Engineering Edge
The challenges faced by ThyssenKrupp Marine Systems, overcome by Flowmaster®
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By T. Zikofsky, ThyssenKrupp Marine Systems
Even today, building a submarine is still far removed from the design processes we know from the automotive engineering-scene. Partly on one hand, this is owing to the high number of integrated systems and parts and on the other hand because of the low numbers of boats ordered by a customer.
The combination of these facts and the very steady process in the defense-business makes it difficult for the engineers to improve systems during an ordered batch. Therefore a well developed 1D CFD tool is needed to acquire all the necessary information for the design process and to ensure the system behavior is defined before the first submarine is commissioned.
FloMASTER provides the functions that help engineers to find the best and most effective solution for pipe design and the needed power consumption of pumps. With this software you are able to calculate pressure drops, NPSH-values, flow rates, fluid velocities, oscillations etc. and furthermore, it is possible to calculate the heat transfer between solids and fluids (interaction).
This article outlines the challenges engineers are faced with in the design process of one of the most complex machines man can build. It will also provide insight as to how FloMASTER is used in the ThyssenKrupp Marine Systems submarine yard in Kiel, Germany.
The Conventional Submarine
A modern conventional submarine uses a non-nuclear air independent propulsion system (AIP) based on a diesel fuel cell combination. The tactical advantages of this system are very low heat and acoustic signatures combined with the ability to stay submerged for long periods. In 2013 the German Navy submarine U32 set a new world record by staying fully submerged longer than ever before, during its trip from Germany to the east coast of the USA.
The fluid systems of the modern HDW Class 212A and 214 Submarines are integrated with nearly 3000 valves and many kilometers of pipeline. These systems are divided into single sub-systems such as cooling seawater systems, fuel-cell systems, ballast system, freeing and compensating systems, fuel oil system, chilled water system, weapon compensating system, fire-fighting system, etc.
Of course, all systems have to be robust and reliable because the conditions at sea can be rough and sometimes hostile, even without battle scenarios.
Simulation of Cooling Systems
There are two simulation scenarios in submarine systems: 1. Checking the performance of an existing system and 2. Designing a completely new one.
The first way is the easiest because, in most cases, there are normally existing CAD-data or isometric drawings of the pipe routing so that the network can be built very quickly. The second way is more difficult because the final routing of the pipes is not clarified in detail. The engineer has to estimate the future routing and has to calculate possible failures. The timescale of a project makes it necessary to order the pumps a long time before the piping arrangement is finished, so the second way must be used.
Building up numeric networks is always a question of time and money. Usually a simple network will be chosen to shorten the time of getting a first solution.
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If one needs an exact solution or a lot of additional features, you have to invest much more time in describing the model and of course the time necessary for transient simulations will increase. In general, it is possible to generate a network according to the specification with a detailed scale of 100%.
At ThyssenKrupp Marine Systems the networks are used for:
- Calculating duty points of pumps
- Calculating NPSH-values;
- Simulating interferences during changes in rotating speeds of pumps (oscillations);
- Checking the thermo management of the system;
- Gathering all information of valve positions for the technical manual;
- Checking the flow speeds in the pipelines according to mechanical and acoustic boards; and
- Calculating the fill quantity of the pipe system to support the weight calculation of the submarine
All before the submarine is to be commissioned, necessary to save time and money.
System Complexity
Currently ThyssenKrupp Marine Systems simulates systems that use water, seawater, fuel oil, air, ideal gas, oxygen and hydrogen as working fluids. For this, and in order to use the internal pipe standard of different pipe diameters, materials, pump curves, pressure drops of valves, flaps and filters are saved in FloMASTER. This makes it easier to define a complex network and to set the necessary boundary conditions. In some networks more than 1,100 components are used.
Figure 3 shows a network with a large number of components. It is possible to calculate the required pump dimension and the fluid distribution in the different flow paths with this system. It is also possible to calculate the temperatures in the pipes because of the thermal heat duty in the heat exchangers. A special feature while using a transient solver is changing the opening position of single valves during the running simulation, so that one can see a live reaction of the system.
Transient Scenarios
In each operating condition of the submarine, the installed cooling pumps have to support a different flow rate. With FloMASTER it is possible to define single operating set-ups for use in parametric studies. Figure 4 shows an example of such a result plot of a transient interference analysis. According to the specification for this cruising condition, pump no. 1 has to provide a constant flow rate although the pumps no. 2, 3 and 4 are starting. It becomes obvious that with the current pipe configuration pump no.1 will not be able to provide the needed flow rate without increasing its rotating speed. With this information the required pump speed of pump no. 1 can be calculated and the best steering and automatic engineering concept for each cruising condition can be found.
Figure 5 shows a network of an exhaust gas system for the diesel generator during submerged cruising condition. Some pipes of the system are flushed with seawater at the exterior wall and some parts are isolated. With this configuration it is possible to calculate the heat exchanges between the pipelines and the sea and also to calculate the temperatures of the exhaust gas in the pipes.
Conclusion
Numerical system simulations in the design process of submarines are a good tool for engineers to find the best solution for a fluid or piping system layout within a minimal amount of time. FloMASTER is the tool of choice for ThyssenKrupp Marine Systems for system analysis. It is also possible to support the submarine crew to find the best system setting by calculating valve positions or pump rotating speeds offline. With the FloMASTER software and the SQL-database a large number of engineers can also work in parallel on a large project, improving efficiency and reducing the design cycle.