An analysis of three-dimensional patterns of experimental detonation cells

The notions of regularity and characteristic width of detonation cells are revisited based on crossed analyses of experimental front-view and longitudinal recordings obtained with the soot-plate technique. Tubes with cross-sections of different shapes, namely round, triangular and square, but the same surface area of 16 cm(2), are used to detonate the stable mixture 2H(2) + O-2 + 2Ar with the initial pressure p(0) varying from 15 kPa to 100 kPa and the initial temperature T-0 = 294 K. The longitudinal recordings show the well-known regular arrangements for this mixture for all cross-section shapes and p(0), but the front -view recordings show irregular patterns except for the square shape and low-enough p(0). There are fewer cells in the round tube, more in the square one, and their average widths and relative differences de-crease with increasing p(0). All front-view cell patterns become irregular and independent of the cross-section shapes with increasing p(0). The cellular dynamics at the walls of a tube is thus not representative of that on the whole detonation front, and longitudinal soot traces alone are not sufficient for describing the cellular structure. An analysis based on graph theory proposes that a tessellation of regular hexagons can model a large set of irregular front-view cells. A cell count on an experimental front-view recording thus defines an average cell width and an intrinsic but high minimum error for this width. Its resulting large sensitivity to the parameters of a simple Arrhenius rate of chemical progress indicates that detailed schemes of chemical kinetics and more advanced conceptual tools than a single length are necessary for characterizing the three-dimensional structures of detonation cells. The Voronoi tessellation supplemented with a physical criterion for the surface density of randomly-distributed point sources could rep-resent a step forward. (C) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.