Integrated Heat Release Research Articles

Investigation of Ammonia/Hydrogen Mixtures and Pilot-Split Strategies in a Laboratory-Scale Radial Swirl Combustor

Abstract The transition to a decarbonized energy future relies on identifying the most suitable alternative fuels that can meet the needs of various energy sectors. While both ammonia and hydrogen are zero-carbon energy vectors, their physical properties and burning characteristics sit on either side of that of natural gas. Hence, mixtures of ammonia and hydrogen are being increasingly looked at as having the potential to fuel current energy systems without requiring significant combustor redesign. However, the combustion characteristics and operation limits for different ammonia/hydrogen mixtures still need to be evaluated. For gas turbine applications in particular, the effect of ammonia/hydrogen mixture composition and operating condition on flame behavior and stability is not well understood. The current work was carried out in a laboratory scale, radial swirl-stabilized turbulent combustor. A systematic study of two ammonia/hydrogen blend ratios (70:30 and 80:20 by volume) and a range of equivalence ratios were tested for different pilot-split ratios, to understand the effect on flame shape, stability and dynamics. Time-resolved pressure and integrated heat release fluctuations were measured to evaluate combustor dynamics, and NH2* chemiluminescence flame images were captured to understand spatial differences in flame structure. When comparing blend ratios, differences were observed in flame macrostructures and combustor dynamics, which could be largely attributed to the considerable difference in the laminar flame speeds of the blends. The addition of pilot generally improved the stability and lean operation for both blend ratios.

Flame-acoustic interactions in high-pressure H2/air diffusion flames

Understanding flame-acoustics interaction (FAI) that can potentially trigger thermoacoustic instabilities is of critical importance to mitigate the serious side effects of pressure oscillations. While FAI has been studied extensively in premixed flames, comparatively less analysis has been conducted to explore FAI mechanisms in non-premixed flames, especially under high-pressure conditions. This study aims to numerically investigate FAI in high-pressure hydrogen/air counterflow diffusion flames, with special attention on the impact of pressure on FAI. To this end, a fully compressible unsteady counterflow solver combined with Navier-Stokes Characteristic Boundary Conditions (NSCBC) is employed to capture the reflection of the acoustic waves at the boundaries. The results show that for all ambient pressures ( 1 ≤ P a ≤ 60 bar) considered here, real-gas effects on flame structures are negligible and that increasing P a would lead to a power-law dependence of density gradient on P a with an exponent of 1.5. Furthermore, all tested cases show acoustic growth with time under fully reflecting boundary conditions, which is found to be pressure dependent. The flames become less resilient to FAI when the pressure (or density gradient) is increased. The results from the spectral analysis show that for all ambient pressures, there is only a single dominant frequency at the low-density fuel side and two dominant frequencies at the high-density oxidiser side. This indicates that the pressure oscillation is more sinusoidal at the H 2 side than that at the air side. Moreover, the phase space portraits of pressure signals indicate that the dynamics of the system are becoming periodic and approaching a limit cycle. It is found that the integrated heat release rate and the pressure fluctuations are partially in phase, which is responsible for the growth of high-frequency acoustic waves at both fuel and oxidiser sides. On the other hand, the significant density gradient contributes to the growth of low-frequency acoustic waves at the oxidiser side due to the high degree of acoustic reflectivity at the density boundary.

