Luca Casarsa

Experimental Investigation of the Aerothermal Performance of a High Blockage Rib-Roughened Cooling Channel

The present study deals with a detailed experimental investigation of the turbulent flow inside a rib-roughened turbine blade cooling channel. The measurements are carried out in a stationary straight channel with high blockage ribs installed on one wall. The main objective is to enhance the understanding and deepen the analysis of this complex flow field with the help of highly resolved particle image velocimetry measurements. A quasi-three-dimensional view of the flow field is achieved, allowing the identification of the main time-averaged coherent structures. The combined analysis of the present aerodynamic results with available heat transfer data emphasizes the role of the mean and fluctuating flow features in the heat transfer process. In particular, the stream wise/ normal to the wall component of the Reynolds stress tensor is shown to be strictly related to the heat transfer rate on the channel surfaces. A correlation to estimate the heat transfer field from the aerodynamic data is presented for the high blockage rib roughened channel flow.

Effects of Rotation and Channel Orientation on the Flow Field Inside a Trailing Edge Internal Cooling Channel

The flow field inside a cooling channel for the trailing edge of gas turbine blades has been numerically investigated with the aim to highlight the effects of channel rotation and orientation. A commercial 3D RANS solver including a SST turbulence model has been used to compute the isothermal steady air flow inside both static and rotating passages. Simulations were performed at a Reynolds number equal to 20000, a rotation number (Ro) of 0, 0.23, and 0.46, and channel orientations of γ = 0, 22.5, and 45, extending previous results towards new engine-like working conditions. The numerical results have been carefully validated against experimental data obtained by the same authors for conditions γ = 0∘ and Ro = 0, 0.23. Rotation effects are shown to alter significantly the flow field inside both inlet and trailing edge regions. These effects are attenuated by an increase of the channel orientation from γ = 0∘ to 45.

Flow field investigations in rotating facilities by means of stationary PIV systems

The flow field inside rotating test sections can be investigated by means of particle image velocimetry (PIV) operated in the phase-locked mode. With this experimental approach, the measurement system is kept fixed and it is synchronized with the periodical passage of the test section. Therefore, the direct output of the PIV measurements is the absolute velocity field, while the relative one is indirectly obtained from proper data processing that relies on accurate knowledge of the peripheral velocity field. This work provides an uncertainty analysis about the evaluation of the peripheral displacement field in phase-locked PIV measurements. The analysis leads to the detection of the levels of accuracy required in the estimation of both the angular velocity and the position of the center of rotation to ensure correct evaluation of the peripheral displacement field. In this regard, a simple methodology is proposed to evaluate the center of rotation position with an accuracy below 1 px. Finally, a procedure to pre-process the PIV images by subtracting the peripheral displacement is described. The advantages of its implementation are highlighted by the comparison with the performance of a more standard methodology where the peripheral field is subtracted from the absolute velocity field and not directly from the PIV raw data.

Effects of Rotation and Buoyancy Forces on the Flow Field Behavior Inside a Triangular Rib Roughened Channel

1 The flow field inside a triangular cooling channel for the leading edge of a gas turbine blade has 2 been investigated. The efforts were focused on the investigation of the interaction between effects of 3 rotation, of buoyancy forces, and those induced by turbulence promoters, i.e. perpendicular square 4 ribs placed on both leading and trailing sides of the duct. PIV and Stereo-PIV measurements have 5 been performed for ReDh=10 , rotation number of 0, 0.2, and 0.6, and buoyancy parameter equal to 0, 6 0.08, and 0.7. Coriolis secondary flows are detected in the duct cross section, but contrary to the 7 smooth case, they are characterized by a single main vortex and are less affected by an increase of 8 the rotation parameter. Moreover, their main topology is only marginally sensitive to the buoyancy 9 forces. Conversely, the features of the recirculation structure downstream the ribs turned out to be 10 more sensitive to both the buoyancy forces and to the stabilizing/de-stabilizing effect on the separated 11 shear layer induced by rotation. 12

Effect of Rotation on a Gas Turbine Blade Internal Cooling System: Experimental Investigation

