Convection heat transfer occurs when thermal energy moves between a solid surface and a fluid that is flowing past it. It is central to engineering systems such as radiators, heat exchangers, electronics cooling, engines, pipes, and HVAC equipment. Unlike pure conduction, convection depends strongly on how the fluid moves near the surface.
Engineers use convection models to predict surface temperatures, heat loss, cooling rates, and equipment performance.
At a heated plate or pipe wall, heat first conducts through a thin layer of fluid touching the surface, then fluid motion carries that energy away. The strength of this process is summarized by the heat transfer coefficient h, which depends on fluid properties, flow speed, geometry, and whether the flow is laminar or turbulent. Newton’s law of cooling, q = hA(Ts - T∞), gives the rate of heat transfer from a surface to a surrounding fluid.
Natural convection is driven by buoyancy from density differences, while forced convection is driven by fans, pumps, or external flow.
Key Facts
- Newton’s law of cooling: q = hA(Ts - T∞), where q is heat transfer rate in watts.
- Heat flux form: q'' = h(Ts - T∞), where q'' is heat transfer per unit area in W/m².
- The heat transfer coefficient h has units W/(m²·K) and measures how effectively convection removes or supplies heat.
- Forced convection uses external motion from a fan, pump, or moving stream, usually increasing h compared with still fluid.
- Natural convection is caused by buoyancy, as warmer fluid becomes less dense and rises while cooler fluid sinks.
- A thinner thermal boundary layer usually means a larger temperature gradient at the wall and stronger convection.
Vocabulary
- Convection
- Convection is heat transfer between a surface and a moving fluid due to both molecular conduction near the wall and bulk fluid motion.
- Heat transfer coefficient
- The heat transfer coefficient h is a proportionality factor that relates surface heat flux to the temperature difference between the surface and the fluid.
- Thermal boundary layer
- The thermal boundary layer is the region of fluid near a surface where the temperature changes from the surface temperature to the free stream temperature.
- Forced convection
- Forced convection is convection caused by externally driven fluid motion such as flow from a fan, pump, blower, or moving vehicle.
- Natural convection
- Natural convection is convection caused by buoyancy forces that arise when temperature differences create density differences in a fluid.
Common Mistakes to Avoid
- Using fluid temperature at the wall instead of the free stream temperature, which gives the wrong temperature difference in q = hA(Ts - T∞).
- Treating h as a universal constant, which is wrong because h changes with fluid type, velocity, geometry, surface condition, and flow regime.
- Ignoring the surface area A, which is wrong because doubling the exposed area doubles the heat transfer rate if h and temperature difference stay the same.
- Confusing natural and forced convection, which is wrong because natural convection is driven by buoyancy while forced convection is driven by external fluid motion.
Practice Questions
- 1 A hot metal plate has area 0.80 m², surface temperature 90°C, surrounding air temperature 25°C, and h = 18 W/(m²·K). Calculate the convective heat transfer rate.
- 2 Water flows through a heated pipe with inside surface area 2.5 m². If h = 650 W/(m²·K), the wall temperature is 70°C, and the bulk water temperature is 40°C, find q.
- 3 A vertical heated plate is cooled first in still air and then with a fan blowing across it. Explain which case should have the larger heat transfer coefficient and why.