A NUMERICAL STUDY ON THE REDUCTION OF GREENHOUSE GASEOUS COMPONENT (CO2) DUE TO THE ADDITION OF H2 IN THE FUEL STREAM OF THE COUNTERFLOW CH4/AIR DIFFUSION FLAME

In this study, a series of 1-D and steady-state numerical simulations have been performed for the prediction of the effect of the addition of H2 on the characteristics of a non-sooting counterflow CH4/Air diffusion flame using detailed chemical reaction model, which is composed of 325 elementary chemical reactions and 53 chemical species. Under the steady-state assumption, a set of one-dimensional transport equations of mass, momentum, species, and energy along with the equation of state has been solved numerically at the atmospheric conditions over the counterflow configuration by exploiting an efficient numerical code, OPPDIF (a Fortran Program for Computing Opposed-Flow Diffusion Flames). The grid adaption technique has been used to achieve better convergence as well as to ensure the maximum accuracy of the simulated results. It is found that the flame temperature is increased due to the addition of H2 with CH4, which is injected into the fuel stream. The elevation in the temperature is caused by the augmentation of the integrated heat release rate of the elementary reactions supported by the active radicals (H, O, and OH), which are generated by the higher reactivity of H2. Besides, it is found that the mole fractions of H2O are increased as the percentage of H2 in the loading fuel (CH4) is increased and also, it is identified that the chain propagating reaction, OH + H2 => H2O + H is dominating one which produces highest amount of H2O. Furthermore, it is noticed that the indirect greenhouse gas or precursor, CO is reduced when H2 is added to CH4. Consequently, the mole fraction of the principle greenhouse gas, CO2 is decreased significantly when the fuel, CH4 percentage is modified by the higher percentage of H2. The sensitivity analysis of elementary reactions reveals the fact that the chemical reaction: OH + CO => H + CO2 is a dominating reaction in producing a lower amount of CO2 when the volume fraction of H2 is increased in the fuel (CH4) stream. In the presence of 75 % H2 in CH4, the pressure-dependent reaction, O + CO (+M) => CO2 (+M) appears as another chemical route that also generates greenhouse gas, CO2 but its contribution is negligibly small.

Impact of the injector lateral offset on the dynamics of a lean premixed flame and the thermoacoustic stability of a burner

The response of a laminar lean premixed flame to excitations based on its position in the combustion chamber and the stability of a burner are numerically investigated. A finite-volume large-eddy simulation method is used to solve the compressible Navier-Stokes equations and a combined G-equation progress variable approach is used to model the flame. Various injector positions in the combustion chamber are investigated in a computational setup with an acoustically non-reflecting outflow boundary condition to analyze the impact on the response of the flame to an external excitation. Due to the changes in the flow field the shape of the flames depends on the displacement offset of the injector. Consequently, the instantaneous, local distribution of the heat release rate fluctuations which is caused by wrinkles on the flame surface is determined by the position of the injector. The heat release rate fluctuations are the dominant source of sound in the investigated configurations. Due to the discrepancies in the local heat release rate fluctuations the flame response to an excitation depends on the lateral offset of the injector. The overall trend of the integrated heat release is the same in most configurations, however, the phase is significantly altered. Since the phase angle of the response of the flame to an excitation determines the stability of a burner, self-excited instabilities can be avoided by adjusting the position of the injector. This is demonstrated using a modified computational domain with an acoustically reflecting outflow boundary condition, which causes the burner to have an acoustic quarter-wave eigenmode. Based on the results of the previous analysis of the flame responses, two injector positions in the combustion chamber are chosen such that the response of the flames is mutually phase-shifted by approximately π. Therefore, it is expected that one of the configurations will lead to a stable burner while the other one exhibits a self-excited instability. The results show that the injector position determines the stability of the burner configuration for the investigated flames.

Thermoacoustic response of fully compressible counterflow diffusion flames to acoustic perturbations

The goal of this research is to study the thermoacoustic response of diffusion flames due to their relevance in applications such as rocket engines. An in-house code is extended to solve the fully compressible counterflow diffusion flame equations, allowing for a spatially- and temporally-varying pressure field. Various hydrogen-air flames with a range of strain rates are simulated using detailed chemistry. After introducing sinusoidal pressure perturbations at the inlet, the gain and phase of various quantities of interest are extracted. As the frequency is increased, the gain of the temperature source term transitions from the perturbed steady flamelet value to a first plateau at intermediate frequencies, and finally to a second plateau at the highest frequencies. At these high frequencies, the gain of the integrated heat release decays to zero, underscoring the importance of compressibility. These three regimes can be identified and explained through a linearization and frequency domain analysis of the governing equations. The validity of the low Mach number assumption and importance of detailed chemistry are assessed.

