The interaction between upstream and downstream film cooling rows in flow field and heat transfer

Heavy-duty engines and gas turbines depend on higher turbine inlet temperature to achieve the desired power output and thermal efficiency. Unfortunately, the high temperatures have resulted in a significantly high cost of cooling and protecting the hot components from damages. Currently, the film cooling method is one of the most common cooling process in engines and turbines. Nevertheless, the cooling effectiveness distribution for single rows of film cooling holes is low, and multiple rows of holes are used to achieve the cooling purpose. Film cooling effectiveness depends on several factors, such as turbulence intensity and boundary layer thickness. Recent studies have revealed that the performance of downstream cooling ejections heavily depends on the upstream ejections. Therefore, a thorough understanding of the interaction between the upstream and downstream cooling films is highly desirable, especially in determining the cooling effectiveness of multiple row cooling films.

Recently, Tsinghua University scientists: Dr. Lang Wang, Professor Xueying Li, Professor Jing Ren, and Professor Hongde Jiang investigated the interaction between the upstream and downstream film cooling rows for evaluating and predicting the cooling effectiveness. Their work is currently published in International Journal of Thermal Sciences.

The research team specifically examined the interaction between one film cooling row and two rows of in-line holes at a small spacing ratio of S/D = 6. The large eddy simulation model is used to analyze both the flow field and heat transfer characteristics, which was validated through an experiment in which a pressure-sensitive paint was used to measure the film cooling effectiveness. In their work, the researchers also analyzed the influence of turbulence on the heat transfer feature and flow fields.

The authors observed the thickening of the downstream boundary layer by the upstream ejection, which increased the penetration depth of the downstream coolant flow and a subsequent thinner coolant coverage of the film cooling ejection. This was attributed to the disturbed horseshoe vortex and strong kidney vortex, which mainly function to increase the lateral diffusion and uplift effects, respectively. Both the second-row ejection and the single row exhibited different cooling performances, with strong evidence of row-to-row interaction. For instance, whereas the first-row coolant jet exhibited broad lateral coverage as a result of entrainment and deflection, the second-row coolant was observed to cover a small area near the centerline. Furthermore, it was worth noting that the first-row film cooling performance was greatly influenced by the flow turbulence intensity than the second-row film cooling because the latter is under the influence of complex vortices generated from the first row.

In a nutshell, the study successfully investigated a row-to-row interaction of the upstream and downstream cooling films in the flow field and heat transfer based on the experiment and large eddy simulation methods. Based on the findings, it can be generally concluded that the film cooing effectiveness the downstream holes is reduced when compared to the first row. In a statement to Advances in Engineering, the authors pointed out that the study results would be of high value in guiding the design of full coverage film cooling.