Modeling of one-dimensional smoldering of polyurethane in microgravity conditions

Results are presented from a model of forward smoldering combustion of polyurethane foam in microgravity. The transient one-dimensional numerical-model is based on that developed at the University of Texas at Austin. The conservation equations of energy, species, and mass in the porous solid and in the gas phases are numerically solved. The solid and the gas phases are not assumed to be in thermal or in chemical equilibrium. The chemical reactions modeled consist of foam oxidation and pyrolysis reactions, as well as char oxidation. The model has been modified to account for new polyurethane kinetics parameters and radial heat losses to the surrounding environment. The kinetics parameters are extracted from thermogravimetric analyses published in the literature and using Genetic Algorithms as the optimization technique. The model results are compared with previous tests of forward smoldering combustion in microgravity conducted aboard the NASA Space Shuttle. The model calculates well the propagation velocities and the overall smoldering characteristics. Direct comparison of the solution with the experimental temperature profiles shows that the model predicts well these profiles at high temperature, but not as well at lower temperatures. The effect of inlet gas velocity is examined, and the minimum airflow for ignition is identified. It is remarkable that this one-dimensional model with simplified kinetics is capable of predicting cases of smolder ignition but with no self-propagation away from the igniter region. The model is used for better understanding of the controlling mechanisms of smolder combustion for the purpose of fire safety, both in microgravity and normal gravity, and to extend the unique microgravity data to wider conditions avoiding the high cost of space-based experiments.     

Combustion mechanism of ultralean rotating counterflow twin premixed flame

In our previous numerical studies [Nishioka Makihito, Zhenyu Shen, and Akane Uemichi. “Ultra-lean combustion through the backflow of burned gas in rotating counterflow twin premixed flames.” Combustion and Flame 158.11 (2011): 2188–2198. Uemichi Akane, and Makihito Nishioka. “Numerical study on ultra-lean rotating counterflow twin premixed flame of hydrogen–air.” Proceedings of the Combustion Institute 34.1 (2013): 1135–1142]. we found that methane– and hydrogen–air rotating counterflow twin flames (RCTF) can achieve ultralean combustion when backward flow of burned gas occurs due to the centrifugal force created by rotation. In this study, we investigated the mechanisms of ultralean combustion in these flames by the detailed numerical analyses of the convective and diffusive transport of the main species. We found that, under ultralean conditions, the diffusive transport of fuel exceeds its backward convective transport in the flame zone, which is located on the burned-gas side of the stagnation point. In contrast, the relative magnitudes of diffusive and convective transport for oxygen are reversed compared to those for the fuel. The resulting flows for fuel and oxygen lead to what we call a ‘net flux imbalance’. This net flux imbalance increases the flame temperature and concentrations of active radicals. For hydrogen–air RCTF, a very large diffusivity of hydrogen enhances the net flux imbalance, significantly increasing the flame temperature. This behaviour is intrinsic to a very lean premixed flame in which the reaction zone is located in the backflow of its own burned gas.     

Transition from forward smoldering to flaming in small polyurethane foam samples

Experimental observations are presented of the effect of flow velocity, oxygen concentration, and a thermal radiant flux on the transition from smoldering to flaming in forward smoldering of small samples of polyurethane foam with a gas/solid interface. The experiments are part of a project studying the transition from smoldering to flaming under conditions encountered in spacecraft facilities, i.e., microgravity, low velocity variable oxygen concentration flows. Because the microgravity experiments are planned for the International Space Station, the foam samples had to be limited in size for safety and launch mass reasons. The feasible sample size is too small for smolder to self-propagate because of heat losses to the surroundings. Thus, the smolder propagation and the transition to flaming had to be assisted by reducing heat losses to the surroundings and increasing the oxygen concentration. The experiments are conducted with small parallelepiped samples vertically placed in a wind tunnel. Three of the sample lateral-sides are maintained at elevated temperature, and the fourth side is exposed to an upward flow and a radiant flux. It is found that decreasing the flow velocity and increasing its oxygen concentration, and/or increasing the radiant flux enhances the transition to flaming and reduces the time delay to transition. Limiting external conditions for the transition to flaming are reported for this experimental configuration. The results show that smolder propagation and transition to flaming can occur in relatively small fuel samples if the external conditions are appropriate. The results also indicate that transition to flaming occurs in the char region left behind by the smolder reaction, and it has the characteristics of a gas-phase ignition induced by the smolder reaction, which acts as the source of both gaseous fuel and heat. A simplified energy balance analysis is able to predict the boundaries between the transition/no transition regions.     

