Flame Heat Research Articles

Flame spread over polyethylene-insulated copper and stainless-steel wires at high pressure

This work studied flame spread over polyethylene (PE)-insulated wires at high pressure with an emphasis on the effect of ambient pressure. Laboratory simulated wires were used as a tested wire, which consisted of semi-transparent low-density PE (LDPE) tube as an insulation and copper (Cu) rod or stainless-steel (SS) tube as a core. Increasing ambient pressure produced much soot, leading to an intensively yellow luminous flame. Flame spread rate for Cu rod increased with ambient pressure, whereas that for SS tube decreased. A qualitative analysis via a simplified flame spread model shows that the flame heat flux increases with ambient pressure, but consequently the flame heating decreases due to the decreased flame heating length. Therefore, the flame spread rate for SS tube decreased with increasing ambient pressure. For Cu rod, however, the core heating is dominant enough to compensate the decreased flame heating, and thus the ambient pressure positively affected the flame spread. During the flame spread, melted insulations were often dripped down, but it reduced with increasing ambient pressure. For Cu rod, no dripping occurred when further increasing ambient pressure, which indicates that there is the dripping limit at a high-pressure region. These findings should contribute to electrical fire safety in elevated pressure environments.

Dynamics of periodically-excited vortices in swirling flames

This work investigates the dynamics of periodically-excited vortices in swirling cold flow and hot flame using high-speed particle image velocimetry, for three different Strouhal numbers, 0.68, 1.02, and 1.70. The periodic upstream perturbation induces coherent vortices in the shear layers both inside and outside of the flame surface. We find that the outer vortex rings (OVRs) play a dominant role in tuning the flame dynamics and heat release, whereas the inner vortex rings (IVRs) are related to the formation of the center recirculation zone (CRZ) and suffer from much stronger dissipation. The evolutions of core vorticity, circulation, trajectory, and convective velocity are quantitatively analyzed for the OVRs to understand two key mechanisms: vortex formation and vortex detachment. The results show that the growing and shedding of the OVRs are governed by the inlet shear layer as well as the variations in inlet velocity and acceleration. Furthermore, both the circulation increment during vortex formation and the vortex convection after detachment are qualitatively captured by existing vortex models, demonstrating the prospect of vortex dynamics in understanding general flame-vortex interaction problems.

Nonlinear analysis of self-sustained oscillations in an annular combustor model with electroacoustic feedback

Self-excited pressure oscillations can occur in combustion systems due to the thermoacoustic coupling between the unsteady acoustics and flame heat release fluctuations. Usually, the knowledge of a Flame Transfer Function is used to predict the onset of thermoacoustic instabilities. However, it is also possible to take advantage of it to model a flame response and study experimentally thermoacoustic phenomena without flames. This is exploited in the present study, in which a novel annular setup for the study of thermoacoustics in annular combustors is presented. The thermoacoustic feedback is replaced by electroacoustic feedback. The pressure fluctuations, measured by a microphone, are delayed and filtered and then sent to a loudspeaker, which produces acoustic perturbations, closing the loop. Each flame model parameter can be varied in a flexible way, which allows to choose combinations of parameters that generate modal behaviours of interest in the experiments. For example, this setup can trigger on demand various configurations which exhibit multiple unstable modes, leading to diverse modal competition scenarios. This allows to assess the multi-input Describing Function method, which is used to predict the frequency and amplitude of each mode contributing to thermoacoustic oscillations, when multiple modes are linearly unstable. The experimental validation of predictions from this method, which can be somewhat cumbersome and expensive in the presence of flames, is facilitated by this setup, in which all parameters and boundary conditions are well known and the noise remains negligible. Prediction uncertainties connected to approximations intrinsic of this method when operating in the vicinity of bifurcation points are also discussed.

