Downloaded from https://www.everyspec.com PROPERT'SES OF MATERIALS I! 2-3. HEAT TRANSSMl A so-called f/ray body will reflect some of Iiscustomar todstinguish threebasic th rain nryrcevd ec h mis- siv peer f agra boy wlldepend ulon paii anoter (3) cond ion hil th iectu absorbed and reflected. It shouid be noted energy ismntcmtl under stying ththea tion to being proportional to ihe fourth pheomeon s osered o b c istnt ith pow-.?r of the temnperature, is also dependent I ~certain physical laws pustulated by v;%rious onhehycaprptisotetodad invetigors Th paticuar aw hic is varies with the wave length.
It is usual to applicable can be established by recourse to ongl th ' diepdenc Te ortespradint poer experimental methods.onteweleg.Thcrepndgeq- Whatever the wrid~ of heat trancfi~, the tion for 1) for a gray body is written miissile designer is bask'.ly interested in ifUtmperzture distribution -ei.her as a function 4", 'cAT, (2 3) f sgae or of space nnd time. The temjpera- wherw t is the emissivity of the gr-y body. I ure distribution is required to deterin The emissivity foi, some engineering mate- heat transfer rates, to make possible the cots,- rials -1. gi'ven in the literature.'" putation of thermal stresses, to deterinine Two final relations should be considered.
the change lin material physical properties, The first is that radiant interchange between or to cGP vith a problem ;Sinmefting, abla 5i',res is governxed by the aiove principles Lion or burhing. if there is a non-absorbing medium between 2-3.1. Radiatic-a. The transfer of heat the surfaces. Second, the radiant interchange - energy by radiation is probably best exerr also dtepends upon tho.. iverse square of the plifled by considering the transfer of heat distanze between the surface-s and the rela- F rom the sun to the earth. Thermal energy tive orientation of one surface with respect Lrom a hot body, the sun, is converted into to another.
If it is assumed that the radiation __/ eleetromagnetic energy which is transr-itted fromn a heated surface has the same flux in thrug spceto a cold body, the earth, all directions, the orientation or "view" be- wharc iisreconverted into thermal eniergy. comes purely geometrical in nature. Terv The radiant energy reaching the cold body factors are also given in the literature.'-- can be absorbed, reflected or transmitted throuf.h the body. Since most e;;gineering 2-3l.2. Convection. 'The trainsmoission of heat materials are opaque, the tronsmiss~vity of by tonvection (liffens materi-illy from the these materials is zero, hence the cf i.ctivity transmission (if heat by radiation.
In the case and absorptivity only need be consiide. of conve-tive heat transfer, heat is conveyed The concept _f a black body is the limiting from one point to another by the movement case of a body with 100O/1 absorptivity and of a fluid Atream; i.e., ihe transport process zero reflectivity. For a given absolute tem- is &ssocidetd w~th the actual movemneat of a perature, no body emits or absorbs more heat fluid streai-n. Convective heat transfer may than a black body.
The r-ate at kvhich the bentta or forced depending upon whether radiant energy is emitted or absorbed by a the fluid motion oc-curs by virtue of the nat- black body is given by the Stefan-Boltzmann 10-111 tendency of the he,.ted, lighter elerneit =rate of radia.-,L enryem~s or ab- blower or othct means are used to produce so tion by T (2 2)ac bofy Bt he fluid to Itsha, or pl hee tine a ~tefan-13olt?mann constant here that natural convective cooling of a sur- 0.174 x 10-4 Btu 1 ( ir) (Wt)(F') face area A a0 the b')n dary of a~ s-olid is A = radiating area, ft" proportionail to (T',, -T_) I %%here T,, arnd T.. T =absolute temperature, F are the surface ind arnbiont fluid tempera- 13