Effect of vent area on vented ammonia-hydrogen-air deflagrations in a 1-m-long duct

Abstract

Although ammonia-hydrogen blended fuel has become a research hotspot, it is essential to recognize that adding hydrogen to ammonia increases the potential explosion hazard before considering the development of ammonia-hydrogen fuel. In this study, the effect of vent area on vented deflagrations of ammonia-hydrogen-air mixtures is investigated in a 1-m-long horizontal rectangular duct with a right end opening at an initial temperature of 293 K and an initial pressure of 101 kPa. A dimensionless vent coefficient Kv is used to define the vent area in this work to clarify the evolution of flame structures and overpressure inside and outside the duct. For a specific Kv, the amplitude of the maximum internal explosion overpressure (Pmax) monitored at the left end of the duct (LE) has a greater difference compared with other pressure monitoring points. The maximum amplitude of Pmax is always reached at the LE for Kv ≤ 3.2, while the difference in the maximum amplitude of Pmax between different monitoring points decreased significantly for Kv > 3.2. With the increasing of Kv from 2.2 to 20.1, Pmax obtained at the right end of the duct (RE) and the center of the duct increases monotonously, but Pmax monitored at the LE shows a non-monotonic increasing trend. A spike structure of penetrating flame bubbles resulting from negative pressure balance is only observed inside the duct for Kv ≤ 5.6. Two types of oscillations (Helmholtz and Acoustic oscillations) can be distinguished in some tests, and only acoustic oscillations of overpressure can be clearly found in tests with Kv > 7.8. With the increasing of Kv from 2.2 to 20.1, the shape of the external fireball will change significantly. As Kv increases from 2.2 to 5.6, the external fireball shapes are always mushroom-shaped. However, the shapes of the fireball become elongated for Kv > 7.8, and even the external fireball presents a jet shape for Kv = 20.1. With the increasing of Kv from 2.2 to 20.1, The maximum external overpressure increases first, thereafter decreases, and finally increases.