Journal of Ship Research, Vol. 52, No. 4, December 2008, pp. 239–248

Journal of Ship Research Study on the Operating Process of an Underwater Diesel Engine Yi Cui, Yilun Zhu, and Kangyao Deng Education Ministry Key Laboratory for Power Machinery and Engineering, School of Mechanical and Power Engineering, Shanghai Jiaotong University

A steady state and transient simulation model for an underwater diesel engine system, including governor, diesel engine, and pipe systems after turbine, is developed and verified. A two-phase flow of exhaust gas and water at the tail pipe is studied with three-dimensional fluid dynamics calculation. A water flooding criterion for an underwater engine is also given by these models and related experiments. Safe operation ranges of the engine are also studied. The histories of relative engine speed, pressure, and Froude number of the tail pipe exhaust gas under starting and stopping processes are studied. The Froude number of the tail pipe exhaust gas is an oscillating phenomena when the engine is starting, which is likely to cause sea water to flow backward into the tail pipe. The opening of the tongue valve must be controlled according to engine back pressure during the stopping process to prevent sea water flooding on the one hand and high back pressure on the other. The underwater diesel engine operating control strategy can be given on the basis of the research work. Keywords: diesel engine; underwater; transient; simulation; water flooding

1. Introduction DIESEL ENGINES are widely used as power for vehicles and ships because of their high thermal efficiency and reliability. The diesel engine is also used in underwater power systems, such as conventional underwater power systems and closed-cycle diesel (CCD) systems (Angenendt et al. 2005, Hawley et al. 1998). In conventional underwater power systems, the diesel engine operates and generates electricity to charge the batteries when a submarine surfaces or cruises just below the surface of the water using a snorkel. In the cruise state using a snorkel, air is sucked into the diesel engine through the snorkel and the exhaust gas is discharged directly into the sea water through a silencer and tail pipe, which can reduce acoustic emissions and infrared signature levels. The operating process of an underwater diesel engine is different from that on the water surface in the following respects: (1) high and variational back pressure and (2) the risk of sea water back flow into the engine. The exhaust gas discharge port or valve is several meters below the surface and the surface level changes with waves. Therefore, the back pressure of the diesel engine exhaust Manuscript received at SNAME headquarters May 16, 2007; revised manuscript received November 20, 2007. DECEMBER 2008

system is high and changes with the surface condition. There exists the risk for sea water to flow into the diesel engine through the underwater discharge port and valve when the engine is running (Zhang et al. 1996, Zhang & Yu 1998). In order to ensure that an underwater diesel engine runs safely, the above-mentioned problems must be solved by experimental or analytical study. Simulation has been used for diesel engine research in an attempt to reduce lengthy and costly prototype testing, particularly for the large diesel engines utilized for marine propulsion (Hetet et al. 1999, Banisoleiman et al. 1993). Different types of simulation models have been developed depending on their objectives. Multidimensional computational fluid dynamics (CFD) models are used to provide a more complete understanding of flow and combustion processes in the cylinder and ducts. Cycle simulation models are used to predict engine behavior under steady and transient operating conditions, to optimize parameters and develop control strategies. The engine cycle simulation model consists of two main parts: an in-cylinder combustion model and a gas exchange and flow model. Two kinds of in-cylinder models, a single-zone model and a multizone model, are available for diesel engine cycle simulation. A single-zone combustion model treats the combustion chamber as a single control volume, and empirical heat release and heat transfer correlations are used to represent the in-

