Zhongran Chi

Optimization of the Hole Exit Shaping of Film Holes Without and With Compound Angles for Maximal Film Cooling Effectiveness

Shaped film holes can achieve higher film cooling effectiveness compared with the simple cylindrical film holes. According to former studies, the geometry of the shaped film holes has significant influence on the cooling performance. In order to maximize the film cooling effectiveness of the shaped holes, a two-level design optimization methodology of the hole exit shaping is developed in the present study.The optimization methodology consists of a parametric design and CFD mesh generation tool called Coolmesh, a RANS CFD solver, a database of film cooling effectiveness distributions, a metamodel, and a genetic algorithm (GA) for evolutionary optimization. A binary parametric representation of the 2D hole exit shaping is initiated based on the B-spline methods. The metamodel can efficiently predict the detailed distribution of film cooling effectiveness using the CFD results in the database, which is continuously updated for higher accuracy. In each first-level iteration, a second-level GA optimization search is carried out coupled with the metamodel, and then the optimal geometry is evaluated using CFD methods and added to the database. An anisotropic turbulence model is applied to the CFD solver for higher accuracy according to a detailed experimental validation using PSP measurements.In the present study, three design optimizations of the shaped holes without and with compound angles are carried out on a flat plate. The optimization methodology can efficiently find the optimal geometries of shaped holes using only hundreds of CFD runs. For the shaped holes with compound angle, the optimized geometry can generate a back flow vortex which interacts with the shear vortex and weakens the mixing of coolant and hot gas, resulting in a higher film cooling effectiveness on the plate.Copyright

Geometrical Optimization and Experimental Validation of a Tripod Film Cooling Hole With Asymmetric Side Holes

A tripod cylindrical film hole with asymmetric side holes is studied numerically and experimentally on a flat plate for higher film cooling effectiveness. First, the influences of geometrical parameters are studied and the optimum configurations of the asymmetric tripod hole are found in a design of experiments (DoE) optimization study based on an improved numerical model for film cooling prediction, in which more than 100 3D computational fluid dynamics (CFD) simulations are carried out. Then, one optimum configuration of the asymmetric tripod hole is examined experimentally using pressure-sensitive paint (PSP) measurements and compared to the experimental results of the simple cylindrical film hole and a well-designed shaped film hole. The flow and heat transferring characteristics of the asymmetric tripod holes were explored from the DoE results. The side holes can form a shear vortex system or an antikidney vortex system when proper spanwise distances between them are adopted, which laterally transports the coolant and form a favorable coolant coverage. According to the experimental results on flat plate, the optimal configuration of the asymmetric tripod hole is significantly better than cylindrical hole, especially at high blowing ratios. Furthermore, it can provide equivalent or even higher film cooling effectiveness than a well-designed shaped hole.

Optimal Arrangement of Combined-Hole for Improving Film Cooling Effectiveness

Film cooling technique is widely used to protect the components from being destroyed by hot mainstream in a modern gas turbine. Combining round-holes is a promising way of improving film cooling effectiveness. A DoE (design of experiment) simulation of 396 cases focusing on the arrangement of the combined-hole with double holes for improving film cooling performance is carried out in this work, and the influence of an aerodynamic parameter, blowing ratio is considered as well. The dimensionless lateral distance (PoD) and compound angle (CA) of the double holes have relative influence on the film cooling performance of the combined-hole unit. At the low blowing ratio, increasing symmetrical compound angle (SCA) has positive influence on the area-average effectiveness (EFF) of the combined-hole. But at the intermediate and large blowing ratio, the influence of SCA on the area-average EFF depends on the range of PoD. At the small PoD, the area-average EFF ascends basically along SCA axis. However, the area-average EFF first ascends and subsequently descends along SCA axis at the large PoD. Asymmetrical compound angle (ACA) is also considered to fit the antikidney vortexes produced in the combined-hole film cooling compared to their ideal schematic. However, the film cooling effect of the cases with ACA is not as good as expected. The area-average EFF of ACA cases locates in the level between that of the adjacent SCA cases. The optimal arrangement of combined-hole unit for improving film cooling effectiveness is relative to the local flow field. The optimal arrangement of PoD and CA for improving the combined-hole film cooling performance is different at different blowing ratios.

