Masahide Kazari

The NEKOMIMI Cooling Technology: Cooling Holes With Ears for High-Efficient Film Cooling

Further improvement of the thermal efficiency of modern gas turbines can be achieved by a further reduction of the cooling air amount. Therefore, it is necessary to increase the cooling effectiveness, so that the available cooling air fulfils the cooling task even if the amount has been reduced. Due to experimental and numerical efforts, it is well understood today that aerodynamic mixing processes are enhanced by counter-rotating vortices (CRV) in the cooling jets and lead to jet lift-off effects. Thus, the film-cooling effectiveness is reduced soon behind the cooling air ejection through the holes. Due to that basic understanding, different technologies for improving film cooling have been developed. Some of them focus on establishing anti-counter-rotating vortices (ACRV) inside the cooling jet that prevent the hot gas from flowing underneath the jet and, thus, avoid the lift-off effect. One of these technologies is the double-jet film cooling (DJFC), invented by the authors, where the special arrangement of two cylindrical holes lead to a cooling jet with such an anti-vortex system. However, beside the advantage that the holes are simple cylindrical holes, one disadvantage is that appropriate supply with cooling air for both holes is sometimes difficult to be established in real configurations. Thus, the authors have followed the idea to transfer the original double-jet film cooling principle to a special configuration with merged holes. Thus, in that case only one air supply is necessary but the anti-vortex effect has been preserved. The derived cooling technology has been named NEKOMIMI technology. The paper explains the principle of that technology. Results from experimental investigations including film cooling effectiveness measurements for the new technology are presented. The results are compared to conventional cooling hole configurations showing the tremendous positive effect in reaching highest film cooling effectiveness for the new configuration at M = 1.5 and partly for M = 1. Numerical investigations for the M = 1.5 case indicate that the existence of the ACRV is the likely reason for the enhanced cooling performance of the new configuration.Copyright

A Parametric Study on the Influence of the Lateral Ejection Angle of Double-Jet Holes on the Film Cooling Effectiveness for High Blowing Ratios

The improvement of the thermal efficiency of modern gas turbines can be achieved by reducing the required cooling air amount. The reduction of the cooling air claims for an improved cooling technology, which assures the protection of the vane and blade airfoil from the hot mainstream flow. Consequently, it is required to increase the cooling efficiency of applied cooling technologies. Streamwise ejection from a cylindrical hole causes kidney vortices, which transport hot gas underneath the cooling jet and leads the cooling jet to lift off the surface. The double-jet film cooling technology represents a solution to establish an anti-kidney vortex, which prevents the double jet from lifting off the surface and raises the lateral spreading of the cooling air. This is achieved by a particular arrangement of simple cylindrical holes to each other. Additionally, the design of double-jet holes reduces significantly the effort of hole manufacturing compared to the effort of manufacturing a shaped hole design. Numerical investigations for blowing ratios from M = 0.5 up to M = 2 and experimental investigations in a test rig prove the proper film cooling ability of the double-jet film cooling technology. Furthermore, this paper presents a numerical parametric study of the double jet film cooling technology. The influence of the lateral ejection angle on the distribution of the cooling film is calculated and analyzed for the blowing ratios of M = 1, M = 1.5 and M = 2. It can be shown that an even higher film cooling effectiveness is reached with the use of the double-jet film cooling technology by an improvement of the hole positions and hole angles than in previous investigations.© 2009 ASME

Heat Transfer Enhancement for Gas Turbine Internal Cooling by Application of Double Swirl Cooling Chambers

