical University of Munich, has been developed and is accessible at http:// www. td. mw. tum. de/ ddt. The database provides researchers working on explosion dynamics with data for theoretical analyses and for the validation of numerical simulations.
The influence of transverse concentration gradients on detonation propagation in H2-air mixtures is investigated experimentally in a wide parameter range. Detonation fronts are characterized by means of high- speed shadowgraphy, OH* imaging, pressure measurements and soot foils. Steep concentration gradients at low average H2 concentrations lead to single-headed detonations. A maximum velocity deficit compared to the Chapman-Jouguet velocity of 9 % is observed. Significant amounts of mixture seem to be consumed by turbulent deflagration behind the leading detonation. Wall pressure measurements show high local pressure peaks due to strong transverse waves caused by the concentration gradients. Higher average H2 concentrations or weaker gradients allow for multi-headed detonation propagation.
OH-PLIF (Planar Laser-Induced Fluorescence) allows for capturing two-dimensional images of flame fronts by visualizing OH radicals. While it is widely used for diagnostics of scientific and technical flames, its application to explosion experiments is more uncommon and has been mostly limited to single-shot PLIF so far. Low speed PLIF systems (around 10 Hz repetition rate) deliver pulse energies of several mJ (pumped dye lasers) or even a few 100 mJ (excimer lasers). Resolving explosion processes and in particular the fast deflagration regime in time requires repetition rates in the kHz range. Pulse energies thereby reduce to about 0.100 mJ for typical commercially available dye laser systems. This work presents—to the best of our knowledge—the first application of HS (High Speed) OH-PLIF to an explosion experiment with fast deflagrations. Our intention is to particularly discuss the potential and limitations of HS OH-PLIF and thereby provide practical guidance.
Extensive knowledge is available on explosions in homogeneous gas mixtures. Mixtures of H2 and air have been investigated particularly in the context of nuclear reactor safety. However, a major current knowledge gap concerns the influence of mixture inhomogeneity. Spatial concentration gradients are omnipresent in real-world accident scenarios. We address this knowledge gap experimentally, reducing complexity by investigating one-dimensional, transverse concentration gradients. In the present paper we analyze transition to detonation. We use highly time-resolved optical diagnostics and pressure measurements to gain insight into the mechanism of transition. Based on the observations we define critical conditions for transition by shock reflection off obstacles by simulating detailed chemical kinetics behind a reflected shock and using the extended second explosion limit as a threshold. Beyond this limit, local explosions occur which initiate transition to detonation. We find that crit ical conditions can be expressed in terms of deflagration overpressure for a wide range of H2 concentrations. This proves true both for homogeneous and non-uniform mixtures.
The influence of water mist on explosion of H2-air mixtures is studied experimentally in a closed channel with rectangular cross section. The investigated range of H2 concentrations covers slow flames, accelerating flames, transition to detonation and detonations. Water loading ratios relevant for severe accident scenarios in nuclear power plants are examined. A system for water mist generation and injection was designed such that the experimental conditions are well defined. The water loading ratio is determined by real-time extinction measurement. Laser diffraction measurement provides the droplet size distribution. Water mist injection results in lower overpressure and retarded transition to detonation compared to dry H2-air mixtures. The effect of 0.11–0.12 kg/m³ water loading ratio is comparable to a decrease in H2 concentration by 0.6 vol. % in the deflagration regime and by 1-2 vol. % in the detonation regime, while transition to detonation is retarded more significantly.
- Apr 2015
The influence of water mist on explosion of H2-air mixtures is studied experimentally in a closed channel with rectangular cross section. The investigated range of H2 concentrations covers slow flames, accelerating flames, transition to detonation and detonations. Water loading ratios particularly relevant for severe accident scenarios in nuclear power plants are examined. A system for water mist generation and injection was designed such that the experimental conditions are well defined. The water loading ratio is determined by extinction measurement in each experiment. Laser diffraction measurements provide the droplet size distribution. Water mist injection results in lower overpressure and retarded transition to detonation compared to dry H2-air mixtures. The effect of a 0.11–0.12 kg/m³ water loading ratio is comparable to a decrease in H2 concentration by 0.6 %vol in the deflagration regime and by 1–2 %vol in the detonation regime, while transition to detonation is retarded more significantly.
- Nov 2014
A methodology for the computationally efficient CFD simulation of hydrogen-air explosions (including transition to detonation) in large volumes is presented. The model is validated by means of the largest ever conducted indoor DDT experiments in the RUT facility. A combination of models is proposed with a particular focus on the influence of flame-instabilities, especially of thermal-diffusive nature, which are crucial for very lean mixtures. Excellent agreement is achieved in terms of flame acceleration. The quality of DDT predictions itself depends on the underlying mechanism. Whereas DDT by shock-focusing is successfully simulated on under-resolved meshes, DDT by local explosions in the vicinity of the turbulent flame brush remains a challenge. Adaptive mesh refinement therefore emerges as a key technique to resolve more of the essential phenomena at reasonable computational costs affordable by industry. Finally, a generic case demonstrates the influence of mixture inhomogeneity, which can promote flame acceleration and ultimately DDT.
We present three-dimensional under-resolved detonation simulations in a large-scale confined geometry (RUT facility at Kurchatov Institute, Moscow). Direct detonation initiation and propagation in a uniform mixture of 25.5 vol.-% hydrogen in air (HySafe experiment HYD 09) is investigated by means of a two-step combustion model originally developed for DDT-simulations. Computational costs are kept at a reasonable level through adaptive mesh refinement. In terms of pressure signals at the channel side walls, we achieve superior results compared to earlier computations. The method proves to reliably predict the destructive potential in real-world accident scenarios.
A current knowledge gap in hydrogen safety research concerns the influence of mixture non-uniformity on explosion processes. Spacial mixture composition gradients are likely to form in real-world accident scenarios. In this work, the influence of such gradients on flame acceleration is experimentally investigated in an entirely closed channel at laboratory scale. Experiments show that concentration gradients can lead to significantly stronger flame acceleration compared to homogeneous mixtures. Optical measurements reveal that flame shapes differ between homogeneous and inhomogeneous mixtures in the unobstructed channel configuration. An enlargement of flame surface area in inhomogeneous mixtures leads to a higher integral reaction rate, which in turn supports flame acceleration. In obstructed channel configurations, however, mixture properties dominate flame propagation. An analytical model based on the integral balancing of mixture properties is proposed and validated by means of experimental data.
- Jan 2014
- 9th International Colloquium on Pulsed and Continious Detonations
Flame acceleration in a closed duct filled with highly explosive hydrogen-air mixtures is investigated using the OH PLIF technique. The applied laser system comprising a pump laser and a dye laser allows for repetition rates up to 40 kHz. Temporal evolution of the flame surface area during the process of flame acceleration is a key parameter. Therefore, a Matlab script was developed for image processing and flame front detection. Results are presented underlining the usefulness of highly time-resolved OH PLIF in transient combustion diagnostics.