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CHAPTER 3

EXPERIMENTAL SETUP DETAILS

The experiments were conducted in the jet facility at the High

Speed Aerodynamics Laboratory, Indian Institute of Technology-Kanpur,

Kanpur, India. The schematic sketch of the jet laboratory is shown in

Figure 3.1 (a). Compressed dry air from the storage tanks is ducted to the

settling chamber through a pressure regulating valve. The stagnant air at

settled equilibrium in the settling chamber was expanded through C-D nozzle

to generate supersonic free jets. The slanted entry C-D nozzles studied in the

present investigation were located in the free jet, within the first cell so that at

the entry to the nozzle the flow is of uniform Mach number. A straight entry

nozzle was also tested in the same field for comparison. In addition to testing

in the free jet supersonic Mach number the straight entry nozzles were tested

by attaching them to the end plate of the settling chamber. For this case the

Mach number at the nozzle entry is incompressible subsonic with Mach

number such that the pressure at the entry is closer to the settling chamber

pressure. All the nozzles were provided with wall pressure taps all along the

length from inlet to exit with an interval of 3 mm each. Few views of one of

the nozzles with wall pressure taps is shown in Figure 3.2. In addition to the

wall pressure distribution the pressure distribution at the center point of the

nozzle exit plane was also measured to assess the pressure loss.

3.1 AIR SUPPLY SYSTEM

A two-stage reciprocating compressor, capable of delivering 360 ft3/min of air at a pressure of 500 psi is being used in this laboratory. The

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compressor is driven by a 150 hp, 3 phase induction motor. A cooling water circuit, driven by an independent pump, cools the compressed air through an inter-cooler. The compressed air is then passed through a pre-filter consisting of porous stone candles to remove contaminants like rust particles and oil droplets. An activated carbon filter is used for finer filtering. The compressed air is dried in a dual-tower semi-automatic silica gel dryer. While one tower is in use, a portion of the dried air is heated and used to reactivate the other. A diaphragm type back pressure valve operated by pressure relief pilot permits the dryer to operate at 500 psi, while the pressure in the storage tanks builds up from atmospheric to the required storage pressure. The compressed air is stored in three tanks, having total capacity of 3000 ft3. The tunnel control section includes a gate valve followed by a pressure regulating valve. The pressure regulating valve is connected to a mixing tube of 3 inch diameter and then to a settling chamber. The layout of the experimental jet facility laboratory is as shown in Figure 3.1 (a).

1. 150hp induction motor 2. Reciprocating compressor 3. Activated charcoal filter and Silica gel dryer units 4. Water cooling unit 5. Storage tanks

6. Gate valve 7. Pressure regulating valve 8 Setting chamber 9 Traversing system 10 Instrumentation desk

Figure 3.1(a) Layout of the Open Jet Facility

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3.2 OPEN JET TEST FACILITY

The experiments were conducted using an open jet facility which consists of a cylindrical settling chamber connected to high pressure storage tanks. Figure 3.1 (b) shows a schematic diagram of the open jet test facility. The air enters the settling chamber through the tunnel section with a gate valve followed by a pressure regulating valve and a mixing length tube of 3 in diameter. The settling chamber is connected to the mixing tube by a wide angle diffuser followed by three screens or closely meshed grids set 3 cm apart for minimizing turbulence at the nozzle inlet. The settling chamber has a constant area circular section of 300 mm inside diameter and 600 mm length. The settling chamber has tappings for stagnation pressure and temperature measurements. The test models are fixed at the end of the settling chamber by a slot holder arrangement, which is a short pipe like protrusion with embedded O-ring to prevent leakage. Model to be studied is placed over the O-ring, over which an annular retaining sleeve with internal threads is

screwed tightly. The settling chamber total pressure ( 0P ), which was the

controlling parameter in this investigation, was maintained constant during a run by controlling the pressure regulating valve. The stagnation pressure ( 0P )

level in the settling chamber gives the different nozzle pressure ratios (NPR), defined as the ratio of stagnation pressure ( 0P ) to the back pressure ( bP )

required for any study. The settling chamber temperature was almost same as the ambient temperature during the test runs and the back pressure is the ambient pressure into which the jets were discharged. The ambient temperature of the room was almost constant within ± 0.5ºC during one experimental run. The stagnation pressure was maintained with an accuracy of ± 0.1%. During the experimental runs, the settling chamber pressure was measured by a pressure transducer. The room temperature was measured by a thermometer. The day-to-day changes in ambient pressure, aP , were measured

by a mercury barometer placed in the laboratory and averaged over the duration of the experiment.

