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.     

Predictions of free jet fires from high pressure, sonic releases

A numerical model for predicting jet fires resulting from high pressure, sonic releases of natural gas is described. The model is based on solutions of the density-weighted forms of the fluid flow equations. It is capable of accurately resolving the near-field shock structure that occurs in these flows through the use of a compressibility corrected version of the k-? turbulence model, and also includes sub-models for the flame lift-off height and a prescribed probability density function/laminar flamelet model of the turbulent non-premixed combustion process. Radiation heat transfer is described using an adaptive version of the discrete transfer method, with solutions of the radiation heat transfer equation obtained using a statistical narrow band approach. The complete model is demonstrated to yield plausible predictions of the structure of both the near-field non-reacting and subsonic combusting zones within wind blown fires, and to provide realistic predictions of flame lift-off heights, mean temperatures, trajectories and the radiation fluxes received about a number of field-scale jet fires.     

Statistical modeling of thermal radiation transfer in buoyant turbulent diffusion flames

A statistical (Monte Carlo) method for radiative heat transfer has been incorporated in CFD modeling of buoyant turbulent diffusion flames in stagnant air and in a cross-wind. The model and the computational tool have been developed and applied to simulate both burner flames with controlled fuel supply rate and in self-sustained pool fires with burning rates coupled with flame radiation. The gas–soot mixture was treated either as gray (using the effective absorption coefficient derived from total emissivity data or the Planck mean absorption coefficient) or as non-gray (using the weighed sum of gray gases model). The comparison of predicted radiative heat fluxes indicates applicability of the gray media assumption in modeling of thermal radiation in case of high soot content. The effect of turbulence-radiation interaction is approximately taken into account in calculation of radiation emission, which is corrected to allow for temperature self-correlation and absorption-temperature correlation. In modeling buoyant propane flames in still air above 0.3 m diameter burner, extensive comparison is presented of the predictions with the measurements of gas species concentrations, temperature, velocity and their turbulent fluctuations, and radiative heat fluxes obtained in flames with different heat release rates. Similar to previously published experimental data, the predicted burning rate of flames above the acetone pools exposed to flame radiation increases with the pool diameter and approaches a constant level for large pool sizes. The magnitude of predicted burning rates is shown to be in agreement with the reported measurements. Augmentation of burning rate of the pool fire in a cross-wind because of increased net radiative heat flux received by the fuel surface and non-monotonic dependence of burning rate on cross-wind velocity, subject to the pool diameter, is predicted. The statistical treatment of thermal radiation transfer has been found to be robust and computationally efficient.     

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.     

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.     

A theoretical framework for calculating full-scale jet fires induced by high-pressure hydrogen/natural gas transient leakage

Previous experimental results on full-scale jet fires induced by high-pressure hydrogen/natural gas transient leakage can only be suitable for solving practical engineering problems, or testing the limitation of previous models. Thus, this paper presents a theoretical framework for the high-pressure hydrogen/natural gas leakage and the subsequent jet fire. The proposed framework consists of a transient leakage model, a notional nozzle model, a jet flame size model, a radiative fraction correlation and a line source radiation model. The framework is validated by comparing the model predictions and experimental measurements of mass flow rate, total flame height and thermal radiation field of hydrogen, natural gas, hydrogen/natural gas mixture jet fires with a flame height up to 100 m. The comparison shows that the theoretical framework can give considerable predictions to properties of full-scale jet fires induced by high-pressure hydrogen/natural gas transient leakage.     

Prediction of state property,flow parameter and jet flame size during transient releases from hydrogen storage systems

The accidental leakage of high-pressure gas storage systems including tank, pipe, etc. can lead to hazardous jet fires resulting in a serious of disastrous events. With the isentropic process assumption on the high-pressure gas leakage or release, the ideal gas equation of state is firstly used to solve the gas transfer problem, and then the Abel-Noble equation of state (AN-EOS) is adopted for the effect of gas molecule volume. Given both the molecule volume and intermolecular attraction should not be ignored for the high-pressure gas, this paper attempts to build the high-pressure gas leakage process model based on the van der Waals equation of state. Together with the available notional nozzle model and the flame size model, the gas leakage process model is used to calculate the gas state property and flow parameter of hydrogen tank leakage and its subsequent jet flame height. The predicted gas mass flow rate, flame height, and gas pressure and temperature are compared to the experimental measurements for validation and the predictions of the model based on ideal gas equation of state and AN-EOS. It is found that the proposed model can give more encouraging results compared to the previous models. The proposed theoretical model shows a great implication for the calculation of other gas tank leakage and can help to predict the thermal radiation field of jet fires.     