Influence of Inclined Angle on Upward Flame Spread over Discrete Extruded Polystyrene Foam

ABSTRACT Upward flame spread over inclined solids is a classical research topic in combustion science and fire safety. However, most of previous works focused on flame spread over continuous solid. In addition, the fire hazard of extruded polystyrene (XPS) thermal insulation materials using on building façade has aroused public concern. In order to develop flame spread theory and the guideline for fire risk assessment of building façade, experimental research on upward flame spread over discrete XPS separated by air intervals under different inclined angles (α = 60°, 75°, 90°, 105°, 120°, and 135°) is conducted in this paper. The influence of inclined angle on discrete flame spread characteristics, including flame shape, flame height, flame spread rate, temperature distribution and heat transfer behavior, are revealed. As α ≤ 90°, there existed a flame vortex in the air interval, which may attribute to the increase of flame spread rate. The average flame height decreased with the increase of inclined angle. The time of flame spread stagnation at intervals rose with the increase of the inclined angle. However, the flame spread rate and melt zone length decreased with the increase of the inclined angle. an empirical formula concerning the flame spread rate and inclined angle was proposed. When α < 90°, the high-temperature zone was close to the wall surface. But when α > 90°, the high-temperature zone was above the air interval and on the XPS bottom. As α ≥ 90°, the average heat flux first dropped and then rose with an increase in inclined angle, while the trend was inverse for α ≤ 90°. A mathematical model was established to predict the flame spread rate of discrete XPS. The experimental results demonstrated the mathematical model was reliable.

Effects of equivalent diameter on vertical PMMA combustion in natural convection environment

SummaryA series of combustion experiments of polymethyl methacrylate (PMMA) plates with two shapes, three thicknesses, and different sizes was performed to investigate the effects of equivalent diameter on combustion characteristics of vertical PMMA in a natural convection environment. Two digital video cameras were used to record the burning process. The temporal variation of mass was recorded by an electronic balance. The influence of equivalent diameter on mass loss rate and flame height was then analyzed theoretically. Convection and radiation heat transfer were calculated and modeled in the theoretical analysis and compared to experiment. The result shows that the theoretical prediction of mass loss rate for each equivalent diameter is in good agreement with the experimental measurements. In addition, the relationship between flame height and heat release rate was analyzed using dimensionless analysis method. It was found that the flame height for vertical PMMA burning in a nature convection environment is dominated by heat release rate and equivalent diameter.

Dynamic self-tuning, flickering and shedding in buoyant droplet diffusion flames under acoustic excitation

We report the mechanism of acoustic-flame coupling for buoyant diffusion flame in droplets. The flame under acoustic excitation exhibits differential varicose and sinuous modes including partial extinction and pinch offs at certain shedding heights. Contrary to traditional buoyant flames, the diameter (d) regression of a droplet results in self-tuning of the instability modes with progression in burning times. In essence, we show that a droplet exhibits multi frequency response with advent of new oscillations modes at different points of the burning lifetime. The acoustic perturbations modulate the flame surface or heat release only at lower frequency bands by feeding energy into the natural vortical stability modes of a buoyant plume. Interestingly, we unearth that a flame is naturally unstable at all frequencies lower than g/d. Acoustic pumping at any such frequency (<g/d) corresponds to a unique convective length scale h such that fflame=g/h. We therefore establish that the flame exhibits multiple convective length scales throughout the burning cycle. We subsequently established that the shedding length scale of the droplet flame shortens by more than 25% with increase in acoustic pressure. We theoretically proved that using critical circulation argument, one can uniquely determine the shedding length scale at any acoustic loading. However, it is interesting to note that while the acoustic pressure amplitude is crucial for determining the convective length at which a flame rolls up, the frequency of excitation has no role to play. In other words, the velocity field introduced by the acoustic pressure is responsible for the shedding of the flame. The acoustic perturbations also lead to bulk oscillation of the flame surface near the forward stagnation zone. This is attributed to the acoustic induced velocity that leads to transient shifting of the stoichiometric surface.

