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The flow through rotating orifices is of interest to the designer of machine incorporating such features. For these systems, the internal air system subsequently often must pass through the rotating components. Excessive supply of internal air results in a degradation of the overall working efficiency of machines while a shortage of it may cause critical damage to the components due to overheating. The need to provide air circulation in rotating system as a method of cooling, chamber sealing and prevention of hot gas entering cavities in materials, has become an important matter in recent industries. Most of the research carried out in the past has been on stationary orifices and only recently has attention shifted to rotating systems. In this paper, the pumping effect of rotating orifices in preventing the seal leakage is investigated then compared to the stationary one. A test rig is designed and built to perform the experiments. The effect of various flow parameters is observed. The proposed method can assist designer to find optimize orifices configuration in preventing non-contact seal leakage.
Keywords: rotating orifices, stationary orifice, seal leakage, pumping effect
The flow through rotating orifices is of interest in rotating equipment such as turbines, compressors, centrifuges, pumps, motors, journal bearings, alternators, and generators. For these systems, the internal air system subsequently often must pass through the rotating components. The internal air system does not contribute directly to the power output instead it is used for important functions like rotor and stator cooling, blade and disc cooling, accessory unit cooling, bearing chamber sealing and prevention of hot gas ingestion into disc cavities . The alternator or motor generates considerable amount of heat and the internal air is used as the cooling medium. Excessive supply of internal air results in a degradation of the overall working efficiency of machines while a shortage of it may cause critical damage to the components due to overheating.
As for previous usage especially in turbines, the labyrinth seal has been used as the method. Nevertheless, the need for a better seal leakage prevention, with a better design, better efficiency, and compact shape has encouraged continuous studies to be done. For rotating orifices, due to the rotation of radial disc there will be a higher mass flow rate to pass through the orifices. This pumping effect theoretically will provide better non-contact seal since the higher mass flow rate is pumping to the seal area.
Most of the previous research concentrated on the flow characteristic over the orifice. The earliest study on rotating orifices was done by Meyfarth and Shine  which studied characteristics over the orifices in term of the tangential speed parameter S, which is the ratio of the tangential speed of the orifice and the axial velocity of the flow into the orifice. Idris et al.  investigated the flow within inclined rotating orifices and found that the most important parameter influencing the discharge coefficient is the angle of incidence. Idris and Pullen  investigated the pumping effect of the rotating orifice and its correlation with the discharge coefficient.
This paper is about investigating the pumping effect of rotating orifices in reducing seal leakage. Several flow parameters such the amount of inlet mass flow rate and the rotation speed of orifices is applied to give the best non contact seal. The result of the rotating orifice is also compared with the stationary orifices.
2. EXPERIMENTAL APPARATUS
A schematic drawing of test rig is shown in Figure 1. The main parts of the test rig includes radial rotating disc with orifice inserts, main shaft, fan, suction cone, discharge chamber, and also the 1520 mm extension pipe .
The air flow pumps into to the test rig by the fan near the inlet suction. The inlet suction has diameter of 100 mm. The extension pipe with 1520 mm long and diameter of 320 mm is used so that the discharge flow from the fan will be fully developed when it reaches the rotating orifices. The outlet section is divided through four of rubber hose having internal diameter of 25 mm, then all of the hoses connected to a discharge chamber with diameter of 100mm.
100 Dia. Swivel wheel
Radial rotating disc with inserts
100 Dia. Rigid wheel
M8 Bolt c/w Hex nut
M10 Bolt c/w s.w &Hex nut
Air discharge chamber
Figure 1: Schematic of the Test Rig
The radial rotating disc has outer diameter of 800 m, while the diameter of the housing is 814 mm, so the seal gap is 7 mm. Four orifices inserts are attached to the disc. The orifices inserts are replaceable so the various geometry of orifice can be used.
The radial rotating disc is connected to the motor through the main shaft that can withstand the rotation up to 15,000 rpm. The motor used is an AC motor with a rated power of 1100 Watt and rated speed of 2870 rev/min. Drive is transmitted to the main shaft via toothed gear and belt, wit a ratio of 4:1.
The air speed at the inlet and outlet is measured using anemometer. Tachometer is used to measure the speed of fan and the rotating radial disc.
