SYNERGISTIC EFFECTS OF LAYERED DOUBLE HYDROXIDE WITH PHOSPHORUS-NITROGEN INTUMESCENT FLAME RETARDANT IN PP/EPDM/IFR/LDH NANOCOMPOSITES

Synergistic effects of layered double hydroxide （LDH） with intumescent flame retardanct （IFR） of phosphorusnitrogen （NP） compound in the polypropylene/ethylene-propylene-diene/IFR/LDH （PP/EPDM/IFR/LDH） nanocomposites and related properties were studied by X-ray diffraction （XRD）, transmission electron microscopy （TEM）, scanning electron microscopy （SEM）, limiting oxygen index （LOI）, UL-94 test, cone calorimeter test （CCT） and thermo-gravimetric analysis （TGA）. The XRD and TEM results show that the intercalated and/or exfoliated nanocomposites can be obtained by direct melt-intercalation of PP/EPDM into modified LDH and that LDH can promote the IFR additive NP to disperse more homogeneously in the polymer matrix. The SEM results provide positive evidence that more compact charred layers can be obtained from the PP/EPDM/NP/LDH sample than those from the PP/EPDM/LDH and PP/EPDM/NP samples during burning. The LOI and UL-94 rating tests show that the synergetic effects of LDH with NP can effectively increase the flame retardant properties of the PP/EPDM/NP/LDH samples. The data from the CCT and TGA tests indicate that the PP/EPDM/NP/LDH samples apparently decrease the HRR and MLR values and thus enhance the flame retardant properties and have better thermal stability than the PP/EPDM/LDH and PP/EPDM/NP samples.

Predicting lean blow-off of bluffbody stabilized flames based on Damkohler number

Lean Blow-Off(LBO) prediction is important to propulsion system design. In this paper,a hybrid method combining numerical simulation and Da(Damk?hler) model is proposed based on bluffbody stabilized flames. In the simulated reacting flow, Practical Reaction Zone(PRZ) is built based on OH radical concentration, and it is considered to be the critical zone that controls LBO.Da number is obtained based on the volume-averaged parameters of PRZ. The flow time scale(s_f)indicates the residence time of the fresh mixture flowing through the PRZ. It is obtained based on the characteristic length and volume-averaged axial velocity of the PRZ. The chemical time scale(s_c) indicates the shortest time needed to trigger the reaction of the mixture. It is obtained by commercial software CHEMKIN through monitoring the transient variation of the reactor temperature. The result shows that the average Da number under different LBO conditions is 1.135(the Da number under each LBO condition ranges from 0.673 to 1.351). This indicates that the flow time scale and chemical time scale are comparable. The combustion is in a critical state where LBO is easy to occur. With the increase of the fuel mass flow rate(the global fuel/air ratio increases from 0.004761 to 0.01095), s_f increases from 0.001268 s to 0.007249 s, and s_c decreases from 0.00124 s to0.00089 s. Accordingly, Da number increases from 1.023 to 8.145, which shows that the combustion becomes more stable. The above results show that the method proposed in the present study can properly predict the LBO limits of combustors, which provides important technical supports for combustor design and optimization.

Thermally stable and flame-retardant poly(vinyl alcohol)/montmorillonite aerogel via a facile heat treatment

Poly（vinyl alcohol）/montmorillonite aerogels with high thermal stability and flame retardancy were prepared with a facile heat treatment method, in which the poly（vinyl alcohol）/montmorillonite aerogels was first prepared by an eco-friendly freeze-drying method, following by a heat treatment process. The structure of the aerogels before and after heat treatment process was characterized by Fourier transform infrared spectroscopy （FTIR） and X-ray photoelectron spectroscopy （XPS）. The thermal stability of the aerogel treated for 3 h at 200 ℃ was improved significantly, which was accompanied by a slight decrease of mechanical property. Meanwhile, cone calorimetry （CC） test revealed that the flame-retardant performance of the heat treated aerogel was improved remarkably.

In situ and ex situ synthesis of poly（vinyl alcohol）-Fe304 nanocomposite flame retardants

Application of flame retardants is limited because of environmental requirements. This work introduces conventional magnetic nanoparticles as a new class of nontoxic and effective flame retardant. Fe3O4 enhanced both the thermal stability and flame retardant properties of a poly（vinyl alcohol） matrix. Nanoparticles were synthesized via a simple precipitation reaction without using an inert atmosphere at room temperature. The effects of different precursors and acrylamide on the morphology of the products were investigated. Nanoparticles exhibited a ferrimagnetic behavior at room temperature. To prepare the magnetic nanocomposite, Fe3O4 nanoparticles were added to the poly（vinyl alcohol）. In the presence of a flame, the magnetic nanoparticles remained together, showed resistance to dripping and protected the polymer matrix. Dispersed nanoparticles play a role of a magnetic barrier layer, which slows product volatilization and prevents flames and oxygen from reaching the sample during decomposition of the polymer.