A detailed aerothermal characterization of an advanced leading edge (LE) cooling system has been performed by means of experimental measurements. Heat transfer coefficient distribution has been evaluated exploiting a steady-state technique using thermochromic liquid crystals (TLCs), while flow field has been investigated by means of particle image velocimetry (PIV). The geometry key features are the multiple impinging jets and the four rows of coolant extraction holes, and their mass flow rate distribution is representative of real engine working conditions. Tests have been performed in both static and rotating conditions, replicating a typical range of jet Reynolds number (Rej), from 10,000 to 40,000, and rotation number (Roj) up to 0.05. Different crossflow conditions (CR) have been used to simulate the three main blade regions (i.e., tip, mid, and hub). The aerothermal field turned out to be rather complex, but a good agreement between heat transfer coefficient and flow field measurement has been found. In particular, jet bending strongly depends on crossflow intensity, while rotation has a weak effect on both jet velocity core and area-averaged Nusselt number. Rotational effects increase for the lower crossflow tests. Heat transfer pattern shape has been found to be substantially Reynolds independent. [DOI: 10.1115/1.4036576]

Rotating Heat Transfer Measurements on a Multi-Pass Internal Cooling Channel: II — Experimental Tests

The present contribution is focused on heat transfer measurements on internal cooling channels of a high pressure gas turbine blade in static and rotating conditions. A novel rig designed for the specific purpose was used to assess the heat transfer coefficients on a full internal cooling scheme of an idealized blade. The channel has a multi-pass design. Coolant enters at the blade hub in the leading edge region and move radially outwards inside a two-sided ribbed channel. The second passage is again a two-sided ribbed channel with a trapezoidal cross section of high aspect ratio, while inside the third leg low aspect-ratio cylindrical pin fins are arranged in a staggered configuration to promote flow turbulence. Inside the third passage, the coolant is progressively discharged at the blade trailing edge and finally at the blade tip. The test model differs with respect to the real design only because there is no curvature due to the blade camber. Conversely, the correct stagger angle of the real blade with respect to the rotation axis is preserved. Experiments were performed for static and rotating conditions with engine similar conditions of Re=21000 and Ro=0.074, both defined at the channel inlet. Transient liquid crystal technique was used for the measurement of the heat transfer coefficient (HTC) on both pressure and suction sides internal surfaces of the channel. From the spatially resolved HTC maps available, it is possible to characterize the thermal performances of the whole passage and to highlight the effect of rotation. INTRODUCTION Many different cooling methods have been developed to ensure that the turbine blade metal temperatures are maintained at a level consistent with safe and economic airfoil service life. Adoption of internal cooling is the starting point to achieve this task. Coolant air is routed to serpentine passages within the airfoil and convectively removes heat from the blade. The spent coolant can be then exhausted in multiple ways: at the blade tip, through cooling holes or slots at the trailing edge or by film cooling holes in multiple locations on the airfoil surface. This necessary cooling flow has a strong impact on the turbine efficiency, therefore improving cooling technology is always a focus point of gas turbine industry research activity, as demonstrated by the large literature available. Inside multi-pass channels different types of turbulent promoters are used to enhance heat transfer. The choice of which kind of turbulator to install is constrained by the channel cross section dimensions and aspect ratio, which in turns depends upon the blade region where the passage is located. Inclined ribs are usually found in the first legs, which have the task to cool leading edge and main body regions. Conversely, inside the thin trailing edge of the airfoil, pin-fin channels are prevalently adopted with combined benefits of structural integrity and heat transfer enhancements. ROTATING HEAT TRANSFER, CURRENT STATE-OF-ART Another important aspect that has to be taken into account in the design process of a cooling passage is the combined effect of turbulators, rotation, and channel orientation. The experimental and numerical results obtained by Hart [1], Lezius and Johnston [2], Speziale [3], and Speziale and Thangam [4] can be considered as the fundamental contributions describing the Coriolis effects on the flow field inside basic channel geometries

Coriolis Effects on the Flow Field Inside a Rotating Triangular Channel for Leading Edge Cooling

