Heat transfer in a rotating rectangular channel with impingement jet and film holes

Abstract

The heat transfer characteristics in a rectangular channel under rotating conditions with impingement jet and film holes was studied. The key features of the geometry are multiple impingement jets and film holes in the leading edge model. The jet Reynolds number (Re_j), jet rotation number (Ro_j) and buoyancy number (Buo) varied from 5000 to 10,000, from 0 to 0.24, and from 0 to 0.57, respectively. Four temperature ratio (TR) cases (0.07, 0.10, 0.13, and 0.16) and three channel orientations (β = 90, 135, and 180) are selected. For all experiment cases, the spacing of jet-to-jet (s/d = 3) and jet-to-target surface (l/d = 3) were held constant. In the non-rotating cases, the heat transfer level of the T2 region (stagnation point), impinged by the jet directly, is the strongest and about twice to the other regions. The overall heat transfer distribution almost does not change significantly as the Re_j varied from 5000 to 10,000. The jet rotation number plays an important role under the rotation state. For the orientation of 135°, the Coriolis force deflects the jet and weakens the heat transfer of the medium radius in the T2 region. No obvious variation in the heat transfer distribution for different temperature ratio under non-rotating and rotating cases was observed. The Coriolis force generated by different rotation directions causes two influences on the flow structure, leading to the different heat transfer distribution. The overall heat transfer level for the orientation of 90° and 135° varies little (5%) with the rotation number, while that of 180° decreases much (25%).

Introduction

With the global industrialization, gas turbines play a vital role over the past few decades, and their techniques have developed a lot. The turbine inlet temperature is continuously increasing to achieve more excellent performance, which far exceeds the thermal limit of blade alloy materials. Internal cooling channels and efficient cooling systems are adopted to guarantee the stable operation of turbine blades. Han et al. [1] have summarized and reviewed many researches on the turbine blade cooling techniques. The cooling methods of the leading edge, middle edge and trailing edge differ depending on the position within the turbine blade. The leading edge works in a more severe condition and needs the impingement cooling method which is more efficient than other methods.

Before the effects of rotation involved, impingement cooling is influenced by many parameters: jet Reynolds number, crossflow, jet-to-jet, jet-to-target surface spacing, temperature differences, and so on. Kercher et al. (1970) [2] made an investigation about the heat transfer on a flat surface with multiple impingement jets. The results revealed that the heat transfer was dominated mainly by the jet Reynolds number and jet-to-target surface spacing. Martin et al. (1977) [3] summarized the application of impingement cooling in the field of engineering through a comprehensive investigation. Empirical equations based on experimental data of arrays of round or slot nozzles were proposed to predict heat and mass transfer coefficients. Metzger et al. (1972) [4] investigated the slot jets in two-dimensional and summarized the correlation of a wide cylindrical cavity to reach the maximum heat transfer level achieved with the best positioning of the jet to target surface. Bunker et al. (1990) [5] researched the regional internal heat transfer in the leading edge regions with impingement jet. The results indicated the heat transfer level increased with about the 0.6 power of Re_j, and with the decrease of jet-to-target spacing. Martin et al. (2013) [6] made an investigation about the stagnation region heat transfer of the leading edge impingement cooling channel by the transient lumped capacitance experiment method. They obtained the Nusselt numbers with temperature differences of jet-to-target surface from 33.3 °C to 222.2 °C and expanded the correlation to evaluate heat transfer coefficients for impingement jet. Taslim et al. (2009–2013) [7,8] investigated the effects of cross-flow on the heat transfer of leading edge model with impingement jet, and revealed that the crossflow reduced the efficiency of impinging cooling both on the stagnation and sidewall regions.

The impingement cooling and film cooling are combined to improve the heat transfer efficiency in the leading edge channel. The presence of film holes usually results in the enhancement of heat transfer performance. Metzger et al. (1990) [9] firstly measured the localized internal heat transfer distribution in the leading edge model with impingement cooling and film cooling. Results indicated that heat transfer mainly depended on jet Reynolds number and the flow extraction rate had little effect. Taslim et al. (2001–2003) [10,11] investigated the heat transfer of the curved and rough target surface with film holes, and revealed that the heat transfer coefficients were elevated by the presence of film holes and the rough target surface which increased the area of the target surface. Andrei et al. (2013) [12] studied the comprehensive effects of impingement jets and mass flow extraction on the heat transfer in the leading edge channels by means of the thermochromic liquid crystals (TLCs). They revealed that the asymmetric mass flow extraction and different crossflow condition almost did not affect the heat transfer. Zhou et al. (2018) [13,14] made an investigation about the effects of the film cooling hole diameter and location on the flow structure and heat transfer of the impingement/effusion cooling by means of numerical simulation. Results showed the diameter and position of the film cooling hole affect the heat transfer and flow structure in the leading edge model significantly.

When the turbine blade working under rotation, the Coriolis forces and centrifugal forces make the flow structure and heat transfer complicated inside the leading edge. Iacovides et al. (2005) [15] experimentally investigated the impingement cooling in the rotating channel of semi-cylinder section and found that rotation weakened the heat transfer level. Hong et al. (2009) [[16], [17], [18]] investigated the heat/mass transfer coefficients for the rotation impingement/effusion cooling system by means of the naphthalene sublimation method. The rotation led to the reduction in heat/mass transfer because of the jet deflection and diffusion. Compared to the flat surface, the more uniform heat/mass transfer is provided on the concave surface. The leading and trailing orientations resulted in the more obvious change of heat/mass transfer distributions than the front orientation due to the jet deflection. Elston et al. (2017) [19] made an investigation about the effect of rotation on impingement jet cooling within a semi-cylindrical surface. The jet was deflected away from the stagnation region, leading the heat transfer coefficients decrease with the jet rotation numbers increasing. Furlani et al. (2016) [20] used the 2D and Stereo PIV methods to investigate the flow structure in the leading edge channel with impinging jets and coolant extraction. When the orientation was 90°, the rotation effect did not have any significant influence on the jet structure because the jet was mainly parallel with the rotation axis. In the next investigation [21] by means of thermochromic liquid crystals (TLCs), results showed that when the rotation number (Ro_j) was up to 0.05, the rotation had a slight effect on the jet velocity core and area average heat transfer coefficients. Burberi et al. [22] conducted numerical simulations using the Hybrid RANS-LES turbulence model, which were in good agreement with the previous experimental results [21]. Cocchi et al. (2019) [23] investigated the cold-bridge-type leading edge cooling system to determine the rotation effects on heat transfer. The rotation reduced the lateral spreading and had a detrimental effect on heat transfer.

As mentioned above, most of the studies about the impingement cooling and film cooling methods are experimental and numerical simulation under non-rotating cases. The combined influences of rotation number and buoyance number in the rotating channel were reported less in the open literature. In addition, the effect of the channel orientation on impingement jet remains to be investigated. Considering the above questions, the experiment is to investigate how the rotation affects the flow structure and heat transfer in leading edge model with impingement jet and film holes. Therefore, the aim of the present work can be generalized as follows:

• Investigate the heat transfer characteristics and flow structure in the leading edge model with impingement jet and film holes under non-rotating and rotating conditions.

• To study the effect of Buoyancy number, the rotation experiments were carried out with the same Re_j and Ro_j but four TR (0.07, 0.10, 0.13, 0.16). Compare the rotation number and buoyancy number method for describing the heat transfer change with rotation.

• Compare the effect of different channel orientations on impingement jet and research the reasons caused the heat transfer change by the Coriolis forces and centrifugal forces.