Wind tunnel investigation of jet fan aerodynamics
Creators: name: Mutama, Kuda Ronald
Publisher: name: University of British Columbia
Year: 2009

description: This study has investigated the performance aerodynamics of jet fans in order to identify and understand the fundamental parameters in their use in mine and tunnel ventilation. Despite their advantages over other ventilation methods, jet fans have not been often used in mining due to an inability to predict performance accurately. They have been used in longitudinal ventilation of vehicle tunnels and other installations with encouraging results despite the fact that such systems have been designed with limited data. The current studies used a wind tunnel to study jet fan ventilation. The fan was simulated by aluminum pipes of different diameters connected to a centrifugal blower. The aluminum pipes were inserted at the entrance section of the wind tunnel and jet outlet velocities ranging between 20 and 40 m/s were used to produce the flow field. In order to study passage wall effects on the flow the jet fan was traversed from a near-wall position to the wind tunnel axis in equal successive steps. The axial pressure development for all positions were determined together with a detailed velocity distribution and the overall entrainment characteristics. Both the magnitudes of the pressure drop and rise depended on the jet fan position from the walls. Near-wall jet fan positions tended to have initially larger pressure drops and lower pressure rises than positions farther from the wall which had lower pressure drops and higher pressure rises. The consequence of this pressure variation was that generally at near wall positions the jet fan entrained more air into the tunnel than at positions farther from the wall. The smaller diameter jet fan produced higher friction losses (as much as 15 % at the wall position) than the larger diameter fan with lower outlet velocity which had about 8 %. The flow field was found to develop rapidly with axial distance. The jet axis velocity developed faster than that of a free jet of the same initial velocity and revealed that jet fans can move air over distances greater than 70 jet fan discharge diameters and still maintain a minimum air velocity of at least 0.5 m/s. For fan positioning at Fp &lt; 0.4, a region of backflow was identified. The backflow fraction was 0.72 and 0.55 for the smaller and larger diameter fan respectively. The performance parameter ξjf of the jet fan determined from pressure and flow (eturainment) ratio considerations QT / Qj (Ptn — Pe ) / (Pj — Ptn) was found to decrease as the jet fan was moved away from the tunnel wall despite higher friction losses at near-wall positions. The jet fan performance parameter is generally below 12 % as verified by mathematical derivations. The larger diameter (lower velocity, Uj = 21.4 mIs) jet fan had ξjf performance values almost twice that of the smaller diameter jet fan (Uj = 40 m/s). The ξjf value ranged between 4.5 to 6 % for the larger diameter fan. High entrainment ratios achieved at near wall positions generally improve jet fan performance. Theoretical equations based on momentum and energy considerations were formulated. These derivations identified a range of flow ratios (n = 0.1 to 0.9) which can be used to design an effective jet fan ventilation system. For each flow ratio (n) there is an optimum area ratio (α) for maximum induction of secondary flow. The present studies have established a procedure for jet fan performance analysis using wind tunnel investigations and have provided useful information for jet fan ventilation design.