An experimental investigation of the swirling flow in a tall-form counter current spray dryer

Abstract

a b s t r a c tThis work studies the air flow in a large swirl counter-current dryer using sonic anemometry. Air velocity and turbulence fields are reported at isothermal conditions and in the absence of particles. In a tall-form unit the structure of the flow is largely influenced by the design of the exit. A contraction originates a central jet and suppresses the formation of recirculation zones despite the vortex acquires a high swirl intensity X (i.e. 1 < X < 2). Access to a full scale tower has permitted to: (a) identify asymmetries owed to the design of inlet and exhaust ducts, (b) present the first detailed turbulence data in production units, characterized by a highly anisotropic field and the axial decay of the turbulence kinetic energy, (c) study the flow stability, identifying the precession of the vortex core and oscillations at a constant Strouhal number and (d) study the impact that a rough wall has in the strength of the swirl. This work presents the first clear evidence of significant friction in spray dryers. The swirl intensity X decays exponentially in the dryer at a rate between 0.08 and 0.09, much higher than expected in pipe flow and independent of Re in the range 10 5 -2.2 Á 10 5 . Production dryers have a large characteristic wall roughness due the presence of deposits, which explains the stronger friction and the discrepancies found in the past between data at full scale or clean laboratory or pilot scale units. It is essential to address this phenomenon in current numerical models, which are validated on laboratory or pilot scale facilities and ignore the role of deposits, thus causing an overprediction of the tangential velocity above 30-40%.

The role of wall deposition and re‐entrainment in swirl spray dryers

“…Their action causes an air vortex to rise into the cylindrical section and the kinetic energy contained in the flow, in particular within the tangential motion, decreases due to the action of the wall shear stress. 26 On reaching the top end of the cylinder, the flow converges into the tubular guard or vortex finder, and exits toward cyclones [23] where the elutriated fines [26] are separated from the exhaust air [25].…”

Residence time distribution of glass ballotini in isothermal swirling flows in a counter-current spray drying tower

“…The selection of this turbulence model was based on a comprehensive validation of the predicted mean and fluctuating air velocity profiles in this tower against experimental data carried out by Ali (2014). Measurements were carried out as part of an experimental program and data were reported in Francia et al (2015a). Generally, a good level of agreement between the predictions and measurements was obtained using steady state conditions.…”

“…4 The air and particle flow dynamics in counter-current spray drying towers are significantly different than those in co-current towers. Counter-current spray dryers involve greater levels of swirl intensity and less flow instability owing to their greater height to diameter ratios (Langrish and Fletcher, 2001;Francia et al 2015a). …”

Modeling and Scaling Up of Industrial Spray Dryers: A Review

“…Extensive experiments and simulations of flow in counter current spray towers for detergent manufacture has been carried out recently by Francia et al (2015aFrancia et al ( , 2015b. The authors studied airflow parameters (air velocity and turbulence) in isothermal conditions (no drying process) in swirl counter-current industrial tower (Figure 2) using a sonic anemometer.…”