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Research project: Transpiration Cooling in Hypersonic Flows

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Transpiration Cooling in Hypersonic Flows

When hypersonic vehicles are flying through the atmosphere tremendous amount of heat loads can be experienced. Thermal protection systems (TPS) are designed to sustain these thermal loads. Traditionally, ablation based TPS have been used to prevent the heat to get to the internal structures.

In the alternative strategies, a coolant is directly injected into the Hypersonic flow, which forms a thin coolant layer on the surface providing cooling. These strategies are supposed to reduce weight and overall cost of TPS. However, it is noted that the direct effusion of coolant fluid through the holes (also called effusion) is not very effective in providing the film cooling as it leads to separation and enhanced mixing inside the boundary layer. To avoid this, a new cooling strategy is explored in the present project, where transpiration-based cooling is envisaged to provide the thermal protection. In this strategy, the coolant is injected into the flow through a porous layer. This leads to a gradually and more uniform distribution of the coolant into the boundary layer and hence higher effectiveness.

The project aims at providing numerical solution to the challenging problem of transpiration cooling in conjunction with the experimental results provided from the Oxford University’s Hypersonic Group. Various injection strategies of coolant-injection are tested, such as slot injection (Fig. 1) and injection using a porous-layer (made-up of ultra-high temperature ceramic (UHTC) material, Fig. 2). The cooling effectiveness of the slot injection is shown in Fig. 3, showing a good match with the experimental results (with similar set-up) for different blowing ratios.

The set-up with porous-layer injection is also studied where the porous layer is modelled as staggered layers of cylinders/spheres in 2D and 3D, respectively. The wall-normal component of velocity is plotted in Fig. 4 (a) for 3D case, which shows the top view of the flow at wall. The fluid close to the porous layer is relatively smooth while it transitions to turbulence farther downstream.A comparison with the slot injection case with similar mass flow rate is shown in Fig. 4 (c) in terms of cooling effectiveness, which shows the greater effectiveness of porous layer in achieving lower wall temperatures specially farther downstream.

Compressible Navier-Stokes equations in 2D and 3D are solved in the finite volume framework AMROC in Cartesian reference system, where spatial derivatives are evaluated using a specially designed hybrid WENO-CD scheme (up to 6th -order accurate) and 3rd order Runge-Kutta method for time integration. Additionally, structured-adaptive-mesh-refinement is achieved using AMROC framework, to capture the region of strong gradients dynamically in the flow. Figure 5 (a) shows the different levels of refinement which can be achieved in the AMROC and (b) shows the switch function (flag) used to identify the regions of the strong gradients and according activates the WENO scheme in the regions of gradient while the central scheme is employed in the smother regions.

1a
Fig.1: (a) Schematic of a slot injection,
1b
Fig 1:(b) Coolant injection through slots into the hypersonic flow.
2a
Fig 2a Schematic of a porous layer
2b
Fig 2b Coolant injection through porous layer into the hypersonic flow
Fig.3
Fig. 3:

Fig. 3: Cooling effectiveness of the slot injector with various blowing ratios compared against the experimental results, showing a good match.

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4a

Fig. 4:(a) v-component of velocity at wall, showing transition to turbulence at downstream locations.

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4b

Fig 4: (b) coolant concentration view from top and side.

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4c

Fig. 4: (c) cooling effectiveness comparison with the slot case showing far better results with porous layer injection.

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Fig. 5: (a) Mesh levels in AMROC shown through the porous layer
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Fig 5: (b) WENO-CD switch flagging detecting the shock structures

Associated research themes

http://transpirationcooling.eng.ox.ac.uk/

Related research groups

Aerodynamics and Flight Mechanics

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