The Combustion Laboratory

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Underwater Propulsion
Investigator: Martin B. Linck


Future propulsion challenges associated with Navy vessels will increasingly deal with underwater propulsion. Underwater combustor systems, associated with torpedoes, missiles, small autonomous underwater vehicles (AUV’s) as well as other types of submersible systems are an area of particular interest. Swirl-stabilized combustion systems are versatile, efficient and reliable. A swirl-stabilized combustor has been investigated under conditions simulating sumbersion.

Underwater Combustor

The experimental section in operation, reacting flow condition. A swirl-stabilized methanol spray flame is visible in the combustor; and the exhaust jet is seen interacting with water in the mixing chamber. The water is not clearly visible due to the shutter speed used to obtain the image. The agitation of the water in the chamber is so violent that it is visible only as a white blur. Baffles and special trap stacks are installed in the mixing chamber to allow optical access. The interaction of the exhaust jet from the combustor with the incompressible water in the mixing chamber can be observed.

A schematic diagram of the test section for simulation of underwater propulsion. The rig consists of two chambers linked by a nozzle block. The upstream chamber functions as a combustor while the downstream structure functions as a mixing chamber.

Swirl Configuration

Schematic diagram of Injector and Fuel Nozzle.

The swirl number, S of a particular assembly can be obtained if the mean axial, radial, and tangential velocities are known as a function of radial location in the combustor. Here, v is the axial mean velocity, w is the tangential mean velocity, and r is the radial location. The above expression is simplified and omits a pressure 
The swirl number of a straight-vane swirl assembly can be approximated as shown in the equation above. Here, dh is the hub diameter of the swirler, and do is the outer diameter of the swirler. The swirl number is thus found to depend primarily on
q, the swirl vane angle.


Inner swirler with twisted vanes. The vanes have a twisted configuration, and the divergence of the vane surface from the axial direction increases with radius. This particular swirler features a 45° angle at the outer vane edge. The swirl number can be calculated using the same equation applied to straight-vane assemblies.
Injector with inner and outer swirlers installed. The outer swirl assembly is a straight-vane-type, featuring a vane angle of 50°.

Effect of Enclosure and Pressurization on Flame Structure

Unenclosed, enclosed, and pressurized flame. It was found that the flame structure depends heavily on the features of the fuel spray and the recirculation region. The flame is generally blue, with regions of orange luminosity near the widest point of the flame. The trailing edge of the flame is orange in color. Enclosed Flame images, showing rapid and slow exposures. The flame photograph of the enclosed, unpressurized flame shown at the left was obtained using a 0.125 s exposure duration. The flame photograph at the right shows the same flame, photographed using a 0.5 s exposure duration. The broad luminous region at right is due to low-frequency instability due to vortex precession.

Effect of Pressure on Swirling Air Flowfield

                                                      Unenclosed and Enclosed PIV Setups


Luminous Spray Flame

Spray Flame Flowfield Distribution

Second PIV Exposure

First PIV Exposure
                                  PIV Diagnostic Development for Luminous Spray Flame


Comparison of Contours of Axial velocity, V. The blue regions in flowfields, near the centerline (X = 0) represent the recirculation regions of the flow in each case. The maximum axial velocities measured in the unenclosed and enclosed, atmospheric-pressure case are near 10 m/s. In the pressurized case the velocities are much lower.The positive 
velocities seen near 
the origin of each 
flowfield are due to the 
effect of the atomization 
air stream. The 
atomization air 
emanating from the fuel 
nozzle is found to have a 
significant effect on the 
shape and structure of the 

Comparison of isothermal axial velocity contours and flame structure in
the unenclosed flame. The blue regions in the flowfield indicate regions of recirculation. The red regions indicate large positive axial velocities.
The structure of the flame near the base of the fuel nozzle is 
seen to correspond closely to the features of the non-reacting air flowfield. 
The flame is stabilized on the boundary of the recirculation region. The 
shape of the recirculation region is similar in the reacting and nonreacting 


Exhaust Jet Dynamic Behavior in a Two-Phase System

Schematics of Flush-Mounted and conical projecting nozzles. The nozzle on the left is flush-mounted with the baseplate of the mixing chamber, and does not project into the mixing chamber. The nozzle on the right features a tapered, conical extension that allowed it to project 40.6 mm into the mixing tank. The conical projecting nozzle was developed in order to examine the effect of nozzle geometry on entrainment and dynamic behavior of the exhaust jet.

Bubble emerging from flush-mounted nozzle. Combustor Pressure: 1 psig.

Bubble emerging from conical nozzle. Combustor Pressure: 1 psig.
Emergence of a bubble from the conical nozzle. 
The maximum diameter of a bubble is approximately
5*DE Combustor Pressure: 1 psig.
Bubble Emergence modes (left) and open-channel modes (right) 
observed at a chamber pressure of 15 psig. Open channel mode 
dominated, but collapsed often. During collapse, a bubble-formation 
mode occurred.


The equation for the Strouhal number, St, where f is the frequency of a periodic instability, d is the diameter of the jet, and Umean is the mean velocity of the jet at the inlet.



        •    The Strouhal number associated with bubble formation and collapse at 1 psig is approximately 0.002
              (vs. 0.2 for single phase gas/gas case)

•    The Strouhal number associated with large-scale events (bubble-fromation) in the jet at 15 psig is also approximately 0.002

The maximum diameter associated with large-scale structures at both pressures is 5 * D
The process of formation and collapse of large-scale structures is apparently buoyancy-driven. Swirl in the combusto air did 
              not affect the exhaust jet

     •    In the case of the high-pressure jet (15 psig), shockwave interaction with the liquid phase appears to affect the jet structure 
              and dynamic behavior

Additional Information

View the entire paper on Underwater Propulsion: Underwater_Propulsion.pdf


Last Edited: May 13, 2006