Flame Heat Research Articles

Flame retardant, heat insulating and hydrophobic chitosan-derived aerogels for the clean-up of hazardous chemicals

Leakage of hazardous chemicals often causes significant casualties, enormous economic losses, and negative social benefits. Presently, fire rescue personnel lack efficient and eco-friendly disposal materials for hazardous chemical leakage accidents. In this study, chitosan (CS) aerogels with excellent flame-retardant performance were prepared via cross-linking by two phosphorus-containing vanillin-based compounds (DV and TV). The as-prepared chitosan aerogels were lightweight and porous. The introduction of DV and TV greatly enhanced the residual char yields of CS at 700 °C and the flame-retardant performance of chitosan aerogels. TCS-5.0 possessed the best flame-retardant performance, indicating that TV was more effective than DV in enhancing the flame-retardant performance of chitosan aerogels. The greatly improved flame-retardant properties could be attributed to TV effectively promoting the residual char formation of chitosan aerogels and reducing the formation of combustible gas phase products. To improve the hydrophobicity of chitosan aerogels, TCS-5.0 was treated with solution immersion to load siloxane molecules on its surface. The water contact angle of HTCS-5.0 was 136.1°. HTCS-5.0 had a high oil absorption multiplicity, absorbing up to 31 times its own weight of chloroform. HTCS-5.0 could continuously absorb organic solvents on the water surface with the assistance of a vacuum pump, indicating that HTCS-5.0 could be used for the clean-up of hazardous chemical leakage accidents.

Facile Synthesis of Phosphorus-Free Biobased Epoxy Resin with Exceptional Flame Retardancy, Mechanical Properties, and Heat Resistance

Facile Synthesis of Phosphorus-Free Biobased Epoxy Resin with Exceptional Flame Retardancy, Mechanical Properties, and Heat Resistance

Improved pool fire-initiated domino effect assessment in atmospheric tank farms using structural response

Domino effects are severe accident scenarios affecting storage tanks and are often initiated by pool fires. Flame engulfment and heat radiation are the two major sources triggering domino effect. Threshold-based and probit-based methods are widely used to assess the possibility and probability of a secondary accident. These methods are also a part of advanced methods devoted to examining synergic or coupling effects. The current work examines (i) how effective the threshold-based methods are and (ii) how accurate the current time to failure (TTF) estimation models are, which are the basis of probit-based methods. The results suggest that threshold-based methods are not pertinent for the quantitative assessment of domino effect and that significant improvement can be made in the existing TTF prediction models using site-specific structural response data. A new set of equations for TTF estimation using data analytics is proposed. Application to 4,080 pool fire scenarios demonstrates that the newly developed model can improve the TTF prediction performance compared to the existing models (around 22% in terms of R2). In addition, a method has been proposed and validated to correlate time with the failure probability for time-dependent domino effect assessment, which is a limitation of probit-based methods.

Experimental study of coupling between the burning behaviors of fuel storage tanks and thin fuel pools

Fires frequently result from fuel spills from burning storage tanks and can represent serious hazards. Previous studies have focused primarily on stand-alone pool or storage tank fires. In contrast, this work investigated storage tank fires combined with thin pool fires, focusing on a tank burning together with a pool, an empty tank in a burning pool and a stand-alone pool fire. The fuel pools are thin to be 4, 6, 8, 12, 16 and 20 mm. An emphasis was put on investigating coupled burning between the tank and pool by analyzing flame images, regression rates and heat transfer. A burning storage tank within a pool fire was found to exhibit an abrupt temperature increase of the heated fuel together with an initial increase in the regression rate in the tank followed by rapid stabilization. Following this, the rate greatly increased because of the surrounding pool fire, then decreased gradually but without achieving a quasi-steady state. Compared with a fuel tank in a pool fire, both an empty tank in a pool fire and a stand-alone pool fire showed higher pool regression rates. The results of this study enrich pool fire knowledge and will assist in developing practical fire preventions.

Lightweight and large-scale rGO reinforced SiBCN aerogels with hierarchical cellular structures exposed to high-temperature environments

SiBCN ceramic aerogel is an ideal potential candidate for ultra-high temperature thermal insulation due to its unique microscopic pore structure combined with the excellent thermal stability of SiBCN ceramic. Here, reduced graphene oxide (rGO) modified SiBCN aerogels (rGO/SiBCN) were prepared through solvothermal, freeze-casting and pyrolysis, and the dimension of the aerogel is up to Φ130 mm × 28 mm. The density of the rGO/SiBCN aerogel is as low as 0.024 g/cm3 and the microstructural regulation is achieved by controlling the rGO content in the aerogel. The hierarchical cellular structure endows the aerogel with a high specific surface area (148.6 m2/g) and low thermal conductivity (0.057 W m−1 K−1). The 10 mm-thick sample exhibits excellent thermal insulation and ablation resistance, as evidenced by its ability to reduce the temperature from ∼1100 °C to ∼180 °C under the intense heat of a butane flame. Moreover, benefiting from the ultrahigh-temperature stability of SiBCN, the rGO/SiBCN aerogel exhibits good thermal stability up to 1200 °C in argon and short-oxidation resistance at 800 °C in air. Therefore, the rGO/SiBCN aerogel with superior overall performance could expand its practical application in high-temperature thermal insulation under extreme environments.

