Effects of nozzle geometry on the reignition by hot gas jets

Abstract

In this paper, the ignition of a hydrogen–air mixture by a jet of burned hot gas (often termed reignition in the field of explosion protection) is studied in a two-chamber experimental configuration. This experiment allows systematic creation of hot gas jets emerging from a thin, long nozzle and them to impinge into an unburned, cold hydrogen–air mixture, possibly causing (re)ignition. In an extensive parametric study, hot exhaust gas jets originating from different nozzle diameters (0.6 mm–1.2 mm) and lengths (25 mm and 70 mm), as well as different pressure ratios along the nozzle (up to 5.5) were tested for their ability to reignite the hydrogen–air mixture. It was investigated how often reignition occurred within 10 tests under each condition. Dependencies between the occurrence of reignition and the mentioned nozzle and jet parameters were determined. It was found that the outcome (reignition or no reignition) can differ for nominally identical experimental paramete rs, which demonstrates the parametric sensitivity of the process. The statistical frequency of reignition occurrence depends on the nozzle diameter as well as on its length. For pressure ratios above the critical value, reignition occurred more often for the longer nozzle (l = 70 mm) at a given diameter. Some of these findings, notably the dependence on the nozzle length, pose a challenge to conventional models.