Multi-Dimensional Platform for Cooling Design of Air-Cooled Turbine Blades

Progresses in conjugate heat transfer simulation method provide a new approach for turbine blade cooling design. The procedure of air-cooled blade design introduced in this paper consists of two levels, a schematic design using 1D flow network calculation method, and a detailed design based on CFX conjugate heat transfer simulation.The program platform for the design method was developed, including an air-cooled blade design program, a 1D cooling structure flow network solver, and conjugate mesh generation tools for air-cooled blade.The design platform contains parametric methods for blade profile, cooling channel, and various cooling structures. The key parametric algorithm named Element Design Method was invented and introduced, which brings about parametric design for complex cooling channel, and accelerates the calculation model generation during the schematic design and detailed design.The flow network solver for schematic design consists of a pressure solving program, a temperature solving program, and a film cooling solving program. The pressure solving program uses linear method to solve the momentum equation, so higher stability of the flow network solver can be achieved. In detailed design, CFD pre-treatment using the commercial software is time-consuming. The mesh generation tools, combined with parametric design programs, can automatically create hexahedron/mixed mesh for turbine cascade and cooling structures with considerable speed and quality, which significantly reduces the difficulty of pre-treatment during detailed design.Copyright

Effect of Radiative Heat Transfer Factor on the Temperature Distribution of a First Stage Vane

As the advanced heavy-duty gas turbine develops, the turbine inlet temperature and pressure have increased quite significantly to achieve better performance. The flow and heat transfer conditions of hot components including combustor and turbine become even more extreme than ever which need corresponding aerodynamic and cooling design development. The issue of combustor-turbine interaction has been proposed as a complicated research topic. Currently the hot streak, turbulence intensity, swirling flow, radiation are the four important factors for combustor-turbine interaction research according to the literature. Especially as the turbine inlet temperature increases, the radiative heat transfer plays a more and more important role.In this paper, a first stage vane is selected for the conjugate heat transfer simulation including radiative heat transfer since it is almost impossible to identify the radiative effect in experiment. The goal is to examine the effects of radiative heat flux and temperature increment caused by radiation. Several radiative factors including the inlet radiation, gas composition, vane surface emissivity and outlet reflection are investigated. The temperature distribution and heat flux enhancement under different conditions are compared, which can provide reference to the turbine heat transfer design. The general information of radiative effect can be summarized by quantitative analysis.Results show that the temperature increases obviously when considering the radiation effect as expected. However, these factors show distinct influence on the vane temperature distribution. The inlet radiation has significant impact on the vane leading edge and pressure side. Besides the gas radiation plays quite uniform on the whole vane surface.Copyright

Coupled Aero-Thermodynamics Optimization for the Cooling System of a Turbine Vane

Cooling system design for the air-cooled turbine is a critical issue in modern gas turbine engineering. Advances in CFD technology and optimization methodology is providing new prospects for turbine cooling system design, that the optimum cooling system of the vanes and blades could be designed automatically by the optimization search coupled with the Conjugate Heat Transfer (CHT) analysis.An optimization platform consists of the Generic Algorithms (GA), a mesh generation tool (Coolmesh), and the CHT solver (ANSYS CFX) is presented in this paper. The optimization study was aimed at finding the optimum cooling structure for the 2nd stage vane of the E3 engine, with acceptable metal temperature distribution and limited coolant amount simultaneously. The vane was installed with impingement and pin-fin cooling structure. The optimization search involved the design of critical parameters of the cooling system, including the size of impingement tube, diameter and distribution of impingement holes, and the size and distribution of pin-fin near trailing edge.The optimization design was carried under two engine operating conditions to explore the affect of different boundary conditions. A constant pressure drop was assumed within the cooling system during each optimization. To make the problem computationally faster, the simulations were approached for the interior only (solid and coolant). A weighed function of temperature distribution and coolant mass flow was used as the objective of the Single Objective Generic Algorithms (SOGA). The result showed that the optimal cooling system configuration with considerable cooling performance could be designed through SOGA optimization without human interference.© 2013 ASME

