The Ducting module gives access to the basic Comflow® functions to calculate the flow and pressure field inside ducts of various geometry. The geometry can be defined using square, circular and triangular blockages. Components that are available in Comflow® are a.o. porous and perforated plates, perforated blocks, porous regions, tube bundles, axial and radial fans.
An example of a heat recovery steam generator (HRSG) is shown below. HRSG are quite large installations (often tens of meters high) in which the excess heat of some (large) gas flow is removed and used to generate steam. The hot gas enters in the picture at the bottom left and the cold gas leaves at the top (possible towards the stack). Water flowing through a set of tube bundles is heated up and converted to steam (this process is not modelled).
The gas flow rates inside an HRSG are often quite large while the total pressure drop over the apparatus is limited to a vety low value, often less than 10 mbars. Therefore, the dynamic pressure of the gas (1/2 r v2) is often in the same order as the pressure drop across the tube bundles. As a result, the distribution of the gas flow over the tubes can easily be quite non-uniform. This can lead to a reduction in the performance of the HRSG, but also to serious damage to the pipes due to local over-heating. Comflow® is a very good tool to assess the distribution of the flow over the pipe bundles and, if the distribution is nonuniform, to test possible solutions (baffles etc.) before they are installed.
In Comflow®, the geometry of the HRSG flow channel (duct) is generated either by 'drawing' rectangles, triangles and circles or by specifying there position and size numerically. The bundles are also inserted and the bundle geometry (tube arrangement, diameter and pitch) is entered. Comflow® treats the tube bundle as a porous but otherwise continuous zone and uses well-known empirical correlations (as given by for instance Idel'chik) to compute the resistance that a tube bundle exerts on the flow, based on the local flow conditions in the tube bundle.
The gas flow at the inlet is specified and at the outlet the pressure is fixed. As medium, exhaust gas is selected; this automatically takes care of the physical constants used in the model (gas density, viscosity etc.). Then the (2-dimensional) HRSG model is divided into a number of computational cells, 50x50 in this case. The CFD model looks like this.
After the CFD computation has converged (which takes about 2 minutes on a 90 MHz Pentium PC) the results can be analyzed. A very useful way of analyzing the computed flow pattern is by a vector plot in which the direction and magnitude of the velocity in each computational cell is drawn.
An alternative is the streamline plot which draws the trajectories (paths) of very small, very light objects in the flow.
In addition to the flow pattern, the (static) pressure contours can be plotted.
The pressure contour plot shows a lense-shaped high pressure region just before the first tube array, caused by the momentum of the inlet flow. This pressure build-up will (and does) cause flow maldistribution over at least the first couple of tube rows.
The flow distribution could be improved by adding e.g. a perforated plate before the tube bundles, but that would (have to) introduce an additional pressure drop that we cannot afford. Another way to improve the flow (apart from altering the general design) is by introducing baffles in the flow. Comflow® can be used to estimate the effect of such baffles and to optimize the number and position of the baffles.
A simple baffle arrangement and its influence on the flow field is shown below.
Of course, this is only an example case and the baffle arrangement could and should be optimized. Also, the calculation should be checked and, if at all possible validated with experimental data.
© 2008 Inudent
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