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.
In thermal conduction, energy is transferred as heat either due to the migration of free electrons or lattice vibrational waves (phonons). There is no mass movement in the direction of energy flow, and heat transfer by conduction depends on 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.
At the surface, it must be emphasized that energy flow occurs purely by conduction, even in conduction. 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 occur at the molecular or macroscopic levels. Due to the mass movement, the rate of energy transfer is higher. The higher the mass movement rate, the thinner the stagnant fluid film layer will be, and the higher the heat flow rate.
It must be noted nucleate boiling at the surface effectively disrupts this stagnant layer. Therefore, nucleate boiling significantly increases the ability of a surface to transfer thermal energy to the bulk fluid.
As was written, heat transfer through a fluid is by convection in the presence of mass movement and conduction in its absence. 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 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.
Heat transfer by convection is more difficult to analyze than heat transfer by conduction because no single property of the heat transfer medium, such as thermal conductivity, can be defined to describe the mechanism. Convective heat transfer is complicated by the fact that it involves fluid motion as well as heat conduction. Heat transfer by convection varies from situation to situation (upon the fluid flow conditions), and it is frequently coupled with the mode of fluid flow. In forced convection, the heat transfer rate through a fluid is much higher by convection than by conduction.
In practice, analysis of heat transfer by convection is treated empirically (by direct experimental observation). Most 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. They 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, and the type of fluid flow. All these conditions affect especially the stagnant film thickness.
Convection involves heat transfer 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.