Drying is an omnipresent operation present in various industrial sectors such as food and agriculture, forestry products, chemicals, and pharmaceuticals. Convective hot air drying with conventional impinging jet nozzles is among the widely used techniques in drying of moist porous materials. However, this method is generally inefficient in drying of fragile materials as a low air flowrate is required to avoid potential damage to the product due to the high exerted force by impinging jet nozzles. To improve the drying efficiency and enhance the product quality, slot jet reattachment (SJR) nozzle has been developed as an alternative technology to the common in-line jet nozzle, slot jet (SJ) nozzle, and perforated plate. SJR nozzle allows for the control of the exerted impingement force by simply changing its exit angle. In the SJR nozzle, the jet is directed outward from the nozzle exit and it then reattaches on an adjacent surface in its vicinity. The turbulent mixing occurs at the boundaries of the free stream induces secondary flow by mass entrainment and causes the flow to reattach the surface in the form of an oval reattachment at close nozzle to surface spacing.

This paper numerically investigates the convective air-drying characteristics of an SJR nozzle and compares it to those of a common SJ nozzle for a moist porous medium. The comparison is made under identical air mass flow rate criterion. The nozzle exit temperature for both SJR and SJ nozzles is kept constant for all the situations at 100°C. Specifically, the effects of SJR nozzle exit angle, exit opening size, and nozzle to surface spacing on the drying of a moist porous medium are studied. The results are presented for stationary and moving reattachment surface in various surface-to-jet velocity ratios under a turbulent flow condition. The governing transport equations within a moist porous medium are coupled with the governing equations of the resultant air flow. The results indicate a much higher drying rates and a lower exerted force magnitude on the porous medium surface with SJR nozzle compared to SJ nozzle. Furthermore, the SJR nozzles are especially suitable for drying applications where it is crucial to protect fragile moist materials from being damaged by direct hot airflow impingement. SJR nozzles provide increased heat transfer rate and a mechanism to control the magnitude and direction of the exerted force on the samples.

 

Keywords: Convective air drying, Slot jet reattachment nozzle, Slot jet nozzle, Moist porous media


Convective Air Drying of Stationary and Moving Moist Porous Medium With Slot Jet Reattachment Nozzle: A Numerical Study

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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