Increased Heat Release Research Articles

Numerical analysis of turbulent flame enhancement by nanosecond repetitively pulsed plasma discharges

Experimental studies of plasma-assisted combustion have shown that Nanosecond Repetitively Pulsed (NRP) discharges are a very efficient way to improve flame stability in lean regimes. These discharges generate a non-equilibrium plasma that induces a local heating and an important production of active species sufficient to enhance the combustion. The aim of this article is to understand the physical and chemical mechanisms involved in plasma-assisted turbulent combustion. High-performance computations of a lean bluff-body turbulent premixed methane-air flame, experimented at EM2C laboratory, are conducted for that purpose. Simulations are performed by combining a semi-empirical plasma model with an analytical combustion mechanism. Without electrical discharges, only a weak reaction zone behind the bluff body is observed. However, when the flame is assisted with the plasma, the flame power and surface increase significantly. A chemical analysis performed at the flame basis shows that the heat and the radical O produced by NRP discharges induce the dissociation of main burnt gases species H2O and CO2 into radicals OH, H, CO, O and species H2, O2. The species with long lifespan OH, H2 and O2 are convected from the center of the bluff-body recirculation zone to the flame front where they are consumed increasing the chemical reactivity. This causes a local increase of heat release rate that attaches the turbulent flame front which can develop downstream.

Effect of immersion time on the combustion characteristics of oil‐impregnated transformer insulating paperboard

AbstractTransformer insulating paperboard is an insulating material commonly used in the oil‐filled equipment in substations, which is usually immersed in the transformer oil. In this study, the effects of immersion time on the combustion characteristics of oil‐impregnated transformer insulating paperboard are studied using the thermogravimetric, UL94 vertical flammability, and cone calorimeter experiments. The experimental results show that the total oil content increases with increasing immersion time and reaches the saturation state in 240 h, with a saturation content of 14.7% ± 0.5%. However, the distribution of oil content inside the paperboard is uneven. At the saturation state, the surface oil content is 35.5%, being about 3 times of the core oil content. The results of UL94 vertical flammability experiments show that the flame spread rate increases with increasing immersion time and is controlled by the total oil content. In the cone calorimeter experiments, the time to ignition is significantly reduced compared to the clean paperboard. Unlike the flame spread rate, the ignition time is controlled by the surface oil content and barely varies with immersion time. The critical heat flux of oil‐impregnated transformer insulating paperboard is found as 3.9 kW/m2, being only 1/3 of clean paperboard. With increasing immersion time, the peak heat release rate increases, while the total heat release, fire growth index, and effective heat of combustion present linear correlations with the total oil content. The CO yield increases with increasing immersion time, and the released CO mainly comes from the combustion of transformer oil.

The Transition and Spread of a Chaparral Crown Fire: Insights from Laboratory Scale Wind Tunnel Experiments

Fire occurring in the chaparral behaves as a crown fire, a dual-layer fire that typically ignites in a dead surface fuel layer and transitions to an elevated live crown layer where it continues to spread. In chaparral fuels including chamise, a dominant species in southern California, flame transition to live crown fuels is associated with higher spread rates and greater fire intensity. Despite the relative importance of surface-to-crown transition and crown fire spread, most fire models represent chaparral fire as surface fire, therefore omitting key behavior processes driving this fire system. The purpose of this study was to characterize transition and spread behavior in chaparral fires modeled experimentally as crown fires. We examined heat release rate in the surface and crown fuel layers, time to transition, flame height, and rate of spread in wind-driven and nonwind-driven fires at two crown base heights. Our results showed that wind increased heat release rate, rate of spread, and flame height. A marked increase in heat release rate was observed in wind-driven fires, where adding wind produced an increase from 328 kW to 526 for a crown base height of 0.6 m and from 243 kW to 503 kW for a crown base height of 0.7 m. Further, crown base height served to decrease heat release rate and rate of spread for wind-driven and nonwind-driven fires.