Development and validation study of a 1D analytical model for the response of reheat flames to entropy waves

Numerical simulations of laminar premixed flames burning hydrogen and methane in spontaneous ignition mode are performed by harmonically exciting the reactants’ temperature at the domain inlet. The results are compared to an analytical model representing the same reactive flow configuration. The model provides a simplified but nevertheless accurate representation of reheat combustion taking place in sequential gas turbine combustors. An analytic expression for autoignition flames transfer functions to entropy waves is derived and used to extend transfer function models from the literature. For validation purposes, results from fully compressible Direct Numerical Simulations (DNS), including a complete representation of the fluctuating acoustic and entropic fields of the reactive flow, are analyzed and compared to incompressible Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations that only take into account the fluctuating entropic field. Methane flames are found to be more sensitive to entropic forcing than hydrogen flames, featuring nonlinear phenomena even for low excitation amplitudes. In the linear regime, all flames behave as predicted by the analytical model and the URANS simulations are found to correctly predict the fluctuating entropic field. The transition from linear to nonlinear flame response is described in detail and its physical mechanisms are explained. Comparisons with results available in the literature show good prediction capabilities, both in terms of flame describing function and integrated heat release rate. Limitations of the proposed analytical model with respect to real combustion systems are discussed and a simple correction is proposed.

Modelling Heat Loss Effects in the Large Eddy Simulation of a Lean Swirl-Stabilised Flame

The flame in a gas turbine model combustor close to blow-off is studied using large eddy simulation with the objective of investigating the sensitivity of including different heat loss effects within the modelling. A presumed joint probability density function approach based on the mixture fraction and progress variable with unstrained flamelets is used. The normalised enthalpy is included in the probability density function to account for heat loss within the flame. Two simulations are presented that use fixed temperature boundary conditions, and use adiabatic and non-adiabatic formulations of the combustion model. The results are compared against the previous fully adiabatic case and experimental data. The statistics for the simulations are similar to the results obtained from the fully adiabatic case. Improved statistics are obtained for the temperature in the near-wall regions. The non-adiabatic flamelet case shows the average reaction rate values at the flame root are approximately 50% smaller in comparison to the adiabatic flamelet cases. This causes the lift-off height to be overestimated. The time series of the lift-off height and the volume integrated heat release rate show that including non-adiabatic flamelets causes the flame to be highly unstable. A higher enthalpy deficit is seen in the near-field regions when the flame root is not present and experiencing some lift-off, suggesting that the flame is more dynamic when including heat loss.

Crust of accreting neutron stars within simplified reaction network

ABSTRACT Transiently accreting neutron stars in low-mass X-ray binaries are generally believed to be heated up by nuclear reactions in accreted matter during hydrostatic compression. Detailed modelling of these reactions is required for the correct interpretation of observations. In this paper, we construct a simplified reaction network, which can be easily implemented and depends mainly on atomic mass tables as nuclear physics input. We show that it reproduces results of the detailed network by Lau et al. very well, if one applies the same mass model. However, the composition and the heating power are shown to be sensitive to the mass table used and treatment of mass tables boundary, if one applies several of them in one simulation. In particular, the impurity parameter Qimp at density ρ = 2 × 1012 g cm−3 can differ for a factor of few, and even increase with density increase. The profile of integrated heat release shown to be well confined between results by Fantina et al. and Lau et al.

