末端气体自燃前燃烧的氧化放热化学动力学模拟

低温化学反应能够产生大量的活性基,并有显著的热量释放,足以使末端气体温度提高到自燃的程度。作者基于低温化学反应机理,建立了一个简易化学动力学模型,当给定混合气的量和初始计算温度时,该模型能预测出混合气氧化反应的温度,累积放热量以及产物浓度随曲轴转角的变化曲线。用此模型预测了ＰＲＦ７５燃料在倒拖发动机上压缩自燃前的氧化放热过程,结果表明,模拟结果与试验结果吻合较好。     

玉米秸焦的低温氧化及动力学特性的恒温量热分析

针对生物质焦的低温氧化和自加热问题,采用恒温量热分析方法对玉米秸250、500℃焦及其原样在4个温度35、45、55、65℃下的低温氧化放热和动力学特性进行了实验研究。放热特性分析表明,3种样品各温度下热流率随时间的变化具有相似的趋势,反应温度对焦放热特性的影响也相似,即随着温度升高,样品的最大放热率和相同时间的累积放热量显著增加。动力学分析表明,烘焙焦(即250℃焦)与原样具有相近的活化能,但指前因子较小,因此烘焙可一定程度上降低自加热倾向;500℃焦的活化能Ea较大,但反应放热对温度变化敏感,因而具有很强的自加热和自燃倾向。     

初始进气充量成分对末端气体自燃化学特性的影响

本文介绍了在一台单缸二冲程爆震实验发动机上，研究进气充量中不同的残余气体成分对焰前反应及自燃的影响的实验结果。结果表明，初始进气充量中含有一定量的残余废气时，抑制了自燃；在残余废气中含有一定量的部分氧化产物时，促进了低温化学反应和自燃。低温化学反应所释放的热量以及活性中间产物的积累导致了终燃混合气的自燃。ＲＯＮ５３汽油的自燃过程表现为两阶段着火特征。     

初始进气充量成分对末端气体自然化学特性的影响

本介绍了一台单缸二冲程爆震实验发动机上，研究进气充量中不同的残余气体成分对焰前的反应及自燃的影响的实验结果。结果表明，初始进气充量中含有一定量的残余废气时，抑制了自燃；在残余废气中含有一定量的部分氧化产物时，促进了低温化学反应和自燃。低温化学反应所释放的热量以及活生中间产物的积累导致了终燃混合气的自燃。ＲＯＮ５３汽油的自燃过程表现为两阶段着火特征。     

丁醇均质混合气与柴油的复合燃烧特性

在一台四缸轻型柴油机上增加一套低压共轨系统,实现了进气道丁醇喷射,研究了不同负荷、不同柴油替代率条件下丁醇均质混合气与柴油的复合燃烧特性.结果表明:丁醇均质混合气与柴油复合燃烧的放热率曲线呈双峰,第一峰是由于丁醇的自燃形成的,第二峰是由于柴油点燃丁醇均质混合气形成的.在丁醇均质混合气过量空气系数不变时,丁醇自燃的始点主要受到缸内气体状态的影响,自燃的放热量与丁醇的循环喷射量呈正相关.复合燃烧的快速燃烧期始点取决于柴油的着火时刻,与丁醇的比例关系不大;快速燃烧期的终点(放热率曲线的分离点)与柴油的循环喷射量呈正相关.丁醇预混合气与柴油复合燃烧的热效率随柴油替代率的增大而增大,且高于纯柴油燃烧模式.     

柴油射流控制压燃放热过程的数值模拟

针对双直喷型柴油射流控制压燃模式的放热过程,通过三维数值模拟耦合简化化学反应动力学机理,对该模式高温放热过程中存在的独立两阶段特性开展研究.结果表明:射流压缩自燃阶段受到柴油射流压缩自燃的主导,同时周围的部分预混合气也参与燃烧.随着缸内温度和压力的升高,达到预混合气的自燃条件,在压缩上止点后6°CA附近,独立区域内预混...     