3. DATA ANALYSIS
The derivations mass flow rates are based from some fluid mechanics equations, such as Bernoulliââ‚¬â„¢s equation, steady flow energy equation and discharge coefficient equation.
If the flow steady, irrotational, inviscid, and incompressible, isentropic flow for a stationary orifice is considered, the derivation of mass flow is based of Bernoulliââ‚¬â„¢s equation.
Figure 2: Flow through sharp-edge orifice
For the flow shown in Figure 2, along with the continuity equation
results in the mass flow equation :
Assuming V1 is small ( i.e A1 >> A2 from mass continuity equation)
Where A = cross sectional area of orifice.
4. ANALYSIS OF THE STATIONARY ORIFICE
The test is performed with passing the fluid through the test rig with stationary orifice. The orifice insert used has hole diameter of 7 mm with hole length 14 mm, and the inclination angle is zero degree. The fan speed of test rig is varied from 160 rpm to 837 rpm, then the mass flow rate at the inlet and outlet is investigated. Table 1 below shows the mass flow rate at the inlet and the outlet and the loss of mass flow rate for each speed of fan.
Table 1: Measured mass flow rate for stationary orifice
Mass flow rate (kg/h)
Figure 3 (a) shows the plot of mass flow rate at the inlet and the outlet for each speed, while the loss of mass flow rate is plotted on Figure 5 (b).
Figure 3: Measured mass flow rate (a) at inlet and outlet, (b) the loss
As seen on table 1, for the slower fan speed, i.e. 160 and 306 rpm, the loss of mass flow rate is about 2.5 ââ‚¬" 2.6 % of inlet mass flow rate, while for the fan speed of 451, 597, 742 and 837 rpm, the loss of mass flow rate increased become 5.7, 5.8, 6, and 5.3 % of inlet mass flow rate respectively.
5. ANALYSIS OF THE ROTATING ORIFICE
The effect of rotating orifice in preventing seal leakage then investigated. The orifice disc is rotated with the speed of motor of 15, 20, and 25 Volt. The fan speed is varied from 160 rpm to 887 rpm also. Table 2 below shows the measured mass flow rate at the inlet and outlet for each fan speed with rotating orifices. The loss of mass flow rate and the percentage to the inlet mass flow rate is also shown.
Table 2: Measured mass flow rate for rotating orifice
Orifice speed (V)
Mass flow rate (kg/h)
From table 2 can be seen that with the rotation of orifice, the inlet mass flow rate for fan speed of 160 rpm to 451 rpm has increased quite significant rather than the increasing of inlet mass flow rate for fan speed of 597 rpm to 887 rpm. It can be revealed that for the lower mass flow rate, the rotation of orifices gives effect to pull the higher mass flow rate into the system.
The curve plot showing the effect of the rotation of orifice to the loss mass flow rate for each fan speed is shown on Figure 4 below.
Figure 4: The effect of rotating orifice speed to the loss of mass flow rate for fan speed of (a) 160 rpm, (b) 306 rpm, (c) 451 rpm, (d) 597 rpm, (e) 742 rpm, and (f) 887 rpm.
As observed from Figure 4 above, the loss of mass flow rate for fan speed of 160, 306, and 451 rpm is increased with the rotation of orifice. While for fan speed of 597, 742, and 887 rpm, the loss of mass flow rate is decreased with the rotation of orifice.
The incoming air mostly goes to the rotating disc, hence the amount of the air through the seal will increasing, resulting the increasing of the mass flow leakage.
A test rig has been designed and built to perform experiments on rotating radial orifices. The purposes of this research are investigating the effect of rotating orifices in preventing seal leakage then comparing to the stationary orifices. The test is performed with various fan speed and radial disc speed. The mass flow rate at the inlet and the outlet is measured using the anemometer. The loss of mass flow rate is then calculated.
The result revealed that the loss of mass flow rate for the inlet mass flow rate lower than 42 kg/h is increased with the rotation of orifices. While for the inlet mass flow rate greater than 55 kg/h the loss of mass flow rate is decreasing with the rotation of orifices. Hence it can be concluded that the pumping effect of rotating orifices is effective in preventing seal leakage at the inlet mass flow rate greater than 55 kg/h. In other hand, for the inlet mass flow rate lower than 42 kg/h the stationary orifice give the best result in preventing seal leakage.