Definition of Convection
- It is the process of heat transfer within a fluid by mixing of one portion of fluid with another.
- Convection is possible only in a fluid medium and is directly linked with the transport of medium itself.
- 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 = Rate of Convective heat transfer
A = Area exposed to heat transfer
h = Coefficient of Convective heat transfer (W/m2 °C)
ts = Surface Temperature
tf = 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.
TYPES OF CONVECTION
Free 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 buoyancy force.
- Nu = f(Gr, Pr)
- Nusselt’s Number (Nu) = (convective heat transfer/conductive heat transfer)
- When fluid is circulated with the help of an external agent e.g. Pumping etc.
- Nu = f(Re, Pr)
- It is used only in 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
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.
- The thermal boundary layer is best described by Prandtl Number.
Pr = molecular diffusivity of momentum / molecular diffusivity of heat
Ratio of kinematic viscosity to thermal diffusivity
Pr = (µcp)/K
K = thermal conductivity
cp = 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.
- It is the ratio of inertia force to viscous force.
Re = ρVL/µ
ρ = fluid density
V= velocity of 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.