Helical ribbon impeller heat transfer

Most of the correlations were taken from Penney (1983) and from tables 5 and 6 in Fasano et al., (1994) for the HE-3 impeller and the bottom head. The correlation for heat transfer coefficients for helical ribbon impellers was taken from Ishibashi et al. (1979). The correlations given by Penney (1983) (p. 879) use the same sources. 

Low viscosity mixing applications can usually be handled efficiently with impeller systems consisting of one or more turbines. To obtain adequate mixing under the laminar flow conditions encountered in high viscosity applications, on the other hand, close-clearance impellers such as anchors and helical ribbons are required. These impellers sweep the whole wall surface of the vessel and agitate most of the fluid batch through physical contact. Helical ribbon impellers are typically used for industrial applications where the viscosity is in the range 20 000 to 25 000 Pa s. Wall scrapers can be mounted on the impeller blades to improve heat transfer. 

Example 14.2 Helical Ribbon Impeller. Determine the process-side heat transfer coefficient for the tank blending design example for a helical ribbon impeller (Bakker and Gates, 1995) ... 

Harry, F. P., and V. W. Uhl (1973). Heat transfer to viscous materials in a vessel with a helical ribbon impeller, presented at the 74th National AIChE Meeting, New Orleans, LA, Mar. 

Figure 5-36 shows the flow pattern in the vertical plane of a vessel equipped with a helical ribbon. A fully structured hexahedral mesh with approximately 100 000 cells was used. The structured 3D mesh was created by extruding and twisting a 2D planar mesh. The fluid is viscous and the impeller Reynolds number is approximately 10. The velocity vectors show that the impeller pumps down at the wall and up in the center. Contours of velocity magnitude on the tank bottom show that there are low velocities in the center and higher velocities near the outside wall. Small circulation loops form between the impeller blades and the vessel wall, as discussed in the general literature. These indicate the need for an even larger D/T or the use of wall scrapers if optimum heat transfer is to be obtained. 

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