The Transient module adds the capablitiy to simulate time-dependent processes using Comflow®. This functionality is added to the existing modules, so using the basic Ducting module with the Transient model, time-dependent flows can be modeled. Not only the flow rate but also inlet or wall temperatures can be specified as a function of time. With the Conjugated Heat Transfer model, transient heat flows through and to solid objects can be modeled.
The following case employs transient modeling and conjugate heat transfer to assess the temperature profile in an insulation layer. Hot exhaust gas (250 degrees Celcius) from a turbine flows through a cylindrical channel. The channel is isolated from the surroundings (winter temperature of minus 10 degrees Celcius) through a layer of isolation material and a thin (5 mm) metal sheet. An additional insulation layer is attached to the inside of the sheet in the first meter of the duct. This material thins out at the downstream edge to get a smooth transition between the sections. At three places, the sheet of steel is fitted to the insulation material by metal pins that extend 15 cm into the material.
As the turbine is started up, the temperature as well as the flow rate of the incoming stream increase. The time-dependent behaviour of the inlet flow and temperature was recorded and is given in the graph below. As the temperature of the flow increases, the temperature of the metal sheet will not increase at the same pace. Therefore, different parts of the sheet will have different temperatures. This will cause stresses in the material that will lead to metal fatique if the cycle is repeated often. To investigate the danger of metal fatigue, the temperature of the sheet was determined at two positions marked 1 and 2. In Comflow®, monitor points were used at these points to follow the temperature in time.
A model is created in Comflow® using the Conjugate Heat Transfer (CHT) module, also utilizing the transient flow module. The picture below shows a screenshot of the Comflow® model with insulation blocks (blue hashed areas), metal sheet (black and white hashed areas), inlet, outlet, wall condition (-10 degrees Celcius, at the top). The inlet temperature and flow are specified as time-dependent curves as given in the graph above. The model has 44 x 22 cells (coarse grid) and the simulation runs for 100 minutes (about 2 timesteps per minute).
As the changes in the flow velocity are very slow compared to the residence time of the model, the flow pattern remains fairly constant over time, only the value of the velocities increase in time. The flow pattern at the beginning of the simulation is shown in the figure below. It can be seen that the transition between the narrow and the wide section of the tube is indeed smooth, but that there is a considerable difference in velocity near the wall.
|Temperature difference profile|
The figure below gives the temperature difference between the points marked above as 1 and 2, from Comflow® and given data. As can be seen, the temperature profile predicted by Comflow® corresponds quite well with the data points, but gives much more detailed information. The model can now be used to optimise the geometry of the duct to decrease the peak temperature difference. This is left as an exercise for the reader.
© 2008 Inudent
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