0022-4502/08/5204-0239$00.00/0

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cylinder processes (Watson et al. 1980, Woschni 1967). Phenomenological multizone models divide fuel spray into many pockets according to their positions; and the processes of each fuel pocket, such as the breakup of liquid fuel, evaporation of fuel droplets, air entrainment of fuel spray, ignition, combustion, and formation of emissions, are calculated along with the crank angle. Because multizone models have considered the distribution of temperature and fuel concentration in the combustion chamber, the model can predict not only performance but also emissions (Hiroyasu et al. 1983, Kuleshov 2005, Cui et al. 2001). Gas exchange and flow models for cycle simulation have been developed into two kinds of methods: the filling and empty method and the one-dimensional fluid flow method. With the filling and empty method one can easily and effectively simulate integrated performance, whereas with the one-dimensional fluid flow method one can study the wave function in intake and exhaust pipes and turbocharging systems. Although extensive diesel engine simulations have been carried out, little has been done on underwater diesel engines, especially considering water flooding. Zhang and Yu (1998) studied the minimum nonflooding speed of a submarine diesel engine and gave a nonflooding criterion by similar analysis and experimental data. In this paper, a hybrid cycle simulation model, developed with GT-POWER engine simulation software (2006), is used to study the operating processes of an underwater diesel engine. The hybrid cycle simulation model consists of a filling and empty engine model and a one-dimensional fluid flow model for a discharge pipe system. The cycle simulation model is verified by engine tests under steady state, starting, and switching processes. A two-phase CFD model for discharging exhaust gas through an underwater pipe is developed by STAR-CD software (2001). The exhaust gas and sea water flow pattern around the tail pipe port is investigated, and a sea water flooding criterion is given for the following underwater operating processes analysis. The underwater engine safe operating range under steady state is studied based on simulation and experimental results. The underwater starting and stopping processes are analyzed and it is found that the suitable tongue valve open and close strategy must be given to prevent sea water back flow into the engine on the one hand and extra-high back pressure on the other hand.

2. Simulation model The arrangement of an underwater diesel engine system is shown in Fig. 1. The end part of the tail pipe is vertical to reduce the submerged depth of the exhaust port. The running cycle of the diesel engine consists of following complicated chemical and physical events: fuel injection, fuel evaporation, air-fuel mixing, ignition and heat release, emission formation, and gas flow (Cui et al. 2001). In order to simulate these complicated processes, some simplification must be used. The filling and empty method is used to model engine cylinder, intake pipe, exhaust pipe, and silencer. Because the dimension of the rear pipe after the silencer is important for underwater engine operation, it is modeled with a one-dimensional method for cycle simulation and a CFD model for detailed two-phase flow pattern study. In order to study the starting, stopping, and transient processes caused by surface storms, Newton’s laws are used to describe the engine and turbocharger dynamics. The behavior of the electronic governor is described with a PID control equation. All these differential equations are solved with corresponding underwater boundary conditions, and the crank angle parameters and cyclebased performance can be obtained. 2.1. Diesel engine simulation model A simple double Wiebe function is used to describe the diesel engine heat release rate (Merker et al. 2006):

冉 冊

冋

1 dx = 共mp + 1兲 ⳯ 6.908 d␸ ␸p ⳯ 共␸ − ␸0兲mp e−6.908

冋

冉 冊

+ 共md + 1兲 ⳯ 6.908

1

␸p

共mp+1兲

共mp+1兲 共␸−␸0兲mp

冉 冊 1 ␸d

⳯ 共␸ − ␸0 − ␸␶兲md e−6.908

册

(1)

xp

共md+1兲

冉 冊 1

␸d

共md+1兲 共␸−␸0−␸␶兲md

兴xd

In this expression, xp, mp, ␸p, xd, md, and ␸d change with such operating conditions as speed, equivalent air-fuel ratio, and so

Nomenclature x ⳱ accumulated heat release fraction ␸ ⳱ engine crank angle (°CA-degree of crank angle) mp ⳱ premixed combustion index md ⳱ diffusion combustion index ␸p ⳱ premixed combustion duration (°CA) ␸p0 ⳱ premixed combustion duration at engine rated condition (°CA) ␸d ⳱ diffusion combustion duration (°CA) ␸d0 ⳱ diffusion combustion duration at engine rated condition (°CA) ␸0 ⳱ heat release starting angle (°CA) ␸␶ ⳱ advancing angle of premixed combustion (°CA) xp ⳱ heat release fraction of premixed combustion

