FLOW AND MIXING CHARACTERISTICS OF
FREE AND CONFINED MULTI-JET
K. Tatsumi1*, E. Shinohara2, M. Tanaka2, P. L. Woodfield3, Y. Kitaoka1 and K. Nakabe1
1
Dept. of Mechanical Engineering and Science Kyoto University, Kyoto, Japan
2
Dept. Mechanical Engineering Osaka Prefecture University, Sakai, Japan
3
Research Center for Hydrogen Industrial Use and Storage, Fukuoka, Japan
Fluid mixing is one of the most essential phenomena in fluid mechanics and is appliedin numerous industrial applications concerning combustions, chemical reactions,
thermal controls, etc. More demands are inevitably rising for developing a mixing
technique which can produce sufficient and effective mixing in a more compact size.
For example, a high performance micro-combustor is one of the key technologies fordeveloping the micro gas turbine (MGT) well-known as the core part of the distributed
power supply system [1, 2]. For this combustor, a good mixing between fuel andoxidizer is required since a failure in having a pertinent mixing will incur both
deterioration of the system efficiency and increase of the emissions of unburned fuel,soot, CO, and NOx from the MGT.
The problem of significant deterioration of the fluid mixing performance is strongly
related to the decrease of flow Reynolds number accompanied by the reduction of thesystem size. Therefore, it is essential to develop an effective mixing method under such
low Reynolds number conditions. For such a purpose, the authors have proposed abaffle-plate-type-multi-jet mixer [2, 3]. In this combustor, as shown in Fig. 1(a), a baffle
plate, to which several nozzle holes are drilled, is introduced where the fuel and air aresupplied from the baffle plate through the nozzles. The air nozzles are circularly
arranged to surround the center-located fuel jet. The mixing performance has beenmarkedly improved in this system mainly due to the increase of the local Reynolds
number and interaction between the jets, and generation of flow recirculation in thedownstream area of the plate.
Three-dimensional numerical simulation solving the flow and concentration fields wasconducted in the present paper for this compact confined multi-jet mixer to evaluate themixing performance. The effects of the nozzle location and size, Reynolds number andvarious upstream conditions on the flow and mixing characteristics are discussed. In
addition to this, the effects of the swirl motion applied by tilting the surrounding jetholes in the circumferential direction, and of the buoyancy are also evaluated.
Furthermore, to have a better understanding of the interaction effects and mixing
mechanism of such multiple jets, the flow characteristics of multi-jet under unconfined(free) conditions, as shown in Fig. 1(b), are also evaluated experimentally [4]. The
velocity distributions using hot-wire anemometry and laser Doppler velocimetry (LDV),(a) Baffle-plate type confined multi-jet
Figure 1. Schematic view of mult-jet.

(b) Multi-free-jet* Corresponding author: Kazuya Tatsumi, phone +81-75-753-5209, fax +81-75-7535209, tatsumi@mbox.kudpc.kyoto-u.ac.jp

and flow visualization were carried out to discuss the effects of the jet flow rate,
external acoustic excitation, cyclic pulsation of the jet flow rate on the flow and mixing
characteristics.
In below are introduced the major results of these multi-jet under confined and freeconditions. Detail discussion will be made in the presentation.
In the confined multi-jet case, the changes of the nozzle-size and location produced alarge effect on the flow pattern and resulting mixing performance. A better mixing wasobtained under the conditions of larger distance or larger momentum difference between
the surrounding jets (air jets) and center jet (fuel jet). This was due to the fact that byincreasing these values, the pressure decreased at the downstream of the center jet, andflow circulations were formed in the area located between the air and center jets.
Enhancement of the mixing between the fuel and air was obtained in this circulation
zone. The results predicted for various upstream conditions also indicated that the baffle
plate could damp out undesirable disturbances included in the upstream flow.
By inclining the surrounding jet holes in the circumferential direction, a strong swirl
motion was obtained in the chamber. However, adding the swirl motion, which is atypical technique applied to large-scale combustors to improve the combustion
performance, incurred a deterioration of the mixing performance due to the interruptionof the radial flow near the baffle plate and increase of pressure near the chamber walls.
Buoyancy effect was small for the baffle plate case without swirl motion when its
operating direction was inline with the central axis. However, when a certain angle
existed between the gravity and central axes, an undesirable asymmetric flow andconcentration patterns appeared that may cause a hot spot and thermal fatigue at thechamber sidewall. Furthermore, buoyancy effect increased when swirl was applied.
On the other hand, in the unconfined free-multi-jet case, it was shown that theinteraction between each jet produced an early transition to turbulent flow comparedwith a single jet case and, hence, enhanced the mixing performance. When an acoustic
wave having an identical frequency to the resonance value of the jet was applied, anearlier transition of the jet from laminar to turbulent flow took place compared with thatin the case without sound, and an effective mixing performance was obtained. Moreover,a bifurcation phenomenon of the center jet was observed, i.e., the center jet was split
into two and then each split jet was entrained to the side jets at the downstream.
In the pulsating-jet case, in which the flow rate was periodically changed, a better
mixing was found in the vicinity of the nozzle exit in the pulsating-jet case evaluatedfrom the results of comparing the time-mean velocity pattern of a single jet with andwithout pulsating motion. On the contrary, further downstream at the location of x/d≤30,
an inverse phenomenon appeared; namely, the case without pulsation showed a better
mixing. This was believed to be related to the instantaneous characteristics of thepulsating jet. Since the Reynolds number of the tested jet was in the range of 1,500 and
was close to the transition Reynolds number, the jet varied between a laminar jet and afully developed turbulent jet in one cycle. When the time-mean value was considered,
the latter jet showed a larger influence in the region close to the jet and vice versa.
[1] K. Suzuki, H. Iwai, J.H. Kim, P.W. Li and K. Teshima, Proc. 12th Int. Heat Transfer
Conference, 583 (2002).
[2] H.S. Choi, Y. Katsumoto, K. Nakabe and K. Suzuki, Fluid Mechanics and Itsapplication Vol. 70, Eds. Polland, A. and Candel, S. 2001, p. 367.
[3] P.L. Woodfield, K. Nakabe and K. Suzuki, Int. J. Heat Mass Transfer, 46 14, 2655
(2003).
[4] K. Tatsumi, E. Shinohara, F. Okamoto, Y. Kitaoka and K. Nakabe, JSME Int. J. B
49-4, 959 (2006).

* Corresponding author: Kazuya Tatsumi, phone +81-75-753-5209, fax +81-75-7535209, tatsumi@mbox.kudpc.kyoto-u.ac.jp