Heat And Mass Transfer - Study Mode
[#126] Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be
Correct Answer
(C) Three
Explanation
Solution: $$frac{{{ ext{Q with shield}}}}{{{ ext{Q without shield}}}} = frac{1}{{{ ext{n}} + 1}}.$$ N = number of parallel shields 0.25n + 0.25 = 1 0.25n = 0.75 n = 3.
[#127] Reynolds number (R N ) is given by (where h = Film coefficient, $$l$$ = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, $$
ho $$ = Density of fluid, c p = Specific heat at constant pressure and $$mu $$ = Coefficient of absolute viscosity)
Correct Answer
(C) $${{ ext{R}}_{ ext{N}}} = frac{{
ho { ext{V}}l}}{mu }$$
Explanation
Solution: Reynolds number (R N ) is given by $${{ ext{R}}_{ ext{N}}} = frac{{
ho { ext{V}}l}}{mu }$$ Where h = Film coefficient, $$l$$ = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, $$
ho $$ = Density of fluid, c p = Specific heat at constant pressure and $$mu $$ = Coefficient of absolute viscosity
[#128] An ordinary passenger aircraft requires a cooling system of capacity.
Correct Answer
(C) 8 TR
Explanation
Solution: An ordinary passenger aircraft requires a cooling system capable of 8 TR capacity and a super constellation requires a cooling system of more than 8 TR capacity
[#129] A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h 1 , h 2 and h 3 respectively. The average heat transfer coefficient for the cube is
Correct Answer
(C) $$frac{1}{{{{ ext{h}}_1}}} + frac{1}{{{{ ext{h}}_2}}} + frac{1}{{{{ ext{h}}_3}}}$$
Explanation
Solution: A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h 1 , h 2 and h 3 respectively. The average heat transfer coefficient for the cube is $$frac{1}{{{{ ext{h}}_1}}} + frac{1}{{{{ ext{h}}_2}}} + frac{1}{{{{ ext{h}}_3}}}.$$
[#130] Moisture would find its way into insulation by vapour pressure unless it is prevented by
Correct Answer
(D) A vapour seal
Explanation
Solution: Moisture would find its way into insulation by vapour pressure unless it is prevented by a vapour seal