Turbulence–Combustion Interactions in Premixed and Non-premixed Flames Generated by Hot Active Turbulent Jets

Direct numerical simulations of turbulent jet ignition (TJI)-assisted combustion of lean hydrogen–air mixtures are performed in a three-dimensional planar jet configuration for various thermo-chemical conditions. TJI is a novel ignition enhancement method which facilitates the combustion of lean and ultra-lean fuel–air mixtures by rapidly and continuously exposing them to high temperature combustion products. Fully compressible gas dynamics and species equations are solved with high order finite difference methods. The hydrogen–air reaction is simulated by a detailed chemical kinetics mechanism. Turbulence–combustion interactions in TJI systems are studied here for different conditions using the flame heat release, temperature, species concentrations, and a newly defined progress variable. Important phenomena such as localized flame extinction/re-ignition and simultaneous existence of premixed/non-premixed flames in TJI-assisted combustion are also investigated.

Modeling of the submicron particles formation and initial layer ash deposition during high temperature oxy-coal combustion

The future use of coal as a fuel for power generation in the US depends on the availability of financially viable technologies for capture and storage of CO2 emissions from power plants. Key second-generation candidates for CO2 capture include high temperature and pressurized oxy-firing of coal, which has the potential to increase efficiency, lower capital costs, avoid air ingress and reduce oxygen requirements. However, unquantified challenges, such as flame behavior, heat transfer, ash transformation, ash deposition and char oxidation, still exist for those technologies. This study specifically focuses on the formation of submicron particles and initial layer ash deposition during high temperature oxy-coal combustion. Previous work has shown that the initial layer deposits are mainly formed of submicron size ash aerosols transported by thermophoresis. Unfortunately, the importance of submicron particle deposition has not received much attention, probably due to the insignificant deposit mass and difficulty in prediction of the submicron particles formation. In this work, models including mineral matter vaporization model, scavenging model and deposition model are developed and applied into a three-dimensional CFD framework to predict the submicron particles formation and subsequent initial layer deposits formation. The model results are comparable to experimental data. The merits of this work are that it has led to the development of a novel approach to predict both submicron particle formation and initial layer ash deposition during oxy-coal combustion.

Oxygen vacancy engineering with flame heating approach towards enhanced photoelectrochemical water oxidation on WO3 photoanode

Abstract Oxygen vacancies are a double-edged sword for photoelectrochemical (PEC) devices and a comprehensive understanding on their role in PEC processes is important. Although there is existing work that illustrates their effect on PEC processes, clarifying the influence of surface and bulk oxygen vacancies, respectively, on the PEC performance is challenging. Herein, we present the fabrication of WO3 photoanodes with tunable bulk and surface oxygen vacancy with a novel two-step flame heating approach that is proving to be a powerful platform for this purpose. We found that both the conductivity and trapping state density of the WO3 photoanode increase with bulk oxygen vacancy density, leading to a volcano-like relationship between the kinetics of charge separation/transport and bulk oxygen vacancy density. Moreover, both the interfacial charge transfer rate constant and the charge recombination rate constant on the surface of the WO3 photoanode increase with surface oxygen vacancy density, which also leads to a volcano-like relationship between the charge injection efficiency and surface oxygen vacancy density. By tuning the surface and bulk oxygen vacancy density simultaneously, the PEC performance of the WO3 photoanode was increased by ca. 10 times, which illustrates the strength of delicate oxygen vacancy engineering in optimizing PEC devices.

Effects of different preheated CO2/O2 jet in cross-flow on combustion instability and emissions in a lean-premixed combustor

To reveal the suppression mechanism of thermoacoustic instability flames under CO2/O2 jet in cross flow. Experiments on the effects of different preheated CO2/O2 jet in cross-flow (JICF) on combustion instability and NOx emissions in a lean-premixed combustor were conducted in a model gas turbine combustor. Two variables of the JICF were investigated—the flow rate and the temperature. Results indicate that combustion instability and NOx emissions could be suppressed when the JICF flow rate increases from 1 to 5 L/min. The average pressure amplitude decreases from 18.6 Pa to 1.6 Pa, and the average NOx emission decreases from 26.4 ppm to 12.1 ppm. But the average pressures amplitude and NOx emissions increase as the JICF temperature grows up. The sound pressure and the flame heat release rate exhibits different mode-shifting characteristics. The oscillation frequency of the sound pressure almost unchanged under JICF injection. However, the oscillation frequency of the heat release rate jumps from 95 Hz to 275 Hz under different JICF temperatures. As the CO2/O2 JICF flow rate arrived 3 L/min, the oscillation frequency of flame heat release rate jumps from 85 Hz to 265 Hz. The color of the flame fronts and roots were changed by the JICF injection. The average length of flame under CO2/O2 JICF cases is shorter than the N2/O2 JICF cases. There are three different modes of flames when the CO2/O2 JICF flow rate varies, and two different modes of flames when the CO2/O2 JICF temperature varies. This article explored the joint effects of different CO2/O2 or N2/O2 JICF on combustion instability and NOx emissions, which could be instructive to the designing of safely and clean combustors in industrial gas turbines.

