An experimental study of natural convection and wall effect in liquid metals contained in vertical cylinders
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A wide v a r i e t y o f m e t a l l u r g i c a l o p e r a t i o n s i n v o l v i n g m e l t i n g o r s o l i d i f i c a t i o n of m e t a l s a n d a l l o y s r e s u l t i n t h e g e n e r a t i o n of n a t u r a l c o n v e c t i o n c u r r e n t s . T h e s e natural convection p r o c e s s e s can have important cons e q u e n c e s b o t h f r o m a k i n e t i c a n d s t r u c t u r a l p o i n t of v i e w . F o r e x a m p l e , m e l t i n g r a t e s of f u r n a c e a n d l a d l e a d d i t i o n s i n s t e e l m a k i n g o p e r a t i o n s c a n be e x t r e m e l y r a p i d due t o t h e g e n e r a t i o n o f n a t u r a l c o n v e c t i o n c u r r e n t s .1 T h e s e c u r r e n t s r e s u l t f r o m t h e m e l t ing m e c h a n i s m w h e r e b y cooler ( m o r e d e n s e ) liquid m e t a l a t t h e m e l t i n g i n t e r f a c e i s d i s p l a c e d by h o t t e r ( l e s s dense) l i q u i d m e t a l from the b u l k . T h u s , u n d e r certain c i r c u m s t a n c e s for example, heavy s t e e l s c r a p c a n be e m p l o y e d i n B a s i c O x y g e n F u r n a c e c o o l i n g o p e r a t i o n s w i t h c o n s e q u e n t e c o n o m i c g a i n s .1 S i m i l a r l y , i n t h e c a s e of s o l i d i f y i n g s t e e l i n g o t s , h i g h l y u n d e s i r a b l e m a c r o s e g r e g a t i o n ( A a n d V t y p e ) of d i s s o l v e d s o l u t e s c a n be a t t r i b u t e d t o n a t u r a l c o n v e c t i o n p r o c e s s e s g e n e r a t e d d u r i n g t h e s o l i d i f i c a t i o n p r o c e s s .2 A s p a r t o f a b a s i c h e a t t r a n s f e r s t u d y on t h e e f f e c t s o f n a t u r a l c o n v e c t i o n in l i q u i d m e t a l s y s t e m s , e x p e r i m e n t s h a v e b e e n c o n d u c t e d in s i m p l e g e o m e t r y w i t h a m i n i m u m of e x t e r n a l p a r a m e t e r s , i . e . , c o n v e c t i v e h e a t t r a n s p o r t in a v e r t i c a l c y l i n d e r h e a t e d f r o m b e l o w w i t h a n a d i a b a t i c side w a l l . T h e f i r s t s t e p i n v o l v e d o b t a i n ing heat t r a n s f e r correlations, and t h e s e are reported in t h e p r e s e n t p a p e r . T h e c o r r e l a t i o n o b t a i n e d i n c l u d e s a t e r m w h i c h t a k e s i n t o a c c o u n t t h e e f f e c t of w a l l p r o x i m i t y o n h e a t t r a n s f e r r a t e s . T h i s wall effect w a s d i s c o u n t e d by G l o b e a n d D r o p k i n 3 on t h e b a s i s o f r e s u l t s o b t a i n e d f r o m less e x t e n s i v e t e s t s in t e r m s o f c y l i n d e r g e o m e t r y . A s s h o w n l a t e r , t h e s e r e s u l t s c a n be a d a p t e d to e s t i m a t e h e a t t r a n s f e r c o e f f i c i e n t s b e t w e e n n a t u r a l l y convecting liquid m e t a l s and a c o l d e r u p p e r s u r f a c e ( e . g . a s o l i d i f y i n g c r u s t on top of 1) a p o o r l y i n s u l a t e d l a d l e of s t e e l o r 2) the u p p e r s u r f a c e s o f s o
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