Vladimir Molkov | University of Ulster (original) (raw)

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Papers by Vladimir Molkov

The report describes the findings of a workshop that was held at the Institute for Energy and Tra... more The report describes the findings of a workshop that was held at the Institute for Energy and Transport (JRC) in Petten Netherlands, on the topic “Gap analysis of CFD modelling of hydrogen release and combustion”. The main topic was divided in 6 sub-topics: release and dispersion,auto-ignition, fires, deflagrations, detonations and DDT, and accident consequences. For each sub-topic, the main gaps in CFD modelling were identified and prioritised.Hydrogen is expected to play an important role in the energy mix of a future low carbon society, (the European Strategic Energy Technology Plan of the European Commission (COM 2007 - 723) and in the Hydrogen, Fuel Cells & Infrastructure Technologies Program-Multi-Year Research, Development, and Demonstration Plan of the USA Department of Energy (DoE 2007).Hydrogen safety issues must be addressed in order to ensure that the wide spread deployment and use of hydrogen and fuel cell technologies can occur with the same or lower level of hazardsan...

This paper is an attempt to give a concise overview of the state-of-the-art in the recent hydroge... more This paper is an attempt to give a concise overview of the state-of-the-art in the recent hydrogen safety research, mostly within the framework of activities performed by the European Network of Excellence HySafe “Safety of Hydrogen as an Energy Carrier” and related projects. The up-to-date knowledge, recent progress and future research are discussed in brief. The large eddy simulation (LES) model developed at the University of Ulster to simulate hydrogen releases and combustion is presented.

The nature of coherent deflagrations in an enclosure, vented into the atmosphere, is analysed. Ex... more The nature of coherent deflagrations in an enclosure, vented into the atmosphere, is analysed. Experimental observations in an empty 547-m3 vented enclosure of the SOLVEX programme are analysed by means of large eddy simulations (LES). The LES model is based on the renormalization group theory and the dilution of the methane-air mixture, as it flows out of the vent, is taken into account. A comparison between simulated and experimental pressure transients and, the dynamics of the propagating flame front has given an insight into the nature of the complex simultaneous interactions between flow, turbulence and combustion inside the enclosure and in expelled gases. LES processing of experimental data unveiled that the substantial intensification of premixed combustion occurs only outside the enclosure , leading to a steep coherent pressure rise in both the internal and the external deflagrations. The external explosion does not affect burning rate inside the enclosure. The LES model sh...

The engineering correlations for assessment of hazard distance defined by a size of fireball afte... more The engineering correlations for assessment of hazard distance defined by a size of fireball after either liquid hydrogen spill combustion or high-pressure hydrogen tank rupture in a fire in the open atmosphere (both for stand-alone and under-vehicle tanks) are presented. The term “fireball size” is used for the maximum horizontal size of a fireball that is different from the term “fireball diameter” applied to spherical or semi-spherical shape fireballs. There are different reasons for a fireball to deviate from a spherical shape, e.g., in case of tank rupture under a vehicle, the non-instantaneous opening of tank walls, etc. Two conservative correlations are built using theoretical analysis, numerical simulations, and experimental data available in the literature. The theoretical model for hydrogen fireball size assumes complete isobaric combustion of hydrogen in air and presumes its hemispherical shape as observed in the experiments and the simulations for tank rupturing at the g...

Hydrogen

Hydrogen jet fires from a thermally activated pressure relief device (TPRD) on onboard storage ar... more Hydrogen jet fires from a thermally activated pressure relief device (TPRD) on onboard storage are considered for a vehicle in a naturally ventilated covered car park. Computational Fluid Dynamics was used to predict behaviour of ignited releases from a 70 MPa tank into a naturally ventilated covered car park. Releases through TPRD diameters 3.34, 2 and 0.5 mm were studied to understand effect on hazard distances from the vehicle. A vertical release, and downward releases at 0°, 30° and 45° for TPRD diameters 2 and 0.5 mm were considered, accounting for tank blowdown. direction of a downward release was found to significantly contribute to decrease of temperature in a hot cloud under the ceiling. Whilst the ceiling is reached by a jet exceeding 300 °C for a release through a TPRD of 2 mm for inclinations of either 0°, 30° or 45°, an ignited release through a TPRD of 0.5 mm and angle of 45° did not produce a cloud with a temperature above 300 °C at the ceiling during blowdown. The re...

