Q1

(a) Differentiate between free and forced convection with examples.

(b) Define Nusselt (Nu). Prandtl (Pr). Reynolds (Re) and Grashof (Gr)

dimensionless numbers. State which numbers are used in free and forced

convection to determine the heat transfer coefficient.

(c) A vertical pipe 75 mm diameter and 1.5 m height is maintained at a

constant temperature of 125 °C. The pipe is surrounded by still

atmospheric air at 25 °C. Find heat loss by natural convection.

Properties of water at 75 °C:

Density = 1.1145 kg/m³

Kinematic viscosity = 21.55 x10-6 m²/s

Prandtl number (Pr) = 0.751

Thermal conductivity (k)= 33.06 x 10-³ W/m K

Grashof number, Gr=

gxBxL³ x AT

V²

Nu = 0.10(GrPr) 0.333

END

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