Improvement of the gas turbine thermal efficiency can be achieved by reducing the cooling fluid amount in internal cooling channels with enhanced convective cooling. Nowadays the state of the art internal cooling technology for thermally high-loaded gas turbine blades consists of multiple serpentine-shaped cooling channels with angled ribs. Besides, huge effort is put on the development of more advanced internal cooling configurations with further internal heat transfer enhancements. Swirl chamber flow configurations, in which air is flowing through a pipe with a swirling motion generated by tangential jet inlet, have a potential for application as such advanced technology. This paper presents the validation of numerical results for a standard swirl chamber, which has been investigated experimentally in a reference publication. The numerical results obtained with application of the SST k-ω model show the best agreement with the experiment data in compare with other turbulence models. It has been found at the inlet region that the augmentation of the heat transfer is nearly seven times larger than the fully developed non-swirl flow. Within the further numerical study, another cooling configuration named Double Swirl Chambers (DSC) has been obtained and investigated. The numerical results are compared to the reference case. With the same boundary conditions and Reynolds number, the heat transfer coefficients are higher for the DSC configuration than for the reference configuration. In particular at the inlet region, the DSC configuration has even higher circumferentially averaged heat transfer enhancement in one section by approximately 41%. The globally-averaged heat transfer enhancement in DSC configuration is 34.5% higher than the value in the reference SC configuration. This paper presents the configuration of the DSC as an alternative internal cooling technology and explains its major physical phenomena, which are the reasons for the improvement of internal heat transfer.Copyright

Numerical and Experimental Characterization of Low NOx Micromix Combustion Principle for Industrial Hydrogen Gas Turbine Applications

The international effort to reduce the environmental impact of electricity generation, especially CO2-emissions requires considerations about alternative energy supply systems. An effective step towards low pollution power generation is the application of hydrogen as a possible alternative gas turbine fuel, if the hydrogen is produced by renewable energy sources, such as wind energy or biomass. The use of hydrogen and hydrogen rich gases as a fuel for industrial applications and power generation combined with the control of polluted emissions, especially NOx, is a major key driver in the design of future gas turbine combustors.The micromix combustion principle allows a secure and low NOx combustion of hydrogen and air and achieves a significant reduction of NOx-emissions. The combustion principle is based on cross-flow mixing of air and gaseous pure hydrogen and burns in multiple miniaturized diffusion-type flames. For the characterization of the jet in cross-flow mixing process, the momentum flux ratio is used.The paper presents an experimental analysis of the momentum flux ratio’s impact on flame anchoring and on the resultant formation of the NOx-emissions. Therefore several prototype test burner with different momentum flux ratios are tested under preheated atmospheric conditions. The investigation shows that the resultant positioning and anchoring of the micro flames highly influences the NOx-formation.Besides the experimental investigations, numerical simulations have been performed by the application of a commercial CFD code. The cold flow simulation results show the mixing of the air and hydrogen after the injection, in particular in the Counter Rotating Vortices (CRV). Furthermore, the hydrogen jet interacts also with another vortex system resulting from a wake flow area behind the combustor geometry. Furthermore, reacting flow simulations have been performed by the application of a Hybrid Eddy Break-Up (EBU) combustion model. The combustion pressure has been varied from atmospheric conditions up to a pressure of 16 bar.The experimental and numerical results highlight further potential of the micromix combustion principle for low NOx-combustion of hydrogen in industrial gas turbine applications.Copyright

Leading Edge Cooling of a Gas Turbine Blade With Double Swirl Chambers

The gas turbine blade leading edge area has locally extremely high thermal loads, which restrict the further increase of turbine inlet temperature or the decrease of the amount of coolant mass flow to improve the thermal efficiency. Jet impingement heat transfer is the state of the art cooling configuration, which has long been used in this area. In the present study, a modified double swirl chambers cooling configuration has been developed for the gas turbine blade leading edge. The double swirl chambers cooling (DSC) technology is introduced by the authors and comprises a significant enhancement of heat transfer due to the generation of two anti-rotating swirls. In DSC cooling the reattachment of the swirl flows with the maximum velocity at the middle of the chamber leads to a linear impingement effect, which is most suitable for the leading edge cooling for a gas turbine blade. In addition, because of the two swirls both suction side and pressure side of the blade near the leading edge can be very well cooled. In this work, a comparison among three different internal cooling configurations for the leading edge (impingement cooling, swirl chamber and double swirl chambers) has been investigated numerically. With the same inlet slots and the same Reynolds number based on hydraulic diameter of channel the DSC cooling shows overall higher Nusselt number ratio than that in the other two cooling configurations. Downstream of the impingement point, due to the linear impingement effect, the DSC cooling has twice the heat flux in the leading edge area than the standard impingement cooling channel.Copyright

