A furnace with a long isothermal graphite tube of diameter D = 12.5 mm is maintained at Tf = 2000 K and is used as a blackbody source to calibrate heat flux gages. Traditional heat flux gages are constructed as blackened thin films with thermopiles to indicate the temperature change caused by absorption of the incident radiant power over the entire spectrum. The traditional gage of interest has a sensitive area of 5 mm2 and is mounted coaxial with the furnace centerline, but positioned at a distance of L = 60 mm from the beginning of the heated section. The cool extension tube serves to shield the gage from extraneous radiation sources and to contain the inert gas required to prevent rapid oxidation of the graphite tube.

(a) Calculate the heat flux (W/m2) on the traditional gage for this condition, assuming that the extension tube is cold relative to the furnace,

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Extension tube Heat flux gage Tube furnace, Ty= 2000 K