Krishna Guntur

Computational Study of Gas Turbine Blade Cooling Channel

It has been a common practice to use cooling passages in gas turbine blade in order to keep the blade temperatures within the operating range. Insufficiently cooled blades are subject to oxidation, to cause creep rupture, and even to cause melting of the material. To design better cooling passages, better understanding of the flow patterns within the complicated flow channels is essential. The interactions between secondary flows and separation lead to very complex flow patterns. To accurately simulate these flows and heat transfer, both refined turbulence models and higher-order numerical schemes are indispensable for turbine designers to improve the cooling performance. Power output and the efficiency of turbine are completely related to gas firing temperature from chamber. The increment of gas firing temperature is limited by the blade material properties. Advancements in the cooling technology resulted in high firing temperatures with acceptable material temperatures. To better design the cooling channels and to improve the heat transfer, many researchers are studying the flow patterns inside the cooling channels both experimentally and computationally. In this paper, the authors present the performance of three turbulence models using TEACH software code in comparison with the experimental values. To test the performance, a square duct with rectangular ribs oriented at 90° and 45° degree and placed at regular intervals. The channel also has bleed holes. The normalized Nusselt number obtained from simulation are validated with that of experiment. The Reynolds number is set at 10,000 for both the simulation and experiment. The interactions between secondary flows and separation lead to very complex flow patterns. To accurately simulate these flows and heat transfer, both refined turbulence models and higher-order numerical schemes are indispensable for turbine designers to improve the cooling performance. The three-dimensional turbulent flows and heat transfer are numerically studied by using several different turbulence models, such as non-linear low-Reynolds number k-omega and Reynolds Stress (RSM) models. In k-omega model the cubic terms are included to represent the effects of extra strain-rates such as streamline curvature and three-dimensionality on both turbulence normal and shear stresses. The finite volume difference method incorporated with the higher-order bounded interpolation scheme has been employed in the present study. The outcome of this study will help determine the best suitable turbulence model for future studies.Copyright

Study of Flow Through a Stationary Ribbed Channel for Blade Cooling

The firing temperature in gas turbine relates itself directly to the power output and the efficiency of the turbine. The higher the firing (operating) temperatures, higher the wall temperature of blades. However, an increase in the firing temperature is limited by the first stage blade material properties. This is because the higher firing temperature may cause a creep rupture, oxidizing, melting and ultimately failing of blades. Prior to blade cooling, the firing temperature was the same as the blade material temperature. Advancements in cooling technology have resulted in high firing temperatures with acceptable material temperatures. To better design the cooling channels and to improve heat transfer, many researchers are studying the flow patterns inside the cooling channels both experimentally and computationally. In this paper, the authors present the performance of three turbulence models using a Computational Fluid Dynamics code in comparison with the experimental values. To test the performance, a square duct was used with rectangular ribs oriented at 90° and 45° degree and placed at regular intervals. The channel also has bleed holes. The wall Nusselt numbers are compared in both the experimental and the computational results after suitable normalization. The Reynolds number is set to 10,000. The interactions between secondary flows and separation lead to very complex flow patterns. To accurately simulate these flows and heat transfer, both refined turbulence models and higher-order numerical schemes are indispensable for turbine designers to improve the cooling performance. The three-dimensional turbulent flows and heat transfer are numerically studied by using several different turbulence models, such as a non-linear low-Reynolds number k-ω and Reynolds Stress (RSM) models. In the k-ω model the cubic terms are included to represent the effects of extra strain-rates such as streamline curvature and three-dimensionality on both normal and shear turbulence stresses. The finite volume difference method incorporated with the higher-order bounded interpolation scheme has been employed in the present study. The outcome of this study helps to determine the best suitable turbulence model for future studies.© 2010 ASME

Experimental and Numerical Evaluation of Geometric Modifications in Gas Turbine Blade Cooling Channel

