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Thermodynamics, Transport, and Fluid Mechanics
CFD MODELING OF SWIRLING COUNTER-CURRENT FLOWS IN INDUSTRIAL SPRAY DRYING TOWERS UNDER FOULING CONDITIONS. Borja Hernández, Blair Fraser, Luis Martin de Juan, and Mariano Martín Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02202 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018
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Industrial & Engineering Chemistry Research
CFD MODELING OF SWIRLING COUNTER-CURRENT FLOWS IN INDUSTRIAL SPRAY DRYING TOWERS UNDER FOULING CONDITIONS. Borja Hernandezab, Blair Fraserb, Luis Martin de Juanb, Mariano Martina,* a Departamento de Ingenieria Quimica. Universidad de Salamanca. Salamanca, Spain. b Procter & Gamble R&D, Newcastle Innovation Centre, Newcastle upon Tyne, UK. ABSTRACT This work presents a novel procedure to predict the airflow pattern under different levels of deposition and Reynolds numbers for swirling flow industrial scale spray drying towers. It improves the accuracy in the prediction of both the hydrodynamics and the effect of the deposition. Initially, steady-state and transient simulations are compared showing that the model can be reduced to steady-state for a certain mesh size. The CFD model is later calibrated using the experimental swirl intensity values under different levels of deposits that have reached a dynamic equilibrium. Then, the model is validated for different Reynolds numbers of operation. Finally, the validated model is applied to examine the vortex behaviour and evaluate the effect of the tower radius reduction. A limit of operation is found for low Reynolds numbers in terms of stability and it is observed that the momentum cannot be only modelled with the radius reduction.
*
[email protected] 1 ACS Paragon Plus Environment
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1. INTRODUCTION Spray drying is a common unit operation to manufacture a wide range of particulate products such as pharmaceuticals, detergents and food, by which a solution is atomized so drops are dried in contact with hot gas. According to the contact, spray dryers are classified as: co-current, countercurrent or mixed flows.1 Co-current dryers are the most analysed type in CFD modelling, at small2 and large scale3. However, CFD research applied to counter-current towers is not as extensive since the number of industrial relevant cases is lower4. In particular, the flow inside the counter-current towers of the present study is characterized by a swirl pattern. This flow pattern presents a more complex fluid dynamics, but it improves the heat and mass transfer efficiency.5 The benefits provided by swirl flows have also been applied to other operations such as combustion,6 centrifugal separation,7,8,9 or heat exchangers with lobed pipes10 or internal swirl generators.11,12 Most of the studies on swirl flows have focused on the instabilities associated, such as the presence of periodical structures13 or the existence of vortex breakdown (VBD).14 VBD induces the formation of recirculation regions that depend on the effect of the downstream boundaries, especially, when the flow suffers a contraction.15 The presence of these instabilities is observed in both swirl cocurrent16 and counter-current drying towers.17 The generation of recirculation regions in large scale counter current towers has been experimentally studied by Francia et al.5 They described the influence of the wall deposits on the modification of the swirl intensity and the existence of recirculation according to the criteria proposed by Escudier et al. 15 Francia et al.5 also described the structure, momentum, velocity and turbulence components of the vortex at different Reynolds numbers and deposition levels. The structure is composed of two vortexes: one internal dominated by the pull towards the low pressure in the top outlet and an external structure across the dead regions, controlled by interactions with the wall. The friction generated in this external vortex depends on the wall deposit levels and it produces a decay in the swirl intensity from the bottom to the top of the tower. 5,17 The swirl decay changes according to the deposits within a range of 200%.5 Previous studies in CFD modelling did not consider the friction generated by the deposits at different levels of deposition, resulting in a lack of accuracy in the simulations under certain conditions for particle dispersion and momentum prediction.20 The addition of roughness given by the deposits requires the entire modelling procedure for spray drying towers18,19 to be changed, because the deposits modify the airflow and the same airflow core-model should not always be used for different levels of deposition. Therefore, in this work a new methodology for the modelling of spray drying towers is presented. However, including this phenomenon is not straightforward. A procedure is developed to account it for and include the effect of deposits in the modelling of the flow inside spray drying towers. 2 ACS Paragon Plus Environment
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Industrial & Engineering Chemistry Research
The validation of CFD models for spray drying towers has traditionally been carried out based on the study of velocity profiles. 21-23 However, the momentum distribution is not regular along the radius, concentrating the mass and momentum at large radial positions. Fluid momentum governs the particle momentum, concentrating them near the wall. Because of this irregular momentum distribution and its direct influence in further modelling stages, a calibration based on swirl intensity is proposed in this work. In fact, since it accounts for the momentum distribution, swirl intensity has been recognised as a more robust method to estimate the momentum of the continuous phase in industrial towers. 5 The rest of the paper is structured as follows. In section 2 a physical description of the flow is provided. In section 3, the CFD modelling methodology to include the effect of the deposits is described. In section 4 the results for the model are reported. This section is completed by applying the CFD model to study vortex characteristics. Finally, in section 5, the conclusions of the work are presented as well as some lines for future work in the area. 2. PHYSICAL DESCRIPTION OF THE FLOW The airflow pattern in the tower was identified as an anisotropic swirling flow.5 In this section we describe the physical laws that model the swirl flow such as the swirl intensity, the VBD and the turbulent components like the Reynolds stresses and anisotropy. 2.1 Swirl intensity Swirling flows are commonly characterized by the swirl intensity or swirl number, S, that quantifies their degree or strength. The swirl intensity was defined by Kitoh25 as the non-dimensional flux of angular momentum, neglecting the contribution of the Reynolds stress and the axial flow development.26 It is normalized by the axial flux of momentum based on superficial velocity, , and tower radius, R, as in eq. (1).
= 2
(1)
2.2 Swirl decay In the case of a flow passing through the tower, the fluid experiences a decay on the swirl intensity due to the wall shear stress (WSS) generated. Kitoh determined the tangential wall shear from the Reynolds averaged angular momentum equation for incompressible, stationary and axially symmetric flow as follows:25 ( ) =
# ! "
− ) + %′'′
* !
(2)
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Where U and W are the time averaged values of the axial and tangential velocity components
are the instantaneous fluctuations, µ is the dynamic viscosity, x and r are the axial respectively, %′'′ # is the flux density of angular momentum per unit mass at radial position and radial positions, r. Turbulent shear stress, %' , and viscous shear stress, )
, !
can be neglected assuming symmetry,
steady state and incompressibility. Thus, for a slow axial development eq. (3) is obtained:
# !
=
(3)
Eq. (3) can be written in dimensionless form using the following transformations: z→ z/D, r→r/D, U→U/Uav and W → W/Uav. Now, the non-dimensional tangential WSS is rearranged as eq. (4): + ( ) =
,-. (!) .0 1
=
2 34 3" 5 *
(4)
6
Eq. (4) indicates that the non-dimensional tangential WSS is a linear function of the axial gradient of swirl intensity. Steenbergen and Voskamp26 concluded that the existence of tangential WSS in the swirl flow causes reduction of the swirl intensity. This explanation justifies the decay predicted by Kitoh 25 in pipes and by Francia et al.5 in swirling flow towers, which is defined as eq. (5): = 789 ∙ ;
