Power number

To characterize behavior of BE12500, power consumption is evaluated at pseudo-steady state as described in Appendix A. We considered the flow field in the BE12500 to be pseudo steady when the axial flow rate and the azimuthal momentum averaged over one impeller revolution were both steady. Figure 4.36 shows the distribution of total force on the turbofoil. It can be observed that distribution of forces is different for the upper and lower surfaces of the blades. Integration of these values over the surface of the impeller allows calculation of the values of power consumption reported in Table 4.11. It can be observed that power consumption is constant in the fully turbulent range (simulations S3 and S4) while a rapid increase is found for the turbofoil in the low Reynolds range. This is a consequence of the wide blade surface of the impeller that determines high friction dissipation especially for high viscosity values (low Reynolds range).

Figure 4.36: Total force distribution on upper (top) and lower (bottom) surface of turbofoil turbine.

Table 4.11: Power consumption calculated for the different simulations. Values are in Watt.

S1	S2	S3	S4
3046.	1551.7	1497.8	1472.6

Values of power consumption are used to calculate the power numbers. Power number values are compared against the power characteristic obtained from Nagata (1975) as described in Appendix B and shown in Figure 4.37. Power number values calculated by the simulations are in good agreement with the power characteristic both in the range of high and low Reynolds.

Figure 4.37: Power number versus Reynolds number: BE12500

It should be noted that power consumption for the turbofoil turbine is larger than the corresponding value for the retreated curved blade impeller, as can be observed in Figure B.4. A larger value of power consumption is observed especially in the laminar range. This suggests that the turbofoil turbine can be conveniently used only in the high Reynolds number range, where its superior pumping capability counterbalances the larger power consumption.