Numerical Simulation of Electrochemical Drilling

Marc Noot

Ph-D research project

Marc Noot was employed as a PhD student in the Scientific Computing Group of professor Mattheij during the period 1993-1997. His research was done for the benefit of the company Eldim b.v. (Lomm, The Netherlands) a manufacturer of gas turbine components.

Introduction

The efficiency of a gas turbine engine is influenced by the temperature at the inlet of the turbine. There is a growing tendency to use higher inlet temperatures which leads to an increase of the heat load on the turbine components. This heat load is caused by the exposure to an enormous heat flux from the burnt gas coming from the combustion chamber. To maintain acceptable safety standards, these components need to be protected against the exposure to heat. Protection methods which have been introduced to prolongate the lifetime of turbine blades are coating, internal cooling and film cooling. In stationary gas turbines these components need to be protected against their severe thermal environment. For this purpose cooling holes are drilled in the blades to increase heat transfer. By introducing multiple ribs on the wall of these holes Eldim hopes to improve the heat transfer. Turbine blades that are designed with this kind of cooling will have multiple cooling holes in the longitudinal direction of the blade.

Using numerical simulation for both the drilling of the holes and the computation of the cooling air flow we try to achieve a better insight in the problem. The main goal of the research project is to investigate the relation between the shape and distribution of turbulators and their effect on the heat transfer in cooling holes. Our approach is to isolate one of those cooling holes in the interior of the blade and to try to study the effect of turbulators on the heat transfer locally with the aid of numerical simulation on a proper turbulence model. Most simulations have been done with the Finite Element Method (FEM).

Numerical Simulation of Electrochemical Drilling

One possibility for producing these holes is drilling. However, since the holes have such a complex shape and their diameter is only about a few millimeters, conventional drilling techniques are not suitable. Therefore an Electrochemical Drilling (ECD) technique is used; i.e. an electrolytic process is employed where an anode moves gradually into the metal. Turbulated cooling holes are drilled by varying process parameters during drilling in a well defined and controlled way. The drilling of these holes requires a large number of well defined experiments on test pieces which are very expensive. Computer simulation of the ECD process may reduce the number of experiments. A model has been developed to determine the effect of parameter variations on the shape of the turbulators.

The ECD process is based on the phenomenon of electrolysis. This is a process where a electric potential difference is imposed on an anode and a cathode. Due to the presence of this electric field the electrolyte, often a sulphuric acid, causes the anode surface to be corroded. After the metal ions have been dissolved and reacted with the negative ions in the solution they are removed by the electrolyte flow. The drill is a conducting cylinder with an insulating coating on the outside and is being lowered into the material with a certain speed while a voltage is applied to it. In this way a cylindrically shaped hole is obtained.

turbulated cooling holes

The current which flows through the electrolyte is due to the movement of ions. The ions in the solution are the "charge-carriers" establishing the current, following the imposed field by the potential difference. Thus the potential gradient need not be equal to zero even if no current is flowing through the solution. The current density is a result of gradients in the electric potential and ion concentrations.

In the figure above the results of a simulation run are presented. At 50 secs intervals of simulation time the shape of the boundary is shown. The shape of the turbulators in this case is not very pronounced. By tuning the process parameters in a different way other shapes can be achieved.

Heat Transfer in Turbulated Cooling Holes

We will focus on internal cooling. This cooling is achieved by compressing relatively cool air through holes in the longitudinal direction of the turbine blades. In order to increase the heat transfer in the holes, the wall of the cooling passage is provided with multiple ribs. These irregularities are called turbulators, after the turbulence they are supposed to cause in the flow. This way the cooling efficiency of these holes is improved in the sense that the amount of contact surface between metal and cooling air is increased and there is also a better heat exchange due to turbulence. It is known that the heat transfer is better than in the laminar case. Moreover less cooling air is needed.

Besides the use of existing numerical packages also own code was developed, written in C++. Using fast iterative methods and ILU-preconditioners the code enables to solve the instationary Navier-Stokes equations in an efficient way. In the Finite Element Method the Taylor-Hood P2-P1 element in combination with the integrated method is used. For time-integration the theta-method has been implemented.

An MPEG movie (1.3Mb) of an instationary flow process. In this animation the filled contour lines of the stream function are displayed. The flow direction is from right to left in the axial direction. The computational domain is a cross section of a axi-symmetric problem. Since this is just a segment in the cooling passage, periodical boundary conditions for the velocity are imposed on both inlet and outlet.