Thermal radiation from cryogenic hydrogen jet fires

The thermal hazards from ignited under-expanded cryogenic releases are not yet fully understood and reliable predictive tools are missing. This study aims at validation of a CFD model to simulate flame length and radiative heat flux for cryogenic hydrogen jet fires. The simulation results are compared against the experimental data by Sandia National Laboratories on cryogenic hydrogen fires from storage with pressure up to 5 bar abs and temperature in the range 48–82 K. The release source is modelled using the Ulster's notional nozzle theory. The problem is considered as steady-state. Three turbulence models were applied, and their performance was compared. The realizable k-ε model showed the best agreement with experimental flame length and radiative heat flux. Therefore, it has been employed in the CFD model along with Eddy Dissipation Concept for combustion and Discrete Ordinates (DO) model for radiation. A parametric study has been conducted to assess the effect of selected numerical and physical parameters on the simulations capability to reproduce experimental data. DO model discretisation is shown to strongly affect simulations, indicating 10 × 10 as minimum number of angular divisions to provide a convergence. The simulations have shown sensitivity to experimental parameters such as humidity and exhaust system volumetric flow rate, highlighting the importance of accurate and extended publication of experimental data to conduct precise numerical studies. The simulations correctly reproduced the radiative heat flux from cryogenic hydrogen jet fire at different locations.     

Performance of hydrogen storage tank with TPRD in an engulfing fire

The performance of a composite hydrogen storage tank with TPRD in an engulfing fire is studied. The non-adiabatic tank blowdown model, including in fire conditions, using the under-expanded jet theory is described. The model input includes thermal parameters of hydrogen and tank materials, heat flux from a fire to the tank, TPRD diameter and TPRD initiation delay time. The unsteady heat transfer from surroundings through the tank wall and liner to hydrogen accounts for the degradation of the composite overwrap resin and melting of the liner. The model is validated against the blowdown experiment and the destructive fire test with a tank without TPRD. The model accurately reproduces experimentally measured hydrogen pressure and temperature dynamics, blowdown time, and tank's fire-resistance rating, i.e. time to tank rupture in a fire without TPRD. The lower limit for TPRD orifice diameter sufficient to prevent the tank rupture in a fire and, at the same time, to reduce the flame length and mitigate the pressure peaking phenomenon in a garage to exclude its destruction, is assessed for different tanks, e.g. it is 0.75 mm for largest studied 244 L, 70 MPa tank. The phenomenon of Type IV tank liner melting for TPRD with lower diameter is revealed and its influence on hydrogen blowdown is assessed. This phenomenon facilitates the blowdown yet requires further detailed experimental validation.     

Effect of heat transfer through the release pipe on simulations of cryogenic hydrogen jet fires and hazard distances

Jet flames originated by cryo-compressed ignited hydrogen releases can cause life-threatening conditions in their surroundings. Validated models are needed to accurately predict thermal hazards from a jet fire. Numerical simulations of cryogenic hydrogen flow in the release pipe are performed to assess the effect of heat transfer through the pipe walls on jet parameters. Notional nozzle exit diameter is calculated based on the simulated real nozzle parameters and used in CFD simulations as a boundary condition to model jet fires. The CFD model was previously validated against experiments with vertical cryogenic hydrogen jet fires with release pressures up to 0.5 MPa (abs), release diameter 1.25 mm and temperatures as low as 50 K. This study validates the CFD model in a wider domain of experimental release conditions - horizontal cryogenic jets at exhaust pipe temperature 80 K, pressure up to 2 MPa ab and release diameters up to 4 mm. Simulation results are compared against such experimentally measured parameters as hydrogen mass flow rate, flame length and radiative heat flux at different locations from the jet fire. The CFD model reproduces experiments with reasonable for engineering applications accuracy. Jet fire hazard distances established using three different criteria - temperature, thermal radiation and thermal dose - are compared and discussed based on CFD simulation results.     

