Heat Transfer Mechanisms: Conduction, Convection, and Radiation
Heat Transfer Mechanisms
Heat Conduction
Heat conduction is a mechanism of energy transfer between two systems linked through the exchange of kinetic energy between their constituent particles, such as molecules, atoms, ions, or free electrons, produced by direct contact. Therefore, heat conduction requires a material medium for transfer. In fluids, heat transfer is closely related to fluid movement. This movement significantly influences the heat transfer process and combines with other mechanisms, including direct contact between solids (conduction) and the effect of radiant energy (radiation).
Fluid Flow and Heat Transfer
When a fluid stream moves relative to a solid wall, the fluid in direct contact with the wall adheres to it, resulting in zero relative velocity. As the fluid moves away from the wall, its velocity increases, following a parabolic profile. If the fluid velocity is sufficiently low, the flow remains smooth and without cross-mixing. This type of motion is called laminar flow. However, if the fluid velocity is high, vortices form within the fluid, leading to strong cross-mixing known as turbulent flow.
Regardless of whether the flow is laminar or turbulent, the fluid velocity near the wall is always lower than the undisturbed fluid velocity due to friction. This region of reduced velocity is called the boundary layer. The fluid’s viscosity acts to resist motion, promoting laminar flow. In contrast, inertial forces acting on the fluid tend to cause turbulence. The transition between laminar and turbulent flow is characterized by the Reynolds number, which represents the ratio of inertial forces to viscous forces.
If the temperature of the wall surface differs from that of the fluid, heat transfer occurs. The direction of heat transfer depends on the temperature difference: heat flows from the hotter to the colder medium. The fluid region near the surface where heat transfer primarily occurs is called the thermal boundary layer. The mechanism of heat transfer through the fluid depends on the type of flow. In laminar flow, heat transfer in the transverse direction occurs mainly through conduction. In turbulent flow, transverse mixing significantly enhances heat transfer.
Heat Convection
Convection is the heat transfer mechanism that occurs within a fluid due to the mass movement of the fluid itself. It involves the simultaneous transport of mass and energy and requires a fluid medium. Fluid motion can be induced by external forces, such as a fan, which is termed forced convection. Alternatively, fluid motion can arise from density differences created by temperature gradients within the fluid, known as natural convection.
The influence of various factors on convection between a solid surface and the surrounding fluid is summarized in an empirical coefficient called the film coefficient, heat transfer coefficient, or surface convection coefficient, represented by ‘h’.
Heat Radiation
Thermal radiation is a process of thermal energy transfer from a system using electromagnetic waves. It originates from the thermal motion of charged particles (electrons or ions) within matter. Thermal radiation can propagate even in a vacuum.
When thermal radiation interacts with a substance, a portion of the energy is absorbed, while the rest is reflected or transmitted. When a body reaches thermal equilibrium with its surroundings, the rate of energy absorption equals the rate at which it emits its own internal energy. A black body is an idealized object that absorbs all incident radiation. The amount of radiation energy emitted per unit time by a black body is given by the Stefan-Boltzmann Law.
Combined Heat Transfer
In practical situations, all three heat transfer modes (conduction, convection, and radiation) occur simultaneously and are generally inseparable. For instance, heat transfer from a warm room to a cold environment involves:
- Convection and radiation from the warm air to the interior wall surface.
- Conduction through the wall’s thickness.
- Convection and radiation from the exterior wall surface to the cold environment.
Pipe Insulation
.- depends on the shape and type of rigid pipe aislamientosecciones (shells) . Supplied in lengths of about 90 m. Circumferential seams of the pieces of insulation shall be staggered henever possible. Longitudinal joints shall be placed up and down alternately.