A P/N/S-containing high-efficiency flame retardant endowing epoxy resin with excellent flame retardance, mechanical properties and heat resistance

In order to improve the flame retardancy while maintaining the excellent mechanical properties and heat resistance of epoxy resin, a novel P/N/S-containing high efficiency flame retardant named TAP was designed and synthesized by a simple one-pot reaction among 2-thenaldehyde, 2-aminobenzothiazole (ABZ) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The chemical structure of TAP was characterized by Fourier transform infrared (FTIR) spectroscopy, 1H and 31P nuclear magnetic resonance (NMR) spectroscopy. Compared to pure EP, the introduction of TAP can reduce the thermal decomposition rate of epoxy resin and increase the char residue. Meantime, when the phosphorus content was only 0.48%, EP/7.5%TAP passed the vertical burning V-0 rating, obtaining a high flame-retardant efficiency. With the addition of 10 wt% TAP, the limiting oxygen index (LOI) value increased from 22.8% of pure EP to 31.5%, while the peak heat release rate (PHRR), total heat release (THR) and total smoke production (TSP) values decreased by 41.26%, 35.70% and 24.03%, respectively. In addition, the glass transition temperature (Tg) of the epoxy resin decreased from 185 °C to 177 °C, while the storage modulus at 30 °C increased. Subsequently, the flexural tests of the samples revealed that the mechanical properties were well maintained. All these results demonstrated that the TAP was a novel high efficiency flame retardant for EP, which enabled the cured EP to maintain excellent heat resistance and mechanical properties.

Experimental and numerical study on the flame characteristics and cooling effectiveness of air-cooled flame holder

For high-performance combustion, it needs to meet the requirements of flame stability and heat protection at a wide operating range. In this work, a novel air-cooled flame holding device containing impingement holes and faced film holes, which can achieve flame stabilization and reduce wall temperature. The cooling air flows out and forms vortices attaching on the back wall, functioning as a thickened film that reduces heat transfer and control the distance between the flame and the back wall. The novel composite cooling scheme worked effectively at reaction state and achieved an almost 20% reduction of wall temperature, and the cooling effect of the 30-deg faced film hole was much better than that of the 60-deg faced film hole. Flame stabilization was influenced by the increasing mass rate of cooling air. The oversize distance of flame stabilizing zone and back wall easily caused flame instability even blow-off. 0.3% and 0.5% mass rate of cooling air were enough to achieve flame stabilization and work effectively to decrease wall temperature. The novel air-cooled flame holder saw both flame stability and cooling effectiveness. Because of its compact design and resistance to thermal ablation, it can be of great importance for an advanced burner.

An extensive numerical study of the burning dynamics of wildland fuel using proposed configuration space

Physics-based flame models capable of predicting small-scale fire behaviors reduce computational power needed for predicting fires of large- and giga-scale. However, classical model correlations are often developed for ‘free fires’ without considering vegetation around. These models may result in inaccurate fire modeling due to wrong ‘prior’ flame shape estimated from θ ~ wind speed. To overcome this defect, three-dimensional small-scale fires with fireline intensity of 100 KW/m are numerically simulated using large eddy simulation. Fire behaviors such as flame tilt angle and heat transfer mechanisms are extensively studied using a newly proposed configuration space {NC, CdLAI}. The former one represents the ratio between fire to wind power, and the latter one considering the vegetation effect is for the first time introduced in flame models. Using the configuration space, two model correlations for flame tilt angle and radiative heat power reaching the unburnt fuels are proposed. The flame tilt angle θ is directly related to CdLAI (CdαsσsHF/2), while inversely related to NC (2gI/ρ0Cp,0T0U03), in contrast to the model proposed for radiative heat power. Comparisons with several classical models evidenced the capability of new flame models in predicting both free and non-free fires. The limits of the validity of the newly proposed models are also discussed.

