A mass-based definition of flame stretch for flames with finite thickness

The flame stretch concept is extended for the case of 3D instationary flames with finite flame front thickness. It is shown that additional contributions to the stretch rate appear apart from the terms which are usually used in flame studies. These extra terms are associated with variations in the mass density along the flame iso-contours and with variations in flame front thickness in time and space. It is finally shown that the following definition for the stretch rate is applicable: K = \/m (dm/dt), denoting the fractional change of mass in an infinitesimally small flame volume.