Combined LDV and Rayleigh measurements in a complex turbulent mixing flow

The c o l d f l o w behind a backward f a c i n g s t e p w i t h i n j e c t i o n of a f o r e i g n b l e e d gas was i n v e s t i ga ted u s i n g a s imultaneous l a s e r Doppler v e l o c i m e t r y Ray le igh s c a t t e r i n g techn ique. The f a c i l i t y s i m u l a t e s t h e f l o w f i e l d i n t h e flame s t a b i l i z a t i o n r e g i o n i n a s o l i d f u e l e d r a m j e t . V e l o c i t i e s and b l e e d gas c o n c e n t r a t i o n s and t h e r e s u l t i n g m i x i n g p a t t e r n were measured. P a r t i c u l a r a t t e n t i o n was p a i d t o t h e covar iance of t h e v e l o c i t y and b l e e d gas c o n c e n t r a t i o n which i s a measure of t h e t u r b u l e n t mass t r a n s p o r t i n t h e f l o w f i e l d . A novel r e d u c t i o n technique was u t i l i z e d i n t h e r e p o r t e d measurements. I n t h i s technique background n o i s e due t o g l a r e , e l e c t r o n i c no ise , and s c a t t e r i n g due t o seed p a r t i c l e s was removed from t h e Ray le igh s c a t t e r i n g s i g n a l . The r e s u l t s o b t a i n e d were i n good agreement w i t h those f r o m an a n a l y t i c a l model based upon a m o d i f i e d k -c code. However, t h e model was found t o somewhat u n d e r p r e d i c t t h e m i x i n g downstream o f rea t tachment and t o somewhat o v e r p r e d i c t t h e v e l o c i t y c o n c e n t r a t i o n covar iance. I n t r o d u c t i o n S o l i d fue led r a m j e t s (SFRJ), as o t h e r ramj e t s , r e q u i r e a f lame anchor ing r e g i o n a t t h e head o f t h e combustion chamber. In t h i s r e g i o n t h e f l o w must be t u r b u l e n t t o f a c i l i t a t e m i x i n g of t h e f u e l and a i r and have a v e l o c i t y lower than t h e b u r n i n g speed of t h e m i x t u r e . Such a f l o w r e g i o n may be generated i n t h e r e c i r c u l a t i o n zone behind a backward f a c i n g s t e p . It i s t h e purpose of t h e work i n t h i s l a b o r a t o r y t o i n v e s t i g a t e t h e complex t u r b u l e n t f l o w f i e l d which r e s u l t s when a f u e l i s i n j e c t e d th rough a porous p l a t e ( t o s i m u l a t e t h e p y r o l y s i s o f a s o l i d f u e l ) i n t o t h e r e c i r c u l a t i o n r e g i o n beh ind a backward f a c i n g s t e p . The p a r t o f t h e s tudy r e p o r t e d here d e a l s w i t h a c o l d f l o w s i m u l a t i o n of t h i s process i n which t h e f u e l was rep laced by CO which has a r e l a t i v e l y l a r g e Ray le igh c r o s s i e c t i o n . Combined l a s e r Doppler v e l o c i m e t r y (LDV) and molecu la r Ray le igh s c a t t e r i n g were used t o determine t h e f low v e l o c i t y and b l e e d gas c o n c e n t r a t i o n d i s t r i b u t i o n s as w e l l as t h e r e s u l t i n g m i x i n g p a t t e r n . I n a d d i t i o n , t h e c o r r e l a t i o n between t h e v e l o c i t y and c o n c e n t r a t i o n f l u c t u a t i o n s r e s u l t e d i n i n f o r m a t i o n about t u r b u l e n t mass t r a n s p o r t i n t h e f l o w f i e l d . The r e s u l t s o f these measurements were compared w i t h t h e p r e d i c t e d f l o w f i e l d o b t a i n e d us ing a m o d i f i e d clk,-a model which was r e p o r t e d elsewhere.