基于SPH方法的射孔枪设计及射孔过程研究

基于SPH方法对射孔枪射孔过程进行了模拟,并实现高速射流情况下的射孔过程仿真。计算了射孔弹弹型不变的情况下以40CrNiMo为材料的枪筒,在不同射流速度下盲孔毛刺高度值,不同射流速度和不同盲孔深度下的穿孔直径以及不同材料下毛刺高度值。通过计算发现,盲孔毛刺高度随射孔速度的增加而降低,且高度值变化幅度越来越小;射孔枪的穿孔直径随射孔速度增大先增大而后减小。给出相应的射孔枪穿孔直径与射孔速度关系的拟合公式,射孔枪的穿孔直径随盲孔深度的减小而增大。对于同样尺寸的枪筒,40CrNiMo材料比45号钢有更高的冲击韧性,产生更小的毛刺和穿孔直径。

Vacuum assisted high-pressure die casting of AZ91D magnesium alloy at different slow shot speeds

The effects of vacuum assistance on the microstructure and mechanical properties of high-pressure die cast AZ91 D alloy at different slow shot speeds were evaluated. Plate-shaped castings of AZ91 D alloy were carried out on a TOYO BD-350V5 cold chamber die casting machine incorporated with a self-improved TOYO vacuum system. It was found that the vacuum pressure in the die cavity at the beginning of mold filling increases with the increase of slow shot speed, following a cubic polynomial curve, resulting in a decline in the porosity-reduction ability of vacuum assistance with the increase of slow shot speed. The externally solidified crystal（ESC） contents in conventional and vacuum die castings behave similar against the slow shot speed. The tensile properties of vacuum die castings were strongly influenced by the ESC content at relative low slow shot speeds. With the increase of slow shot speed, the influence of the gas porosity level in vacuum die castings would get prominent.

Effect of multi-step slow shot speed on microstructure of vacuum die cast AZ91D magnesium alloy

Two multi-step （two-step and three-step） slow shot speeds were used in the vacuum die casting process of AZ91D magnesium alloy. The vacuum pressure variation in the die cavity before mold filling was monitored by using a pressure sensor. The microstructures of the produced castings were analyzed with optical microscope and image analysis software. The experimental results demonstrate that, the vacuum pressure in the die cavity at the beginning of mold filling is significantly reduced by using three-step slow shot speed, resulting in a low gas porosity level in the produced castings. At an appropriate multi-step slow shot speed, the dwell time of the liquid metal in the shot sleeve before mold filling can be reduced and the flow of the liquid metal in the shot sleeve at the later stage of the slow shot process can be restrained, which cause a low externally solidified crystal content in the produced castings.

Mixing times in single and multi-orifice-impinging transverse(MOIT) jet mixers with crossflow

We study the macromixing behavior of single and multi-orifice-impinging transverse(MOIT) jet mixers with crossflow, in particular, the overall mixing time and the back-splash mixing time of the injected flow with the crossflow, using the PLIF technique. It is found that for a given mixer configuration, there is a critical jet-tocrossflow velocity ratio rcat which the back-splash begins to occur. Further increase in the velocity ratio r leads to sharp increase in the back-splash mixing time, which can offset the intensification of the downstream mixing. The dimensionless overall mixing time decreases as r increases to reach either a plateau or a local minimum, and the corresponding r value represents the optimal velocity ratio roptfor the macromixing. The momentum ratio of the two liquid streams is a key factor determining rcand ropt. For a larger scale mixer, a higher momentum ratio is required to achieve the optimal macromixing with the minimum dimensionless overall mixing time.

A patchy-saturated rock physics model for tight sandstone based on microscopic pore structures

The wave-induced local fluid flow mechanism is relevant to the complex heterogeneity of pore structures in rocks.The analysis of the local fluid flow mechanism is useful for accurately describing the wave propagation characteristics in reservoir rocks.In the exploration and production of hydrocarbon reservoirs,the real stratum may be partially saturated with a multi-phase fluid mixture in general.Therefore,it is of great significance to investigate the wave velocity dispersion and attenuation features in relation to pore structures and fluids.In this work,the characteristics of fabric microstructures are obtained on the basis of pressure dependency of dry rock moduli using the effective medium theory.A novel anelasticity theoretical model for the wave propagation in a partially-saturated medium is presented by combining the extended Gurevich squirt-flow model and White patchysaturation theory.Numerical simulations are used to analyze wave propagation characteristics that depend on water saturation,external patchy diameter,and viscosity.We consider a tight sandstone from the Qingyang area of the Ordos Basin in west China and perform ultrasonic measurements under partial saturation states and different confining pressures,where the basic properties of the rock are obtained at the full gas saturation.The comparison of experimental data and theoretical modeling results shows a fairly good agreement,indicating that the new theory is effective.