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Home Mechanical Engineering Heat Transfer Lab Determination of Heat Transfer Coefficient Through Natural Convection
Heat Transfer Lab Lab Experiments

Determination of Heat Transfer Coefficient Through Natural Convection



Aim

To determine the surface Heat transfer coefficient along the length of the tube by Natural convection.

Diameter of the cylinder (D) : 35mm

Length of the tube (L) : 500mm

Duct size : 250 mm x 250 mm x 700 mm

No. of thermocouples : 8

Temperature indicator : 0-400 °C

Apparatus Required:

  • Natural convection experimental setup
  • Stabilized power supply
  • Stop Clock

Procedure:

  1. Switch 'ON' The heater and Set an input Voltage (60V) Using the voltage regulator.
  2. The heater inside the vertical tube heats the steel tube through the conduction. As the tube gets heated, the heat from the outer surface of tube transfer to the surrounding by natural convection.
  3. Allowing the heater to heat the tube, observe the readings of thermocouple at different locations at equal intervals of time until the system reaches steady state and tabulate them.
  4. Choosing appropriate formulae for a natural convection of a horizontal tubes, external surface from HMT data book, calculate the heat transfer coefficient.

Observations:

S. No. Voltage (V) Volts Current (I) Amps Temperature (°C)/T1(°C)/T2(°C)/T3(°C)/T4(°C)/T5(°C)/T6(°C)/T7(°C) Ambient Temp/T8(°C)/Ta(°C) Heat transfer coefficient (ha) W/m2K
1
2
3
4
5

Formula Used

Ref:-Heat and Mass Transfer Data book by Kothandaraman & Subramaian, New Age Publishers.

Heat input:       (q) = V*l Watts

Average surface temp (Ts) = T1+T2+T3+T4+T5+T6+T7/7

=---------------------- °C

Ambient air temp (Ta) = T 8 °C

Mean film temp (Tf) =Ts + Ta /2 °C

=l /Tf in K

Properties of air at 'Tf' from the at and Mass Transfer data book

Grashof No     Gr = gβ Δ TL 32 Δ

Δ T = Ts - Ta(°C)

Gr Pr = ___ _ __

Nusselt No,     Nu = 0.13 (Gr Pr) 1/3

Heat transfer coefficient, (ha) = NuK/D W/m 2K

Result:

The surface of Heat transfer coefficient along the length of the tube is (ha) ___ _ __

Viva-Voce Questions

What is the range of ‘h’ for Natural convection in gases & liquids?

Range of ‘h’for natural convection in gases is 3-25W/m2 -k & for liquids it is 50- 350W/m2 -k.. Boundary layer is a thin layer at the surface where gradients of both velocity & temperature are large.

What is Nusselt Number?

Nusselt Number represents the enhancement of heat transfer through a fluid layer as a result of convection relative to conduction across a same layer. Larger the Nu, more effective is convection.

What is a boundary layer?

Boundary layer is a thin layer at the surface where gradients of both velocity & temperature are large.

What is meant by laminar flow ?Laminar flow:

Laminar flow is sometimes called stream line flow. In this type of flow, the fluid moves inlayers and each fluid particle follows a smooth continuous path. The fluid particles in each layer remain inan orderly sequence without mixing with each other.

Define Convection.

Convection is a process of heat transfer that will occur between a solid surface and a fluid medium when they are at different temperatures.

What is meant by Newtonion and non-newtonion fluids?

The fluids which obey the Newton’s law of viscosity are called Newtonion fluids and those which do not obey are called no-newtonion fluids.