Optimization of a two-step CH4/air reaction mechanism in a CO2-enriched environment for high-fidelity combustion simulations

In the gas turbine framework, the adoption of carbon capture and storage (CCS) systems coupled with strategies to improve the exhaust CO2 content is a promising technology to abate the carbon footprint of such machines. However, any departure of the oxidant from the air can compromise the accuracy of the conventional models to represent the combustion process. In this work, the effect of the CO2 enrichment of the mixture on an atmospheric premixed swirled flame is investigated by means of large eddy simulation (LES), comparing the numerical predictions with the experimental results. The high-fidelity numerical model features a dedicated global reaction mechanism derived through an in-house optimization procedure presented in this study. The chemical scheme is obtained by optimizing a widely used CH4–air two-step mechanism to improve key flame parameters such as the laminar flame speed and thickness and the resistance of the flame to the stretch with moderate CO2 dilution. The numerical results are analyzed in terms of flame shape, heat losses, and pressure fluctuations, showing a promising agreement with the experimental measurements and demonstrating the capabilities of the numerical model for CO2-diluted combustion.

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.

Experimental investigation on the combustion characteristics and thermoacoustic instability of liquid spray flame with blended fuel

This study experimentally investigates the combustion characteristics and thermoacoustic instability of two fuels, ethanol and RP-3, at five different blending ratios. The flame images, root temperature of blended fuel flame and pollutant emissions were photographed and tested. The experimental results indicate that high-pressure pulsation amplitude and airflow disturbance can enhance droplets’ evaporation and combustion processes, resulting in a reduction in flame length. Lower evaporation temperature ensures a higher temperature of the flame root while increasing the blend ratio of RP-3 and increasing the global equivalence ratio can increase the temperature level inside the combustion chamber. The root temperature of ethanol flame can reach over 1400 K. Fuel characteristics and burner structure can influence the coupling between pressure pulsation and flame heat release pulsation, leading to differences in the amplitude of combustion instability. The shorter the outlet length of the burner, the wider the stability range of the flame. Moreover, the increase in total burner length results in a shift of the oscillation dominant frequency towards lower frequencies. Emission characteristic curves reveal that high-pressure pulsation amplitude can improve the combustion efficiency of blended RP-3 fuel but are unfavorable for the complete combustion of ethanol. Meanwhile, under the dilution effect of the airflow, the emissions of NOx and CO at the outlet gradually decrease as the airflow increases.

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.

Practice Design of Ship Thin Section Considering Prevention of Welding-Induced Buckling

_ The lightweight fabrication of thin-walled cabin sections is popular for advanced ships, and the dimensional tolerance generated by welding buckling significantly influences the fabrication accuracy and schedule with poststraightening. A typical thin section employed in the superstructure of a high-tech passenger ship is considered the research object. Conventional fabrication procedures and welding conditions were examined beforehand with combined thermal elastic-plastic and elastic FE computations based on the theory of welding inherent deformation, while welding buckling was represented with identical behavior compared with fabrication observation. Actually, there are usually two methods to prevent welding buckling with advanced fabrication. Stiffeners with optimized geometrical features and excellent elasticity moduli were assembled to enhance the rigidity of the ship thin section, and less welding inherent deformation with advanced welding methods can be employed to reduce mechanical loading. Computational results show that either less in-plane welding inherent strain or higher structural rigidity can reduce the magnitude of welding-induced buckling, and avoid the generation of welding-induced buckling during the lightweight fabrication. Keywords ship thin section; welding buckling; FE computation; welding driving force; structural rigidity

Unstable flame structure adjacent to discrete wall with narrowly-ventilated air cavity

This paper investigates the interaction between a discrete non-reactive wall with a narrow ventilated cavity and a turbulent diffusion flame. The analysis is based on both experiments and large eddy simulations (LES). Firstly, the prediction is compared with experiments on two reference configurations: a continuous wall and a discrete system. The analysis was carried out to establish the validity of models to capture the boundary-layer flame and heat exchange. Overall, the predictions are in reasonable qualitative and quantitative agreement with experimental results. The article then examines the spacing effect of inert discrete elements on flame characteristics. The analysis is conducted for a vertical and horizontal layout. The orientation and spacing of the elements affect the stability of the flow structure and fire dynamics. It produces larger or smaller flames, which affect the flame-to-element heat transfer. Correlations were developed to predict flame height and heat flux. The study's findings can aid further fundamental studies to predict and mitigate the flame spread risk over such systems with combustible elements.

