Hydrogen jet flames

A critical review and rethinking of hydrogen jet flame research is carried out. Froude number only based correlations are shown to be deficient for under-expanded jet fires. The novel dimensionless flame length correlation is developed accounting for effects of Froude, Reynolds, and Mach numbers. The correlation is validated for pressures 0.1–90.0 MPa, temperatures 80–300 K, and leak diameters 0.4–51.7 mm. Three distinct jet flame regimes are identified: traditional buoyancy-controlled, momentum-dominated “plateau” for expanded jets, and momentum-dominated “slope” for under-expanded jets. The statement “calculated flame length may be obtained by substitution the concentration corresponding to the stoichiometric mixture in equation of axial concentration decay for non-reacting jet” is shown to be incorrect. The correct average value for non-premixed turbulent flames is 11% by volume of hydrogen in air (range 8%–16%) not stoichiometric 29.5%. All three conservative separation distances for jet fire are shown to be longer than separation distance for non-reacting jet.     

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 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.     

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.     

Chemical effects on molecular species concentrations in turbulent fires

The measurement and modeling of molecular species concentrations in turbulent pool and buoyant jet flames is described. The experimental parameters included burner diameter (2.8 mm jet nozzle, 190, 381, and 762 mm pools), theoretical combustion heat release rate (10–283 kW), lip size (0–25 mm), and fuel (CH₄, C₃H₈). Time-averaged species concentrations were obtained through axial and radial sampling probe traverses. A novel sampling probe was developed which provides a constant mass flow of flame gases that is not biased toward either hot or cold gas eddies.Local concentrations of major gas species (fuel, O₂, CO, CO₂, H₂O, N₂) in the fire are correlated by the mixture fraction, which is the fraction of atomic species present which originated in the supplied fuel. The correlation appears to be independent of pool diameter, lip size, and heat release rate. These turbulent correlations differ from the corresponding curves for laminar flames primarily due to composition broadening resulting from time average measurements of widely fluctuating components. We obtained higher than expected concentrations of CO and CO₂ in centerline measurements near the fuel source. An attempt is made ot explain these findings based on non-equal species diffusivity and local radiative extinction.The correlations obtained in this work form the basis for two closely related models: (1) for predicting mean species concentrations in turbulent flames by weighting laminar data with an assumed pdf of the mixture fraction, and (2) the chemical scaling of turbulent pool fires using Froude modeling principles. These applications are briefly discussed.     

Mathematical modeling of agricultural fires beneath high voltage transmission lines

This paper presents a mathematical model for agricultural fires based on a multi-phase formulation. The model includes dehydration and pyrolysis of agricultural fuel and pyrolysis products. The model considers a homogeneous distribution of the agricultural solid fuel particles, interacting with the gas flow via source terms. These terms include: drag forces, production of water vapour and pyrolysis products, radiative and convective heat exchange. A multi-phase radiative transfer equation for absorbing-emitting medium is considered to account for the radiative heat exchange between the gas and solid phases of the fire. The main outputs of the present model are most important to study the influence of agricultural fire occurring beneath high voltage transmission lines. The agricultural fire causes a flashover due to the ambient temperature rise and soot accumulation on the insulator of these transmission lines. Numerical results of the present model are obtained for flat grassland fires to study the effects of wind velocity, solid fuel moisture content and ignition length on some selected fire outputs. These outputs include the temperature, velocity, soot volume fraction fields of the gas phase, together with fire propagation rate and flame geometry. The numerical results are compared to the available experimental work in the literature.     

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.     

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.     

