Experimental investigation of effusion and transpiration air cooling for single turbine blade

Highlights

• First experimental study of effusion and transpiration cooling on the single blade.

• Quantitative investigation of the overall cooling effectiveness on the blade.

• Qualitative investigation of flow structure via smoke-laser sheet visualization.

• Transpiration cooling achieves superior cooling than effusion and internal cooling.

Abstract

A great number of studies have been conducted on a film cooling for turbine blades, which is to prevent thermal damage on blades originated from high turbine inlet temperature. However, film cooling with several rows of cooling-holes results in lifting-off of coolant film and limited cooling on a restricted area due to flow reattachment. In this study, effusion and transpiration cooling were applied to the single C3X blade. A multiple hole-array with a diameter of 0.5 mm was fabricated by the electric discharging machining, and a porous structure with an equivalent pore diameter of 40 μm was manufactured by the 3-D metal additive manufacturing. Experiments were performed in the high-temperature subsonic wind tunnel, which has a freestream temperature of 100 °C and a velocity of 20 m/s. The surface temperature of blades was measured using infrared thermometry with a specially designed protocol to eliminate background radiation errors from the surroundings. Also, the outflow of coolant from blades was investigated with smoke-laser sheet visualization. The overall cooling effectiveness was quantitatively analyzed on the pressure-side, suction-side, and leading-edge of blades. Due to the enhancement of convective cooling through porous media, transpiration cooling achieves 34% and 25% higher cooling effectiveness than effusion and internal cooling each.

Introduction

The gas turbine combined cycle has been widely used and developed since it is one of the power generation systems that have great advantages in both the environmental and economic aspects [1]. According to the second law of thermodynamics, the efficiency of a gas turbine cycle increases with the turbine inlet temperature (TIT). The TIT has been increasing over the years due to real economies of higher efficiency at higher firing temperatures. The increase of TIT makes the turbine blade being exposed in thermally severe conditions, leading to thermal damage on it [2]. Therefore, in the past, to alleviate the problems caused by the thermal load on the blade, internal cooling, external film cooling, and thermal barrier coating (TBC) have been studied [3], [4]. Recently, as TIT continues to increase further, advanced cooling techniques, such as combined cooling (film cooling with jet impingement) and micro cooling (effusion and transpiration cooling), have drawn attention.

To cool down the heated blade through internal cooling, structures for heat exchange and flow control, such as serpentine passage with ribs, also called turbulence promoters [5], [6], pin fin arrays [7], [8], and jet impingements [9], [10] were installed to enhance heat transfer inside the blades. Recently, to enhance the internal cooling performance, the geometry of ribs and fillets inside the cooling passage was optimized through the evolutionary algorithm [11], which has been in the spotlight of late, with considering the heat transfer coefficient, pressure drop, and the area of cooling passage as the multiple objective functions. Additional to the internal cooling, film cooling has also been actively studied to further enhance the cooling performance by forming a film with cooling air on the blade surface to obstruct the heat transfer from the hot mainstream [12], [13]. Major design parameters for film cooling are hole size, pitch [14], [15], arrangement [16], shape [17], [18], and the injection angle of holes [19], [20]. Through combining the film cooling and the jet impingement, called conjugate heat transfer, several studies showing the improved overall convective heat transfer on the blade have been also reported [21], [22].

Effusion cooling has been suggested to overcome the limitation of film cooling [23], [24]. Differences between the film and effusion cooling are the diameter and the number of cooling holes [25]. With multiple arrays of micro-sized cooling holes, the effusion cooling could generate uniform cooling film on the entire surface and also shows better utilization of coolant due to more efficient convective cooling through densely arranged holes. Ngetich et al. [26] investigated the performance of double-wall effusion cooling geometry through the three-dimensional (3-D) conjugate simulation that can quickly predict the blade cooling performance distribution with only a small wall element by combining existing correlations. Murray et al. [27] demonstrated the overall cooling effectiveness of double-wall effusion cooling geometry through the specially designed heat transfer experimental facility with infrared thermography. They validated the developed conjugate computational fluid dynamics model to interpret the flow structure of the double-wall geometry. In the same vein as effusion cooling, transpiration cooling using micro-sized porous structures has also drawn attention. Randomly dispersed porous acts like densely spaced micro-sized holes like that of effusion cooling. Therefore, transpiration cooling could also produce a uniform insulating layer of cooling film on a broader surface. Furthermore, due to the large porosity of the structure, a much larger heat transfer area through the porous interface than micro-machined cooling holes could significantly enhance the convective cooling [28], [29], [30]. Liu et al. [31] investigated the impact of particle diameter and thermal conductivity of a solid matrix on the transpiration cooling performance with sintered porous flat plates composed of bronze and stainless steel. However, the transpiration cooling using the porous structure has difficulty in manufacturing and controlling pore size. To overcome the difficulty, Liu et al. [32] and Xu et al. [33] demonstrate the transpiration cooling through the sintered woven wire mesh structures with the control of porosity and blowing ratio. Besides, with the recent development of additive manufacturing technology using metal 3-D printing, Min et al. [34] and Huang et al. [35] experimentally investigated the cooling effectiveness of transpiration cooling in a flat plate geometry produced through precisely controlled additive manufacturing of various porous structures.

Unlike the previous studies that investigate the cooling performance on a simple flat plate geometry, the present paper provides the experimental investigation of the overall cooling effectiveness on the single C3X blade where the effusion and transpiration cooling was applied. Through the electric discharging machining (EDM) and 3-D metal additive manufacturing, effusion holes and porous structure were manufactured on the blade surface. Infrared (IR) thermometry was employed to measure the temperature distribution on the surface. Nowadays, IR thermometry has been widely used due to its intuitive thermal image as well as no external disturbance with non-contact measurement. However, the indiscriminate use of IR thermometry without a theoretical understanding causes a large measurement error. Therefore, in this work, a specially designed test section and a unique temperature conversion protocol were used to measure the surface temperature of a turbine blade. The overall cooling effectiveness of three different blades with internal cooling, effusion cooling, and transpiration cooling was analyzed and compared with measured surface cooling temperature. The flow structure of the coolant released from the blade surface was visualized for effusion and transpiration cooling to improve the qualitative interpretation of the overall cooling effectiveness on the blade surface. This study is expected to be the groundwork for providing insight into the application of micro cooling techniques to experiment with a cascade of blades and the actual turbine system further.