Sensitivities of direct numerical simulations to chemical kinetic uncertainties: spherical flame kernel evolution of a real jet fuel

A systematic computational study is conducted to understand the propagation of uncertain chemical kinetic parameters into turbulent premixed flame simulations. Two snapshots of the cross-section of a spherical flame kernel in homogeneous isotropic turbulence is extracted from a three dimensional direct numerical simulation (DNS) and serve as the initial conditions for a series of subsequent two-dimensional DNS that sample and quantify the impact of chemical kinetic uncertainties on the integrated heat release rates and certain species concentrations. The target configuration features real jet fuel chemistry modeled by a 31-species reduced reaction model. The importance of each reaction is first ranked, by comparing the normalized reaction flux based on a snapshot from the DNS. A two-step process is then taken where initial Monte Carlo screening is performed by perturbing 99 reaction rate constants simultaneously. Following this, a subset of 19 reactions is studied using a larger collection of 2D DNS simulations. As the major quantity of interest, the integrated heat release rate is consistently the most sensitive to reactions R8 (H+O2=O+OH) and R32 (CO+OH=CO2+H). When combined with a consideration of the reaction rate uncertainty, R71 (CH3+OH=CH2*+H2O) consistently impacts the quality of flame prediction most prominently. For the case starting with a strong burning flame, the rate limiting reactions are found to be not significantly different between the turbulent and laminar conditions, although the magnitudes of sensitivity coefficients are amplified for certain reactions under turbulent conditions due to an enlarged thermochemical space. For the case initialized with a near-extinction flame, more upstream and/or competing reactions with R8 and R32 are observed. Another manifestation of the enlarged thermochemical space in turbulent flames is the wider range of influential reactions for integrated mass of CO, in comparison with the laminar condition where the sensitivity is dominated by R32. For H and C2H2, reactions R71, R42 (HCO+O2=CO+HO2) and R40 (HCO+M=CO+H+M) are identified to impact the model predictions for such species as acetylene in the most significant manner.

A diesel/natural gas dual fuel mechanism constructed to reveal combustion and emission characteristics

As natural gas is an ideal alternative fuel with low cost and clean combustion, this study constructs a mechanism of diesel/natural gas dual fuel, couples it with CFD, and then discusses the effects of natural gas replacement rates on engine combustion and emission under medium load conditions. The detailed mechanism of natural gas is simplified by two methods into a reduced mechanism, which are verified to prove that it can accurately predict ignition delay, laminar flame speed, temperature and pressure. The study also combines the reduced mechanism with the 95/5vv diesel mechanism to obtain a dual-fuel mechanism. This dual fuel mechanism is verified through chemical kinetics calculations, to predict the ignition delay, laminar flame speed, important free radicals and components. Furthermore, this mechanism is verified through coupling CFD, to predict that the combustion pressure and integrated heat release agreed with the experimental data. Results show that the dual-fuel mechanism displays its correctness, effectiveness and applicability. Calculations within the range of 0–60% natural gas replacement rates indicate a regular combustion sequence; at 40% replacement rate, NO emissions goes down by 6.8%, and soot emissions decrease by 92.9%. Yet when the replacement rate of natural gas is over 55%, combustion in cylinder deteriorates.

Simulations of flame propagation during the ignition process in an annular multiple-injector combustor

PurposeIgnition process is a critical issue in combustion systems. It is particularly important for reliability and safety prospects of aero-engine. This paper aims to numerically investigate the burner-to-burner propagation during ignition process in a full annular multiple-injector combustor and then validate it by comparing with experimental results.Design/methodology/approachThe annular multiple-injector experimental setup features 16 swirling injectors and two quartz tubes providing optical accesses to high-speed imaging of flames. A Reynolds averaged Navier–Stokes model, adaptive mesh refinement (AMR) and complete San Diego chemistry are used to predict the ignition process.FindingsThe ignition process shows an overall agreement with experiment. The integrated heat release rate of simulation and the integrated light intensity of experiment is also within reasonable agreement. The flow structure and flame propagation dynamics are carefully analyzed. It is found that the flame fronts propagate symmetrically at an early stage and asymmetrically near merging stage. The flame speed slows down before flame merging. Overall, the numerical results show that the present numerical model can reliably predict the flame propagation during the ignition process.Originality/valueThe dedicated AMR method together with detailed chemistry is used for predicting the unsteady ignition procedure in a laboratory-scale annular combustor for the first time. The validation shows satisfying agreements with the experimental investigations. Some details of flow structures are revealed to explain the characteristics of unsteady flame propagations.

