Nafiz H. K. Chowdhury

Heat Transfer and Pressure Drop Measurements in High Solidity Pin Fin Cooling Arrays With Incremental Replenishment

Leading edge heat loads on turbine airfoils can be reduced by increasing the diameter of the leading edge. The lower external heat transfer and more generous curvature may allow for cooling this region internally. Large diameter leading edge regions are expected to exhibit a relatively broad region with nearly constant heat transfer. However, the ability of internal passages to cool a surface diminishes with distance as cooling air picks up thermal energy within the passage. Two novel internal cooling geometries have been designed and tested, which incrementally replenish cooling air by using impingement holes distributed along the array. These cooling methods have been compared to a baseline high solidity passage in terms of both array heat transfer and pressure drop. Heat transfer rates and pressure drop have been determined on a row by row basis to provide a means to assess their ability to sustain adequate cooling levels across the entire leading edge region. The authors believe turbine airfoil designs integrating large diameter leading edge regions with properly designed internal passages have the potential to eliminate showerhead cooling arrays in many industrial gas turbine applications. This change is especially beneficial in environments where fuel or air impurities have the potential to clog leading edge showerhead cooling arrays. Heat transfer and pressure drop measurements were acquired in a bench scale test rig. Reynolds numbers ranged from approximately 5000 to 60,000 for the constant height channel arrays based on the pin diameter and the local maximum average velocity across a row. The high solidity pin fin arrays have an axial spacing (X/D) of 1.074 and a cross channel spacing (S/D) of 1.625. The constant section pin fin arrays have channel height to diameter ratios of 0.5. Each array has eight rows of pins with six pins per row in a staggered arrangement. Heat transfer testing was conducted using a constant temperature boundary condition.

A Predictive Model for Preliminary Gas Turbine Blade Cooling Analysis

The growing trend to achieve a higher Turbine Inlet Temperature (TIT) in the modern gas turbine industry requires, in return, a more efficient and advanced cooling system design. Therefore, a complete study of heat transfer is necessary to predict the thermal loadings in the turbine vane/blade. To estimate the metal temperatures, it is important to simulate the external hot gas flow condition, the conduction in the blade material, and the internal coolant flow characteristics accurately and simultaneously. As a result, proposing novel, quicker, and more convenient ways to study the heat transfer behavior of gas turbine blades is of absolute necessity. In the current work, a predictive model for the gas turbine blade cooling analysis in the form of a computer program has been developed to answer this need. The program is capable of estimating distribution of coolant mass flow rate, internal pressure and metal temperature of a turbine blade based on external and internal boundary conditions. The simultaneous solutions result from the coupled equations of mass and energy balance. The model is validated by showing its accuracy to predict the temperature distributions of a NASA E3 blade with an uncertainty of less than +/−10%. Later, this paper documents the overall analysis for a set of different boundary conditions with the same blade model (E3) and demonstrates the capability of the program to extend for other cases as well.Copyright

The Response of High Intensity Turbulence in the Presence of Large Stagnation Regions

Relatively small scale turbulence is known to intensify in the presence of a stagnation region due to the elongation of these eddies by the mean strain field of the approach flow. Experimental evidence also demonstrates that the large scale eddies are blocked as they approach presence of the stagnation surface. Recent heat transfer measurements suggest that very high intensity turbulence or turbulence in the presence of very large scale leading edge regions may not be as strongly influenced by the stagnation region strain field. Understanding the physics of turbulence is critical to the improvement of turbulence models which are used to predict the surface heat load in gas turbine hot sections.This paper documents the response of high intensity turbulence in the approach flow of two large cylindrical leading edge regions. Measurements of turbulence intensity, scale, spectra, and dissipation have been acquired for five elevated levels of turbulence in the approach flow of two large diameter (0.1016 m and 0.4064 m) leading edge regions. Generally, three influences were observed. Initially, in the presence of the largest cylinder the smaller scale higher intensity turbulence showed increased decay due to longer effective convection times. Secondly, dissipation levels, as estimated from the inertial subrange of the one-dimensional spectra, initially decreased then increased as the strain field intensified in the presence of the stagnation regions. Finally, the measurements indicated that the energy in the low wave number spectra was increasingly blocked in the near wall region of the leading edge.Copyright