Investigating the effect of impinging jet and rib usage on the cooling performance of the channel surface

Jet impingement cooling (JIC) is a technique commonly used to cool surfaces exposed to high temperatures, such as the leading edges of gas turbine blades. In this method, a high-speed fluid jet is directed towards the target surface, increasing heat transfer on the surface and preventing material overheating. Jet impingement cooling is a widely employed cooling method in energy production and aviation, especially with advancements in gas turbine technology leading to higher temperatures for turbine blades. As the maximum operating temperature of the target surface continuously increases due to advancements in gas turbine technology, it becomes crucial to prevent excessive local surface temperatures. Elevated temperatures in specific regions can not only diminish the aerodynamic efficiency of the blades but may also contribute to an increased likelihood of structural deformations and deformities. The consequence of failing to achieve uniform cooling on the target surface extends beyond immediate operational challenges, exerting a substantial impact on the overall performance and longevity of the gas turbine blades. Non-uniform cooling introduces a host of detrimental effects that compromise the functionality and structural integrity of the blades. In order to enhance heat transfer on the target surface and achieve a uniform distribution of heat transfer in the internal cooling channels of gas turbine blades, structures such as pins and ribs are preferred on the surface. Pins and ribs act as key elements in this thermal enhancement strategy. By introducing these features on the surface, the flow dynamics of the cooling fluid are modified, promoting increased convective heat transfer. The pins create localized turbulence in the fluid flow, enhancing the heat exchange process and preventing the formation of stagnant boundary layers. This, in turn, helps in achieving a more uniform distribution of cooling across the target surface. In addition to improving heat transfer efficiency, the presence of pins and ribs contributes to the prevention of hot spots and ensures that the cooling effect extends consistently throughout the internal cooling channels. This becomes especially crucial in preventing localized overheating, which could lead to material degradation, reduced performance, and a shorter lifespan of the blade. In this study, an extended jet impingement model with a flat target surface was employed. The flat target surface was initially roughened with elliptical and slot-shaped pins, and the effects of pin arrangements on heat transfer performance were numerically investigated. Various pin heights were examined for variable nozzle lengths, different pin arrangements, and Reynolds numbers (Re) in a rectangular channel flow. The pins were arranged in a staggered and circular pattern on the target surface to enhance heat transfer and achieve a uniform distribution of heat transfer. The numerical results and solution procedure were validated by comparing them with experimental data from the literature. The analyses were conducted using the SST k-ω turbulence model, considering boundary conditions. The numerical model accurately predicted heat transfer and pressure drop for both smooth and pin-roughened target surfaces. Average Nusselt numbers (Nu) and pressure drops were numerically determined for different dimensionless pin heights (Hp/d = 0.167, 0.417, 0.667), various pin arrangements (R1, R2, R3), dimensionless nozzle lengths (G/d = 1.0, 2.0, 6.0), and Reynolds numbers (16250, 27100, 32500). The numerical results were compared with the orifice plate model, i.e., the traditional impinging jet model (G/d = 6.0 and Hp/d = 0). For the elliptical-pinned flat target surface, a maximum increase of 35.82% in the average Nu number was observed at Re = 16250 for R2_G/d = 1.0 ve Hp/d = 0.167. Optimal parameters for the entire range of Reynolds numbers, considering flow characteristics and thermal performance criteria (TPC), were determined as R3_G/d = 1.0 and Hp/d = 0.167. Additionally, increasing the number of elliptical pins improved the homogeneity of the local heat transfer coefficient distribution. Furthermore, when a slot pin geometry was used, the maximum heat transfer increase was calculated as 33.2% for parameters R1_G/d = 1.0 and Hp/d = 0.167. Experimentally, to reduce the interaction between the jet flow and crossflow and ensure that jets extended towards the target surface in the potential core region, jets were elongated using nozzles. Finally, to enhance convective heat transfer and turbulence intensity within the channel, the target surface was roughened with conical pins immediately after the jets. The effects of dimensionless nozzle heights (G/d = 1.0, 2.0, 3.0, 6.0), dimensionless conical pin heights (Hc/d = 0.67, 1.00 and 1.33) on heat transfer and flow characteristics were determined. Experimental data were obtained using Sakarya University Mechanical Engineering TLC Laboratory, and liquid crystal thermography was used to determine the heat transfer coefficients on the identified