CFD HVAC (Heat Ventilation & Air Conditioning) simulations in AnsaldoBreda

AnsaldoBreda da anni si appoggia alle metodologie CAE per migliorare le prestazioni e le caratteristiche dei suoi prodotti, in modo da portare sul mercato treni e cazzorre di prima qualità, con riferimento, nello specifico, all'aspetto della loro climatizzazione. Sperimentare virtualmente permette, tra l'altro, di accertare il rispetto degli standard internazionali, sia negli aspetti prettamenti tecnici, che in quelli propri della sicurezza e dell'affidabilià, in riferimento ad una amplissima varietà di condizioni ambientali di utilizzo e di scenari operativi, inclusi incidenti, malfunzionamenti, incendi, e simili. Il 'case history' che qui presentiamo tratta delle simulazioni HVAC condotte su un treno. I modelli messi a punto hanno permesso, non solo di ricreare un fenomeno complesso, ma di intervenire immediatamente in fase di progettazione per apportare cambiamenti essenziali in tempi ridottissimi, a favore dell'efficienza e della qualità del prodotto, vanto della AnsaldoBreda.

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Ansaldobreda with the objective to supply the best in class product to its costumers has started several years ago the implementation of CAE methodologies to gain more insight on the performance of their trains and coaches which are sold on a worldwide basis.
Modern train rail coaches require the design of the highest standards to fulfil legal regulations and technical specifications. Furthermore due to the fact that a modern train can travel for thousands of kilometres, the specification and standards to be satisfied are both a function of local country and legislations, and obviously of the latitude since a train must be operative in climate condition ranging for example from Scandinavia to Southern Europe.

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CAE application which can be applied to rail coaches range from FEM analysis for static, thermal and dynamic structural tests, crash simulations and acoustic simulation. To the CFD field where of particular interest are HVAC simulation for simulating thermal comfort, heat transfer simulations and many more.
This article describes the HVAC activity on thermal comfort carried out on a railway motor-coach.

Railway–Motor Coach
With the objective to yield the best product possible to its costumers Ansaldobreda has carried out a series of simulations on the motor coach.

The motor-coach has to withstand the summer severe ambient conditions.

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Such severe climatic conditions include a 50 Celsius degrees outside air temperature, radiative heat flux from the summer sun, all appliances in the bar turned on and releasing hest, and the coach filled to the maximum capacity of passengers, with the human bodies treated as a heat source. All lights are assumed to be turned on and acting as heat sources. Furthemore the train is assumed to be not moving, hence the heat transfer coefficient toward the outside environment is minimum.

As the next pictures show this motor-coach is a two floor coach, hence with passengers on the bottom floor as well as the top floor. The two floors are connected with stairs, and at one end of the coach at an intermediate level a bar is present, where all the additional internal heat source from the kitchen appliances of the bar appliances such as coffee-machines, ovens, etc. are located.

The Ansaldobreda engineers expected severe conditions, especially on the top floor where a lot of radiate heat flux is coming in form the sun, and also where buoyant effects are bound to carry most of the hot recirculation air.
Hence a revised duct system is first simulated.
A first study to simulate the air flow distribution in the ducts is carried out from the pressure plenum downstream the fan-coil of the ventilators, to the ducts supplying air to the bar, the top floor and the bottom floor, as shown in the next pictures. This study is necessary in order to calculate the mass flow out of each duct, which will the applied to the ventilation of the coach volume.

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The system is simulated with a CFD analysis with the actual porous slots area in the pressure plenum (coloured parts in the picture), which distribute as a function of the area and the porosity the air in the three volumes left and right for a total of six different mass flow distributions. As mentioned above since a lot of heat is expected on the hot floor, the pressure air slots are dimensioned to supply more cool air to the top floor, proportional to the enthalpy increase estimated.
Total pressure needed to supply the required mass flow on all six systems is shown in the next picture.
Subsequently a much more complex model of the coach is built in order to verify that the mass flow distributions from the ducts actually deliver the desired comfort.
The model is shown in the next picture with the unstructured mesh applied. All materials have been taken into account with their thickness and conductivity.
Refinement of the mesh have been applied to all inlets, all seats and details of the stairs and the furniture of the bar for a total of 6 million elements between tetrahedrons and prisms.
The CFD model solve for momentum and continuity as well as turbulence, and heat transfer.
In this case the heat transfer model took into account several physical aspects:

• Differential air temperature yielding natural convection toward the top floor
  
• Non adiabatic walls
  
• Radiant heat transfer from surface to surface as well as through the volume

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The result is an accurate prediction of the flow filed as well as the temperature field which is the main object of investigation.
The next pictures show the streamlines ventilating the whole coach and the temperature filed on the mid section, where it can be seen the maximum temperatures near the roof where most of the radiative heat flux is concentrated.

Conclusion
CAE simulation, in particular CFD in this case, have proved to be able to simulate a complex phenomenon: air recirculation with buoyant effects, non adiabatic walls, and radiative heat transfer, in a complex geometry: coach including chairs, bar with furniture and appliances, stairs and other details.

The post-processing is actually a virtual reality system where the design engineer can asses the status of the actual geometry and take decision on the modifications.
Furthermore a parameterization of the model is often possible, hence simulating an additional features or changing scenarios of the simulation for environment purposes can be extremely rapid, with a matter of just a couple of hours work with Boolean operations with modern day pre-processors, requiring just extra CPU time to rerun the case.

The employment of these modern scientific techniques in the design process by the AnsaldoBreda engineers has allowed them to gain awareness on the thermal performance of their product (coach) and verify with a virtual system the soundness of the HVAC plant.
This ultimately allows AnsaldoBreda to supply the best in class products and to eventually spot out critical areas.

Ing. Petiti, Barsi, Chiti
ANSALDO BREDA
Ing. Fontana, Damiani, Bucchieri EnginSoft
info@enginsoft.it