Heat and mass transfer from jet impingement flow for drying machines

Abstract

Multiple impinging jets are widely used for their enhanced heat and mass transfer characteristics, especially for drying of material such as paper and textiles. Efficient transport characteristics between a drying medium and the material being dried as well as the jet impingement force acting on the force sensitive products (i.e. paper, fabrics) or force sensitive surfaces (i.e. painted, coated) are very critical in the industrial drying applications for the overall performance of the operation. Many variables and influencing factors and parameters need to be considered for the proper design of such multiple impinging jet systems. At present, a complete understanding of the effect of all the design and operating parameters is lacking. Designers should optimize the design parameters of industrial drying equipment to achieve the minimum capital costs (size of the apparatus) and running costs (energy consumption). In the framework of this thesis, the heat and mass transfer rates and pressure force between the multiple impinging round jets and moving surface are investigated numerically. Numerical simulations have been conducted to characterize the impinging jet heat and mass transfer and pressure force on a moving surface with an array of impinging round jets varying the following parameters: jet Reynolds number, nozzle-to-surface distance, jet-to-jet spacing, jet exit angle, relative surface curvature and relative surface velocity. Then, the key controlling design variables such as the nozzle-to-surface distance and the jet-to-jet spacing both normalized by the jet exit diameter (i.e., H/d and S/d), the jet exit velocity, the surface velocity and the jet exit angle have been considered for a design optimization. Correlations describing the average Nusselt number and pressure force coefficient for single row jet configurations impinging on the flat and curved surfaces have been developed. The present work illustrates that it is possible to address the effects of the complex flow phenomena as encountered in industrial drying process using generic CFD codes with good results. Thus complex and extensive computations based on elaborate and time consuming turbulence modelling (i.e. RSM or LES) may be avoided as well the expensive measurements of jet arrays impinging on moving surfaces. Using the correlation equations derived from the computations the dependencies for the main parameters over a wide range may be easily be determined. The analysis of the flow fields showed many different phenomena interacting, leading to different flow regimes. In order to assess these phenomena further investigations, preferably based on LES and DNS modelling and flow measurements are needed.

Keywords: Multiple jets, Heat transfer, Pressure force, Energy consumption, Surface motion, Surface curvature, Optimization