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xd ⳱ heat release fraction of diffused combustion ␣ ⳱ in-cylinder stoichiometric air fuel ratio ␣0 ⳱ in-cylinder stoichiometric air fuel ratio at rated condition of engine ␶i ⳱ ignition delay (ms) n ⳱ engine speed (r/min) nTK ⳱ rotation speed of turbocharger (r/min) n0 ⳱ engine rated speed (r/min) T ⳱ in-cylinder temperature (K) p ⳱ in-cylinder pressure (bar) Je ⳱ moment inertia of engine shaft system (kgm2) JTK ⳱ moment inertia of turbocharger shaft (kgm2) Me ⳱ engine torque (Nm) Ml ⳱ engine load torque (Nm)

MT ⳱ turbine output torque (Nm) MK ⳱ compressor load torque (Nm) z(i) ⳱ fuel rack position at time point i (mm) e(i) ⳱ engine speed error at time i (r/min) ⌬t ⳱ time step in calculation (s) KP ⳱ proportional factor in PID controller KI ⳱ integration factor in PID controller KD ⳱ differential factor in PID controller Fr ⳱ Froude number ⳱ (v/√gD) Bo ⳱ Bond number ⳱ (␳gD2/␴) v ⳱ exhaust gas velocity (m/s) g ⳱ gravitational acceleration (m/s2) D ⳱ diameter of tail pipe (m) ␳ ⳱ water density (kg/m3) ␴ ⳱ surface tension of water (N/m) F ⳱ volume fraction

JOURNAL OF SHIP RESEARCH

Fig. 1

Arrangement of underwater diesel engine system

forth, which are expressed in empirical formulas obtained from a large number of experimental data (Watson & Marzouk 1977): xp = 1 − 0.99 Ⲑ ␣0.35␶i0.24

␸p = ␸p0 ␸d = ␸d0

冉冊 冉 冊冉冊 n n0

0.25

␣0 ␣

0.6

n n0

Fr =

(2) (3)

Bo =

v

公gD ␳gD2 ␴

(9)

(10)

0.5

(4)

Ignition delay is calculated with the following equation:

␶i = C1 + C2 exp

speed. Last, the amount of exhaust gas emitted to the cabin is also affected by the speed. Having a reasonable nonflooding minimum engine speed is beneficial for maneuverability, safety, and the environment in the cabin. Seawater flooding at the tail exhaust pipe is a very complicated two-phase flow process, which is affected by many factors, such as exhaust gas velocity, diameter of the tail pipe, water depth, surface tension, and viscosity of water and gas. According to a previous study, the gas-water two-phase flow pattern in similar situations is related to Froude and Bond number (Zhang et al. 1996, Zhang & Yu 1998, Valencia et al. 2004, Ousaka et al. 2006), which are defined as:

冉 冊

C3 −C p 4 T

(5)

where C1, C2, C3, and C4 are positive constants. The performance of the underwater diesel engine can be obtained with the above empirical combustion model, although the model cannot be used to study the exhaust emissions. The gas flow rates between volumes are calculated with thermodynamic nozzle flow rate formulas. The engine and turbocharger rotation speed are evaluated using Newton’s second law: dn 60 = 共M − Ml兲 dt 2␲Je e

(6)

60 dnTK = 共M − MK兲 dt 2␲JTK T

(7)

In this case, the Bond number is much greater than 1; therefore, the surface tension effects on the two-phase flow pattern can be omitted. As a result, the Froude number is used in this paper as a criterion to estimate the inception of flooding. In order to determine the Froude number for water flooding, a CFD model is developed and shown in Fig. 2. The dimensions of the model are constructed according to those of the submarine tail pipe and the operating condition using the snorkel. In the model, a pipe is linked to the bottom of a tank filled with water. The diameter of the pipe is 210 mm; the length, width, and height of the tank are 5 m, 2 m, and 4 m, respectively. The depth of water is given, and above the water surface is atmosphere. Exhaust gas flows from the pipe into the water tank at a given velocity. The model is a highly nonlinear, free surface, two-phase flow problem, and a commercial software STAR-CD is used to solve the problem.

