Convection Heat Transfer Formula and Calculation Examples

📚 Quick Study Guide

• 🔥 Convection is heat transfer due to the movement of fluids. This movement can be natural (due to buoyancy) or forced (due to a fan or pump).
• 📏 Newton's Law of Cooling governs convective heat transfer: $q = hA(T_s - T_\infty)$, where:
  • $q$ is the heat transfer rate (W)
  • $h$ is the convection heat transfer coefficient (W/m²K)
  • $A$ is the surface area (m²)
  • $T_s$ is the surface temperature (K or °C)
  • $T_\infty$ is the fluid temperature (K or °C)
• 💨 The convection heat transfer coefficient, $h$, depends on fluid properties, flow velocity, and geometry.
• 🔢 Dimensionless numbers like the Nusselt number ($Nu$), Reynolds number ($Re$), and Prandtl number ($Pr$) are crucial for correlating and predicting $h$.
• 🧪 Empirical correlations are often used to determine $h$ for specific geometries and flow conditions.

Practice Quiz

1. Which of the following is the primary mechanism of heat transfer in convection?

   1. Conduction
   2. Radiation
   3. Fluid motion
   4. Phase change

2. What does 'h' represent in Newton's Law of Cooling?

   1. Heat flux
   2. Thermal conductivity
   3. Convection heat transfer coefficient
   4. Surface area

3. Which dimensionless number is most directly related to convective heat transfer?

   1. Reynolds number
   2. Prandtl number
   3. Nusselt number
   4. Grashof number

4. For forced convection, what is the main driving force for fluid motion?

   1. Buoyancy
   2. External force (e.g., fan, pump)
   3. Temperature difference
   4. Surface tension

5. A hot plate with a surface temperature of 120°C is cooled by air at 25°C. If the convection heat transfer coefficient is 20 W/m²K and the surface area is 0.5 m², what is the rate of heat transfer?

   1. 475 W
   2. 950 W
   3. 1187.5 W
   4. 2375 W

6. In natural convection, what force drives the fluid motion?

   1. Pressure gradient
   2. Buoyancy forces
   3. Viscous forces
   4. Inertial forces

7. Which of the following scenarios typically results in a higher convection heat transfer coefficient?

   1. Laminar flow
   2. Turbulent flow
   3. Stagnant fluid
   4. Vacuum