Within uids, the heat transfer takes place through a combination of molecular conduction and energy transportation created by the motion of uid particles. This mode of heat transfer is known as convection. The heat exchange rate in uids by convection is much higher than the heat exchange rate in solids through conduction. This difference becomes more prominent in geothermics because rocks have very low-thermal conductivities compared to metals and other solids. Convection processes inside the Earth can be of two broad types: free and forced.
Free or natural convection refers to the free motion of a uid and is solely due to differences in the densities of the heated and cold particles of a uid. The origin and intensity of free convection are solely determined by the thermal conditions of the process and depend on the kind of uid, temperature, potential and volume of the space in which the process takes place. Forced convection occurs under the in uence of some external force. Flow of water in hot springs and heat transport due to volcanic eruptions are examples of forced convection (advection). Forced convection depends on the physical properties of the uid, its temperature, ow velocity, shape
and size of the passage in which forced convection of uid occurs. Genera lly speaking, forced convection may be accompanied by free convection, and the relative in uence of the latter increases with the difference in the temperatures of individual particles of the uid and decreases with the velocity of the forced ow. The in uence of na tural convection is negligible at high- ow velocity.
In problems dealing with the transmission of heat through the process of convection, the uid under consideration is usually bounded on one or more sides by a solid. Let at any given time, Ts be the temperature of the solid at its boundary with the uid and TN the uid temperature at a far-off yet unspeci ed point. In accordance with Newton’s law of cooling, the amount of heat owing would be proportional to the temperature difference and could be expressed as
q ¼ hðTs T1 Þ
where h is the heat transfer coef cient. The heat is transferred by convection and consequently the heat transfer coef cient depends, in general, upon the thermal boundary condition at the solid– uid boundary. However, under many situations, hcan be estimated satisfactorily when the uid dynamics of the ow system is known.
Free or natural convection refers to the free motion of a uid and is solely due to differences in the densities of the heated and cold particles of a uid. The origin and intensity of free convection are solely determined by the thermal conditions of the process and depend on the kind of uid, temperature, potential and volume of the space in which the process takes place. Forced convection occurs under the in uence of some external force. Flow of water in hot springs and heat transport due to volcanic eruptions are examples of forced convection (advection). Forced convection depends on the physical properties of the uid, its temperature, ow velocity, shape
and size of the passage in which forced convection of uid occurs. Genera lly speaking, forced convection may be accompanied by free convection, and the relative in uence of the latter increases with the difference in the temperatures of individual particles of the uid and decreases with the velocity of the forced ow. The in uence of na tural convection is negligible at high- ow velocity.
In problems dealing with the transmission of heat through the process of convection, the uid under consideration is usually bounded on one or more sides by a solid. Let at any given time, Ts be the temperature of the solid at its boundary with the uid and TN the uid temperature at a far-off yet unspeci ed point. In accordance with Newton’s law of cooling, the amount of heat owing would be proportional to the temperature difference and could be expressed as
q ¼ hðTs T1 Þ
where h is the heat transfer coef cient. The heat is transferred by convection and consequently the heat transfer coef cient depends, in general, upon the thermal boundary condition at the solid– uid boundary. However, under many situations, hcan be estimated satisfactorily when the uid dynamics of the ow system is known.
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