Effect of Chemistry on Temperature Boundaries of Gas dynamic-Chemistry Coherence Region for the Detonation Initiation in Systems with Induction Time Gradient I.A. Zaev 1* , C.A.Mechik 2 , I.A. Kirillov 1, 2 , S. Van der Hoeven 3 , D. Roekaerts 3 1 Moscow Institute of Physics and Technology, Moscow, Russia 2 RRC “Kurchatov Institute”, Moscow, Russia 3 Delft University of Technology, Delft, The Netherlands Abstract In the present work we study the effect of hydrogen/air kinetics on the boundaries of the coherence region, introduced in Ref. [1]. The role of the initial value of the equivalence ratio and initial pressure are evaluated in parametric computations. A simplified model for two basic stages of the combustion process, which are important for detonation initiation, is proposed (“constant heat release approximation” model). On the basis of the theoretical assumptions about processes in the “hot spot” system of [1], for different equivalence ratios the critical conditions for detonation initiation are represented in the phase plane of the hot spot initial conditions ( ). gradT T , 0 * Corresponding author: zaev@8ka.mipt.ru Associated Web site: http://www.mipt.ru Proceedings of the European Combustion Meeting 2005 Introduction In Ref. [1] it was shown that the process of detonation initiation by “hot spots” can be characterized by a critical parameter, namely the spatial size of the coherence region, which is a region within the hot spot where the gas-dynamic and chemical kinetic processes are coherent in sense, pointed out by J. Lee [2]. The initial temperature profile in the system under consideration is shown in Figure 1. The temperature in the center is assumed to be and the ambient temperature is . The total size of the spot is equal to 0 T 00 T L 2 . The temperature gradient is given by the relation ( ) L T T T grad 00 0 − − = . The kinetic (induction time, ) and thermodynamic (coustic speed, ) parameters of the combustible mixture have been used in the definitions of and . Therefore, from a practical viewpoint, it is important to investigate how the initial mixture composition (equivalence ratio) and initial conditions (pressure) influence the values of and . For accuracy using a detailed kinetic model is required. Figure 1 Initial temperature profile of the hot spot The coherence region exists for every hot spot if temperature in its center is sufficiently high. Coherence region can be characterized by upper and lower temperature boundaries. The lower boundary is determined by the post-shock mechanism of detonation formation. And, depending on the initial hot spot conditions, the upper boundary of the coherence region is between two temperatures, and . This is described in detail in Ref. [1]. 1 T 2 T 1 T 2 T The definition of the temperatures and is based on the spontaneous flame concept, developed by Zel’dovich [3]. A spontaneous flame was considered as a sequence of local adiabatic constant volume explosions. For such a flame the propagation velocity was defined as the inverse of the induction time gradient. is the temperature at the location where the local value of the spontaneous flame speed is equal to the Chapman-Jouguet velocity. is the temperature at the position where the local spontaneous flame velocity is equal to the local acoustic speed. 1 T 2 T 1 T 2 T Specific Objectives i t a V 1 T 2 T 1 T 2 T Another task is the prediction of the critical conditions for detonation initiation for all types of H 2 /air mixtures. Based on the theoretical assumptions on the processes in the system described in [1], and the results of the computations of this work, a way of plotting the diagram for critical conditions for detonation initiation for different equivalence ratios in the phase plane of “hot spot” initial conditions is proposed. gradT T , 0