Enhancing heat protection performance of firefighters’ protective clothing: A comparative analysis of fabric and knit structure in the lining of multi-layered construction

A firefighting protective clothing item is an important piece of equipment that can protect firefighters from the risk of burns during firefighting and rescue activities. In this study, a method to improve the heat protection performance of firefighter protective clothing lining was proposed, while consulting with textile experts. The protective performance effects of flame and radiant heat were investigated according to the quilting spacing (25.4, 50.8, and 76.2 mm) of the lining and weaving method (fabric or knitted) used for the multi-layered structure of the protective clothing. The experiments conducted to evaluate the impact of different weaving methods on the protective performance of firefighter clothing against flame, radiant heat, and combined flame/radiant heat revealed that the knitted lining exhibited excellent protection performance. Specifically, the knitted lining demonstrated a delayed effect in temperature increase, with maximum delays ranging from approximately 2.2–4.0 s for flame protection experiments, and 5.7–7.4 s for radiant heat protection experiments. For knitted lining, one thread is woven in an interlocking manner; thus, the heat resistance is higher and thermal conductivity is lower owing to the formation of a space compared that for a fabric lining. The results suggest that the weaving method affects the heat transfer according to the formation of space and change in the thickness of the clothing. The findings of this study could be useful for many purposes, such as cost effective and quick analysis of the expected performance of a design or weaving method.

Near blowout instabilities and identification based on multivariate analysis with high-speed multi-species optical diagnostic techniques

Early prediction and depth analysis of lean blowouts are desirable for both aeroengine and stationary gas turbines. This study used a combination of high-speed multi-species optical diagnostic techniques and multivariate analysis methods to investigate near-blowout instabilities and identify combustion conditions. The flame root dynamic behavior and heat release pulsation characteristics were investigated using advanced modal decomposition methods. Results showed that a near-blowout flame exhibited a characteristic frequency (∼200 Hz) or “fingerprint” related to the temporal and spatial coupling characteristics of CH and OH radicals. Furthermore, flame image feature screening and multivariate fusion for combustion condition identification are reported for the first time. Significant differential features and effective fusion models were investigated and discussed. The identification accuracy of the multivariate fusion feature model compared with that of the geometric feature model improved by more than 24 %. Moreover, simplified fusion models of low-order moment and geometric features were constructed with relatively high identification accuracy (99 %) and low relative standard deviation value (14.1 %). These results demonstrate the significant potential of combining multi-species optical diagnostic techniques and multivariate analysis methods, thereby expanding their application to the field of combustion science.

Ignition limit of EPS foam by a hot particle under cross wind

The ignition of the building insulation materials by a hot particle is a typical spot fire phenomenon, but the scientific understanding is still limited. In this work, a hot steel spherical particle (6-16 mm and 800-1200 °C) was dropped onto the low-density expandable polystyrene (EPS) foam with an external airflow velocity of 0-4 m/s to obtain the ignition limit at the flash point and fire point. Airflow provides an alternative shortcut transition of unstable flash flame to a strong fire point and fuel burnout, because airflow increases the oxygen supply and flame heating rather than cooling the particle. As the airflow velocity increases, both flash and fire points first become easier to reach because airflow facilitates the mixing of pyrolysates and oxygen in the Smothering Regime. When the airflow velocity increases to the Thermal Regime, the delay time remains stable. Further increasing the airflow velocity to the Chemical Regime, the ignition delay time slightly increases until the airflow blows off the flash flame by cooling the particle or blowing away the flammable mixture from the hot surface. Such a competitive effect of airflow on hot particle ignition is also qualitatively verified by theoretical analysis. Flame retardants inside EPS foam do not change the flash ignition but inhibit the transition to fire point and burnout, even under the assistance of airflow. This work enhances the comprehension of the complex interactions between flash points and fire points in the spotting or hot-particle ignition of the building facades.

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.

