What Is Convection?

 

[caption id="attachment_210" align="alignright" width="300" caption="convection heat transfer"][/caption]

What Is Convection:

Convection is the mechanism of heat transfer occurs as a result of movement of fluid on a macroscopic scale. I.e. heat transfer due to the mixing of elements in fluid or the heat transferred from a solid surface to the moving fluid.

There are several factors, on which heat transfer by convection depends on, such as fluid thermal conductivity, fluid density, fluid velocity, solid surface roughness, temperature difference between fluid and solid surface, moving fluid turbulence, etc. however, as a general rule, it has been experimentally proven that the higher the fluid velocity, the higher is the convective heat transfer coefficient [some times called as film conductance, because of its relation to the conduction process].

Difference Between Conduction And Convection:

It generally doesn’t make sense trying to differentiate between the conduction and convection; as it is the same energy, which is transferred by the combined action of conductivity and the movement of the fluid. Initially, the energy is delivered from solid to the fluid at the solid-fluid interface by conduction then the fluid stream absorbs and transfers energy as convection.

Classification Of Convective Heat Transfer Coefficient:

Convective heat transfer is classified as:

• Forced convection

In forced convection, the fluid is forced to flow by external means, such as fans, stirrers, etc. generally, the magnitude or rate of heat transfer in force convection is greater then that of natural convection. In this mode of heat transfer, the heat transfer coefficient, h, mainly depends on the fluid velocity.

• Free convection

Free convection is also called as natural convection, i.e. fluid flows naturally because of the gravitational and buoyancy forces.

Newton’s Cooling Law For Heat Convection:

Newton’s law of cooling is considered as the basic law for convection; which is stated as:

“The heat transfer per unit area by convection is directly proportional to the temperature difference between solid and fluid which, using proportionality constant called the heat transfer coefficient, i.e.

\[Q=hA(T_{fluid}-T_{solid})\]

Where,

h = Convective heat transfer coefficient; W/m².^oC

 

Dimensionless Numbers Used For Convection Heat Transfer Analysis:

• Reynolds Number

Reynolds number is related to the flow of fluids; specially the transition of flow from laminar flow to turbulent flow conditions. This dimensionless number is used to describe whether the flow is laminar or turbulent; hence this is the main step for the convection heat transfer analysis.

\[Re=\ \frac{\rho DV}{\mu }\]

Where,

ρ = density of fluid

V = average fluid velocity

D = tube diameter [internal]

µ = dynamic viscosity of fluid

• Nusselt Number:

This is actually the empirical correlation of the tube size along with the flow conditions.

\[Nu=\ \frac{hL}{k_f}\]

Where,

h = connective heat transfer coefficient.

L = characteristic length of the tube

k_f = thermal conductivity of fluid

• Prandtl Number

It is the ratio of the kinematic viscosity (υ) to the thermal diffusivity (α). It represents the thermo-physical property of fluid, and is independent of flow conditions.

\[Pr=\frac{\upsilon }{\alpha }=\frac{{cp}_{\upsilon }}{k_f}\]

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Reference books:

• Kirk Othmar, “ Encyclopedia Of Chemical Technology”, vol. 12, 4^th ed. , “Heat Exchange Technology”.


• J.P. Holman, “Heat Transfer”, 10^th edition.


• Eduardo Cao, “Heat transfer In Process Engineering”, chap. 4

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