As mentioned earlier, the same cross-flow fan housing configuration used by Harloff and Wilson [13] was also built and tested in the Turbopropulsion Laboratory at NPS. The work at NPS included not only the free-delivery condition (i.e. ambient back pressure), but also off-design performance (back pressure greater than ambient). Off-design performance data were obtained by adding an extension to the exhaust duct with a butterfly throttle valve shown in Fig. 21[19] and [44]. One variation of the fan housing was also considered, specifically blanking off the two cavities.
Fig. 23 shows plots of total pressure ratio and isentropic compression efficiency as a function of corrected mass flow rate for rpm ranging from 2000 to 5000, taken from Yu et al. [19]. Due to facility constraints, the rpm range studied by NPS is much lower than the test conditions of Harloff and Wilson presented in Fig. 22. Also shown in the same figures are the performance curves without the butterfly throttle valve (denoted as open throttle), which were confirmed by the authors to be comparable to those measured by Harloff [41]. Note that adiabatic compression efficiency values above 70% were also obtained. In addition to studying the off-design performance of this cross-flow fan configuration, the research group at NPS also investigated the effects of modifying the two cavities. Their results showed that when the cavities are blanked-off, a significant gain in efficiency was obtained at the off-design (near stall) conditions at the expense of a small reduction in mass flow rate and total pressure ratio [42]. There was no discussion on the effect of the PVC on the size and location of the eccentric vortex during throttling in Ref. [42].