Question

Consider a refrigeration truck traveling at 90 km/h at a location where the air temperature is 27°C. The refrigerated compartment of the truck can be considered to be a 2.7 m-wide, 2.4-m-high, and L-m-long rectangular box. The outer surface of the truck is coated with a low-emissivity material, and thus radiation heat transfer is very small. Use the following thermal properties for air: k = 0.02514 W/m.°C; v = 1.516x10m/s; Pr=0.7309 L = 4.75 m. a) (10 pts.) Is the flow at the end of the boundary layer laminar or turbulent? b) (10 pts.) Which is the average Nusselt number? c) (30 pts.) Which is the rate of heat transfer gained from the roof of the truck if the average temperature of the outer surface of the refrigeration compartment of the truck is 25°C. L (in m) 2.4 m Refrigeration truck

Answer 1

fruct Too roof of h 247 [2.7m 2 dir -L -4.75m properties of air V = go Emph Izom o k=0.02514 Wimk Too=270 27 ambient temp pr=0.7309 To asuja teip of touck = 256 se=1516880 m Reynolds no les VLula bil 2 = 25X4.75 1.5168107 Re=78.331x105 5X10 Aurse the since at the end of length Re=78.331x105 boundary layer is turbulent at the end For fuobulent flow avg. Muselt no is given by in No=hL = *0.036 (Re) -4Pr) = 0.036 ( 18.33//05 (0.7309/02 10629-738 Jan 0-33 k NU © He af rat gaired Qgafred Now NU IL = 0.036 (Red K 10029.738 hx47 (Pr ) 0.02514 ha beat toampo coefficient

: h=86.259 W/m²K =56.259 W/m²k i ang heut gain. neglecting radiation gain (Qars) = h A ( Toortsi =56-259 X(LABI (2725) 56-259 (4.75X207) (2) (Qavg) garant = 1443.4 m hos gain _tes - -