Experimental investigation on flame spread over jet fuel with influence of external heat radiation

The development of liquid fires commonly accompanies with the external heat radiation. This subject concentrates on the laboratory-scale experiments (100 cm × 4.4 cm × 2.0 cm) on flame spread over jet fuel of RP-3 with radiant heat fluxes of 1.1, 1.69, 2.25, 2.8 and 3.6 kW/m2. The results show that the flame behaviors, subsurface flow and heat transfer mechanism depend strongly on the intensity of external heat flux. The measurements of flame pulsation frequency and flame spread rate verify that they increase significantly initially but augment slightly afterwards with the externally applied heat flux, indicating that the heat flux is principally used for raising the liquid’s temperature in the initial stage of flame spreading. The heat exchanges involving flame spread process, namely, external heat radiation, convective heat flux, conductive heat flux, flame radiation, and re-radiant heat losses are analyzed. The energy calculation model confirms that the total heat flux is approximately equal to the sum of heat fluxes from radiant panel and convective flow. The scaling analysis of velocity of subsurface flow proves that it should be linearly related to the externally applied heat flux in the correlation of us∼q˙er2/3.

Experimental study of flame heat transfer in a vertical turbulent wall fire

This paper presents experimental measurements in a vertical turbulent wall fire, with a primary goal of developing a deep understating and database for heat transfer on solid surfaces where fuel pyrolyzes. An enhanced porous-plate vertical wall-fire burner was developed, and experiments were conducted with various mass transfer rates of ethylene to measure flame temperature, radiative power, spectral emission, radiative and convective heat flux. The maximum mean flame temperature is found to decrease with height, and the temperature profile in the outer flame boundary layer to be self-similar. The radiant power and heat flux show a linear increase vertically in the pyrolysis region and the peak shifts to higher evaluation with a greater fuel flow rate. Soot emission contributes a dominant fraction to thermal radiation. On the other hand, the convective heat flux promptly reduces from the flame leading edge in the laminar region at the bottom of the wall-burner and remains relatively steady in the downstream turbulent region. Convective heat flux dominates the heat transfer in the lower height while radiative heat transfer is more prominent in the upper region of the wall.

NOx emission reduction reaction of ammonia-hydrogen with self-sustained pulsating oscillations

Unlike hydrocarbon fuel, ammonia (NH3) is an alternative but promising carbon-free renewable fuel. The utilization of NH3 as an energy resource can effectively reduce greenhouse gas CO2 emission. Here we explore interdisciplinary physics of ammonia-hydrogen-fueled combustion-sustained pulsating oscillations and its impact on NOx emission via numerical simulations. A longitudinal combustor with both ends acoustically open is modeled. The premixed flame and constant-temperature heat exchangers are implemented upstream and downstream respectively. With the model validated, it is applied to gain insights on the effects of 1) the mass flow rate ṁ of the fuel mixture, 2) the hydrogen mass fraction wH2 relative to the fuel mixture, and 3) the temperature TH of the heat exchangers. It is found that nonlinear pulsating oscillations could be generated, depending on ṁ Furthermore, the oscillation amplitude and dominant mode frequency are shown to strongly depend on ṁ andwH2. In addition, as wH2 is reduced from 45% to 15%, intermittent oscillations with a period of 100 s are observed to superimpose on the acoustic resonance nature of the combustor with a time scale of 10−2 s. Finally, varying temperatures of the heat exchanger are found to affect NOx generation and decrease the amplitude of the pulsating oscillations somehow.