Electronic governors are used in underwater diesel engines. A PID control algorithm is used in governor simulation (Gerstle & Merker 1998): z共i兲 = z共i − 1兲 + KP关e共i兲 − e共i − 1兲兴 + KI⌬te共i兲 KD + 关e共i兲 − 2e共i − 1兲 + e共i − 2兲兴 ⌬t

(8)

KP, KI, and KD are estimated according to governor parameters and engine test results provided by the manufacturer. 2.2. Model of exhaust gas tail pipe When a submarine cruises with a diesel engine just below the surface using a snorkel, the exhaust gas discharge port of the tail pipe is below the surface. The seawater will flow backward into the tail pipe and do harm to the engine when the engine speed is below a nonflooding minimum speed. Therefore, the nonflooding minimum speed is an important index for the submarine. Whether the submarine power system can be started or stopped with a single diesel engine is determined by the speed. The temporary settle speed and speed for malfunction are also determined by the DECEMBER 2008

Fig. 2

Two-phase flow STAR-CD model for tail pipe JOURNAL OF SHIP RESEARCH

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The volume of fluid (VOF) methodology is used in STAR-CD to solve the submerged gas injection problem. According to this, the interface between the liquid and gas phase is captured by the distribution of a scalar F, which is defined as the ratio of liquid volume over the total volume in a computational cell. The volume fraction F is obtained from the following transport equation: ⭸F + uជ ⭈ ⵜF = 0 ⭸t

(11)

In order to maintain the flow interface sharpness, equation (11) is solved using a higher-order compressive scheme, the compressive interface capturing scheme for arbitrary meshes (CICSAM). The physical properties in any given cell are either purely representative of one of the phases, or representative of a mixture of the phases, depending on the volume fraction values. The continuum surface force (CSF) model is used to model the force due to surface tension acting on the gas-liquid interface. The high Reynolds number k-␧ model modified for multiphase is used to describe turbulence in both phases. The model convergence is described in section 3. As the nonflooding criterion is established by the CFD model, the exhaust gas parameters at the tail pipe port, such as velocity, temperature, pressure, and Froude number, are calculated with a hybrid cycle simulation model. Because the exhaust gas and water two-phase flow in the tail pipe is complicated and could not be simulated directly with the hybrid cycle simulation model, the following assumptions are made: • The flow of gas and water in the tail pipe is one dimensional. • The phase change of water or gas is not considered. • The separation face between water and gas is a moving plane that is normal to the pipe axis. Based on these assumptions, one-dimensional Navier-Stokes equations are used to describe the exhaust gas and sea water flow in the tail pipe system. A staggered grid, finite volume method is used to solve these partial differential equations. The two-phase one-dimensional model of the tail pipe is integrated with the filling and empty model of the engine to perform the system cycle simulation. The established hybrid cycle simulation model is shown in Fig. 3.

3. Model validation The hybrid cycle simulation model must be validated before it is used to predict steady and transient underwater diesel engine performance. The regarded engine displacement is 63.3 L, and rated power is 1,000 kW at 1,800 r/min. The engine test rig has been established. With electronic controlled valves mounted after the exhaust silencer, the test rig is capable of simulating underwater operating conditions. The variation of controlled back pressure after the silencer is ±2 kPa. The engine test data and simulated results under different load and back pressure conditions (shown in Table 1) in rated speed are shown in Figs. 4 to 7. Operating conditions 1 to 4 are full load conditions with increasing back pressure. Operating conditions 5 to 7 are 75%, 50%, and 10% loads, respectively. It can be seen that the simulation results coincide with test data quite well. The histories of engine speed and fuel rack position during the starting process on the water surface