International Journal of Hydrogen Energy, 2017

Engineering tools for calculation of hazard distances for cryogenic hydrogen jets are currently m... more Engineering tools for calculation of hazard distances for cryogenic hydrogen jets are currently missing. This study aims at the development of validated correlations for calculation of hazard distances for cryogenic unignited releases and jet fires. The experiments performed by Sandia National Laboratories (SNL) on jets from storage temperature in the range 46-295 K and pressure up to 6 bar abs are used to expand the validation domain of the correlations. The Ulster’s under-expanded jet theory is applied to calculate parameters at the real nozzle exit. The similarity law for concentration decay in momentum-dominated jets is shown to be capable to reproduce experimental data of SNL on 9 unignited cryogenic releases. The accuracy of the similarity law to predict experimentally measured axial concentration decay improves with the increase of the release diameter. This is thought due to decrease of the effect of friction and minor losses for large release orifices. The dimensionless fla...

The large eddy simulation (LES) model is suggested to simulate a wrinkled premixed flame propagat... more The large eddy simulation (LES) model is suggested to simulate a wrinkled premixed flame propagation in large-scale gaseous explosions. The model combines filtered progress variable equation to model flame front propagation and gradient method for combustion rate modelling. Renormalisation group theory was employed to simulate subgrid scale wrinkling, to calculate effective turbulent viscosity, effective Prandtle and Shmidt numbers. The model was applied to simulate: a) stoichiometric hydrogen-air deflagration in 2.3 diameter closed spherical vessel, b) initial stage of 10.5% (vol.) methane-air deflagration in 547 m3 vented rectangular enclosure. The suggested technique was capable to resolve explicitly down to the order of LES filter size the flame front wrinkling (cells), caused by hydrodynamic instability, and to reproduce correctly the flame front propagation velocity observed in experiment. Wrinkling factor and fractal dimension of the developing flame front structure were obta...

Energies, 2021

In this paper the multi-phenomena deflagration model is used to simulate deflagrative combustion ... more In this paper the multi-phenomena deflagration model is used to simulate deflagrative combustion of several fuel–air mixtures in various scale closed vessels. The experimental transient pressure of methane–air, ethane–air, and propane–air deflagrations in vessels of volume 0.02 m3, 1 m3, and 6 m3 were simulated. The model includes key mechanisms affecting propagation of premixed flame front: the dependence of laminar burning velocity of concentration, pressure, and temperature; the effect of preferential diffusion in the corrugated flame front or leading point concept; turbulence generated by flame front itself or Karlovitz turbulence; increase of the flame front area with flame radius by fractals; and turbulence in the unburned mixture. Laminar velocity dependence on concentration, pressure, and temperature were calculated using CANTERA software. Various scale and geometry of used vessels induces various combustion mechanism. Simulations allow insight into the dominating mechanism....

Turbulent non-premixed flame stability limits are very important for safety considerations, espec... more Turbulent non-premixed flame stability limits are very important for safety considerations, especially for design of thermally activated pressure relief devices (TPRD), e.g. to prevent flame blow-off and hydrogen accumulation in the enclosure. There is a body of research on parameters of lift-off and blow-out for turbulent non-premixed flames. Hydrogen safety engineering requires validated contemporary tools, such as CFD models, that can be used for the design of TPRD, which parameters will provide either the stabilisation of flame or its blow-off, including the effect of wind. The applied in this study CFD model is based on the eddy dissipation concept (EDC) sub-model for combustion, which incorporates a detailed chemistry mechanism with 37 chemical reactions, and the RNG k-epsilon sub-model for turbulence modelling. The model has been successfully applied to simulation of spontaneous ignition during the sudden release of hydrogen into the air, and the numerical study of indoor jet...

