Natural convection heating process

The dependence of natural convection heat transfer on the aforementioned parameters can be established based on the physics of the process. Let us assume that a vertical wall is in contact with a fluid. The wall temperature Tw is higher than the fluid temperature T. When a unit volume of fluid contacts the hot wall, the fluid receives energy from the wall due to molecular collisions. The fluid molecules begin to move with a higher velocity. The initial fluid volume expands. From this description one can conclude that energy transfer should depend on the parameters T, Tw, P, and cp. 

Natural convection heat transfer occurs when a solid surface is in contact with a gas or liquid which is at a different temperature from the surface. Density differences in the ffuid arising from the heating process provide the buoyancy force required to move the ffuid. Free or natural convection is observed as a result of the motion of the fluid. An example of heat transfer by natural convection is a hot radiator used for heating a room. Cold air encountering the radiator is heated and rises in natural convection because of buoyancy forces. The theoretical derivation of equations for natural convection heat-transfer coefficients requires the solution of motion and energy equations. 

We now know that the cooling of the baffles by the rising cold vapour takes place via an efficient heat transfer process, because the natural convective heat transfer for the vapour is greatly enhanced (perhaps by as much as tenfold or more) in the vertical temperature gradient above the liquid. 

To get to the surface, the heat in-flow is first absorbed by a process of natural convective heat transfer creating an upward flow of less dense superheated liquid there is no boiling and also no evaporation at the point where the heat is absorbed. At a vertical wall, the flow of superheated liquid assumes the form of a boundary layer inomediately adjacent to the wall, in a layer about 1-5 mm thick. Heat transfer from the heated wall to the liquid by such a boundary layer flow is very effective and is well documented in many texts on heat transfer [6,7]. 

Convection. Heat transfer by convection arises from the mixing of elements of fluid. If this mixing occurs as a result of density differences as, for example, when a pool of liquid is heated from below, the process is known as natural convection. If the mixing results from eddy movement in the fluid, for example when a fluid flows through a pipe heated on the outside, it is called forced convection. It is important to note that convection requires mixing of fluid elements, and is not governed by temperature difference alone as is the case in conduction and radiation. 

Heat transfer by convection occurs as a result of the movement of fluid on a macroscopic scale in the form of eddies or circulating currents. If the currents arise from the heat transfer process itself, natural convection occurs, such as in the heating of a vessel containing liquid by means of a heat source situated beneath it. The liquid at the bottom of the vessel becomes heated and expands and rises because its density has become less than that of the remaining liquid. Cold liquid of higher density takes its place and a circulating current is thus set up. 

In conduction, heat is conducted by the transfer of energy of motion between adjacent molecules in a liquid, gas, or solid. In a gas, atoms transfer energy to one another through molecular collisions. In metallic solids, the process of energy transfer via free electrons is also important. In convection, heat is transferred by bulk transport and mixing of macroscopic fluid elements. Recall that there can be forced convection, where the fluid is forced to flow via mechanical means, or natural (free) convection, where density differences cause fluid elements to flow. Since convection is found only in fluids, we will deal with it on only a limited basis. Radiation differs from conduction and convection in that no medium is needed for its propagation. As a result, the form of Eq. (4.1) is inappropriate for describing radiative heat transfer. Radiation is... 

The exptl values of the critical temp lie between the values calcd for purely conductive purely convective heat transfer, indicating the complex nature of the transfer in this case. It was concluded that the effective heat transfer coefficient increases during the decomp process due to the stirring of liq phase by the gaseous products which are involved... 

The transfer of heat by convection is also an important component of the indirect cooling of the process. Natural convection currents result from localized heating/cooling effects and the tendency of hot fluids to rise above colder fluids, while forced convection, utilizing a pump, enables higher rates of heat transfer to occur (within the limits of the heat-exchanger design). 

A deeper insight into the physical nature of the process. By representing experimental data in a dimensionless form, physical states (e.g. turbulent or laminar flow range, suspension state, heat transfer by natural or by forced convection, and so on) can be delimited from each other and the limits quantified. In this manner, the domain of individual physical quantities also becomes apparent. 

Conduction is treated from both the analytical and the numerical viewpoint, so that the reader is afforded the insight which is gained from analytical solutions as well as the important tools of numerical analysis which must often be used in practice. A similar procedure is followed in the presentation of convection heat transfer. An integral analysis of both free- and forced-convection boundary layers is used to present a physical picture of the convection process. From this physical description inferences may be drawn which naturally lead to the presentation of empirical and practical relations for calculating convection heat-transfer coefficients. Because it provides an easier instruction vehicle than other methods, the radiation-network method is used extensively in the introduction of analysis of radiation systems, while a more generalized formulation is given later. 

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