The flow field inside a rotating smooth radial channel with a triangular shaped cross section is investigated. Test conditions resemble those pertaining to the passages used for the internal cooling of the gas turbine blades leading edge. Heat transfer data are also available from the literature on the same geometry and at comparable working conditions and have been profitably used for a combined aerothermal analysis. The model consists of a straight smooth channel with an equilateral triangle cross section. The rotation axis is aligned with one of the triangle bisectors. Two dimensional particle image velocimetry (PIV) and stereo-PIV were used in order to characterize the inlet flow (in static conditions) and the rotation-induced secondary flow in the channel cross section at Re = 20,000, Ro = 0.2 and Re = 10,000, Ro = 0.4. A wider range of working conditions (Re = 10,000–40,000, Ro = 0.2–0.6) was explored by means of Reynolds averaged Navier–Stokes (RANS) simulations carefully validated by the available PIV data. The turbulence was modeled by means of the shear stress transport (SST) model with a hybrid near-wall treatment. The results show that the rotation-induced flow structure is rather complicated and show relevant differences compared to the flow models that have been considered thus far. Indeed, the secondary flow turned out to be characterized by the presence of two or more vortex cells, depending on channel location and Ro number. No separation or reattachment of these structures is found on the channel walls but they have been observed at the channel apexes. The stream-wise velocity distribution shows a velocity peak close to the lower apex and the overall flow structure does not reach a steady configuration along the channel length. This evolution is fastened (in space) if the rotation number is increased while changes of the Re number have no effect. Finally, due to the understanding of the flow mechanisms associated with rotation, it was possible to provide a precise justification of the channel thermal behavior.

Comparison of Rankine Cycles for Micro-CHP Generation Based on Inward Flow Radial Turbine or Scroll Expander

This contribution aims to analyze micro-CHP units based on Rankine cycles. Two types of expander are considered: a small scale inward flow radial turbine and a volumetric scroll type expander. This latter, should allow to overcome the limitation imposed by a standard steam-turbine that arise when the required shaft-power is very low. Moreover, the scroll expander will also allow to easily treat wet steams, which must be avoided when considering a turbo-expander. The final aim is to deduce which one of the two types of expander is more suitable, with a specified target performance and the availability of a certain hot source. In order to define the thermodynamic expansion process, the analysis uses a one-dimensional model of the radial turbine, previously developed by the authors, and of an estimation of the scroll expander efficiency. Also, the analysis is carried out for different working fluids, such as water, and two organic fluids, cyclohexane and toluene. Through the discussion of the results, for a specified set of constraints (e.g. expander inlet temperature, temperature of condensation, expander geometrical parameters) it is possible to deduce important indications on the most suitable expander for a given cycle layout.Copyright

Flow field inside a leading edge cooling channel with turbulence promoters in rotating conditions

The present work deepens the analysis of the flow field inside a triangular equilateral channel with turbulence promoters, perpendicular to the radial direction, on both leading and trailing sides, under rotation and both isothermal and nonisothermal conditions (i.e. with centrifugal buoyancy forces). Simulations have been performed at constant Re = 10,000, Ro = 0–0.2–0.6, and Bo = 0–0.08–0.7, the latter corresponding to 80℃ temperature difference between fluid and walls. These conditions match those of the particle image velocimetry measurements, used for comparison against predictions. After proper validation, the numerical modeling helped with the assessment of the flow field evolution along the radial extension of the cooling channel. It has been possible to determine the path of the coolant throughout the channel and localize where the heat transfer would have been enhanced/decreased by secondary flow structures, with respect to the stationary case. Furthermore, a rather Bo-independency of the flow field in this kind of geometry has been confirmed. The analysis presented in this paper finds support from the thermal data available from the open literature, which is rich of thermal analysis indeed, but lacks a detailed description of internal flow fields.

Gas turbine blade internal cooling: design, development, and validation of a new rig for heat transfer measurements under rotation.

The contribution describes part of the work carried out on a wider research project aimed to set up a new tool to study rotational effects on the heat transfer distribution inside realistic cooling passages for gas turbine blades. Transient thermochromic liquid crystals (TLC) measurement technique is chosen in order to obtain spatially resolved heat transfer data. This obliges to perform the transient measurements with a cold temperature step on the coolant flow, in order to replicate correctly the buoyancy effects induced by rotation. This target is achieved by a new facility which components and working principle have been the subject of previous contributions. In the present paper, the progresses made in the development of the data processing methodology are described at first. Successively, a first step into the demanding rig and methodology validation process is commented by exploiting the results of a wide test campaign on a simple cooling channel geometry.