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are shown in Fig. 8. It can be seen that the control strategy of the electronic governor is a step function when the engine speed is very low, with the rack position decreasing with speed increasing. The engine speed can increase to rated speed in about 10 seconds with the speed regulation strategy on the water surface. The histories of engine speed and fuel rack position during the load acceptance process from 0 to 60% and then to 100% at rated speed are shown in Fig. 9. During the load acceptance process, the engine speed decreases first and then recovers under the function of the governor; the variation range of engine speed is within 5%. Through the comparison between measurement and calculated data under steady and transient processes, it is concluded that the cycle simulation model can predict steady and transient conditions very well. Numerical models used in the exhaust tail pipe CFD modeling are similar to those in papers of Valencia et al. (2004, 2006). Valencia has studied submerged gas injection into a liquid in a model of a copper converter using the VOF and standard k-␧ turbulence models implemented in the commercial solver Fluent. Experiments have also been carried out, and the experimental results have proved the predictability of the numerical model. In this paper, although the detailed two-phase flow patterns at the exhaust tail pipe have not been tested, the nonflooding Froude number has been obtained by experiments and hybrid cycle simulation. The nonflooding Froude number is 11.5 from experiments and 13.6 from CFD simulation, which proved the reliability of the exhaust tail pipe multiphase flow simulation.

4. Simulation and analysis of underwater engine performance 4.1. The criterion for water flooding As described in section 2, the gas-water two-phase flow pattern at the tail pipe outlet port is determined by the Froude number. The Froude number of the inception of flooding is given by twophase flow CFD simulation. Flow patterns at different Froude numbers are studied. It is found that the seawater enters into the pipe after the exhaust gas bubble leaves the port when the Froude number is less than 13.6. The seawater that entered into the tail pipe will flow down under the function of gravity. The flooding phenomena will disappear when the Froude number increases. Therefore, the Froude number 13.6 is a critical point for seawater flooding. Figure 10 is the flow pattern at the tail pipe port when the Froude number is 13.6, where light gray represents the gas phase and dark gray the liquid phase. Experiments have been carried out on the engine test stand to test the nonflooding minimum engine speed without load. The engine exhaust tail pipe has been submerged in a water pool. Because the water is static and the submerged length of tail pipe is short, the cooling effect of water is omitted. When the exhaust pipe port is 2.4 m under the surface, seawater flooding does not happen when the relative engine speed (speed/rated speed) is 0.72, whereas flooding will happen when the relative engine speed is 0.69. When the exhaust pipe port is 2.9 m under the surface, flooding will happen when the relative engine speed is 0.72, and flooding does not happen when the relative engine speed is 0.75. Cycle simulation for the engine system is also carried out to

JOURNAL OF SHIP RESEARCH

Fig. 3

GT-POWER hybrid cycle simulation model

study the experimental conditions and determine the exhaust gas velocity at the tail pipe port. It is found that the Froude number at the tail pipe port is 11.3 when the relative speed is 0.72 and port depth is 2.4 m, and the Froude number is 11.5 when the relative speed is 0.75 and port depth is 2.9 m.

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Comparing the CFD results with the results of cycle simulation and testing, it is found that the nonflooding Froude number is about 11.5 with tests and 13.6 with CFD simulation. From the view of safety, the Froude number value 13.6 is used as a criterion for nonflooding.

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Table 1

Operation conditions No. of conditions

Load (%) Back pressure after silencer (mbar)

1

2

3

4

5

6

7

100 1,165

100 1,365

100 1,565

100 1,672

75 1,585

50 1,528

10 1,480

Fig. 6

Fig. 4

Measured and calculated boost pressure

Fig. 7

Fig. 5

Measured and calculated exhaust temperature after turbine

Measured and calculated brake specific fuel consumption

Measured and calculated cylinder exhaust gas mean temperature

4.2. The nonflooding operation range under steady state The pipes of the engine exhaust system are cooled by seawater when running underwater. The cooling condition affects exhaust gas velocity greatly, which must be considered carefully in cycle simulation. The Froude number and temperature at the tail pipe port when submarine cruising with a single engine with no load under different conditions are simulated and shown in Table 2. In order to prevent flooding, the relative engine speed must be higher 244

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Fig. 8

Engine start process on water surface

than 0.94 when running with a single diesel engine without load. Cooling the exhaust gas with seawater will decrease the gas velocity and reduce noise level radiated from the pipe vent, but tends toward flooding. JOURNAL OF SHIP RESEARCH