Modelling of species in hood fires by conditional moment closure

Carbon monoxide, the chief killer in fires, and other species are modelled for a series of enclosure fires. The conditions emulate building fires where CO is formed in the rich, turbulent, nonpremixed flame and is transported frozen to lean mixtures by the ceiling jet which is cooled by radiation and dilution. Conditional moment closure modelling is used and computational domain minimisation criteria are developed which reduce the computational cost of this method. The predictions give good agreement for CO and other species in the lean, quenched-gas stream, holding promise that this method may provide a practical means of modelling real, three-dimensional fire situations.     

A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH₄) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H₂–air and CH₄–H₂–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NO_x emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NO_x emissions in the CH₄ flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH₄ flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH₄ flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.     

The response of buoyant laminar diffusion flames to low-frequency forcing

Buoyant jet diffusion flames are frequently used to investigate phenomena associated with flares or fires, such as the formation and emission of soot, polycyclic aromatic hydrocarbons (PAH), and carbon monoxide (CO). To systematically investigate the influence of transient vortex-flame interactions on these processes, laminar jet flames may be periodically forced. Previous work has demonstrated that forcing the fuel stream at a (low) frequency close to the natural buoyant instability frequency will trigger the production of vortices on the air side of the high-temperature reaction zone, coupling the overall flame response to the forcing frequency. In the work reported here, measurements in methane/air and ethylene/air slot flames show that over a substantial range of forcing frequencies and amplitudes, the dominant, air-side vortex production is locked at precisely one-half the excitation frequency of the fuel stream. This phenomenon is examined in detail through the utilization of several laser diagnostic techniques, yielding measurements of both the frequency response of the flames and phase-locked images of the internal flame structure. Under some conditions the subharmonic response of the flame leads to transient separation of the PAH and soot layers from the surrounding high-temperature flame zone, potentially affecting the soot formation and radiation processes. This data should provide useful information for comparison with detailed modeling aimed to improve the understanding of the complex nature of the buoyant instability in jet flames.     

Modelling lifted hydrogen jet fires using the boundary layer equations

The focus of this paper is the simulation of lifted hydrogen jet fires. The computational modelling of rim-stabilised fires is mature. This is not the case for lifted jets where computational studies have focussed on understanding the mechanism for the location of the combusting flame base and good agreement between predicted and measured flow properties is not universal.The simulation of hydrogen jet fires is an interesting and current area of research due to the sensitivity to global warming and the potential to address this problem with “The Hydrogen Economy” concept. To utilise hydrogen successfully requires the development of robust and accurate models to investigate new techniques for assessing the safety of operations involving hydrogen to ensure the inherent hazards of using hydrogen do not negate the benefit of reduced CO₂ emissions.The paper presents preliminary findings of an investigation into modelling lifted hydrogen jet fires using the boundary layer equations. The use of the boundary layer equations means that any model calibration is rigorous confident that the numerical error is negligible. A number of lift-off models based on the flamelet quenching concept and the turbulence time-scale are implemented and evaluated using available experimental data taken from the open literature. The lift-off models based on flamelet quenching use multi-flamelet libraries for the state relationships used to calculate the mean flow properties. Initial results suggest that a lift-off model based on the flamelet quenching concept incorporating the small-scale strain rate gives reasonable agreement with the measured lift-off compared to the other modelling approaches considered. This is an interesting result as it contrasts with earlier studies of methane and propane lifted jets, where the large-scale strain rate gave better agreement between observation and theory. This would suggest the appropriate strain rate model for a particular fuel and jet could be related to the residence time of the jet or the bulk strain rate.     

Experimental study of the emissivity of flames resulting from the combustion of forest fuels

Heat transfer by radiation is taken into account in most models that predict the propagation of forest fires. This heat transfer mechanism is normally formulated according to the Stefan–Boltzmann law in terms of flame temperature and flame emissivity. This study focused on flame emissivity. Experimental studies carried out to compute the emissivity of the flames generated during the combustion of forest fuels were reviewed, thereby highlighting differences in methodologies and results. Since the results of these studies with regard to the exponential relationship between flame emissivity and flame thickness were not in agreement, two methods based on IR imagery were used in the present study to calculate flame emissivity values. Nine circular fuel beds with a diameter of 0.3–2.5 m were prepared with common Mediterranean species and burned as stationary fires. An exponential correlation between flame emissivity and flame thickness was observed for both methods. According to the results of this study, only flames thicker than 3.2 m would exhibit an emissivity close to that of a blackbody (0.9), and the associated extinction coefficient would be 0.72. A long-term retardant product was used to treat the fuel of two of the nine tests that were carried out and no effect on flame emissivity was observed.     