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Figure 3.1(b) Schematic Representation of the Open Jet Test Facility

Figure 3.2 Photographic View of Nozzle Attachment

3.3 INSTRUMENTATION FOR PRESSURE MEASUREMENT

3.3.1 Pressure Probe

A standard sharp edged pitot probe was used for pitot pressure

measurement in the supersonic streams delivered by the nozzles in the present

investigation. The accuracy of the pressure measured depends on the probe

shape, flow Reynolds number, the magnitude of transverse shear, turbulence

intensity and length scale, the orientation with respect to the mean flow

direction and the Mach number.

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A pitot tube having inner diameter 0.4 mm and outer diameter 0.6

mm was used in the present investigation. Schematic diagram of the pitot

probe and its attachment system is shown in Figure 3.3. The C-D nozzles used

were of exit diameter 10 mm. The corresponding ratio of exit diameter of the

nozzle to the probe outer diameter is 16.67. Thus a reasonable spatial

resolution 277.8 (>64) (ratio of nozzle exit area to the probe cross-sectional

area) was ensured. The pitot pressure measured in the supersonic regime is

the total pressure ( 02P ) behind the bow shock that stands ahead of the probe.

Thus, it is not the actual total pressure ( 01P ). If the actual total pressure is

required one has to correct for the pressure loss across the shock. Since the

supersonic jet core is wave dominated, the Mach number in the core varies

from point to point and also the shocks in different cells are of varying

strength. Therefore, no attempt is made to correct the measured total pressure

for shock loss. It has to be emphasized that, in supersonic regions there is

some measurement error due to probe interference with shock structure and so

the measured pressure data in supersonic regions should be considered only

qualitative and good enough for comparative study. In all the pressure

measurements the sensing probe stem was oriented parallel to y-axis with the

sensing probe facing the jet axis (x-axis).

Figure 3.3 Schematic Diagram for Pitot Probe

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The pitot probe is mounted on a three-dimensional traverse. The

traverse has six degrees of freedom, which also includes a probe-yawing

mechanism. The traverse has linear resolution of 0.1 mm in all the three-

dimensions i.e., thus the positioning accuracy of the probe was within 0.1

mm in all the three axes x, y and z. A view of the jet test facility along with

the traverse is shown in Figure 3.4.

Figure 3.4 A View of the Jet Facility Along with Traverse

3.4 PRESSURE TRANSDUCER AND APPLICATION

SOFTWARE

The pitot pressure sensed by the probe was measured using a

Pressure System Inc. (PSI) model 9016, 16-channel transducer (interfaced

with a Pentium 4 computer loaded with VI based software for data

acquisition) is shown in Figure 3.5. The model 9016 transducer is capable of

measuring pressures up to 300 psi, which is approximately 20 atm. The

accuracy of the transducer (after re-zero calibration) is specified to be

±0.15% full scale. Also, transducer off-set error was eliminated by performing

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a re-zero calibration prior to every run. The application software developed

using the Lab VIEW links the host computer to the pressure scanner via

TCP/IP communication. The application software performs all the required

functions like initialize, reset, re-zero calibration and read pressure.

Figure 3.5 PSI Model 9016 16- Channel Pressure Transducer

3.5 EXPERIMENTAL MODEL

A C-D nozzle with an area ratio 4 (i.e. AAe = 4) was used in the

present investigation. Based on one-dimensional theory the test C-D nozzle

has an exit Mach number of 2.94. The nozzle inlet diameter is 15 mm, throat

diameter is 5 mm and exit diameter is 10 mm. The convergent section length

is 24 mm and divergent portion is 46 mm long. Either sides of the wall 24

tapings (12 each side) are taken to record the static pressure. The tapings are

of inner diameter 1 mm and outer diameter 1.5 mm. Schematic diagram of

C-D nozzle with dimensions is given in Figure 3.6(a). The nozzle was

fabricated of aluminium and attached to flange of cast iron with the help of

bolts and the whole assembly was placed on the model holder of the settling

chamber. Experiments were carried out for different NPRs.