Fig. 9

0–60%–100% loading process on water surface

4.3. Underwater engine starting process analysis Starting the engine under water is different from surface operation. The tongue valve at the tail pipe is closed before engine starting to prevent water flooding. If the tongue valve is opened too early when starting the engine, sea water will flood; on the contrary, if the tongue valve is opened too late when starting the engine, the back pressure is high, which makes the engine starting process difficult. Therefore, the control strategy and opening time of the tongue valve must be decided carefully to prevent water flooding and excessively high back pressure. The single engine starting processes with the exhaust tail pipe port 2.9 m under the water surface with different valve opening times are shown in Figs. 11, 12, and 13. The cooling effect of seawater has been considered in the calculation. In Figs. 11 to 13, the tongue valve was opened immediately when pressure before the valve reached 1.4, 1.6, and 1.8 bar, respectively. Under these control strategies, the tongue valve was opened at 3.2, 4.3, and 5.1 seconds, respectively, after engine starting. Shortly after the valve opening, pressures before the valve reached peak values of 1.67, 1.8, and 1.94 bar, respectively. The variations of Froude number at the tail pipe port are also shown in Figs. 11 to 13 with the limiting value of 13.6. The Froude number increases from zero to a high value and fluctuates for several seconds before reaching steady values in all three cases. All these starting processes can be divided into four phases, as shown in Fig. 14: (1) period before valve opens, (2) period from valve opening to water discharged from tail pipe, (3) period from water discharged from tail pipe to engine

Fig. 10 DECEMBER 2008

reaching rated speed, (4) period from engine reaching rated speed to reaching steady state. In phase 1 of the starting process, the increasing pressure before the opening of the valve is due to exhaust gas accumulated in the exhaust system. In phase 2, water is pushed out of the tail pipe by the pressure difference between exhaust gas and water. The water discharging rate increases as less water is left in the tail pipe. As water is drained out of the tail pipe in phase 3, the velocity of exhaust gas at the tail pipe port fluctuates for about 2.4 s because of the wave function of gas, and then increases with engine speed. In phase 4, as engine speed increases to rated speed, the speed governor of the engine acts on the fuel pump rack, and the fuel supply decreases quickly to nearly zero to prevent engine overspeed, which reduces the energy and velocity of the exhaust gas as a result. Under the function of the governor, the injected fuel mass then increases to its steady value; as a result, the velocity of exhaust gas increases also to its steady value. Although larger than 13.6 at steady state, the Froude number is less than 13.6 in some time windows during the engine starting process, which presents the possibility of water flooding. According to the calculation results shown in Figs. 11 to 13, the time windows for Froude numbers less than 13.6 cannot be eliminated by delaying the valve opening time. But the width of the time windows can be controlled with valve opening time. If the width of the time windows is reduced to a certain level and there is no time for the water counter flow from the external tongue valve or tail pipe port to the engine, the starting process will be safe. Delaying the valve opening time and having a sensitive speed governor will help to prevent water flooding. 4.4. Underwater engine stopping process analysis When the engine is stopping, no fuel is injected into the cylinders and the engine will continue rotating for several seconds with inertia. Although engine speed decreases gradually, the tail pipe back pressure drops abruptly because of the cutting of the fuel supply. The exhaust gas velocity and Froude number at the tail pipe port drop almost simultaneously with the back pressure dropping and the cutting off of the fuel, which is shown in Fig. 15. Because the Froude number drops so quickly below the nonflooding critical value of 13.6, the external tongue valve must be closed at the same time that the fuel supply is cut off. Figure 16 shows the histories of relative speed, back pressure before the tongue valve, and the Froude number at the tail pipe port after fuel is cut off and

Two-phase flow pattern at tail pipe port (Froude number = 13.6) JOURNAL OF SHIP RESEARCH

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Table 2

Froude number and temperature at tail pipe port Condition

1

Relative engine speed Load (%) Cooling condition Relative engine speed Load (%) Cooling condition Relative engine speed Load (%) Cooling condition

2

3

1 0 With 0.94 0 With 1 0 Without

Froude Number

Temperature (°C)

