柴油发动机燃油跨/超临界喷射特性数值仿真

doi:10.12382/bgxb.2022.1012

摘要/Abstract

摘要：

针对柴油发动机工作条件下喷雾特性变化复杂的问题,利用计算流体动力学软件CONVERGE,采用大涡模拟(Large Eddy Simulation,LES)方法进行跨/超临界柴油机燃料正庚烷的射流喷雾特性仿真分析,对LES中湍流亚格子模型进行深入研究,并从质量分数、密度和温度三方面对比跨/超临界射流的喷雾特性。研究结果表明:随机湍流分散模型开启会使喷雾贯穿距的预测值更接近试验值,当湍流分散系数c_ps=0.16时模型预测的贯穿距比c_ps=0.05更加贴合试验的结果;跨临界射流存在更加类似于液态的区域,升温时从环境吸收更多热量;与跨临界射流相比,超临界射流喷雾的温度扩散更加迅速,并且喷雾前端温度接近环境温度;跨临界射流中类似液态区域形成了较大的密度分层,抑制喷雾扩散,超临界射流喷雾外层的絮状结构比跨临界射流出现得更早,絮状结构范围更大,径向扩散作用更强;两种状态下温度、密度和质量分数分布呈现一定的对应关系,即温度、质量分数和密度都在中心轴线靠近喷油器处最大,随着轴向距离和径向扩散的增大而减小。

关键词: 柴油发动机, 燃油, 跨/超临界喷射, 喷雾特性, 随机湍流分散模型

Abstract:

For the complex changes in spray characteristics of diesel engine under working conditions, this paper uses CFD software CONVERGE to simulate and analyze the jet spray characteristics of n-heptane fuel in transcritical/supercritical diesel engine by using large eddy simulation method, and conducts an in-depth study on turbulent subgrid model of LES. The spray characteristics of the transcritical/supercritical jet are compared from mass fraction, density and temperature. The results show that the stochastic turbulence dispersion model can make the predicted values of gas phase and liquid phase penetration distances closer to the test values, and the predicted gas and liquid phase penetration distances for turbulent dispersion coefficient c_ps=0.16 are more consistent with the test results than those for c_ps=0.05. Compared with the transcritical jet, the temperature diffusion of supercritical jet spray is faster, and the temperature of spray front is close to the ambient temperature. The similar liquid region in the transcritical jet forms a larger density stratification, which inhibits spray diffusion. Therefore, the flocculent structure in the spray outer layer of supercritical jet appears earlier than that of the transcritical jet. The flocculent structure has a larger range, and the density is closer to the surrounding environment. It is also found that the distributions of temperature, density and mass fraction in the two states show a certain corresponding relationship, that is, the temperature, mass fraction and density are all maximum at the central axis close to the injector, and decrease with the increase of axial and radial distances.

Key words: diesel engine, fuel, transcritical/supercritical jet, spray characteristics, stochastic turbulence dispersion model

中图分类号:

TK421

刘宇浩, 解方喜, 刘成军, 刘宇, 洪伟. 柴油发动机燃油跨/超临界喷射特性数值仿真[J]. 兵工学报, 2024, 45(5): 1416-1425.

LIU Yuhao, XIE Fangxi, LIU Chengjun, LIU Yu, HONG Wei. Numerical Simulation of Transcritical/supercritical Injection Characteristics of Diesel Engine Fuel[J]. Acta Armamentarii, 2024, 45(5): 1416-1425.

图/表 13

表1 计算模型的选择

Table 1 Calculation model selection

模型类别	模型名称
气态状态方程	PR方程
求解器	PISO(基于密度)
分布类型	均匀分布
湍流扩散模型	O’Rourke
碰撞模型	NTC
碰壁模型	Wall Film
破碎模型	KH-RT(KH-ACT修正)
湍流模型	LES模拟
蒸发模型	Frossling modeling模型

图1 定容燃烧弹几何模型

Fig.1 Geometric model of constant volume combustion bomb

表2 定容弹初始条件

Table 2 Initial conditions of constant volume combustion bomb

初始条件	数值
定容弹温度/K	1000
定容弹压力/MPa	4.33
燃料质量/mg	17.8
N₂体积分数/%	89.71
CO₂体积分数/%	6.52
H₂O体积分数/%	3.77

表3 算例代称及对应参数修正与选择

Table 3 Another names for calculation cases and corresponding parameters correction and selection

算例	燃油喷射温度/K	燃油喷射压力/MPa	燃料质量/mg
Case1	373	150	17.8
Case2	541	150	17.8

图2 基础网格分别为1mm、2mm、4mm的贯穿距对比

Fig.2 Comparison of penetration distance when the base mesh is 1mm/2mm/4mm

图3 湍流分散系数cps分别为0/0.05/0.16时不同时刻喷雾图像

Fig.3 Spray images at different moments when turbulent dispersion coefficient cps is 0/0.05/0.16

图4 cps分别为0、0.05、0.16时的喷雾贯穿距曲线

Fig.4 Spray penetration distance curves for cps=0/0.05/0.16

图5 喷雾贯穿距仿真值与试验值对比

Fig.5 Comparison of simulated and experimental spray penetration distances

图6 试验(左)与仿真(右)的正庚烷质量分数对比

Fig.6 Comparison of experiment (left) and simulated (right) n-heptane mass fractions

图7 各个时刻跨/超临界射流喷雾温度分布

Fig.7 Temperature distribution of transcritical/supercritical jet sprays at each time

图8 0.3ms时刻跨(左)/超(右)临界射流温度分布放大图

Fig.8 Enlarged views of the temperature distributions of transcritical(left)/supercritical(right) jets at 0.3ms

图9 各个时刻跨/超临界射流喷雾密度分布

Fig.9 Density distribution of transcritical/supercritical jet spray at each time

图10 各个时刻跨/超临界射流喷雾质量分数分布

Fig.10 Mass fraction distribution of transcritical/supercritical jet spray at each time

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