2

Geometry at 50% span

Stator 1 Rotor 1 Stator 2 Rotor 2

Number of Blades Meridional Chord (in.) Aspect Ratio Solidity

Camber Angle (deg) Thickness/Chord (%)

24 32 28 32

16 21 21 18

The facility features a constant tip diameter of 19.18 in. (0.49 m), a hub/tip ratio of 0.739, and a nominal rotational speed of 2500 rpm. Figure 2.2 depicts the turbine blade row configuration, with a model of the actual turbine blades and cross sections shown in Figure 2.3. The design point predicted midspan static pressure distributions are presented in Figure 2.4.

Air at ambient temperature and pressure is drawn into the turbine inlet by a Novenco 800 centrifugal blower powered by a Marathon 100-hp (75-kW) 3565-rpm electric induction motor. The turbine inlet is an annular contraction with an area ratio of nine and a length-to-inlet diameter ratio of 0.75. Downstream of the turbine is an annular diffuser with an area ratio of 1.79. A 12-ft (3.7 m) length of 20-in. (0.5 m) diameter PVC pipe connects the turbine cascade to the blower. A Badger Meter, Inc. type BVT-IF 20-in. (0.5 m) diameter venturi flow meter is located just upstream of the blower inlet. At the blower inlet, air flow rates of 3,000-5,400 cfm (1.4 - 2.6 m3/s) are regulated by a variable inlet vane damper allowing blade row axial velocities ranging from 40-110 ft/sec (12.2-33.5 m/s) to be achieved. A Remote Controls, Inc. model RC40SR-60 90°

2.1.2 Main Air Flow System rotation spring return pneumatic actuator is used for damper vane actuation, controlled by a Kytronics, Inc. model DLU80 digital positioner. The Turbine rotational speed is monitored with a Honeywell model UDC 3000 process controller which closes the damper should the turbine rotational speed exceed 2,700 rpm. The blower provides a maximum suction of -2.4 psig (16.5 kPa) at blower inlet. Figure 2.5 schematically depicts the air flow system.

2.1.3 Turbine Loading System

Turbine loading is controlled by a Go-Power Systems model EDP-316 waterbrake dynamometer. At the highest turbine loading 40 hp (30 kW) is absorbed by the loading system at a steady torque of approximately 70 ft-lb (95 m-N) when operating at 2500-rpm. A Morse type HV chain drive transmits power from the turbine shaft to the water brake.

Turbine loading is regulated by control of the water flow rate through coarse-adjustment and fine-adjustment needle valves. Both the flow rate and pressure supplied to the water brake vary with control valve setting. As the current studies mandated tight speed regulation, a closed-cycle water system was installed to insulate the system from variations in water main pressure. A Gould model JS09 1-hp jet pump is used to supply pressures of 0-70 psig (0-482.6 kPa) at flow rates as high as 35 gpm (2.1 L/s) to the water brake. The pump draws from an 80 gallon (300 liter) supply reservoir. A bypass line allows a continual stream of water to be pulled through the pump even when the water break supply is highly restricted, thus preventing a pump motor thermal overload and unexpected pump shutdown. Such a water pump shutdown would result in uncontrollable turbine loading and possible turbine over-speed.

Because the water-brake performance is sensitive to exit pressure restriction, the discharge water must exit below the water brake to atmospheric pressure. To achieve this, a discharge pan is employed along with a scavenge pump to transfer discharge water to the supply reservoir. A schematic of the water system for the water brake is shown in Figure 2.6. The thermal energy added to the water system from the extracted turbine work must be removed to keep the system within the individual component temperature limits. As ambient convection is not sufficient, warm water is continually drained from the system and made-up with cool water from the laboratory mains.

2.2 Turbine Operation

Steady turbine operation is described by dimensionless work flow parameters that serve as set points and are quantified during turbine operation.

2.2.1 Performance Parameters

The turbine operating point is specified by the midspan values of the blade loading coefficient Y and flow coefficient O. These dimensionless parameters set the aerodynamic loading of the turbine stage:

The blade loading coefficient Y represents the specific work extracted from a turbine nondimensionalized by the square of the wheel speed. The work term is calculated from the difference in rotor inlet (1) and exit (2) tangential velocities at midspan (M). The turbine stage through-flow is represented by the flow coefficient, which is the ratio of the area-average axial velocity Ux at the inlet to the wheel speed. These dimensionless parameters form the turbine operating map and for ideal flow are related as the stage characteristic:

where P is the relative flow angle at the rotor blade inlet (1) and exit(2).

2.2.2 Performance Instrumentation

Figure 2.7 schematically diagrams the turbine performance instrumentation. Stagnation temperature measurements are made with three thermistor probes sensing midspan temperature at Stations 0, 2, and 4. The thermistor probes, Figure 2.8, are constructed from Omega model 44032 precision thermistors matched by the manufacturer to a standardized resistance-temperature curve to within ±0.2 °F (±0.2 °C) accuracy. Temperature is determined by first measuring the thermistor resistance via a voltage divider circuit, then determining the temperature from the standardized resistance-temperature curve.

Stagnation pressure measurements are made with a United Sensor model PAC-8-KI Pitot probe and two stagnation pressure probes that are aligned with the mean flow to sense midspan pressures at Stations 0, 2, and 4 respectively. The pressure probes are constructed of stainless steel tubing and connected by pneumatic tubing to a 5 psi (34.5 kPa) Scanivalve Corporation differential pressure transducer. The Scanivalve sensor is calibrated with a water manometer. Pitot and stagnation pressure probe geometries are depicted in Figure 2.9.

The volumetric flow rate measurements for determining the flow coefficients are made with a venturi flow meter. The following equation correlates the manufacturer's venturi flow calibration.

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