Heat transfer study on gas turbine blades is very important due to the resultant increase in cycle thermal efficiency. This study is focused on the heat transfer effects on a reference nozzle guide vane and test rig component preparation in heat and power technology division at KTH. In order to prepare the current test rig for heat transfer experiments, some feature should be changed in the current layout to give a nearly instant temperature rise for heat transfer measurement. The heater mesh component is the main component to be added to the current test rig. Some preliminary design parameters were set and the necessary power for the heater mesh to achieve required step temperature rise was calculated. For the next step, it is needed to estimate the heat transfer coefficient and the other parameters for study on the reference blade using numerical methods. Boundary layer analysis is very important in heat transfer modeling so to model the reference blade heat transfer and boundary layer properties, a 2D boundary layer code TEXSTAN is used and the velocity distribution around the vane was set to an input dataset file. After elements refinement to ensure the numerical accuracy of TEXSTAN code, various turbulence modeling was check to predict the heat transfer coefficient and boundary layer assessments. It was concluded from TEXTAN calculations that both suction and pressure side have transition flow while for the suction side it was predicted that the flow regime at trailing edge is fully turbulent. Based on the Abu-Ghannam –Shaw Transition model and by the aid of shape factor data, momentum Reynolds number and various boundary layer properties, it was concluded that the pressure side remains in transient region.

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Increased efficiency and durability of gas turbine components is driven by demands for reduced fuel consumption and increased reliability in aircraft and power generation applications. The complex flow near the endwall of an axial gas turbine has been identified as a significant contributing factor to aerodynamic loss and increased part temperatures. Three-dimensional (non-axisymmetric) contouring of the endwall surface has been shown to reduce aerodynamic losses, but the effect of the contouring on endwall heat transfer is not well understood. This research focused on understanding the general flow physics of contouring and the sensitivity of the contouring to perturbations arising from leakage features present in an engine. Two scaled low-speed cascades were designed for spatially-resolved measurements of endwall heat transfer and film cooling. One cascade was intended for flat and contoured endwall studies without considering typical engine leakage features. The other cascade modeled the gaps present between a stator and rotor and between adjacent blades on a wheel, in addition to the non-axisymmetric endwall contouring. Comparisons between a flat and contoured endwall showed that the contour increased endwall heat transfer and increased turbulence in the forward portion of the passage due to displacement of the horseshoe vortex. However, the contour decreased heat transfer further into the passage, particularly in regions of high heat transfer, due to delayed development of the passage vortex and reduced boundary layer skew. Realistic leakage features such as the stator-rotor rim seal had a significant effect on the endwall heat transfer, although leakage flow from the rim seal only affected the horseshoe vortex. The contours studied were not effective at reducing the impact of secondary flows on endwall heat transfer and loss when realistic leakage features were also considered. The most significant factor in loss generation and high levels of endwall heat transfer was the presence of a platform gap between adjacent airfoils. / Ph. D.

The present study focuses on local heat transfer characteristics on the tip and near-tip regions of a turbine blade with a flat tip, tested under transonic conditions in a stationary, 2-D linear cascade consisting of seven blades, the three center blades having a variable tip clearance gap. The effects of tip clearance and exit Mach number on heat transfer distribution were investigated on the tip surface using a transient infrared thermography technique. In addition, thin film gages were used to study similar effects on the near-tip regions at 94% based on engine blade span of the pressure and suction sides. The experiments were conducted at the Virginia Tech transonic blow-down wind tunnel facility with a seven-blade linear cascade. Surface oil flow visualizations on the blade tip region were carried-out to shed some light on the leakage flow structure. Experiments were performed at three exit Mach numbers of 0.7, 0.85, and 1.05 for two different tip clearances of 0.9% and 1.8% based on engine blade span. The exit Mach numbers tested correspond to exit Reynolds numbers of 7.6 x 105, 9.0 x 105, and 1.1 x 106 based on blade true chord. The tests were performed with a freestream turbulence intensity of 12%. Results at 0.85 exit Mach showed that an increase in the tip gap clearance translates into a 12% increase in the heat transfer coefficients on the blade tip surface. Similarly, at 0.9% tip clearance, an increase in exit Mach number from 0.85 to 1.05 also led to a 24% increase in heat transfer on the tip. High heat transfer was obtained at the leading edge area of the blade tip, and an increase in the tip clearance gap and exit Mach number augmented this leading edge heat transfer. At 94% of engine blade span on the suction side near the tip, a peak in heat transfer was observed in all test cases at an s/C of 0.66 due to the onset of a downstream leakage vortex. At the design condition, this peak represents an increase of a factor of 2.5 from the immediate preceding s/C location. An increase in both the tip gap and exit Mach number resulted in an increase, followed by a decrease in the near-tip suction side heat transfer. On the near-tip pressure side, a slight increase in heat transfer was observed with increased tip gap and exit Mach number. In general, the suction side heat transfer is greater than the pressure side heat transfer as a result of the suction side leakage vortices. / Master of Science

