Giovanna Barigozzi

An experimental investigation of stator induced unsteadiness on centrifugal impeller outflow

Detailed flow measurements were taken in a centrifugal turbomachine model to investigate the aerodynamic influence of the vaned diffuser on the impeller flow. The model consists of an unshrouded centrifugal impeller with backswept blades and a rotatable vaned diffuser, which enables a continuous variation of the vaned diffuser location with respect to the measuring points. Phase-locked ensemble-averaged velocity components have been measured with hot-wire probes at the impeller outlet for 30 different relative positions of the probe with respect to the diffuser vanes. The data also include the distribution of the ensemble-averaged static pressure at the impeller front end, taken by means of miniature fast response pressure transducers flush-mounted at the impeller stationary casing. By circumferentially averaging the results obtained for the different circumferential probe locations, the periodically perturbed impeller flow has been split into a relative steady flow and a stator-generated unsteadiness. The results for the different probe positions have also been correlated in time to obtain instantaneous flow field images in the relative frame, which provide information on the various aspects of the diffuser vane upstream influence on the relative flow leaving the impeller.

Fan-shaped hole effects on the aero-thermal performance of a film-cooled endwall

The present paper investigates the effects of a fan-shaped hole endwall cooling geometry on the aero-thermal performance of a nozzle vane cascade. Two endwall cooling geometries with four rows of holes were tested, for different mass flow rate ratios: the first configuration is made of cylindrical holes, whereas the second one features conical expanded exits and a reduced number of holes. The experimental analysis is mainly focused on the variations of secondary flow phenomena related to different injection rates, as they have a strong relationship with the film cooling effectiveness. Secondary flow assessment was performed through downstream 3D aerodynamic measurements, by means of a miniaturized 5-hole probe. The results show that at high injection rates, the passage vortex and the 3D effects tend to become weaker, leading to a strong reduction of the endwall cross flow and to a more uniform flow in spanwise direction. This is of course obtained at the expense of a significant increase of losses. The thermal behavior was then investigated through the analysis of adiabatic effectiveness distributions on the two endwall configurations. The wide-banded thermochromic liquid crystals (TLC) technique was used to determine the adiabatic wall temperature. Using the measured distributions of film-cooling adiabatic effectiveness, the interaction between the secondary flow vortices and the cooling jets can be followed in good detail all over the endwall surface. Fan-shaped holes have been shown to perform better than cylindrical ones: at low injection rates, the cooling performance is increased only in the front part of the vane passage. A larger improvement of cooling coverage all over the endwall is attained with a larger mass flow rate, about 1.5% of core flow, without a substantial increase of the aerodynamic losses.

High sensitivity measurements of thermal properties of textile fabrics

A new testing apparatus is proposed to measure the thermal properties of fabrics made from polymeric materials. The calibration of the apparatus and the data acquisition procedure are considered in detail in order to measure thermal conductivity, resistance, absorption and diffusivity constants of the tested fabric samples. Differences between dry and wet fabrics have been carefully detected and analyzed. We have developed a new measurement protocol, the ThermoTex protocol, which agrees with the UNI EN 31092 standard and entails accurate quantification of the experimental errors according to a standard statistical analysis, thus allowing a rigorous investigation of the physical behavior of the phenomena involved. As a consequence, our equipment exhibits great potential for optimizing the thermal comfort of fabrics, according to the market demand, thanks to the possible development of a predictive phenomenological theory of the effects involved.

Simulation of Solarized Combined Cycles: Comparison Between Hybrid Gas Turbine and ISCC Plants

The present paper investigates two different Solarized Combined Cycle layout configurations. In the first scheme, a solarized gas turbine is coupled to a solar tower. Pressurized air at compressor exit is sent to the solar tower receiver before entering the GT combustor. Here temperature is increased up to the nominal turbine inlet value through natural gas combustion. In the second CC layout, solar energy is collected by line focusing parabolic trough collectors and used to produce superheated steam in addition to the one generated in the heat recovery boiler. The goal of the paper is to compare the thermodynamic performance of these CSP technologies when working under realistic operating conditions. Commercial software and in-house computer codes were combined together to predict CSP plant performance both on design and off-design conditions. Plant simulations have shown the beneficial effect of introducing solar energy at high temperature in the Joule-Brayton cycle and the drawback in terms of GT performance penalization due to solarization. Results of yearly simulations on a one hour basis for the two considered plant configurations are presented and discussed. Main thermodynamic parameters such temperatures, pressure levels, air and steam flow rates are reported for two representative days.

