Chapter 6 - Heat transfer characteristics of non-Newtonian fluids in pipes

Abstract

In many chemical and processing applications, fluids need to be heated or cooled and a wide range of equipment may be utilized. The examples of the equipment include double pipe, shell, and tube heat exchangers; and stirred vessels fitted with cooling coils or jackets. Sometimes, heat is generated in the process, as in extrusion, which is extensively carded out in the polymer and food industry. It may also be necessary to reduce the rate at which heat is lost from a vessel or to ensure that heat is removed at a sufficient rate in equipments, such as screw conveyors. In most applications, it is the rate of heat transfer within the process equipment that is of principal interest. However, with thermally sensitive materials, the temperature profiles must be known and maximum permissible temperatures must not be exceeded. Because of their high consistencies, non-Newtonian materials are most frequently processed under conditions of laminar flow. Furthermore, shear stresses are generally so high that viscous generation of heat can rarely be neglected, and the temperature dependence of the rheological properties adds to the complexity of the mass, momentum, and energy balance equations. The most important thermo-physical properties of non-Newtonian fluids are thermal conductivity, density, specific heat, surface tension, and coefficient of thermal expansion. While the first three characteristics enter into virtually all heat transfer calculations, surface tension often exerts a strong influence on boiling heat transfer and bubble dynamics in non-Newtonian fluids. Much of the research activity in this area is related to the transfer of heat to inelastic non-Newtonian fluids in laminar flow, in circular and non-circular ducts.