The cavitation phenomena seems to not affect the rotordynamic forces, especially at low flow rates.

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Conclusions

The results here presented of the experimental characterization of fluid-induced rotordynamic forces on the DAPROT3 inducer and on the VAMPDAP pump conducted in Alta’s CPRTF under different operating conditions (cavitation numbers, flow coefficients and flow temperatures) confirmed the good agreement between the results obtained in the tests carried out at constant and at slowly ramping whirl speeds. Experiments at variable whirl speed allowed for the spectral characterization of the radial and tangential components of the rotordynamic force as functions of the whirl frequency and for the accurate assessment of the relevant maxima/minima, as required for the identification of the most dangerous operational conditions of the machine.

For what concern DAPROT3 inducer it has been found that lower flow rates are usually associated to higher rotordynamic forces and a more stable conditions. The results for normal and tangential forces show that a deviation from the “classical” trend, obtained by Brennen for centrifugal pumps, has been observed at flow coefficients higher than the design value. This findings are qualitatively in agreement with the results obtained at Caltech in experiments on simple helical inducers and confirmed by previous experiments at Alta on high-head variable-pitch, tapered-hub inducers similar to the DAPROT3 of the present investigation. Earlier studies clearly identified the synchronization between the whirl motion of the inducer eccentricity with the average rotation of the inlet backflow as the main phenomenon responsible for the observed behavior of the rotordynamic forces in cavitating inducers. This effect is especially important in operation at high-load, large tip clearances and reduced cavitation numbers, as a consequence of the combined effects of the intensification of the tip leakage flow and the asymmetric flow perturbations generated by the interaction of whirl eccentricity with the inlet flow cavitation. These findings are fully consistent with the results of present experiments on the DAPROT3 inducer, where the tip clearance is comparatively high (7.7% of the mean blade height) and the observed deviations of the rotordynamic force spectra from the expected behavior is mostly manifest at high-loads under heavy cavitation conditions. Consequently, these phenomena are also believed to be responsible for the observed features of the rotordynamic force spectra measured in present experiments on the DAPROT3 inducer.

The cavitation phenomena seems to not affect the rotordynamic forces, especially at low flow rates.

Nevertheless a certain influence has been noted on higher flow rates where a particular phenomenon arises for negative whirl frequency ratios at highly cavitating condition probably governed by a coupling effect between them. Indeed, in these operating conditions the normal force presents a wider stabilizing region with lower intensity at high whirl ratios. A similar trend has been observed for hot flow condition in which the deviation from noncavitating results has been found in the presence of a moderate cavitation whereas no deviation has been observed at highly cavitating condition. These results confirmed the importance of thermal cavitation effects which can significantly affect and alter the fluid-induced rotordynamic force in certain operating conditions.

Apart from this particular phenomena, the presence of a hot flow is usually associated with a

significantly increase in rotordynamic force, especially for normal force, at high whirl frequency ratios

both at negative and positive whirl speeds.

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Finally, the rotordynamic forces seems to slightly affect the hydraulic efficiency. A maximum variation of 2% has been found for the inducer operating under different whirl speeds at fixed flow rate. This influence is even lower for the inducer operating at design, where at noncavitating condition the efficiency can reach a loss of 1%. The same behavior has been demonstrated for other flow coefficients. Hence the major variations have been verified at low flow rate. The principal reason for this is that the whirl motion interacts with the tip leakage flow, which is the main responsible in loss of efficiency.

Moreover, the influence of the temperature on the hydraulic efficiency can be observed as a shift of the experimental curve thus the same trends are verified also in the presence of a hot flow. Hence the substantial effect of the temperature is to decrease the hydraulic efficiency by a maximum of 2%

regardless the whirl ratio, depending from the operating conditions (cavitation number, flow coefficient).

The second part of the experimental campaign dealt with the VAMPDAP pump. It has been found that a deviation from the classic trend at positive whirl speeds is still present, according to the presence of the DAPROT3 as a part of the mixed flow pump and mounted upstream with respect to the VAMPIRE. The results of this pump are in accordance with the experimental diagrams evaluated from the two separate configurations and the rotordynamic force obtained is a combination between the two components:. the maxima and minima of tangential and normal rotordynamic forces are verified approximately at the same whirl ratios of the inducer but the presence of the centrifugal pump modify their intensity and sometimes their stabilizing behavior. Moreover the rotordynamic forces at very negative whirl ratios appear to be much higher with respect to the forces evaluated for the inducer and the pump alone, thus leading to interpret all these facts given by a whirl-dependent combination of the two contributions.

Nevertheless, maxima and minima appear also for high flow rates even if neither the inducer nor the pump show this behavior. No maxima or minima are present in case of either DAPROT3 or VAMPIRE in stand-alone configuration at same flow rate and inlet pressure. Hence a probable interaction may arise between the centrifugal pump and the inducer due to presence of the inducer itself which gives rise to a strong azimuthal velocity component to the flow entering the pump and which can alter the load on the pump blades.

VAMPDAP pump presented a low dependence from the cavitation effect and significant variations have been found only for high flow rates where the rotordynamic force decreases at negative whirl ratios. On the other hand the effect of a decrease in the flow rate develops stronger rotordynamic forces at negative whirl ratio whereas a shift in the maxima and minima has been detected in the positive case.

The effect of the temperature has been found to be similar to the results obtained from DAPROT3 inducer, thus increasing the rotordynamic force at high whirl speeds, whatever the direction of rotation.

For what concern the hydraulic efficiency of the VAMPDAP, the effect of the rotordynamic forces have been demonstrated to be less than 1% and therefore mostly negligible, whereas the cavitation phenomena surprisingly slightly increases the hydraulic efficiency of the machine at design.

Finally, in case of a hot flow as a working fluid, the efficiency does not present relevant differences

except for a few specific operating conditions at which thermal cavitation play an important role.