FILM COOLING CALCULATIONS WITH AN ITERATIVE CONJUGATE HEAT TRANSFER APPROACH USING EMPIRICAL HEAT TRANSFER COEFFICIENT CORRECTIONS

Open Access
Author:
Dhiman, Sushant
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 15, 2010
Committee Members:
  • Savas Yavuzkurt, Thesis Advisor
Keywords:
  • Conjugate Heat transfer
  • Film Cooling
  • Gas Turbine
  • CFD
Abstract:
An iterative conjugate heat transfer technique was developed and automated to predict the temperatures on film cooled surfaces such as flat plates and turbine blades. Conventional approaches using a constant wall temperature to calculate heat transfer coefficient and applying it to solid as a boundary condition can result in errors of around 14% in internally cooled blades,as shown by previous research work. This indicates a need for conjugate heat transfer calculation techniques. However, full conjugate film cooling calculations also suffer from inability to correctly predict heat transfer coefficients in the near field of film cooling holes and require high computational cost making them impractical for component design in industrial applications. Iterative Conjugate Heat Transfer (ICHT) analysis is a compromise between these two techniques where the external flow convection and internal blade conduction are loosely coupled. The solution obtained from solving one domain is used as boundary condition for the other. This process is iterated until convergence. Flow and heat transfer over a film cooled blade is not solved directly, instead convective heat transfer coefficients resulting from external convection on a similar blade without film cooling and under the same flow conditions are corrected by use of experimental data to incorporate the effect of film cooling in the heat transfer coefficients. The effect of conjugate heat transfer is taken into account by using this iterative technique. Unlike full Conjugate Heat Transfer (CHT), the ICHT analysis doesn’t require solving a large number of linear algebraic equations at once. It uses two separate meshes for external convection and blade conduction and thus problem can be solved in lesser time using less computational resources. A demonstration of this technique using a commercial CFD solver FLUENT is presented for simulations of film cooling on flat plates and C3X turbine blade. Results are presented in form of film cooling heat transfer coefficients and surface temperature distribution which are compared with results obtained from conventional approach where in which heat transfer data is usually obtained on test surface maintained at a constant temperature or heat flux condition. For uncooled surfaces, the deviations were as high as 3.5% between conjugate and conventional technique results for the wall temperature. For film cooling simulations on a flat plate using the ICHT approach showed deviations up to 10% in surface temperature compared to constant wall temperature technique for a high temperature difference case and 3% for a low temperature difference case, since surface temperature is not constant over the surface when conjugate heat transfer is considered. In case of 2D full conjugate heat transfer simulation performed on a non film cooled C3X vane reasonable agreement to experimental data was observed for mid-span wall temperature distribution. Average deviations of around 6% on the pressure side and 7% on the suction side from experimental data were obtained. Film cooling calculations performed on the C3X blade using the ICHT technique predicted wall temperature distribution in excellent agreement with the data. Temperature distribution obtained using ICHT were within 2 to 3 % of the experimental data i.e. well within the experimental uncertainty, while conventional technique predicted the same within 6 to 8% of the data. In all, for wall temperature distribution, an overall improvement of around 7% on the pressure side and 10% on the suction side over conventional technique was obtained using ICHT. Results thus show that the ICHT technique is an effective tool for performing accurate film cooling calculations which reduces computational cost and complexity involved in meshing discrete film cooling holes. Moreover, it is physically more appropriate than conventional technique since it considers the effect of blade metal conduction.