Stabilization mechanisms of lifted flames in a supersonic stepped-wall jet combustor

Flame stabilization is the key to extending scramjets to hypersonic speeds;accordingly,this topic has attracted much attention in theoretical research and engineering design.This study performed large eddy simulations(LESs)of lifted hydrogen jet combustion in a stepped-wall combustor,focusing on the flame stabilization mechanisms,especially for the autoignition effect.An assumed probability density function(PDF)approach was used to close the subgrid chemical reaction source.The reliability of the solver was confirmed by comparing the LES results with experimental data and published simulated results.The hydrogen jet and the incoming stream were first mixed by entraining large-scale vortices in the shear layer,and stable combustion in the near-wall region was achieved downstream of the flame induction region.The autoignition cascade is a transition of fuel-rich flame to stoichiometric ratio flame that plays a role in forming the flame base,which subsequently causes downstream flame stabilization.Three cases with different jet total temperatures are compared,and the results show that the increase in the total temperature reduces the lift-off distance of the flame.In the highest total temperature case,an excessively large scalar dissipation rate inhibits the autoignition cascade,resulting in a fuel-rich low-temperature flame.

Modelling ignition probability with pairwise mixing-reaction model for flame particle tracking

Reduced order models for ignition analysis can offer insights into ignition processes and facilitate the combustor optimization.In this study,a Pairwise Mixing-Reaction(PMR)model is formulated to model the interaction between the flame particle and the surrounding cell mixture during Lagrangian flame particle tracking.Specifically,the model accounts for the two-way coupling of mass and energy between the flame particle and the surrounding shell layer by modelling the corresponding turbulent mixing,chemical reaction and evaporation process if present.The state of a flame particle,e.g.,burnt,hot gas or extinguished,is determined based on particle temperature.This model can properly describe the ignition process with a spark kernel being initiated in a nonflammable region,which is of practical importance in certain turbine engines and has not been rigorously accounted for by the existing models based on the estimation of local Karlovitz number.The model is integrated into an ignition probability analysis platform and is demonstrated for a methane/air bluff-body flame with the flow and fuel/air mixing characteristics being extracted from a non-reacting simulation.The results show that for the spark location being at the extreme fuellean outer shear layer of the recirculation zone,PMR can yield ignition events with a significant number of active flame particles.The mechanisms for the survival of the initial flame particles and the entrainment of the survived flame particles into the recirculation zone are analyzed.The results also show that the ignition probability map from PMR agrees well with the experimental observation:a high ignition probability in the shear layer of the recirculation zone near the mean stoichiometric surface,and low ignition probabilities inside the recirculation zone and the top stagnation region of the recirculation zone.The parametric study shows that the predicted shape of the ignition progress factor and ignition probability is in general insensitive to the model parameters and the model is adequate for quantifying the regions with high ignition probabilities.

A forced ignition probability analysis method using kernel formation analysis with turbulent transport and Lagrangian flame particle tracking

A forced ignition probability analysis method is developed for turbulent combustion,in which kernel formation is analyzed with local kernel formation criteria,and flame propagation and stabilization are simulated with Lagrangian flame particle tracking.For kernel formation,the effect of turbulent scalar transport on flammability is modelled through the incorporation of turbulenceinduced diffusion in a spherically outwardly propagating flame kernel model.The dependence of flammability limits on turbulent intensities is tabulated and serves as the flammability criterion for kernel formation.For Lagrangian flame particle tracking,flame particles are tracked in a structured grid with flow fields being interpolated from a Computational Fluid Dynamics(CFD)solution.The particle velocity follows a Langevin model consisting of a linear drift and an isotropic diffusion term.The Karlovitz number is employed for the extinction criterion,which compares chemical and turbulent timescales.The integration of the above two-step analysis approach with non-reacting CFD is achieved through a general interpolation interface suitable for general unstructured CFD grids.The method is demonstrated for a methane/air bluff-body flame,in which flow and fuel/air mixing characteristics are extracted from a non-reacting simulation.Results show that the computed ignition probability map agrees qualitatively with experimental results.A reduction of the ignition probability in the recirculation zone and a high ignition probability on the shear layer of the recirculation zone near the mean stoichiometric surface are well captured.The tools can facilitate optimization of spark placement and offer insights into ignition processes.

