Regimes of heat transfer (conditions under which an industrial process takes place):
A fluid being heated or cooled may be flowing in laminar flow, in turbulent flow, or in the transition range between laminar and turbulent flow. Also, may be flowing in forced or natural convection. In some instances more than one flow type may occur in the same stream; for instance, in laminar flow at low velocities, natural convection may be superimposed on forced laminar flow.
The direction of flow of the fluid may be parallel to that of the heating surface, so that boundary layer separation does not occur; or the direction of flow maybe perpendicular or at an angle to the heating surface, and then boundary layer separation often occurs.At ordinary velocities the heat generated from fluid friction is usually negligible in comparison with the heat transferred between the fluids. In most cases friction heating may be neglected. It may be important in operations involving very viscous fluids such as the injection molding of polymers.
Because the conditions of flow at the entrance to a tube differ from those well downstream from the entrance, the velocity field and the associated temperature field may depend on the distance from the tube entrance. Also, in some situations the fluid flows through a preliminary length of unheated or uncooled pipe, so that the fully developed velocity field is established before heat is transferred to the fluid, and the temperature field is created within an existing velocity field.
Finally, the properties of the fluid—-viscosity, thermal conductivity, specific heat, and density-—are important parameters in heat transfer. Each of these, especially viscosity, is temperature-dependent. Since the temperature varies from point to point in allowing stream undergoing heat transfer, a problem appears in the choice of temperature at which the properties should be evaluated.
Because of the various effects noted above, the entire subject of heat transfer to fluids without phase change is complex arid in practice is treated as a series of special cases rather than as a general theory. All cases considered in this chapter do, however, have a phenomenon in common: In all the formation of a thermal boundary layer, analogous to the hydrodynamic boundary layer. It profoundly influences the temperature field and so controls the rate of heat flow.
HEAT TRANSFER TO FLUIDS WITHOUT PHASE CHANGE