2.4. Performance analysis and discussion
We conclude this part of the article by reviewing the calculated results for our test case fan and investigate the effect of vortex motion during throttling to demonstrate one of the many challenging aspects of cross-flow fan analysis. Porter [35] showed that the size, strength, and position of the vortex depend on the fan operating point and housing design, greatly affecting fan stability. Two main types of behavior occur during throttling: (1) motion and expansion of the vortex along the impeller periphery, and (2) motion into the impeller interior. Porter showed experimentally that the peripheral path produces stable performance, and fans using a simple round nose vortex wall and logarithmic spiral rear wall exhibit such characteristics. In contrast, he showed that if the vortex propagates into the interior the fan performs poorly and becomes unstable. We will explore the favorable stabilizing case using the mean-line analysis and demonstrate the need for a fully coupled analysis for cross-flow fan prediction.
The test fan geometry is shown in Fig. 10, and its approximate flow region behavior is shown in Fig. 16 based on flow visualization (Fig. 9) and other data. Condition (1) is a high flow rate case where we focus our initial attention to correlate the analysis with the fan performance test data, and conditions (1), (2), (3), (4) and (5) cover the range of operation φ=0.35 to 1.08. Fig. 17 shows the calculated performance for the fan in relation to test data for the assumption of both constant and variable flow regions. Throughout the analysis the diffuser efficiency is fixed at 0.8 and the vortex power factor is 0.7. It can be seen that the calculated results for fixed flow regions (dashed lines) nicely match the test data at high flow rates. In terms of pressure coefficients this is a direct result of the cascade performance adjustments described in Section 2.2 (in this analysis pressure rise is entirely due to action in region A). However, the calculated power for region A alone, λA is significantly low relative to test, and vortex power dissipation must be incorporated and adjusted through the factor, kC. Consequently, overall correlation with test data for the fixed flow region case is accomplished by prescribing the region flow boundaries and then inferring the cascade characteristics and vortex power based on the modeling framework. The intent then is to use the adjusted model to examine variations in fan operation.