IGNITION IN NONPREMIXED COUNTERFLOWING HYDROGEN VERSUS HEATED AIR - COMPUTATIONAL STUDY WITH SKELETAL AND REDUCED CHEMISTRY

Nonpremixed ignition of counterflowing H-2 against hot air is studied numerically with emphasis on developing simplifying approximations to the conservation equations governing this system. We derive and examin e a number of different ''skeletal'' and ''reduced'' chemical reaction mechanisms that are used to simplify the full kinetic mechanism consi sting of 9 species and 19 bidirectional elementary reaction steps. It is found that the use of inherently homogeneous approximations such as the steady-state or partial equilibrium approximations in the derivat ion of reduced reaction mechanisms can lead to significant errors in t his inhomogeneous system. We demonstrate that reaction step R11 (H + H O2 --> 2OH) plays a critical step in kinetically controlled H-2-air ig nition, and present a 6-step skeletal mechanism which represents the s mallest set of elementary reactions that will provide proper turning p oint behavior in the first and second ignition limits. A new sensitivi ty analysis methodology is introduced that quantifies the sensitivity of the system response, particularly near the ignition turning point, to important parameters in the conservation equations. We also examine the so-called decoupled ignition environment approximation in which i gnition takes place within a static ignition environment, consisting o f the temperature and major species concentration profiles. that is de coupled from and can be solved independently of the minor species prof iles. (C) 1998 by The Combustion Institute.