非绝热正向阴燃波的结构研究

本文通过活化能渐近分析方法研究了非绝热正向阴燃波的两种典型结构，即反应滞后结构和反应前导结构，比较了当来流空气流量发生变化时两种结构的主要特征参数的变化规律。研究结果表明，相比较于绝热情况，当考虑阴燃向外界的对流热损失效应时，两种正向阴燃结构随来流空气流量变化时的一维响应均表现为两个解分支，相应地存在一个熄灭极限，对应...     

Numerical investigation of low-frequency instability and frequency shifting in a scramjet combustor

The combustion instability in a laboratory-scale direct-connect hydrogen-fueled scramjet combustor is investigated numerically. The numerical simulation has been carried out using a delayed detached eddy simulation (DDES) with a detailed reaction mechanism. The computational framework has high fidelity by applying multi-dimensional high order accurate schemes for handling convective and viscous fluxes. The field data were accumulated up to 100 milliseconds on each case to capture sufficiently the repetitive behavior of low-frequency instability of order of 100 Hz. The numerical results exhibit the formation/dissipation of pressure and shock wave induced by continuous heat release in the combustor. This motion of pressure/shock wave, so-called upstream-traveling shock wave, presents repeated dynamics between isolator and combustor with a period of several milliseconds. With this periodic hydrodynamic characteristic, the upstream-traveling shock wave interacts with the boundary layer and injected fuel stream affecting fuel/air mixing and burning, and finally inducing the combustion instability in a scramjet combustor. Frequency analysis derived major instability frequencies of 190 Hz and 450 Hz in the isolator and combustor for low and high equivalence ratios, respectively. Current numerical results present the underlying flow physics on the shifting of the instability frequency by changing the equivalence ratio observed by the previous experimental studies. The fact that an instability frequency exists homogeneously from isolator to combustor informs that the combustion instability of scramjet engine is the fully coupled flow/combustion dynamics throughout the engine on a macroscopic scale.     

A numerical study on extinction behaviour of laminar micro-diffusion flames

We conducted a numerical study on the fluid dynamic, thermal and chemical structures of laminar methane–air micro flames established under quiescent atmospheric conditions. The micro flame is defined as a flame on the order of one millimetre or less established at the exit of a vertically-aligned straight tube. The numerical model consists of convective–diffusive heat and mass transport with a one-step, irreversible, exothermic reaction with selected kinetics constants validated for near-extinction analyses. Calculations conducted under the burner rim temperature 300 K and the adiabatic burner wall showed that there is the minimum burner diameter for the micro flame to exist. The Damköhler number (the ratio of the diffusive transport time to the chemical time) was used to explain why a flame with a height of less than a few hundred microns is not able to exist under the adiabatic burner wall condition. We also conducted scaling analysis to explain the difference in extinction characteristics caused by different burner wall conditions. This study also discussed the difference in governing mechanisms between micro flames and microgravity flames, both of which exhibit similar spherical flame shape.     

