Design of a tip appendage for the control of tip leakage vortices in axial flow fans
Date
2021-03
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: The design of a tip appendage for the passive control of tip leakage vortices inaxial flow fans is investigated. A periodic three-dimensional numerical model of a1.542 m (casing) diameter subsonic axial flow fan, designed for use in air-cooledheat exchangers (ACHEs), is developed using the open-source CFD libary Open-FOAM (OF) v1806. The implemented meshing strategy, which uses a combina-tion of the cross-platform libraries cfMesh and SnappyHexMesh (SHM), is found to sufficiently validate against experimental data with a root-mean-square error(RMSE) of 3.8 Pa in total-to-static pressure rise, 129.6 W in fan shaft power consumption, and 1.5 percentage points in total-to-static efficiency for the datum fan case.
Steady and unsteady numerical computations are used to determine the effect of tip clearance on fan performance characteristics. An increase of 15.6 percent intotal-to-static pressure rise and a 5.1 percentage point increase in total-to-static efficiency is found with a two-thirds reduction in tip clearance at the fan’s design point flow rate. Normalized helicity contours are used in the visualization and quantification of the flow near the blade tip. The findings show a reduction in the relative tip leakage flow (TLF) velocity,WTLF, and tip leakage vortex (TLV) exit trajectory angle, βTLV, along the blade chord with a reduction in tip clearance.
These two flow characteristics are numerically investigated with the aim of con-trolling them through implementation of two tip appendage designs, these being a constant thickness (CT) end-plate and novel trailing edge (TE) end-plate de-sign. A numerical comparison between the two designs indicates that the new TE end-plate concept improves the fan performance characteristics through exhibiting control of the relative TLV exit trajectory angle. Consequently, this design is chosen with the aim of further improving the fan aerodynamic performance characteristics through optimization.
The Design and Analysis of Computer aided Experiments (DACE) method is used in the construction of the Kriging based surrogate model’s database. The resulting database is coupled with an Efficient Global Optimization (EGO) algorithm which completes the workflow of the multi-objective multi-point (MOMP) optimization process. The Pareto-front of non-dominated solutions is used to guide the optimal design selection, on which the experimental evaluations are based. The results of the optimized design indicate improved fan performance characteristics at greater than peak efficiency flow rates. This design is found to increase the datum fan’s design point performance characteristics by a value of 32.90 percent in total-to-static pressure rise and a 7.66 percentage point increase in total-to-static efficiency at the fan’s design speed of 722 rpm.
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
AFRIKAANSE OPSOMMING: Raadpleeg teks vir opsomming
Description
Thesis (PhD)--Stellenbosch University, 2021.
Keywords
Tip leakage flow, Axial fans, Heat exchangers, End-plates, UCTD, Air-cooled heat exchange