Red hues designate hot areas, while regions with blue hues are cold. A hot, less-dense lower boundary layer sends plumes of hot material upwards, and likewise, cold material from natural convection heat transfer pdf top moves downwards. Convection is usually the dominant form of heat transfer in liquids and gases.
Thermal expansion of fluids may also force convection. In other cases, natural buoyancy forces alone are entirely responsible for fluid motion when the fluid is heated, and this process is called “natural convection”. An example is the draft in a chimney or around any fire. After heating has stopped, mixing and conduction from this natural convection eventually result in a nearly homogeneous density, and even temperature. The convection heat transfer mode comprises one mechanism. This motion is associated with the fact that, at any instant, large numbers of molecules are moving collectively or as aggregates. Such motion, in the presence of a temperature gradient, contributes to heat transfer.
It is customary to use the term convection when referring to this cumulative transport and the term advection when referring to the transport due to bulk fluid motion. 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. 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. Further classification can be made depending on the smoothness and undulations of the solid surfaces. Not all surfaces are smooth, though a bulk of the available information deals with smooth surfaces.
Wavy irregular surfaces are commonly encountered in heat transfer devices which include solar collectors, regenerative heat exchangers and underground energy storage systems. They have a significant role to play in the heat transfer processes in these applications. Since they bring in an added complexity due to the undulations in the surfaces, they need to be tackled with mathematical finesse through elegant simplification techniques. Also they do affect the flow and heat transfer characteristics, thereby behaving differently from straight smooth surfaces. For a visual experience of natural convection, a glass filled with hot water and some red food dye may be placed inside a fish tank with cold, clear water.
The convection currents of the red liquid may be seen to rise and fall in different regions, then eventually settle, illustrating the process as heat gradients are dissipated. Convection-cooling is sometimes loosely assumed to be described by Newton’s law of cooling. The law applies when the coefficient is independent, or relatively independent, of the temperature difference between object and environment. In classical natural convective heat transfer, the heat transfer coefficient is dependent on the temperature. However, Newton’s law does approximate reality when the temperature changes are relatively small.
The convective heat transfer coefficient is dependent upon the physical properties of the fluid and the physical situation. Based on a work by Newton published anonymously as “Scala graduum Caloris. This page was last edited on 10 January 2018, at 04:00. Colors closer to red are hot areas and colors closer to blue are cold areas. Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place.