Study on the variation rule of smoke back‐layering length of tunnel ceiling jet induced by strong fire plume

AbstractTunnel fire is a major research topic in tunnel safety. Because of the tunnel's narrow and enclosed structure, smoke movement plays an important role in the investigation of the tunnel fire. In this paper, the Froude similarity principle is used to study the variation rule of the smoke back‐layering length of tunnel ceiling jet induced by strong fire plume with different heat release rates, longitudinal ventilation velocities, and effective heights of fire source analyzed. It is found that the smoke back‐layering length of tunnel ceiling jet induced by strong fire plume increases with the increase of effective height of fire source and decreases with the increase of longitudinal ventilation velocity. When the heat release rate is relatively small (), the smoke back‐layering length of the tunnel ceiling jet induced by strong fire plume increases as the heat release rate increases. However, for the larger heat release rate (), the smoke back‐layering length of the tunnel ceiling jet induced by strong fire plume is mainly related to the longitudinal ventilation velocity. Based on Thomas's model and Li's model, by introducing the correction coefficient, correction formulas of the smoke back‐layering length of tunnel ceiling jet induced by strong fire plume changing with the effective height of fire source are proposed.

Numerical Simulation on the Effect of Fire Shutter Descending Height on Smoke Extraction Efficiency in a Large Atrium

In this study, a series of numerical simulations were carried out to investigate the effect of fire shutter descending height on the smoke extraction efficiency in a large space atrium. Based on the full-scale fire experiments, this paper carried out more numerical simulations to explore factors affecting the smoke extraction efficiency in the atrium. The smoke flow characteristics, temperature distribution law and smoke extraction efficiency of natural and mechanical smoke exhaust systems were discussed under different heat release rates and fire shutter descending heights. The results show that the smoke spread rate and the average temperature of the smoke are higher with a greater heat release rate. After the mechanical smoke exhaust system is activated, the smoke layer thickness and smoke temperature decrease, and the stable period of heat release rate is shorter. In the condition of natural smoke exhaust, the smoke extraction efficiency increases exponentially with the increase of heat release rate and the descending height of the fire shutter, and the maximum smoke extraction efficiency is 48.8%. In the condition of mechanical smoke exhaust, the smoke extraction efficiency increases with the increase of mechanical exhaust velocity. When the velocity increases to the critical value (8 m/s), the smoke extraction efficiency is essentially stable. The smoke extraction efficiency is increased first with the increase of fire shutter descending height and then has a downward trend when the descending height drops to half, and the maximum smoke extraction efficiency is 70.3% in the condition of mechanical smoke exhaust. Empirical correlations between the smoke extraction efficiency and the dimensionless fire shutter descending height, the dimensionless heat release rate and the dimensionless smoke exhaust velocity have been established. The results of this study can provide a reference for the design of smoke prevention and exhaust systems in the atrium.

Barents-Kara sea-ice decline attributed to surface warming in the Gulf Stream

Decline in winter sea-ice concentration (SIC) in the Barents-Kara Sea significantly impacts climate through increased heat release to the atmosphere. However, the past Barents-Kara SIC decrease rate is underestimated in the majority of Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models. Here we show that climate model simulations can reproduce the Barents-Kara SIC trend for 1970–2017 when sea surface temperature (SST) variability in the Gulf Stream region is constrained by observations. The constrained warming of the Gulf Stream strengthens ocean heat transport to the Barents-Kara Sea that enhances the SIC decline. The linear trends between the SIC and SST are highly correlated in the CMIP6 ensemble, suggesting that the externally forced component of the Gulf Stream SST increase explains up to 56% of the forced Barents-Kara SIC trend. Therefore, future warming of the Gulf Stream can be an essential pacemaker of the SIC decline.

Impact of different nano additives on performance, combustion, emissions and exergetic analysis of a diesel engine using waste cooking oil biodiesel

