In order to clarify the effect of rotor inlet geometry of half-ducted propeller fan on performance and velocity fields at rotor outlet, the experimental investigation was carried out using a hotwire anemometer. Three types of inlet geometry were tested. The first type is the one that the rotor blade tip is fully covered by a casing. The second is that the front one-third part of blade tip is opened and the rest is covered. The third is that the front two-thirds are opened and the rest is covered. Fan test and internal flow measurement at rotor outlet were conducted about three types of inlet geometry. At the internal flow measurement, a single slant hotwire probe was used and a periodical multisampling technique was adopted to obtain the three-dimensional velocity distributions. From the results of fan test, the pressure-rise characteristic drops at high flowrate region and the stall point shifts to high flowrate region, when the opened area of blade tip increases. From the results of velocity distributions at rotor outlet, the region with high axial velocity moves to radial inwards, the circumferential velocity near blade tip becomes high, and the flow field turns to radial outward, when the opened area increases. 1. Introduction A lot of axial fans, which are small size and low pressure rise, are used in our daily life. Some common examples are a room ventilation fan, a radiator fan in car engine room, a cooling fan of outdoor unit of a room air conditioner, a power unit cooling fan of personal computer, and so on. They are classified into several types using the relationship of relative location between rotor blade raw and outer casing. The authors call these classified types as follows: they are ducted type, half-ducted type, semiopen type, and open type, as shown in Figure 1. The semiopen type fan, which is test fan type, has the outer casing only rear part of rotor blade tip region. In cases of semiopen type fan, inflow direction is axial and radial, that is, a part of blade tip works as blade leading edge. This inflow pattern causes complex flow at rotor inlet. Figure 1: Types of small axial fan and image of inlet flow pattern. It is well known that the vortices in fans have a large effect on fan performance and its noise level. Therefore, many researches [1–7] for the vortices in fans have been conducted. However, these researches are mainly archived for the ducted type fans, while for other types of fans there are few researches. There are studies for low-pressure axial fans as follows. For the room ventilation fan, Fukano et al. [8, 9] have
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