Cooling channels are essential components of the modern gas turbine engines. Enhanced cooling techniques are allowing for increase in firing temperatures and thereby improving power and efficiency. Air bled from the compressor is passed through serpentine passages within the blade to take away the heat and keep the blade under operating temperatures. Heat transfer in the channel can be improved by increasing the turbulence in the coolant flow. U-Bends in the serpentine channels cause secondary flow and Coriolis effects. Apart from this, the channel walls are impregnated with turbulators such as ribs, pins and dimples. These turbulators cause flow disturbance near the wall improving the heat transfer. Designing the cooling system requires detailed understanding of the flow patterns in the channel. Numerical techniques help understand the flow in detail providing 3-dimensional velocity profiles and other parameters. Experimental data helps validate the numerical models and improves the reliability of the numerical results. In this research, experiments were performed to determine the Nusselt number distribution in a cooling channel and numerical simulations were performed to compare and predict the heat transfer in other channels.

Numerical Comparison of Heat Transfer and Pressure Drop in Gas Turbine Blade Cooling Channels With Dimples and Rib-Turbulators

In order to enhance the performance of a gas turbine and to maintain the blade material within operating temperature range, cooling channels are made within the blade materials that extract the heat. The walls of these cooling channels are usually enhanced with some sort of turbulence generators — ribs and dimples being the most common. While both the geometries provide improvement in enhancing the heat transfer, dimples usually have a lower pressure drop. It is essential to improve the heat transfer rate with a minimal pressure loss. In this study, the heat transfer and pressure loss are determined numerically and combined to show the effect of both in channels with ribs and dimples on one wall of the channel. Similar geometric and boundary conditions are used for both the turbulators. Reynolds numbers of 12,500 and 28,500, based on the hydraulic diameter are used for the study. The Reynolds-Stress Model was used for all the computations as a turbulence model by employing Fluent.Copyright

Self-Healing Technology for Compressor and Turbine Blades

The problem of cracking in steam turbines and compressor blades is one of the major problems associated with their design. Not only in these areas, but many other parts are suffering from the same problem of cracking due to excess stress, fatigue and high temperatures. One of the recent solutions to this is the use of specialized alloys, ceramics and metal matrix composites with improved high temperature strength and fatigue limits, but these materials will also suffer from cracking. Another potential method of improving the fatigue life of steam turbine and compressor blades is through the use of self-healing metals. There has been a recent interest in making self-healing metal composites that heal any cracks with little or no human interaction. One of the ways to achieve this is to send a healing agent (generally a low melting alloy or uncured resin) through hollow passages made in the matrix. When a crack appears, the healing agent flows into the crack, solidifies and closes the crack, effectively healing it. Recently, researchers at UW-Milwaukee have extended the concept of self-healing to metals, and have synthesized self-healing aluminum and solder alloys. This method may be used for the blades of steam turbines and compressors. The hollow passages can be made in a similar method that cooling passages of the gas turbine blades are made. Proper material choice will result in good bonding between the healing agent and the walls of a crack. This study deals with the advantages, ease of use and other considerations for this method to be used in practice. The main problems that need to be over come are materials selection, loss of strength due to addition of hollow channels, and the need to develop methods to initiate healing by flow of the healing agent into a crack. Plausible solutions to these are also discussed.Copyright

Study of Uniform Heating and Efficiency of Paper Mill Drying Procedures

The Paper Mill Project goal is to improve upon the uniform heating of a continuous paper sheet flowing across a heat source for the purpose of drying the paper. The project tests different porous media to achieve uniform heat distribution across the paper. Existing designs overheat the paper to overcome the uneven heat distribution caused by uneven flow velocities; current designs also achieve low overall efficiency. This project attempts to test the effect porous media has on creating even heat distributions across the paper sheet, and which type of porous media achieves the greatest overall efficiency. This project was conducted in two phases; the first phase tests the different porous medium drying a single sheet conveying at two different sheet velocities. The second phase tested the different porous medium drying four stationary strips of paper for a length of one minute.Copyright

