Investigation of instability and turbulence effects on gas explosions: experiments and modelling

Safe design of industrial facilities requires models that can predict the consequences of accidental gas explosions in complex geometries with sufficient accuracy. Computa- tional fluid dynamics (CFD) models are widely used for consequence assessment in the process industries. The primary mechanism for flame acceleration during gas explo- sions in congested geometries is the positive feedback between turbulence generated in the reactant mixture, especially in shear and boundary layers from explosion-driven flow past obstacles and walls, and enhanced combustion rates. Additionally, a range of instability phenomena can significantly increase flame acceleration in gas explosions. This doctoral project addresses the following research question: "how can the sub- grid representation of flame acceleration mechanisms due to instability effects and flow past obstructed regions be improved in a CFD tool used for consequence assessment of gas explosions?". The thesis presents and validates new sub-grid models developed for the CFD tool FLACS, focusing on the following flame acceleration mechanisms: (i) the influence of the hydrodynamic and thermal–diffusive instabilities (intrinsic in- stabilities) on flame acceleration in the initial phase of a gas explosion, (ii) the influ- ence of thermal–diffusive effects on the rate of turbulent combustion for different fuels and mixture concentrations, (iii) the role of the Bénard–von Kármán (BVK) instability downstream of bluff-body obstacles in explosion-induced flow, (iv) how the Rayleigh– Taylor (RT) instability developing on a flame front that is accelerated over an obstacle or a vent opening may enhance the combustion rate, and (v) how flexible obstructions with very small components (in the form of vegetation) induce flame acceleration in gas explosions. There was a lack of available experimental work describing the rela- tive importance of several of the aforementioned effects. Therefore, three experimental campaigns were designed and conducted as part of the doctoral study. Experimen- tal findings thus constitute a significant part of the original scientific contribution of the present work; these can be used to develop sub-grid models for any consequence model system. Three additional campaigns, performed by other research groups, were simulated in order to validate the new sub-grid models. The first experimental campaign presented in the thesis concerned explosion exper- iments performed in a large-scale, empty, vented enclosure comprising two chambers separated by a doorway. Simulations indicate that model performance might improve by introducing the effect of the RT instability. Furthermore, the results suggest that the model should account for thermal–diffusive instability effects both in the initial phase of a gas explosion and in the regime of turbulent combustion. The onset and growth rate of intrinsic instabilities in gas explosions are closely linked to the value of the Markstein number of the fuel-air mixture. The second experi- mental campaign presented in the thesis was performed as part of the doctoral study to explore the effect of varying the fuel concentration on overpressures and flame speeds in a series of propane-air explosions. The variations in the fuel concentration effec tively change the Markstein number of the mixture. The dissertation compares exper- imental results with predictions from a version of FLACS that includes a Markstein number-dependent combustion model, developed as part of the doctoral project. For negative Markstein numbers, the new model version performs significantly better than the original model. This work addresses mechanisms (i) and (ii). The thesis elaborates on instability effects relevant for scenarios where explosion- induced flow interacts with partial confinement and obstacles, focusing on BVK and RT instabilities in particular, i.e. mechanisms (iii) and (iv). A third experimental cam- paign was performed as part of the present study to investigate the relative contribution of vortex shedding, caused by the BVK instability, to the overpressure generation in gas explosions with a single obstacle inserted. Vortex shedding was observed in the baseline laboratory-scale experiments, and then suppressed by two passive flow con- trol methods. The most effective configuration reduced the maximum overpressures by approximately 32 %. The results can be used to model the flame surface area in- crease downstream of obstacles in any consequence model system. However, further experimental work, building on the findings of the present doctoral study, is required to formulate a sub-grid model for mechanism (iii). A sub-grid model for flame wrinkling due to the RT instability (mechanism (iv)), based on the linearised growth rate of instabilities on an accelerated flame front, was implemented in FLACS. The thesis presents updated model results for two series of vented explosion experiments, obtained with a version of FLACS that includes both the Markstein number-dependent burning velocity model and the RT instability effect. The first test case involved a series of fuel-lean hydrogen-air explosions. The second test case revisited the campaign performed in the twin-compartment enclosure that was presented in the very beginning of the thesis. A third test case was included to inves- tigate whether the sub-grid models that were initially developed and validated for ex- plosions with a high degree of confinement and idealised obstacle configurations could produce improved results also for explosions in complex geometries with a low degree of confinement. Therefore, a series of large-scale natural gas-air explosions performed in a full-scale offshore module was simulated. All three test cases were performed by other research groups. The sub-grid models developed as part of the present doctoral study significantly improve the representation of several of these experiments. The fourth experimental campaign was motivated by a need for an improved ap- proach to sub-grid modelling of the effect of vegetation on gas explosions in CFD tools, i.e. mechanism (v). Recent accidents have shown that this is highly relevant for risk assessment for onshore process facilities. The presence of foliage on spruce branches notably enhanced maximum overpressures in the experiments, suggesting that obstruc- tions of very small dimensions may contribute significantly to the flame acceleration. Highly flexible, fractal-like obstructions require additional modelling considerations. Based on the experimental results, the effects of foliage and flexibility were included in FLACS simulations by constructing congestion blocks, representing the effective drag area that is expected to produce flame acceleration. This modelling approach success- fully reproduces the experimental trends. In conclusion, the results of the experimental campaigns presented in this thesis have improved the understanding of several important physical effects related to flame acceleration in industrial-scale explosions. Furthermore, the thesis demonstrates how this knowledge may be used to model gas explosions more accurately