切向气流对激光辐照树脂基复合材料的影响

 为了考察切向强迫气流对激光辐照下树脂基复合材料热响应的影响,基于边界层换热理论,研究了切向气流与靶面的对流换热系数和热分解气体对表面热交换的覆盖效应,并用有限差分法对激光辐照下树脂基复合材料的1维热响应模型进行数值求解。数值计算表明：高速切向气流的存在会加速靶材表面与外部环境的热交换,从而明显降低激光对靶材的加热效率;边界层换热理论给出的对流换热系数和覆盖因子是合理有效的,适用于数值模拟切向气流对激光辐照下树脂基复合材料热响应的影响;向靶材表面溢出的热分解气体对靶材表面与外部环境的热交换有一定的抑制作用,但影响较小,基本可以忽略不计。     

Fingering instability in combustion: an extended view

We detail the experimental situation concerning the fingering instability that occurs when a solid fuel is forced to burn against a horizontal oxidizing wind. The instability appears when the Rayleigh number for convection is below criticality. The focus is on the developed fingering state. We present direct measurements of the depletion of oxygen by the front as well as new results that connect heat losses to the characteristic scale of the instability. In addition, we detail the experimental system, elaborate (qualitatively and quantitatively) on the results that were previously presented, and discuss new observations. We also show that the same phenomenological model applies to electrochemical deposition.     

Characteristic analysis of low-velocity gas filtration combustion in an inert packed bed

This paper investigates the low-velocity filtration combustion of lean methane–air mixtures occurring in inert packed beds by using a modified one-temperature model, considering the axial thermal diffusion owing to the convective gas–solid heat transfer. Based on the scaling analysis of various transport terms in different conservation equations, a high-activation energy asymptotic method is applied in the flame zone and results in a set of powerful analytical solutions for combustion macrocharacteristics under the fully developed conditions. These are then combined with the eigenvalue method of the modified one-temperature model in the whole flow region to study the flame behaviour analytically and numerically. Our results have shown that the combustion wave velocity is a key characteristic parameter in the filtration combustion process. Compared with other existing theoretical results, the present analytical solutions demonstrate the intricate relationships among the combustion wave velocity, the flame speed, the peak flame temperature and the effects of the variable thermo-physical properties, and show better prediction performance for the combustion wave velocity, the flame speed and the peak flame temperature. Excellent agreements with experimental results have been observed, especially for very lean filtration combustion with stream-wise propagating combustion fronts.     

Influence of double diffusive effects on miscible viscous fingering

Miscible viscous fingering classically occurs when a less viscous fluid displaces a miscible more viscous one in a porous medium. We analyze here how double diffusive effects between a slow diffusing S and a fast diffusing F component, both influencing the viscosity of the fluids at hand, affect such fingering, and, most importantly, can destabilize the classically stable situation of a more viscous fluid displacing a less viscous one. Various instability scenarios are classified in a parameter space spanned by the log-mobility ratios R(s) and R(f) of the slow and fast component, respectively, and parametrized by the ratio of diffusion coefficients δ. Numerical simulations of the full nonlinear problem confirm the existence of the predicted instability scenarios and highlight the influence of differential diffusion effects on the nonlinear fingering dynamics.     

壁面轴向导热对微细管内对流换热的影响

本文通过数值解析的方法研究了考虑壁面轴向导热时微细管内的对流换热。结果表明,当管外为对流换热边界条件时,管内充分发展对流换热的Nu依然在3.66～4.36之间。但若忽略壁面轴向导热,采用一维热阻模型整理微细管内对流换热的实验数据将会导致错误的结论。     

Effect of structural conduction and heat loss on combustion in micro-channels

This paper presents a simple analytical model for the effects of heat exchange within the structure of a micro-channel combustor, and heat loss from the structure to the environment. This is accomplished by extending reasoning similar to that employed by Mallard and Le Chatelier in their thermal theory for flame propagation. The model is used to identify some of the basic parameters that must be considered when designing an efficient micro-combustor and its predictions are compared with the results of a numerical simulation of stoichiometric premixed combustion of a hydrogen–air mixture stabilized between two parallel plates. The simulation incorporates a one-dimensional continuity/energy equation solver with full chemistry coupled with a model for thermal exchange in the structure. The results show that heat exchange through the structure of the micro-combustor can lead to a broadening of the reaction zone. Heat loss to the environment decreases the broadening effect and eventually results in flame quenching. This behaviour, which arises from the thermal coupling between the gas and the structure, influences the maximum achievable power density of microscale combustors.     

