1. INTRODUCTIONGENERAL ASSUMPTIONS

Nozzles and diffusers interchange fluid velocity and fluid static pressure. In a subsonic flow, a converging nozzle transforms a high-pressure, low-velocity flow into a high velocity, low-pressure jet. A subsonic diffuser transforms a high-velocity, low-pressure jet into a low-velocity, high-pressure flow. A venturi is the union of a nozzle and a diffuser.

The basic assumptions employed in the analysis of nozzles and diffusers in this chapter are:

1. The fluid is homogeneous and Newtonian; viscous shear stress is proportional to the applied velocity gradient.

2. The nozzle and diffuser walls are rigid.

3. The flow is steady.

4. There is no heat transfer either through solid boundaries or within the fluid itself.

5. The boundary surfaces are uniform and smoothly convergent for nozzles and uniform and smoothly divergent for diffusers. The change in flow area per unit axial distance is a small fraction of the flow area.

6. The entrance flow velocity is less than the speed of sound. Mach numbers above 1 are not considered.

The theoretical nozzle analysis in this chapter is based on adiabatic frictionless flow through an ideal nozzle. Since the frictional losses are often no more than a few

percent of the flow energy in a smoothly rounded nozzle at Reynolds numbers in excess of 104, this theoretical analysis can be an excellent approximation of the actual flow. The theoretical estimates of the nozzle flow can be corrected to incorporate frictional losses using experimentally)' measured coefficients.

Inviscid (i.e., frictionless) theoretical analysis is not generally adequate to predict diffuser flow. Diffuser flow is often strongly three dimensional owing to the growth and separation of boundary layers at the diffuser walls. Nonideal behavior such as separation, unsteady flow, and stall limits the diffuser performance. As a result, most diffuser design data are obtained experimentally, although advanced numerical viscous considerable promise.

1.1 THEORETICAL NOZZLE

A nozzle is a converging duct. The nozzle is to convert the potential energy differential in pressure between two points into the kinetic energy of fluid flow.

Nozzles consisting of smoothly rounded faces, such as shown in Fig. 1-1, can devices with discharges as high as 99% ideal in a high-Reynolds-number.

The flow through a nozzle, such as shown in fig1-1, is driven from the high pressure, the nozzle inlet toward the low static pressure at the minimum flow area of the nozzle.

Maximum Flow Rate. The maximum flow velocity at the nozzle throat is Mach I, the speed of sound. Decreases in pressure beyond the pressure at the nozzle exit required to produce sonic flow at the throat serve only to establish shock waves in the nozzle exit . Neither increases in the inlet pressure nor decreases in the outlet pressure can increase the velocity in the throat of a converging nozzle beyond the speed of sound. The maximum flow is limited by the speed of sound in the throat because pressure waves cannot be transmitted upstream through the nozzle, to increase inlet flow, any faster than the speed of sound. Once sonic velocity is established in the nozzle throat, all downstream parameters such as downstream pressure or downstream nozzle geometry are completely isolated from any influence on the nozzle inlet flow.

This can be seen by considering the mass flow rate through the nozzle,

(a)Trends are with axial distance in the direction of flow. Flow is adiabatic and frictionless (isentropic). The trends for velocity, pressure, temperature, Mach number, and density are reversed for supersonic flow. .

(b)Tends to limit of M =1

Application:. Nozzles can be used to compute the pressure, density, temperature, and mass flow at any point along a frictionless subsonic nozzle in terms of the reference isentropic stagnation values and one additional parameter to fix the flow rate. For example, if the nozzle forms an inlet from a large reservoir of static fluid, then the isentropic stagnation pressure is the static pressure in the reservoir p0. The isentropic stagnation density, temperature, and speed of sound are similarly the reservoir ambient values. If the nozzle outlet static pressure p is known, or postulated, the ratio p / po can be computed and the Mach number at the nozzle outlet can be immediately read. The outlet density, temperature, and velocity can be computed using the same line with the known stagnation values:

The area ratio A/A. is read from the same line . The Mach number and property ratios at any other nozzle area are found by the line corresponding to the area ratio

Since A*, like po, To, o and eo, will be constant through the nozzle. A is the outlet area, A1 is the nozzle area at the point of interest, and A* is the minimum nozzle area for

the mass flow at sonic velocity.

