Numerical investigation of the effect of wavy target surface on heat transfer in extended jet impact cooling

Gas turbines are among the most critical components of modern energy generation systems, operating at high temperatures and occupying a unique position in advanced engineering applications. One of the most effective ways to enhance turbine efficiency is by increasing the temperature of the working fluid at the turbine inlet. However, this is directly constrained by the thermal limits of the superalloy materials used in turbine blade manufacturing. Despite advances in material technology, the maximum temperatures that current materials can withstand remain below the operating temperatures encountered in gas turbines. Therefore, effective cooling of turbine blades is essential to preserve material integrity and ensure sustained high-efficiency performance of the system. The geometry of gas turbine blades exhibits considerable complexity depending on the flow conditions. Temperature and heat flux distributions along the blade are non-uniform, necessitating the application of different cooling techniques in different regions. Convective cooling through internal channels is commonly employed in the mid-span regions, while film cooling techniques are typically applied on external surfaces. Pin-fin cooling is used on the trailing edge, whereas jet-impingement cooling is implemented particularly on the leading-edge regions directly exposed to hot gas flow. Jet-impingement cooling is widely used in gas turbines, electronic thermal management, and concentrated solar power systems due to its ability to generate high local Nusselt numbers. In jet-impingement cooling systems, the target surface geometry has a significant influence on the flow structure and heat transfer. Most studies in the literature have assumed flat target surfaces, and the effects of wavy or roughened surfaces on heat transfer have been only partially investigated. However, modifying the target surface geometry directly affects the flow characteristics, increasing local turbulence intensity and vortex formation, which can substantially enhance heat transfer. Wavy surfaces alter the spreading behavior of the jet upon impact, dynamically affecting flow direction and velocity distribution, thereby modifying both the magnitude and distribution of heat transfer. In this thesis, the effect of wavy target surface geometries on heat transfer in jet-impingement cooling is investigated numerically. The study aims to determine the influence of wave forms (peak, valley, leftward, or rightward) on heat transfer coefficients and to assess their contributions to the flow structure. The analysis was conducted using a single-row jet arrangement with six jets. The jet exit velocity was set to 48,75 m/s, and the Reynolds number was fixed at Re = 32,500. Under these constant flow conditions, eight different target surface models were analyzed: • Base model (flat surface) • A1 model (peak-type wavy) • A2 model (continuous peak-type wavy) • B1 model (leftward wavy) • C1 model (rightward wavy) • C2 model (continuous rightward wavy) • D1 model (valley-type wavy) • D2 model (continuous valley-type wavy) For each model, wave height (A) and wavelength (λ) were defined with different geometric ratios to better observe jet impingement and spreading behavior. The simulations were performed using ANSYS Fluent via Computational Fluid Dynamics (CFD) methodology. The flow was modeled as incompressible, steady, and turbulent, with the k-ω SST (Shear Stress Transport) turbulence model employed due to its proven ability to accurately predict boundary layer separation and reattachment in jet-impingement flows. Mesh independence studies were conducted to ensure that results were not sensitive to mesh density. The H/D ratio (nozzle-to-target distance / jet diameter) was kept constant so that flow behavior depended solely on surface geometry. Solutions were iterated until convergence criteria were satisfied, and flow fields, temperature distributions, and local Nusselt numbers were evaluated for each model. The numerical results demonstrated a pronounced effect of target surface geometry on heat transfer performance. Compared to the flat Base model, all wavy surfaces exhibited higher average Nusselt numbers. Geometric variations induced by the wavy surfaces caused the jet to change direction after impact, generating strong secondary flows and vortical structures. In particular, peak-type models (A1, A2) showed increased local velocity gradients as the jet slid over the wave peaks, resulting in a notable enhancement in heat transfer coefficients. Continuous wavy models (A2, C2, D2) allowed the jet to redirect more systematically and spread energy over a wider surface area, resulting in higher average Nusselt numbers than their non-continuous counterparts. Specifically, the C2 model (continuous rightward wavy) facilitated jet alignment with the flow direction, producing a more uniform temperature distribution across the target surface. Valley-type surfaces (D1, D2) trapped part of the jet flow in wave troughs, creating local turbulence and recirculation zones. While this enhanced heat transfer in certain areas, flow weakening in other regions limited the overall increase in average Nusselt number. Nevertheless, these models still outperformed the flat surface in terms of heat transfer. Overall, wavy surfaces provided a 10–35% increase in heat transfer relative to flat surfaces. Increasing wave height strengthened jet-surface interactions, enhanced turbulence energy, and consequently increased heat transfer coefficients. However, excessively high wave amplitudes caused instabilities in jet impingement behavior, reducing heat transfer in localized regions. Therefore, determining the optimal wave height and wavelength is critical for maximizing heat transfer performance. In conclusion, the numerical analyses conducted in this study indicate that target surface geometry plays a decisive role in the heat transfer performance of jet-impingement cooling systems. Wavy surfaces enhance turbulence and more effectively distribute jet energy across the surface, resulting in higher heat transfer coefficients compared to flat surfaces. Continuous wavy geometries, in particular, maintained flow stability while providing more uniform temperature distributions. This work demonstrates that optimizing surface geometry is a highly effective passive method for enhancing heat transfer in jet-impingement cooling systems. The findings offer valuable insights for the cooling of gas turbine blades, thermal management of electronic components, and the design of high-heat-flux systems, contributing to improved system efficiency and material durability in future energy technologies