General information on convection

Convection:

Convection is heat transport with a flowing medium. This may be a gas or a liquid (both are fluids). A distinction is made between free convection in which the fluid rises due to density differences due to heating and forced convection in which the fluid flows as a result of a technical device.

Temperature differences are unavoidable in everyday life due to a wide variety of mechanisms. Starting with the heating of surfaces by sunlight through to physical activity which causes us to sweat, to the technical use of heat:
Convection occurs practically everywhere due to the atmosphere that surrounds us.

Convection describes the transfer of heat by moving fluid particles. In free convection, this motion is caused by the density differences resulting from warming. The hot fluid expands and flows upwards. New, cool fluid from the surroundings flows in. Free convection requires undisturbed inflow and outflow of the surrounding fluid.

The illustration on the left shows a candle. The heated air rises due to free convection. In this case the air is heated by the burning wax.

In forced convection there is incident flow of the heat-transferring surface. This can be facilitated by an external technical device, such as a fan. But it can also be exploited, for instance, by the head wind of vehicles. This results in an increased speed compared to free convection. The quicker transport away of the hot fluid causes a higher temperature gradient from the hot surface to the fluid, and thus a better heat transfer.

The image shows a vehicle engine. The hot cylinder is cooled by flowing water (A) (water pump not shown). The heated water is cooled by the ambient air in the cooler (B). As described above, the fan does not necessarily have to be running.

Regardless of the exact process, in convection there are common features:

The hot surface dissipates heat to the fluid. This occurs directly at the surface by conduction into the fluid. The higher the temperature difference, the higher the strive for equalisation.

The heat now flows in two directions:

Perpendicular to the surface into the fluid.

Parallel to the surface with the motion of the fluid.

If heat transfer is to be influenced, then these two heat fluxes must be influenced. This cannot be done separately.

Flow formation is directly linked to this topic. Both of the sub-sections below look at heat transport with the flow. The flow formation in pipe flow examined below is widely studied and forms the basis for understanding heat transport. The heat is carried along with the flow. The examination focuses on the directions of movement.