An un-insulated air conditioning duct of rectangular cross section $$1\,\,m \times 0.5\,m,$$ carrying air at $${20^ \circ }C$$ with a velocity of $$10 m/s,$$ is exposed to an ambient of $${30^ \circ }C$$. Neglect the effect of duct construction material. For air in the range of $${20-30^ \circ }C,$$ data are as follows: thermal conductivity $$=0.025 W/m.K;$$ viscosity $$ = 18\mu Pa.s;$$ Prandtl number $$=0.73;$$ density $$= 1.2$$ $$kg/{m^3}.$$. The laminar flow Nusselt number is $$3.4$$ for constant wall temperature conditions and, for turbulent flow, $$Nu = 0.023\,\,R{e^{0.8}}\,{\Pr ^{0.33}}.$$

The heat transfer per meter length of the duct, in watts, is

The following figure was generated from experimental data relating spectral black body emissive power to wave length at three temperatures $${T_1},{T_2}$$ and $${T_3}\left( {{T_1} > {T_2} > {T_3}} \right).$$

The conclusion is that the measurements are

A solid cylinder (surface $$2$$) is located at the center of a hollow sphere (surface $$1$$). The diameter of the sphere is $$1m,$$ while the cylinder has a diameter and length of $$0.5m$$ each. The radiation configuration factor $${F_{11}}$$ is.

Hot oil is cooled from $${80^ \circ }C$$ to $${50^ \circ }C$$ in an oil cooler which uses air as the coolant. The air temperature rises from $${30^ \circ }C$$ to $${40^ \circ }C$$. the designer uses a $$LMTD$$ value of $${26^ \circ }C$$. the type of heat exchanger is