Investigation of OH* chemiluminescence and heat release in laminar methane–oxygen co-flow diffusion flames

OH* chemiluminescence is one of the major spontaneous emission in flames, and often applied in combustion diagnostics to indicate flame structure, strain rate, equivalence ratio, heat release rate, etc. In this work, OH* chemiluminescence in the laminar methane–oxygen co-flow diffusion flames was investigated. A high resolution ultra-violet imaging system was used to capture the OH* chemiluminescence images. Numerical simulations of the experimental cases were performed based on OH* chemiluminescence reaction mechanism. The numerical results show good agreement with the experimental measurements. It's found that there are two OH* distribution zones in laminar methane–oxygen co-flow diffusion flames. Analysis on the production pathway of OH* chemiluminescence shows that the reaction H + O + M = OH* + M (R1) is the major formation reaction of OH* chemiluminescence in laminar methane–oxygen diffusion flames. The increase of diluent addition in oxidizer will lead to the dominant OH* production pathway changing from the reaction R1 to the reaction CH + O2 = OH* + CO (R2). The OH* distribution characteristics under different global oxygen-fuel equivalence ratios indicate that OH* chemiluminescence can be employed as an appropriate indicator to characterize the combustion condition. Moreover, the correlation between integrated heat release rate and integrated OH* concentration is derived for the oxygen-deficient flames. The integrated heat release rate can be predicted in terms of integrated OH* concentration, methane flow rate and global oxygen-fuel equivalence ratio.

Analysis of transient blow-out dynamics in a swirl-stabilized combustor using large-eddy simulations

In the present work, the flame structure and dynamics of a swirl-stabilized methane/air flame near and at blow-out conditions are investigated using large-eddy simulations (LES). To this end, simulations at stable conditions and during a transient blow-out sequence are performed employing a flamelet/progress variable (FPV) model and a thickened flame (TFLES) approach with finite-rate chemistry. Good agreement is obtained for velocities, OH mass fraction profiles, and statistics between LES and measurements at conditions near blow-out. Lift-off height comparisons reveal that the TFLES model predicts a significantly less stable flame compared with the FPV model, which is consistent with the stability limits observed for the two models. Subsequently, transient blow-out simulations are performed and examined. This analysis is guided by employing early warning indicators from dynamical system analysis to identify critical transition points and precursors that trigger the onset of blow-out. It is found that the variance of the integrated heat release is a sensible quantity as an early warning signal in detecting blow-out. Detailed comparisons are then performed between the two models to examine the difference in terms of the blow-out mechanism. The comparison reveals that the different phases of the transient blow-out sequence are qualitatively similar for the two models. However, local extinction phenomena are more significant for the TFLES model, reducing the stabilizing feedback by hot combustion products and leading to a total blow-off time, which is three times shorter compared to FPV.

