Effects of Hub Profiles on Efficiency and Flow of Jet Fan with Single Stage Impeller

The traditional jet fans have equipped with the tandem impellers and the exclusive motors, and such designs are asso ciated with the expensive initial cost. This serial research proposes how to simplify the fan profile for reducing the initial cost without the performance deteriorations. The paper discusses the effects of the hub profiles installed in the tradi tional/commercial jet fan, on the performances in the several type single-stage work. The hub with the long tail corn gives the best efficiency, and not only the stay vanes but also the cooling fins deteriorate more or less the performances, accompanying the impeller works as follows. The efficiency at the reverse rotation of the original impeller, namely at the flow condition running from the outlet to the inlet, is doubtlessly lower than one at the original rotation. The unique cascade, where the leading and the trailing edges of the blade are alternated in the tangential direction, was prepared in anticipation of improving the performances. These results advise the desirable profile of the jet fan equipp ed with the single-stage impeller, and the numerical simulation provides the optimum blade profile for the bidirectional flow.

• Jet Fan; Bidirectional Flow; Impeller; Initial Cost; Performance

1. Introduction

The jet fan installed in the driveway tunnel is required to exhaust the injurious gasses in the bi-directions, namely the inlet and/or the outlet, at the emergency such as fires, traffic accidents and so on. Then, the traditional jet fan is generally equipped with the two-stage impellers which are effective to one direction, and each impeller is driven by the exclusive and particular motor. The impeller profiles have been optimized by many researchers and engineers [1-4], but above assemblages make not only the dimensions large but also the initial cost expensive.

To overcome such weak points, this serial research proposes how to simplify the jet fan profile for reducing the initial cost without the performance deteriorations as possible. This paper discusses the effect of the hub profiles, installed in the traditional/commercial jet fan, on the performances, while the several type impellers are in the single-stage work.

2. Model Jet Fan and Experiments

2.1. Model Jet Fan

Figure 1 shows the model jet fan where the length of the casing is 1.129 m, the duct diameter is 0.304 m with the hub ratio 0.41. The model jet fan was designed so as to meet the downsized dimensions of the standardized jet fan with the bore 1.025 m [5]. The motor is, however, set on the outside of the casing and drives the single impeller through the pulley system with the torque meter and the tachometer because the large sized motor was provided for the experiments. The rotational speed of the impeller was kept constant at 1700 min⁻¹ by the inverter control. The flow is measured with the 3-holes pitot tube as the steady state conditions, at the Sections S1-S6 shown in Figure 1.

Three kinds of hub and stay vanes are prepared, the profiles and position of are shown in Figure 2, which have (a) the long tail corn with the half convergent angle 12 degrees (Long Tail Hub), (b) the short tail corn with the angle 24 degrees (Short Tail Hub) and (c) have not the tail corn (Suddenly Enlargement Hub). The cylindrical hub with the tail corn is supported by the three thin piano wires at two positions. Figure 3 shows the aircooling fin. The impeller and the stationary nose hub overhang the duct from the bearing box supported by the stay vanes at the inlet, while the nose hub profiles are the same as the tail hubs at the radius larger than the bearing box.

2.2. Impeller Profiles

Figure 4 shows the original blade profile of the model impeller composed of 8 blades with the tip diameter 0.302 m, which is appropriated from the traditional/ commercial jet fan with the tandem impellers. The blade element has no camber but is twisted to make the swirling velocity component constant in the radial direction. Three kinds of the impellers/cascades were prepared in

References

1. M. Nishi, K. Yoshida, K. Kojima and H. Ishii “Aerodynamic Performance of a 2-Stage Jet Fan (Usage of a Rotor with Symmetric Blades),” Journal of Society of Mechanical Engineers, No. 38, 2004, pp. 129-130.

2. S, Oba, “Development of the High Velocity Jet Fan,” Dengyosya Technical Review, Vol. 31, No. 1, 2007, pp. 9-13.

3. M. Johkoh, T, Nishioka and T. Kanno, “Development of High-Speed Low-Noise Jet Fan for Modern Tunnel Ventilation Systems,” Proceedings of 9th Asian International Conference on Fluid Machinery, Jeju, 16-19 October 2007, No. AICFM9-056.

4. H, Hayashi, “Effects of Blade Geometry on Flow and Noise Characteristics of Jet Fan,”Proceedings of 9th Asian International Conference on Fluid Machinery, Jeju, 16-19 October 2007, No. AICFM9-143.

5. K, Hayashi, Y. Niizeki and S. Itoh, “JF-1000 High-Velocity Jet Fan,” Toshiba Review, Vol. 61, No. 8, 2006, pp. 32-35.

6. T, Kanemoto, “Discussion of Numerical Flow Simulation for Counter-Rotating Cascades,” Vol. 1, IFFM Publishers, Gdansk, 2001.