Numerical and experimental investigation of flow patterns in scraped surface heat exchangers
Multi-scale and/or multi-disciplinary approach to process-product innovation
CFD & Chemical Engineering- II (T3-4b)
Keywords: CFD, scraped surface heat exchanger, shear rates, viscous flow, PIV
Numerical and experimental investigation of flow patterns in scraped surface heat exchangers
Mourad YATAGHENE(1) , Francine FAYOLLE(1)*, Jack LEGRAND(2)
GEPEA - UMR CNRS 6144
(1) ENITIAA, Rue de la géraudière, BP 82225, 44322 NANTES cedex 3, France
(2) Université de Nantes, CRTT, BP 406, 44602 SAINT NAZAIRE Cedex, France
Scraped surface heat exchangers (SSHE) are often used in the food industry to process highly viscous fluids. However, they are known for their high shearing properties, which can influence the product texture at the outlet. It was also shown before (Fayolle et al. 2005) that in the case of non Newtonian shear-thinning fluids, the flow rate of a small layer of fluid close to the exchange surface can increase dramatically and leads to a lower residence time of that particular layer, and then a lower treatment in the case of pasteurization processes.
A numerical investigation of SSHEs was undertaken using the commercial CFD code FLUENT in order to characterize the flow patterns for Newtonian and Non-Newtonian fluids. Simulation results were validated with experimental shear rates measured using electrochemical method and conducted on the exchange surface as well as in the inlet and outlet bowl (Mabit et al., 2005).
Simulations were carried out in standard geometries of SSHE. Two different flow configurations were treated separately: first, the inlet bowl which is not reached by the scraping blades, and second the exchanger itself, with two scraping blades.
In the inlet bowl, velocity profiles calculated in the whole bowl show a dead zone where clogging can occur. The presence of this dead zone was already suspected by electrochemical measurements and is confirm by the simulation. CFD calculation can then be an interesting tool for process designers in order to create the most appropriate inlet and suppress this dead zone. Velocity profile given at the end of the inlet bowl can thus be used to initialize the calculation in the exchanger itself. Profiles obtain for different model fluids (Newtonian HV45 and non Newtonian CMC) are discussed and compared with PIV measurements.
In the exchange part itself, simulation is more difficult because of the particular geometry of the floating blades. A grid refinement in the zone localized between the tip of the blades and the stator has to be used. A numerical resolution of the laminar isothermal steady-state flow using multiple reference frames (MRF) approach was first attempted in a 2D cross section of the exchanger. A comparison of predictions with experimental measurements was carried out and gave good agreement at the exchange surface during blade scraping. Axial flow has then to be taken into account in order to model the whole system correctly and to introduce thermal treatment, which is attempted by a 3D simulation.
References:
Mabit, J., F. Fayolle and J. Legrand (2003) Chem. Eng. Science 58(20): 4667-4679
Fayolle F. Mabit J. and Legrand J. (2005) J. Appl. Electrochem. 35, 487-498
See the full pdf manuscript of the abstract.
Presented Tuesday 18, 15:40 to 16:00, in session CFD & Chemical Engineering- II (T3-4b).