NOZZLE DISCHARGE COEFFICIENT

Definition. The actual flow rate through a nozzle seldom, if ever, equals the theoretical flow rate. Dissipation due to viscous friction and blockage of flow area by the boundary layer tend to reduce the flow rate below the theoretical ideal. A discharge coefficient, denoted by the symbol C, is introduced to correct the isentropic, one-dimensional theoretical flow rate:

Actual Mass Rate of Flow

C = ----------------------------------------

Theoretical Mass Rate of Flow

1.2 THEORETICAL DIFFUSER PERFORMANCEA diffuser is an expanding duct. The primary objective of a diffuser is to recover fluid static pressure from a fluid stream while reducing the flow velocity. The fluid slows as it passes through a diffuser, and a portion of the kinetic energy of the flow is converted into the potential energy of pressure. An efficient diffuser is one which converts the highest possible percentage of kinetic energy into pressure within a given restriction on diffuser length or expansion ratio.

In a diffuser, the pressure gradient opposes the flow . As a result the boundary layer in a diffuser decelerates and thickens rapidly, and it can separate from the diffuser walls to form large unsteady eddies that block the diffuser flow. The separation of flow from the diffuser walls is called diffuser stall, and it virtually always degrades the diffuser pressure recovery. Thus, the limit of diffuser performance is largely governed by boundary layer growth and the onset of stall.

Consider the control volume of figure ,which is bounded by the interior walls of the diffuser. The flow is steady and incompressible. Conservation of mass for this control volume with constant density,

UA= constant

Fig. Constant surface for a diffuser with a free discharge. The boundary layer is retarded at the diffuser walls. 1.3 VENTURI TUBES Venturi tubes are the union of nozzle and diffuser, as shown in figure, The purpose of the venture tube is to create a region of low static pressure at the venture throat which cab be used to draw in second fluid, as in a venture carburetor, or to generate a pressure differential between the throat static pressure and the static pressure in the contiguous pipe line, as in a venture flow meter.

The mass flow rate through a venture tube can be predicted by the same formulae for nozzles that are developed. For compressible flow the mass rate is

( 1 )

( 2 )

( 3 )

Three figures of venture tubes designs

Discharge Coefficients for Venturi Tube Flowmeters.Notation: D = diameter of contiguous pipe; d = throat diameter of venturi; U = average flow velocity through contiguous pipe; Ud = average velocity through throat; v = kinematic viscosity

2. LBS ACTUATOR

This actuator is the most powerful in our line. One version of this actuator can provide up to 600 pounds of traverse force. The one half inch probe ball screw actuator is used where a larger probe is required and when larger drive forces are needed on the probe. This actuator is rugged and stands up well under vibration and elevated temperature. The actuator can be fitted with a water cooled base for mounting in high temperature locations. Stepping motors, which have the advantage of being electrically noise free and being able to operate over a wide range of speed, can also be provided with this actuator.2.1 Typical Specifications: LBS ACTUATOR TRAVERSE Range: 6,12,16,20,24,30,36 inches

Force: 300 pounds (1334.40 N)

Speed: 5 inches/minute (127 mm/minute)

Hysteresis: 0.002 inches (.05 mm)

Linearity: 0.1% of full scale

Motor: Globe 166AI00-9, 1/12 hp. (D.C.)

Superior M063-FD06 (Stepping Motor)ANGLE

Range: 180, 360 degrees

Torque: 10 foot-pounds (13.50 Nm)

Speed: 20 seconds for 180 degrees

Hysteresis: 0.2 degrees

Linearity: 0.1% of full scale

Motor: Globe 100AI08-8, 1/30 hp. (D.C.)

Superior M063-FD06 (Stepping Motor)

GENERAL

Probe Diameter: 1/2 inch maximum (12.70 mm)

Mounting: 1.75 inch (44.45 mm) tubular base for

mounting in socket mount

Potentiometers: Ten turn, 1000 ohm, 0.1% linearity

Wiring: Teflon

Weight: 20 pounds (9.07 kg)

.