14.6

56.2

13.6

56.6

17.1

100

Fig. 13

Underwater starting process with valve opening timing 3

Fig. 11 Underwater starting process with valve opening timing 1

Fig. 14

Fig. 12 Underwater starting process with valve opening timing 2

the tongue valve is closed immediately at 0 s. The engine relative speed decreases gradually because of friction and pumping loss. The pressure before the tongue valve increases gradually and reaches a peak value of 3.15 bar at 12.7 s because of the exhaust gas accumulated in the exhaust system. After 12.7 s, the pressure in the exhaust pipe system decreases slowly. One reason is that the backward flow rate of gas from the exhaust system into the intake 246

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Four different phases during starting under water

system during valve overlap increases because of the high pressure difference and slow engine speed. The other reason is that the cooling effect increases along with the engine speed decreasing. After the closure of the tongue valve, the exhaust gas Froude number at the tail pipe port fluctuates around the zero point because of the compressibility of the retained exhaust gas in the tail pipe after the tongue valve. Excessive exhaust gas retained in the exhaust pipe system during engine stopping will leak to the cabin and harm the crew. In order to reduce the retained exhaust gas and pipe system pressure during engine stopping, the tongue valve control strategy is studied. As shown in Fig. 17, when exhaust gas pressure is higher than 1.4 bar, the tongue valve opens immediately; otherwise, the tongue valve closes immediately. When the tongue valve is closed, the exhaust gas velocity after the valve drops sharply and water flooding occurs, but the exhaust gas velocity after the tongue valve increases quickly and the Froude number exceeds 13.6 after the tongue valve opens again. Then water that previously entered into the tail pipe would be expelled out of the pipe by high exhaust gas velocity. If the time interval JOURNAL OF SHIP RESEARCH

Fig. 15

Engine stopping process with tongue valve opened Fig. 17

Engine stopping process with tongue valve control strategy

• The exhaust gas velocity and Froude number at the tail pipe port fluctuate during engine starting, which may result in water flooding. Delaying the valve opening time and having a sensitive speed governor will help prevent water flooding. • The tongue valve opening must be adjusted or closed at the same time that the fuel is cut off during the engine stopping process to prevent water flooding. • The tongue valve control strategy must be used to prevent water flooding and excessively high back pressure during engine stopping.

References Fig. 16 Engine stopping process with tongue valve closed at 0 s

between water flooding and discharging is short enough, sea water will not enter the engine. Therefore, the control strategy of the tongue valve must be specified carefully.

5. Conclusions A hybrid cycle simulation model for an underwater diesel engine and a real pipe system, a two-phase exhaust gas and water flow model in the tail pipe, are developed in this paper. The models are verified by test results and used for study of underwater diesel engine operation under steady and transient processes. The criterion for water flooding at the tail pipe is established with a limiting Froude number. The minimum operating speed without water flooding under steady state is given. The starting and stopping processes with different tongue valve control strategies are also studied. The following conclusions are reached: • In order to prevent water flooding, the minimum Froude number of the exhaust gas at the tail pipe port is 13.6 under the studied conditions. • Under the condition of a single engine with no load operation under a tail pipe port water depth of 2.9 m, the minimum relative speed is 0.94 for safe operation. DECEMBER 2008

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two-phase flow in inclined pipes, Nuclear Engineering and Design, 236, 1282–1292. STAR-CD 3.15. 2001 User manual, Computational Dynamics Limited. VALENCIA, A., PAREDES, R., ROSALES, M., GODOY, E., AND ORTEGA, J. 2004 Fluid dynamics of submerged gas injection into liquid in a model of copper converter, International Communications in Heat and Mass Transfer, 31, 1, 21–30. VALENCIA, A., ROSALES, M., PAREDES, R., LEON, C., AND MOYANO, A. 2006 Numerical and experimental investigation of the fluid dynamics in a Teniente type copper converter, International Communications in Heat and Mass Transfer, 33, 302–310. WATSON, N., AND MARZOUK, M. 1977 A Non-linear Digital Simulation of a Turbocharged Diesel Engine Under Transient Conditions, SAE 7710123.

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