四冲程汽油机 CAI 燃烧放热率模型研究

通过大量试验数据分析,提出了适用于四冲程汽油机可控自燃（ControlledAutoIgnition,CAI）燃烧的放热率模型。研究发现汽油机CAI燃烧在低温时有少量放热,因此,放热率模型包括两个阶段——低温缓慢自燃阶段和高温迅速放热阶段,与一维发动机模拟软件GT-Power相耦合后在相同试验条件下着火时刻及平均指示压力PIMEP的计算结果与试验结果偏差较小。此模型可用于进行发动机预测、设计以及控制策略的研究。     

火花点火发动机末端混合气自燃化学的实验研究

本文介绍碳氢燃料的自燃化学过程的实验研究结果。试验研究是在一台倒拖发动机上进行的。实验中研究了进气温度、燃料辛烷值、发动机转速和压缩比对燃料氧化和自燃的影响。对实验中的燃料自燃变化规律以及自燃从无到有的连续循环间的放热过程进行了总结，并对实验中的一些特殊现象进行了可能的解释，指出焰前反应中间产物的某些特性及其对焰前反应的重要影响。     

火花点火发动机末端混合气自燃化学的实验研究

本文介绍碳氢燃料的自燃化学过程的实验研究结果。试验研究是在一台倒拖发动机上进行的。实验中研究了进气温度，燃料辛烷值，发动机转速和压缩比对燃料氧化和自燃的影响，对实验中的燃料自燃变化规律以及自燃从无到有的连续循环间的放热过程进行了总结，并对实验中的一些特殊现象进行了可能的解释，指出焰前反应中间产物的某些特性及其对焰前反应的重要影响。     

用加速量热仪研究参比燃料的低温氧化特性(Ⅱ)

基于用加速量热仪(ARC)对参比燃料与空气的混合物在低温条件下的氧化特性的实验结果,对ARC实验结果进行了热力学和化学动力学分析．根据能量平衡方程,推导出在较小的样品质量下,考虑到密闭容器中各种基本反应物质的比热容随温度变化时,热惰性因子的准确表达式,算出参比燃料在反应过程中的实际自放热率,并根据化学动力学分析求出正庚烷和异辛烷低温氧化动力学参数．对正庚烷和异辛烷的自放热率分析发现,它们都出现双峰现象,并分别在589．4K和585．7K时有最大放热率,随后在598．5K和594．8K结束自放热．     

The overall reaction concept in premixed,laminar, steady-state flames. II. Initial temperatures and pressures

In a previous paper, we examined the adequacy of the overall reaction model for premixed, laminar, one-dimensional, steady-state flames. The single reaction model gave quite accurate results for burning velocity. The temperature and heat release profiles were also generally accurate. The accuracy of the major species profiles varied from fair to good. However, the optimal overall kinetic parameters varied with the equivalence ratio.In this paper, the adequacy of an overall reaction model for premixed, laminar, one-dimensional, steady-state flames is examined for variations in initial temperature and pressure. The single reaction model gives quite accurate results for burning velocity; temperature and heat release profiles are also generally accurate; major species profiles are reproduced with fair to good accuracy. The optimal overall parameters change with initial temperature and pressure. However, a single set of parameters is found to be accurate over a limited range of initial temperatures and pressures.     

Measurements of wood pellets self-heating kinetic parameters using isothermal calorimetry

Wood pellets release heat and gases during storage. A TAM air isothermal calorimeter was used to measure the rate of heat release by bulk pellets at temperatures ranging from 30 to 50 °C. The results showed that self-heating rate at the tested temperature range strongly depended on the reaction temperature and the age of pellets. Heat release rate increased as the reaction temperature increased but the rate was not sensitive to pellets moisture content. The activation energy of the self-heating reaction increased with the age of pellets. A simple global kinetic model for self-heating was developed from the experimental data. The model can be applied to simulate the self-heating process in pellets storage containers at different storage temperatures and age of stored pellets.     