The Effect of an Upstream Ramp on Cylindrical and Fan-Shaped Hole Film Cooling: Part II — Adiabatic Effectiveness Results

The present companion papers report the results of an experimental investigation on possible beneficial effects of an upstream ramp on discrete hole film cooling. This analysis was carried out on a flat plate model. Two hole geometries have been considered: cylindrical and fan-shaped with conical expanded exit. To compare different cooling schemes, a combined aero-thermal analysis was performed. Tests have been carried out at low speed and low inlet turbulence intensity level, with blowing ratios varied in the range 0.3–1.0. Part I was focused on the aerodynamic analysis: it has been shown that the introduction of a ramp is always detrimental, as it gives a strong loss increase (+5%). Fan-shaped hole was instead the best solution, as it gives losses comparable with the cylindrical one, reduced turbulence mixing and jet dilution. Part II of this paper faces the thermal analysis. The thermal behaviour of the cooled surface has been analysed using the wide banded TLC’s technique, so to obtain adiabatic effectiveness distributions. Additional air temperature measurements have been carried out by traversing a thermocouple downstream of injection holes. The upstream ramp was found to provide a thermal protection improvement (+40%) only at low blowing rate in the case of cylindrical hole. The application of a ramp upstream of a fanshaped hole was instead detrimental for all blowing conditions. The fan shaped hole geometry with no ramp resulted to be the best solution also in terms of adiabatic effectiveness (50% higher than the cylindrical one at BR = 0.5).Copyright

Endwall Film Cooling Effects on Secondary Flows in a Contoured Endwall Nozzle Vane

The present paper investigates the effects of endwall injection of cooling flow on the aerodynamic performance of a nozzle vane cascade with endwall contouring. Tests have been performed on a 7 vane cascade with a geometry typical of a real gas turbine nozzle vane. The cooling scheme consists of four rows of cylindrical holes. The same cooling scheme, applied to a flat endwall, was already investigated by the authors. Tests have been carried out at low speed (M2is = 0.2) with a low inlet turbulence intensity level (1.0 %) and with a coolant to mainstream mass flow ratio varied in the range from zero (solid endwall) to 2.5%. Energy loss coefficient, secondary vorticity and outlet angle distributions were computed from 5-hole probe measured data. Contoured endwall results, with and without film cooling, were compared to planar endwall data. Endwall contouring was responsible for a significant overall loss decrease, thanks to the reduction of both profile and planar side secondary flows losses; a loss increase on the contoured side was instead observed. Like as for the planar endwall, even for contoured endwall coolant injection modifies secondary flows, reducing their intensity, but the relevance of the changes is reduced. Nevertheless for all the tested injection conditions, secondary losses on the contoured side are always higher than in the planar case, while contoured cascade overall losses are lower. A unique minimum overall loss injection condition was found for both tested geometries, corresponding to an injected mass flow rate of about 1.0%.

The Effect of an Upstream Ramp on Cylindrical and Fan-Shaped Hole Film Cooling: Part I — Aerodynamic Results

The present companion papers report the results of an experimental investigation on possible beneficial effects of an upstream ramp on discrete hole film cooling. This analysis was carried out on a flat plate model. Two hole geometries have been considered: cylindrical and fan-shaped with conical expanded exit. To compare different cooling schemes, a combined aero-thermal analysis was performed. Tests have been carried out at low speed and low inlet turbulence intensity level, with blowing ratios varied in the range 0.3–1.0. The aerodynamic investigation has been performed through the measurements of discharge coefficients and detailed flow field measurements. Surveys were carried out by traversing a flattened Pitot tube. Additional turbulence measurements have been carried out by means of hot wire traverses. All this information, together with the adiabatic effectiveness results presented in Part II of this paper, allowed drawing a comprehensive picture of the complex aero-thermal flow field in the injection region. The upstream ramp provided a moderate improvement in the case of cylindrical holes, as it allows the coolant to diffuse someway, before interacting with the mainflow, but it produced also a significant increase of aerodynamic losses. The fan shaped hole geometry without the ramp resulted to be the best solution, as it provides a coolant injection with a good lateral spreading and a low turbulence level.Copyright

