Marco Surace

Correlative Analysis of Effusion Cooling Systems

Gas turbine cooling has steadily acquired major importance whenever engine performances have to be improved. Among various cooling techniques, film cooling is probably one of the most diffused systems for protecting metal surfaces against hot gases in turbine stages and combustor liners. Most recent developments in hole manufacturing allow us to perform a wide array of microholes, currently referred to as effusion cooling. Though some drawbacks of such a concept still need to be solved (manufacturing costs, holes blockage, and then system reliability), its potential is now worth investigating. This paper presents the validation of a simplified numerical two-dimensional conjugate approach through a comparison with the experimental results of effectiveness for an effusion plate. A preliminary test is performed with the steady-state technique, using thermochromic liquid crystal wide-band formulations. Results are obtained in terms of local distributions of adiabatic effectiveness. Average values are compared with calculations to validate the numerical code. Then, the design of experiment approach is used to perform several conjugate numerical tests (about 180), so as to derive the behavior of different effusion plates in terms of overall effectiveness and mass flow rate. Data are analyzed in detail, and a correlative approach for the overall effectiveness is proposed.

Film Cooling System Numerical Design: Adiabatic and Conjugate Analysis

Film cooling is certainly the most diffused system to protect metal surface against hot gases, both in turbogas blades and combustors. Although being very diffused, there are still several aspects of its behavior which need a better understanding. Mainly, the performance of multi-row holes configurations are still estimated correcting single-row correlations. Heat transfer coefficient modifications due to the presence of injected coolant are hard to evaluate, and even now few studies in literature take into account this factor. The present work is a detailed numerical study of some effects of film cooling. 3D CFD-RANS simulations have been performed to infer interesting trends of adiabatic superposition effects and conjugate heat transfer performances. In particular, several calculations have been carried out to evaluate single row and multi-row film cooling behavior in terms of heat transfer coefficient, overall and adiabatic effectiveness. Test were conducted with blowing ratios between 0.5 and 5.5, coolant Reynolds from 1000 to 16000.Copyright

Automated CFD Analysis Within the

The design of state-of-the-art combustion chambers is based on a multitude of design rules. To use this knowledge more effectively and to accelerate the combustor design process an automated combustion chamber design tool is being developed within the European project INTELLECT D.M. (Inte grated Le an L ow E mission C ombust or D esign M ethodology). Due to the automation of the design process the time required to set up a new preliminary combustion chamber design is reduced from weeks to hours. The development of the automated preliminary combustor design tool is described in [1]. The focus of this paper is on new developments of the design system PRECODES (pre liminary co mbustor de sign s ystem) including automated mesh generation and CFD simulation. Design rules and parameters are formalized and stored within an EXCEL database. The combustor layout process including the calculations of cooling air mass flows and the zonal layout is done automatically using this database. The layout process has to be iteratively adjusted in order to find an optimal design due to the nonlinear interdependence of some of the design variables. The EXCEL database provides information for two parametric CAD models. The first parametric model includes the flame tube, pre-diffuser, cowl, metering panel, heatshield and the casing. Therefore it is relatively complex and only used for weight approximation and visualization purposes. The second CAD model is a generic model of the flame tube providing the basis for the automatic CFD mesh generation and CFD simulations. The CAD geometry is transferred to the commercial grid generator ICEM-CFD via the ICEM internal direct CAD interface. Based on the CAD geometry a multiblock structured mesh is generated automatically. Due to the utilization of the same blocking master model for different flame tubes varying in combustor size and orientation, and size and position of the mixing holes the mesh topology differs only marginally between different designs. Thus the CFD simulations are well comparable. Different combustor configurations are generated based on input parameter changes, i.e. changing the pressure level, the zonal stoichiometry or the maximum allowable material temperatures. An overview of the present results and the potentials of applying the automated combustor design tool PRECODES is presented.Copyright

Impingement Cooling for Modern Combustors: Experimental Analysis and Preliminary Design

The aim to reach very low emission limits has recently changed several aspects of combustor fluid dynamics. Among them, combustor cooling experienced significant design efforts to obtain good performances with unfavorable conditions. This paper deals with experimental research and 1D numerical simulations of impingement cooling from multiple jet arrays, performed in the first two years of the European research project LOPOCOTEP. Geometries are derived from typical LPP combustor cooling configurations, i.e. small and sparse holes, due to low coolant mass flow rate and high pressure losses (compared to typical blade cooling parameters). Tests are performed with both transient and steady-state technique, using TLC (Thermochromic Liquid Crystal) wide-band formulations. Results are obtained in terms of local distributions of heat transfer coefficient and effectiveness. Average values are compared with calculations to validate numerical code and derive useful indications on effectiveness variation for different blowing rate. It is accordingly found that literature results almost match the measurements. It is also shown that measured row-by-row effectiveness values can be usefully employed in a preliminary design stage. Data concerning holes discharge coefficients are also presented.Copyright

Preliminary Evaluation of Turbulence Level Influence in Heat Transfer Measurements

Heat transfer coefficients have often been experimentally measured, taking into account Nusselt number as a function of Reynolds and Prandtl number. Most experimenters spend their effort to control turbulence level, set it to different values, or keep it unchanged during the tests, as it’s not easy to predict how its initial level may change final results. The aim of this work is to add some comprehension on how different turbulence incoming levels may affect heat transfer measurements, and when it’s possible or not to neglect such effects. Experimental setup features different duct geometries, and thermocromic liquid crystals coupled with hot-wire anemometers are used as main measurement techniques. Tests were performed for Reynolds number from 10000 to 50000 and turbulence level from 3% to 12%. Several turbulence manipulators were adopted, including aluminum foams and multi-perforated plates, and results show some interesting dependences of heat transfer from both turbulence level and grid features.Copyright

A New Concept of Impingement Cooling for Gas Turbine Hot Parts and Its Influence on Plant Performance

Significant improvements in gas turbine cooling technology are becoming harder as progress goes over and over. Several impingement cooling solutions have been extensively studied in past literature. An accurate and extensive numerical 1D simulation on a new concept of sequential impingement was performed, showing good results. Instead of having a single impingement plate, we used several perforated plates, connecting the inlet of each one with the outlet of the previous one. Main advantages are: absence of the negative interaction between transverse flow and last rows impinging jets (reduced deflection); better distribution of pressure losses and heat transfer coefficients among the different plates, especially when pressure drops are significant and available coolant mass flow rate is low (lean premixed combustion chamber and LP turbine stages). Practical applications can have a positive influence on both cooled nozzles and combustion chambers, in terms of increased cooling efficiency and coolant mass flow rate reduction. Calculated effects are used to analyze main influences of such a cooling system on global performances of power plants.© 2003 ASME

EXPERIMENTAL STUDIES ON TURBULENCE BEHIND POROUS FOAMS

Assessment of turbulence level at the entrance of a wind tunnel or a measurement section is always a key-point. In particular the variation of turbulence features with Reynolds number needs to be kept under control to ensure constant turbulence level, or at least to be conscious of its change. Moreover, most heat transfer experimental results may be greatly influenced by flow turbulence, in turn giving importance to an accurate evaluation. Classical screens and perforated plates were already extensively studied in literature. Our experimental survey considered several turbulence manipulators. Particularly, besides the ordinary perforated plates, some metal foams were analyzed, whose application in flow turbulence control is nearly unknown. Hotwire anemometers were employed along with a digital data acquisition system to acquire turbulence data. Relation between foam porosity and turbulence level was investigated, inferring useful correlations. Additional test on classical perforated plates were carried on mainly to validate the whole facilities reliability. Nomenclature