EXPT NO: NAME:

DATE: ROLL NO:

DETERMINATION OF HEAT TRANSFER COEFFICIENT BY NATURAL

CONVECTION

AIM:

1. To determine the average surface heat transfer coefficient for a vertical tube losing heat by natural convection.

2. To compare it with the value obtained from empirical relations.

THEORY:

Convection is a mode of heat transfer, generally takes place in liquid and gases. Consider a

fluid over a heated surface, the molecules of fluid adjacent to the surface, absorb heat and

become hot. On heating, the molecules become lighter due to decrease in density, they rise up

and the cold molecules of higher density come down in contact of heated surface. In this

way, a motion of molecules set up in fluid due to density gradient.

Natural convection heat transfer is extensively used in the following areas of engineering

1. Cooling of transformers, transmission lines and rectifiers.

2. Heating of houses by stream or electrical radiations.

3. Heat loss from steam pipe lines in power plants and heat gain in refrigerant pipe

lines in air conditioning applications.

4. Cooling of reactor core in nuclear power plants.

5. Cooling of electronic devices (chips, transistors) by finned heat sinks.

APPARATUS:

The apparatus consists of a vertical stainless steel tube enclosed in a rectangular

duct. Front side of the duct is made of transparent section to facilitate visual

observation. An electrical heating element embedded in a copper tube act as the

heat source. The surface temperature is measured at different heights using

thermocouples. The surface of the tube is polished to minimize the radiation

losses. A voltmeter and an ammeter enable the determination of wattage

dissipated by the heater. The chamber temperature also can be measured.

500 mm

THEORY

Experimental heat transfer coefficient hexpt = Q/ A(Ts- Ta) W/m2K.

Where Q = heat input in Watts

A = surface area in m2

Ts = surface temperature in 0C

Ta = ambient temperature in 0C

Theoretical value of heat transfer coefficient ht = Nu k/ L

Where, Nu = Nusselts number

k = thermal conductivity in W/m0C

L = length of pipe in m.

Nusselts number can be calculated by using free convection correlations for vertical cylinders.

Nusselts Number, Nu = ht L/K = 0.53 [GrPr]1/4

for Gr.Pr< 105,

Nu = ht L/K = 0.56 [GrPr]1/4

for 105

OBSERVATIONS:

SI

No

Heat Input Q in W

Thermocouple Readings in C hexpt

W/m2K

ht

W/m2K V

(v)

I

(A)

Q=VI

(W)

T1 T2 T3 T4 T5 T6 T7 Ambient

Temperature

T8(Ta)

SAMPLE CALCULATION FOR SET NO:

1. To find out the average heat transfer coefficient

hexpt = Q/ A(Ts- Ta) W/m2K

Where

V = V

I = A

Q= heat input, VI = = W

d = outer diameter of cylinder = 0.044 m

L = length of cylinder = 0.5 m

A = surface area in m2= dL= = m2

Ts = surface temperature in 0C

TS = (Tl + T2 + T3 + T4 + T5 + T6 + T7) /7 = oc

Ta = ambient temperature in0C= T8

hexpt = = W/m2 K

2. To find out the theoretical heat transfer coefficient

Grashof Number, Gr = L3g (Ts T8) /

2

Where - Volumetric coefficient of thermal expansion K-1

For ideal gas, = 1/Tf

where Tf - absolute film temperature at which properties are taken from data book.

g = acceleration due to gravity in m/s2

Ts = surface temperature= K

Ta = ambient temperature= K

Tf= (Ts+Ta)/2 + 273 K= K

L = length of cylinder = m

= kinematic viscosity= m2/s

Grashof Number, Gr =

Prandtl number, Pr = Cp/ K

Where, = dynamic viscosity of air = kg/m-s

Cp = specific heat of fluid = kJ/kg0C

K = thermal conductivity of air = W/m0C

Rayleigh Number, Ra = Gr Pr = =

Nusselt Number, Nu = =

(use the co-relation)

Theoretical heat transfer coefficient, ht = Nu k/ L

= W/m2K

RESULT:

Experimental heat transfer coefficient, hexpt =

Theoretical heat transfer coefficient, ht=

INFERENCE:

PRECAUTION:

1. Keep the dimmer stat to zero position before start.

2. Increase the voltage slowly.

3. Do not increase power input above 150V.