Biodiesel is derived from waste cooking oil (WCO) by transesterification. Methyl ester was prepared by mixing diesel and biodiesel oils as 20% by volume. Nano particles as TiO2, Al2O3 and CNTs were blended with biodiesel blend at different concentrations of 25, 50, and 100 mg/l to enhance the physicochemical fuel characteristics to obtain clean and efficient combustion performance. An experimental setup was incorporated into a diesel engine to investigate the influence of these nano-materials on engine performance, exergy analysis, combustion characteristics and emissions using WCO biodiesel-diesel mixture. Enriching methyl ester mixture with 100 ppm titanium, alumina and CNTs (B20T100, B20A100 and B20C100) increased the thermal efficiency by 4%, 6% and 11.5%, respectively compared to B20. Biodiesel blending with nano additives B20T100, B20A100 and B20C100 decreased the emissions of CO (11%, 24% and 30%, respectively), HC (8%, 17% and 25%, respectively) and smoke (10%, 13% and 19%, respectively) compared to B20. However, the noticeable increase of NOx was estimated by 5%, 12% and 27% for B20T100, B20A100 and B20C100, respectively. Finally, the results showed the rise in peak cylinder pressure by 5%, 9% and 11% and increase in heat release rate by 4%, 8% and 13% for B20T100, B20A100 and B20C100, respectively. The fuel exergy of B20T100, B20A100 and B20C100 are lower than biodiesel blend B20 by 6.5%, 16% and 23% but the exergetic efficiency are increased by 7%, 19% and 30% at full load about B20.

Influence of Palm Oil Biodiesel Blend Preheating on Performance Parameters, Emissions and Combustion Characteristics in a Stationary Diesel Engine

This research evaluates the effect of preheating in different blends of palm oil biodiesel in a stationary diesel engine. For the research development, the characteristics of performance, emissions (CO, CO2, NOx, HC, and smoke opacity), combustion chamber pressure, and heat release rates are evaluated. Experimental tests were conducted at three load conditions (50%, 75%, and 100%) and three biodiesel blends (BP5%, BP10%, and BP15%). To study the effect of preheating, a fuel injection temperature of 30 °C to 70 °C was set. The results obtained show a 6.40% increase in BSFC and a 4.41% reduction in engine BTE when running with biodiesel blends. However, preheating in biodiesel blends led to lower BSFC and higher BTE by 2.08% and 2.12%. The preheating of palm oil biodiesel (BP5%, BP10%, and BP15%) favored the decrease in CO, HC, and engine smoke opacity emission levels by 29%, 37.31%, and 7.84%. The additional presence of oxygen molecules and the increase in fuel injection temperature contribute to the formation of NOx emissions. The increase in injection temperature causes an increase in peak combustion pressure levels and heat release rate by 2.53% and 2.41%. In general, preheating palm oil biodiesel is a promising strategy to minimize the negative effects caused by the high density and viscosity of this type of fuel.

Investigation for Effects of Jet Scale on Flame Stabilization in Scramjet Combustor

Jet scale affects the mixing and combustion of fuel and inflow. With the increase in the scale of scramjet combustors, the study of large-scale jets is particularly significant. The effects of jet scale on flame stability in scramjet combustors were studied by direct-connect combustion experiments. In this paper, the flame distribution characteristics of different jet scales were compared by using similar jet/inflow momentum ratios. The inflow Mach numbers were 2.4 and 3.0, and the total temperature was 1265 K and 1600 K, respectively. The results show that, when the equivalence ratio increases, the combustion intensity increases. Under the condition of same momentum ratio, the increase of jet scale is conducive to fuel injection into the core mainstream, increasing heat release, and the flame stabilization mode will change from cavity stabilization mode to jet-wake stabilization mode. Increasing the distance between jet orifices is not beneficial to combustion, and may even lead to blowoff.

Effect of injection timing on combustion, emission and particle morphology of an old diesel engine fueled with ternary blends at low idling operations

Currently, idling emissions and old vehicle emissions have attracted more and more attention. Many countries have made corresponding policies to limit these emissions. In this study, to investigate the combustion, engine performance, and emission characteristics of an old diesel engine fueled with diesel-palm biodiesel-ethanol (DPE) ternary blended fuels according to various multiple injection strategies under low idling conditions, a series of experiments were carried out on an old common-rail direct injection (CRDI) diesel engine. The engine speed and load were fixed at 750 rpm (the lowest speed) and 30 Nm, respectively. Two test modes of A (only pilot injection timing was variable) and B (only main injection timing was variable) were comparative studied. The experimental results show that the main-pilot injection timings and DPE ternary blends have significant impacts on the engine performance, combustion and characteristics. Overall, as the ethanol concentrations in DPE ternary blends increase, brake specific fuel consumption (BSFC), maximum heat release rate (HRRmax), ignition delay, carbon monoxide (CO) and hydrocarbon (HC) emissions increase, while nitrogen oxides (NOx) and smoke emissions are simultaneously reduced. The high volatility, high latent heat of evaporation and high oxygen content of ethanol may play a major role in reducing NOx and smoke emissions. In addition, the primary particle diameter of all tested fuels is distributed in a narrow range of 18–24 nm, and the addition of ethanol is beneficial to reduce the particle diameter.