2.2 INSTALLATION:

The installation of the Actuator includes allotting mechanical space on the test rig sufficient to mount the Actuator, heat avoidance problems, and getting the proper electrical cables and transducers hooked up to the Actuator and probe. It should be determined whether there is sufficient space to mount the LBS Actuator to the test rig in the desired location. Outline diagrams of the Actuator are shown in D5015 and D5016B. The first diagram shows the top view clearance of the Actuator while the second shows the side views. Sometimes the Actuator is attached directly to the test rig and at other times it is mounted onto another Third Motion Actuator. If the latter is the case, then the Actuator base mounting hole pattern should be compared with the mounting hole pattern on the Third Motion Actuator for compatibility. The outline diagrams of the Actuator and Third Motion Actuator should be viewed to determine any potential mechanical interference between Actuator movement and test rig obstructions. The LBS Actuator mounts to the test rig by way of a mounting base. This base is shown in D5061. There is a clearance hole in the middle of the base flange for a 1/2" diameter probe. This hole should mount over a similar clearance hole in the test rig mounting surface. Four (4) 1/4" diameter mounting holes for the base are spaced at 90 intervals around the probe hole on a 2.750" diameter circle. The test rig mounting surface generally is drilled with four blind 1/4-20 or 1/4-28 tapped holes to accept the Actuator base. Actuator weight and mounting position might be a possible factor in mounting if the mounting surface on the test rig is not substantial. The weight of the LBS Actuator is 20 or more pounds. If the Actuator is mounted horizontally, the weight of the Actuator, probe, and transducers might cause a distortion in the test rig case. The installer should also check to make sure that the probe to be used is long enough to fit the test rig conditions and the Actuator. The probe will be required to go through the center of the LBS Actuator ball screw. Generally the probe length for an LBS Actuator must be a minimum of the "0" dimension on 05016B.2.3 Temperature Considerations for Installation

The LBS Actuator has a number of components built into it that are plastic, etc., in which temperature considerations should be taken into account if too high or Iowan environmental temperature is to be encountered. For temperature environments above 75 C, precautions will have to be taken to prevent high temperatures from effecting the Actuator. The motors and readout devices should not be subjected to environments above 75 C. Temperatures can be reduced on an Actuator by using deflectors to eliminate radiant heat buildup. If heat conduction from the test rig is a problem, a cooling spacer can be mounted between the test rig and the Actuator base. Water can be run through the spacer to carry away the excess heat. Water flow rates should be adjusted to keep the temperature below 170 F or 75 C.2.4 Electrical InstallationThe model LBS Actuator has one electrical cable to connect the Actuator to its operating control (or controls). This cable contains the traverse and angle motor drive and traverse and angle position readout circuits. If an Automatic Angle control or a TAC control is used, then a transducer extension cable and transducer valve extension cable are also used. The electrical cables should be routed from the control room to the Probe Actuator location on the test rig. These cables can be up to 100 or 200 feet long without causing any problems. Make sure the cables are not routed next to any hot surfaces on the test rig. Also make sure the cables do not have any stress on the end connectors as the Actuator motions move during a run.

Wiring diagrams in this manual show the standard Actuator wiring diagrams. Extension cables for connection to controllers are shown in the control operating manuals.2.5 Probe Installation

The probe is installed by inserting the probe through the ball screw that is incorporated into the vertical height of the Actuator. The probe tip exposed to the flow medium is inserted through the collet and the length of the screw. It emerges out the bottom of the Actuator by the base. The probe is held firmly in position by the probe collet. The probe collet is a split clamp collet. It is shown in D5214. This collet is made for 1/2" diameter probes. If a different diameter is to be used up to 1/2", then a special collet should be ordered.2.6 Calibration of the Actuator

Actuator calibration consists of checking the traverse and angle motion position verses their respective electronic readouts at the control.

