Higher turbine inlet temperatures (TIT) are required for higher overall efficiency of gas turbine engines. Due to the constant push towards achieving high TIT, the heat load on high pressure turbine components has been increasing with time. Gas turbine airfoils are equipped with several sophisticated cooling technologies which protect them from harsh external environment and increase their operating life and reduce the maintenance cost. The turbine airfoils are coated with thermal barrier coatings (TBCs) and the external surface is protected by film cooling. The internals of gas turbine blades are cooled by relatively colder air bled off from the compressor discharge. Gas turbine internals can be divided into three broad segments – Leading edge section, (2) mid-chord section and (3) trailing edge section. The leading edge of the airfoil is subjected to extreme heat loads due to hot main gas stagnation and high turbulence intensity of the combustor exit gases. The leading edge is typically cooled by jet impingement which cross-over the rib turbulators in the feed chamber. The mid-chord section of the turbine airfoils have serpentine passages connected via. 180° bends, and they feature turbulence promotors which enhance the heat exchange rates between the coolant and the internal walls of the airfoil. The trailing edge section is typically cooled by array of pin fins. On one hand, the coolant routed through the internal passages of turbine airfoil help maintain the airfoil temperatures within safe limits of operation, the cooled air comes at a cost of loss of high pressure air from the compressor section. The aim of this study is to develop internal cooling concepts which have high thermal hydraulic performance, i.e. to gain high levels heat transfer enhancement due to cooling concepts at lower pumping power requirements. Experimental and numerical studies have been carried out and new rib turbulator designs such as Criss-Cross pattern, compound channels featuring uniquely organized ribs and dimples, novel jet impingement hole shapes have been developed which have high thermal-hydraulic performance.
Further, gas turbine blades rotate at high rotational speeds. The internal flow routed thought the serpentine passages are subjected to Coriolis and centrifugal buoyancy forces. The combined effects of these forces results in enhancement and reduction in heat transfer on the pressure side and suction side internal walls. This leads to non-uniformity in the heat transfer enhancement which leads to non-uniform cooling and increase in the sites of high and low internal wall temperatures. Development of cooling concepts which have high thermal hydraulic performance under non-rotating conditions is important, however, under rotation, the heat transfer characteristics of the internal passages is significantly different in an unfavorable way. So the aim of the turbine cooling research is to have concepts which provide highly efficient and uniform cooling. The negative effects of rotation has been addressed in this study and new orientation of two-pass cooling channels has been presented which utilizes the rotational energy in favor of heat transfer enhancement on both pressure and suction side internal walls.
Present study has led to several new cooling concepts which are efficient under both stationary and rotating conditions. / Ph. D. / Higher turbine inlet temperatures lead to higher overall efficiency of gas turbines. Hence, the high pressure stages of turbine sections, which are downstream of the combustor section, have significant thermal load. The turbine inlet temperatures can be as high as 1700°C and turbine airfoil material melting point temperature is around 1000°C. In order to protect the blade for the harsh environment, relatively colder air (~700°C) bled off from the compressor discharge is routed through the internal cooling passages of turbine airfoils. The coolant bled from the compressor section contributes the reduction in the performance of the engine. Hence, the aim of the turbine cooling research is to achieve high rates of heat transfer at relatively lower pumping power requirements. In order to enhance the heat transfer rates from between the hot internal walls of airfoil and the coolant, turbulence promotors are typically installed in the mid-section of the airfoil which features serpentine passages interconnected by 180° bends. Present study is focused on development of highly efficient concepts for internal flows in turbine airfoils.
The other aspect of internal cooling research is focused on characterization of heat transfer under rotating conditions. Coriolis force and centrifugal buoyancy forces lead to non-uniform cooling and the heat transfer rates are significantly different under rotating conditions compared to non-rotating conditions. Present study utilizes detailed measurements of heat transfer coefficients under rotating conditions for the development of cooling designs for two-pass ribbed channels where rotational effects can be used in favor of heat transfer enhancement, leading to enhanced and more uniform cooling of internal walls.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/78358 |
| Date | 18 July 2017 |
| Creators | Singh, Prashant |
| Contributors | Mechanical Engineering, Ekkad, Srinath V., Qiao, Rui, Tafti, Danesh K., Ng, Wing Fai, Lowe, K. Todd |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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