Measuring Detonation Cell Size in Gaseous Hydrocarbon-Air Mixtures at Elevated Temperatures and Oxygen-Deficient Conditions

This study uses a detonation tube of 66.7 mm diameter and 6 m length with soot foil to measure the detonation cell size of hydrocarbon fuel (ethylene) with air at elevated initial temperatures (300-590 K) and reduced oxygen concentration (12-21 % vol) at various equivalence ratios (0.6-1.1) and atmospheric pressure. Nitrogen was used as diluent to reduce the oxygen concentration. This simulates non-ideal detonation condition from partial deflagration of fresh reactants. The detonation wave speed before the soot foil section was measured as well. The cell sizes were measured manually from soot foil photographs. The results showed that as the equivalence ratio and temperature increases, the cell size decreases, increasing detonability. At equivalence ratio of 0.8 with atmospheric oxygen concentration, increasing the temperature from 300 K to 590K decreased the average cell size by a factor of 3. At equivalence ratio of 0.6, only 590 K cases detonated, with some cases exhibiting spiral mode with cell sizes fifth of a normal case. With reduced oxygen in stoichiometric conditions, elevated initial temperature increased detonability and decreased cell sizes. At 15% vol oxygen, cell sizes were decreased by a factor of two when the temperature was increased from 480 to 590 K. At 13% vol oxygen, only high temperature cases detonated, with cell sizes reaching the diameter of the tube. The results showed the competing effects of reduced oxygen and elevated temperature for detonability and detonation cell size.