Experimental results are presented for accelerated flames in gas mixtures with vertical concentration gradients in a smooth and an obstructed tube. Hydrogen-air mixtures with a global hydrogen content between 13 and 30 vol.-% are investigated at an initial pressure of1atm. An injection mechanism is described that is capable of creating homogeneous mixtures as well as mixtures with vertical concentration gradients inside the tube. It is shown that the vertical concentration gradient has a major influence on flame acceleration (FA), indicated by the change of maximum flame velocity and pressure. Mixtures of the same hydrogen content can produce three different flame regimes, viz. slow flames, fast flames, and detonations, depending on the concentration gradients present. The peak over-pressure at the end of the tube can increase up to two orders of magnitude compared to homogeneous mixtures. It is shown that the maximum (local) concentration is a good criterion to compare different mixtures. The transition from slow flames to fast flames and even the deflagration-to-detonation transition (DDT) in different mixtures can be correlated well using the maximum fuel concentration near the upper wall.
flame acceleration, inhomogeneous mixture, vertical concentration gradient, hydrogen