Prediction of the failure probability of the overhead power line exposed to large-scale jet fires induced by high-pressure gas leakage

A thermal failure model (TFM) is proposed to predict the failure probability of Aluminum Conductor Steel-Reinforced (ACSR) typed power line close to a large-scale jet fire of leaked high-pressure gases. It introduces a newly developed method for heat transfer from jet fires and a distribution model for conductor failure probability via IEEE Standard 738–2012. Comparisons covering van der Waals equation, jet flame length correlations (Chamberlain, Schefer, Molkov and Bradley) and thermal radiation models (point source, multi-point source and line source) were made to illustrate priority with respect to experimental measurement of large hydrogen and natural gas jet fires. Results show that a theoretical framework incorporating van der Waals equation, Molkov's correlation for jet flame length, radiative fraction model and point source model is adequately precise to predict high-pressure leakage process, total flame length and received radiant heat flux (far-field). Predicted total flame lengths of a large jet fire for nearby power lines within 50–200 m to the accident site correspond well to reported results and the conservative hazard ranges are predicted based on harm criteria of wood and Probit equations. In simulations, an acceptable safety distance for power line carrying 907 A and below is determined to be 150 m.     

Predicting radiative characteristics of hydrogen and hydrogen/methane jet fires using FireFOAM

A possible consequence of pressurized hydrogen release is an under-expanded jet fire. Knowledge of the flame length, radiative heat flux as well as the effects of variations in ground reflectance is important for safety assessment. The present study applies an open source CFD code FireFOAM to study the radiation characteristics of hydrogen and hydrogen/methane jet fires. For combustion, the eddy dissipation concept for multi-component fuels recently developed by the authors in the large eddy simulation (LES) framework is used. The radiative heat is computed with the finite volume discrete ordinates model in conjunction with the weighted sum of grey gas model for the absorption/emission coefficient. The pseudo-diameter approach is used in which the corresponding parameters are calculated using the formulations of Birch et al. [24] with the thermodynamic properties corrected by the Able-Noble equation of state. The predicted flame length and radiant fraction are in good agreement with the measurements of Schefer et al. [2], Studer et al. [3] and Ekoto et al. [6]. In order to account for the effects of variation in ground surface reflectance, the emissivity of hydrogen flames was modified following Ekoto et al. [6]. Four cases with different ground reflectance are computed. The predictions show that the ground surface reflectance only has minor effect on the surface emissive power of the smaller hydrogen jet fire of Ekoto et al. [6]. The radiant fractions fluctuate from 0.168 to 0.176 close to the suggested value of 0.16 by Ekoto et al. [6] based on the analysis of their measurements.     

A study of barrier walls for mitigation of unintended releases of hydrogen

Hydrogen jet flames resulting from ignition of unintended releases can be extensive in length and pose significant radiation and impingement hazards. Depending on the leak diameter and source pressure, the resulting consequence distances can be unacceptably large. One possible mitigation strategy to reduce exposure to jet flames is to incorporate barriers around hydrogen storage and delivery equipment. An experimental and modeling program has been performed at Sandia National Laboratories to better characterize the effectiveness of barrier walls to reduce hazards. This paper describes the experimental and modeling program and presents results obtained for various barrier configurations. The experimental measurements include flame deflection using standard and infrared video and high-speed movies (500 fps) to study initial flame propagation from the ignition source. Measurements of the ignition overpressure, wall deflection, radiative heat flux, and wall and gas temperature were also made at strategic locations. The modeling effort includes three-dimensional calculations of jet flame deflection by the barriers, computations of the thermal radiation field around barriers, predicted overpressure from ignition, and the computation of the concentration field from deflected unignited hydrogen releases. The various barrier designs are evaluated in terms of their mitigation effectiveness for the associated hazards present. The results show that barrier walls are effective at deflecting jet flames in a desired direction and can help attenuate the effects of ignition overpressure and flame radiative heat flux.     

