We were tasked with supplying a Finite Element Model (FEM) of a new Engine Thrust Reverser structure followed by static and fatigue stress analysis of some of the critical structural components.
The engineering challenges included the construction of a fully operational Thrust Reverser FEM, subjected to all critical flight and ground load conditions. These conditions included internal pressure, external pressure, thermal loading and inertial loading. The FEM results were subsequently used as part of the Static and Fatigue check stress analysis. The FEM and stress calculations formed a crucial part of the Thrust Reverser’s certification and airworthiness process.
According to regulation, the mechanical ventilation system should achieve a rate of 6 Air Changes per Hour (ACH) around all areas of the car park and 10 ACH where car engines are running and queues can form. Extract fans should be arranged to prevent stagnated areas where stale air can accumulate.
During smoke clearance the ventilation system should achieve 10 ACH in the area where the fire is located. Also air velocities along escape routes should not exceed 5m/s to avoid impeding occupants escaping and furthermore, pressures on doors should ideally be negative and low.
Computational Fluid Dynamics
A CAD model was constructed, as illustrated in Figure 1. The model contained all the major features of the carpark design including pillars and vehicles to ensure that it was representative of the actual design. Figure 2 shows an example of the mesh around the ceiling mounted impulse fan.
The software used for this project was the ANSYS Fluent commercial CFD system. The geometry was meshed with a 3D Poly-Hexcore Mesh down to a size of 0.005m on the boundary surfaces. This mesh technology contains fewer cells than older approaches but has more degrees of freedom in the mesh allowing for faster and more accurate solutions. The simulation was run using a RANS solver using the K-Omega SST model.
The CFD analysis allowed both the airflow and the age of air to be examined at various levels in the plant room. The number of air changes per hour could also be examined. The boundary conditions could be modified if required for changes such as whether the plant rool entrance was open or closed. The impulse fans could be relocated if the results did not meet the system regulation requirements.
Simulations were conducted for both Pollution Control and Smoke Clearance and a snapshot of the results from both scenarios are shown below. The differences can be seen in the velocity contours for the two scenarios where the fans operate differently. It can be seen that the velocity of the air at the plant room entrance and the stairwell escape route is less than 5 m/s during Smoke Clearance. It was also possible to demonstrate that air pressures on escape doors were all negative and did not exceed requirements during the Smoke Clearance scenario.