Novel Gas Turbine Blade Leading Edge Cooling Configuration Using Advanced Double Swirl Chambers

The Double Swirl Chambers (DSC) cooling technology, which has been introduced and developed by the authors, has the potential to be a promising cooling technology for further increase of gas turbine inlet temperature and thus improvement of the thermal efficiency. The DSC cooling technology establishes a significant enhancement of the local internal heat transfer due to the generation of two anti-rotating swirls. The reattachment of the swirl flows with the maximum velocity at the center of the chamber leads to a linear impingement effect on the internal surface of the blade leading edge nearby the stagnation line of gas turbine blade. Due to the existence of two swirls both the suction side and the pressure side of the blade near the leading edge can be very well cooled. In this work, several advanced DSC cooling configurations with a row of cooling air inlet holes have been investigated. Compared with the standard DSC cooling configuration the advanced ones have more suitable cross section profiles, which enables better accordance with the real blade leading edge profile. At the same time these configurations are also easier to be manufactured in a real blade. These new cooling configurations have been numerically compared with the state of the art leading edge impingement cooling configuration. With the same configuration of cooling air supply and boundary conditions the advanced DSC cooling presents 22–26% improvement of overall heat transfer and 3–4% lower total pressure drop. Along the stagnation line the new cooling configuration can generate twice the heat flux than the standard impingement cooling channel. The influence of spent flow in the impinging position and impingement heat transfer value is in the new cooling configurations much smaller, which leads to a much more uniform heat transfer distribution along the chamber axial direction.Copyright

Highest-Efficient Film Cooling by Improved NEKOMIMI Film Cooling Holes: Part 2 — Hot Gas Flow Conditions

In modern gas turbines, the film cooling technology is essential for the protection of the hot parts, in particular of the first stage vanes and blades of the turbine, against the hot gases from the combustion process in order to reach an acceptable life span of the components. As the cooling air is usually extracted from the compressor, the reduction of the cooling effort would directly result to an increased thermal efficiency of the gas turbine. Understanding of the fundamental physics of film cooling is necessary for the improvement of the state-of-the-art. Thus, huge research efforts by industry as well as research organizations have been undertaken to establish high efficient film cooling technologies. It is common knowledge today that film cooling effectiveness degradation is caused by secondary flows inside the cooling jets, i.e. the Counter-Rotating Vortices (CRV) or sometimes also mentioned as kidney-vortices, which induce a lift-off of the jet. Further understanding of the secondary flow development inside the jet and how this could be influenced, has led to hole configurations, which can induce Anti-counter-rotating Vortices (ACRV) in the cooling jets. As a result, the cooling air remains close to the wall and is additionally distributed flatly along the surface. Beside different other technologies, the NEKOMIMI cooling technology is a promising approach to establish the desired ACRV. It consists of a combination of two holes in just one configuration so that the air is distributed mainly on two cooling air streaks following the special shape of the generated geometry. The original configuration was found to be difficult for fabrication by advanced machining processes. Thus, the improvement of this configuration has been reached by a set of geometry parameters, which lead to configurations easier to be manufactured but preserving the principle of the NEKOMIMI technology. Within a numerical parametric study several advanced configurations have been obtained and investigated under hot gas flow conditions. By systematic variation of the parameters a further optimization with respect to highest film cooling effectiveness has been performed. The best configuration outperforms the basic configuration by more than 20% regarding the overall averaged adiabatic film cooling effectiveness.© 2013 ASME