On disintegration of near-limit cellular flames

A strongly non-linear geometrically-invariant model for the dynamics of near-limit cellular flame is proposed, where the flame evolution is governed by a system of equations for the flame interface and its temperature. The model generalizes its earlier weakly non-linear version pertinent to a mildly perturbed planar flame. Numerical simulations of the new model show that at sufficiently high levels of heat losses the cellular flame resulting from the diffusive instability exhibits a tendency toward self-fragmentation, quite in line with direct numerical simulations of the associated reaction-diffusion system.     

Quantum thermal transport from classical molecular dynamics

Using a generalized Langevin equation of motion, quantum thermal transport is obtained from classical molecular dynamics. This is possible because the heat baths are represented by random noises obeying quantum Bose-Einstein statistics. The numerical method gives asymptotically exact results in both the low-temperature ballistic transport regime and the high-temperature strongly nonlinear classical regime. The method is a quasiclassical approximation to the quantum transport problem. A one-dimensional quartic on-site model is used to demonstrate the crossover from ballistic to diffusive thermal transport.     

Reaction driven convection around a stably stratified chemical front

A vertical stratification of a light and hot fluid over a heavy and cold one is expected to be stable with regard to buoyancy-driven convection. Here we show that chemical reactions can trigger convection around chemical fronts even in cases where concentration and heat both contribute to a stable density stratification. The balance between intrinsic thermal and solutal density gradients initiated by a spatially localized reaction zone and double diffusive mechanisms are at the origin of a new convective instability, the mechanism of which is explained by a displaced particle argument. Linear stability analysis of a reaction-diffusion-convection model confirmed by nonlinear simulations delimits the instability region in the parameter space spanned by the thermal and solutal Rayleigh numbers. Experimental systems in which to test our theoretical predictions are proposed.     

Analysis of ignition of a porous energetic material

A theory of ignition is presented to analyse the effect of porosity on the time to ignition of a semi-infinite porous energetic solid subjected to a constant energy flux. An asymptotic perturbation analysis, based on the smallness of the gas-to-solid density ratio and the largeness of the activation energy, is utilized to describe the inert and transition stages leading to thermal runaway. As in the classical study of a nonporous solid, the transition stage consists of three spatial regions in the limit of large activation energy: a thin reactive–diffusive layer adjacent to the exposed surface of the material where chemical effects are first felt, a somewhat thicker transient–diffusive zone and, finally, an inert region where the temperature field is still governed solely by conductive heat transfer. Solutions in each region are constructed at each order with respect to the density-ratio parameter and matched to one another using asymptotic matching principles. It is found that the effects of porosity provide a leading-order reduction in the time to ignition relative to that for the nonporous problem, arising from the reduced amount of solid material that must be heated and the difference in thermal conductivities of the solid and gaseous phases. A positive correction to the leading-order ignition-delay time, however, is provided by the convective flow of gas out of the solid, which stems from the effects of thermal expansion and removes energy from the system. The latter phenomenon is absent from the corresponding calculation for the nonporous problem and produces a number of modifications at the next order in the analysis arising from the relative transport effects associated with the gas flow.     

