Effect of surface conditions on the normal spectral emittance of titanium and zirconium between 1000 and 1800 K
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N. B A R I A A N D R E N A T O
G. B A U T I S T A
The n o r m a l s p e c t r a l e m i t t a n c e of Ti and Z r was o b t a i n e d in the t e m p e r a t u r e r a n g e 1100 to 1800 K and wavelength 0.645 ~ f o r e l e c t r o c h e m i c a l l y p o l i s h e d , m e c h a n i c a l l y p o l i s h e d and 60 g r i t g r o u n d s u r f a c e s a t a p r e s s u r e b e l o w 10 -e m m Hg. The e m i t t a n c e at the m e l t i n g point of both m e t a l s was o b t a i n e d b y e x t r a p o l a t i o n of an e m p i r i c a l equation f i t t e d to the e x p e r i m e n t a l d a t a o b t a i n e d f o r the e l e c t r o c h e m i c a l l y p o l i s h e d s u r f a c e and was found to b e in good a g r e e m e n t with r e p o r t e d v a l u e s . The c o n s i d e r a b l e g r a i n g r o w t h on the s u r f a c e a f f e c t e d the e m i t t a n c e . The e m i t t a n c e was found to i n c r e a s e with t e m p e r a t u r e up to 1300K and then d e c r e a s e d with f u r t h e r i n c r e a s e in t e m p e r a t u r e .
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r e c e n t y e a r s , the use of m a t e r i a l s a t v e r y high t e m p e r a t u r e s h a s g a i n e d m o r e and m o r e i m p o r t a n c e , and a s a r e s u l t t h e r e h a s b e e n an i n c r e a s e d i n t e r e s t in t h e i r high t e m p e r a t u r e p h y s i c a l p r o p e r t i e s . One p r o p e r t y which is v e r y i m p o r t a n t a t high t e m p e r a t u r e s is t h e r m a l r a d i a t i o n , which is the m a i n mode of h e a t t r a n s f e r at t e m p e r a t u r e s above 1000 K. F o r a given s u b s t a n c e t h e r m a l r a d i a t i o n is a function of the r a d i a t i o n s u r f a c e a r e a , i t s t e m p e r a t u r e and e m i t t a n c e . E m i t t a n c e of a s u b s t a n c e is a m e a s u r e of the t h e r m a l energy emitted by that substance at any temperature, and c a n b e e i t h e r t o t a l o r s p e c t r a l , d e p e n d i n g on w h e t h e r the r a d i a t i o n m e a s u r e d is o v e r a l l o r a n a r r o w b a n d of the e n t i r e s p e c t r a l r e g i o n . It c a n a l s o b e c h a r a c t e r i z e d a s h e m i s p h e r i c a l o r d i r e c t i o n a l depending upon w h e t h e r the r a d i a t i o n b e i n g e m i t t e d is m e a s u r e d on the e n t i r e h e m i s p h e r e a b o v e the s u r f a c e o r in a p a r t i c u l a r d i r e c t i o n a t an a n g l e to the s u r f a c e . The n o r m a l s p e c t r a l e m i t t a n c e is d e f i n e d a s the r a t i o of the e m i t t e d r a d i a n t flux o v e r a n a r r o w band of w a v e l e n g t h ~ and k +dk, in a d i r e c t i o n n o r m a l to the s u r f a c e , at a given t e m p e r a t u r e , to that e m i t t e d b y a b l a c k b o d y u n d e r the s a m e c o n d i t i o n s of wavelength and t e m p e r a t u r e . E m i t t a n c e i s a function of the s u r f a c e t e m p e r a t u r e , w a v e l e n g t h of r a d i a t i o n , s u r f a c e r o u g h n e s s and s u r f a c
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