Chapter 5: Convection


5.1 Overview of Convection

Heat transfer using movement of fluids is called convection. In natural convection, the flow is induced by the differences between fluid densities which result due to temperature changes. Forced convection uses externally induced flow, such as wind.

The heat transfer rate for convection is given by the following equation:

 
(Eq. 5.1)

where h is the convection coefficient, A is the surface area, and Tsurface and T are the surface and ambient temperatures, respectively. The convection coefficient is a measure of how effective a fluid is at carrying heat to and away from the surface. It is dependent on factors such as the fluid density, velocity, and viscosity. Generally, fluids with higher velocity and/or higher density have greater h.

5.2 Natural Convection

Density of fluid changes with temperature. In general, fluids expand as the temperature rises, and thus the density decreases (density is the mass per unit volume). Warm fluids therefore are more buoyant than cooler fluids. A hot object will heat the surrounding fluid, which rises due to the buoyancy force. The warm fluid is then replaced by cool (unheated) fluids. Similarly, cool objects will draw heat away from the surrounding fluid, which then fall due to the increased density. The cool fluid is then replaced by the warm fluid, initiating convective currents.

For a hot horizontal plate surrounded by air, convection currents form when the air above the plate start to rise, as shown in Figure 5.1. The air below the plate, however, cannot rise because the plate is blocking the flow. The heated fluid under the plate will eventually escape through the sides of the plate; however, the convective flow below the plate is very small compared to the flow on top. In general, natural convection is more pronounced (has a higher h) on the topside of a hot plate or the bottom side of a cold plate.


Figure 5.1 Natural convection around a horizontal hot plate

The convection coefficient for natural convection in gas is generally between 1 W/m2K and 20 W/m2K; typical values for liquids fall between 100 W/m2K and 1000 W/m2K.

5.3 Forced Convection

Fluid flow caused by a fan or any other external forces create forced convection. Forced convection is generally more efficient than natural convection due to the faster velocity of the currents. In forced convection, buoyancy has little effect on the direction of flow.

The relation between flow velocity, direction, and temperature is illustrated in Figure 5.2 for a hot, horizontal surface under forced convection. Since the buoyancy does not affect the flow, the bottom side of the plate will have the same patterns. Close to the surface, the flow velocity is inevitably slowed down due to friction. Right at the surface, the velocity is actually zero. This region of retarded flow is called the boundary layer. The region of warm airflow is generally well within the velocity boundary layer, and is called the thermal boundary layer.
Figure 5.2 Forced convection currents

The convection coefficient for forced convection in gasses generally range between 50 W/m2K and 250 W/m2K. For liquids, values start around 100 W/m2K, and can be as high as 10,000 W/m2K.

5.4 Convection in Ovens

Conventional ovens use natural convection to heat foods while baking. Ovens typically contain two heating elements, on top and bottom of the oven. During baking, the bottom element heats up, which heats the air inside the oven. The hot air rises and creates a current, which helps to distribute heat throughout the oven. Natural convection currents are easily blocked by large pans, creating non-uniform temperatures within the oven.

Convection ovens improve temperature distribution by using a fan, located inside the oven, to create forced convection currents. The forced convection currents efficiently mix the air inside an oven, creating uniform temperatures even in the presence of large pans. Furthermore, the increased airflow results in a higher convection coefficient, which reduces cooking time.

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