1) Please reply with details about the Fluent model. Was it Transient or Steady State? If it was Steady State, then there are no oscillating pressures that would be relevant to a Harmonic Response analysis. If Fluent was a Transient solution, those results are not useful for a Harmonic Response. The Fluent Transient pressure load would be useful to drive a Transient Structural model.
2) All loads in a Harmonic Response are sinusoidally time-varying. You specify three items: Amplitude, Phase and Frequency for the load. I think the relevant load is a 20 Hz unbalanced fan/motor attached to the piping. You need to obtain the unbalanced force created by that mechanical system.
3) There are two methods to compute a Harmonic Response: MSUP and Full. MSUP uses a Modal analysis to decompose the problem to a much smaller model but that comes with limitations, such as the inability to apply both Frequency and Spatially varying loads. You can do that in Full, but I don't think you have the correct load in the first place.
4) In Workbench, if you link the Solution cell of a Modal analysis to the Setup cell of a Harmonic Response, you are doing MSUP. If you don't have that link, you are doing Full.
5) If you have only one load, then the phase angle is not relevant. The ANSYS Help page in the Mechanical APDL section 4.4 (link below) has a good example of when phase angle is important in Figure 4.3
Follow these instructions to use the link above on the Student license.