Pressure loads and structural response of the BNL high-temperature detonation tube

Abstract

The high-temperature detonation tube facility being designed at Brookhaven National Laboratory must withstand dynamic pressure loads. These loads are associated with both detonations and deflagration-to-detonation transition (DDT). The present report documents the results of computations of the pressure loads and structural response. Structural response considerations indicate that radial motion of the tube is sufficiently rapid that the tube actualkly responds to the peak pressure behind the wave. The structure moves sufficiently rapidly (strain rates between 5 and 40 s^-1) that strain rate effects on the material may be significant. The sudden nature of the applied load produces a much larger deformation and stress within the tube than would the same peak pressure applied statically. This dynamic effect is determined by simple single degree of freedom computations based on pressure histories calculated from gasdynamic simulations. An equivalent static pressure is computed from the calculated stresses. The ratio of the equivalent static pressure to the peak dynamic pressure (the dynamic load factor) ranged between 1.4 and 2.0. For normal operation as a detonation tube, this means that the equivalent maximum static pressure is twice the reflected Chapman-Jouguet (CJ) detonation pressure. During DDT, the peak pressures can be up to 80 atm in the region between the shock and the flame. The magnitude of the peak pressure produced during reflection depends critically on the details of the process. A reasonable estimate is 160 atm. Pressures as high as this have been experimentally observed in a few cases and are consistent with 20% overdriven detonations. The peak pressure generated during detonation reflection, both CJ and overdriven, is extremely localized and influences the design only in the vicinity of the end. The structure is also stiffer at the ends due to the presence of the the welded flanges and the bolted cover plate. It is difficult to estimate the relative frequency of such high pressure loads but prior experience indicates that catastrophic failures rarely occur when DDT occurs in tubes designed for only CJ detonations. I recommend that the tube be designed around the normal operational load of an equivalent static pressure of 100 atm. This static pressure will result in stresses comparable to those produced by the normal relfection of steady CJ detonation. The baseline design is a tube 20 m in length, 12 inches in diameter, and constructed from 316L stainless steel; the material of choice for high temperature operation with H2. This results in a tube wall thickness of 3/4 inch, 900 lb. class flanges, 2" thick end plates (flat), and 16 fasteners of 1-1/4 inch diameter for each joint. Note that this wall thickness is conservative for this design pressure since the allowable stress of 100 MPa (15ksi) will not be achieved until the equivalent static pressure is 127 bar. Although the stresses produced by the most severe DDT events exceed the allowable stress in this design, they are less than the tensile strength of the material. In the most severe events calculated, the tube would deform by would not catastrophically fail.

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