Comparative analysis of methods for heat losses in turbulent premixed flames using physically-derived reduced-order manifolds

Heat losses have the potential to substantially modify turbulent combustion processes, especially the formation of pollutants such as nitrogen oxides. The chemistry governing these species is strongly temperature sensitive, making heat losses critical for an accurate prediction. To account for the effects of heat loss in large eddy simulation (LES) using a precomputed reduced-order manifold approach, thermochemical states must be precomputed not only for adiabatic conditions but also over a range of reduced enthalpy states. However, there are a number of methods for producing reduced enthalpy states, which invoke different implicit assumptions. In this work, a set of a priori and a posteriori LES studies have been performed for turbulent premixed flames considering heat losses within a precomputed reduced-order manifold approach to determine the sensitivity to the method by which reduced enthalpy states are generated. Two general approaches are explored for generating these reduced enthalpy states and are compared in detail to assess any effects on turbulent flame structure and emissions. In the first approach, the enthalpy is reduced at the boundary of the one-dimensional (1D) premixed flame solution, resulting in a single enthalpy deficit for a single premixed flame solution. In the second approach, a variable heat loss source term is introduced into the 1D flame solutions by mimicking a real heat loss to reduce the post-flame enthalpy. The two approaches are compared in methane–air piloted turbulent premixed planar jet flames with different diluents that maintain a constant adiabatic flame temperature but experience different radiation heat losses. Both a priori and a posteriori results, as well as a chemical pathway analysis, indicate that the manner by which the heat loss is accounted for in the manifold is of secondary importance compared to other model uncertainties such as the chemical mechanism, except in situations where heat loss is unphysically fast compared to the flame time scale. A new theoretical framework to explain this insensitivity is also proposed, and its validity is briefly assessed.     

基于对流传热模型的LD端面泵浦圆形激光晶体的热效应研究

通过对半导体端面泵浦棒状Nd∶YAG晶体的热效应进行了理论分析,研究了端面抽运圆形截面激光晶体内部温度场,建立了符合条件的激光晶体热模型。考虑晶体侧面与冷却液之间的对流传热,以及晶体端面与外界非绝热边界条件,从而建立更为合理的边界条件,得出更符合实际的晶体的温度分布场。研究结果表明,考虑端面的对流传热后,计算的晶体中心温度降低,而相应的热焦距稍有增加;空气传热系数增加时,晶体中心温度明显降低,热焦距显著增加,减弱了晶体热效应。     

Smolder waves, smolder spots, and the genesis of tribrachial structures in smolder combustion

We examine the evolution of reverse smolder waves with edges, motivated by the existing literature on edge-flames. An advancing smolder edge proves to be of little interest, and a well-defined invariant structure does not emerge following initial transients. A retreating smolder edge also lacks invariant structure, but retreats at a reasonably well-defined speed and can lead to novel structures and novel evolutions. Thus, a transient propagating smolder spot can be generated; and a tribrachial structure evolves from this spot with a tail that has the form of a forward smolder wave, and two leading branches, fuel-lean and fuel-rich reverse smolder waves. High temperatures and high reaction rates accompany these evolutions, and it is noted that this could lead to flaming (gas-phase) combustion.     

The role of local thermal non-equilibrium in modelling smouldering combustion of organic liquids

A one-dimensional numerical model of smouldering combustion was developed in order to better understand smouldering and accurately predict forced, upwards, self-sustained smouldering for the purposes of treating hydrocarbon-contaminated soil. The role of local thermal non-equilibrium was explored via a new heat transfer correlation obtained specifically for conditions typical of smouldering hydrocarbon-contaminated soil. The model was calibrated to a smouldering experiment and then confidence in the model was gained by independent simulations of additional experiments. The smouldering chemistry was represented by a two-step kinetic mechanism, with the results indicating that this simple framework was sufficient to reproduce the main features of self-sustained smouldering. Local thermal non-equilibrium was demonstrated to be significant in smouldering, with an average normalized temperature difference of 36% between the air and the sand/fuel. Moreover, incorporating the new non-equilibrium correlation provided accurate predictions, particularly in the heat transfer-dominated regions preceding and trailing the front. Results further demonstrated that the most widely used correlation in the literature effectively ensures local thermal equilibrium and such models could not reproduce the experiments.