Experimental Investigation on the Performance of an Aviation Piston Engine Fueled with Bio-Jet Fuel Prepared via Thermochemical Conversion of Triglyceride

Bio-jet fuels prepared by the thermochemical conversion of triglyceride can be used as complete substitutes of jet fuels. A bio-jet fuel prepared as a substitute of the RP-3 jet fuel and the RP-3 jet fuel itself were, respectively, used to fuel a small aviation piston engine. The characteristic tests of the engine were carried out, and the performances of the power, economy, emissions, and heat release law of the engine fueled with the two fuels were analyzed. The feasibility of the bio-jet fuel as a substitute for the RP-3 jet fuel was proved by the experimental results, which show that when the engine is fueled with the bio-jet fuel, the power and economy performance do not deteriorate; however, the HC emissions increase at small and medium throttle openings, while at large throttle openings, the performances of power and economy decreases, the emissions of HC and NOx increase, and the CO emission decreases. The bio-jet fuel is more prone to spontaneous combustion than the RP-3 jet fuel, so knock combustion would be more likely to occur at large throttle openings, and large cooling air flux is required to cool the cylinder because spontaneous combustion would increase heat release.

Are solid-state batteries safer than lithium-ion batteries?

<h2>Summary</h2> All-solid-state batteries are often assumed to be safer than conventional Li-ion ones. In this work, we present the first thermodynamic models to quantitatively evaluate solid-state and Li-ion battery heat release under several failure scenarios. The solid-state battery analysis is carried out with an Li₇La₃Zr₂O₁₂ solid electrolyte but can be extended to other configurations using the accompanying spreadsheet. We consider solid-state batteries that include a relatively small amount of liquid electrolyte, which is often added at the cathode to reduce interfacial resistance. While the addition of small amounts of liquid electrolyte increases heat release under specific failure scenarios, it may be small enough that other considerations, such as manufacturability and performance, are more important commercially. We show that short-circuited all-solid-state batteries can reach temperatures significantly higher than conventional Li-ion, which could lead to fire through flammable packaging and/or nearby materials. Our work highlights the need for quantitative safety analyses of solid-state batteries.

Long-term effects of ilmenite on a micro-scale bubbling fluidized bed combined heat and power pilot plant for oxygen carrier aided combustion of wood

Small-scale solutions for the application of solid biomass fuels for the generation of heat and power with fluidized beds are hardly available. The main issues comprise agglomeration and slagging, incomplete burnout of combustibles and low efficiencies. During so-called oxygen carrier aided combustion, an oxygen carrier replaces the conventional bed material and balances the local and temporal oxygen supply in the fluidized bed. This results in a more homogeneous combustion and increased heat release in the fluidized bed area. Especially for small-scale bubbling fluidized bed solutions, these effects require an adapted plant setup. However, applications of oxygen carriers in small-scale bubbling fluidized bed boilers are barely reported. Process parameters differ significantly from large-scale circulating fluidized beds regarding resident times, particle volume fractions and size distributions. Hence, this research examines the enhancing effect of 50 wt% ilmenite in a 45 kWth bubbling fluidized bed fired combined heat and power plant. Long-term measurements in a field test environment promise a potential decrease of the carbon monoxide emissions of up to 75 %. Lower fluidized bed temperatures and apparently poor combustion conditions even enhance the positive effect of ilmenite. Moreover, the porous ilmenite particles interact with ash compounds such as potassium, calcium and magnesium. The migration of potassium into the inner particle mitigates the formation of low-melting layers around the particle. Calcium and magnesium form titanium oxide phases in the outer particle layer, below an iron enriched particle shell. Thereby, ilmenite prevents the formation of sticky particle layers, which addresses agglomeration and slagging issues. The particle shell is subject of ongoing attrition leading to a constant entrainment of active material from the combustion chamber. Therefore, a continuous addition of fresh material is essential to sustain the beneficial impact of the oxygen carrier. The optimal operation period for the oxygen carrier additive is supposed to cover 1–2 weeks.

