Diesel engines used for the main power supplier of submarine normally run in high
back pressure and low intake pressure, causing unstable performances.
Furthermore, when a submarine runs under the sea the exhaust pipe of the diesel
engine is under the seawater. Once the lowest pressure in the exhaust pipe is
not sufficient to push all the water out, the water will flow into the exhaust
pipe and damage the diesel engine. Modeling can provide a useful guide for
designing diesel engines, intake and exhaust pipes, and turbocharging systems to
avoid water flowing into diesel engine. However, existing simulation methods
cannot well simulate the exhaust system of an underwater diesel engine, in which
the interface between the liquid water and the exhaust gas is variable. To
overcome the drawbacks of existing simulation methods in handling the variable
interface between the two phases, a variable interface finite volume method
(FVM) is proposed, and a corresponding model is developed in this work. This is
the major contribution of this work. A detailed model description and numerical
treatment of governing equations are given. The new model is validated using the
experiment conducted in this work on the procedure of gas pushing water in a
pipe. The validation results show that the variable interface FVM is effective
and reliable. Due to the complexity of the exhaust gas flow at the tailpipe,
three-dimensional (3D) flow at the exit of exhaust pipe under different exhaust
gas speeds is studied. Results show that, when the exhaust gas speed is below 20
m/s, after the bubble leaves the exit, a part of seawater will flow into the
exhaust pipe and flow down along the pipe wall under gravity. With the increase
in speed, this phenomenon disappears. Using the newly developed one-dimensional
(1D) and 3D model, the 16V-MTU396SE84 underwater diesel engine’s performance was
simulated under different back pressures. Also, the effect of silencer’s volume
on the stability of diesel engine’s exhaust system was studied. Simulation
results show that, with the increase in exhaust back pressure, the excess air
factor becomes smaller, combustion turns worse, combustion pressure and maximum
in-cylinder pressure become lower, the combustion temperature, maximum
temperature, and brake specific fuel consumption go up. In addition, silencer’s
volume is very important to the stability of engine performance. The bigger the
silencer, the more stable the exhaust system. The flow in the 16V-MTU396SE84
diesel engine’s exhaust pipe under the seawater was also calculated. Simulation
results are consistent with engine tests showing that when the engine runs under
full load the exhaust gas pressure and the pushing water speed in the exhaust
pipe are high, whereas in the part load, the exhaust gas pressure and the water
speed become a little lower. The correct results of these simulated performances
of underwater marine diesel engines indicate that the models newly developed in
this work are reliable.
