Convection Heat Transfer
By
Dr. SUDARSHAN B
Asst. Prof., Mechanical Engg., Dept.
Research Scholar, IISc.
6/24/2025 Convection heat transfer 1
, Basic mechanism:
bulk fluid
motion
(advection)
Convection
includes
energy
random motion
transfer
of fluid molecules
(conduction or
diffusion)
convection heat transfer strongly depends on;
dynamic viscosity μ, thermal conductivity k,
density ρ, specific heat Cp,
fluid velocity V, geometry,
surface roughness, type of fluid flow etc…
where
h convection heat transfer coefficient, W/m2 °C
As heat transfer surface area, m2
Ts temperature of the surface, °C Convection is the most complex mechanism of heat transfer…??
T temperature of the fluid sufficiently far from the
surface, °C
6/24/2025 Convection 2
,The Convection Boundary Layers (Forced Convection):
The Velocity Boundary Layer
• The quantity is termed the boundary layer thickness, defined as the value of y for which u = 0.99u.
The boundary layer velocity profile refers to the manner in which u varies with y through the boundary layer.
• Fluid flow is characterized by two distinct regions:
a thin fluid layer (the boundary layer) in which velocity gradients and shear stresses are large
a region outside the boundary layer in which velocity gradients and shear stresses are negligible.
• With increasing distance from the leading edge, the effects of viscosity penetrate farther into the free stream
and the boundary layer grows ( increases with x).
no-slip condition
friction coefficient
6/24/2025 Convection 3
, The Thermal Boundary Layer
• The region of the fluid in which these temperature gradients exist is the thermal boundary layer, and its
thickness t is typically defined as the value of y for which the ratio [(Ts - T)/(Ts - T∞)] = 0.99.
• With increasing distance from the leading edge, the effects of heat transfer penetrate farther into the free
stream and the thermal boundary layer grows
no-temperature-jump condition
• at the surface, there is no fluidmotion and
energy transfer occurs only by conduction
• conditions in the thermal boundary layer, which
strongly influence the wall temperature gradient
(at y=0) determine the rate of heat transfer across
the boundary layer.
• Since (Ts - T∞) is a constant, independent of x,
while δt increases with increasing x, temperature
gradients in the boundary layer must decrease with
increasing x. Accordingly, the magnitude of (at
y=0) decreases with increasing x, and it follows
that and h decrease with increasing x.
6/24/2025 Convection 4
By
Dr. SUDARSHAN B
Asst. Prof., Mechanical Engg., Dept.
Research Scholar, IISc.
6/24/2025 Convection heat transfer 1
, Basic mechanism:
bulk fluid
motion
(advection)
Convection
includes
energy
random motion
transfer
of fluid molecules
(conduction or
diffusion)
convection heat transfer strongly depends on;
dynamic viscosity μ, thermal conductivity k,
density ρ, specific heat Cp,
fluid velocity V, geometry,
surface roughness, type of fluid flow etc…
where
h convection heat transfer coefficient, W/m2 °C
As heat transfer surface area, m2
Ts temperature of the surface, °C Convection is the most complex mechanism of heat transfer…??
T temperature of the fluid sufficiently far from the
surface, °C
6/24/2025 Convection 2
,The Convection Boundary Layers (Forced Convection):
The Velocity Boundary Layer
• The quantity is termed the boundary layer thickness, defined as the value of y for which u = 0.99u.
The boundary layer velocity profile refers to the manner in which u varies with y through the boundary layer.
• Fluid flow is characterized by two distinct regions:
a thin fluid layer (the boundary layer) in which velocity gradients and shear stresses are large
a region outside the boundary layer in which velocity gradients and shear stresses are negligible.
• With increasing distance from the leading edge, the effects of viscosity penetrate farther into the free stream
and the boundary layer grows ( increases with x).
no-slip condition
friction coefficient
6/24/2025 Convection 3
, The Thermal Boundary Layer
• The region of the fluid in which these temperature gradients exist is the thermal boundary layer, and its
thickness t is typically defined as the value of y for which the ratio [(Ts - T)/(Ts - T∞)] = 0.99.
• With increasing distance from the leading edge, the effects of heat transfer penetrate farther into the free
stream and the thermal boundary layer grows
no-temperature-jump condition
• at the surface, there is no fluidmotion and
energy transfer occurs only by conduction
• conditions in the thermal boundary layer, which
strongly influence the wall temperature gradient
(at y=0) determine the rate of heat transfer across
the boundary layer.
• Since (Ts - T∞) is a constant, independent of x,
while δt increases with increasing x, temperature
gradients in the boundary layer must decrease with
increasing x. Accordingly, the magnitude of (at
y=0) decreases with increasing x, and it follows
that and h decrease with increasing x.
6/24/2025 Convection 4
