Gilles Leroy

Numerical Optimization of Turbomachinery Bladings

An optimization process is used to design bladings in turbomachinery. A gradient-based method is coupled to Navier-Stokes solvers and is applied to three different bladings. A new rotor blade of a transonic compressor is designed by using a quasi three-dimensional approach, with a significant efficiency improvement at the design point. The off-design behavior of this new compressor is also checked afterwards. The same quasi three-dimensional approach is used on a stator blade of a turbine, but the whole stage is computed in this case. The losses are locally reduced, proving the good sensitivity of the solver. Finally, a new three-dimensional rotor blade of a compressor is designed by applying deformation functions on the initial shape. The efficiency is improved over a wide range of mass flow. The whole results indicate that the optimization process can find improved design and can be integrated in a design procedure.

Study and Control of a Radial Vaned Diffuser Stall

The aim of the present study is to evaluate the efficiency of a boundary layer suction technique in case of a centrifugal compressor stage in order to extend its stable operating range. First, an analysis of the flow pattern within the radial vaned diffuser is presented. It highlights the stall of the diffuser vanes when reaching a low massflow. A boundary layer separation in the hub-suction side corner grows when decreasing the massflow from the nominal operating point to the surge and finally leads to a massive stall. An aspiration strategy is investigated in order to control the stall. The suction slot is put in the vicinity of the saddle that originates the main separating skin-friction line, identified thanks to the analysis of the skin-friction pattern. Several aspiration massflow rates are tested, and two different modelings of the aspiration are evaluated. Finally, an efficient control is reached with a removal of only 0,1% of the global massflow and leads—from a steady-state calculations point of view—to an increase by 40% of the compressor operating range extent.

Unsteady Analysis of Inter-Rows Stator-Rotor Spacing Effects on a Transonic, Low-Aspect Ratio Turbine

The aerodynamic performances of an axial turbine are affected by the distance between the stator and the rotor. Previous studies have shown different trends, depending mainly on whether the turbine is subsonic or not. The present paper aims at improving the understanding of the effect of rows spacing on the flow through a transonic turbine.A one-stage, low aspect ratio, high pressure turbine case is investigated using CFD. Steady and unsteady phase-lagged RANS computations are performed on this configuration with different inter-blade rows distances. The results are successfully compared with experimental data from a cold air turbine rig. Entropy production balances are used to emphasize the main loss areas and the loss variations caused by changes in inter-blade rows distance. Two techniques are compared for computing these balances, and one of them appears to perform much better. The flow features causing these losses are then identified. Finally, an optimal inter-rows spacing is found. It is a compromise between the losses created by strong stator-rotor interactions at small inter-rows gaps and the losses generated at the endwalls in the inter-rows space at large distances.Copyright

Numerical Investigation of the Flow in a Radial Vaned Diffuser Without and With Aspiration

An analysis of the flow in a centrifugal compressor vaned diffuser from the nominal operating point of the compressor stage to a point near surge was conducted. Study of performance coefficients, and then of the skin-friction pattern, reveals the growth of a corner stall between the hub wall and the suction side of the vane when moving the operating point towards surge.Considering the location of the skin-friction pattern singular elements, a boundary layer suction technique has been developed and then numerically tested. The hub wall corner stall was controlled, and performances predicted near surge have been significantly modified, as well as the flow structures when reaching the limit of numerical stable operating range: the major change in the topology of the flow occurs now within the impeller, in the splitter leading edge region, and let think about a leading role of the main blade tip clearance vortex in the instabilities release. The surge massflow seems to have been significantly reduced.Copyright

Numerical Investigation of High-Pressure Turbine Environment Effects on the Prediction of Aerothermal Performances

Aerothermal prediction for the high-pressure turbine is challenging because of the complex environment that interacts with the turbine: hot-streak migration, unsteady flow phenomena, fluid/solid thermal coupling and technological details (squealer tip, coolant ejections, fillets, etc.). There is a need to compare their relative impacts on the blade temperature and turbine efficiency prediction. This is the main purpose of this paper. URANS simulations of the flow have been performed with a structured flow solver in a one stage high-pressure turbine. The baseline simulation takes into account the squealer tip and an inlet condition representative of a hot streak generated by the combustion chamber. Other technological details (coolant ejections and fillets) and fluid/solid thermal coupling on the rotor blade are alternatively considered in the simulation in order to quantify their relative contribution. The Chimera technique is used to ease the integration of technological details. The conjugate heat transfer (CHT) problem is solved by means of a code coupling where fluxes and temperatures are exchanged at the blade surface between the fluid dynamics solver and the solid thermal code. Results shows that rotor blade fillets have a little impact on both the blade temperature and the turbine efficiency (less than 1%). On the contrary, taking into account external cooling leads to a modification of radial distribution of loss and loading coefficients and reduces the efficiency by 2%. The blade temperature is also impacted, mainly on the suction side where differences of several per cent with the baseline case are observed. Fluid/solid coupling mainly affects the blade temperature prediction by homogenizing it which induces differences around 3% with the baseline case. To complete the analysis, a post-processing that includes a computation of local entropy production terms is used. It shows that the entropy production is mainly due to turbulent dissipation and allows to identify the reduction of efficiency of the case with cooling as an additional production of entropy where the cooling flow mixes with the main flow.Copyright

3D Simulation of Coupled Fluid Flow and Solid Heat Conduction for the Calculation of Blade Wall Temperature in a Turbine Stage

A critical problem in high pressure turbine of modern engine is the vane and blade reliability as it is subjected to high thermal constraints. Actually the flow entering the turbine presents high level of stagnation temperature as well as great radial and circumferential temperature gradients. Considering that a small variation of the blade temperature leads to a strong reduction of its life duration, accurate numerical tools are required for prediction of blade temperature. Because of the complexity of the flow within a turbomachine, the blade wall temperature is heterogeneous and a fluid/solid coupling may improve wall temperature prediction. This study presents a coupling strategy of a Navier Stokes flow solver and a conduction solver to predict blade temperature. Firstly, the method is applied to the well documented NASA C3X configuration. The influence of the fluid/solid interface boundary condition is studied with regards to the wall temperature and heat flux prediction as well as to the computational efficiency. The predicted wall temperature is in good agreement with the experimental results. The method is finally applied to the prediction of the blade temperature of a high pressure turbine representative of a modern engine. Adiabatic and coupled results are compared and discussed.Copyright