Characterization of high-pressure,underexpanded hydrogen-jet flames

Measurements were performed to characterize the dimensional and radiative properties of large-scale, vertical hydrogen-jet flames. This data is relevant to the safety scenario of a sudden leak in a high-pressure hydrogen containment vessel and will provide a technological basis for determining hazardous length scales associated with unintended hydrogen releases at storage and distribution centers. Jet flames originating from high-pressure sources up to 413 bar (6000 psi) were studied to verify the application of correlations and scaling laws based on lower-pressure subsonic and choked-flow jet flames. These higher pressures are expected to be typical of the pressure ranges in future hydrogen storage vessels. At these pressures the flows exiting the jet nozzle are categorized as underexpanded jets in which the flow is choked at the jet exit. Additionally, the gas behavior departs from that of an ideal-gas and alternate formulations for non-ideal gas must be introduced. Visible flame emission was recorded on video to evaluate flame length and structure. Radiometer measurements allowed determination of the radiant heat flux characteristics. The flame length results show that lower-pressure engineering correlations, based on the Froude number and a non-dimensional flame length, also apply to releases up to 413 bar (6000 psi). Similarly, radiative heat flux characteristics of these high-pressure jet flames obey scaling laws developed for low-pressure, smaller-scale flames and a wide variety of fuels. The results verify that such correlations can be used to a priori predict dimensional characteristics and radiative heat flux from a wide variety of hydrogen-jet flames resulting from accidental releases.     

Experiment study on the pressure and flame characteristics induced by high-pressure hydrogen spontaneous ignition

A series of experiments were conducted to study the pressure and combustion characteristics of the high-pressure hydrogen during the occurrence of spontaneous ignition and the conversion from spontaneous ignition to a jet fire and explosion. Different initial conditions including release pressure (4–10 MPa), tube diameter (10/15 mm), and tube length (0.3/0.7/1.2/1.7/2.2/3 m) were tested. The variation of the pressure and flame signal inside and outside of the tube and the development of the jet flame were recorded. The experimental results revealed that the minimum ignition pressure required for self-ignition of hydrogen at different tube diameters decreased first and then increased with the extension of tubes. The minimum ignition pressure for tubes diameters of 10 mm and 15 mm is no more than 4 MPa and the length of the tubes is L = 1.7 m. The minimum release pressure required for spontaneous ignition of a tube D = 15 mm is always lower than that of a tube D = 10 mm at the same tube length. When the spontaneous ignition occurred, it did not absolutely trigger the jet fire. The transition from spontaneous ignition to a jet fire must go through the specific stages.     

Evaluation of barrier walls for mitigation of unintended releases of hydrogen

Hydrogen jet flames resulting from ignition of unintended releases can be extensive in length and pose significant radiation and impingement hazards. Depending on the leak diameter and source pressure, the resulting consequence distances can be unacceptably large. One possible mitigation strategy to reduce exposure to jet flames is to incorporate barriers around hydrogen storage and delivery equipment. While reducing the extent of unacceptable consequences, the walls may introduce other hazards if not properly configured. An experimental and modeling program has been performed at Sandia National Laboratories to better characterize the effectiveness of barrier walls to reduce hazards. This paper describes the experimental and modeling program and presents results obtained for various barrier configurations. The experimental measurements include flame deflection using standard and infrared video and high-speed movies (500 fps) to study initial flame propagation from the ignition source. Measurements of the ignition overpressure, wall deflection, radiative heat flux, and wall and gas temperature were also made at strategic locations. The modeling effort includes three-dimensional calculations of jet flame deflection by the barriers, computations of the thermal radiation field around barriers, predicted overpressure from ignition, and the computation of the concentration field from deflected unignited hydrogen releases. The various barrier designs are evaluated in terms of their mitigation effectiveness for the associated hazards present. The results show that barrier walls are effective at deflecting jet flames in a desired direction and can help attenuate the effects of ignition overpressure and flame radiative heat flux.     

Updated jet flame radiation modeling with buoyancy corrections

Radiative heat fluxes from small to medium-scale hydrogen jet flames (<10 m) compare favorably to theoretical predictions provided the product species thermal emittance and optical flame thickness are corrected for. However, recent heat flux measurements from two large-scale horizontally orientated hydrogen flames (17.4 and 45.9 m respectively) revealed that current methods underpredicted the flame radiant fraction by 40% or more. Newly developed weighted source flame radiation models have demonstrated substantial improvement in the heat flux predictions, particularly in the near-field, and allow for a sensible way to correct potential ground surface reflective irradiance. These updated methods are still constrained by the fact that the flame is assumed to have a linear trajectory despite buoyancy effects that can result in significant flame deformation. The current paper discusses a method to predict flame centerline trajectories via a one-dimensional flame integral model, which enables optimized placement of source emitters for weighted multi-source heat flux prediction methods. Flame shape prediction from choked releases was evaluated against flame envelope imaging and found to depend heavily on the notional nozzle model formulation used to compute the density weighted effective nozzle diameter. Nonetheless, substantial improvement in the prediction of downstream radiative heat flux values occurred when emitter placement was corrected by the flame integral model, regardless of the notional nozzle model formulation used.     