Deflagration regimes of laminar flames modeled after the ozone decomposition flame

Methods of activation-energy asymptotics are employed to investigate regimes of combustion of steady, planar, adiabatic deflagrations involving a four-step kinetic mechanism modeled after that of the ozone decomposition flame. The analysis demonstrates the occurrence of previously known regimes having flame structures that involve a nonreactive preheat zone followed by a narrow reactive-diffusive zone, in which a steady-state approximation for the reaction intermediary may or may not apply and downstream from which a recombination zone may or may not exist. In addition, a new regime is identified having a two-zone flame structure in which the intermediary is generated in a downstream zone that obeys a steady-state approximation for temperature and diffuses into an upstream zone where the primary heat release occurs. In this regime convection, diffusion, and reaction all are important in both zones, and heat release persists in the preheat zone all the way to the cold boundary. For the ozone flame new results for burning velocities are given and regimes are identified as functions of pressure, initial temperature, and initial ozone concentration.     

正庚烷均质压燃燃烧反应化学动力学数值模拟研究

应用零维详细化学反应动力学模型，对正庚烷均质压燃燃烧反应的化学反应动力学过程进行了数值模拟研究，分析了在内燃机边界条件下影响其燃烧反应的关键基元反应、关键中间产物以及自由基。结果表明，正庚烷的燃烧过程由高温反应和低温反应两个阶段组成，高温反应阶段又可以分为蓝焰反应和热焰反应两个阶段。正庚烷氧化反应首先经过脱氢反应，第一次加氧异构化后的第二次加氧是低温反应的必经途径，其产物的两次分解是低温反应阶段OH自由基的主要来源；蓝焰反应阶段主要是甲醛氧化成CO的过程，H2O2的热分解是控制该阶段反应最重要的基元反应，也是OH自由基的主要来源；热焰反应主要是CO氧化成CO的过程；CO的生成途径是：低温反应生成的甲醛(CH2O)脱氢生成HCO，HCO氧化生成CO，OH是CO氧化为CO2和正庚烷脱氢反应最重要的自由基。     

Isolated n-heptane droplet combustion in microgravity: “Cool Flames” – Two-stage combustion

Recent experimentally observed two stage combustion of n-heptane droplets in microgravity is numerically studied. The simulations are conducted with detailed chemistry and transport in order to obtain insight into the features controlling the low temperature second stage burn. Predictions show that the second stage combustion occurs as a result of chemical kinetics associated with classical premixed “Cool Flame” phenomena. In contrast to the kinetic interactions responsible for premixed cool flame properties, those important to cool flame droplet burning are characteristically associated with the temperature range between the turnover temperature and the hot ignition. Initiation of and continuing second stage combustion involves a dynamic balance of heat generation from diffusively controlled chemical reaction and heat loss from radiation and diffusion. Within the noted temperature range, increasing reaction temperature leads to decreased chemical reaction rate and vice versa. As a result, changes of heat loss rate are dynamically balanced by heat release from chemical reaction rate as the droplet continues to burn and regress in size. At reaction temperatures below the turnover, heat loss over takes the heat release rate and extinction occurs. Should heat release exceed heat loss as the temperature increases to that for hot ignition, initiation of a high temperature burning phase may be possible. Parametric study on factors leading to initiation of the second stage burning phenomena are studied. Results show that both carbon dioxide and helium diluents can promote initiation of low temperature burning at smaller initial drop diameters than found with nitrogen as diluent. Small amounts of carbon dioxide and helium in the ambient is sufficient to activate the phenomena. The chemical kinetics dictating the second stage combustion and extinction process is also discussed.     