Spray–turbulence–chemistry interactions under engine-like conditions

Turbulent multiphase combustion plays an important role in both nature (e.g., volcanos and pool fires) and industry (e.g., industrial furnaces, aeroengines, and internal combustion engines). It is a highly complex multiscale and multi-physicochemical process in which interactions between the dispersed and continuous phases, phase change, droplet collisions, evaporation, mixing, heat transfer, and chemical reactions occur simultaneously. In recent years, significant progress has been made in understanding the mechanisms of spray flames and their behaviors in combustion engines. This paper covers key and representative developments in the area of turbulent spray combustion with a focus on spray–chemistry–turbulence interactions. The effects of turbulence–chemistry, spray–turbulence, and spray–chemistry interactions on the spray process, ignition, flame stabilization and emission are comprehensively discussed at elevated pressures and temperatures. Furthermore, spray–radiation and spray flame–wall interactions, which are important to engine performance and emission characteristics, are scrutinized. Supercritical spray flames and turbulent spray flames in dual-fuel engines are also discussed. Finally, outlooks and further challenges for the research field are outlined.     

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.     

Radiation intensity imaging measurements of methane and dimethyl ether turbulent nonpremixed and partially premixed jet flames

Quantitative time-dependent images of the infrared radiation intensity from methane and dimethyl ether (DME) turbulent nonpremixed and partially premixed jet flames are measured and discussed in this work. The fuel compositions (CH₄/H₂/N₂, C₂H₆O/H₂/N₂, CH₄/air, and C₂H₆O/air) and Reynolds numbers (15,200–46,250) for the flames were selected following the guidelines of the International Workshop on Measurement and Computation of Turbulent Nonpremixed Flames (TNF Workshop). The images of the radiation intensity are acquired using a calibrated high speed infrared camera and three band-pass filters. The band-pass filters enable measurements of radiation from water vapor and carbon dioxide over the entire flame length and beyond. The images reveal localized regions of high and low intensity characteristic of turbulent flames. The peak mean radiation intensity is approximately 15% larger for the DME nonpremixed flames and 30% larger for the DME partially premixed flames in comparison to the corresponding methane flames. The trends are explained by a combination of higher temperatures and longer stoichiometric flame lengths for the DME flames. The longer flame lengths are attributed to the higher density of the DME fuel mixtures based on existing flame length scaling relationships. The longer flame lengths result in larger volumes of high temperature gas and correspondingly higher path-integrated radiation intensities near and downstream of the stoichiometric flame length. The radiation intensity measurements acquired with the infrared camera agree with existing spectroscopy measurements demonstrating the quantitative nature of the present imaging technique. The images provide new benchmark data of turbulent nonpremixed and partially premixed jet flames. The images can be compared with results of large eddy simulations rendered in the form of quantitative images of the infrared radiation intensity. Such comparisons are expected to support the evaluation of models used in turbulent combustion and radiation simulations.     

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.     

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.     

A consideration of methods of determining the radiative characteristics of jet fires

The radiative characteristics of jet fires is usually expressed through the use of a fraction of heat radiated, which is primarily a property of the fuel being considered. It is generally determined from experimental data of incident radiation around a fire and then derived by using a model of the incident radiation in terms of the fraction of heat radiated. Popular approaches include the single point source model where the flame is represented by a single point usually located halfway along the flame, or use of an idealised flame shape, such as a cylinder or cone, and deriving the flame surface emissive power which is closely related to the fraction of heat radiated. However, these modelling approaches may provide erroneous results for the fraction of heat radiated if incident radiation data in the near-field is used, and the fraction of heat radiated derived using one modelling approach may not be applicable to another approach without some adjustment. This paper explores the inherent near-field and far-field behaviour of different modelling approaches and the resulting impact on the fraction of heat radiated derived from each modelling approach using incident radiation data. A weighted multi-point source approach model was found to replicate both near-field and far-field behaviour well and capable of deriving the true fraction of heat radiated. Four idealised shapes were considered and it was found that the true fraction of heat radiated would need to be adjusted for use with these models even in the far-field, and some shortcomings in near-field behaviour were identified, which would suggest that some weighting of the surface emissive power over different regions of the flame would be needed. Finally, an idealised shape with hemispherical point sources distributed over its surface was considered and this model behaved well in both the near-field and far-field.     

中心回燃式燃油锅炉燃烧室实验研究

中心回燃式锅炉自引进以来一直没有一种理想的燃烧室传热计算方法，本文根据中心回燃式锅炉燃烧室内射流火焰与回流烟气之间存在着强烈的掺混，认为燃烧室传热计算中不应忽略对流换热。提出了适合中心回燃式锅炉燃烧室传热的计算方法，并通过实验加以验证。