Experimental Investigation of Heat Transfer Dependency on Conjugate and Convective Thermal Boundary Conditions in Pin Fin Channel

The present work experimentally quantifies the effects of thermal boundary conditions, i.e., conjugate and convective boundary conditions, on heat transfer performance for the pin fin channel in trailing edge of gas turbine blade. The geometry of pin fin arrays is typical of x/D=y/D=2.5 and H/D=1.For conjugate case, model is constructed with a relatively high conductivity material so that the Biot number of the model matches engine condition. Uniform heat flux is imposed along the external wall of pin fin arrays and highly resolved temperature distributions of internal wall is obtained with steady liquid crystal, meanwhile external temperature is measured through thermocouples. For convective case, model is constructed with low thermal conductivity material to ensure the usage of transient liquid crystal to obtain heat transfer coefficients of the internal wall on the same configuration.Both the measurements are used as boundary conditions to conduct simulation of solid part of pin fin array. Internal and external wall non-dimensional temperature distributions, as well as isothermal lines distribution, of the two cases are compared, results indicate that it will produce large errors in temperature predictions without considering conjugate effect. Further analysis are made about the mechanism of thermal boundary conditions in determining wall temperature, which demonstrates the necessity of taking conjugate heat transfer effect in turbine cooling design.Copyright

Effect of Corrugated Orifice and Pin-Fin on Multiple Array Impingement Cooling With Low Nozzle to Target Distance

Impingement cooling is widely used in turbine vanes and combustors. With the increase of turbine inlet temperature, high heat transfer coefficient and low pressure drop are required for cooling structures. A series of new impingement configurations combined with corrugated orifice and pin-fins were developed in the present work. Both transient liquid crystal (TLC) and pressure measurement were applied on the impingement cooling structures. A 3D numerical method was also used for conjugate heat transfer simulation. Corrugated orifice helps decrease the pressure drop by decreasing the speed of cross flow. Experimental data show that, corrugated orifice is helpful in reducing pin-fin induced pressure drop but contributes little to heat transfer. Pin-fins increases both the heat transfer and pressure drop lightly. Conjugate heat transfer simulation shows that pin-fins significantly reduce the metal temperature by conduction. Structures with pin-fins can make a good use of the large surface area of corrugated orifices.© 2014 ASME

Cooling Structure Optimization for a Rib-Roughed Channel in a Turbine Rotor Blade

Cooling design for the air-cooled turbine blades is a critical issue in modern gas turbine engineering. Advances in CFD technology is providing new prospects for turbine cooling design, as the optimum cooling structures of the blades could be designed through the optimization search coupled with the Conjugate Heat Transfer (CHT) analysis. In this paper, the optimization study for the rib arrangement of a rib-roughed channel in a rotor blade is discussed.The optimization study introduced is realized utilizing a parametric analysis platform, which consists of the parametric design and mesh generation tool and the commercial CHT solver ANSYS CFX. For the optimization study, firstly a group of Design of Experiments (DoE) analysis of a rib-roughed rectangular channel is performed in order to find the optimum rib arrangement and to explore the objective of the optimization search. Then, the optimization search of the optimum rib arrangement is performed for a rib-roughed channel within a rotor blade based on the multi-island Genetic Algorithms (GA) of iSIGHT. During optimization search, a constant pressure drop is assumed within the cooling system, and the CHT simulations are approached for the interior only in order to make the search computationally faster.According to the DoE analysis, minimizing the averaged wall temperature on blade surface is chosen as the optimization objective for the design of rib arrangement. The results of the GA search shows that the optimal rib arrangement with best cooling performance can be decided, and the optimal mass flow rate for the cooling channel is found simultaneously. The optimum schemes of the rib arrangement found by the DoE analysis and GA search are quite identical, which further validates the feasibility of design optimization for the blade cooling structure with the GA and CHT simulations.Copyright