Research progress on solid-fueled Scramjet

The solid-fueled Scramjet is an interesting option for supersonic combustion ramjet.It shows significant advantages such as simple fuel supply and compactness,avoiding the complex system of tanks and pipelines that encountered in liquid-fueled Scramjets.The solid-fueled Scramjet could be the simplest air-breathing engine for the hypersonic flight regime.This paper presents a comprehensive and systematic review of the research progress on solid-fueled Scramjet in various institutes and universities.It summarizes a progress overview of three types of the solid-fueled Scramjet,which covers a wealth of landmark numerical and experimental results.Based on this,several relevant key technologies are proposed.Several inherent scientific issues are refined,such as the mixing mechanism of multi-phase flow and supersonic airflow,ignition and combustion mechanism of the condensed phase in a supersonic airflow,and coupling mechanism of gas and solid phase in a supersonic flow.Finally,the historical development trend is clarified,and some recommendations are provided for future solid-fueled Scramjet.

Experimental research of air-throttling ignition for a scramjet at Ma 6.5

An experimental investigation on ignition characteristics with air-throttling in an ethylene-fueled scramjet under flight Ma 6.5 conditions was conducted.The dynamic process of air-throttling ignition was explored systematically.The influences of throttling parameters,i.e.,throttling mass rate and duration,were investigated.When the throttling mass rate was 45% of the inflow mass rate,ambient ethylene could be ignited reliably.The delay time from ignition to throttling was about 45–55 ms.There was a threshold of throttling duration under a certain throttling mass rate.It was shorter than 100 ms when the throttling mass rate was 45%.While a 45%throttling mass rate would make the shock train propagate upstream to the isolator entry in about10–15 ms,four lower throttling mass rates were tested,including 30%,25%,20%,and 10%.All of these throttling mass rates could ignite ethylene.However,combustion performances varied with them.A higher throttling mass rate made more ethylene combust and produced higher wall pressure.Through these experiments,some aspects of the relationships between ignition,flame stabilization,combustion efficiency,and air-throttling parameters were brought to light.These results could also be a benchmark for CFD validation.

Experimental study of hysteresis and catastrophe in a cavity-based scramjet combustor

Hystereses and catastrophes were experimentally investigated in a cavity-based scramjet combustor.The inflow Mach number was 3.0.Fuel Equivalence Ratio(ER)was continuously regulated with multi-steps to explore influences of historical regulation directions on combustion states.Two divided hysteresis loops with catastrophes were observed.By 1-D flow estimations,the first loop occurred with shock-free/separated scramjet mode transitions,while the second kept in the separated scramjet mode.This breaks through the traditional knowledge that hysteresis and catastrophe were certainly related to ramjet/scramjet mode transitions.The first hysteresis and catastrophes were attributed to flame stabilization mode transitions between the cavity shearlayer stabilized and the jet-wake stabilized,with flow separation establishment/vanishment upstream the cavities.The obvious variations of flame and shock/separation structures meant large wall-pressure changes in the expansive duct,and generated obvious thrust catastrophes.Besides,transition ER and catastrophe were larger in historical ER-increasing path because combustion efficiency became obviously larger as flow separation established.Difference of critical transition ERs meant the first hysteresis.The second hysteresis and catastrophes in the jet-wake stabilized mode were attributed to flame/shock interaction mode transitions between the flame/shock weak interaction mode and intensive interaction mode.Each transition caused slightly stronger/weaker flame interacting with slightly larger/smaller flow separation,which meant small wall-pressure changes in the expansive duct,and thus thrust catastrophe was unobvious.Hysteresis occurred as the critical transition ER was slightly higher in historical ER-increasing path because of slightly lower combustion efficiency under slightly smaller separation.

Recent progress in electric-field assisted combustion: a brief review

The control of combustion is a hot and classical topic.Among the combustion technologies,electric-field assisted combustion is an advanced technology that enjoys major advantages such as fast response and low power consumption compared with thermal power.However,its fundamental principle and impacts on the flames are complicated due to the coupling between physics,chemistry,and electromagnetics.In the last two decades,tremendous efforts have been made to understand electric-field assisted combustion.New observations have been reported based on different combustion systems and improved diagnostics.The main impacts,including flame stabilization,emission reduction,and flame propagation,have been revealed by both simulative and experimental studies.These findings significantly facilitate the application of electric-field assisted combustion.This brief review is intended to provide a comprehensive overview of the recent progress of this combustion technology and further point out research opportunities worth investigation.