Flame Heat Release Rate Research Articles

Low-temperature reactivity, extinction, and heat release rate of non-premixed cool flame at elevated pressures

Understanding the fundamental characteristics of high-pressure cool flames is crucial for the development of advanced and efficient low-temperature combustion engine technologies. The pressure dependency of multi-oxygen addition branching reactions in low-temperature chemistry significantly influences the dynamics, structure, and reactivity of cool flame. This study investigates the non-premixed cool flame of diethyl ether (DEE) at elevated pressures. The results show that pressure rise promotes low-temperature chemistry and significantly extends the extinction limit of cool flame. It is found that the cool flame heat release rate is correlated with the product of pressure and the square root of the pressure-weighted strain rate, Q∼aP·P, which is different from that of hot flames, Q∼aP. The radical index concept for atmospheric cool flames is extended to high-pressure cool flames allowing to decouple the mass and thermal transports from the chemical kinetics term to evaluate the fuel reactivity at elevated pressures. The radical index shows that the low-temperature reactivity of DEE is enhanced with the pressure and is higher than n-dodecane by a factor of 19, 18.3, and 16.4 for 1, 3, and 5 atm, respectively. Kinetic analysis reveals that pressure rise results in QOOH stabilization and promotions of the second O2 addition and the chain-branching reaction pathway for multiple OH radical productions.

Size scale effect on mass burning flux and flame behavior of solid fuels

The study investigates the horizontal fuel size effect on free-burning fires for PMMA plates and wood cribs. The fuel size effect on mass burning flux and flame behavior is mainly discussed and compared with typical pool fires. For the PMMA plate and liquid pool, when the fuel size is small (< 10 cm), either the 3D sidewall burning of PMMA or the container wall heated by flame can promote the burning flux at the horizontal projection area. As the fuel size increases, these side wall burning or heating effects decrease, causing the drop in burning flux with fuel scale for both the PMMA plate and liquid pool. For small-scale wood cribs, fire cannot self-sustain due to the large airflow cooling. With the increase in wood crib size, the burning rate first remains constant and then gradually increases, driven by the enhanced internal radiation. As the fuel size increases above 20–30 cm, the flame radiation dominates the burning flux for all fuel types. Fire dynamics simulator (FDS) was adopted to simulate the horizontal size effect by setting a varied fire source (horizontal projection) area. First, the flame geometry and heat release rate (HRR) of simulations were validated against experimental results. Subsequently, the validated fire model generates cases covering a broad range of fire scales. Finally, a new correlation of flame height with the fire heat release rate and fuel size is proposed, and its prediction capability is validated in the numerical fire modeling. This study quantifies the size effect on the burning rate for common solid fuels and provides valuable information for the numerical modeling of multi-scale fires.

Spray swirling flame dynamic under combined modulation of air and fuel

Combustion instability is still a main challenge in developing advanced gas turbine combustors. It is essential to conduct instability control once it occurs. In this paper, liquid fuel modulation by a high-speed on/off valve is used to control flame oscillation generated by loudspeakers. Spray swirling flame dynamic under the combined modulation of air and fuel is experimentally studied. Flame dynamical characteristics and coherent structures at different excitation phase delays are discussed with nonlinear time-series analysis and proper orthogonal decomposition method. Experimental results show flame heat release rate fluctuation is decreased by 86% at 40° while flame instability is intensively enhanced at 220°. Phase portraits and recurrence plots show the flame is in a chaotic oscillation with low amplitude aperiodic oscillations at 40° while it suffers from the largest fluctuation at 220°. Difference in flame dynamic is caused by the destructive (out of phase) and constructive (in phase) interference of heat release rate fluctuations induced by air excitation and fuel modulation. Flame returns back to its nature dynamical behavior under out-of-phase modulation. Longitudinal modes of the spray flame are enhanced while the spiral modes are inhibited under in-phase modulation.

Synchronization under saturable nonlinearity.

In this theoretical work, we introduce a nonlinear gain saturation law representative of the experimentally observed properties manifested by phenomena ranging from aeroacoustic shear layers in self-sustained cavity oscillations to flame heat release rate in thermoacoustic instabilities. Furthermore, this type of saturable gain may be relevant for a wider class of physical systems, such as active laser media in photonics. The nonlinearity discussed herein governs the fullscale behavior of a self-oscillator exhibiting linear loss under large amplitude perturbations, in contrast to the cubic damping and linear gain of the Van der Pol model. A distinctive characteristic of the proposed equation is the simple, well behaved gain term in the slow timescale dynamics.