Endwall Film Cooling Through Fan-Shaped Holes With Different Area Ratios

The present paper reports on the aero-thermal performance of a nozzle vane cascade, with film cooled endwalls. The coolant is injected through four rows of cylindrical holes with conical expanded exits. Two endwall geometries with different area ratios have been compared. Tests have been carried out at low speed (M = 0.2), with coolant to mainstream mass flow ratio varied in the range 0.5–2.5%. Secondary flow assessment has been performed through 3D aerodynamic measurements, by means of a miniaturized 5-hole probe. Adiabatic effectiveness distributions have been determined by using the wide banded thermochromic liquid crystals (TLC) technique. For both configurations and for all the blowing conditions, the coolant share among the four rows has been determined. The aerothermal performance of the cooled vane have been analyzed on the basis of secondary flow effects and laterally averaged effectiveness distributions; this analysis was carried out for different coolant mass flow ratios. It was found that the smaller area ratio provides better results in terms of 3D losses and secondary flow effects; the reason is that the higher momentum of the coolant flow is going to better reduce the secondary flow development. The increase of the fan-shaped hole area ratio gives rise to a better coolant lateral spreading, but appreciable improvements of the adiabatic effectiveness were detected only in some regions and for large injection rates.Copyright

Fan-Shaped Hole Effects on the Aero-Thermal Performance of a Film Cooled Endwall

The present paper investigates the effects of a fan-shaped hole endwall cooling geometry on the aero-thermal performance of a nozzle vane cascade. Two endwall cooling geometries with four rows of holes were tested, for different mass flow rate ratios: the first configuration is made of cylindrical holes, whereas the second one features conical expanded exits and a reduced number of holes. The experimental analysis is mainly focused on the variations of secondary flow phenomena related to different injection rates, as they have a strong relationship with the film cooling effectiveness. Secondary flow assessment was performed through downstream 3D aerodynamic measurements, by means of a miniaturized 5-hole probe. The results show that at high injection rates, the passage vortex and the 3D effects tend to become weaker, leading to a strong reduction of the endwall cross flow and to a more uniform flow in spanwise direction. This is of course obtained at the expense of a significant increase of losses. The thermal behavior was then investigated through the analysis of adiabatic effectiveness distributions on the two endwall configurations. The wide banded TLC’s technique was used to determine the adiabatic wall temperature. Using the measured distributions of film cooling adiabatic effectiveness, the interaction between the secondary flow vortices and the cooling jets can be followed in good detail all over the endwall surface. Fan-shaped holes have been shown to perform better than cylindrical ones: at low injection rates, the cooling performance is increased only in the front part of the vane passage. A larger improvement of cooling coverage all over the endwall is attained with a larger mass flow rate, about 1.5% of core flow, without a substantial increase of the aerodynamic losses.Copyright

Application of Unsteady Computational Fluid Dynamics Methods to Trailing Edge Cutback Film Cooling

The main purpose of this numerical investigation is to overcome the limitations of the steady modeling in predicting the cooling efficiency over the cutback surface in a high pressure turbine nozzle guide vane. Since discrepancy between Reynolds-averaged Navier–Stokes (RANS) predictions and measured thermal coverage at the trailing edge was attributable to unsteadiness, Unsteady RANS (URANS) modeling was implemented to evaluate improvements in simulating the mixing between the mainstream and the coolant exiting the cutback slot. With the aim of reducing the computation effort, only a portion of the airfoil along the span was simulated at an exit Mach number of Ma2is = 0.2. Three values of the coolant-to-mainstream mass flow ratio were considered: MFR = 0.66%, 1.05%, and 1.44%. Nevertheless the inherent vortex shedding from the cutback lip was somehow captured by the URANS method, the computed mixing was not enough to reproduce the measured drop in adiabatic effectiveness η along the streamwise direction, over the cutback surface. So modeling was taken a step further by using the scale adaptive simulation (SAS) method at MFR = 1.05%. Results from the SAS approach were found to have potential to mimic the experimental measurements. Vortices shedding from the cutback lip were well predicted in shape and magnitude, but with a lower frequency, as compared to particle image velocimetry (PIV) data and flow visualizations. Moreover, the simulated reduction in film cooling effectiveness toward the trailing edge was similar to that observed experimentally.