PARAMETRIC STUDY FOR JET IMPINGEMENT HEAT TRANSFER ENHANCEMENT USING DIFFERENT ROUGHNESS ELEMENTS

Abstract

Gas turbines are utilized for a variety of industrial applications, such as electric power generation (large land-based gas turbines), driving compressors and pumps, and providing thrust for aircraft propulsion (jet engines). To maximize the engine’s efficiency, the rotor inlet temperature (TIT) needs to continuously increase beyond its current maximum of 1700 oC, which is far beyond the softening limit of turbine blades and vanes at 1000 oC. Thus, the cooling of hot gas path components is vital to prevent component failure due to thermal stress. Over the past decades, turbine cooling technologies have rapidly advanced. Regions under critical heat loads from the combustor gases, such as the leading edge of rotating blades and the leading edge and mid-cord regions of stator vanes, require aggressive. Jet impingement is commonly used in these areas. Jet impingement cooling provides a very high local heat transfer rate at the expense of a high pressure loss penalty. Further enhancement of jet impingement heat transfer has been achieved by introducing roughness elements to the target surface. Therefore, continued research to evaluate the performance of different roughness elements is necessary. In this dissertation, experimental investigations on the heat transfer, crossflow, and discharge coefficients in a rectangular impingement channel roughened with pin-fins, strip-fins, orW-shaped ribs were conducted using a steady-state, copper plate experimental method. The experimental study was supplemented by numerical flow field visualizations using ANSYS Fluent software to better understand the complex flow behavior. This parametric study examines the associated effects of varying the following key parameters: jet Reynolds number (Rejet = 10k – 70k), jet-to-target surface spacing (z/d = 3 and 6), pin-fin height (H/d = 1.5 and 2.75), strip-fin height (H/d = 1.5 and 2.75), and W-rib pitch (P/e = 18, 9 and 6). The results revealed important information regarding the heat transfer performance of each investigated configuration. Experimental results showed that all channel configurations have similar discharge coefficients, which decrease at a higher rate beyond Rejet = 50k when increasing the height of pin-fin or strip-fin, the number of W-rib rows, or the jet-to-target surface spacing. Additionally, the pressure drop in the impingement channel increases with the addition of roughness elements. The results indicated that the heat transfer augmentation from increasing fin height or number of W-ribs is less than the augmented surface area. Therefore, it is recommended to optimize the height and position of installed roughness elements for best heat transfer performance. Furthermore, the numerical results demonstrated highly non-uniform velocity distributions at the near-target surface regions, signifying non-uniform heat transfer. The highest heat transfer enhancement was obtained in the narrow impingement channel with long strip-fins.