Experimental investigation on the effects of transverse injection distribution scheme on dual flame dynamics subjected to flow disturbances

Combustion instability has become a major problem in low-emission annular gas turbine combustors. It is of practical importance to investigate the effects of transverse nozzle distribution scheme on multi-flame dynamics subjected to external acoustic perturbations. In this study, the responses of heat release rate perturbations from dual propane/air laminar premixed flames to axial acoustic perturbations have been experimentally measured. Specifically, the velocity disturbance upstream the flame base has been measured by two-microphone method and the flame heat release rate perturbation is obtained by the photomultiplier tube with CH* filter lens. The dynamic evolution of dual flames is captured by the high-speed camera and then analyzed with the proper orthogonal decomposition (POD) method. Results indicate that there is a critical distribution distance, where the peak gain of the dual flame transfer function is the largest. And the direct injection dual flame transfer function can behave as the low-pass filter. Furthermore, the dual flame transfer function behaves nonlinearly with the increase of the injection angle. The dual flame transfer function for large injection angle can behave as the hybrid mode of the low-pass filter and the band-pass filter. The POD analysis reveals that the coupling effect of gas thermal expansion and stretch effect would contribute to the interaction between dual flame front dynamics. The transverse injection distribution scheme (dual flame distance and injection angle) has a significant effect on the dual flame front dynamics, which in turn affects the flame heat release rate disturbance. The bulk mean flow velocity can enhance the interaction effect between dual flame front dynamics.

Dynamic Response Mechanism of Ethanol Atomization–Combustion Instability under a Contrary Equivalence Ratio Adjusting Trend

This study experimentally explored the effects of equivalence ratio settings on ethanol fuel combustion oscillations with a laboratory-scale combustor. A contrary flame equivalence ratio adjusting trend was selected to investigate the dynamic characteristics of an ethanol atomization burner. Research findings denote that optimizing the equivalence ratio settings can prevent the occurrence of combustion instability in ethanol burners. In the combustion chamber, the sound pressure amplitude increased from 138 Pa to 171 Pa and eventually dropped to 38 Pa, as the equivalence ratio increased from 0.45 to 0.90. However, the sound pressure amplitude increased from 35 Pa to 199 Pa and eventually dropped to 162 Pa, as the equivalence ratio decreased from 0.90 to 0.45. The oscillation frequency of the ethanol atomization burner presents a migration characteristic; this is mainly due to thermal effects associated with changes in the equivalence ratio that increase/decrease the speed of sound in burnt gases, leading to increased/decreased oscillation frequencies. The trend of the change in flame heat release rate is basically like that of sound pressure, but the time-series signal of the flame heat release rate is different from that of sound pressure. It can be concluded that the reversible change in equivalence ratio will bring significant changes to the amplitude of combustion oscillations. At the same time, the macroscopic morphology of the flame will also undergo significant changes. The flame front length decreased from 25 cm to 18 cm, and the flame frontal angle increased from 23 to 42 degrees when the equivalence ratio increased. A strange phenomenon has been observed, which is that there is also sound pressure fluctuation inside the atomized air pipeline, and it presents a special square waveform. This study explored the equivalence ratio adjusting trends on ethanol combustion instability, which will provide the theoretical basis for the design of ethanol atomization burners.

Experimental investigations of the fire behavior of composites impacted by kerosene/air turbulent flame

