For the fan of the HPOTP, pressure measurements on the front shroud of a fan and a fanless diffuser showed that the pressure fluctuations of the front shroud increased with the presence of the diffuser wheels. Ensemble average unsteady pressure measurements of the 77 right, center and the left three-hole tube pressure tab for Impeller Zl. Ensemble averaged unsteady total pressure measurements for 104 Impeller Zl and Diffusers S and T at mid-height of the diffuser channel.
Ensemble averaged unsteady total pressure measurements for 105 Zl runners and S and T diffusers at mid-height of the diffuser channel. Measurements were made at Caltech's Rotor Force Test Facility. On the left edge of the photo you can see the pressure tap used for reference pressure (upward pressure) for steady pressure measurements.
The position of the leading edge of the full and partial runner blades is indicated. The geometry of pressure taps for pressure measurement on diffuser blades and rotor blades is shown in Figure 2.19. It can be seen that both the magnitude and the phase of the two total average total pressure measurements are almost identical.
This new data acquisition system was used for most rotor blade pressure measurements.
DIFFUSER
The total pressure measurements of the outflow centrifugal impellers were made, as previously mentioned, with a total pressure probe (these measurements will be discussed in Chapter 4). The probe head of the three-hole tube was located at the height of the middle channel of the Diffuser T. In Figures 3a and 3b, the left and right holes of the three-hole tube were cut by about one diameter of lead pipe (0.035 in ).
The stable calibration of the three-hole tube was performed in an air jet (see Figure 3.4). During calibration, the three-hole tube was fixed in such a way that the three-hole tube tip remained in the same position in the nozzle, regardless of the angle of rotation. It can be seen that for the left and right tap the pressure measurements are monotonic, while for the middle hole they are similar to the calibration curve of the total pressure probe (Figure 2.20).
The three-hole pipe was located in the middle of the channel height of the diffuser and was aligned with an angle of 20 degrees (see Figure 3.3b). The radial gap between the three-hole tube and the rear edge of the impeller during the measurements was, as for the stable measurements, 4% and 7% of the impeller outlet radius for Impeller R and 8% and 11% of the impeller outlet radius for Impeller Zl. The unsteady measurements were made for each tap in the three-hole pipe (right, middle and left tap) in a separate run.
A spectrum of the unstable pressure measurements at the left pressure valve of the three-hole tube using Impeller Zl is shown in figure 3.12. Again, the region of high total pressure on the suction side of the partial impeller is not resolved by the three-hole tube. However, the total pressure profile from the suction side of the full fan blade to the pressure side of the partial fan blade is.
This may be due to non-uniform flow effects around the tip of the three-hole tube. If we take as a criterion the occurrence of minimum total pressures during the passage of the rotor blades, the phase difference between the total pressure probe and the tube with three holes is practically not noticeable. The quotient of the thus defined characteristic length, s*, of the fluctuations and the width of the three-hole tube is approximately 2.
Turning first to measurements of fluctuating pressure and total pressure in the absolute reference frame, P~v and fJ;,av', it can be seen that distinct minima occur on the pressure side of the trailing edge of the impeller blade. The maximum total pressure occurs in the first half of the blade channel (between the suction side of the blade and the center of the blade channel), while the maximum pressure occurs in the second half of the blade channel (between the center of the blade channel and the pressure side of the blade).
CEHTER PRESSURE TAP
LEFT PRESSURE TAP
FLUCTUATING DYHA"IC PRESSURE
FLUCTUATIHG PRESSURE
FLUCTUATING PRESSURE
FLUCTUATING FLOW ANGLE (STATIONARY FRAME)
FLUCTUATING DYNA"IC PRESSURE
FLUCTUATING DYNAMIC PRESSURE
The presence of the diffuser vanes therefore increases the magnitude of the total pressure. For Impeller R, the magnitude of the total pressure fluctuations varied widely between the cochlear sheath and the cochlear hub. Diffuser vane pressure measurements for the pump impeller of the SSME's HP OTP.
But even at the trailing edge of the blade, the fluctuations are still periodic with the passage frequency of the rotor blades. Spectra from the unsteady blade pressure measurements showed that the pressure fluctuations were periodic with the passage frequency of the impeller blades. This strong increase in blade pressure fluctuations near the shroud was not found for Impeller R.
The magnitude of the ensemble-averaged vane pressure fluctuations for Impeller R and for Impeller Zl is shown in Figure 6.18. On the trailing half of the diffuser vane, the fluctuations are also greater for Impeller R. The dashed pairs of lines indicate the passage of the pressure side and the suction side of the trailing edge of the impeller beyond the leading edge of the diffuser vane.
That smallest radial gap between the fan blade trailing edge and the diffuser fan leading edge is 5% of the fan blade trailing edge radius. It can be seen that the size of the fluctuations varies significantly during one fan revolution. In figure 6.25, the magnitude of the blade pressure fluctuations (at all three fan blade pressure taps) is versus.
However, it was found that the magnitude of the fluctuations on the pressure and suction side is very different. Dashed lines indicate the passage of the center of the trailing edge of the rotor blade past the leading edge of the diffuser blade. Turning now to the ensemble average measurements, we can see that the magnitude of the pressure fluctuations varies significantly during one revolution of the rotor.
The magnitude of the largest pressure fluctuations at the pressure tap on the blade pressure is ~ 0.2. Regardless of the specific diffuser, the largest fluctuations were found to occur at the fan blade trailing edge.
