Vision of tunnel
1 March 2012
The simulation of these systems of ventilation of the tunnels turns into all a challenge because of the complex physical phenomena involved in the process of design, as well as to the big dimension that can have the tunnels to design that it can reach several kilometres. In comparison with the results derived of the physical proofs, the computational dynamics of fluent (CFD) is able to simulate with big precision patterns of flow and falls of pressesure in tunnels of any length. Zitron Spain, specialist in the design and construction of systems of ventilation of tunnels that guarantee the hygiene against fires and the quality of the air for travellers and personnel of maintenance, has a wide experience in CFD. This experience contributes him a factor diferenciador has allowed him win cientos of agreements of ventilation and design of tunnels of renown along his 45 years of experience including projects such as the one of the Tunel of Guadarrama in Spain, the one of Saint Gottardo in Switzerland and of Rennsteig in Germany. Zitron Also has participated in more than 100 stations of metre of Madrid with systems of ventilation of emergency.
The simulation, a solution to the complexity of the system
Because of the complexity of said systems, results however, difficult and costly build maquetas to scale on which realise proofs that evaluate the potential answer of the designs of the systems of ventilation because the system of ventilation can exert an impact on the transversal section, form, alineación, number and size of the devices of ventilation of the tunnel and especially the work of civil engineering required for his construction. The simulation however, allows to evaluate the performance of alternative configurations and determine how have to use the system, so much in normal conditions as under emergency, all this in lower time and to lower cost that the physical proofs.
For the simulation, Zitron uses the software Ansys Fluent CFD because it provides a rank excepcionalmente wide of physical models, necessary to be able to predecir with precision the performance of the system of ventilation of tunnels. It is more, the solution can escalarse with the nodules of necessary process in function of the different magnitudes. It does a decade, the first application by part of the company required simply the estimate of the fall of pressesure through the circuit of ventilation. Later, Zitron evolved quickly to realise simulation of tunnels to scale natural of until several cientos of metres of length. The researchers validated said studies with measures of speed to scale natural in a lot of tunnels of roads, metre and railway industry. The initial results of the simulation showed totally in accordance with the measurements of field, asi that Zitron uses at present the software of simulation of dynamics of fluids like main tool of design and to be able to obtain perspective on the performance of the systems of ventilation proposed. This has provided to Zitron a competitive advantage to strengthen his position of leader in the sector of the simulation of tunnels.
The set up of the simulation
Zitron Uses also simulation of fires to evaluate the quality of the longitudinal ventilation. The engineers define a model of propagation of fires inside the tunnel in a strategically chosen location to take into account the conditions in the most adverse case. A number of longitudinal fans, situated in series along the tunnel, activate with a retardo of 60 seconds from the start of the ignición of the fire. The simulation allows predecir the capacity of the fans to direct the smoke to one of the exits. The tunnel has to be free of smoke in a period of determinate time, usually of around 600 seconds. Other key results of the simulation are the maps of temperature and distribution of the speeds along the tunnel.
The geometries of a tunnel are used to to be quite standardised, with a transversal area constant. The position and capacity of the fans, as well as the location and size of the possible fire are the main variables in the design. Like the software CAD of Zitron finds integrated cheese cheese with the platform Workbench of Ansys, any change in the model of design updates automatically also in the model of simulation. The software of simulation of engineering Workbench facilitates to the engineers of Zitron the realisation of parametric simulations, consistent in a series of traced with different values for one or more parameters of design, as for example the number of fans, distance between them and location of the fire.
Because of the length of the tunnels, the mallado also results critical. In vital areas, such as the location of the fire or in the downward ascending/discharge of the fans, the mesh has to be quite fine to capture with precision the physical phenomena. In other areas however, the cells can be of greater dimension to reduce the total number of cells. Zitron Also has carried out several analyses of sensitivity to understand which are the effects of the size and type of cell in the solution. The researchers concluded that they have to use meshes structured to range the orientation of the cells with the steering of the flow and avoid like this the numerical diffusion. It has showed that a size of cell of 0,25 metres situated in the vicinities of the location of the fire provides precise gradients of speed and temperature. The cells can reach a maximum of 5 metres of length between the location of the fire and the fans, since in these areas, the flow ranges with the mesh. The methods of scanning and mallado MultiZona available in the technological platform of simulation of engineering Workbench allow mallado hexaédrico structured with inputs minimum by part of the user and in this way so much the size like the growth of the cell can force through the mallado local.
One of the inputs that also is key is the capacity of the fire and the sources of smoke or contaminantes. The team of Zitron also has compared several methods, from the terms source simpler of heat and mass until the mechanism of reaction more sophisticated. The results of both strategies have showed in consonance, whenever the volume of the fire fulfil the relation of 1 megavatio by cubic metre. Thus and by comfort, generally uses a source of heat and source of product of combustion for the simulations. Also for the modelado of the turbulences, Zitron has evaluated several models Reynolds-averaged Navier–Stokes, although it has observed that the attainable model k-ε it describes with greater precision the patterns of flow and the transfer of heat in applications of ventilation of tunnels.
The calculation of the temperature of the smoke
The propagation of the smoke calculates with a diagram of advance over time. The standard approximation for transitory simulations is a diagram of advance over time (ITA) iterative. For a step of determinate time, all the equations resolve iterativamente until they fulfil the criteria of convergence. The advance to the following step of time, usually needs of a number of iteraciones external and, consistently, a considerable effort of calculation. The version of the software of simulation Fluent 6.2 already included a diagram of advance of time no iterative (NITA), that has gone improving in the back versions. Only it is necessary an only iteración external by each temporary jump, what accelerates the transitory simulations by a factor of between two and five times.
Although the capacity of the total fire can adjust to compensate artificially the impact owing to the radiation, the precise calculation of the temperature of the wall in the vicinities of the fire has to incorporate a modelado of radiation. The pertinent radiation of sources of heat located by the fire is not easy to be able to characterise because surfaces of the walls and the components of the fluids, for example the smoke, form part of the radiation. The model of orderly discreet is here the option preferred because it allows to describe sources of heat located while they consider the means of participation with properties multibanda. Historically, this model was available only for diagrams ITA what ralentizaba the time of total calculation. However with the last version of the software of simulation Fluent, the application of the model of orderly discreet has extended to diagrams Nita, what allows to increase the rapidity of the method of the iteración individual by temporary jump.
Thanks to the solution Ansys CFD Zitron can obtain a very reliable perspective of the performance of the systems of ventilation with the possibility that the software can go incorporating and evolving in function of the needs of simulation of the company. The experience of Zitron with the software of simulation CFD has provided him an important competitive advantage in the engineering of hygiene against fires in the tunnels.