Forced convection heating process

The discussion and analyses of Chap. 5 have shown how forced-convection heat transfer may be calculated for several cases of practical interest the problems considered, however, were those which could be solved in an analytical fashion. In this way, the principles of the convection process and their relation to fluid dynamics were demonstrated, with primary emphasis being devoted to a clear understanding of physical mechanism. Regrettably, it is not always possible to obtain analytical solutions to convection problems, and the individual is forced to resort to experimental methods to obtain design information, as well as to secure the more elusive data which increase the physical understanding of the heat-transfer processes. 

Forced convection heat transfer is probably the most common mode in the process industries. Forced flows may be internal or external. This subsection briefly introduces correlations for estimating heat-transfer coefficients for flows in tubes and ducts flows across plates, cylinders, and spheres flows through tube banks and packed beds heat transfer to nonevaporating falling films and rotating surfaces. Section 11 introduces several types of heat exchangers, design procedures, overall heat-transfer coefficients, and mean temperature differences. 

The limit Pe 0 yields the pure conduction heat transfer case. However, for a fluid in motion, we find that the pure conduction limit is not a uniformly valid first approximation to the heat transfer process for Pe 1, but breaks down far from a heated or cooled body in a flow. We discuss this in the context of the Whitehead paradox for heat transfer from a sphere in a uniform flow and then show how the problem of forced convection heat transfer from a body in a flow can be understood in the context of a singular-perturbation analysis. This leads to an estimate for the first correction to the Nusselt number for small but finite Pe - this is the first small effect of convection on the correlation between Nu and Pe for a heated (or cooled) sphere in a uniform flow. 

D. G. Thomas, Enhancement of Forced Convection Heat Transfer Coefficient Using Detached Turbulence Promoters, Ind. Eng. Chem. Process Des. Dev. (6) 385-390,1967. 

The objective of this investigation was to determine the effects of vibration on heat 1 transfer and scaling mechanisms related to saline water conversion processes. During the initial phases of this study the effect of both vibration of the heat transfer surface and resonant acoustic vibrations in water on forced convection heat transfer was explored. Forced convection heat transfer was considerably more influenced by a vibrating heat transfer surface than by a standing acoustic wave in the flow medium. The major portion of this study was therefore concentrated on forced convection heat transfer from a vibrating heat transfer surface. 

The manufacture of large monofilaments, certainly larger than 100 denier, must be carried out in a water-quench process because of the inherent limitation of forced convective heat-transfer with gases. Conventional screw extruders are used for melting and conveying the polypropylene resin, just as in the case of the melt-spinning processes described above. Often, however, there are no metering pumps used in the process. 

The main mechanism of air-to-particle heat transfer during heating, cooUng, and drying processes of grains is forced convection. The forced convection heat transfer coefficient (surface conductance), h, is determined from the Nusselt number, Nu ... 

R. Rathnasamy, J. H. Arakeri, K. Srinivasan, Experimental investigation of forced convective heat transfer to air, liquids and liquid mixtures in a long narrow channel. Proceedings of the Institution of Mechanical Engineers Part E -Journal of Process Mechanical Engineering, 2005, 219, 311-317. 

The use of wind as a renewable energy source involves the conversion of power contained in moving air masses to rotating shaft power. These air masses represent the complex circulation of winds near the surface of Earth caused by Earth s rotation and by convective heating from the sun. The actual conversion process utilizes basic aerodynamic forces, ie, lift or drag, to produce a net positive torque on a rotating shaft, resulting in the production of mechanical power, which can then be used directly or converted to electrical power. 

In this section the correlations used to determine the heat and mass transfer rates are presented. The convection process may be either free or forced convection. In free convection fluid motion is created by buoyancy forces within the fluid. In most industrial processes, forced convection is necessary in order to achieve the most economic heat exchange. The heat transfer correlations for forced convection in external and internal flows are given in Tables 4.8 and 4.9, respectively, for different conditions and geometries. 

A hot-water heating system forces water into pipes, or arrangements of pipes called registers that warm from contact with warm water. Air in the room warms from contact with the pipes. Usually, the pipes are on the floor of a room so that warmer, less dense air around the pipes rises somewhat like a helium-filled balloon rises in air. The warmer air cools as it mixes with cooler air near the ceiling and falls as its density increases. This process is called convection and the moving air is referred to as convection current. The process of convection described here is pipe-to-air and usually does a better job of heating evenly than in an air-to-air convection system—the circulation of air by fans as in a forced-air heating system. 

When the fluid displaced is accelerated by wind or artificial means the process is called forced convection. With forced convection the rate of heat transfer is increased - substantially so in many cases. 

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. 

Forced-Convection Flow. Heat transfer in pol3rmer processing is often dominated by the uVT flow advectlon terms the "Peclet Number" Pe - pcUL/k can be on the order of 10 -10 due to the polymer s low thermal conductivity. However, the inclusion of the first-order advective term tends to cause instabilities in numerical simulations, and the reader is directed to Reference (7) for a valuable treatment of this subject. Our flow code uses a method known as "streamline upwindlng" to avoid these Instabilities, and this example is intended to illustrate the performance of this feature. 

---