Enhancing energetic performance of metal-organic complex-based metastable energetic nanocomposites by spray crystallization

Energetic metal-organic complexes have been involved in nanothermites as novel oxidants. However, the existing preparation methods often lead to mixing inhomogeneity and small contact area of ingredients, the reactivity and functionality of the novel energetic nanocomposites are still limited. In this work, spray crystallization (SC) method was used to prepare novel energetic nanocomposites, the high-energy metal-organic complex [Ni(CHZ)3](ClO4)2 (CHZ = 1,3-diaminourea) was composited with nano-aluminum (n-Al). Results showed that n-Al/[Ni(CHZ)3](ClO4)2 energetic nanocomposites prepared by SC method increased heat release to 2977.6 J/g and peak pressure to 3.91 MPa with higher pressurization rate (1324.06 MPa/s), decreased sensitivity thresholds (>100 mJ) to electrostatic discharge (ESD) and enhanced detonation ability compared with [Ni(CHZ)3](ClO4)2 alone and physically mixed (PM) n-Al/[Ni(CHZ)3](ClO4)2. These results proved that it is significant to introduce energetic metal-organic complexes with inherent high energy in new-concept n-Al/energetic metal-organic complexes nanocomposites through SC method for a better performance of its application.

Flammability and flame spread behavior of common fuels in Chinese historical buildings: An experimental research

ABSTRACT A feature of Chinese historical buildings is the preservation of many traditional customs, such as using candles and incense burners, and decorating the rooms with some textiles. Therefore, there are both many fire sources and combustible materials indoors, which puts a huge threat to fire safety. Although some researchers have experimentally studied the ignition and fire spread characteristics of several combustible materials, to better conduct fire risk assessment in historical buildings, the dynamic burning behavior of common fuels should be further analyzed. This paper carried out experimental investigations for flammability and fire spread behaviors of combustible materials usually used in Chinese historical buildings, which are made of both natural fiber and synthetics. Considering practical applications, the size of each sample is chosen from standard-testing size to real size. It is summarized that there are large deviations among different materials due to their ingredients, textures, structures, etc. The natural-based materials are easy to be ignited and burn up quickly, and some synthetics reveal sharp increase in heat release rate. For the synthetics, as it can melt during burning, the flame spread process is accompanied by dripping. Once there are some combustibles near it, the dripping part is liable to ignite the combustibles and cause damage to cultural relics. Based on these knowledges, fire hazards of different materials in Chinese historical buildings are evaluated.

Effect of cetane coupled injection parameters on diesel engine combustion and emissions

Experiments on the effects of different cetane coupled injection parameters on diesel engine combustion and emissions were conducted on a supercharged 4-cylinder direct injection diesel engine. The results showed that with the increase of cetane number (CN), the peak in-cylinder pressure and heat release rate of fuel decrease, the coupling injection pressure increase, CO and HC emissions decrease, NOx emissions increase, the number concentration and total mass of particulate matter (PM) decrease. As the CN increases, the peak in-cylinder pressure decreases, the coupling injection timing is delayed, the peak heat release rate increases, brake thermal efficiency (BTE) decrease, brake specific fuel consumption (BSFC) increases, CO and HC emissions increase, NOx emissions decrease, and the number concentration and total mass of PM increases. Fuel characteristics and injection timing significantly influence ignition delay time and combustion duration, where CN = 53.9 fuel has no pre-injection to release heat at different injection timing, which has a more substantial impact on in-cylinder pressure and heat release rate.

