A detailed aerothermal characterization of an advanced leading edge (LE) cooling system\ud has been performed by means of experimental measurements. Heat transfer coefficient\ud distribution has been evaluated exploiting a steady-state technique using\ud thermochromic liquid crystals (TLCs), while flow field has been investigated by means of\ud particle image velocimetry (PIV). The geometry key features are the multiple impinging\ud jets and the four rows of coolant extraction holes, and their mass flow rate distribution is\ud representative of real engine working conditions. Tests have been performed in both\ud static and rotating conditions, replicating a typical range of jet Reynolds number (Rej),\ud from 10,000 to 40,000, and rotation number (Roj) up to 0.05. Different crossflow conditions\ud (CR) have been used to simulate the three main blade regions (i.e., tip, mid, and\ud hub). The aerothermal field turned out to be rather complex, but a good agreement\ud between heat transfer coefficient and flow field measurement has been found. In particular,\ud jet bending strongly depends on crossflow intensity, while rotation has a weak effect\ud on both jet velocity core and area-averaged Nusselt number. Rotational effects increase\ud for the lower crossflow tests. Heat transfer pattern shape has been found to be substantially\ud Reynolds independen
Increasing turbine inlet temperature is one of the main strategies used to accomplish the demand for increased performance of modern gas turbines. Thus, optimization of the cooling system is becoming of paramount importance in gas turbine development. Leading edge (LE) represents a critical part of cooled nozzles and blades, given the presence of the hot gases stagnation point, and the unfavorable geometrical characteristics for cooling purposes. This paper reports the results of a numerical investigation, carried out to support a parallel experimental campaign, aimed at assessing the rotation effects on the internal heat transfer coefficient (HTC) distribution in a realistic LE cooling system of a high pressure blade. Experiments were performed in static and rotating conditions replicating a typical range of jet Reynolds number (10,000–40,000) and Rotation number (0–0.05). The experimental results consist of flowfield measurements on several internal planes and HTC distributions on the LE internal surface. Hybrid RANS–large eddy simulation (LES) models were exploited for the simulations, such as scale adaptive simulation and detached eddy simulation, given their ability to resolve the complex flowfield associated with jet impingement. Numerical flowfield results are reported in terms of both jet velocity profiles and 2D vector plots on two internal planes, while the HTC distributions are presented as detailed 2D maps together with averaged Nusselt number profiles. A fairly good agreement with experiments is observed, which represents a validation of the adopted modeling strategy, allowing an in-depth interpretation of the experimental results.
The ever increasing performance requirements of modern aeroengines necessitate the development of effective ways to improve efficiency and reduce losses. Casing temperature control is particularly critical from this point of view, since thermal expansion directly affects the blade tip clearance and thus the associated leakages. To limit the turbine tip flows, active clearance control (ACC) systems have been implemented over the last decades. These systems are usually based upon impingement cooling, generated by a series of perforated manifolds enclosing the turbine casing. When dealing with aeroengine low pressure turbines, the current trend in increasing the engine bypass ratio, so as to enhance the system propulsive efficiency, pushes the limits of ACC traditional design performance. The reduction of the pressure head at the ACC system inlet requires lower nozzle-to-target distances as well as denser impingement arrays to compensate the reduction of the jets' Reynolds number. Literature correlations for the impingement heat transfer coefficient estimation are then out of their confidence range and also RANS numerical approaches appear not suitable for future ACC designs. In this work, methodologies for the development of accurate and reliable tools to determine the heat transfer characteristics of low pressure ACC systems are presented. More precisely, this paper describes a custom designed finite difference procedure capable of solving the inverse conduction problem on the target plate of a test sample. The methodology was successfully applied to an experimental setup for the measurement of the thermal loads on a target plate of a representative low pressure ACC impinging system. The experimental outcomes are then used to validate a suitable numerical approach. Results show that RANS model is not able to mimic the experimental trends, while scale-resolving turbulence models provide a good reconstruction of the experimental evidences, thus allowing to obtain a correct interpretation of flow and thermal phenomena for ACC systems.
In the present work, two different impingement/effusion geometries have been investigated, both having staggered hole configuration and an equal number of impingement and effusion holes. The first geometry, which is designed in case of low coolant availability, has impingement hole pitch-to-diameter ratios of 10.5 in both orthogonal directions, a jet-to-target plate spacing of 6.5 hole diameters, with effusion holes inclined of 20 • with respect to the target surface. The second geometry, which is designed in case of high coolant availability, has impingement hole pitch-to-diameter ratios of 3.0, a jet-to-target plate spacing of 2.5 diameters and normal effusion holes. For each geometry, two relative arrangements between the impingement and effusion holes have been investigated, as well as various Reynolds numbers for the sparser geometry.The experimental investigation has been performed by applying a transient technique, using narrow band thermochromic liquid crystals (TLCs) for surface temperature measurement. A CFD analysis has also been performed in order to support interpretation of the results. Results show unique heat transfer patterns for every investigated geometry. Weak jet-jet interactions have been recorded for the sparser array geometry, while intense secondary peaks and a complex heat transfer pattern are observed for the denser one, which is also strongly influenced by the presence and position of effusion holes. For both the geometries, effusion holes increase heat transfer with respect to impingement-only, which can be mainly attributed to a reduction in flow recirculation for the sparser geometry and to the suppression of spent coolant flow for the denser one.
This paper reports the results of an experimental campaign involving heat transfer measurements on the target surface of an impingement jet array. Test were performed with the help of an open loop wind tunnel test rig, housing a model of the cooling system. The model general layout consists of an impingement channel, designed as a straight duct with rectangular section. A side of the channel is a steel impingement plate, while the opposite side acts as the impingement target surface and is composed of an electrically heated Inconel sheet supported by a thin steel plate. The coolant flow is provided by a plenum located upstream the impingement plate. The combined use of an inverter controlled electric fan and four rotary vanes vacuum pumps allows air circulation inside the model. Convective heat transfer coefficient on the impingement target surface is evaluated through a steady-state technique. The temperature of the target surface is measured through IR thermography: the outer side of the target surface is painted with a high-emissivity black coating and is observed by an IR camera; the inner temperature is then obtained through a simple finite difference model of the target plate. In the present work, different impingement layouts were tested (3 ≤ Sx/d ≤ 10, 3 ≤ Sy/d ≤ 20, 2.5 ≤ H/d ≤ 3.33) for different values of jet Reynolds number (2000 ≤ Rej ≤ 19000). Heat transfer results show a good agreement with the existing correlations, thus providing a validation for the adopted measurement technique, and extend the investigation to holes pitch values outside from correlations.
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