摘要
枪炮发射时,高温、高压火药燃气出口后快速膨胀形成复杂的膛口流场,并产生一系列危害现象,如膛口冲击波、膛口焰以及气流对弹丸运动的干扰等。研究膛口流场的动力学机理及影响因素对提高武器性能、减小气流危害等具有重要的应用价值。
以数值计算为主并结合实验,开展了膛口流场的冲击波与化学反应现象及其影响因素的研究。流场的基本方程采用含化学反应的ALE形式Navier-Stokes方程,计算网格为分区贴体结构化网格。采用有限体积法离散方程,对流项和时间项分别采用AUSM+格式和Runge-Kutta法求解,湍流模型为k-ε模型,化学反应机理采用C-H-O-N9组分12步基元反应,并使用MPI方法进行分区并行计算。
(1)在膛口射流与冲击波的研究中,不考虑湍流和化学反应的影响,数值模拟了包含高速运动弹丸和初始射流的膛口流场,分析了初始流场对火药燃气流场的影响。结果表明,初始流场超音速核心区域的方向性导致了火药燃气流出膛口后在各个方向上具有不同的膨胀特性,其在轴向上的膨胀速度大于侧向的膨胀速度,从而生成了冠状冲击波。此外,初始流场的存在提高了近膛口区域气体的滞止压力。
(2)通过改变装药量、弹丸质量和口径等发射条件,计算了9种条件下后效期内膛口射流对弹丸速度的扰动。结果表明弹丸在后效期内的增速最大可达3%以上,最大速度出现在弹丸越过马赫盘后;改变发射药装药量对弹丸后效期增速率的影响较小;在一定范围内,增速率随弹丸质量的增加而线性减小
(3)对膛口反应流的形成机理及膛口装置的影响进行了分析。在本文计算条件下,膛口流场化学反应的分为两个阶段:第一阶段内化学反应现象主要发生在两侧涡环内;第二阶段内,射流边界向轴线收缩并与外部涡环脱离,在双剪切层作用下射流边界失稳并发生弯曲和破碎,射流边界中的空气被卷入马赫盘下游轴线附近的高温火药燃气区域内,并在该区域内发生化学反应。筒形和锥形喷管对膛口反应流的影响主要表现为:一方面火药燃气在喷管内膨胀并加速,膛口马赫盘减弱,火药燃气穿过马赫盘后的温度相对降低;另一方面射流边界的稳定性得到提高,火药燃气和空气的混合作用被抑制。
(4)在高空环境下的膛口冲击波场变化规律研究中,实验和数值结果都表明,在一定范围内,膛口冲击波场的超压峰值随着膛口环境压力的变化近似线性降低。有超音速来流时,膛口冲击波影响范围为锥形区域,且影响范围内的超压值比无来流时更大。膛口化学反应现象在高空条件下也得到了抑制。
During the gun firing process, a complicated muzzle flow field will be formed after the high-pressure and high-temperature propellant gas exits the muzzle. A series of harmful phenomena occur throughout development of the muzzle flow, such as muzzle blast and flash, the perturbation of the projectile velocity. The dynamics process and affecting factors of muzzle flow need to be studied in order to improve the weapon performance and control the hazards.
The numerical and experimental methods are employed to study the complex phenomenon of muzzle flow and its affecting factors. Time-dependent Navier-Stokes equations with chemical reaction, which were cast in the Arbitrary Lagrangian-Elerian (ALE) framework, are used in the numerical investigations. Finite volume method is used for space discretization. The flux across the cell interface is calculated by using AUSM+scheme, solutions are advanced in time by explicit two step Runge-Kutta method, and the C-H-O-N reaction mechanism with9components and12elementary reactions is performed to solve the reacting source terms, and MPI method is used for the parallel computing.
1) The effect of precursor flow on the propellant flow field is investigated based on the numerical results. It shows that, the bow shock is produced because of the directivity of precursor flow. The propellant gas expanding velocity in the axial direction is much higher than in the radial direction in the early stage. And the maximal stagnation pressure in the area near the muzzle withe considering the precursor flow increases by twice more than that without considering the precursor flow.
2) Perturbation of the projectile velocity in the after effect period is calculated under different conditions. The results indicate that the velocity reaches the maximum value after the projectile crosses the Mach disk, which could be increased more than3%on the muzzle velocity. The charge weight exerts little effect on the rate of velocity change, and the projectile mass scale is almost linear with the rate of velocity change.
3) Under the calculation condition, there are two stages for the chemical reaction in the muzzle flow. In the first stage, the reaction takes place in the vortex rings in the side of the jet flow. In the second stage, the boundary layer separates from the vortex ring and the K-H instability is caused by the shear layer, and the reaction area moves to the downstream of the Mach disk which is near the axis. Nozzle has two effects on the muzzle reacting flow:first, the strength of Mach disk is decreased with reduction of gas temperature after the expansion in the nozzle; second, the jet contact surface is more stable, which suppresses the mixing of propellant gas and air in some extent.
4) Based on the experimental and numerical results, the investigation on the overpressure of muzzle blast with different ambient pressure is performed. Both of them reveal that the overpressure of the muzzle blast is almost linear with the ambient pressure within a certain range. When the gun is in supersonic incoming flow, the muzzle shock wave is limited in a cone-shape area, and the overpressure value in this area is much higher than that in the static environment.
引文
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