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
- Set up the heated cylinder/plate in still air (no blower). Switch on the heater and set power to 40–60 W.
- Wait until steady-state is reached (temperature readings stabilise over 5 minutes).
- Record surface temperature (T_s) and ambient temperature (T_∞).
- Record power input Q = V × I from the wattmeter.
- Calculate h = Q / (A × (T_s − T_∞)).
- 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
- Switch on the blower. Set the air velocity using the regulator and measure with an anemometer.
- Maintain the same heater power as in natural convection.
- Allow steady state. Record T_s, T_∞, and air velocity V (m/s).
- Calculate h = Q / (A × (T_s − T_∞)) for forced convection.
- 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
| Mode | Power Q (W) | T_s (°C) | T_∞ (°C) | Air Velocity (m/s) | h (W/m²K) | Nu (experimental) |
|---|---|---|---|---|---|---|
| Natural Convection | 0 (still) | |||||
| Forced Convection |
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
Related: Heat Transfer Lab Equipment | Heat Conduction Experiment | Shell and Tube Heat Exchanger | Engineering Lab Equipment Guide
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