Mechanism on Convection
Mechanism on Convection
In thermal conduction, energy is transferred as heat either due to the migration of free electrons or lattice vibrational waves (phonons). There is no movement of mass in the direction of energy flow. Heat transfer by conduction is dependent upon the driving “force” of temperature difference. Conduction and convection are similar in that both mechanisms require the presence of a material medium (in comparison to thermal radiation). On the other hand they are different in that convection requires the presence of fluid motion.
It must be emphasized, at the surface, energy flow occurs purely by conduction, even in conduction. It is due to the fact, there is always a thin stagnant fluid film layer on the heat transfer surface. But in the next layers both conduction and diffusion-mass movement in the molecular level or macroscopic level occurs. Due to the mass movement the rate of energy transfer is higher. Higher the rate of mass movement, thinner the stagnant fluid film layer will be and higher will be the heat flow rate.
It must be noted, nucleate boiling at the surface effectively disrupts this stagnant layer and therefore nucleate boiling significantly increases the ability of a surface to transfer thermal energy to bulk fluid.
As was written, heat transfer through a fluid is by convection in the presence of mass movement and by conduction in the absence of it. Therefore, thermal conduction in a fluid can be viewed as the limiting case of convection, corresponding to the case of quiescent fluid.
Convection as a Conduction with Fluid Motion
Some experts do not consider convection to be a fundamental mechanism of heat transfer since it is essentially heat conduction in the presence of fluid motion. They consider it to be a special case of thermal conduction, known as “conduction with fluid motion”. On the other hand, it is practical to recognize convection as a separate heat transfer mechanism despite the valid arguments to the contrary.
In practice, analysis of heat transfer by convection is treated empirically (by direct experimental observation). Most of problems can be solved using so called characteristic numbers (e.g. Nusselt number). Characteristic numbers are dimensionless numbers used to describe a character of heat transfer and can be used to compare a real situation (e.g. heat transfer in a pipe) with a small-scale model. Experience shows that convection heat transfer strongly depends on the fluid properties dynamic viscosity, thermal conductivity, density, and specific heat, as well as the fluid velocity. It also depends on the geometry and the roughness of the solid surface, in addition to the type of fluid flow. All these conditions affect especially the stagnant film thickness.
Convection involves the transfer of heat between a surface at a given temperature (Twall) and fluid at a bulk temperature (Tb). The exact definition of the bulk temperature (Tb) varies depending on the details of the situation.
- For flow adjacent to a hot or cold surface, Tb is the temperature of the fluid “far” from the surface.
- For boiling or condensation, Tb is the saturation temperature of the fluid.
- For flow in a pipe, Tb is the average temperature measured at a particular cross-section of the pipe.