Experimental heat transfer data has been collected at engine representative conditions in this work to use in future work to improve computational models. Tests were carried out in a transonic cascade wind tunnel with the data collected using thin film gages. All of the necessary development to use the thin film gages has been completed, including construction of electronics and analysis tools to reduce the data. Gage installation and calibration techniques have been successfully implemented for the current research facility and those procedures have been documented. Heat transfer tests were carried out at engine design speed as well as conditions both above and below design speed. The resulting effect of different Reynolds numbers on heat transfer has been studied and the data collected has been compared with computer predictions, analytical correlations, and data from other published literature to validate the results obtained. Finally, it has been shown that a transient analysis technique can be used to process the data for gages that do not exhibit steady results during the quasi-steady test run. This transient technique resulted in data that agrees well with published literature and analytical correlations. / Master of Science

This paper experimentally investigates the effect of high freestream turbulence intensity, turbulence length scale, and exit Reynolds number on the surface heat transfer distribution of a turbine blade at realistic engine Mach numbers. Passive turbulence grids were used to generate freestream turbulence levels of 2%, 12%, and 14% at the cascade inlet. The turbulence grids produced length scales normalized by the blade pitch of 0.02, 0.26, and 0.41, respectively. Surface heat transfer measurements were made at the midspan of the blade using thin film gauges. Experiments were performed at exit Mach numbers of 0.55, 0.78 and 1.03 which represent flow conditions below, near, and above nominal conditions. The exit Mach numbers tested correspond to exit Reynolds numbers of 6 x 105, 8 x 105, and 11 x 105, based upon true chord. The experimental results showed that the high freestream turbulence augmented the heat transfer on both the pressure and suction sides of the blade as compared to the low freestream turbulence case. At nominal conditions, exit Mach 0.78, average heat transfer augmentations of 23% and 35% were observed on the pressure side and suction side of the blade, respectively. / Master of Science

A primary motivation for this research comes from the need to improve the ability of polymer-matrix composites to withstand lightning strikes. In particular, we are concerned with lightning strike damage in composite wind turbine blades. The direct effects of lightning strike on polymer-matrix composites often include rapid temperature rise, melting or burning at the lightning attachment points, and mechanical damage due to lightning-induced magnetic force and acoustic shock wave. The lightning strike damage accumulation problem is essentially multiphysic. The lightning plasma channel discharges an electric current up to 200 kA, inducing a severe heat flux at the surface of the composite structure, as well as generating Joule heating through the composite structure. The resulting electro-thermo-mechanical response of the composite structure may include matrix degradation and decomposition, delamination, and fiber breakage and sublimation, thus leading to catastrophic failure. The existing studies related to the lightning strike damage in composites ignored the lightning channel radius expansion during the initial lightning discharge and lacked adequate treatment of material phase transitions. These assumptions significantly simplify the mathematical treatment of the problem and affect the predictive capabilities of the models. Another common feature of these limited studies is that they all focused on carbon-fiber-reinforced polymer-matrix (CFRP) composites, which are electrically conductive. In the present thesis, the thermal responses and thermal ablations in a non-conductive glass-fiber-reinforced polymer-matrix (GFRP) composite wind turbine blade and in a conductive CFRP composite wind turbine blade are studied, respectively. In the case of non-conductive GFRP composite wind turbine blade, prior to the thermal response and thermal ablation analysis, a finite element analysis is performed to calculate the electric field due to lightning stepped leader to estimate the dielectric breakdown of the non-conductive composite wind turbine blade. The estimation of dielectric breakdown is used to determine whether Joule heating needs to be included in the problem formulation. To predict the thermal response and thermal ablation in the composite structure due to lightning strike, a physics-based model describing surface interaction between the lightning channel and the composite structure has been developed. The model consists of: (i) spatial and temporal evolution of the lightning channel as a function of the electric current waveform; (ii) temporary and spatially non-uniform heat flux and current density (in the case of electrically conductive CFRP composite or if dielectric breakdown occurs in the case of non-conductive GFRP composite) generated at the composite structure; and (iii) nonlinear transient heat transfer problem formulation for layered anisotropic composites that includes the moving boundary of the expanding lightning channel and the phase transition moving boundary associated with instantaneous material removal due to sublimation. The model has been employed to investigate the thermal responses and thermal ablations in a GFRP composite laminated panel used in a Sandia 100-meter all-glass baseline wind turbine blade (SNL 100-00) and a typical CFRP composite laminated panel subjected to lightning strike. The temperature-dependent directional material properties for both the GFRP and CFRP composites have been determined in this thesis using a micromechanics approach based on the experimental data for fibers and resin. An integrated Matlab-ABAQUS numerical procedure features the aforementioned aspects (i), (ii), and (iii) of the developed model. The obtained results include the evolution of temperature fields in the composite laminated panel and the progressive shape change of the composite laminated panel due to thermal ablation. The predictions of thermal ablation in the CFRP composite laminated panel are validated by reported experimental results.

