Tutorial $1$: Calculate the loss coefficient $U_L$ for $1$ meter long cylindrical receiver of parabolic trough collector of $60-mm$ at $200C.$ The absorber surface has an emittance of ${}_r=0\mathrm{.}31.$ The absorber is covered by a glass tubular cover $90$ mm in outer diameter and $4$ mm thick and has transmissivity of $0\mathrm{.}88.$ The space between the absorber and cover is evacuated. The wind speed is $5\frac{m}{s}$ and the sky and air temperatures are $2$ and $10C,$ respectively.

Take air properties at an average temperature $286\mathrm{.}5K,=1\mathrm{.}232k\frac{g}{m^3},=1\mathrm{.}794{10}^{-5},k=0\mathrm{.}2046\frac{W}{m}.K$