The Round Turbulent Jet Objective: The purpose of this experiment to study the behavior of turbulent air flow that exit from round pipe and to know the velocity profile of the jet that exits from that pipe. Introduction: The behavior of a jet as it mixes into the fluid which surrounding it has importance in many engineering applications. The exhaust from a gas turbine or cars is an obvious example. In this experiment we establish the shape of an air jet as it mixes in a turbulent manner with the surrounding air. If the Reynolds number of a jet is sufficiently small, the jet remains laminar for some length. In this case the mixing with the surrounding fluid is very slight, and the jet retains its identity. Laminar jets are important in certain fluidic applications, where a typical diameter of pipe may be equal to 1 mm. The schematic diagram of turbulent jet is shown s hown in Fig. (1).
Fig. (1)
The jet starts where fluid emerges uniformly at speed U from the end of a thin-walled tube of cross-sectional radius R, placed in the body of a large volume of surrounding fluid. The sharp velocity discontinuity at the
edge of the tube gives rise to an annular shear layer which almost immediately becomes turbulent. The width of a viscous layer increases in the downstream direction as shown in that diagram. For a short distance from the end of the tube the layer does not extend right across the jet, so that at section 1 there is a core of fluid moving with with the undisturbed velocity U at the inside. Further Further downstream the shear layer extends right across the jet and the velocity u o on the jet axis starts to fall as the mixing continues until ultimately the motion is completely dissipated. Description of Apparatus and Procedure: The round jet is produced by discharging air from the air box through a short tube as indicated in Fig.(2). The inlet of the tube is rounded to prevent separation so that a substantially uniform velocity distribution is produced at the tube tube exit. A traversing mechanism is supported on the tube so that a pitot tube may be brought to any desired position in the jet. Measurements are normally made in one plane.
Fig. (2)
The pitot tube is first brought into the plane of the exit of the jet tube and the scale readings are noted for which the axial position (X=0) and the radial position (r=0) are zero. The latter may be obtained by taking the average of the readings when the tube is set in line with one side and then
the other side of the tube. The pressure P o in the air box is then brought to a convenient value and traverses are made at various axial stations along the length of the jet. Theory: The atmospheric pressure at Duhok city is equal to 950 mbar, or o 2 p atm 95000 N / m , and the temperature of air inside the duct is equal to15 C, =
then the absolute temperature of air is equal to 288K. So the air density is can be calculated using the ideal gas law as follows: p " air ! R ! T …………………………………………………………..(1.1) =
Or: " =
96000 287 ! (15 + 273)
=
1.14kg / m 3 …………………………………………(1.2)
The total pressure can be measured at end of pipe by using fine Pitot tube and manometer as follows: ! H ……………………………………………………………(1.3) " P t
=
water
Where: H is the manometer pressure head in mm. The uniform velocity V in (m/s) at end of tube can be estimated as follows: P t
1 =
2
" ! V
2
……………………………………………………….……(1.4)
Then the Reynolds number at end section of the pipe is equal to: Re
V " D =
…………………………………………………………..….(1.5)
!
2
Where ! is the air kinematic viscosity (m /s). The pressure values (P o) at different (X) sections can be measured by using fine pitot tube and manometers as follows: P o
1 =
2
2
" ! vo ………………………………………………………….…(1.6)
While, the pressure values (P) at different (r) sections can be measured by using fine pitot tube and manometers as follows: P
1 =
2
" ! v
2
………………………………………………………….…(1.7)
Sample of Calculations:
Diameter of tube, D= 51.6 mm. Radius of tube, R=25.8 mm. -5
2
Kinematic viscosity, ! =1.48"10 m /s. ……….. Constant Total head, H = mm, Total pressure, P " ! H =
Uniform velocity, Then, V
2p =
P
=
water
1 =
2
" ! V
2
=
!
Readings and Results:
Table (1) Velocity distribution along a center line of the jet (X) . X# (mm) 0 50 100 150 200 250 300 350 400
h (mm)
p (N/m)
v
!
(m/s)
!
Notes: All elevations or pressure readings (h) must must subtracted from (100 mm).
Table (2) Velocity distribution at various (r) sections of the jet. r mm
X=75 mm ! h ! mm
o
0 10 20 30 40 50 60 70 80
X=150 mm ! h ! mm o
X=225 mm ! h ! mm o
X=300 mm ! h ! mm o
X=375 mm ! h ! mm o
Discussions: 1. At any value of (x) on the centerline, the velocity is starting to change along the jet axis? 2. Is the flow velocity increased or decreased as we keeping away from the tube end?
3. Show the relationship between x (mm) and velocity ratio
!
!
on (x-y)
diagram. 4. Draw the relationship between
! !!
and r (mm) on (x-y) diagram using
scale paper, for five (x) values. 5. Find the value of constant constant (c) if you know that it value value can be estimated from the following equation: v
c =
V
X
6. Estimate the value oijkl;f Reynolds number R at each (x) value. e
7. Find the values of (r) and (x) which makes the velocity ratio (