Actuator position readout is usually picked up on the Actuator by a potentiometer or an encoder. Encoder devices are digital and have no positive mechanical stops. Potentiometers have precision ten mechanical turns. The ten turns are usually geared to the full scale motion of the Actuator angle or traverse motion. Any movement beyond the full scale motion will result in potentiometer damage.WARNING: When calibrating the Actuator motions, any change in the potentiometer mechanical settings should be done carefully to make sure that the potentiometer is not driven beyond its end points. Adjustable limit switch settings should be checked to limit range so that potentiometer damage cannot take place. Many Actuator angle motions have either mechanical counters or vernier dials to help calibrate the angle motion mechanical positions with the electrical. Standard angle motion ranges are: 500, 1020, 1800, or 3600. Check the Model number of the Actuator to see what the angle range is. Calibration procedure for the Actuator angle motion is outlined in the control manual. Actuator traverse motions usually have mechanical counters to help calibrate the Actuator mechanical and electrical readout. The traverse calibration procedure is outlined in its control manual. If the traverse gear box cover is disengaged from the traverse gearbox, care must be taken in returning the gearbox cover to the Actuator. The gearbox cover has the traverse limit switch assembly mounted on top of it. This limit switch assembly is coupled to the gear on the traverse potentiometer by way of a drive wheel with a pin extending from it. The pin engages a slot in the potentiometer drive gear. If the drive wheel on the limit switch assembly is turned while the cover is disengaged, or if the Actuator traverse motion has been moved while the cover is not in place, the traverse limit switch setting will have been changed. Traverse limit switch positions should then be checked and reset upon the replacement of the traverse gearbox cover. Failure to do this may result in permanent damage to the potentiometer.2.7 General Notes: Actuator parts are made out of both aluminum and steel components. Aluminum parts are made with a tempered alloy for machinability and strength. Most aluminum parts incorporated into the Actuator have been anodized. Steel parts are generally plated for protection against oxidation. Gears are made of stainless steel alloys. Worms in the motor drive gear trains are generally hardened steel to prevent excessive wear. Ball bearing screws are not normally lubricated. Lubrication of the ball screw will cause problems by attracting dirt. This will do more harm than non-lubrication. Actuator gearboxes are lubricated with a ROCOL MTS1000 grease. This grease is good over a temperature range from -300 C to + 1800 C. It might be necessary from time to time to clean old grease from the Actuator gearing and regrease. The time between regreasing the Actuator gearing is dependent on the temperature environment the Actuators are run in.

LBS Top View Clearance Installation - D5015

LBS Top View Clearance Installation ( Trav Only) - D5015A

LBS Side View Clearance Installation - D5016D

LBS Side View Clearance Installation ( Trav Only) - D5016A

LBS Actuator Mounting Base - D5061

LBS Actuator Standard Collet - D5214

3.0 WORK INSTRUCTIONS FOR CALIBRATION OF VENTURIMETER3.1 SCOPE:

This Work instructions details out the key processes, various procedure for calibration for Venturimeters up to 500mm of upstream diameter for low speed air flow applications. This work instructions also addresses the various controls exercised and the key measurement parameters involved in the calibration procedure.

3.2 PROCEDURE:3.2.1 Input required:

Manufacturing drawing of the venturimeter.

Venturimeter to be tested.

Work order from Commercial department, if applicable.

Visual inspection of the sample venturimeter for deciding worthiness of calibration as follows

Smoothness of surface and circularity of ducting.

Perpendicularity of surface pressure tapping nipples with respect to the axis.

3.2.2 Methodology Followed

This section covers the methodology followed for the calibration pf venturimeter and can be categorized as follows; Requirement of equipment.

Requirement of instrumentation and measurement parameters.

Calibration requirements of instrumentation.

Venturimeter Calibration set up preparation.

Venturimeter Calibration procedure.

Data recording and calculation details.

Report preparation.

Requirement of equipment Calibration of the venturimeter is carried in the low speed calibration test rig situated in Open Circuit Facility of Turbo machinery laboratory. The test rig includes a settling chamber, contraction cone and ducting in which flow is uniform. The air supply to the test rig is given by the centrifugal blower driven by a 50kw motor. Flow is controlled by inlet guide vane mechanism. Venturimeter is fitted at the down stream of the ducting. Suitable discharge ducting is fitted at the end of the venture meter for flow stabilization.Requirement of instrumentation and measurement parameters The following pre-calibrated instruments / items are required for the calibration

RTD with display unit.

Micromanometer ( 0-2000 mmwc range with 0.1 mm resolution )

Selector box.

Traversing mechanism with controller.

Three holed cylindrical / wedge probe.

Dry and wet bulb temperature.

Absolute pressure transducer ( 0-1200 mbar )

Calibration requirements of instrumentation All the instruments and measuring equipments listed in above are required to calibrated in a setup where traceability is maintained to the national or international standards.Venturimeter Calibration set up preparation

Venturimeter is to be inspected for any burrs or rust which will be cleaned by emery/filling.

Checking of pressure tap nipples at the upstream and venture throat for blockage and cleaning as needed.

Connection of Venturimeter in the ducting as detailed.

Setting up of traversing mechanism along with aerodynamic probe for flow measurement at the upstream of venturimeter.