Complex systems can be divided into simpler substructures. Determining the properties of each subcomponent by experimental procedures is practical and can serve to verify or calibrate finite element models. In this work, an existing model of a wind turbine blade was improved by use of experimental data. Such a blade is a subpart of a complete wind turbine. For calibration purpose, several material tests were made in order to determine the stiffness and mass properties. Later on, vibration tests of the blades were conducted and compared with simulation results of the improved model. Geometry variability within sets of blades was also studied. The blade twist angles and the center of gravity positions were found to vary moderately, which accounts for differences in blades’ dynamic behavior. Correlations between experimental data and analytical model results were very high for the first eight modeshapes. That is, according to the Model Assurance Criterion the calibrated model achieves a high-quality representation of reality. However, torsional modes in the computer model occur at a higher frequency than the experimental ones. Substructuring of the turbine allows the blades to be modeled and validated independently of the other substructures and can later be incorporated into a complete model of the turbine.

Experimental research has been completed using a custom designed and built 4m diameter wind turbine in a university operated wind facility. The primary goals of turbine testing were to determine the power production of the turbine and to apply the particle image velocimetry (PIV) technique to produce flow visualization images and velocity vector maps near the tip of a blade. These tests were completed over a wide range of wind speeds and turbine blade rotational speeds. This testing was also designed to be a preliminary study of the potential for future research using the turbine apparatus and to outline it's limitations. The goals and results of other large scale turbine tests are also briefly discussed with a comparison outlining the unique aspects of the experiment outlined in this thesis. Power production tests were completed covering a range of mean wind speeds, 6.4 m/s to 11.1 m/s nominal, and rotational rates, 40 rpm to 220 rpm. This testing allowed the total power produced by the blades to be determined as a function of input wind speed, as traditionally found in power curves for commercial turbines. The coefficient of power, Cp, was determined as a function of the tip speed ratio which gave insight into the peak power production of the experimental turbine. It was found, as expected, that the largest power production occurred at the highest input wind speed, 11.1 m/s, and reached a mean value of 3080 W at a rotational rate of 220 rpm. Peak Cp was also found, as a function of the tip speed ratio, to approach 0.4 at the maximum measurable tip speed ratio of 8. Blade element momentum (BEM) theory was also implemented as an aerodynamic power and force prediction tool for the given turbine apparatus. Comparisons between the predictions and experimental results were made with a focus on the Cp power curve to verify the accuracy of the initial model. Although the initial predictions, based on lift and drag curves found in Abbot and Von Doenhoff (1959), were similar to experimental results at high tip speed ratios an extrapolation of the data given by Hoffman et al. (1996) was found to more closely match the experimental results over the full range of tip speed ratios. Finally PIV was used to produce flow visualization images and corresponding velocity maps of the chord-wise air flow over an area at a radius ratio of 0.9, near the tip of a blade. This technique provided insight into the flow over a blade at three different tip speed ratios, 4, 6 and 8, over a range of wind speeds and rotational rates. A discussion of the unique aspects and challenges encountered using the PIV technique is presented including: measuring an unbounded external flow on a rotating object and the turbulence in the free stream affecting the uniform seeding and stability of the flow.