Numerical Study on Flow and Heat Transfer of a Cooled First Stage Vane of a Gas Turbine

This paper presents a numerical simulation of composite cooling on a first stage vane of a gas turbine, in which gas by fixed composition mixture is adopted. To investigate the flow and heat transfer characteristics, two internal chambers which contain multiple arrays of impingement holes are arranged in the vane, several arrays of pin-fins are arranged in the trailing edge region, and a few arrays of film cooling holes are arranged on the vane surfaces to form the cooling film. The coolant enters through the shroud inlet, and then divided into two parts. One part is transferred into the chamber in the leading edge region, and then after impinging on the target surfaces, it proceeds further to go through the film cooling holes distributed on the vane surface, while the other part enters into the second chamber immediately and then exits to the mainstream in two ways to effectively cool the other sections of the vane. In this study, five different coolant flow rates and six different inlet pressure ratios were investigated. All the cases were performed with the same domain grids and same boundary conditions. It can be concluded that for the internal surfaces, the heat transfer coefficient changes gradually with the coolant flow rate and the inlet total pressure ratio, while for the external surfaces, the average cooling effectiveness increases with the increase of coolant mass flow rates while decreases with the increase of the inlet stagnation pressure ratios within the study range.

Measurements and Modelling of Ingress in a New 1.5-Stage Turbine Research Facility

In gas turbines, hot mainstream flow can be ingested into the wheel-space formed between stator and rotor discs as a result of the circumferential pressure asymmetry in the annulus; this ingress can significantly affect the operating life, performance and integrity of highly-stressed, vulnerable engine components. Rim seals, fitted at the periphery of the discs, are used to minimise ingress and therefore reduce the amount of purge flow required to seal the wheel-space and cool the discs. This paper presents experimental results from a new 1.5-stage test facility designed to investigate ingress into the wheel-spaces upstream and downstream of a rotor disc. The fluid-dynamically-scaled rig operates at incompressible flow conditions, far removed from the harsh environment of the engine which is not conducive to experimental measurements. The test facility features interchangeable rim-seal components, offering significant flexibility and expediency in terms of data collection over a wide range of sealing-flow rates. The rig was specifically designed to enable an efficient method of ranking and quantifying the performance of generic and engine-specific seal geometries. The radial variation of CO2 gas concentration, pressure and swirl is measured to explore, for the first time, the flow structure in both the upstream and downstream wheel-spaces. The measurements show that the concentration in the core is equal to that on the stator walls and that both distributions are virtually invariant with radius. These measurements confirm that mixing between ingress and egress is essentially complete immediately after the ingested fluid enters the wheel-space and that the fluid from the boundary-layer on the stator is the source of that in the core. The swirl in the core is shown to determine the radial distribution of pressure in the wheel-space. The performance of a double radial-clearance seal is evaluated in terms of the variation of effectiveness with sealing flow rate for both the upstream and the downstream wheel-spaces and is found to be independent of rotational Reynolds number. A simple theoretical orifice model was fitted to the experimental data showing good agreement between theory and experiment for all cases. This observation is of great significance as it demonstrates that the theoretical model can accurately predict ingress even when it is driven by the complex unsteady pressure field in the annulus upstream and downstream of the rotor. The combination of the theoretical model and the new test rig with its flexibility and capability for detailed measurements provides a powerful tool for the engine rim-seal designer.

Uncertainty Quantification of Thermocouple Air Temperature Measurement in Highly Radiative Environment: Application to Turbofan Engine Compartment

This paper focuses on thermocouple air temperature measurement uncertainty due to the radiative fluxes present in the engine compartment where engine case skin temperature can exceed 900 K. To really measure air temperature, the convective heat flux in the thermocouple bead must be predominant. This is why heat shields are used in order to reduce the radiative heat flux on the bead. However, in engine compartment, the heat shield orientation must be optimized since numerous hot walls surround the thermocouple. In order to evaluate the impact of badly oriented heat shields and to provide a data bank for numerical simulation validations, a heated wind tunnel has been used. It has been shown that the uncertainty on the thermocouple temperature can reach dozens of degrees depending on the air speed and the heat shield orientation. Furthermore a specific 3D thermocouple model has been build and validated by comparison with the lab measurements. Then this thermocouple 3D model has been integrated in the whole engine compartment aero-thermal model in order to quantify the uncertainty of the thermocouple air temperature measurement in the real engine environment.