The structure, axial concentration decay, and formation of flammable hydrogen-air mixture are stu... more The structure, axial concentration decay, and formation of flammable hydrogen-air mixture are studied numerically for a case of hydrogen release from a round nozzle and a plane nozzle with aspect ratio 200. Simulations of the underexpanded hydrogen jet structure for both circular and plane nozzles were conducted using the computational fluid dynamics in two stages: firstly from the nozzle across the shocks’ structure, and secondly downstream from the Mach disk to the far field. The results confirmed the phenomenon of “switching” axes known for plane jets of finite aspect ratio: the faster mixing in the plane of a minor axis makes the jet in this plane wider than in the plane of a major axis. The faster hydrogen-air mixing takes place only in the near field close to the nozzle. Downstream the plane nozzle jet structure tends to reproduce that of the axisymmetric jet. In the far field both plane and round nozzle jets resemble the similarity law for axial concentration decay in round j...

The paper presents nomograms as engineering tools for assessment of hazard distances from a blast... more The paper presents nomograms as engineering tools for assessment of hazard distances from a blast wave after rupture of stand-alone or onboard hydrogen cylinder in a fire. The nomograms were built basing on the validated model, which includes the contribution of combustion of hydrogen released after a tank rupture to the blast wave strength. Two categories of the nomograms were developed. The first category is simplified and designed for the use by first responders, working on the scene of accident. These nomograms are constrained by fixed harm to people and damage to buildings. The other category of nomograms is for the use by hydrogen safety engineers. These nomograms give more flexibility to users, allowing to apply various harm/damage criteria accepted in different national and international standards. It was emphasised, that harm/damage criteria differ from one country to another. This naturally gives the different hazard distances. The need for international efforts to unify t...

The report describes the findings of a workshop that was held at the Institute for Energy and Tra... more The report describes the findings of a workshop that was held at the Institute for Energy and Transport (JRC) in Petten Netherlands, on the topic “Gap analysis of CFD modelling of hydrogen release and combustion”. The main topic was divided in 6 sub-topics: release and dispersion,auto-ignition, fires, deflagrations, detonations and DDT, and accident consequences. For each sub-topic, the main gaps in CFD modelling were identified and prioritised.Hydrogen is expected to play an important role in the energy mix of a future low carbon society, (the European Strategic Energy Technology Plan of the European Commission (COM 2007 - 723) and in the Hydrogen, Fuel Cells & Infrastructure Technologies Program-Multi-Year Research, Development, and Demonstration Plan of the USA Department of Energy (DoE 2007).Hydrogen safety issues must be addressed in order to ensure that the wide spread deployment and use of hydrogen and fuel cell technologies can occur with the same or lower level of hazardsan...

This paper is an attempt to give a concise overview of the state-of-the-art in the recent hydroge... more This paper is an attempt to give a concise overview of the state-of-the-art in the recent hydrogen safety research, mostly within the framework of activities performed by the European Network of Excellence HySafe “Safety of Hydrogen as an Energy Carrier” and related projects. The up-to-date knowledge, recent progress and future research are discussed in brief. The large eddy simulation (LES) model developed at the University of Ulster to simulate hydrogen releases and combustion is presented.

The nature of coherent deflagrations in an enclosure, vented into the atmosphere, is analysed. Ex... more The nature of coherent deflagrations in an enclosure, vented into the atmosphere, is analysed. Experimental observations in an empty 547-m3 vented enclosure of the SOLVEX programme are analysed by means of large eddy simulations (LES). The LES model is based on the renormalization group theory and the dilution of the methane-air mixture, as it flows out of the vent, is taken into account. A comparison between simulated and experimental pressure transients and, the dynamics of the propagating flame front has given an insight into the nature of the complex simultaneous interactions between flow, turbulence and combustion inside the enclosure and in expelled gases. LES processing of experimental data unveiled that the substantial intensification of premixed combustion occurs only outside the enclosure , leading to a steep coherent pressure rise in both the internal and the external deflagrations. The external explosion does not affect burning rate inside the enclosure. The LES model sh...

