Convection Heat Transfer

Why it matters

Convection heat transfer is essential in many engineering applications, such as heating and cooling systems, automotive engines, and electronic devices. Understanding convection helps engineers design systems that efficiently manage heat, ensuring safety and performance.

Key ideas

• Convection is the transfer of heat between a solid surface and a fluid (liquid or gas) in motion. It combines conduction within the fluid and the bulk movement of the fluid.
• Types of Convection:
  • Natural Convection: Occurs due to buoyancy forces caused by density differences in the fluid due to temperature variations.
  • Forced Convection: Occurs when an external source, like a pump or fan, induces fluid motion.
• Newton's Law of Cooling: Describes the rate of heat transfer in convection, stating that the heat transfer rate is proportional to the temperature difference between the surface and the fluid.
• Heat Transfer Coefficient (h): A crucial parameter in convection, representing the convective heat transfer per unit area per unit temperature difference.

The coefficient h depends on geometry, fluid properties, velocity and flow regime; it is not solely a material property. For spatially varying conditions integrate h(T_s-T_fluid) over the surface. Use an appropriate bulk fluid temperature for internal flow.

Formulas

• q = h·A·(T_s - T_∞)
  • q: Heat transfer rate (W)
  • h: Heat transfer coefficient (W/m²·K)
  • A: Surface area (m²)
  • T_s: Surface temperature (K)
  • T_∞: Fluid temperature far from the surface (K)

Worked example

Given:

• Surface temperature, T_s = 80°C = 353.15 K
• Fluid temperature, T_∞ = 30°C = 303.15 K
• Surface area, A = 2 m²
• Heat transfer coefficient, h = 25 W/m²·K

Find: Heat transfer rate, q

1. Identify the formula: q = h·A·(T_s - T_∞)
2. Substitute the values: q = 25 W/m²·K · 2 m² · (353.15 K - 303.15 K)
3. Calculate the temperature difference: 353.15 K - 303.15 K = 50 K
4. Calculate the heat transfer rate: q = 25 · 2 · 50 = 2500 W

Final Answer: 2500 W

Common mistakes

• Confusing natural and forced convection.
• Mixing temperature scales. A difference in degrees Celsius has the same numerical value as a difference in kelvins.
• Misidentifying the surface area involved in heat transfer.

For GATE ME

Questions often involve calculating the heat transfer rate using given parameters or determining the heat transfer coefficient. Practice problems involving both natural and forced convection scenarios.

Quick check

1. What is the primary difference between natural and forced convection?
2. How does the heat transfer coefficient affect the rate of convection?
3. Can a Celsius temperature difference be used in Newton’s cooling law?

Answers: 1. Natural convection is driven by buoyancy forces, while forced convection is driven by external means. 2. A higher heat transfer coefficient increases the rate of convection. 3. Yes: T_s - T_∞ has the same numerical value in °C and K; absolute kelvins are needed in radiation fourth powers.