360
R. Naren Shankar et al.
1 Introduction
Co-flowing jets (CFJ) finds application in various engineering devices. It is used
in fire fighting hoses to spray water to a greater distance, it is also used in exhaust
chimneys to transport polluted air to a greater height in welding torch to increase
the length of the jet torch and so on. In all these applications the surrounding jet
elongates the potential core of the inner jet. Co-flow jets are also used in aerospace
applications like injectors and exhaust nozzles. Co-flow is considered as a passive
control of the primary jet because jet can be controlled without any external power
source for example in high bypass turbofan engines the bypass jet (Secondary jet)
suppresses primary jet noise. In this case, chevrons are used to suppress the noise
which has its own drawbacks like manufacturing complexity. Tabs are also used
to control jets which creates a blockage in the flow that results in thrust loss [1],
hence co-flow by varying separation distance can be suggested as a vital alternative
compared to chevrons and tyres as passive controls.
Separation distance variation at low subsonic Mach numbers has been reported in
plenty in open literature as listed in Naren et al. [2]. Separation distance variation at
subsonic and sonic correctly expanded regimes have also been reported [2–7]. But
very few researches have performed sonic under expanded CFJ [8, 9]. Even in these
studies, the central jet is sonic under expanded and the surrounding jet is subsonic.
The characteristics of overexpanded co-flowing jet were studied by Sharma et al.
[10] and reported the increase in core length up to 104% when the jet is surrounded
by another jet. The jet exit characteristics of Mach 2 jet surrounded by a Mach 1.6
co-flowing jet are studied by De Satyajit and Rathakrishnan [11]. The present study
is the effort to analyse sonic under expanded CFJ in both primary and secondary jets
with varying separation distance which has not been reported in studied ever before.
2 Numerical Detail
The CFJ nozzle used in the current study has a primary convergent nozzle with inlet
diameter 20 mm and exit diameter 10 mm with a length of 35 mm. The secondary
duct is maintained with a constant width of 5 mm. The geometry is created in CATIA
V5 R18, meshed with ICEMCFD, and analysed in ANSYS CFX 16. A course grid
with 5.5 lakh nodes, medium grid with 11 lakh nodes and fine grid with 22 lakh
nodes were created for mesh independence study.
Based on the mesh independence study, medium mesh with 11 lakh nodes is
selected for the study. The simulation has been validated with the experiment and has
good agreement with experimental results [8] as shown in Fig. 1. A primary nozzle of
length 60 mm, 20 mm inner diameter and 10 mm outer diameter is used. Secondary
duct with a uniform width of 5 mm throughout the length is used. Separation distance
is maintained as 3 and 15 mm between the nozzle and the duct [2]. Mesh has been
imported to CFX and 0 atm. pressure has been given to ideal gas in fluid section. SST
R. Naren Shankar et al.
1 Introduction
Co-flowing jets (CFJ) finds application in various engineering devices. It is used
in fire fighting hoses to spray water to a greater distance, it is also used in exhaust
chimneys to transport polluted air to a greater height in welding torch to increase
the length of the jet torch and so on. In all these applications the surrounding jet
elongates the potential core of the inner jet. Co-flow jets are also used in aerospace
applications like injectors and exhaust nozzles. Co-flow is considered as a passive
control of the primary jet because jet can be controlled without any external power
source for example in high bypass turbofan engines the bypass jet (Secondary jet)
suppresses primary jet noise. In this case, chevrons are used to suppress the noise
which has its own drawbacks like manufacturing complexity. Tabs are also used
to control jets which creates a blockage in the flow that results in thrust loss [1],
hence co-flow by varying separation distance can be suggested as a vital alternative
compared to chevrons and tyres as passive controls.
Separation distance variation at low subsonic Mach numbers has been reported in
plenty in open literature as listed in Naren et al. [2]. Separation distance variation at
subsonic and sonic correctly expanded regimes have also been reported [2–7]. But
very few researches have performed sonic under expanded CFJ [8, 9]. Even in these
studies, the central jet is sonic under expanded and the surrounding jet is subsonic.
The characteristics of overexpanded co-flowing jet were studied by Sharma et al.
[10] and reported the increase in core length up to 104% when the jet is surrounded
by another jet. The jet exit characteristics of Mach 2 jet surrounded by a Mach 1.6
co-flowing jet are studied by De Satyajit and Rathakrishnan [11]. The present study
is the effort to analyse sonic under expanded CFJ in both primary and secondary jets
with varying separation distance which has not been reported in studied ever before.
2 Numerical Detail
The CFJ nozzle used in the current study has a primary convergent nozzle with inlet
diameter 20 mm and exit diameter 10 mm with a length of 35 mm. The secondary
duct is maintained with a constant width of 5 mm. The geometry is created in CATIA
V5 R18, meshed with ICEMCFD, and analysed in ANSYS CFX 16. A course grid
with 5.5 lakh nodes, medium grid with 11 lakh nodes and fine grid with 22 lakh
nodes were created for mesh independence study.
Based on the mesh independence study, medium mesh with 11 lakh nodes is
selected for the study. The simulation has been validated with the experiment and has
good agreement with experimental results [8] as shown in Fig. 1. A primary nozzle of
length 60 mm, 20 mm inner diameter and 10 mm outer diameter is used. Secondary
duct with a uniform width of 5 mm throughout the length is used. Separation distance
is maintained as 3 and 15 mm between the nozzle and the duct [2]. Mesh has been
imported to CFX and 0 atm. pressure has been given to ideal gas in fluid section. SST