Influence of wall friction on flow regimes and scale-up of counter-current swirl spray dryers

Abstract

We study the vortex flow in swirl dryers across full scale designs and conditions. Three air flow regimes are related to the effects of friction and design features. The characteristics of deposits are vital to explain the evolution of the vortex. Friction reduces the swirl drastically and it has a major impact on the turbulence. Flow regime prediction facilitates the use of new control and scale-up criteria. a b s t r a c tThe structure of the vortex flow in swirl spray dryers is investigated after having fouled the walls with deposits typical of detergent manufacture. The range of Re and swirl intensity Ω characteristic of industry are studied using three counter-current units of varying scale and design. The friction with the deposits increases the flow turbulence kinetic energy and causes a drastic attenuation of the swirl and as a result, the vortex breaks down in the chamber forming recirculation regions (i.e. areas of reverse flow). Three flow regimes (1) no recirculation, (2) central and (3) annular recirculation have been identified depending on the swirl intensity. New control and scale up strategies are proposed for swirl dryers based in predicting the decay and the flow regime using the unit geometry (i.e. initial swirl intensity Ω i ) and experimental decay rates function of the coverage and thickness of deposits. The impact in design and numerical modelling must be stressed. Adequate prediction of the swirl is vital to study fouling and recirculation, which surely play an important part in the dispersion and aggregation of the solid phase. Current models have no means to replicate these phenomena, and yet, in this case neglecting the deposits and assuming smooth walls would result in (a) over-prediction of swirl velocity up to 40 À 186% (b) under-prediction of turbulent kinetic energy up to 67 À85% and (c) failure to recognise recirculation areas.

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

“…Different flow regimes have been correlated to the X range that prevails in the chamber [33], but in the case treated here of ''cleaned'' walls, U z remains positive and shows two maxima: a central jet associated to the diameter of the vortex finder and one nearby the wall, related to the centrifugal inertia. As the flow approaches the top the velocity of the jet increases from 1:1 to 1:4U av for z 6 5.7D to values > 5-6U av close to the top.…”

“…This work however maintains homogeneous roughness with a twofold purpose: (a) replicate the best scenario in industry given by recently ''cleaned'' walls, and (b) ensure an easier numerical reproduction. Further experimental work [33] investigates the role of the deposits at the wall, comparing this reference to the flow field established under deposits of different thickness and coverage. In this case, reproducible conditions were obtained by cleaning the inner surface with an automatic ring designed to ensure the layer of deposits is thinner than 0.006D.…”

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.…”

“…A precession of the jet coming close to the walls promoting particles to hit the wall, with frequency of around 1 Hz was observed. Francia et al (2015b) investigated the effect of wall friction on flow regimes and the structure of the vortex flow in three counter-current units of varying scale and design. For the industrial dryer the authors analyzed air flow pattern for three different amount of the deposit on the wall; clean walls, small deposition (e.g., for short term production), heavy deposition (long term production, no automatic cleaning)…”

“…Swirl intensity decays exponentially due to high wall roughness produced by the deposits, neglecting the role of wall deposits might cause an over prediction of the tangential velocity around 30-40%. 8 Francia et al (2015b) Three countercurrent units of varying scale and design, l/d=10.58, l/d=2.97, l/d=2.87 Sonic anemometer, no drying process Strong effect of wall friction on flow regimes and the structure of the vortex flow, 3 regimes: clean walls, small deposition, heavy deposition. Neglecting the deposits results in "over-prediction of swirl velocity up to 40÷186%, under-prediction of turbulent kinetic energy loss up to 67÷85% and failure to recognize recirculation areas" 9 Wawrzyniak et al (2012a) l=37, d=6, con-current, no swirl flow Anemometer, no drying process High flow instability…”