HEAT AND MASS TRANSFER IN TURBULENT DRYING PROCESSES — EXPERIMENTAL AND THEORETICAL WORK

Abstract

ABSTRACT The main objective in this work is to study and deduce a governing equation for net mass transfer in moist air and turbulent flow. Development of simple and reliable steady state models for turbulent moist air-drying has been considered to be quite well covered in literature. However, the lack of necessary background information concerning classical drying models is now being rectified through research carried out with new approaches, which are initiated by advancement in laboratory equipment. The known and trusted models are combined with coupled momentum, heat and mass transfer equations creating a reliable governing mass transfer equation for use in turbulent moist air drying processes, i.e. the advanced drying model (ADM). The ADM is a relatively user friendly and robust model, and it is well-suited for identifying transfer coefficients from boundary layer measurements, for example in modem high intensity paper drying machines. The advanced drying model is analysed and verified with the specially designed experimental apparatus described in this article. The deduced mass transfer equation is then presented and experimentally verified to clarify why the use of Stefan's diffusion equation should be avoided when calculating high drying intensities in turbulent flow. Finally, when applied to a wide drying range, the classical drying models require parameters which have been experimentally verified. Therefore, a comprehensive knowledge of governing mass transfer mechanisms will also reduce the large number of necessary drying experiments. The advanced drying model, which includes variable physical properties and transport coefficients, allows the simulation of many geometrical shapes and drying configurations and therefore provides a tool for optimising drying processes in a new manner.