In this research, we conducted an experimental analysis of a Kerosene/air burner designed according to the FAA's proposed ISO 2685 standard. The burner serves as a vital tool for generating flames and burnt gases, simulating real-world conditions relevant to fire safety studies. The core objective of this study was to comprehensively characterize the burner's behavior. Through a series of experiments, we investigated the influence of the equivalence ratio on several key parameters, including the spatial distribution of gas temperature (monitored using thermocouples), heat flux (measured with heat flux gauges), and the composition of gas emission species. Notably, our findings revealed a significant correlation between the equivalence ratio and these parameters. Specifically, we observed that as the equivalence ratio increased, so did the flame temperature, heat flux, and heat release rate. This increase continued until a critical equivalence ratio value of 1.03 was reached, resulting in a flame temperature of 1101.5°C and a heat flux of 140 kW/m2. Furthermore, our study encompassed a pyrolysis test on various composite materials, including Carbon-phenolic, Carbon-BMI, and Carbon-PEKK. The comparative analysis of these materials highlighted the superiority of Carbon-PEKK, which exhibited the lowest mass loss, the highest back-face temperature without experiencing significant material delamination, and the lowest concentration of gas emission species. Importantly, our investigation also delved into the influence of the equivalence ratio on the flow's turbulence characteristics, a critical factor in comprehending the burner's performance and its interactions with materials in the context of fire safety scenarios. These findings contribute to a better understanding of combustion dynamics, which is invaluable for enhancing fire safety measures and developing more efficient flame-generating systems.

Experimental study on conical flame transfer functions considering velocity profiles

This paper investigates the linear thermoacoustic dynamic response of laminar premixed stoichiometric propane/air flames under different oncoming flow velocity profiles. Experiments focused on four nozzle thickness-to-diameter ratios: 0.75, 1.5, 2.5 and 3.75, and three different mean bulk velocities: 1.1m/s, 1.2m/s and 1.3m/s. As the nozzle thickness-to-diameter ratio increases, the velocity profile transitions from a developing velocity profile to a fully developed velocity profile. The flame transfer function (FTF) gain exceeds unity at low frequencies. Notably, the effect of the velocity profile is mainly manifested in the moderate frequency range where the FTF gain decreases. When the velocity profile approaches the resonance frequency of the corresponding small orifice, the decline in FTF gain becomes less pronounced. For fully developed velocity profiles, the decrease in mean bulk velocity leads to an increase in FTF gain at low frequencies, but a more rapid decrease at higher frequencies. Additionally, this paper also investigates segmented flame transfer functions (segFTFs) at various axial positions along the flame. With the increase in frequency, the effect of flame front disturbance on FTF changes from the overall flame displacement caused by flame root stand-off distance, to the effect of flame front wrinkle counteracting effect. The different velocity profiles significantly impact the flame dynamic response, thereby affecting the flame heat release rate and consequently the FTF.

Experimental investigation of the plasma-assisted spray combustion of methanol/water mixtures

To achieve the goal of net zero by 2050, carbon dioxide emissions must be reduced using carbon–neutral fuels, and methanol is a feasible choice for gas turbines, IC engines, and furnaces. Methanol can be prepared by bio-process or thermal catalytic processes, but the crude product contains much water. Crude methanol must be dehydrated into high-grade methanol to enable its use as fuel. The dehydration process is the most energy-intensive step in methanol production. Therefore, this study investigated the feasibility of adding different proportions of water to methanol as a fuel for plasma-assisted spray combustion by using low-power gliding arc plasma. Based on the current sprayer setup, using plasma to sustain water-content methanol is more energy-efficient than refining it. The results delineate that plasma can sustain the combustion of methanol in a spray, even when the methanol contains 60 % water. The length and lift-off height of the flame are related to the water content of the fuel. To illustrate this distinguishing phenomenon, this study used CH and OH fluorescence to conduct a more in-depth analysis. The distribution of CH* radicals indicated that the gliding arc plasma enhanced the flame heat release rate, which may be why the flammability limit extended to greater proportions of water with methanol. It's fascinating to observe that blended fuel with higher water content can result in higher OH chemiluminescence signals. This suggests that the dissociation of water vapor, caused by the thermal effect of the gliding arc, plays a crucial role at the flame base locations. In addition, the flammability limits, emissions, and combustion efficiency of methanol/water spray combustion with and without plasma assistance were all examined.

The impact of equivalence ratio on the fire characteristics of Kerosene/air flame produced by NexGen burner for aeronautic application

This paper provides an experimental investigation of a Kerosene/air flame (produced by the NexGen burner designed by the FAA, and here calibrated for ISO 2685/AC20-135 standards fire tests conditions), which is used to generate flame/burnt gases impinging on material samples in the field of fire safety. The purpose of this study is to characterize this burner, and experimental means are implemented to better understand the effects of the equivalence ratio on the spatial distribution of the gas temperature (thermocouples) and the heat flux (heat flux gauge). A parametric study was carried out by varying the global equivalence ratio (ER); highlighting a flickering phenomenon, a relation between the ER and the heat flux, and determining empirical correlations. Hence, the measured flame temperature, heat flux, and heat release rate increase up to a critical value of equivalence ratio equal to 1.03 (flame temperature (1101.5 °C) and heat flux (140 kW/m2).

