Case Studies in Thermal Engineering 27 (2021) 101221 2 with the actual dimensionless temperature by defining the tangential velocity recovering coefficient, which is verified with the experimental results. Moreover, a linear relationship between the dimensionless temperature drop and the effective swirl ratio at the nozzle outlet can be deduced further. Especially, this relationship shows a good agreement between the predicted values and numerical results. Since the dimensionless temperature

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... p decreases with the increase of the dimensionless power consumption; an important discovery can be concluded that the algebraic sum of these two physical quantities is always equal to 1. Therefore, the results can offer a reference for the design and optimization of gas turbine cover-plate type pre-swirl system. Introduction The role of the pre-swirl system is used to provide cooling air at an appropriate level of mass flow rate and pressure in the field of gas turbine engines [1] . As one of the major components in the internal air system, the pre-swirl system can be applied to offer the low-temperature airflow into the turbine rotating blade [2, 3] . The pre-swirl system is usually divided into cover-plate system and direct transfer system. Generally, the pre-swirl nozzle is installed inside the stator side of the pre-swirl system. The pre-swirl nozzles play a very important role in the system: it swirls the cooling air in the rotor rotation direction, and this reduces the work done by the rotor to the cooling air as well as the relative total temperature of the air supplied to turbine blades [4] . Especially, the temperature drop in the pre-swirl system comes from the acceleration of the expansion of the nozzle [5] . Then, the swirling flow effect is of significance for aerodynamic and thermodynamic characteristics in the flow field [6] . As well as, the swirl flow is of concern for heat transfer characteristics [7, 8] . Therefore, it is worthwhile to reveal the flow and temperature characteristics in the pre-swirl system so as to improve the cooling effect in the secondary air system. It is necessary to conduct an experimental and theoretical analysis of the direct-transfer type pre-swirl system. Then, Meierhofer et al. [9] were the first to conduct the experimental investigation on a direct transfer pre-swirl system. They found that the system temperature drop is closely associated with the circumferential velocity at the pre-swirl nozzle outlet. And then, the temperature drop effectiveness was concluded to be as a function of the rotational speed of the turbine disc and the effective swirl ratio of the pre-swirl nozzle outlet [10] . As a performance evaluation index, the discharge coefficients of the supply hole and pre-swirl nozzle were measured in the direct-transfer type pre-swirl system [11] . On the hand, the discharge coefficients of the supply hole can be determined by the relative reference frame. On the other hand, the discharge coefficient of the supply hole inside the direct-transfer system also can be measured, considering the effect of the work done by the rotor. Furthermore, Chew et al. [12] studied the nozzle discharge behavior and system temperature drop in a cover-plate system. Gei et al. [13] presented that the measured temperature drop was significantly lower than the ideal value. And in addition to the pre-swirl nozzle, more flow losses occurred in the rotating components downstream of the nozzle. Hence, the effect of mass flow rate, swirl ratio, the number of the nozzles, and rotating speed on total pressure loss was investigated in a pre-swirl rotor-stator cavity [14] . Then, Bricaud et al. [15] analyzed thermodynamic loss of a direct transfer pre-swirl system by measuring discharge coefficients, total temperature, and velocity. Benim et al. [16] [17] [18] reported a numerical analysis of the direct-transfer type pre-swirl system. Especially, the numerical results were qualified with experimental test results and also proved to be sufficiently accurate [19, 20] . It can be concluded that the pressure loss in the rotating parts is the dominant loss source, especially for the supply hole. Thus, to reduce system entropy increase, the swirl ratio at supply hole inlet could be as close to the ideal value of 1 as possible. For some researches on the cover-plate type pre-swirl system, a combined experimental and analytical study for a cover-plate system was carried out by Karabay et al. [2]. They discovered that the cooling air in the cover-plate cavity (rotor-rotor cavity) would flow radially outward following the free vortex law, when the air flow rate in the system was large enough. Chew et al. [21] measured and analyzed the pre-swirl nozzle discharge coefficient and temperature drop of the cover-plate pre-swirl system. Lewis et al. [22] investigated the effect of the pre-swirl nozzles at the radial location on heat transfer. And, Liu et al. [23] revealed the effect of pre-swirl angles and inlet positions based on the experimental study. It was discovered that the cavity pressure distribution is evidently influenced by the inlet position and the pre-swirl angle. Furthermore, the discharge coefficient of pre-swirl nozzle increases as the pressure ratio enhances. Recently, the effect of the pre-swirl nozzle closure ratio on the unsteady flow of the pre-swirl system was studied by Lei et al. [24, 25] . The aerodynamic performance of the receiver hole on the influence of the pre-swirl system temperature drop has been reported by Gong et al. [26, 27] . The one-dimensional predicted model was built by Liu et al. to predict the velocity of the sealing flow and the swirl ratio at the receiver hole inlet [28] . It can be summarized that adjusting the swirl ratio distribution in the system is important to reduce the rotor entropy increase, and improve the system cooling performance. To get a high temperature drop, it is interesting to note that the impeller was designed and installed in the cover-plate cavity [29] . Especially, the effect of impellers was also researched with three-dimensional CFD analyses [30] [31] [32] . Gupta et al. [30] pointed out that the rotating impellers can enhance the swirl flow inside the cover-plate cavity and reduce the relative velocity at supply hole. Hence, the effect of impellers in the low radius pre-swirl system can contribute to a greater pressure rise in the rotor and a lower pressure loss at the supply hole inlet, which is beneficial to improve the system cooling performance. However, the impellers will also increase the power consumption of the system so as to result in a work loss of the engine. Because the application of impellers in high radius pre-swirl systems is severely limited due to the radial space limitation. Simultaneously, thermal management and power saving operations are important evaluation indexes. Therefore, the power consumption is worthy of further study in the pre-swirl system. As can be concluded, there have been few experimental results reported on the measurement of the airflow temperature and