Spatial and radiative properties of an open-flame hydrogen plume

Considerable effort is being directed toward updating safety codes and standards in preparation for production, distribution, and retail of hydrogen as a consumer energy source. In the present study, measurements were performed in large-scale, vertical flames to characterize the dimensional and radiative properties of an ignited hydrogen jet. These data are relevant to the safety scenario of a sudden leak in a high-pressure hydrogen containment vessel. Specifically, the data will provide a technological basis for determining hazardous length scales associated with unintended releases at hydrogen storage and distribution centers. Visible and infrared video and ultraviolet flame luminescence imaging were used to evaluate flame length, diameter and structure. Radiometer measurements allowed determination of the radiant heat flux from the flame. The results show that flame length increases with total jet mass flow rate and jet nozzle diameter. When plotted as a function of Froude number, which measures the relative importance of jet momentum and buoyancy, the measured flame lengths for a range of operating conditions collapse onto the same curve. Good comparison with hydrocarbon jet flame lengths is found, demonstrating that the non-dimensional correlations are valid for a variety of fuel types. The radiative heat flux measurements for hydrogen flames show good agreement with non-dimensional correlations and scaling laws developed for a range of fuels and flame conditions. This result verifies that such correlations can be used to predict radiative heat flux from a wide variety of hydrogen flames and establishes a basis for predicting a priori the characteristics of flames resulting from accidental releases.     

Investigation of heat transfer characteristics of hydrogen combustion process in cylindrical combustors from microscale to mesoscale

In the field of micro and mesoscale combustion, the feature of flame-wall thermal coupling is of great significance because of its small scale nature. Thus, this work provides a comprehensive heat transfer analysis in cylindrical combustors from the perspective of numerical simulation. The combustor has a fixed length-to-diameter aspect ratio of 10, and the channel diameter is scaling up from 1 mm to 11 mm to explore the influence of chamber dimension on heat transfer and flame structure. The distribution of convective and radiative heat flux on inner surface, contribution of thermal radiation are given. Moreover, the role of radiation in flame structure is analyzed, and the convective and radiative heat losses are quantitatively analyzed. We find that radiative heat flux is smaller compared to convective heat flux, and the proportion of radiative heat flux becomes larger with an increasing diameter. Thermal radiation does not change the flame structure when the diameter is less than 3 mm. When the diameter is greater than 5 mm, thermal radiation changes the location of flame front. The heat loss becomes larger at a smaller diameter, and heat loss ratio can reach approximately 73.6% in the combustor with diameter of 1 mm.     

Experimental study on flame height and temperature profile of buoyant window spill plume from an under-ventilated compartment fire

Fire experiments were carried out in a scale model, consisting of an 0.8 m cubic fire compartment with six window like geometries and an attached 3 m (wide) × 5 m (high) façade wall. A propane porous gas burner with controlled fuel supply rate was the fire source. Gas temperature profiles were measured inside the compartment and near the façade wall. The outside spill flame heights were recorded by a CCD Digital camera. Temperature and flame heights are correlated with heat release rate and the window geometry using physically non-dimensional analysis. The steady gas temperatures inside the compartment are determined by an overall energy balance between the heat release rate inside the compartment and the wall conduction and opening radiation heat losses using an effective overall heat loss coefficient. Flame heights on the façade are non-dimensionally correlated by the excess fuel heat release rate outside the enclosure and a characteristic length scale for the window. These results agree with previous results in the literature. Vertical gas temperatures near the façade wall outside the enclosure are non-dimensionally correlated with the total convective heat flow rate above the flames and the same characteristic window length scale as the flame height, with the additional necessary determination of a virtual origin of the convective flow above the flame. These results and correlations are new and a significant improvement over previous results in the literature.     

