Convection Heat Transfer Process

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Convection Heat Transfer Notes

Convection Definition

• It is the process of heat transfer within a fluid by mixing one portion of the fluid with another.
• It is possible only in a fluid medium and is directly linked with the transport of the medium itself.
• The mixing motion of the fluid tells us about the effectiveness of heat transfer by convection method.
• Newton’s Law of Cooling governs the convective heat transfer between a surface and an adjacent fluid.

Q= hA(t_s-t_f)

Q = Rate of Convective heat transfer

A = Area exposed to heat transfer

h = Coefficient of Convective heat transfer (W/m² °C)

t_s = Surface Temperature

t_f = Fluid Temperature

Factors on which ‘h’ depend

• A geometry of Surface.
• Nature of fluid flow.
• Thermodynamic and transport properties (e.g. viscosity, density, etc.).
• Prevailing thermal conditions.

Read More- Modes of Heat Transfer

Types of Convection

Free Convection or Natural Convection

• When fluid flows or circulates by virtue of the natural differences in densities of hot and cold fluids.
• The difference in density of the hot and cold fluids causes the fluid to flow in an upward direction.
• The force causing this flow is known as the buoyancy force.
• Nu = f(Gr, Pr)
• Nusselt’s Number (Nu) = (convective heat transfer/conductive heat transfer)

Forced Convection

• When fluid is circulated with the help of an external agent e.g. Pumping etc.
• Nu = f(Re, Pr)

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Dimensionless Numbers related to Convection Processes

Grashoff Number

• It is used only in the natural convection heat transfer process.
• It is the ratio of the product of inertia force and buoyancy force to the square of viscous force.

Gr = (Inertia Force × Buoyancy Force) / (Viscous Force)²

(ρ²βgΔθL³)/µ²

g = Acc. Due to gravity

β = Coefficient of Volume Expansion

Δθ = Temperature difference

ρ = density of the fluid

L = Characteristic Length of the geometry

µ = Dynamic or Absolute Viscosity

γ = Kinematic Viscosity of the fluid (ρ/ µ)

• Grashoff Number provides the main criteria in determining whether the fluid flow is laminar or turbulent in Natural Convection.
• Grashoff Number plays the same role in free convection as Reynolds Number in forced convection.
• For Vertical plates, critical Grashoff Number is 10⁹.

Prandtl Number

• The thermal boundary layer is best described by Prandtl Number.

Pr = molecular diffusivity of momentum / molecular diffusivity of heat

OR

The ratio of kinematic viscosity to thermal diffusivity

Pr = (µc_p)/K

K = thermal conductivity

c_p = Specific Heat

• The Prandtl Numbers of fluids range from less than 0.01 for liquid metals to more than 100000 for heavy oils.
• Heat diffuse quickly in liquid metals (Pr<<1) and very slowly in oils (Pr>>1) relative to momentum.

Reynolds Number

• It is the ratio of inertia force to viscous force.

Re = ρVL/µ

ρ = fluid density

V= velocity of a fluid

• Reynolds number tells the relative predominance of inertia force over the viscous force.
• It is an important criterion in forced convection heat transfer.
• A higher value of Re means a greater contribution of inertia force.

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