6.2 Convection
6.2.1 General Principles
Convection involves heat transfer caused by a moving fluid in contact with a warm
surface, which then warms further, becomes less dense, and rises by buoyancy.
Such a process involving the movement of different density fluids is referred to as
free or natural convection. Heat transfer by forced convection occurs when the fluid
movement is mechanical, e.g., under atmospheric wind or mechanical ventilation.
This happens, for example, when a heated metal plate cools more rapidly when
placed in front of a fan than when exposed to still air. In both cases, heat transmission between the solid surface and the fluid takes place by instantaneous contact
between the surface molecules and those in the adjacent fluid. If convective heat
through a fluid causes a change in temperature, sensible heat is transferred. Sensible
heat is transported, for example, from a hot to a cooler surface, by turbulent eddies
and released into the surrounding atmosphere after mixing. Latent heat is transferred if, in the absence of a temperature change, convection causes a change in the
state of a substance (e.g., liquid to vapor), Latent heat can be released into the air
from humid air mass which condenses with the formation of clouds.
Heat convection in environmental systems occurs with the displacement of the air
in the active surface close to the ground. This can be soil or another solid surface,
e.g., a layer of vegetation where solar radiation is absorbed. The active surface is the
atmospheric layer where the main radiation exchanges occur. It is usually warmer
than the ambient air during the daytime with positive radiation balance and colder
than the air during the night-time with negative radiation balance.
It is well-known that fluids provide resistance to flow over a flat surface due to
drag resistance to movement between fluid layers. This resistance is stronger in
lubricating oils than in water. Viscosity (Chap. 2 and Annex II) is used to quantify
flow resistance between the various layers and between the fluid and contact surface. Although less apparent in gases, this property is common to both liquids and
gases. Viscosity is the motion quantity that corresponds to flow velocity (Holman
1983). In laminar flow, fluid molecules move from one layer to another, transporting motion corresponding to the velocity flow. Fluid motions are transported
from high to lower velocity zones, generating shear stress in the flow direction.
The dynamic or absolute viscosity l, in Nsm
−2 units, is defined as the proportionality constant between the vertical gradient of horizontal velocity and the
induced shear stress. The kinematic viscosity, m, is defined as
m ¼ l=q
ð6:15Þ
with m
2 /s units, where q is the fluid density.
When a fluid flows over on a flat surface at temperature T f , and velocity v f , the
particles in contact with the surface remain fixed due to viscosity and retard the
movement of adjacent fluid layers located at higher levels (Chap. 2 and Annex II).
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6 Heat and Mass Transfer Processes
6.2.1 General Principles
Convection involves heat transfer caused by a moving fluid in contact with a warm
surface, which then warms further, becomes less dense, and rises by buoyancy.
Such a process involving the movement of different density fluids is referred to as
free or natural convection. Heat transfer by forced convection occurs when the fluid
movement is mechanical, e.g., under atmospheric wind or mechanical ventilation.
This happens, for example, when a heated metal plate cools more rapidly when
placed in front of a fan than when exposed to still air. In both cases, heat transmission between the solid surface and the fluid takes place by instantaneous contact
between the surface molecules and those in the adjacent fluid. If convective heat
through a fluid causes a change in temperature, sensible heat is transferred. Sensible
heat is transported, for example, from a hot to a cooler surface, by turbulent eddies
and released into the surrounding atmosphere after mixing. Latent heat is transferred if, in the absence of a temperature change, convection causes a change in the
state of a substance (e.g., liquid to vapor), Latent heat can be released into the air
from humid air mass which condenses with the formation of clouds.
Heat convection in environmental systems occurs with the displacement of the air
in the active surface close to the ground. This can be soil or another solid surface,
e.g., a layer of vegetation where solar radiation is absorbed. The active surface is the
atmospheric layer where the main radiation exchanges occur. It is usually warmer
than the ambient air during the daytime with positive radiation balance and colder
than the air during the night-time with negative radiation balance.
It is well-known that fluids provide resistance to flow over a flat surface due to
drag resistance to movement between fluid layers. This resistance is stronger in
lubricating oils than in water. Viscosity (Chap. 2 and Annex II) is used to quantify
flow resistance between the various layers and between the fluid and contact surface. Although less apparent in gases, this property is common to both liquids and
gases. Viscosity is the motion quantity that corresponds to flow velocity (Holman
1983). In laminar flow, fluid molecules move from one layer to another, transporting motion corresponding to the velocity flow. Fluid motions are transported
from high to lower velocity zones, generating shear stress in the flow direction.
The dynamic or absolute viscosity l, in Nsm
−2 units, is defined as the proportionality constant between the vertical gradient of horizontal velocity and the
induced shear stress. The kinematic viscosity, m, is defined as
m ¼ l=q
ð6:15Þ
with m
2 /s units, where q is the fluid density.
When a fluid flows over on a flat surface at temperature T f , and velocity v f , the
particles in contact with the surface remain fixed due to viscosity and retard the
movement of adjacent fluid layers located at higher levels (Chap. 2 and Annex II).
168
6 Heat and Mass Transfer Processes