Characteristic of cryogenic hydrogen flames from high-aspect ratio nozzles

Unintentional leaks at hydrogen fueling stations have the potential to form hydrogen jet flames, which pose a risk to people and infrastructure. The heat flux from these jet flames are often used to develop separation distances between hydrogen components and buildings, lot-lines, etc. The heat flux and visible flame length is well understood for releases from round nozzles, but real unintended leaks would be expected to be from higher aspect-ratio cracks. In this work, we measured the visible flame length and heat-flux characteristics of cryogenic hydrogen flames from high-aspect ratio nozzles. Heat flux measurements from 5 radiometers were used to assess the single-point vs the multi-point methods for interpretation of heat flux sensor data, finding the axial distance of the sensor for a single-point heat flux measurement to be important. We compare the flame length and heat flux data to flames of both cryogenic and compressed hydrogen from round nozzles. The aspect ratio of the release does not affect the flame length or heat flux significantly, for a given mass flow under the range of conditions studied. The engineering correlations presented in this work enable the prediction of flame length and heat flux which can be used to assess risk at hydrogen fueling stations with liquid hydrogen and develop science-based separation distances for these stations.     

Investigating the effect of computational grid sizes on the predicted characteristics of thermal radiation for a fire

Recently, the phenomenological modeling of fires has been shifted from the engineering application of correlation-based methods to the computational fluid dynamics (CFD) techniques. Therefore, the majority of this paper is to investigate the effects of grid sizes on the predicted radiative characteristics involved in a fire using CFD simulations, with the aim of selecting the appropriate grid size under the consideration of prediction accuracy and computing cost. Based on the present simulations, the predicted flame height increases as the decreasing grid size and would approach to a quasi-steady value if the simulation grids are adopted to be small enough. Similar results are also revealed in the radiative heat flux behaviors. The predicted distributions of radiative heat fluxes have no significant variations as the grid size is reduced to some small value. Several experiments of small pool fires with various diameters (20–38 cm) are conducted to assess the present CFD predictions. Using the appropriate grid size, the predicted results for radiative heat fluxes and flame heights show good agreement with the experimental data for different-size pool fires. This grid size suggested in this paper could assist the CFD simulations of pool fires in obtaining the accurate enough predictions with reasonable computing time.     

Experimental investigation on the heat transfer of an impinging inverse diffusion flame

This paper presents the results of an experimental study on the heat transfer characteristics of an inverse diffusion flame (IDF) impinging vertically upwards on a horizontal copper plate. The IDF burner used in the experiment has a central air jet surrounded circumferentially by 12 outer fuel jets. The heat flux at the stagnation point and the radial distribution of heat flux were measured with a heat flux sensor. The effects of Reynolds number, overall equivalence ratio, and nozzle-to-plate distance on the heat flux were investigated. The area-averaged heat flux and the heat transfer efficiency were calculated from the radial heat flux within a radial distance of 50 mm from the stagnation point of the flame, for air jet Reynolds number (Re_air) of 2000, 2500 and 3000, for overall equivalence ratios (Φ) of 0.8–1.8, at normalized nozzle-to-plate distances (H/d_IDF) between 4 and 10. Similar experiments were carried out on a circular premixed impinging flame for comparison.It was found that, for the impinging IDF, for Φ of 1.2 or higher, the area-averaged heat flux increased as the Re_air or Φ was increased while the heat transfer efficiency decreased when these two parameters increased. Thus for the IDF, the maximum heat transfer efficiency occurred at Re_air = 2000 and Φ = 1.2. At lower Φ, the heat transfer efficiency could increase when Φ was decreased. For the range of H/d_IDF investigated, there was certain variation in the heat transfer efficiency with H/d_IDF. The heat transfer efficiency of the premixed flame has a peak value at Φ = 1.0 at H/d_P = 2 and decreases at higher Φ and higher H/d_P. The IDF could have comparable or even higher heat transfer efficiency than a premixed flame.     