Computational fluid dynamics simulations of hydrogen releases and vented deflagrations in large enclosures

This paper presents model predictions obtained with the CFD tool FLACS for hydrogen releases and vented deflagrations in containers and larger enclosures. The paper consists of two parts. The first part compares experimental results and model predictions for two test cases: experiments performed by Gexcon in 20-foot ISO containers (volume 33 m3) as part of the HySEA project and experiments conducted by SRI International and Sandia National Laboratories in a scaled warehouse geometry (volume 45.4 m3). The second part explores the use of the model system validated in the first part to accidental releases of hydrogen from forklift trucks inside a full-scale warehouse geometry (32 400 m3). The results demonstrate the importance of using realistic and reasonably accurate geometry models of the systems under consideration when performing CFD-based risk assessment studies. The discussion highlights the significant inherent uncertainty associated with quantitative risk assessments for vented hydrogen deflagrations in complex geometries. The suggestions for further work include a pragmatic approach for developing empirical correlations for pressure loads from vented hydrogen deflagrations in industrial warehouses with hydrogen-powered forklift trucks.publishedVersio

CFD Analysis of Explosions with Hydrogen-Methane-Air Mixtures in Congested Geometries

Hydrogen is an enabler for de-carbonising the energy system in Europe by 2050. In the UK, several projects are investigating the feasibility of gradually blending hydrogen into the natural gas pipelines with the aim to reach 100% hydrogen in the gas network. However, the safe use of hydrogen as a fuel presents different challenges than conventional hydrocarbon-based fuels Advanced consequence models are powerful tools that can be used to support the design process and estimate the consequences of potential accidents. This paper analyses the predictive capabilities of two combustion models for explosion for hydrogen, methane and hydrogen-methane blends. The analysis involves the default combustion model in the commercial version (FLACS-CFD v21.2), and a new combustion model implemented in an in-house development version where the model for premixed turbulent combustion incorporates Markstein number effects (FLACS-CFD v21.2 IH). Experiments performed by Shell in unconfined pipe-racks, some of which were part of the EU funded project NaturalHy, are considered. The simulation results from both versions of FLACS-CFD are within a factor of 2 of the overpressures observed in the experiments. However, FLACS-CFD v21.2 IH appears to give an improved representation of the overpressure trend with variations in the hydrogen equivalence ratio observed in the experiments.publishedVersio

Simulating vented hydrogen deflagrations: Improved modelling in the CFD tool FLACS-hydrogen

Under embargo until: 2021-10-06This paper describes validation of the computational fluid dynamics tool FLACS-Hydrogen. The validation study focuses on concentration and pressure data from vented deflagration experiments performed in 20-foot shipping containers as part of the project Improving hydrogen safety for energy applications through pre-normative research on vented deflagrations (HySEA), funded by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCH 2 JU). The paper presents results for tests involving inhomogeneous hydrogen-air clouds generated from realistic releases performed during the HySEA project. For both experiments and simulations, the peak overpressures obtained for the stratified mixtures are higher than those measured for lean homogeneous mixtures with the same amount of hydrogen. Using an in-house version of FLACS-Hydrogen with the numerical solver Flacs3 and improved physics models results in significantly improved predictions of the peak overpressures, compared to the predictions by the standard Flacs2 solver. The paper includes suggestions for further improvements to the model system.acceptedVersio