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The geometric details of the C-D nozzle kept in supersonic stream are shown in Figure 3.6 (b). The test C-D nozzle shown in Figure 3.6 (b) was provided with 23 static pressure ports that are equally-spaced in the axial direction. The row of ports starts at 2 mm from the inlet and ends at 2 mm before the nozzle exit with an interval of 3 mm each. The tap nearest the exit was used to indicate nozzle exit pressure. Without disturbing the pressure ports 15º, 30°, 45° and 57° cut were given at the inlet of the C-D nozzle. For each cut at inlet different runs were made by placing the C-D nozzle with its inlet in the uniform parallel flow issuing out of a Mach 2.0 Laval nozzle. The slant entry C-D nozzle was kept in the potential core region of a supersonic jet issuing from a properly contoured Laval nozzle to generate uniform Mach 2 flow. The Mach 2 Laval nozzle is of rectangular cross-section with 25 cm height and 15 cm width. To study the effect of freestream Mach number on the flow separation the 15° slanted entry nozzle alone is tested with Mach 1.6 and Mach 1.8 in addition to Mach 2 test. The C-D nozzle is placed very close to the exit plane of the Laval nozzle to ensure that the C-D nozzle is in the potential core region of the supersonic jet. Photographic views of nozzle are shown in Figures 3.6(c) and 3.6(d). The approximate location of the driven nozzle at the exit of the driving nozzle for a given slant angle α is shown in Figure 3.6(e).

Figure 3.6 (a) Schematic Diagram of C-D Nozzle Attached to Stagnant

Chamber

All dimensions are in mm

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Figure 3.6(b) Schematic Diagram of C-D Nozzle Kept in Supersonic

Stream

Figure 3.6 (c) Photographic Views of C-D Nozzle

3.6 EXPERIMENTAL PROCEDURE

For the investigation of flow separation pressure readings are taken

along the wall of the nozzle for different locations with the help of tapings

which are connected to the pressure transducer whose ports were connected to

a computer. Again for the calibration of Mach number, pressure readings are

taken at the exit with a pitot probe in order to compare the design and

delivered Mach number.

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Figure 3.6 (d) Photographic View of Slanted Entry C-D Nozzle Placed in Mach 2 Laval Nozzle

Figure 3.6 (e) Schematic of Model Setup Showing the Positions of Both

the Nozzles

3.7 SHADOWGRAPH SYSTEM Shadowgraph is suitable for visualizing the flow field where strong gradients exist. Supersonic and underexpanded jet flows produce large density gradients that lead to variations in optical refractive index of the light that passes through the supersonic flow. Shadowgraph system at the High Speed

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Aerodynamics Lab of Indian Institute of Technology-Kanpur was used in this study for visualizing the waves present in the jet flow field. The flow pattern was filmed using the shadowgraph system with a Helium spark arc light source in conjunction with 150 mm concave mirror. The concave mirror is mounted on a stand. The light source was collimated by the condenser lens and was then brought to the concave mirror. The parallel beam from the mirror was made to pass through the jet flow field and projected on the screen. The photographs of shadowgraph images of shock-train on the screen were taken directly by using a CCD camera. The surface finish of the mirror is λ/6 and the focal length is 1.6 m. The arrangement of mirror and light source are shown in Figure 3.7. When flow takes place through the flow field the light beam will be refracted wherever there is a density gradient. However, if the density gradient everywhere in the flow field is constant, all light rays would deflect by the same amount, and there would be no change in the illumination of the picture on the screen. Only when there is a gradient in density gradient there will be tendency for light rays to converge or diverge. In other words, the variations in illumination of the picture on the screen are proportional to the second derivative of the density.

Figure 3.7 Schematic Diagram for Shadowgraph System

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3.8 DATA ACCURACY The room temperature was almost constant with a maximum variation of ± 0.5º C during one experimental run. The stagnation pressure was maintained with an accuracy of ± 0.1%. The uncertainty in the measurement of pitot pressure was estimated to be ± 1.8% and that of probe position was ± 3.5%. The nozzle dimensions were accurate within ± 1.6%. All the measurements were found to be repeatable within ± 3.0%. The maximum uncertainty involved in the calculation of local Mach number M was estimated to be ± 3.5%.