Laser interferometric detection of heat release rate perturbation field of acoustically perturbed laminar premixed flames

Laser interferometry, which was originally used to measure density perturbation, has been previously used to determine the heat release rate perturbation, which is a key factor in the analysis of combustion instability. However, its spatial accuracy is still unsatisfied. In the present work, an improved fringe Fourier analysis method is proposed to process the results obtained from the laser interferometry measurement, aiming to accurately determine the spatial distribution of flame heat release rate perturbation. The measurement of heat release rate perturbation was performed on a acoustic perturbed laminar premixed flame, with the burner outlet diameter of 8 mm. Interference patterns generated by two laser beams with a diameter of 70 mm were recorded at a frame rate of 6000 Hz. A spatial dependent fringe processing method, referred as the improved fringe Fourier analysis, was developed to demodulate the interference phase from the fringe patterns. The proper orthogonal decomposition analysis was performed on these results to analyze the component of interference signals. Validation of the present technique was conducted by comparing the heat release rate perturbation field resulting from the laser interferometric technique to those from the CH* chemiluminescence from the flame. Comparisons were performed both for the global and local values, and the low-frequency natural oscillation effects were analyzed. Finally, a refraction correction method based on background-oriented Schlieren is proposed to compensate the beam deflection caused by flame.

Modeling hydrogen–diesel dual direct injection combustion with FGM and transported PDF

This work extends the transported probability density function (PDF) method to model hydrogen–diesel dual direct injection (H2DDI) combustion, where a H2 jet is ignited by a small pilot diesel jet flame. The work is motivated by previous H2DDI engine tests where stable compression ignition engine operation was reported with up to 90 % H2 supply by energy share. To help explain this high performance and provide a tool to help further engine optimization, the Eulerian Monte Carlo Fields (EMCF) solution method is employed with which a transported PDF equation is solved in combination with Flamelet Generated Manifold (FGM) for high accuracy and computational efficiency. In all cases, the pilot fuel ignites before interacting with the H2 jet and thus reaction rate and chemical composition are computed as the weighted average of the corresponding values taken for two separated FGM tables generated for the two fuels. Pressure measurements and high-speed schlieren imaging performed in a preburn-type optical constant volume combustion chamber (CVCC) with n-heptane (nC7H16) pilot fuel were used to validate the EMCF+FGM model. The application focus is how H2 ignition and combustion is affected when the nC7H16 is injected prior to or after the main H2 jet. EMCF+FGM computed heat release rate and flame structure were compared with experimental data and results from conventional FGM model based on presumed PDF. When applied to H2DDI combustion, the EMCF+FGM model successfully reproduces flame structures and heat release rate under different injection strategies, including the transition towards partially-premixed combustion when the nC7H16 pilot follows the main H2 injection. Moreover, analysis of heat release rate from different stochastic fields can provide a useful indication about cyclic variability and combustion stability.

Radiative characteristics of large-scale fire whirl: An experimental study

Flame radiative characteristics, depending on the flame type, flame scale, and fuel type, are key parameters that determine fire spread and thermal hazards. Accurate prediction of flame radiation in large-scale natural fires requires detailed information on radiative characteristics. This paper presented an experimental study on the radiative properties of large-scale fire whirls with RP-5 aviation fuel. The maximum flame height and mean width reached 21.40 m and 3.60 m, respectively. Unlike large-scale free pool fires, the radial stabilization effect of the fire whirl suppressed the unstable toroidal vortex structures and the smoke production significantly. Most of the flame was high-temperature and high-emissive power areas in the large-scale fire whirl, and the emissive power exceeded 100 kW/m2 on about 60 % flame area. The flame temperature and emissive power first increased with height, then stabilized and decreased in the upper part of the fire whirl. The peak flame temperature, mean flame temperature, and mean emissive power all increased with an increase in the flame width. The mean emissive power ranged from 93.06 to 157.47 kW/m2, 2.3–3.9 times the values in free pool fires of the same scale. The solid flame radiation model could effectively predict the instantaneous radiative heat fluxes from large-scale fire whirls to distant external targets with the corrections of atmospheric transmissivity. The semi-empirical correlation of the radiative fraction based on the turbulent dissipation showed that neither the flame size nor heat release rate affects the radiative fraction, consistent with the our and other researchers’ experimental data from medium to large-scale fire whirls.

