Experimental investigation of the developed focusing elements for generating extreme pressures in a diaphragmless shock tube

High-pressure shock waves of several tens of MPa are of importance in many applications, such as material fabrication and testing, combustion chemistry and metrology. Controlled and predetermined shock waves can be generated by a shock tube, where the sudden expansion of a high-pressure driver gas into a low-pressure driven gas caused by the instantaneous opening of a connection between two sections produces a compression shock wave that propagates through the driven gas. The reflection of the shock wave from the end wall of the driven section of the shock tube generates a rapid pressure rise, which in conventional straight shock tubes with a uniform cross-section is limited to a few MPa. This paper presents and experimentally investigates two configurations of the shock-wave focusing element that were designed on the basis of shock dynamics theory to effectively enhance the strength of the shock waves generated in the shock tube and therefore increase the generated driven end-wall pressures. The wall shape of the first configuration consists of a concave transformation segment that smoothly transforms the incident planar shock wave into a spherical converging shock wave and a convex transformation segment that further smoothly transforms the spherical shock wave back to planar, which then propagates into the straight outlet tube to the end wall. The convex transformation segment in the second configuration was replaced by a conical segment that further converges and accelerates the spherical shock wave formed in the concave segment towards the end wall and thereby additionally amplifies its strength. The results show that by converging the shock waves with the designed shock-wave focusing elements, the upper pressure limit of the shock tubes is increased from a few MPa up to 13.4 MPa and 44 MPa, respectively.