Hypergolic ignition response to oxidizer droplet properties

The hypergolic ignition response to oxidizer droplet properties is an important consideration for oxidizer injector design in hybrid rocket motors. Hence, the present work focused on hypergolic ignition characterization as a function of droplet properties and impact dynamics for a promising propellant. The propellant for the present work consisted of rocket grade hydrogen peroxide (RGHP) as oxidizer, high-density polyethylene (HDPE) as fuel and sodium borohydride (NaBH4) as the hypergolic additive embedded in the HDPE matrix. This propellant was chosen for this detailed study since it showed promising results in prior studies, exhibiting low (<10 ms) ignition delays. For this work, in a drop-test setup, RGHP droplet diameters between 1.90±0.11 mm and 4.35±0.26 mm were dropped on the HDPE + NaBH4 fuel samples at velocities between 0.3 m/s to 1.93 m/s. The ensuing ignition event was studied using both visible and infrared high-speed imaging. Post-impact droplet parameters were determined to enable a deeper analysis of observed phenomena. While ignition delay was the focus of this work, flame spread areas and heat release were also studied. Higher impact velocities generally led to lower ignition delays while splashing oxidizer drops increased probability of larger flame areas and higher heat release. Additionally, the initial ignition/heat release event was found to originate in a small region at the thinnest area of the post-impact droplet. Surface temperature measurements were also performed using infrared imaging. The data suggested that the initial heat release was found to be likely due to RGHP-NaBH4 reaction with no contribution from RGHP decomposition. Regression and random forest machine learning analysis found ignition delay depended strongly on the minimum post-impact drop layer thickness. Based on the data analysis, a simple mathematical model was developed whose results matched experimentally observed trends. The study is expected to further understanding of the processes that control hypergolic ignition in addition to aiding rocket engineers design better oxidizer feed systems. Novelty and SignificanceThis work constitutes a novel in-depth study on the influence of drop impact properties on hypergolic ignition. Using innovative techniques, this work is the first to show the overwhelming importance of residual droplet layer thickness on ignition delay. Additionally, a simple mathematical model based on that finding performed well against experimental data which is of substantial significance, given the complex and multi-parametric nature of hypergolic ignition. Finally, the work establishes that operating in the splashing regime enhances heat release and flame spread. The candidate propellant for this work was hydrogen peroxide and high-density polyethylene with embedded NaBH4. However, the findings of this study are likely generalizable over a range of solid-liquid hypergolic propellants involving solid fuels with embedded energetic materials, of which there are many. The findings of this work can enhance understanding of hypergolic ignition in addition to aiding in better rocket injection system design.

Wildland surface fire spread: Mechanism transformation and behavior transition

Surface fire is the most prevalent type of wildland fire and dominates the cost of wildland firefighting. The high-intensity burning behaviors in surface fires stem from the significant interaction of combustion with heat transfer and fluid flow under complicated fuel, meteorology, and topography conditions. Over the past several decades, research has resulted in incrementally enhanced insights into the mechanism transformations and behavior transitions related to surface fires, with theories and models developed and validated. These advances have improved the fundamental understanding of surface fire evolution significantly. However, the current knowledge is still inadequate for dealing with practical surface fire problems. In this paper, we specifically address two complexities associated with fire growth. First, the fuel preheating mechanisms involving various forms of flame heat transfer may change under different conditions. Second, a surface fire may transition to other intensely burning behaviors by interacting with environmental conditions. For the first complexity, we initially discuss the fuel preheating mechanism, flame propagation across discrete fuels, and flame geometry transformation in wind-driven fire spread. Then, we examine the firelines interaction and its induced junction fire. Followed are the fireline transformations under different topography conditions, focusing on the flame attachment in upslope terrain and the fireline behaviors in canyon terrain. For the second complexity, we highlight the transitions between surface fire and other behaviors with distinct mechanisms, including smoldering, crown fire, fire whirl, and spot fire. Despite significant advances, more understanding of the mechanisms underlying fuel preheating processes and fireline behaviors is needed. Furthermore, we need more exploration into the scaling law of combustion dynamics of surface fires and the non-steady transitions to other intense burning behaviors. All these challenges have proven an elusive goal for future work to enhance surface fire predictive capabilities.

Thermal response and pyrolysis behavior of carbon fiber/phthalonitrile composites under one-sided butane flame heating: Experimental and numerical analysis

The present work innovatively investigates the one-sided flame heating thermal response and pyrolysis behavior of carbon fiber/phthalonitrile (Cf/PN) composite laminates. The effectiveness of the Abaqus/UMATHT subroutine in studying the temperature and reaction mechanism-dependent material thermal performance is proved. A numerical model which accounts for the correction of pyrolysis depth and multi-stage pyrolysis kinetics parameters is established to generate massive data to replace repeated experiments. By analyzing the temperature and temperature rise rate distribution, the total mass loss ratio, and the post-heating micromorphology in the thickness section, the pyrolysis characteristics of the material are discussed. Finally, the rear surface temperatures corresponding to different surface heat flux, specimen thickness and heating time are numerically predicted to provide potential engineering guidance for the application of phthalonitrile composite materials in thermal protection and load-bearing integrated structures.