models. Optimal nozzle length and conical pin geometry were determined to achieve the best thermal performance. Additionally, the proposed model resulted in approximately a minimum 5% increase in average heat transfer coefficient on the target surface with negligible pressure loss. The aforementioned studies mainly focused on flat target surfaces, while the internal surfaces of gas turbine blades often have a concave geometry. The pin configurations mentioned can also be applied on concave surfaces, especially to enhance heat transfer and create a more homogenous temperature distribution. Consequently, pin configurations on concave surfaces can be adopted as a strategy to improve the overall heat transfer performance of jet impingement cooling systems, similar to the approach for flat target surfaces. In this context, a channel flow with a semi-circular shape and a jet array was used to roughen a concave surface with elliptical and aerofoil pins. The fundamental goal of these studies was to determine the heat transfer performance and flow characteristics of elliptical and aerofoil pin combinations with an extended jet hole and to demonstrate their applicability in cooling a turbine blade. Additionally, in a different study, the interaction between ribs and extended jets was investigated for the same model. Experimental data from the literature were used to validate the numerical solution procedure, and the low-Re corrected SST k-w turbulence model was preferred for solving the turbulence equations. In numerical studies of models where elliptical pins were placed on concave target surfaces, a fixed three-row (R3) pin arrangement was preferred, and analyses were conducted for different Reynolds numbers (Re = 5000, 15000, and 25000), dimensionless nozzle-target surface distances (0.5 ≤ G/d ≤ 8.0), and dimensionless orifice plate-target surface distances (H/d = 4.0 and 8.0). For physical models with aerofoil pins mounted on concave surfaces, H/d was taken as 8.0. Furthermore, an evaluation was made regarding the effect of single-row (R1), double-row (R2), and triple-row (R3) pin arrangements along the concave surface on convective heat transfer. Average Nu numbers, local Nu contours, flow characteristics, and thermal performance criteria on smooth and pinned target surfaces were thoroughly examined. The results showed that reducing G/d and roughening the surface with pins increased both local and area-averaged Nu numbers compared to the traditional impinging jet configuration. For Re = 25000 at H/d = 8.0, maximum heat transfer enhancement of 55.68% was achieved with an extended jet (G/d = 0.5) and elliptical pin-roughened surface design. Additionally, the highest TPC value for the elliptical-pinned model was calculated as 1.10 for G/d = 2.0 and H/d = 8.0 at Re = 25000. For the aerofoil pin design, the highest heat transfer increase on the concave surface was 52.81% for the R2 pin arrangement at Re = 25000 and G/d = 0.5. The maximum TPC value was obtained as 1.12 for the R2 pin arrangement at Re = 25000 and G/d = 2.0. In a different study, it was observed in the analysis of smooth concave surfaces that a dead flow region occurred between adjacent impinging jets due to the gradual jet array and the curvature effect of the concave surface. This region led to low heat transfer areas on the concave surface. To overcome this disadvantage, rectangular-sectioned V-shaped ribs (VSR) were placed in regions where the stagnation point occurred. The effect of different dimensionless rib heights (Hr/d = 0, 0.2, 0.3, and 0.4), dimensionless nozzle-target surface distances (G/d = 0.5, 2.0, 4.0, and 8.0), and rib angles (α = 30˚, 45˚, 60˚, and 90˚) on the concave surface was numerically investigated to increase convective heat transfer, reduce thermal stress, and achieve a more uniform heat transfer distribution. The results indicated an increase in overall heat transfer and a more uniform heat transfer compared to the traditional jet impingement configuration. The most significant increase in heat transfer due to the combination of extended jets with VSR was calculated as 47.23% for Hr/d = 0.2 and α = 45˚ with a reduction of G/d to 0.5. Additionally, the highest TPC value in the proposed model was determined as 1.07 for G/d = 2.0, Hr/d = 0.2, and Re = 25000. In light of the findings, it is apparent that surfaces enhanced with pin configurations substantially influence the uniformity of both average and local heat transfer coefficients within extended jet impingement cooling systems. This innovative approach holds the potential to mitigate the deleterious impacts of thermal stresses on the pertinent surface, thereby prolonging the operational life of the blade material. As a result, the cooling performance of the surface demonstrates improvement when compared to a smooth surface, accompanied by a noteworthy reduction in local temperatures, effectively minimizing the risk of regional overheating. The development of additive manufacturing technologies makes it possible to use such geometries in the internal cooling channels of gas turbine blades, contributing significantly to the literature based on the results of this study.