Radiation from Gas-Jet Diffusion Flames in Microgravity Environments

This paper presents the f i r s t demonstrat ion o f q u a n t i t a t i v e f lamerad ia t ion measurement i n m ic rog rav i t y environments, w i th t h e o b j e c t i v e of s tudy ing the inf luences and c h a r a c t e r i s t i c s o f r a d i a t i v e t rans fe r on the behavior o f gas je t d i f f u s i o n flames w i t h poss ib le a p p l i c a t i o n t o spacecra f t f i r e de tec t ion . Laminar d i f f u s i o n flames o f propane, burning i n quiescent a i r a t atmospheric pressure, a re s tud ied i n the 5.18Second Zero-Gravi ty F a c i l i t y o f NASA Lewis Research Center. Rad ia t i on from these flames i s measured us ing a wide-view angle, thermop i le -de tec tor radiometer, and comparisons a re made w i t h normalg r a v i t y f lames. The r e s u l t s show that t h e r a d i a t i o n l e v e l i s s i g n i f i c a n t l y h ighe r i n microg r a v i t y compared t o normal -g rav i ty environments due t o l a r g e r flame s ize , enhanced soot formation, and entrapment o f combustion products i n the v i c i n i t y of t he flame. These e f f e c t s a re the consequences of the removal o f buoyancy which makes d i f f u s i o n the dominant mechanism o f t ranspor t . The r e s u l t s show t h a t longer t e s t t imes may be needed t o reach steady s t a t e i n m ic rog rav i t y environments. 4' *Paper AIAA-91-0719, presented a t t he A I A A 29th Aerospace Sciences Meeting, Reno, Nevada, January 7-10, 1991. tSenior S c i e n t i s t ; Member A IAA. Author t o whom correspondence should be sent t o t h e f o l l o w i n g address: Ohio 44135. SAIC/NASA LeRC, MS 500-217, Cleveland, +Chief S c i e n t i s t ; Member A I A A . 'Manager, Zero-Gravi ty F a c i l i t y . "NASA P r o j e c t S c i e n t i s t . This paper is deelared a work of the U.S. Governmml and is no1 subjccl Io copyright protection in the Unilcd Stater. 1. I n t r o d u c t i o n The term " d i f f u s i o n flame" c l a s s i f i e s those types of flames i n which t h e f u e l and o x i d i z e r a re n o t premixed. The bas i c mechanisms which c o n t r o l these flames (and combustion i n genera l ) i nc lude the coupled processes of mixing. chemical k i n e t i c s i n c l u d i n g soot fo rmat ion and d i s p o s i t i o n , flame r a d i a t i o n , and buoyancy-induced convect ion. A major problem impeding our understandinq o f f lame behavior i s t h e masking e f f e c t o f buoyancy on flames t h a t a re s tud ied i n a normal -g rav i ty environment. The behavior o f f i r e s on e a r t h i s determined by t h e combined convec t ive and d i f f u s i v e ef fects, where buoyancy-induced convec t ion masks the processes o f chemica l -d i f f us lona l i n t e r a c t i o n t h a t a re fundamental t o t h e understanding of combustion phenomena. I n ze ro -g rav i t y environments, the buoyant fo rce i s e l im ina ted , and t h e remaining physico-chemical processes a re respons ib le fo r t he d i f f e r e n t behavior of flames observed i n the microg r a v i t y experiments conducted to-date. ' Recent s tud ies o f m i c r o g r a v i t y laminar d i f f u s i o n flames have shown some unique, and i n most cases, unexpected phenomer~a . ~ -~ For example, i n o rde r t o s tudy t h e e f fec ts o f m i c r o g r a v i t y on f l ammab i l i t y and e x t i n c t i o n o f flames i n the nearl i m i t regions, t e s t s were conducted under reducedoxygen2 and/or reduced-pressure3 cond i t i ons . The r e s u l t s show t h a t flame c h a r a c t e r i s t i c s , c o l o r , l um inos i t y , and soo t ing behav io r a re s t r o n g l y a f fec ted i n m i c r o g r a v i t y compared t o normal g rav i ty . 2 Soot ing was e l im ina ted i n m i c r o g r a v i t y propane flames a t 16% oxygen, and the flames were e n t i r e l y blue, whereas t h e i r normal -g rav i ty counterpar ts were yel low, luminous, and very s i m i l a r t o flames under atmospheric cond i t i ons . Th is has an impor tan t imp l i ca t i on , namely, reduced r a d i a t i v e hea t ing and reduced hazard of flame spreading t o the sur round ing combust ib le m a t e r i a l s by reduc ing the oxygen concen t ra t i on i n microg r a v i t y environments. I n a d d i t i o n , f l ammab i l i t y l i m i t s may be quite d i f f e r e n t i n m i c r o g r a v i t y and normal -g rav i ty flames under i d e n t i c a l cond i t i ons o f pressure and oxygen concent ra t ion . s tudy was conducted t o i n v e s t i g a t e the e f f e c t s o f r a d i a t i v e l o s s on flame behavior and c h a r a c t e r i s t i c s . The present