【作者】 韦静思；

【导师】 舒歌群；

【作者基本信息】 天津大学 ， 动力机械及工程， 2010， 博士

【摘要】 热声耦合的本质是空间中热能与声能相互转化的现象,目前,国内外热声领域的研究主要集中在热声制冷,火箭推进器等方向。在内燃机领域,虽然研究者们已认识到燃烧室的压力振荡与燃烧过程尤其是爆震燃烧关系十分紧密,然而由于内燃机燃烧过程中声学振荡的复杂性,目前声波在燃烧室中的具体作用如何仍然没有定量描述,因此对内燃机燃烧中的热声耦合问题进行研究具有重要意义。本文首先以火花点火发动机为研究对象,通过分析所引用的可视化发动机高速摄影燃烧照片,应用声学软件SYSNOISE建立了能描述燃烧室结构特征的声模态计算模型并进行了频响分析。基于已燃区和未燃区等燃烧特征,建立声学时域计算模型,开展单点和多点激励下的燃烧室压力波瞬态响应计算,初步研究了燃烧室压力场的时域特性和变化规律。随后,通过对柱坐标系波动方程已有的经典解法的讨论,可知经典解法在求解内燃机燃烧过程中声场的困难,得出必须采用数值方法求解的结论。之后,将准维模型与声学模型相耦合,通过合理简化,采用数值解法并编制程序进行计算,与实验结果对比表明,该方法能粗略反应内燃机燃烧过程中的热声耦合性质。为更深入了解燃烧过程中压力的声学振荡现象,在分析KIVA的流场求解算法基础上,根据KIVA的流场求解方程组推导出能够运用于内燃机燃烧过程中热声耦合计算的波动方程。该方程表明内燃机燃烧室的声波受到多个源项的激励,包括化学反应放热、粘性耗散、湍流产生项和耗散项、热传导、焓耗散等,具有三维非稳态瞬变的特点,必须与KIVA的模拟相结合进行求解。根据所得方程构造相应的差分方程,给出了在与KIVA相结合时计算的稳定性条件及其数学证明。推导圆筒结构混合气-气缸-水三层结构的声波反射系数的表达式,以具体应用与热声耦合的计算过程中。根据所得的波动方程编制计算程序,在燃烧模拟过程中实时获取对燃烧室声场的各种激励源,得出了各源项的定量化计算结果,容易看出化学反应放热是燃烧过程声波产生的主要激励源。计算了某汽油机各种爆震工况的特性,通过与实测爆震时气缸压力振荡的对比,表明用波动方程计算内燃机燃烧过程中声场是可行的。提取多个特征点的振荡历程,经分析知燃烧室不同点的振荡在频率分布和幅值上相差较大,各种声学结构和燃烧参数对燃烧中的声学振荡有明显的影响。在分析了内燃机燃烧过程中的热声耦合现象与现有热声装置的异同后,本文将波动方程与燃烧计算软件KIVA3V相结合,根据热声理论,定量的计算了热声耦合在内燃机燃烧过程中的作用。对于文中汽油机,引用高速摄影图片对燃烧过程进行描述,与计算结果进行对比分析。结果表明,爆震发生前,热声效应的作用是增进燃烧过程的不稳定性;而在爆震发生后,热声效应的作用是消耗振荡能量,使系统趋于稳定。爆震时的声压突增能达到数个兆帕的量级,此时由于声学波动造成的温度波动可超过200K。最后,利用耦合程序对一台柴油机燃烧过程进行声学计算和热声学计算,得出了其燃烧过程中燃烧室内的声场波动和温度波动。更多还原

【Abstract】 The essence of thermoacoustic is a phenomenon of reciprocal transformation of thermal energy and acoustic energy in medium. At the present time, both in China and other countries, the thermoacoustic field concentrates its attention on thermoacoustic refrigeration and rocket propellers. Although the researchers in the field of internal combustion engine have recognized that the pressure oscillation play a great role in combustion chamber, especially in knock conditions. However, there is still no definite theory about the role because of complicacy of the acoustic wave in cylinder during combustion. Therefore, it is of great importance to study the thermoacoustic problems during the combustion of an engine.In this paper, a SI engine is chosen as research object. By analyzing the high-speed photographs taken from an optical engine, a model which can be used to describe acoustic characteristics of structure of the combustion chamber of the optical engine is established to calculate the acoustic modes of the chamber and the frequency responses is analyzed in the acoustic software SYSNOISE. Base on the characters of burnt and unburned regions of the chamber, a time domain model is built to calculate the transient responses of pressure wave in the combustion chamber under Single-point stimulation and multi-point stimulation.Subsequently, according to discussion of existing classical solution of wave equation in cylindrical coordinate, it is revealed that one will face awkwarddifficulties when the classical method is adopted to simulate the acoustic wave during combustion of an IC engine. Thus a quasi-dimensional model is applied to couple with an acoustic model. With rational simplifications made to the coupled models, a procedure is compiled and used to calculate the heat release and pressure oscillation. The results indicate that the thermoacoustic characteristics can be explored in this way approximately.For profound comprehension of the acoustic oscillations of pressure wave in combustion, with analysis of arithmetic in fluid field embedded in KIVA, a wave equation is deduced based on the equations set that is included in KIVA for fluid solution, which can be applied on the theromacoustic computation during the combustion of an IC engine. The wave equation shows that, in an IC engine, acoustic wave is stimulated by multiple source-items, such as chemical reactions、viscous dissipation、turbulence generation energy and the rate its dissipation、heat transfer and enthalpy dissipation. To solve this three-dimensional transient wave equation, the calculation should be combined with KIVA simulation. For the wave equation, corresponding difference equation is constituted and the stability conditions while it is coupled with KIVA calculation and mathematics justifications are provided. Aformula is derived for the reflect coefficients for a three layer cylindrical structure of mixed gas- cylinder-water, which is applied in the thermoacoustic computation.Based on the wave equation, subroutines are programmed and real-time quantitative results of the stimulating source items in combustion chamber are obtained during the combustion simulation. Through the results, it is obviously that the chemical reaction is the mainly source to the acoustic wave in combustion. Then various knock conditions of a SI engine are computed. By comparison of cylinder pressure data and the simulation results, a conclusion can be drawn that it is feasible to apply the wave equation on calculation of acoustic field in an IC engine. Oscillating courses is extracted from several specific points at different position, showing that acoustic structure and combustion parameters influence the acoustic wave in combustion explicitly.After the contrast of discrepancies and similarities between IC engine combustion and general thermoacoustic equipments, the wave equation is coupled with KIVA3V to predict the quantificational thermoacoustic effect in IC engines according to thermoacoustic theories. For the SI engine in this paper, high-speed photographs are quoted to denote the development of flame in contrast with the contours from simulation. By Rayleigh integral, we can conclude that before the time of knock, thermoacoustic effect can enhance the instability of the IC engine’s combustion system, while it will drive the system to uniformity after knock. When the knock takes place, the steep increase of acoustic pressure may reach the magnitude of mega Pascal and temperature fluctuation will surpass 200 K due to the acoustic wave. In the end, the couple procedure is introduced in acoustic and thermoacoustic computation to figure out its acoustic wave and temperature fluctuation in the chamber.更多还原