FLOW PROPERTIES IN AN AXIALLY ROTATING DIFFUSER

1

University of Ljubljana
Faculty of
Mechanical Engineering
FLOW PROPERTIES IN AN AXIALLY ROTATING DIFFUSER
Tom Bajcar, Brane irok, Marko Hocevar, Ferdinand
Trenc, Matej Novak
2

University of Ljubljana
Faculty of
Mechanical Engineering
Abstract

• Study of rotating diffuser flow properties
  through
• - analysis of kinematics (LDA)
• - analysis of heat transfer (IR camera).
• Significant kinematic pattern due to rotation.
• Heat transfer locally enhanced or inhibited due
  to rotation.
• Comparison with rotating straight pipes.

3

University of Ljubljana
Faculty of
Mechanical Engineering
Introduction
Figure 1b Rotating straight pipe.
Figure 1a Stationary axial diffuser.

• Axial pressure gradient ?p/?x
• Flow separation at sufficiently high values
  of the diffuser divergence angle (gt 12).

• Radial pressure gradient ?p/?r ? ?ut2/r
  
• Existence of a recirculation zone in the
  longitudinal axis of the flow at a sufficiently
  high rotation frequency
• Laminarization of the axial velocity profiles
  due to rotation
• Inhibited heat transfer between the inner pipe
  flow and the pipe wall.

4

University of Ljubljana
Faculty of
Mechanical Engineering
Kinematics experimental set-up
Figure 3 Diffuser.
Divergence angle ? 18º Length of the
conical part L 165 mm Inlet diameter di
60 mm Outlet diameter do 112.27 mm.
Figure 2 Experimental rig.
5

University of Ljubljana
Faculty of
Mechanical Engineering
Kinematics experimental set-up
Figure 5 Transverse sections (dimensions in mm).
Figure 4 Traverse lines (inlet and outlet of the
diffuser).
Table 1 Operating conditions.
6

University of Ljubljana
Faculty of
Mechanical Engineering
Kinematics - results
- Tangential and radial velocity components -

Figure 6 Distribution of the time averaged mean
tangential velocity ut at the diffuser outlet (f
52.8 Hz, Rei 5.84?103).
Figure 7 Distribution of the time averaged mean
radial and tangential velocities (in m/s)
outlet transverse section (f 52,8 Hz, Re 5.84
103).
7

University of Ljubljana
Faculty of
Mechanical Engineering
Kinematics - results
- Axial velocity components -
Figure 8 Distribution of the time averaged mean
axial velocity components at Rei 2.01?104 a)f
0 Hz b)f 52.8 Hz.
8

University of Ljubljana
Faculty of
Mechanical Engineering
Kinematics - results
- Flow visualization -
Figure 9 Flow visualization at Re 5.84 ?103.
9

University of Ljubljana
Faculty of
Mechanical Engineering
Thermodynamics experimental set-up
Figure 10 Experimental rig.
10

University of Ljubljana
Faculty of
Mechanical Engineering
Thermodynamics experimental set-up
Table 2 Operating conditions.
Figure 11 IR image of the diffuser.
11

University of Ljubljana
Faculty of
Mechanical Engineering
Thermodynamics - results
- Transverse section A -
Figure 12 Section A, Re 5.84103.
Figure 13 Section A, Re 2.01104.
12

University of Ljubljana
Faculty of
Mechanical Engineering
Thermodynamics - results
- Transverse section D -
Figure 14 Section D, Re 5.84103.
Figure 15 Section D, Re 2.01104.
13

University of Ljubljana
Faculty of
Mechanical Engineering
Thermodynamics - results
Re 1.17104
Figure 16 f 0 Hz.
Figure 17 f 52.8 Hz.
14

University of Ljubljana
Faculty of
Mechanical Engineering
Thermodynamics - results
Re 5.84103
Re 1.17104
Re 2.01104
Re 2.81104
Figure 15 Temperature profiles on the inner
diffuser wall surface.
15
University of Ljubljana
Faculty of
Mechanical Engineering
Conclusions

• Flow inside the rotating diffuser is divided
  into a rotating part (adjacent to the wall) and a
  non-rotating part (similar to the rotating
  pipes)
• All velocity components rise in the vicinity of
  the rotating wall, together with the axial
  velocity component (not common to the rotating
  pipes)
• No laminarization of the axial velocity
  components was observed in the rotating diffuser
  (not common to the rotating pipes)
• Rotation helps spreading the flow in the radial
  direction, thus preventing the flow separation
  from the wall (similar to the rotating pipes)
• Rotation obviously enhances heat transfer from
  the inner diffuser flow to the diffuser wall at
  sufficiently high rotating frequency to Re number
  ratios (not common to the rotating pipes) and
  generally inhibits it otherwise (similar to the
  rotating pipes).