Pool fire dynamics: Principles,models and recent advances

Pool fire is generally described as a diffusion combustion process that occurs above a horizontal fuel surface (composed of gaseous or volatile condensed fuel) with low (∼zero) initial momentum. Fundamentally, this type of diffusion combustion can be represented by basic forms ranging from a small laminar candle flame, to a turbulent medium-scale sofa fire, and up a storage tank fire, or even a massive forest fire. Pool fire research thus not only has fundamental scientific significance for the study of classical diffusion combustion, but also plays an important role in practical fire safety engineering. Therefore, pool fire is recognized as one of the canonical configurations in both the combustion and fire science communities. Pool fire research involves a rich, multilateral, and bidirectional coupling of fluid mechanics with scalar transport, combustion, and heat transfer. Because of the unabated large-scale disasters that can occur and the numerous and complex 'unknowns' involved in pool fires, several new questions have been raised with accompanying solutions and old questions have been revisited, particularly in recent decades. Significant developments have occurred from a variety of different perspectives in terms of pool fire dynamics, and thus the scientific progress made must be summarized in a systematic manner. This paper provides a comprehensive review of the basic fundamentals of pool fires, including the scale effect, the wind effect, pressure and gravity effects, and multi-pool fire dynamics, with particular focus on recent advances in this century. As the fundamentals of pool fires, the theoretical progress made with regard to burning rates, air entrainment, flame pulsation, the morphological characteristics of flames, radiation, and the dimensional modelling are reviewed first, followed by new insights into the fluid mechanics involved, radiative heat transfer and combustion modeling. With regard to the scale effect, recent experimental and theoretical advances in internal thermal transport and fluid motions within the liquid-phase fuel, lip height effects, and heat transfer blockage are summarized systematically. Furthermore, new understandings of aspects including heat feedback and the burning rate, flame tilt, flame length and instability, flame sag and base drag, and soot and radiation behavior under wind, pressure and gravity effects are reviewed. The growing research into the onset and the merging dynamics of multiple pool fires in the last decade is described in the last section, this research will be helpful in the mitigation of threatening outdoor massive (group) fires. This review provides a state-of-the-art survey of the knowledge gained through decades of research into this topic, and concludes by discussing the challenges and prospects with regard to the complex coupling effects of heat transfer, with the fluid and combustion mechanics of pool fires in future work.     

Lengths of laminar jet diffusion flames under elevated gravity

There are two prevalent scaling relationships for lengths of laminar jet diffusion flames on circular burners. Experimental studies of earth-gravity and microgravity flames generally invoke a linear relationship between normalized flame length and Reynolds number. In contrast, most studies conducted at elevated gravity have correlated flame lengths with a function of Reynolds and Froude numbers. An important distinction between these scalings is that the Reynolds scaling indicates that stoichiometric flame length is independent of gravity level, whereas the Reynolds–Froude scaling indicates that length decreases with increased gravity. The present work examines the ability of both approaches to correlate laminar hydrogen, methane, ethane, and propane flame lengths for a range of 1–15 times earth gravity. The Reynolds scaling is shown to accurately correlate the length measurements at both earth gravity and elevated gravity. The Reynolds–Froude scaling also correlates the measurements, but its theoretical basis is less rigorous, it does not account as accurately for variations in fuel flowrate, it does not admit microgravity flames, and past predictions of its behavior at low and high Froude number are not supported even with the present extension of Froude number to over eight orders of magnitude. It is shown that the observed reduction in luminosity length at elevated gravity can be attributed to soot interference and that stoichiometric flame length is independent of gravity except in the approach to microgravity.     

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.     

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.     

Investigation of soot transport and radiative heat transfer in an ethylene jet diffusion flame

Numerical and experimental investigations highlighting the heat and mass transfer phenomena in a laminar co-flowing jet diffusion flame have been carried out. The fuel under consideration is ethylene, with ambient air as the co-flowing oxidizer. The diffusion flame is modeled using a 17-step reduced reaction mechanism with finite rate chemistry and the effects of soot on the radiative heat transfer of the flame have been demonstrated. Soot growth and oxidation processes are studied using a two-equation transport model, while the radiative heat transfer is modeled using the P1 approximation. An in-house finite volume code has been developed to solve the axi-symmetric Navier–Stokes equations in cylindrical coordinates, along with the soot mass fraction, soot number density, energy and species conservation equations. Comparison of predictions with experimental results shows reasonable agreement with regard to the flame height and temperature distribution. A parametric study is also presented, which illustrates the effects of the fuel jet Reynolds number and the flow rate of co-flow air.     