Monte Carlo modeling of radiative transfer in a turbulent sooty flame

The calculation of radiative transfer within a sooty turbulent ethylene-air diffusion jet flame has been carried out by using a Monte Carlo method and an accurate CK model for the gases. The influence of the turbulence-radiation interaction (TRI) has been studied. In the TRI modeling, the radiative properties of the assumed homogeneous turbulent structures are randomly obtained from a multidimensional probability density function (PDF) of the reaction progress variable, of the mixture ratio and of the soot volume fraction. This joint PDF is obtained from an Eulerian-Lagrangian turbulent combustion model and the sizes of the turbulent structures are directly derived from a k-? model. In the considered flame, the TRI effect is an increase of the radiative heat loss by about 30%. The radiative heat loss becomes almost equal to one-third of the chemical heat release. Soot particles play the most important role in the global radiative heat loss but the influence of gaseous species like CO₂ and H₂O can be important in the local energy balance.     

Combustion of suspended fine solid fuel in air inside inert porous medium: A heat transfer analysis

Present work is a numerical analysis of combustion of submicron carbon particles inside an inert porous medium where the particles in form of suspension in air enter the porous medium. A one-dimensional heat transfer model has been developed using the two-flux gray radiation approximation for radiative heat flux equations. The effects of absorption coefficient, emissivity of medium, flame position and reaction enthalpy flux on radiative energy output efficiency have been presented. It is revealed that in porous medium the combustion of suspended carbon particles is similar to premixed single phase gaseous fuel combustion except the former has shorter preheating temperature zone length. Use of porous ceramic having high porosity and made of Al₂O₃ or ZrO₂ with stabilized flame position operated nearer to downstream end will ensure radiative output maximum and minimum at downstream and upstream end, respectively.     

Experimental evaluation of barrier walls for risk reduction of unintended hydrogen releases

Hydrogen-jet flames resulting from ignition of unintended releases can be extensive in length and pose hazards associated with radiation and impingement onto objects, combustible materials and people. Depending on the leak diameter and source pressure, the resulting consequence distances can be unacceptably large. One possible mitigation strategy to reduce exposure to jet flames is to incorporate barriers around hydrogen storage and delivery equipment. While reducing the extent of unacceptable consequences, the walls may introduce other hazards if not properly configured. An experimental program has been implemented to better characterize the effectiveness of barrier walls at risk mitigation. This paper describes the experiments and presents results obtained for various barrier configurations. The measurements include flame deflection using standard and infrared video, high-speed movies (500 fps) to study initial flame propagation from the ignition source, overpressure levels due to ignition, wall deflection, radiative heat flux, and gas and wall temperatures. The various barrier designs are evaluated in terms of their mitigation effectiveness for the associated hazards present. The results show that barrier walls are effective at deflecting flames in a desired direction. While barrier walls can result in increased overpressures and radiative heat flux in the vicinity of the wall, they can also attenuate the effects of these hazards in surrounding areas if properly implemented.     

Investigation on combustion mode and heat release in a model scramjet engine affected by shocks

A model scramjet engine in which the 1.0 Ma hydrogen jet mixes and reacts with the 2.0 Ma surrounding airstream is investigated using large eddy simulation. The flame structure is analyzed with a focus on the relationship between premixed/diffusion combustion mode and heat release in the supersonic reacting flow. The flame filter is used to evaluate the contributions to heat release rate by different combustion modes qualitatively and quantitatively. Results show that the heat is released from a combination of premixed combustion mode and diffusion combustion mode even when the fuel and airstream are injected into the combustor separately. Local mode-transition occurs as the supersonic jet flame propagates and interacts with shocks. The diffusion combustion mode dominates during the ignition stage and the premixed combustion becomes dominant during the intensive combustion region. When the shock wave impinges on the flame, the combustion area decreases a little due to the compression effects of the shock. However, the heat release rate is significantly improved in the interaction region since the shock could increase the air entrainment rate by directing the airflow toward the fuel jet and enhance the mixing rate by inducing vorticity due to baroclinic effects, which is good for flame stabilization in the supersonic flow. For the present case, 33.3% of the heat is released by diffusion combustion and 66.7% of the heat is released by premixed combustion. Thus the premixed combustion mode is dominant in terms of its contributions to heat release in the model scramjet engine.