Performance Testing of an Axial Flow Fan Designed for Air-Cooled Heat Exchanger Applications

Abstract

An axial flow fan developed in the previous study is tested in order to characterize its performance. The M-fan, a 7.3152 m diameter rotor only axial flow fan was designed to perform well under the challenging operating conditions encountered in air-cooled heat exchangers. Preliminary computational fluid dynamics (CFD) results obtained using an actuator disk model (ADM) as well as a periodic three dimensional model indicate that the fan meets the specified performance targets, with an expected total-to-static efficiency of 59.4% and a total-to-static pressure rise of 114.7 Pa at the operating point. Experimental tests are performed on the M-fan in order to determine its performance across a full range of flow rates. A range of fan configurations are tested in order to ascertain the effect of tip clearance, blade angle, and hub configuration on fan performance. Due to the lack of a suitable facility for testing a large diameter fan, a scaled 1.542 m diameter model is tested on the ISO 5801 type A fan test facility at Stellenbosch University. A Reynolds-averaged Navier–Stokes CFD model representing the M-fan in the test facility is also developed in order to provide additional insight into the flow field in the vicinity of the fan blades. The results of the CFD model will be validated using the experimental data obtained. Both the CFD results and the experimental data obtained are compared to the initial CFD results for the full scale fan, as obtained in the previous study, by means of fan scaling laws. Experimental data indicate that the M-fan does not meet the pressure requirement set out in the initial study at the design blade setting angle of 34 deg. Under these conditions, the M-fan attains a total-to-static pressure rise of 102.5 Pa and a total-to-static efficiency of 56.4%, running with a tip gap of 2 mm. Increasing the blade angle is shown to be a potential remedy, improving the total-to-static pressure rise and efficiency obtained at the operating point. The M-fan is also shown to be highly sensitive to increasing tip gap, with larger tip gaps substantially reducing fan performance. The losses due to tip gap are also shown to be overestimated by the CFD simulations. Both experimental and numerically obtained results indicate lower fan total-to-static efficiencies than obtained in the initial CFD study. Results indicate that the M-fan is suited to its intended application, however, it should be operated with a smaller tip gap than initially recommended and a larger blade setting angle. Hub configuration is also shown to have an influence on fan performance, potentially improving performance at low flow rates.

Design of a Tip Appendage for the Control of Tip Leakage Vortices in Axial Flow Fans

“…The differences in numerical and experimental results achieved in this study are comparable to that obtained by Corsini and Sheard (2007a) and Wilkinson et al (2019) who made use of open-source and commercial software packages, OF and ANSYS FLUENT, respectively. This suggests that the proposed open-source meshing strategy may be appropriate in modelling the scale M-fan.…”

“…This is in order to determine an acceptable variation in the numerical results across the evaluated range of volumetric flow rates. Table 4.1 compares the RM SE of the numerical results to the maximum standard deviation of the experimental test facility (as used by Wilkinson et al, 2019). 4.1, it can be seen that only the 6 mm numerical result lies within the experimental test facility's maximum standard deviation.…”

“…Table 4.2 compares the numerical and experimental result differences of the scale M-fan to various fans found in literature, evaluated at their respective design point flow rates. (Wilkinson et al, 2019) 4.0 mm 3.77 0.012 AC90/6 (Corsini and Sheard, 2007a) 3.5 mm 1.50 0.020…”

“…As such, it would make sense to likewise adjust the design point flow rate when varying the tip clearance to compensate for this change in diameter. The various design point flow rates (as determined in Appendix A) for the scale M-fan at various tip clearances are given in In order to illustrate this effect further, comparison is made between using a constant design point flow rate (as done by Wilkinson et al (2019) using the 14.57 m 3 /s design point flow rate of the geometrically similar 6 mm tip clearance case) as opposed to the current method of adjusting the flow rate when altering the blade tip diameter. The experimental results of the 4 mm tip clearance case using the two aforementioned methods are given in Table 5.2, with the geometrically similar 6 mm tip clearance case included for comparison.…”

“…However, during scale experimental testing the M-fan was found unable to meet the required design specifications. A possible cause for this could be related to tip clearance effects (as suggested by Wilkinson et al, 2019) as the M-fan was initially designed with a zero tip clearance and as such did not take tip clearance effects into account. As a result, the current study will focus on improving the performance of the M-fan through introduction of a design which considers the tip region.…”