Film Cooling Effectiveness Comparison Between Shaped- and Double Jet Film Cooling Holes in a Row Arrangement

The improvement of the thermal efficiency of modern gas turbines can be achieved by reducing the needed cooling air amount. Consequently it is required to increase the cooling efficiency of applied cooling technologies. Streamwise ejection from a cylindrical hole causes kidney vortices which transport hot gas underneath the cooling jet and leads the cooling jet to lift off from the surface. Cooling performance is highly increased by using the shaped hole technique, which weakens the kidney vortex structure. However the formation of secondary flows can not completely be avoided by using shaped holes instead of cylindrical holes. Another promising film cooling technology is the double-jet film cooling, which prevents the cooling jet from lifting off the surface and raises the lateral spreading of the cooling air by generating an anti kidney vortex. This paper presents a comparison of the film cooling effectiveness between the shaped film cooling technique and the novel double jet film cooling technique for the high blowing ratios M = 1, M = 1.5 and M = 2. Various geometries of fan-shaped holes with lateral expansion angles of 10°, 14° and 18° are used for the comparison study. It can be shown that the shaped hole row arrangement provides higher cooling values in a slight region near to the hole exits, while the double jet film cooling technique shows a obvious cooling advantage in the further downstream area for high blowing ratios. Furthermore recent results of an on-going experimental comparison study between the double jet, trench and cylindrical technique are presented, which proves the advantage capability of the double jet film cooling.Copyright

Experimental and Numerical Study on Optimizing the DLN Micromix Hydrogen Combustion Principle for Industrial Gas Turbine Applications

Combined with the use of renewable energy sources for its production, hydrogen represents a possible alternative gas turbine fuel for future low emission power generation. Due to the difference in the physical properties of hydrogen compared to other fuels such as natural gas, well-established gas turbine combustion systems cannot be directly applied to Dry Low NOx (DLN) hydrogen combustion.The DLN Micromix combustion of hydrogen has been under development for many years, since it has the promise to significantly reduce NOx emissions. This combustion principle for air-breathing engines is based on cross-flow mixing of air and gaseous hydrogen. Air and hydrogen react in multiple miniaturized diffusion-type flames with an inherent safety against flash-back and with low NOx-emissions due to a very short residence time of the reactants in the flame region.The paper presents an advanced DLN Micromix hydrogen application. The experimental and numerical study shows a combustor configuration with a significantly reduced number of enlarged fuel injectors with high thermal power output at constant energy density. Larger fuel injectors reduce manufacturing costs, are more robust and less sensitive to fuel contamination and blockage in industrial environments.The experimental and numerical results confirm the successful application of high energy injectors, while the DLN Micromix characteristics of the design point, under part load conditions and under off-design operation are maintained. Atmospheric test rig data on NOx emissions, optical flame structure and combustor material temperatures are compared to numerical simulations and show good agreement.The impact of the applied scaling and design laws on the miniaturized Micromix flamelets is particularly investigated numerically for the resulting flow field, the flame structure and NOx formation.© 2015 ASME

Experimental and Numerical Investigations of the NEKOMIMI Film Cooling Technology

The improvement of the thermal efficiency of modern gas turbines can be achieved by reducing the required cooling air amount. Therefore, it is necessary to increase the cooling effectiveness, so that the available cooling air fulfils the cooling task even if the amount has been reduced. Due to experimental and numerical efforts, it is well understood today that aerodynamic mixing processes are enhanced by counter rotating vortices (CRV) in the cooling jets and lead to jet liftoff effects. Novel film cooling technologies focus on establishing anti-counter-rotating-vortices (ACRV) inside the cooling jet that prevent the hot gas from flowing underneath the jet and, thus, avoid the lift-off effect. One of these technologies is the NEKOMIMI film cooling, which is derived from the original double-jet film cooling (DJFC).Copyright