Heat Transfer Lab. Experiment 4

Experiment 4

OBJECT: - To determine the surface heat transfer coefficient for a vertical tube losing heat by natural convection.

INTRODUCTION: - When a hot body is kept in still atmosphere, heat is transferred to the surrounding fluid by natural convection. The fluid layer in contact with the hot body gets heated; rise up due to the decrease in its density and the cold fluid rushes in to take place. The process is continuous and the heat transfer takes place due to the relative motion of hot cold fluid particles.

APPARATUS: - 

SPECIFICATIONS: -
Diameter of cylinder (d) = 38 mm
Length of cylinder (L) = 500 mm
Thermal conductivity of tube material (k) = W/mK
Perimeter of cylinder = m
Cross section area of cylinder = m2

FORMULAS: -
The heat transfer coefficient is given by:

where,

h = Average surface heat transfer coefficient (W/ m²K)

q = Heat transfer rate (Watts)

A_s = Area of heat transferring surface = πd (m²)

Ts_av = Average surface temperature = (T1 + T2 + T3+ T4 + T5 + T6)/6

T_a = Ambient temperature in the duct = T7

The surface heat transfer coefficient, of a system transferring heat by natural convection depends on the shape, dimensions and orientation of the fluid and the temperature difference between heat transferring surface and the fluid. The dependence of h on all the above-mentioned parameters is generally expressed in terms on non-dimensional groups as follow:

OBSERVATIONS: -

Voltage reading,         V =               (Volts)

Ammeter reading,       I =             (Amperes)

Thermal Conductivity of air at film temperature T_f, k_air =             W/mK    

Density of air at film temperature T_f, ρ_air =           kg/m³

Dynamic viscosity of air at film temperature T_f, =            kg/ms

Prandtl No. of air, Pr = 0.712

T_air = T₇ =                    ºC 

CALCULATION: -

Average surface temperature of the fin, Ts_av = (T1+T2+T3+T4+T5+T6)/6

The fluid properties are evaluated at the film temperature given by, 

Experimental heat transfet coefficient is given as

 Theoretical heat transfer coefficient is given as

PRECAUTIONS: -

1. Operate the changeover switch of temperature indicator, gently. 
2. Make sure that the steady state is reached before taking the final reading. 
3. See that throughout the experiment, the blower is OFF 

RESULT: - 
Percentage difference between theoretical and experimental value =