Influence of radiative effects on the buoyancy-induced flows in High-Pressure compressor disks

The research concerns the optimization of the secondary flow used for cooling the turbine disks in conditions close to the actual operation. This secondary air circuit, passing through the HP compressor, consumes up to 6% of the fuel. Thus, a better understanding of the strong coupling between flow structures and heat transfer (convection, conduction and radiation) developing within the inter-disk spaces modeling a turbomachine HP compressor is crucial. The flow and heat transfer depend most strongly on the disk surface temperature distribution, and radiation effect between the solid surfaces in the cavity can be significant. In addition, buoyancy-induced flow is a strongly conjugate problem: the temperature distribution on the disks affects the flow in the cavity, and vice versa. This buoyancy-induced flow is not only unsteady and three-dimensional it is also unstable.

It is proposed to investigate the flows and heat transfer in a simplified inter-disk cavity taking into account not only the radiative effects but also conductive exchanges between the solid parts and the fluid in order to establish a more accurate thermal balance. The objective is to provide, for a range of parameter values approaching actual situations, a precise description of the temperature distribution as well as the flow structures inside and at the outlet of the inter-disk cavity. The results will also serve to better guide future experiments, in order to avoid the excessive cost of assembling a test bench under conditions useful to designers. The study will be carried out by direct numerical simulations, from a 3D cylindrical computation code based on High Order Compact (HOC) scheme and dedicated to isothermal incompressible flows. This multidomain code is parallelized by means of a hybrid approach MPI / OpenMP.

The proposed subject will require modifications of this code to reach the above mentioned goals. The first step will be to implement the low Mach number approximation to take into account the variation of density resulting from the (relatively moderate) temperature differences between the "cold" axial air and the outer shroud heated by the hot main flow. The second phase of the development will consist in including the conductive exchanges between the solid parts and the fluid. So, the temperature does not need to be prescribed at walls, allowing for a more realistic representation of the very strong coupling between the flows and the heat transfer to the walls. At this stage, a number of simulations will be carried out in order to establish a first mapping of the temperature field distribution and structures of the associated flow. The objective of this first series of simulations is to detect areas of high temperature, particularly near the walls to optimize the implementation of the radiative model. Once this preliminary mapping is obtained, we will proceed to the implementation of the model taking into account the radiative effects. This step is indeed the most important and the most difficult in terms of modeling. An important work of optimization, especially on the treatment of the terms resulting from the radiative effects, should be made to obtain reasonable computational cpu times.

To succeed, the candidate will have a good background in fluid dynamics, heat transfer and numerical modelling. Competence in parallel computing is strongly desirable.

Submit CV with motivation and recommendation letters to A. Randriamampianina