(11b) Investigations on Heat Transfer in Dilute Two-Phase Flow during Pneumatic Conveying

Multi-phase flows are present in a large amount of technical unit operations, especially in chemical, food and power industries. For many of these processing steps using hydrodynamic calculations exist good models to describe multi-phase-flow properties in e.g. heat exchangers, reactors or dryers. Especially for fluidized bed reactors many publications show the effort to describe heat-transfer phenomena for the disperse phase system gas-solid where essential progress took place within the last decades. Several mechanisms for heat-transfer differentiated in particle-particle, particle-gas and particle-wall contact have been well evaluated.

Less attention was spent on heat transfer phenomena within dilute pneumatic conveying systems, although this operation step is essential for material handling. Approaches from related disciplines like fluidized bed reactors are not directly transferable because of obvious increased air velocities up to the point of turbulence. Drying a product, followed by pneumatic conveying into a hopper or filling station, it is necessary to know the exact product temperature as possible to prevent caking or degradation of the bulk solids in which the particle temperature is not measurable during conveying.

Within the scope of this present work, experiments and approaches were carried out to determine the heat-transfer of a two-phase flow during pneumatic conveying with focus on heat convection and heat conduction. The experiments were carried out for different types of material and particle sizes respectively mass loadings varied over a wide range of scale. Furthermore, air volume flow rate was constant for several conveying gas velocities, with Reynolds numbers from non- to turbulent regime. A novel measurement device was developed capable to detect the heat transfer and successfully implemented into the testing setup. In addition, during the measurements it was possible to detected changes in flow profile as well as sedimentation effects. Finally based on data analysis, an empirical correlation is determined to predict the heat conduction of two-phase flow for the design of pneumatic conveying systems with attention to heat transfer.