Study on CO2* chemiluminescence of CH4/O2 co-flow diffusion flame based on the chemical kinetic mechanism

Chemiluminescence of CO2* is suitable for characterizing flame properties such as flame structure, equivalence ratio and heat release rate. However, experimental studies and numerical simulations of CO2* are not yet complete. In this work, the two-dimensional distributions of CO2* chemiluminescence were obtained by a high-resolution CCD camera. Numerical simulation results of CO2* were performed based on the detailed reaction kinetics mechanism of CO2* and were in good agreement with that of experiments. The differences of CO2* reactions at different axial positions were studied. The main CO2* formation reaction R2(CH + O2 = CO2*+H) at the burner outlet and R1(O + CO + M = CO2*+M) at the core reaction zone were determined. On the fuel side, the R1 reaction rate was mainly affected by temperature. On the oxidizer side, the R1 reaction rate was affected by temperature and reactant mole fraction. The main quenching reaction was R5(CO2*+H2O = CO2 + H2O). The oxygen-fuel equivalence ratio changed the reaction rate of formation and quenching reactions mainly by affecting temperature and reactant mole fraction.

Melt-flowing of expanded polystyrene in large-scale discrete flame spread

Large-scale experiments were performed to investigate the influence of the melting flow of thermoplastic materials on discrete downward flame spread in façade fires. Expanded polystyrene, one of the flammable insulation materials broadly used in buildings, was selected as the study object. The fuel coverage (f = hfuel/(hfuel + hb)) was adjusted via barrier heights (hb = 30, 50, 70, and 90 cm) to explore its impact on flame propagation and heat release rate. The discrete flame spread process with burning flows was clarified into five stages, and the critical fuel coverage for preventing flame spread was found to be 0.40–0.46. The heat release rate of the entire burning process was measured, revealing that fuel coverage significantly impacted the HRR curve shape and peak value by influencing the formation time of pool fires above and below the barrier. Additionally, the properties of burning flows were characterised by area and HRR. A proportional relationship was established between the pool fire height and spill fire area. This research provides a novel observation of melting flows developed from burning thermoplastic material and uncovers the influence of melting flows on flame propagation in façade fires.

Upward flame spread behaviour of cladding materials on a medium-scale ventilated façade experimental setup with a single combustible wall

A parametric experimental study was performed to characterise the fire spread dynamics in a simplified ventilated façade using a medium-scale testing rig comprised of a non-combustible and a combustible cladding wall (1800 × 600 mm). Three different cavity widths and four different cladding materials were tested. Measurements of the flame height, the incident heat flux on the non-combustible cavity wall and oxygen consumption calorimetry were performed. A strong relationship between flame height and heat release rate was found for the growth phase of the fire. It has been shown that the time for encapsulation failure and subsequent cladding material core ignition decreased as the cavity width was reduced since the heat transfer to the walls was enhanced. The increase in the heat transfer to the opposite wall with all the materials could lead to external heat fluxes above the critical heat flux for ignition of a number of combustible cladding materials. This highlights the importance of considering the interaction of the products used in the façade and its geometry for the design of façade assemblies when accounting for the fire performance of the system. The results also show the need to understand the impact of the interaction between the design variables and the system performance, since the material performance observed at bench-scale may fail to capture the performance in heat transfer and flame spread scenarios observed at a system scale.