Computational Study of Heat Transfer Distribution in a Square Channel with Rib Turbulators and Bleed Holes

The objective of this study is to clearly understand the flow and the heat transfer mechanisms in a gas turbine cooling system by performing a computational analysis of the turbulent heat transfer of an internal ribbed channel. To test the performance, a square duct with rectangular turbulence promoting ribs oriented at 60o inverted V ribs and placed at regular intervals on one of the walls with bleeding holes has been simulated. The normalized Nusselt number obtained from simulation are validated against experiments. The Reynolds number is set at 12,500 for both simulation and experiment. The three-dimensional turbulent flows and heat transfer are numerically studied by using a turbulence model, nonlinear low-Reynolds number k-ω model. In k-ω ω ω ω model the cubic terms are included to represent the effects of extra strain-rates such as streamline curvature and threedimensionality on turbulence normal and shear stresses. The finite volume difference method incorporated with the higher-order bounded interpolation scheme has been employed in the present study. The outcome of this study helps determine the best suitable turbulence model for future studies.

Comparison of Turbulence Models With Experiment for Heat Transfer in a Cooling Channel With Dimples

Turbine blade cooling is one of the most important developments in gas turbine history. Development of blade cooling enabled increase in firing temperature and in turn improving the efficiency. Different cooling channel geometries have been tested for improving the heat transfer efficiency. Smooth channels, channels with sharp bends, channels with ribs and other such tabulators were used to improve the turbulence and thereby increasing the efficiency. In the recent developments, hemi-spherical dimples are being considered instead of ribs as dimples have less pressure loss. This paper compares numerical computations of the heat transfer characteristics of a dimpled rectangular channel with published experimental values. Reynolds number ranges from 5000 to 40,000. For the numerical computations two different turbulence models, k-e and k-ω models are used, the software for the simulation was Fluent and grid generation was achieved by Ansys workbench. The dimpled channel results are normalized with the smooth channel results.Copyright

Uniform Heating Device for a Paper Mill Process

Paper mills use elaborated drying process using hot rollers to dry the paper. In the recent past, use of hot air for drying has gained some interest. The main disadvantage of this method is non-uniform air temperature, which will cause lateral shrinkage of the paper. This paper discusses a new drying system. The process incorporates a porous medium to ensure uniform temperature by flattening the velocity distribution. The new system requires less equipment pieces and utilized less space for the drying process. A cloth is used in place of paper to increase the repeatability of the experiment. This experiment utilized a conveyor system to transport the cloth within the heater section. Variables concerning velocity and temperature values of the heated air, and heater intensity were adjusted to produce the different drying conditions. Forced air propane heater was used as the heat source. Design modifications were made so that the heater is more suitable for this process. The results showed significant improvement of the velocity and feasibility of extending this technology to the actual scale.Copyright

Computational Study of Heat/Mass Transfer Characteristics in a Ribbed Cooling Channel in a Stationary Gas Turbine Blade

†It is a common practice to incorporate flow channels within a gas turbine blade for cooling purposes. This is done to prevent any damages to the blades from the high temperature gases. From thermodynamic principles it is known that the higher inlet temperatures lead to higher efficiencies, but to achieve these high temperatures, it is necessary that the blade material sustain the thermal stresses. The common methods of cooling are using internal cooling passages and film cooling. The internal cooling passages are being studied extensively, both experimentally and computationally. The understanding of the flow and heat transfer characteristics is very important for optimal design of the channels. Different geometries are used. Turbulence enhancers are used to enhance the turbulence and hence the heat-transfer. The present work is a computational study of a flow channel with ribs along the walls. The objective of the current study is to validate different computational models with the experimental results. The model used for the experiment is modeled with minor changes to accommodate simpler mesh and stable numerical simulation. The model is studied using the k – � and k – � numerical models and the results are compared with the experiment. This study gives an insight into what models are more suited for various types of simulations and ultimately eliminating the need for validating and predict using the numerical simulation. This study concludes to say that the k-� model predicts better results compared to the k-� model.