Figure 22. Laboratory data validating tapered-surface CFD results (Figure 21.)

Shown in Figures 23-25 are the computed heat removal effects of shaft (rotational) speed, barrier fluid throughflow, and thermofluid properties (as reflected by Prandtl Number), respectively. To check the effect of shaft speed, a water throughflow rate of 1.4 l/m was assumed. Generally speaking, the greater the shaft speed, the higher the Reynolds Number and level of turbulence, effects leading to improved heat removal efficiency, as illustrated

Figure 23. Barrier fluid cooling efficiency versus shaft rotational speed.

by Figure 23. In Figure 24 we see a similar trend with respect to throughflow, again for water and a shaft speed of 183 rad/s (1750 rpm). Finally, Figure 25 indicates a dramatic loss of cooling efficiency with increasing Prandtl Number, Pr, of the barrier fluid. The Prandtl Number is a dimensionless, temperature-dependent fluid property/parameter

Figure 24. Barrier fluid cooling efficiency versus rate of throughflow.

defined as the product of the specific heat and thermal conductivity of the fluid divided by the fluid viscosity. (It can also be physically interpreted as the ratio of momentum diffusivity to thermal diffusivity of the fluid.) Three commonly used barrier fluids - water, a 50/50 weight-percent mixture of Ethylene Glycol and water, and a commonly

Figure 25. Barrier fluid cooling efficiency versus Prandtl Number.

used synthetic oil - were all evaluated for an inlet fluid temperature of 38 C, and a shaft rotational speed of 377 rad/s (3600 rpm).

Figure 26 shows the axial flow circulation pattern computed for the case of synthetic oil having a viscosity of about forty times that of water at 38 C. As clearly seen, the reduced communication between the cooler fluid in the flow channel and the warmer fluid near