Natural and Forced Convection Experiment — Heat Transfer Coefficient Determination

Aim of the Experiment

To determine the heat transfer coefficient for natural (free) and forced convection from a heated surface, and to compare the heat transfer rates in both modes.

Apparatus Required

• Natural and forced convection apparatus (heated cylinder or flat plate)
• Blower / fan for forced convection
• Thermocouples or temperature sensors at heater surface and at various distances
• Anemometer (for air velocity measurement in forced convection)
• Wattmeter / power supply (ammeter + voltmeter)
• Stopwatch

Theory

Convection is the transfer of heat by the movement of a fluid. When fluid motion is caused only by density differences due to temperature gradients, it is called natural (free) convection. When fluid is forced over the surface by a fan or pump, it is called forced convection.

Newton’s Law of Cooling: Q = h × A × (T_s − T_∞)

Where: h = convective heat transfer coefficient (W/m²K), A = surface area (m²), T_s = surface temperature (°C), T_∞ = ambient temperature (°C).

Natural convection is characterised by the Grashof number (Gr) and Prandtl number (Pr). Forced convection is characterised by the Reynolds number (Re) and Prandtl number (Pr).

Procedure — Natural Convection

1. Set up the heated cylinder/plate in still air (no blower). Switch on the heater and set power to 40–60 W.
2. Wait until steady-state is reached (temperature readings stabilise over 5 minutes).
3. Record surface temperature (T_s) and ambient temperature (T_∞).
4. Record power input Q = V × I from the wattmeter.
5. Calculate h = Q / (A × (T_s − T_∞)).
6. Calculate Grashof number Gr = gβ(T_s − T_∞)L³/ν² and compare Nu_exp = h × L/k with correlation Nu = C × (Gr × Pr)^n.

Procedure — Forced Convection

1. Switch on the blower. Set the air velocity using the regulator and measure with an anemometer.
2. Maintain the same heater power as in natural convection.
3. Allow steady state. Record T_s, T_∞, and air velocity V (m/s).
4. Calculate h = Q / (A × (T_s − T_∞)) for forced convection.
5. Calculate Re = V × L/ν and Nu_exp = h × L/k. Compare with Nu = 0.664 × Re^0.5 × Pr^(1/3) for laminar flow over a flat plate.

Observation Table

Result

Heat transfer coefficient — Natural convection: h = _______ W/m²K. Forced convection: h = _______ W/m²K. Forced convection gives significantly higher h, confirming that fluid motion greatly enhances heat transfer rates.

Internal Links

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