Drømmenes poetiske skapelse En studie av fem «drømmedikt» av Tone Hødnebø

Denne avhandlingen tar for seg dikt av samtidsforfatteren Tone Hødnebø. Jeg har analysert fem dikt som er hentet fra tre av Hødnebøs diktsamlinger: «I» fra Larm (1989), «Lysskye affærer» fra Pendel (1997) og «BYGNINGEN KRENGER», «NÅR DU KOMMER HJEM» og «MEN ALT» fra Stormstigen (2002). De fem diktene omhandler drømmer, og det er nettopp drømmer som står sentralt i problemstillingen. Problemstillingen jeg har arbeidet ut fra, er: Hva skaper «drømmediktene» til Tone Hødnebø? For å svare på problemstillingen har jeg tatt utgangspunkt i Sigmund Freuds arbeid med drømmer, og jeg bruker særlig hans begreper, «forskyvning», «fortetning» og det uhyggelige for å forklare hva som skapes i diktene. I tillegg bruker jeg Otto M. Rheinschmiedt, en psykoanalytiker i tradisjonen etter Freud, sine tanker om drømmer som multitemporale. Maurice Blanchot bidrar med et litteraturfilosofisk perspektiv på drømmer, der det er særlig hans idéer om drømmer som noe som eksisterer på «utsiden», det stedet der «alt har forsvunnet» trer frem og der den sovende ikke finner hvile, som blir viktig. Metodekapitlet omhandler den analytiske prosessen rundt analysen av diktene. Diktene til Tone Hødnebø er særdeles knappe og gåtefulle, og har krevd at arbeidet med analysene måtte gjøres i ulike faser, noe som har vært en tidkrevende prosess. I denne avhandlingen er jeg ikke opptatt av drømmenes latente betydning, men den manifeste. Det vil si at jeg har forholdt meg til hvordan diktene som dikt fremstiller drømmer, og ikke drømmenes underliggende betydning. Analysene har vist at hvert drømmedikt skaper noe som er helt unikt, nemlig svimmelhet i diktet «BYGNINGEN KRENGER», uhygge i diktet «NÅR DU KOMMER HJEM», fortetning i diktet «I», en form for endeløshet i diktet «MEN ALT» og avmakt i diktet «Lysskye affærer». Likevel har diktene til felles at de skaper uhyggelige rom, situasjoner relasjoner og hendelser. I oppgavens drøftingsdel knyttes analysene til perspektiver fra Freud og Blanchot, med særlig vekt på det uhyggelige

Internal pressure gradient errors in sigma-coordinate ocean models: The finite volume and weighted approaches

Terrain-following (sigma-coordinate) models are widely used. They are often advantageous when dealing with large variations in topography, and give an accurate representation of the bottom and top boundary layers. However, near steep topography, the use of these coordinates can lead to a large error in the internal pressure gradient force. Using finite differences is the traditional way of discretising the equations. However, it is possible to integrate over horizontal cells by using a finite volume approach instead. In this work, we will interpret the traditional computation of the internal pressure in the Princeton Ocean Model (Blumberg and Mellor, 1987) as a finite volume method. We will include additional points in the computantional stencil and derive higher order finite volume methods. The standard 2nd order POM method will also be combined with the rotated grid approach from Thiem and Berntsen, 2006. We will investigate the possibility of using an 'optimal weighting', with weights that depend on the topography, the stratification, and the grid size. All methods will be applied to an idealised test case - the seamount problem

Investigation of instability and turbulence effects on gas explosions: experiments and modelling