Numerical study on hydrogen and thermal storage performance of a sandwich reaction bed filled with metal hydride and thermochemical material

Hydrogen storage and release process of metal hydride (MH) accompany with large amount of reaction heat. The thermal management is very important to improve the comprehensive performance of hydrogen storage unit. In present paper, thermochemical material (TCM) is used to storage and release the reaction heat, and a new sandwich configuration reaction bed of MH-TCM system was proposed and its superior hydrogen and thermal storage performance were numerically validated. Firstly, the optimum TCM distribution with a volume ratio (TCM in inner layer to total) of 0.4 was derived for the sandwich bed. Then, comparisons between the sandwich reaction bed and the traditional reaction bed were performed. The results show that the sandwich MH-TCM system has faster heat transfer and reaction rate due to its larger heat transfer area and smaller thermal resistance, which results in the hydrogen storage time is shortened by 61.1%. The heat transfer in the reaction beds have significant effects on performance of MH-TCM systems. Increasing the thermal conductivity of the reaction beds can further reduce the hydrogen storage time. Moreover, improving the hydrogen inflation pressure can result in higher equilibrium temperature, which is beneficial for the enhancing heat transfer and hydrogen absorption rates.     

Two-dimensional modeling of a salt-gradient solar pond with wall shading effect and thermo-physical properties dependent on temperature and concentration

In this paper,the behavior of a salt-gradient solar pond with the square cross-section has been studied experimentally and numerically.A small-scale solar pond were designed and built to provide quantitative data.A two-dimensional,transient heat and mass transfer model has been solved numerically by using finite-control-volume method.In this study,all the thermo-physical properties are variable as the function of temperature and salt concentration.Numerical results as obtained for the experimental pond have been satisfactorily compared and validated against measured data.Furthermore,the wall shading effect has been elaborated to improve the agreement between two sets of results.The temperature of the storage zone is predicted well by the model.It also can be observed that the initial concentration profile is preserved with time.The stability of the pond in time has been investigated in order to distinguish the critical zones.Finally,the application of an energy analysis gives an efficiency of about 12%for the pond.     

Hydrogen release from ammonia borane dissolved in an ionic liquid

Hydrogen release from ammonia borane (AB) dissolved in an ionic liquid (IL) is analyzed using batch, isothermal reaction data. It is found that an Avrami-Erofeyev type model, normally used in solid-gas systems, can be modified for this two-step reaction to describe the measured kinetics of hydrogen release and the observed double peaks in release rates. The kinetic model indicates that temperatures in excess of 200 °C are needed for complete conversion (release of 2.35 H₂-equivalents) in a reasonably compact flow reactor with <20 s AB/IL residence time. A non-isothermal, plug flow reactor model indicates that heat transfer to ethylene glycol at 80 °C alone is not sufficient to control the peak temperature to <250 °C in the exothermic decomposition reaction. The peak temperature, however, can be controlled by partial recycling of the spent AB/IL mixture. The reactor can also be operated adiabatically to obtain complete conversion while limiting the peak temperature through recycle. In a flow reactor where the feed stream is heated by mixing with the recycle stream there is a minimum recycle ratio for complete conversion. This minimum recycle ratio is a function of the reactor outlet temperature, which is determined by heat transfer.     

Sensitivity of diesel engine thermodynamic cycle calculation to measurement errors and estimated parameters

The use of thermodynamic models for the calculation of the heat release law from the experimental in-cylinder pressure signal has been a common practice as a way to study the combustion process of internal combustion engines. However, the results of this procedure depend mainly on two factors: the validity of the assumed hypothesis and the quality of the measurement of the experimental parameters (both mean and instantaneous ones) used as input data. In this work a sensitivity study of a thermodynamic diagnostic model is presented, with the objective of evaluating the influence of errors in the measuring techniques or in the estimation of parameters on the main results, such as mean gas temperature, heat release and rate of heat release. In order to eliminate the effect of the uncertainty associated with the combustion process itself, the study focused on motored engine conditions. Otherwise, the effect of the inadequacy of the assumed hypothesis was eliminated by using as input in the diagnostic thermodynamic model the results from a predictive thermodynamic model with the same hypothesis as the diagnostic one, instead of an experimental pressure signal.