Leak proof Connection of three pressure signals from the cylindrical probe and one static tap at the same traverse plane to the selector box using tubing.

Leak proof Connection of pressure signals from venturimeter upstream and throat to the selector box using tubing.

Leak proof Connection between selector box output port and micro manometer.

Setting up of RTD at Venturi up stream and connection to digital display.Venturimeter Calibration procedure Visual inspection of the pressure signal lines for leakage or blocks.

Start of blower for mechanical run and for system stabilization.

Run the blower at different speeds ( at least five ) by regulating the IGV and make the following measurements: Ambient pressure (Barometric pressure ) Dry and wet bulb temperatures. Wall static pressure at the probe location.

Three pressure signals from the probe.

Venturi upstream and throat pressures.

Venturi upstream flow temperature.

Recording of primary data as above and data analysis

Preparation of the detailed report.

Data recording and calculation details The primary data sheet is given and data shall be recorded in the format manually at present. The calculation details are programmed in MS Excel and the raw data is fed into the program for calculation.

Report preparation

The calibration report shall be issued as per the given format.3.3 Test Verification and Validation

The following methodology will be followed to verify the correctness of the test data:

The uncertainty will be calculated as per procedure.

The calibration constant K will be compared with previous data, if the size of the venturimeter is same and variation of the order of 5% will be accepted considering geometry variation and uncertainty. In case of non conformance, the test will be repeated with through checking of primary signals for leakage/blockage.

In case of new size of venturimeter , the test will be repeated three times and repeatability should be within 2%. Nonconformance will be dealt in the same manner as given above. The validation of the calibration results will be achieved for a repeatability of calibration factor within 2% and uncertainty within 2%.

3.4 Measure of Completion

The measure of completion would be the submission of the calibration report to the commercial departments as per the schedule given in the work order. Internal measure of the productivity shall be as per the procedure.

Fig.Calibration of Venturimeter with traverse mechanism

Test Results:

CALIBRATION OF VENTURIMETER

4. TESTING PROCEDURE4.1 The testing will be carried out as per the following steps: 1. The test-valve will be connected to the test setup.

2. The blower will be started with an IGV setting.

3. Checks will be made using liquid soap that no air leaks out of the gaskets or packings between the flanges.

4. The setting angles of the guide vanes at blower inlet will be adjusted such that the required air velocity is obtained at test valve inlet.

5. The pitot static probe shall be aligned with the flow. The measures with the probe shall be made at 8 locations, at each diameter as per BS Standard; perpendicular to each other across the section of measuring pipe.6. The average of the measured values shall be calculated.

7. The pitot-static probe will be set to the average reading point~

8. The differential pressure across the test valve will be measured using a micromanometer or water manometer as also the differential pressure of straight pipe at 8D location by using micromanometer or water manometer.

9. The total and static pressures from the pitot static probe will be measured.

10. The static pressure at valve inlet (P1) will be measured

11. The temperature of the air inside the testing pipe at points near the pitot-static probe and valve inlet will be measured.

12. Steps 4 through 11 will be repeated for dif ferent flap openings. Valve angle opening will be adjusted with increments of 5 degrees up to maximum valve opening angle.NOTE:

In case of disc fluttering, after opening the flap to obtain the required air flow rate, the spindle of the test valve may be held to stop fluttering at the particular position.4.2 CALCULATION OF VOLUME FLOW RATE OF AIR:The measurements from the pitot-static probe shall be used for calculating the air flow rate at each valve opening.

Calculation of air flow rate :

CONCLUSION:The Calibration of Venturimeter has done by the traversing mechanism. In the traverse mechanism with the help of probe and micromanometer the required parameters has measured. The Dry and Wet bulb Temperatures are required in the calculation of density and volume air flow. BIBILOGRAPHY:

Olson. A.T. , Nozzle Discharge Coefficients Compressible flow J. fluid eng

Weber, H . E. , Boundary layer Calculation for Analysis and Design J. fluids Engg.

Keith, T. G., and J.E.A.John, Calculated Orifice Plate Discharge Coefficients at low Reynolds numbers J. fluids Engg.

Head ,V.P., Improved Expansion Factors for Nozzle, Orifices and Variable Area Meters J. fluids Engg.

Jordon. D, and M.D. Mintz, Air tables. McGrew-Hill, New york, 1965.

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