Safe design of industrial facilities requires models that can predict the consequences of accidental gas explosions in complex geometries with sufficient accuracy. Computa- tional fluid dynamics (CFD) models are widely used for consequence assessment in the process industries. The primary mechanism for flame acceleration during gas explo- sions in congested geometries is the positive feedback between turbulence generated in the reactant mixture, especially in shear and boundary layers from explosion-driven flow past obstacles and walls, and enhanced combustion rates. Additionally, a range of instability phenomena can significantly increase flame acceleration in gas explosions. This doctoral project addresses the following research question: "how can the sub- grid representation of flame acceleration mechanisms due to instability effects and flow past obstructed regions be improved in a CFD tool used for consequence assessment of gas explosions?". The thesis presents and validates new sub-grid models developed for the CFD tool FLACS, focusing on the following flame acceleration mechanisms: (i) the influence of the hydrodynamic and thermal–diffusive instabilities (intrinsic in- stabilities) on flame acceleration in the initial phase of a gas explosion, (ii) the influ- ence of thermal–diffusive effects on the rate of turbulent combustion for different fuels and mixture concentrations, (iii) the role of the Bénard–von Kármán (BVK) instability downstream of bluff-body obstacles in explosion-induced flow, (iv) how the Rayleigh– Taylor (RT) instability developing on a flame front that is accelerated over an obstacle or a vent opening may enhance the combustion rate, and (v) how flexible obstructions with very small components (in the form of vegetation) induce flame acceleration in gas explosions. There was a lack of available experimental work describing the rela- tive importance of several of the aforementioned effects. Therefore, three experimental campaigns were designed and conducted as part of the doctoral study. Experimen- tal findings thus constitute a significant part of the original scientific contribution of the present work; these can be used to develop sub-grid models for any consequence model system. Three additional campaigns, performed by other research groups, were simulated in order to validate the new sub-grid models. The first experimental campaign presented in the thesis concerned explosion exper- iments performed in a large-scale, empty, vented enclosure comprising two chambers separated by a doorway. Simulations indicate that model performance might improve by introducing the effect of the RT instability. Furthermore, the results suggest that the model should account for thermal–diffusive instability effects both in the initial phase of a gas explosion and in the regime of turbulent combustion. The onset and growth rate of intrinsic instabilities in gas explosions are closely linked to the value of the Markstein number of the fuel-air mixture. The second experi- mental campaign presented in the thesis was performed as part of the doctoral study to explore the effect of varying the fuel concentration on overpressures and flame speeds in a series of propane-air explosions. The variations in the fuel concentration effec tively change the Markstein number of the mixture. The dissertation compares exper- imental results with predictions from a version of FLACS that includes a Markstein number-dependent combustion model, developed as part of the doctoral project. For negative Markstein numbers, the new model version performs significantly better than the original model. This work addresses mechanisms (i) and (ii). The thesis elaborates on instability effects relevant for scenarios where explosion- induced flow interacts with partial confinement and obstacles, focusing on BVK and RT instabilities in particular, i.e. mechanisms (iii) and (iv). A third experimental cam- paign was performed as part of the present study to investigate the relative contribution of vortex shedding, caused by the BVK instability, to the overpressure generation in gas explosions with a single obstacle inserted. Vortex shedding was observed in the baseline laboratory-scale experiments, and then suppressed by two passive flow con- trol methods. The most effective configuration reduced the maximum overpressures by approximately 32 %. The results can be used to model the flame surface area in- crease downstream of obstacles in any consequence model system. However, further experimental work, building on the findings of the present doctoral study, is required to formulate a sub-grid model for mechanism (iii). A sub-grid model for flame wrinkling due to the RT instability (mechanism (iv)), based on the linearised growth rate of instabilities on an accelerated flame front, was implemented in FLACS. The thesis presents updated model results for two series of vented explosion experiments, obtained with a version of FLACS that includes both the Markstein number-dependent burning velocity model and the RT instability effect. The first test case involved a series of fuel-lean hydrogen-air explosions. The second test case revisited the campaign performed in the twin-compartment enclosure that was presented in the very beginning of the thesis. A third test case was included to inves- tigate whether the sub-grid models that were initially developed and validated for ex- plosions with a high degree of confinement and idealised obstacle configurations could produce improved results also for explosions in complex geometries with a low degree of confinement. Therefore, a series of large-scale natural gas-air explosions performed in a full-scale offshore module was simulated. All three test cases were performed by other research groups. The sub-grid models developed as part of the present doctoral study significantly improve the representation of several of these experiments. The fourth experimental campaign was motivated by a need for an improved ap- proach to sub-grid modelling of the effect of vegetation on gas explosions in CFD tools, i.e. mechanism (v). Recent accidents have shown that this is highly relevant for risk assessment for onshore process facilities. The presence of foliage on spruce branches notably enhanced maximum overpressures in the experiments, suggesting that obstruc- tions of very small dimensions may contribute significantly to the flame acceleration. Highly flexible, fractal-like obstructions require additional modelling considerations. Based on the experimental results, the effects of foliage and flexibility were included in FLACS simulations by constructing congestion blocks, representing the effective drag area that is expected to produce flame acceleration. This modelling approach success- fully reproduces the experimental trends. In conclusion, the results of the experimental campaigns presented in this thesis have improved the understanding of several important physical effects related to flame acceleration in industrial-scale explosions. Furthermore, the thesis demonstrates how this knowledge may be used to model gas explosions more accurately