LES of Nonlinear Saturation in Forced Turbulent Premixed Flames

The mechanisms for nonlinear saturation of a bluff-body stabilised turbulent premixed flame are investigated using LES with the transported flame surface density (TFSD) approach to combustion modelling. The numerical simulation is based on a previous detailed experimental investigation. Results for both the unforced non-reacting and reacting flows are validated against experiment, demonstrating that the fundamental flow features and predicted flame structure are well captured. Key terms in the FSD transport equation are then used to describe the generation and destruction of flame surface area for the unforced reacting flow. In order to investigate the non-linear response of the unsteady heat release rate to acoustic forcing, four harmonically forced flames are considered having the same forcing frequency (160 Hz) but different amplitudes of 10 %, 25 %, 50 % and 64 % of the mean inlet velocity. The flame response is characterised using the Flame Describing Function (FDF). Accurate prediction of the FDF is obtained using the current approach. The computed forced flame structure matches well with the experiment, where effects of shear layer rollup and growth of the vortices on the flame can be clearly observed. Transition to nonlinearity is also observed in the computed FDF. The mechanisms leading to the saturation of the flame response in the higher amplitude case are characterised by inspecting the terms in the FSD transport equation at conditions when the integrated heat release is at its maximum and minimum, respectively. Pinch-off and flame rollup can be seen in snapshots taken at the phase angle of maximum integrated heat release. Conversely, intense vortex shedding and flame-sheet collapse around the shear-layer, as well as small-scale destruction of flame elements in the wake, can be seen in snapshots taken at the phase angle of minimum integrated heat release. The pivotal role of FSD destruction on nonlinear saturation of the flame response is confirmed through the analysis of phase-averaged terms in the FSD transport equation taken at different locations. The phase-averaged subgrid curvature term is found to concentrate in the cusps and downstream regions where flame annihilation is dominant.

Heat release rate estimation in laminar premixed flames using laser-induced fluorescence of CH2O and H-atom

The present work demonstrates the feasibility of heat release rate imaging using the laser-induced fluorescence (LIF) of atomic hydrogen (H-atom) and formaldehyde (CH2O) in laminar premixed flames. The product of H-atom LIF and CH2O LIF signals is evaluated on a pixel-by-pixel basis and is compared with that of the OH × CH2O technique. These results for equivalence ratio ranging from 0.8 to 1.1 are compared with computations of one-dimensional freely-propagating flames. The performance of these markers is studied based on the following two aspects: the spatial accuracy of the local heat release rate and the trend in the total heat release rate with equivalence ratio. The measured trend in the spatial distribution of radicals and the deduced heat release rate agree well with the computational values. The variation in the spatially integrated heat release rate as a function of equivalence ratio is also investigated. The results suggest that the trend in the variation of the integrated heat release rate and the spatial location of heat release rate can be evaluated by either of these markers. The OH-based marker showed certain sensitivity to the chemical mechanism as compared to the H-atom based marker. Both the OH-based and H-atom based techniques provide close estimates of heat release rate. The OH based technique has practical advantage when compared to the H-atom based method, primarily due to the fact that the H-atom LIF is a two-photon process.

Thermoacoustic Instability in a Rijke Tube with a Distributed Heat Source

A Rijke tube with a distributed heat source is investigated. Driven by the widely existing thermoacoustic instability in lean premixed gas turbine combustors, this work aims to explore the physicochemical underpinning and assist in the elucidation and analysis of this problem. The heat release model consists of a row of distributed heat sources with individual heat release rates. The integrated heat release rate is then coupled with the acoustic perturbation for thermoacoustic analysis. A continuation approach is employed to conduct the bifurcation analysis and capture the nonlinear behaviour inherent in the system. Unlike the conventional approach by the Galerkin method, the acoustic equations are originally discretized using the Method of Lines (MOL) to build up a dynamic system. The model is first validated and shown to yield good predictions with available experimental data. Influences of multiple heat sources, time delay, and heat release distribution are then studied to reveal the extensive nonlinear characteristics involved in the case of a distributed heat source. It is found that distributed heat source plays an important role in determining the stability of a thermoacoustic system.

