Date of Award

May 2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Ryoichi S Amano

Committee Members

Robert M Cuzner, John R Reisel, Deyang Qu, Istvan G Lauko

Keywords

Gas Turbine Cooling Systems, Heat and Mass Transfer, Internal Cooling Systems, Internal Flow, Turbulence modeling, Turbulent flow and heat transfer

Abstract

Gas turbines are used in various systems, including in power plants, marine, and aircraft propulsion engines. The airflow in gas turbines can reach temperatures as high as 2000 K that exceeds the softening points for most high-performance blade materials, such as special steels, titanium alloys, and super-alloys. Preventing thermal failure in the turbine-blades has driven many technological advances in materials science and mechanical design. In this thesis, the main focus is on improving the performance of the internal cooling channel technology. In this study, the optimized thermal behavior of the coolant gas and surfaces in turbines by incorporating novel internal cooling channels is modeled. Various computational simulations for modeling the minute details of the fluid dynamics of the coolant in blade airfoil channels are performed. This computation is done by the Large Eddy simulation method and implementing a conjugate heat transfer model. Many computational parameters are included to capture the details of airflow and hot surface interactions. These parameters are materials surface properties, Coriolis and centrifugal buoyancy forces due to rotation, effects of separating/reattaching shear layers, and the secondary flows induced by the bending regions. The maximum blade temperature and its impact on the heat transfer coefficient (The Nusselt number) are determined.

In this thesis, the following major challenges that were not considered in earlier investigations were targeted

1- A heat transfer method to present the heat transfer distribution and flow behavior near realistic gas turbine operating conditions.

2- Turning region turbulator design that causes less pressure drop along the channel and enhances the total heat transfer coefficient.

3- The combined physical and geometrical effects on heat transfer efficiency.

4- Validating the numerical results with an experimental set up that can provide similar gas turbine operating conditions.

5- Heat transfer correlation for calculating local Nusselt number for a developing flow passing the channel.

This study deliberates the channel turning effect and revealed the importance of this phenomena on the cooling channel's overall heat transfer performance and pressure distribution. Turning guide vanes that enhance the heat transfer coefficient in the turning region of the channel are designed. This study found the optimum guide vane design and rib turbulator configuration that led to a 1.5 times increase in the heat transfer coefficient of the system. Also, it is found that the Nusselt number strongly depends on the Reynolds number. In order to explain the relationship between the Nusselt number and Reynolds number, the Nusselt number is characterized by smooth wall channels and developed a new correlation between the Nusselt number and the Reynolds number. The Nusselt number calculated based on this proposed relationship captures the developing flow and turning the region's effects on the local Nusselt number. It also has a better agreement with the numerical and experimental results. This correlation can be a reference for similar studies.

For the experimental heat transfer investigation, the experimental setup was built, and it reached the maximum rotation speeds up of 1500 rpm. Achieving this rotational speed is mechanically challenging and is considered substantial to similar experimental setups. This experimental setup enabled the execution and evaluation of the numerical predictions. For the test section in this study, the same size geometry that modeled in the numerical study is designed and built. The corresponding boundary conditions used in the numerical study are implemented in the experiment. An innovational method to record temperature measurement in the rotational motion is used. A series of miniature portable wireless thermocouple connectors and data loggers, and a wireless receiver to measure and collect temperature values at various points along the channel were implemented. Different thermocouple at the different channel’s cross-section location, taking the average temperature in each cross-section, are utilized. This method provided real-time data at high speed. Air bulk temperature is calculated at each cross-section by using the recorded experimental data. This method of calculating the Bulk temperature before investigating the heat transfer along the channel is one of the novelties of the experimental study. After measuring the heated wall temperature and bulk temperature along the channel, the heat transfer coefficient in both stationary and rotational motion is calculated. Results from this study are in good agreement with the numerical results that approved the accuracy of the test section.

Available for download on Sunday, May 22, 2022

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