<Li> Free or natural convection: when fluid motion is caused by buoyancy forces that result from the density variations due to variations of thermal ± temperature in the fluid . In the absence of an internal source, when the fluid is in contact with a hot surface, its molecules separate and scatter, causing the fluid to be less dense . As a consequence, the fluid is displaced while the cooler fluid gets denser and the fluid sinks . Thus, the hotter volume transfers heat towards the cooler volume of that fluid . Familiar examples are the upward flow of air due to a fire or hot object and the circulation of water in a pot that is heated from below . </Li> <Li> Forced convection: when a fluid is forced to flow over the surface by an internal source such as fans, by stirring, and pumps, creating an artificially induced convection current . </Li> <P> In many real - life applications (e.g. heat losses at solar central receivers or cooling of photovoltaic panels), natural and forced convection occur at the same time (mixed convection). </P> <P> Internal and external flow can also classify convection . Internal flow occurs when a fluid is enclosed by a solid boundary such when flowing through a pipe . An external flow occurs when a fluid extends indefinitely without encountering a solid surface . Both of these types of convection, either natural or forced, can be internal or external because they are independent of each other . The bulk temperature, or the average fluid temperature, is a convenient reference point for evaluating properties related to convective heat transfer, particularly in applications related to flow in pipes and ducts . </P>

What are three natural processes that depend on convection