AdvancedTopic 86 of 105Thermodynamics & Fluids
Convection
Heat transfer by fluid motion, q=hA(Ts-T∞).
In depth
Convection combines conduction and fluid motion: q=hA(T_s-T_∞), with h from correlations (Nu=hL/k). Depends on flow regime, geometry and fluid properties; typical h: air natural 5–25, forced air 25–250, water forced 500–10 000 W/m²K, boiling 2500–100 000.
Key points
- Newton's law of cooling
- Boundary layer thickness
- Heat transfer coefficient is not a material property
ExampleA 100 W chip with 0.01 m² and h=20 W/m²K rises 500 K without a heat sink.
Convection describes heat transfer between a surface and a moving fluid, which governs cooling of engines, radiators and electronics.
What is Convection?
Heat transfer by fluid motion, q=hA(Ts-T∞).
Why is it important to understand convection?
Convection describes heat transfer between a surface and a moving fluid, which governs cooling of engines, radiators and electronics.
What is the heat transfer coefficient?
A value h that gives heat flow per unit area per degree of temperature difference between the surface and the fluid.
What drives convection?
Fluid motion, either due to buoyancy from temperature differences or from an external pump or fan.
How is Convection used in practice?
Typical applications include estimating the heat loss from a hot pipe in still air, calculating the cooling of a chip by airflow over its heat sink and choosing a heat transfer coefficient for water flowing past a tube.
What should I learn before Convection?
Study "Thermal resistance" first (topic 85 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
What are the key points of Convection?
Newton's law of cooling. Boundary layer thickness. Heat transfer coefficient is not a material property.
Can you give an example of Convection?
A 100 W chip with 0.01 m² and h=20 W/m²K rises 500 K without a heat sink.
What comes after Convection?
The next topic is "Natural and forced convection", which builds directly on this one.
How difficult is Convection?
It is classed as Advanced in Thermodynamics & Fluids (topic 86 of 105). It is an advanced topic. It assumes you are comfortable with the intermediate topics in Thermodynamics & Fluids.