Coordinate Systems and Integration Limits for Global Flame Transfer Function Calculations

This technical note describes the influence of coordinate systems and integration limits on global flame transfer functions (FTF). This work is motivated by recent work to model both the local space-time dynamics of the flame, as well as its globally integrated heat release, in response to flow and mixture property variations [1-9]. The front tracking approach forms the basis of the flame response modeling used here, where the flame position is the zero contour of the implicit function, , whose dynamics are described by [1, 2, 10-12],

Flame Leading Edge and Flow Dynamics in a Swirling, Lifted Flame

Flames in high swirl flow fields with vortex breakdown often stabilize aerodynamically in front of interior flow stagnation points. In contrast to shear layer stabilized flames with a nearly fixed, well-defined flame attachment point, the leading edge of aerodynamically stabilized flames can move around substantially as a result of both the inherent dynamics of the vortex breakdown region and externally imposed oscillations. Motion of this flame stabilization point relative to the flow field may have an important dynamical role during combustion instabilities, as it creates flame front wrinkles and heat release fluctuations. For example, a prior study has shown that nonlinear dynamics of the flame response at high forcing amplitudes were related to these leading edge dynamics. This heat release mechanism exists alongside other flame wrinkling processes, arising from such processes as shear layer rollup and swirl fluctuations. This article describes an experimental investigation of flow forcing effects on the dynamics of the leading edge of a swirl stabilized flame. Flame and flow dynamics were characterized using high-speed particle image velocimetry (PIV) and CH* chemiluminescence imaging. A range of forcing conditions was achieved by varying the forcing frequency, amplitude, and acoustic field symmetry. These results show that the flame leading edge motion is dominated by the natural flow instabilities, particularly the precession of the recirculation zone around the centerline. The fluctuations in leading edge motion at the excitation frequency are much smaller than these natural motions, but are still on the order of the fluid particle displacement associated with the external excitation, indicating that they exhibit an important influence on local flame wrinkling and heat release. Flame response modeling shows that the global, spatially integrated heat release response is controlled by three factors—vortical disturbances, acoustic flow disturbances, and flame leading edge motion. The vortical flow motions dominate the flame response, and the flame leading edge motion is a minor contributor to the overall heat release response. This is an important result, as it shows that the significant motions of the flame leading edge actually have little dynamical significance for understanding the spatially integrated, forced response of the flame.

Propagation, dissipation, and dispersion of disturbances on harmonically forced, non-premixed flames

This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to a similarly excited premixed flame. Bulk axial or transverse excitation, in either case, leads to the excitation of wrinkles on the flame that propagate axially. Inclusion of axial diffusion in the non-premixed case, and burning velocity stretch sensitivity in the premixed case, cause wrinkle dissipation and dispersion. There are important differences in spatially integrated unsteady heat release dynamics between premixed and non-premixed flames. For general Strouhal numbers, mass burning rate fluctuations are the dominant contributor to non-premixed flame heat release fluctuations, while area fluctuations are the dominant contributor to premixed flame heat release fluctuations. Moreover, the heat release response of non-premixed flames rolls off much slower with frequency, O(St−1/2) compared to O(St−1) for premixed flames, and, hence, are more sensitive to flow perturbations than premixed flames at high Strouhal numbers. The asymptotic tendencies of the non-premixed flame, however, are largely controlled by the near burner exit region with high transverse gradients and, thus, are expected to be quite sensitive to burner exit details and finite chemistry effects.

Heat release rate measurements of burning mining vehicles in an underground mine

Heat release rates from two full-scale fire experiments with mining vehicles in an underground mine are presented. The mining vehicles involved were a wheel loader and a drilling rig typical for mining operations. The calculated peak heat release rate of the loader was 15.9MW and occurred after approximately 11min from ignition. The calculated peak heat release rate of the drilling rig was 29.4MW and occurred after approximately 21min from ignition. The heat release rate was calculated from measured data of gas concentrations of oxygen, carbon monoxide and carbon dioxide, measured gas velocity and measured gas temperatures. The fuel load of the wheel loader consisted mainly of the tyres, the hydraulic oil and the diesel fuel. The fuel load of the drilling rig consisted mainly of the hydraulic oil and the hydraulic hoses. The calculated heat release rate curves were controlled by comparing the summed up energy contents of the participating components with the integrated heat release rate curves.