Drømmenes poetiske skapelse En studie av fem «drømmedikt» av Tone Hødnebø

Denne avhandlingen tar for seg dikt av samtidsforfatteren Tone Hødnebø. Jeg har analysert fem dikt som er hentet fra tre av Hødnebøs diktsamlinger: «I» fra Larm (1989), «Lysskye affærer» fra Pendel (1997) og «BYGNINGEN KRENGER», «NÅR DU KOMMER HJEM» og «MEN ALT» fra Stormstigen (2002). De fem diktene omhandler drømmer, og det er nettopp drømmer som står sentralt i problemstillingen. Problemstillingen jeg har arbeidet ut fra, er: Hva skaper «drømmediktene» til Tone Hødnebø? For å svare på problemstillingen har jeg tatt utgangspunkt i Sigmund Freuds arbeid med drømmer, og jeg bruker særlig hans begreper, «forskyvning», «fortetning» og det uhyggelige for å forklare hva som skapes i diktene. I tillegg bruker jeg Otto M. Rheinschmiedt, en psykoanalytiker i tradisjonen etter Freud, sine tanker om drømmer som multitemporale. Maurice Blanchot bidrar med et litteraturfilosofisk perspektiv på drømmer, der det er særlig hans idéer om drømmer som noe som eksisterer på «utsiden», det stedet der «alt har forsvunnet» trer frem og der den sovende ikke finner hvile, som blir viktig. Metodekapitlet omhandler den analytiske prosessen rundt analysen av diktene. Diktene til Tone Hødnebø er særdeles knappe og gåtefulle, og har krevd at arbeidet med analysene måtte gjøres i ulike faser, noe som har vært en tidkrevende prosess. I denne avhandlingen er jeg ikke opptatt av drømmenes latente betydning, men den manifeste. Det vil si at jeg har forholdt meg til hvordan diktene som dikt fremstiller drømmer, og ikke drømmenes underliggende betydning. Analysene har vist at hvert drømmedikt skaper noe som er helt unikt, nemlig svimmelhet i diktet «BYGNINGEN KRENGER», uhygge i diktet «NÅR DU KOMMER HJEM», fortetning i diktet «I», en form for endeløshet i diktet «MEN ALT» og avmakt i diktet «Lysskye affærer». Likevel har diktene til felles at de skaper uhyggelige rom, situasjoner relasjoner og hendelser. I oppgavens drøftingsdel knyttes analysene til perspektiver fra Freud og Blanchot, med særlig vekt på det uhyggelige

Computational fluid dynamics simulations of hydrogen releases and vented deflagrations in large enclosures

This paper presents model predictions obtained with the CFD tool FLACS for hydrogen releases and vented deflagrations in containers and larger enclosures. The paper consists of two parts. The first part compares experimental results and model predictions for two test cases: experiments performed by Gexcon in 20-foot ISO containers (volume 33 m3) as part of the HySEA project and experiments conducted by SRI International and Sandia National Laboratories in a scaled warehouse geometry (volume 45.4 m3). The second part explores the use of the model system validated in the first part to accidental releases of hydrogen from forklift trucks inside a full-scale warehouse geometry (32 400 m3). The results demonstrate the importance of using realistic and reasonably accurate geometry models of the systems under consideration when performing CFD-based risk assessment studies. The discussion highlights the significant inherent uncertainty associated with quantitative risk assessments for vented hydrogen deflagrations in complex geometries. The suggestions for further work include a pragmatic approach for developing empirical correlations for pressure loads from vented hydrogen deflagrations in industrial warehouses with hydrogen-powered forklift trucks