The engineering correlations for assessment of hazard distance defined by a size of fireball afte... more The engineering correlations for assessment of hazard distance defined by a size of fireball after either liquid hydrogen spill combustion or high-pressure hydrogen tank rupture in a fire in the open atmosphere (both for stand-alone and under-vehicle tanks) are presented. The term “fireball size” is used for the maximum horizontal size of a fireball that is different from the term “fireball diameter” applied to spherical or semi-spherical shape fireballs. There are different reasons for a fireball to deviate from a spherical shape, e.g., in case of tank rupture under a vehicle, the non-instantaneous opening of tank walls, etc. Two conservative correlations are built using theoretical analysis, numerical simulations, and experimental data available in the literature. The theoretical model for hydrogen fireball size assumes complete isobaric combustion of hydrogen in air and presumes its hemispherical shape as observed in the experiments and the simulations for tank rupturing at the g...

Hydrogen

Hydrogen jet fires from a thermally activated pressure relief device (TPRD) on onboard storage ar... more Hydrogen jet fires from a thermally activated pressure relief device (TPRD) on onboard storage are considered for a vehicle in a naturally ventilated covered car park. Computational Fluid Dynamics was used to predict behaviour of ignited releases from a 70 MPa tank into a naturally ventilated covered car park. Releases through TPRD diameters 3.34, 2 and 0.5 mm were studied to understand effect on hazard distances from the vehicle. A vertical release, and downward releases at 0°, 30° and 45° for TPRD diameters 2 and 0.5 mm were considered, accounting for tank blowdown. direction of a downward release was found to significantly contribute to decrease of temperature in a hot cloud under the ceiling. Whilst the ceiling is reached by a jet exceeding 300 °C for a release through a TPRD of 2 mm for inclinations of either 0°, 30° or 45°, an ignited release through a TPRD of 0.5 mm and angle of 45° did not produce a cloud with a temperature above 300 °C at the ceiling during blowdown. The re...

International Journal of Hydrogen Energy, 2017

Engineering tools for calculation of hazard distances for cryogenic hydrogen jets are currently m... more Engineering tools for calculation of hazard distances for cryogenic hydrogen jets are currently missing. This study aims at the development of validated correlations for calculation of hazard distances for cryogenic unignited releases and jet fires. The experiments performed by Sandia National Laboratories (SNL) on jets from storage temperature in the range 46-295 K and pressure up to 6 bar abs are used to expand the validation domain of the correlations. The Ulster’s under-expanded jet theory is applied to calculate parameters at the real nozzle exit. The similarity law for concentration decay in momentum-dominated jets is shown to be capable to reproduce experimental data of SNL on 9 unignited cryogenic releases. The accuracy of the similarity law to predict experimentally measured axial concentration decay improves with the increase of the release diameter. This is thought due to decrease of the effect of friction and minor losses for large release orifices. The dimensionless fla...

The large eddy simulation (LES) model is suggested to simulate a wrinkled premixed flame propagat... more The large eddy simulation (LES) model is suggested to simulate a wrinkled premixed flame propagation in large-scale gaseous explosions. The model combines filtered progress variable equation to model flame front propagation and gradient method for combustion rate modelling. Renormalisation group theory was employed to simulate subgrid scale wrinkling, to calculate effective turbulent viscosity, effective Prandtle and Shmidt numbers. The model was applied to simulate: a) stoichiometric hydrogen-air deflagration in 2.3 diameter closed spherical vessel, b) initial stage of 10.5% (vol.) methane-air deflagration in 547 m3 vented rectangular enclosure. The suggested technique was capable to resolve explicitly down to the order of LES filter size the flame front wrinkling (cells), caused by hydrodynamic instability, and to reproduce correctly the flame front propagation velocity observed in experiment. Wrinkling factor and fractal dimension of the developing flame front structure were obta...