A method of determining flame radiation fraction induced by interaction burning of tri-symmetric propane fires in open space based on weighted multi-point source model

The interaction of multiple fires may lead to a higher flame height and more intense radiation flux than a single fire, which increases the possibility of flame spread and risks to the surroundings. Experiments were conducted using three burners with identical heat release rates (HRRs) and propane as the fuel at various spacings. The results show that flames change from non-merging to merging as the spacing decreases, which result in a complex evolution of flame height and merging point height. To facilitate the analysis, a novel merging criterion based on the dimensionless spacing S/z_c was proposed. For non-merging flames (S/z_c >0.368), the flame height is almost identical to a single fire; for merging flames (S/z_c ≤0.368), based on the relationship between thermal buoyancy B and thrust P (the pressure difference between the inside and outside of the flame), a quantitative analysis of the flame height, merging point height, and air entrainment was formed, and the calculated merging flame heights show a good agreement with the measured experimental values. Moreover, the multi-point source model was further improved, and radiation fraction of propane was calculated. The data obtained in this study would play an important role in calculating the external radiation of propane fire.     

Theoretical analysis on flame dimension in turbulent ceiling fires

In this paper, theoretical analysis based on boundary layer theory on flame dimension in turbulent ceiling fires is carried out. The turbulent ceiling fire is developed from a downward round injection source beneath an unconfined inert ceiling. A correlation between the dimensionless flame diameter and the dimensionless heat release rate is obtained, C₁(S/d) ∼ C₂Q^13/4. This relation for turbulent ceiling fires is correlated to experimentally measured flame diameters for ceiling fires. Based on the limited data available, the agreement with experiments is very good. Additional experiments are needed to further verify the validity of this relation.     

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.     

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.     

Experimental study on spark ignition of non-premixed hydrogen jets

The leaks of pressurized hydrogen can be ignited if an ignition source is within a certain distance from the source of the leaks, and jet fires or explosions may take place. In this paper, a high speed camera was used to investigate the ignition kernel development, ignition probability and flame propagation along the axis of hydrogen jets, which leaked from a 3-mm-internal-diameter nozzle and were ignited by an electric spark. Experimental results indicate that for successful ignition events, the ignition delay time increases with an increase of the distance between the nozzle and the electrode. Ignitable zone of the hydrogen jets is underestimated if using the predicted hydrogen concentration along the jets centerline. The average rate of downstream flame decreases but that of the upstream flame increases with the electrode going far from the nozzle.     

Predicting the main geometrical features of horizontal rectangular source fuel jet fires

The main geometrical features of horizontal jet fire with rectangular source fuel have seldom been revealed in the past, especially the rectangular orifice with same area but different aspect ratios. In order to better understand the rectangular jet fire, a set of numerical simulations were carried out by rectangular source fuel with same rectangular orifice area S (4 cm²) but different aspect ratios (orifice length to orifice width: L/W = 1, 2, 4) to investigate the flame shape, flame length and flame width. The simulated flame lengths and flame widths were compared with previous experimental data and calculated values using the Thornton model. The non-dimensional flame length and flame width were defined, in which the flame geometrical features were found in relation to the orifice aspect ratio and fuel jet velocity. Results show that the flame length and flame width increases with fuel jet velocity, while the flame length decreases with aspect ratio n for same orifice area, but the flame width increases simultaneously. The simulated data agree well with previous experimental data, but the predictions by Thornton model are larger than simulated and previous experimental values. The modified Thornton model is proposed considering both orifice shape and aspect ratio to apply to rectangular jet fire.     

A re-examination of entrainment constant and an explicit model for flame heights of rectangular jet fires

Flame heights of buoyant turbulent jet fires produced by rectangular nozzles whose aspect ratio varied from 1:1 to 1:71 are investigated experimentally in this work. The change of the entrainment constant parameter C₁ with aspect ratio is discussed based on the comprehensive data obtained. It is found the value of C₁ does not need to be transformed from 0.179 to 0.444 with an increase in aspect ratio from axisymmetric one to linear one as proposed previously in the classic correlation due to limited data, a change which might be misleading. It is revealed to in fact change little with rectangular fire source aspect ratio and can be constantly taken as 0.185. A new explicit model to predict flame heights for given heat release rates of rectangular jet fires is then proposed, which is shown to be in good agreement with the measured values for different source aspect ratios.