Convective Heat Transfer

Heat energy transferred between a surface and a moving fluid with different temperatures - is known as convection.

In reality this is a combination of diffusion and bulk motion of molecules. Near the surface the fluid velocity is low, and diffusion dominates. At distance from the surface, bulk motion increases the influence and dominates.

Convective heat transfer can be

• forced or assisted convection
• natural or free convection

• Conductive Heat Transfer

Forced or Assisted Convection

Forced convection occurs when a fluid flow is induced by an external force, such as a pump, fan or a mixer.

Natural or Free Convection

Natural convection is caused by buoyancy forces due to dens ity differences caused by temperature variations in the fluid. At heating the density change in the boundary layer will cause the fluid to rise and be replaced by cooler fluid that also will heat and rise. This continues phenomena is called free or natural convection.

Boiling or condensing processes are also referred to as a convective heat transfer processes.

• The heat transfer per unit surface through convection was first described by Newton and the relation is known as the Newton's Law of Cooling.

The equation for convection can be expressed as:

q = h_c A dT                                         (1)

where

q = heat transferred per unit time (W, Btu/hr)

A = heat transfer area of the surface (m², ft²)

h_c = convective heat transfer coefficient of the process (W/(m^2oC, Btu/(ft² h ^oF))

dT = temperature difference between the surface and the bulk fluid (^oC, F)

Heat Transfer Coefficients - Units

• 1 W/(m²K) = 0.85984 kcal/(h m^{2 o}C) = 0.1761 Btu/(ft² h ^oF)
• 1 Btu/(ft² h ^oF) = 5.678 W/(m² K) = 4.882 kcal/(h m^{2 o}C)
• 1 kcal/(h m^{2 o}C) = 1.163 W/(m²K) = 0.205 Btu/(ft² h ^oF)

• Overall Heat Transfer Coefficients

Convective Heat Transfer Coefficients

Convective heat transfer coefficients - h_c - depends on type of media, if its gas or liquid, and flow properties such as velocity, viscosity and other flow and temperature dependent properties.

Typical convective heat transfer coefficients for some common fluid flow applications:

• Free Convection - air, gases and dry vapors : 0.5 - 1000 (W/(m²K))
• Free Convection - water and liquids: 50 - 3000 (W/(m²K))
• Forced Convection - air, gases and dry vapors:  10 - 1000 (W/(m²K))
• Forced Convection - water and liquids:  50 - 10000 (W/(m²K))
• Forced Convection - liquid metals:  5000 - 40000 (W/(m²K))
• Boiling Water : 3.000 - 100.000 (W/(m²K))
• Condensing Water Vapor: 5.000 - 100.000 (W/(m²K))

• Heat Exchanger Heat Transfer Coefficients 

Convective Heat Transfer Coefficient for Air

The convective heat transfer coefficient for air flow can be approximated to 

h_c = 10.45 - v + 10 v^1/2                                     (2)

where

h_c = heat transfer coefficient (kCal/m²h°C)

v = relative speed between object surface and air (m/s)

Since

1 kcal/m²h°C = 1.16 W/m²°C

- (2) can be modified to

h_cW = 12.12 - 1.16 v + 11.6 v^1/2                                     (2b)

where

h_cW = heat transfer coefficient (W/m²°C)

Note! - this is an empirical equation and can be used for velocities 2 to 20 m/s.

• Convective Air Flow from a single Heat Source

Example - Convective Heat Transfer

A fluid flows over a plane surface 1 m by 1 m. The surface temperature is 50^oC, the fluid temperature is 20^oC and the convective heat transfer coefficient is 2000 W/m^2oC. The convective heat transfer between the hotter surface and the colder air can be calculated as

q = (2000 W/(m^2oC)) ((1 m) (1 m)) ((50 ^oC) - (20 ^oC))

    = 60000 (W)

    = 60 (kW)

Convective Heat Transfer Calculator

Convective Heat Transfer Chart

• Convective Heat Transfer Chart (pdf)