Laboratory Infra-Red Thermal Assessment of Laser-Sintered High-Pressure Nozzle Guide Vanes to De-Risk Engine Design Programmes

The continuing maturation of metal laser-sintering technology (DMLS) presents the opportunity to de-risk the engine design process by experimentally down-selecting HPNGV cooling designs using laboratory tests of laser-sintered — instead of cast — parts to assess thermal performance. Such tests could be seen as supplementary to thermal-paint-test engines, which are used during certification to validate cooling system designs. In this paper, we compare conventionally cast and laser-sintered titanium alloy parts in back-to-back experimental tests at engine-representative conditions over a range of coolant mass flow rates. Tests were performed in the University of Oxford Annular Sector Heat Transfer Facility. The thermal performance of the cast and laser-sintered parts — measured using new infra-red processing techniques — is shown to be very similar, demonstrating the utility of laser-sintered parts for preliminary engine thermal assessments. We conclude that the methods reported in this paper are sufficiently mature to make assessments which could influence engine development programmes.

Modeling of Compressor Drum Cavities With Radial Inflow

Reynolds-Averaged Navier-Stokes (RANS) computations have been conducted to investigate the flow and heat transfer between two co-rotating discs with an axial throughflow of cooling air and a radial bleed introduced from the shroud. The computational fluid dynamics (CFD) models have been coupled with a thermal model of the test rig, and the predicted metal temperature compared with the thermocouple data. CFD solutions are shown to vary from a buoyancy driven regime to a forced convection regime, depending on the radial inflow rate prescribed at the shroud. At a high radial inflow rate, the computations show an excellent agreement with the measured temperatures through a transient rig condition. At a low radial inflow rate, the cavity flow is destabilized by the thermal stratification. Good qualitative agreement with the measurements is shown, although a significant over-prediction of disc temperatures is observed. This is associated with under prediction of the penetration of the axial throughflow into the cavity. The mismatch could be the result of strong sensitivity to the prescribed inlet conditions, in addition to possible shortcomings in the turbulence modeling.

Numerical Characterization of Hot Gas Ingestion Through Turbine Rim Seals

This paper deals with a numerical study aimed at the characterization of hot gas ingestion through turbine rim seals. The numerical campaign focused on an experimental facility which models ingress through the rim seal into the upstream wheel-space of an axial-turbine stage. Single-clearance arrangements were considered in the form of axial- and radial-seal gap configurations. With the radial-seal clearance configuration, CFD steady-state solutions were able to predict the system sealing effectiveness over a wide range of coolant mass flow rates reasonably well. The greater insight of flow field provided by the computations illustrates the thermal buffering effect when ingress occurs: for a given sealing flow rate, the effectiveness on the rotor was significantly higher than that on the stator due to the axial flow of hot gases from stator to rotor caused by pumping effects. The predicted effectiveness on the rotor was compared with a theoretical model for the thermal buffering effect showing good agreement. When the axial-seal clearance arrangement is considered, the agreement between CFD and experiments worsens; the variation of sealing effectiveness with coolant flow rate calculated by means of the simulations display a distinct kink. It was found that the “kink phenomenon” can be ascribed to an over-estimation of the egress spoiling effects due to turbulence modelling limitations. Despite some weaknesses in the numerical predictions, the paper shows that CFD can be used to characterize the sealing performance of axial- and radial-clearance turbine rim seals.

Conjugate Heat Transfer Simulation of the Intercooled Compressor Vanes Based on Discontinuous Galerkin Method

Convective cooling channels are applied in a two-dimensional compressor vane to use the intercooling method to improve the efficiency of Brayton cycle and reduce the temperature of the vane. In this paper, we analyze the effect of coolant to the aerodynamic performance and heat transfer performance of the main stream and the vane. For the case of a two-dimensional compressor vane NACA65-(12A2I8b)10, the vane which has five convective cooling channels has been numerically simulated in different test conditions by discontinuous Galerkin (DG) method. The coolant is supercritical carbon dioxide whose pressure is 10MPa. Conjugate heat transfer method has been used in this paper. The numerical simulation result is similar to the experiment data and has been compared with the result of the vane without cooling channels to prove the effect of cooling channels. Cooling channels have large effect on the distribution of temperature and heat transfer coefficient. In addition, the relationship between Nu and Re on the fluid-solid interface has been analyzed and a suitable empirical equation has been obtained. This work analyzes the effect of intercooling system in the compressor and give several advice on future engineering applications in aero engines and gas turbines.