Experimental investigation on the influence of biogas in diesel engine powered with silkworm larval litter and cashew nut fruit

ABSTRACT The poor thermal properties of biogas, such as spark and flame point and heating value, cause the auto-ignition problem in an internal combustion engine. This increases the consumption of fuel and hydrocarbon emissions. It is required to improve the engine output aspects and minimize emissions in diesel engine mode. In this regard, an attempt was made in which biogas was produced from the mixture of silkworm larval litter and cashew nut fruit. Different blends of diesel and biogas were used to power a single-cylinder four-stroke diesel engine. The engine has experimented at a constant compression ratio of 16 and speed of 1500 rpm with varying load conditions of 20%, 40%, 60%, 80%, and 100%. Various parameters such as brake thermal efficiency, brake specific fuel consumption, heat release maximum pressure were determined at different loading conditions. Exhaust gases were subjected to emission analysis. From the results, it was observed that brake thermal efficiency was reduced from 22% (100% diesel) to 17% (70% diesel+30% biogas) at 80% load condition. There was a reduction in NOx and CO levels when the engine was operated with dual-fuel mode.

Hot Gas Forming of Aluminum Alloy Tubes Using Flame Heating

Hot metal gas forming (HMGF) is a desirable way for the automotive industry to produce complex metallic parts with poor formability, such as aluminum alloys. A simple hot gas forming method was developed to form aluminum alloy tubes using flame heating. An aluminum alloy tube was heated by a flame torch while the tube was rotated and compressed using a lathe machine and simultaneously pressurized with a constant air pressure. The effects of the internal pressure and axial feeding on expansion and wall thickness distribution were examined. The results showed that the proposed gas forming method was effective for forming aluminum alloy tubes. It was also indicated that axial feeding is a vital parameter to prevent reductions in wall thickness by supplying the material flow during the forming process.

Visualisation and performance evaluation of biodiesel/methane co-combustion in a swirl-stabilised gas turbine combustor

While dual fuel firing of power generation combustion systems can improve the fuel flexibility of such systems, several studies on compression ignition engines have also shown a positive impact on NOX and PM emissions. Previous multiphase fuel combustion studies for combustion turbines are limited, thus the present study addresses that gap by fuelling a model swirl stabilised gas turbine combustor with a blend of waste cooking oil-derived biodiesel and methane. Methane was increasingly injected into swirling combustion air flow while simultaneously reducing the biodiesel spray flowrate across a pressure atomiser, thus maintaining an overall equivalence ratio of 0.7 while delivering a thermal power output of 15 kW in all cases, except for flame stability range trials. Direct flame imaging, CH* and C2* chemiluminescence imaging, post combustion emissions as well as stability performance of the flames were evaluated. NOX emissions were found to decrease by 29% and unburnt hydrocarbons increased by 10% as the fraction of methane in fuel mix increased to 30%. Further, flame images suggest increased wrinkling and perturbing of the flame front as gas fraction of the biodiesel/methane flame increases. However, the temporal variation of integral intensity of CH* and C2* species chemiluminescence point to at least an 8% improvement in flame stability when 30% of flame heat output is supplied by methane compared to neat biodiesel burn. Also, it was found that flame stability limits reduce as methane partly replaces biodiesel in the flame.

An analytical model for the transversely forced flame transfer functions of conical and V-flame dynamics

Heat release rate perturbations of two-dimensional asymmetric premixed slot flames subjected to transverse perturbations, so called flame transfer functions (FTFs), are theoretically studied in this paper. The proposed models of both the two-dimensional conical flame and V-flame are studied based upon the linear analysis. The flame front dynamic response under the transverse disturbance is captured and solved using the G-equation model. The analytical solution of the FTF is derived using different oncoming velocity perturbation models, in order to reveal the flame heat release rate response mechanism to the transverse disturbances. In the two-dimensional conical flame model, the flame front tip moves with the transverse perturbations, which in turn also affects the FTF. In the two-dimensional V-flame model, the flame front tips are intersected by the flame branches and the unburned gas boundaries; this also contributes to the different behaviours of the FTF. The linear flame tip dynamics and the explicit analytical models of the FTFs of both the conical flame model and the V-flame model are proposed and validated. Results show that the transverse disturbance propagation velocity model has a significant effect on the FTF. The movement of the flame tip is also affected by the transverse convection models.