Ind. Eng. Chem. Res. 1993,32, 575-576
575
Local Heat Transfer in a Mixing Vessel Using a High-Efficiency Impeller Seungjoo Haam and Robert S. Brodkey' Department of Chemical Engineering, The Ohio State University, 140 Columbus, Ohio 43210-1180
W.19th Avenue,
Julian B. Fasanot Chemineer Znc., P.O. Box 1123, Dayton, Ohio 45401
A previously reported computer-based, local heat-transfer sensor system was utilized to determine process-side heat-transfer coefficients associated with mixing with a high-efficiency impeller. Each impeller (of different diameters) was evaluated to determine its effects on the mixing parameters of the tank. A 0.38-m vessel with a liquid depth of 0.46 m was used. The off-bottom clearance of the impeller was T/6 (6.35 cm). Impeller speed was varied to provide a range of Reynolds numbers. The exponents for the normally accepted empirical heat-transfer equation were examined for each component.
Introduction The rate at which heat is transferred from an agitated vessel depends upon the agitator type, vessel design, process conditions, and the phases of the process fluids. Our ongoing research is on the heat transfer at the wall of an agitated vessel equipped with a dished bottom and four baffles. In the previous paper (Haam et al., 19921, the measuring technique was developed and results were reported for the well-known Rushton impeller. The present note reports on using the system to establish a heat-transfer correlation for a typical high-efficiency impeller (Chemineer Model HE-3). Literature Review Based on the previous review of the literature and equations, empirical corrections of the form NNu
= hT/k = kl(NRe)"(Nfi)b(P/Pw)C
are usually sufficient to represent the data collected. The Nusselt number ( N N ~in) our work represented the local heat transfer about midway between the fluid surface and the bottom and between the baffles. Haam et al. (1992) showed that this value was a reasonable estimate for the side wall heat transfer.
Experimental Procedure The facilities used have been described in the paper cited and will not be repeated here. Different values of Reynolds numbers were obtained by varying the revolutions per minute (rpm) for any given set of conditions,i.e., impeller diameter and fluid temperature. In addition, eight DIT and six C/T ratios were studied. The four baffles were 3.18 cm each in width. The Chemineer Model HE-3 impeller was used throughout this study. The HE-3 impeller is an open-blade hydrodynamic fluid foil axial impeller (see sketch in Figure 1) that gives very close to an axial flow, much more so that the more conventional 4 5 O pitched flat blade impeller. Heat transfer for such impellers has not been previously reported. In all runs the Z/Tratio was maintained at a little over 1. The water temperature was also measured manually by inserting a portable thermocouple meter sensor into the water prior +
Technical Director.
0888-5885/93/2632-0575$04.00/0
Figure 1. Sketch of a Chemineer HE-3high-efficiency impeller.
to each run. This value was then used to determine the approximate rpm for the desired Reynolds number. The raw data were taken at the conditions cited by using an averaging mode of 3000 data points for each of the 8 channels recorded. The raw data were then converted to processing variables and nondimensional numbers.
Results The main objective of the experimental runs was to establish the constants in the following equation:
The results are based on 131observationsmade for 16 000 < N R
