An Investigation Of The Channel Profile Design For Shock Wave Enhancement

Strong shock wave generation in a shock tube has important academic and engineering application significance.Driving techniques for the shock tube method share a common idea,i.e.to increase driving capability by increasing the pressure and the sound speed of the driver gases,but the demanded shock intensity may go beyond the technical limits.To get rid of the difficulties in the driver side,an area contraction in the driven section of a shock tube became one of the most effective methods to further increase the intensity of the incident shock wave,particularly for a certain initial driven pressure that is not allowed to be very low.Complex shock-wall interactions will be produced in a conventional contraction channel,which not only reduces shock enhancement level,but also seriously affects the uniformity of the shock front and followed flow.Great efforts of the geometry optimization for the contraction channel wall profile have been made to weaken the complex interactions,but a theory or design criterion is highly expected to maximally eliminate the complex wave interactions along the moving shock front.In view of this requirement,the following works have been done.1.A reverse method of two-dimensional wall profile design for planar shock wave enhancement is developed.Based on shock dynamics theory,a contraction channel with a designed concave-oblique-convex wall profile is proposed to obtain a smooth planar-to-planar shock transition with shock intensity amplification.A typical example is presented with a combination of experimental and numerical methods,where the shape of transmitted shock is almost planar and the post-shock flow is almost uniform.Analyses of wall profile input parameters illustrate that the contraction ratio is crucial to shock enhancement level,and the wall profile design is insensitive to initial shock strength variations.2.For high shock Mach number,a modified method of high-temperature gas effects involving three specific heat ratios(3-? model)is proposed.The method not only obtain acceptable accuracy of calculation,but also retains analytic formulas applying for shock dynamics theory system.Firstly on the basis of traditional 2-y correction method that includes the specific heat ratio ahead of and behind a shock wave,some equivalent parameters that signify variations in specific heat ratio and gas composition across a shock wave are introduced to reasonably take high-temperature gas effects into account.Meanwhile,a database considering thermochemical equilibrium is applied to obtain the numerical replacement values of the equivalent parameters and post-shock specific heat ratio,which will be transferred to the analytic formulas in the wall profile design process.Using the 3-y model to deduce moving shock relations and Chester-Chisnell-Whitham relation,whose results fit very well with the numerical solutions of equilibrium flow and numerical simulation.These modified relations are successfully applied to the reverse method of channel profile design.3.In order to enhance the shock wave further,an orthogonal cross-section contraction method is presented to obtain a larger contraction ratio in a certain space.Specifically,an area reduction can be achieved by successively decreasing the height and width of the channel to achieve the effect of shock enhancement in the three-dimensional scale,which is also convenient for two-dimensional observation of the flow field.Experimentally,a smooth planar-to-planar shock transition is also obtained after twice area reduction with shock intensity amplification greatly.4.Shock induced ignition phenomenon is studied in the contraction channel with the designed concave-oblique-convex wall profile.Our contraction channel is converted smoothly into a straight one when the shock intensity reaches the ignition temperature.It insures that the airflow is moving downstream without reflected waves,which can provide a stable and controllable ignition environment.Under the experimental conditions,there are two independent ignition positions in the flow direction,which are closer to each other as the increase of initial shock intensity and can be merged into one position.Experimentally,the ignition is more likely to occur near the wall in the vertical direction.The experimental ignition time is in good agreement with the numerical simulation results.An obstacle downstream the outlet is able to speed up the processes of ignition occurrence and combustion wave development.