Detailed Experimental Characterization of Heat Transfer Coefficient Over the Internal Cooling Passages of an Additive Manufactured Turbine Airfoil

This report describes the detailed experimental study to characterize the local heat transfer coefficient distribution over the internal cooling passages of a simplified generic airfoil. The airfoil is manufactured through additive manufacturing based on actual geometry and dimensions (1X scale model) of row one airfoil, applicable in large gas turbine system. At the mainbody section, the serpentine channel consists of three passages without any surface features or vortex generators. Both the leading edge and trailing edge sections are subjected to direct impingement. The trailing edge section is divided into three chambers, separated by two rows of blockages. This study employs the well-documented transient liquid crystal technique, where the local heat transfer coefficient on both pressure and suction sides is deduced. The experiments were performed at varying Reynolds number, ranging from approximately 31,000–63,000. The heat transfer distribution on the pressure side and suction side is largely comparable in the first and third pass, except for the second pass. Highest heat transfer occurs at the trailing edge region, which is ultimately dominated by impingement due to the presence of three rows of blockages. A cursory numerical calculation is performed using commercially available software, ANSYS CFX to obtain detailed flow field distribution within the airfoil, which explains the heat transfer behavior at each passage. The flow parameter results revealed that the pressure ratio is strongly proportional with increasing Reynolds number.

Impingement Cooling in Narrow Rectangular Channel With Novel Surface Features

This paper describes a detailed experimental investigation of narrow jet impingement channel with surface features. Three novel surface features: aerofoil shaped dimple cavities on the target plate, chevron elements extending between the jet issuing plate and the target plate and 45 degree wedges mounted on the jet-issuing plate, are proposed. The narrow rectangular channel is 254 mm × 57.2 mm × 19.1 mm (10” × 2.25” × 0.75”) in dimensions and consists of five jets with a constant diameter, D of 9.525 mm (0.375”). The inter-jet spacing and jet-to-target plate distance is 4D and 2D, respectively. Three test cases with different novel surface features are proposed, and the effect of these surface features on the distribution of heat transfer coefficient on the target plate is characterized using the transient liquid crystal technique. In the first test case, dimpulated surface features are introduced on the target plate. The second case consists of chevron elements which extend between the jet issuing plate and the target plate, while the third case has 45 degree wedges mounted on the jet-issuing plate. The smooth jet impingement channel is used as a baseline case for comparison of the heat transfer coefficient distribution on the target plate. The Reynolds number is defined based on the jet diameter, D and bulk velocity of the jet. The experiments were performed at Reynolds number ranging between 61,000 to 98,000. In order to gain a better insight of the flow field within the channel for each of these features, a steady state numerical simulation was performed for each case using the commercially available software, ANSYS CFX. The boundary conditions for these simulations were set as close to the experimental conditions as possible. For turbulence closure, the Shear Stress Transport (SST) model was used which has been shown to be reasonably accurate with moderate computational costs. The numerical results are in favorable trend compared to the values obtained through experimentation. However, in certain regions, the SST turbulence model has overpredicted the heat transfer coefficient values. The results show that the first test case with dimpulated surface features exhibits the highest heat transfer enhancement among all the tested configurations. This enhancement is approximately 25 percent higher than that of the baseline case. The presence of the chevron elements has minimized the deflection of the jets due to crossflow, but, inhibited the spreading of the impinging jets on the target plate. This, in turn, has reduced the local heat transfer performance quite substantially. In case of the 45 degree wedges, the heat transfer enhancement was augmented at the downstream, which was ultimately caused by the diversion of the crossflow towards the target plate.

Effect of Surface Roughness on Conjugate Heat Transfer of a Turbine Vane

For the purpose of researching the effect of surface roughness on boundary layer transition and heat transfer of turbine blade, a roughness modification approach for γ-Reθ transition model was proposed based on an in-house CFD code. Taking surface roughness effect into consideration, No. 5411 working condition of Mark II turbine vane was simulated and the results were analyzed in detail. Main conclusions are as follows: Surface roughness has little effect on heat transfer of laminar boundary layer, while has considerable effect on turbulent boundary layer. Compared with smooth surface, equivalent sand roughness of 100μm increases the temperature for about 28.4K on suction side, reaching an increase of 5%. Under low roughness degree, effect of shock wave dominants on boundary layer transition process on suction side, while above the critical degree, effect of surface roughness could abruptly change the transition point.