Observation of reactive shear layer modulation associated with high-frequency transverse thermoacoustic oscillations in a gas turbine reheat combustor experiment

This paper presents the investigation of high-frequency thermoacoustic oscillations and associated flame dynamics in an experimental gas turbine reheat combustor at atmospheric pressure. Examination of dynamic pressure measurements reveals bursts of high-frequency periodic oscillations which appear randomly amidst stochastic fluctuations in the reheat combustor. Analysis of the flame dynamics during these bursts of periodic behaviour reveals that increased heat release in the reactive shear layers of the reheat flame is associated with greater thermoacoustic driving potential. This redistribution of heat release is likely due to the stochastic nature of auto-ignition kernel formation. To determine the underlying flame-acoustic coupling mechanism behind the driving potential, phase-resolved flame dynamics over the acoustic cycle are investigated which reveal the presence of an oscillatory heat release pattern associated with the first transverse eigenmode. An in-phase interaction between the acoustic field and these heat release oscillations in the shear layer regions indicates that this phenomenon likely constitutes a thermoacoustic driving mechanism. This is an important step towards the development of models for high-frequency thermoacoustic driving mechanisms relevant to reheat combustion systems, which will allow accurate prediction and mitigation of thermoacoustic instabilities in future designs.

Insight into flickering/shedding in buoyant droplet-diffusion flame during interaction with vortex

The interaction of incident vortex with buoyant diffusion flame of a pendant fuel droplet (n-dodecane) is investigated in the present study. The flame stretching/shedding events occur depending on vortex strengths, which are characterized using vortex Reynolds number (Rev). Different flame response regimes have been identified for different vortex strengths. The velocity scale associated with the vortices is higher when compared to natural convection velocity leading to forward extinction, followed by flame lift-off. The flame lift-off increases with Rev, and partial extinction occurs at very high Rev. In all the cases, the flame oscillates/sheds during the vortex interaction, which causes a momentary increase in the flame heat release rate. As the vortex moves past the flame, the oscillations dampen gradually, and the flame regains the natural buoyant behavior. The high-velocity scales imposed during vortex interaction make the flow momentum-dominant, resulting in a low Richardson number (Ri). Since, the flickering frequency scaling valid for Ri>>1 is not obeyed, a new scaling has been proposed to incorporate the vortex momentum effect by using vortex circulation instead of gravity. Only a portion of the incident vortex circulation contributes to the shedding as some of it is lost to forward extinction and lift-off events. Thus, the reduced vortex velocity scale is used to estimate the critical circulation. The flame shedding height calculated using new scaling in the critical circulation equation agrees with experimental observations.

Megafires in a Warming World: What Wildfire Risk Factors Led to California’s Largest Recorded Wildfire

Massive wildfires and extreme fire behavior are becoming more frequent across the western United States, creating a need to better understand how megafire behavior will evolve in our warming world. Here, the fire spread model Prometheus is used to simulate the initial explosive growth of the 2020 August Complex, which occurred in northern California (CA) mixed conifer forests. High temperatures, low relative humidity, and daytime southerly winds were all highly correlated with extreme rates of modeled spread. Fine fuels reached very dry levels, which accelerated simulation growth and heightened fire heat release (HR). Model sensitivity tests indicate that fire growth and HR are most sensitive to aridity and fuel moisture content. Despite the impressive early observed growth of the fire, shifting the simulation ignition to a very dry September 2020 heatwave predicted a &gt;50% increase in growth and HR, as well as increased nighttime fire activity. Detailed model analyses of how extreme fire behavior develops can help fire personnel prepare for problematic ignitions.

Technologies integration to achieve brake thermal efficiency beyond 55% for HD diesel engines

This study focuses on the technologies integration to aim about 60% indicated thermal efficiency (ITE) with a single-cylinder HD diesel engine, roughly equivalent to 55% brake thermal efficiency (BTE) with multiple-cylinder engines. A new thermodynamic cycle concept and a new heat insulation structure were discussed as the key integrated technologies. Otto cycle is the best thermodynamic cycle for ideal thermal efficiency. However, significant increase in heat release rate around top dead center results in higher cooling loss and sometimes in deterioration of late combustion under a higher compression ratio without any improvement in ITE. The newly proposed cycle is the hybrid of isobaric cycle and following steep pressure increase up to peak firing pressure (PFP) constraint from the timing when in-cylinder volume change rate becomes significant, which suppresses the average gas temperature. For further cooling loss reduction, the new in-cylinder wall insulating concept was investigated. The in-cylinder insulating effect was much significant by applying not only on the piston crown but also on the cooling side of piston. Although the test engine hasn’t been optimized yet, ITE was almost reached at about 60% by means of the multiple-injector concept with compression ratio of 23.5:1.