Investigation of the flame properties and fire behavior of carbon‐reinforced PEKK, BMI and phenolic composites impacted by Jet‐A1/air flames

AbstractThis paper provides an experimental investigation of a kerosene/air burner (the NexGen burner designed on the FAA's proposed ISO 2685 standard), which is used to generate flame/burnt gases impinging on material samples in the field of fire safety. The purpose of this study is to characterize this burner, and experimental means are implemented to better understand the effects of the equivalence ratio on the spatial distribution of the gas temperature (thermocouples), the heat flux (heat flux gauge), and gas emission species. Hence, the measured flame temperature, heat flux, and heat release rate increase up to a critical value of equivalence ratio equal to 1.03. Furthermore, a pyrolysis test was carried out on composite materials and the results of the comparative analysis of carbon‐phenolic, carbon‐BMI, and carbon‐PEKK materials show that carbon‐PEKK had the lowest mass loss, highest back‐face temperature without significant material delamination, and the lowest concentration of gas emission species.

Experimental and numerical investigations of forced response of multi-element lean-premixed hydrogen flames

Understanding the distinctive thermoacoustic properties of multi-element lean-premixed hydrogen flames is crucial for the development of hydrogen-capable gas turbines. Extending our earlier investigations of the self-excited dynamics of a cluster of lean-premixed pure hydrogen-air flames, here we explore the dynamic response to harmonic velocity perturbations, using an integrative approach combining loudspeaker-forced direct measurements and LES-based numerical simulations. Electronically-excited OH intensity distribution, generally assumed to be equivalent to the flame's heat release rate, shows an anchored conical reaction zone. Numerical simulations of heat release rate contours, however, reveal the formation of a more concentrated thin annulus region resembling an inverted V shaped layer, created by preferential diffusion of hydrogen molecules. This difference between OH-intensity distribution and heat release rate contours suggests consequential uncertainties in surrogate-dependent flame transfer function evaluations; the transfer function gains from measured data are well defined by a fast-decaying low-pass filter behavior, but the simulation result exhibits slowly-decaying large gain with slight overshoot over the entire range of frequency. Because the systematic uncertainties associated with the estimation of the flame's heat release rate cannot be accurately quantified, we rely on a reduced-order network modeling framework combined with direct measurement to test the accuracy of the prediction of the growth of self-sustained pressure fluctuations. We demonstrate that LES-based transfer functions predict the system's stability more accurately, and this suggests that the calculated heat release rate data are closer to the hydrogen flame's true heat release response than the wavelength-specific light intensity measurement. This assessment enables us to identify structurally simple moderate transverse oscillations of local regions containing atypically concentrated energy as pivotal processes controlling high-frequency hydrogen combustion dynamics.

Investigation of the correlation between OH*, CH* chemiluminescence and heat release rate in methane inverse diffusion flame

The characterization of the heat release rate is of great importance for studying the combustion process. In this work, the correlation between heat release rate and OH*, CH* chemiluminescence in methane inverse diffusion flame is explored with a numerical simulation over a wide range of oxygen/fuel equivalence ratios and methane flow rates. It is found that the flame heat release rate is mainly related to the formation and consumption of the species OH, C2H2, CH3, CH4, CO, CO2, H, H2O and O. The ground state OH concentration gradient is correlated with the heat release rate distribution, and the peak location of the gradient in the ground state OH concentration aligns with the peak location of the heat release rate. The outline of the OH* distribution is consistent with the profile of the maximum of the OH concentration gradient. CH* is used to indicate the main distribution of the heat release rate, and the outline of the OH* distribution coincides with the outline of the heat release rate.

Flame behavior and temperature distribution of coupled fire induced by the interaction of hydrogen jet flame and pool flame

Once fires occur unexpectedly at the oil-hydrogen integrated refueling stations, there is a high likelihood that they will escalate into a coupled fire, resulting in unpredictable casualties and property losses. In this work, experiments on coupled fire induced by hydrogen jet flame and a heptane pool flame were conducted with different nozzle diameters and release pressures. It has been found that there is a critical diameter above which the hydrogen jet can be ignited instantly. This critical diameter is determined by the release pressure and the location of the pool flame. The heat release rate is greater than that of the single pool flame, and it reaches its highest point at 0.2 MPa. Compared to a single fire, the height of the flame decreases while the length of the flame increases, a prediction model is proposed for the dimensionless flame length and width, taking into account the ratio of the heat release rate of the hydrogen jet flame to the heat release rate of the pool flame. The temperature of the coupled fire reaches its highest point at 0.2 MPa and increases as the nozzle diameter. And it decreases with the increase in the distance when Y= 0 m. However, it increases with the increase of the distance when Y= 0.05 m due to the inclination of the flame.