Energies, 2021

In this paper the multi-phenomena deflagration model is used to simulate deflagrative combustion ... more In this paper the multi-phenomena deflagration model is used to simulate deflagrative combustion of several fuel–air mixtures in various scale closed vessels. The experimental transient pressure of methane–air, ethane–air, and propane–air deflagrations in vessels of volume 0.02 m3, 1 m3, and 6 m3 were simulated. The model includes key mechanisms affecting propagation of premixed flame front: the dependence of laminar burning velocity of concentration, pressure, and temperature; the effect of preferential diffusion in the corrugated flame front or leading point concept; turbulence generated by flame front itself or Karlovitz turbulence; increase of the flame front area with flame radius by fractals; and turbulence in the unburned mixture. Laminar velocity dependence on concentration, pressure, and temperature were calculated using CANTERA software. Various scale and geometry of used vessels induces various combustion mechanism. Simulations allow insight into the dominating mechanism....

Turbulent non-premixed flame stability limits are very important for safety considerations, espec... more Turbulent non-premixed flame stability limits are very important for safety considerations, especially for design of thermally activated pressure relief devices (TPRD), e.g. to prevent flame blow-off and hydrogen accumulation in the enclosure. There is a body of research on parameters of lift-off and blow-out for turbulent non-premixed flames. Hydrogen safety engineering requires validated contemporary tools, such as CFD models, that can be used for the design of TPRD, which parameters will provide either the stabilisation of flame or its blow-off, including the effect of wind. The applied in this study CFD model is based on the eddy dissipation concept (EDC) sub-model for combustion, which incorporates a detailed chemistry mechanism with 37 chemical reactions, and the RNG k-epsilon sub-model for turbulence modelling. The model has been successfully applied to simulation of spontaneous ignition during the sudden release of hydrogen into the air, and the numerical study of indoor jet...

The structure, axial concentration decay, and formation of flammable hydrogen-air mixture are stu... more The structure, axial concentration decay, and formation of flammable hydrogen-air mixture are studied numerically for a case of hydrogen release from a round nozzle and a plane nozzle with aspect ratio 200. Simulations of the underexpanded hydrogen jet structure for both circular and plane nozzles were conducted using the computational fluid dynamics in two stages: firstly from the nozzle across the shocks’ structure, and secondly downstream from the Mach disk to the far field. The results confirmed the phenomenon of “switching” axes known for plane jets of finite aspect ratio: the faster mixing in the plane of a minor axis makes the jet in this plane wider than in the plane of a major axis. The faster hydrogen-air mixing takes place only in the near field close to the nozzle. Downstream the plane nozzle jet structure tends to reproduce that of the axisymmetric jet. In the far field both plane and round nozzle jets resemble the similarity law for axial concentration decay in round j...

The paper presents nomograms as engineering tools for assessment of hazard distances from a blast... more The paper presents nomograms as engineering tools for assessment of hazard distances from a blast wave after rupture of stand-alone or onboard hydrogen cylinder in a fire. The nomograms were built basing on the validated model, which includes the contribution of combustion of hydrogen released after a tank rupture to the blast wave strength. Two categories of the nomograms were developed. The first category is simplified and designed for the use by first responders, working on the scene of accident. These nomograms are constrained by fixed harm to people and damage to buildings. The other category of nomograms is for the use by hydrogen safety engineers. These nomograms give more flexibility to users, allowing to apply various harm/damage criteria accepted in different national and international standards. It was emphasised, that harm/damage criteria differ from one country to another. This naturally gives the different hazard distances. The need for international efforts to unify t...