| 1. |
- Siddique, Waseem
(författare)
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Design of Internal Cooling Passages: Investigation of Thermal Performance of Serpentine Passages
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gas turbines are used to convert thermal energy into mechanical energy. The thermal efficiency of the gas turbine is directly related to the turbine inlet temperature. The combustion and turbine technology has improved to such an extent that the operating temperature in the turbine inlet is higher than the melting temperature of the turbine material. Different techniques are used to cope with this problem. One of the most commonly used methods is internal cooling of the turbine blades. Conventionally air from the compressor is used for this purpose but due to higher heat capacity, steam can be used as coolant. This opens up the possibility to increase the gas temperature. In the case of a combined cycle power plant, its availability provides a good opportunity to be used as a coolant. The trailing edge of the gas turbine blades is an important region as it affects the aerodynamics of the flow. The aerodynamics demands a sharp and thin trailing edge to reduce profile losses. The conventional method is the release of a lot of cooling air though a slot along the airfoil trailing edge. However in the case of internal only cooling designs, the coolant is not allowed to leave the channel except from the root section to avoid mixing of the gas in the main flow path with the coolant and loss of cooling medium. The challenge is to design an inner cooling channel, with the cooling medium entering and leaving the blade at the root section, which reduces the metal temperatures to the required values without an increase of the profile losses and at acceptable cooling flow rate and pressure drop. This thesis presents Computational Fluid Dynamic (CFD) based numerical work concentrated firstly on the flow and heat transfer in two-pass rectangular channels with and without turbulator ribs. The aspect ratio of the inlet pass was reduced to accommodate more channels in the blade profile in chord-wise direction. Additionally, the divider-to-tip wall distance was varied for these channels. Their effect on heat transfer and pressure drop was studied for smooth as well as ribbed channels. It was followed by a numerical heat transfer study in the trapezoidal channel. Different RANS based turbulence models were used to compare the numerical results with the experimental results. Further, new designs to enhance heat transfer in the channel’s side walls (named as trailing edge wall) were studied. These include the provision of ribs at the trailing edge wall only, inline arrangement of ribs at the bottom as well as at the trailing edge wall and a staggered arrangement of these ribs. The final study was a conjugate heat transfer problem with an aim to propose the best internal cooling channel design to reduce the metal temperature of the trailing edge surface for the given thermal and flow conditions. A number of different options were studied and changes were made to get the best possible channel design. The results show that for a two-pass rectangular channel (both smooth and ribbed), the reduction in inlet channel aspect ratio reduces the pressure drop. For a smooth channel the reduction in the width of the inlet pass does not affect the heat transfer enhancement at the inlet pass and outlet pass regions. In case of ribbed channels, heat transfer decreases at the tip and bend bottom with decrease in the width of the inlet pass. Among different turbulence models used to validate numerical results against experimental results for case of trapezoidal channel, the low-Re k-epsilon model is found to be the most appropriate. Using the turbulence model that yields results that are closest to the experimental data, the staggered arrangement of ribs at the trailing edge wall is found to have maximum thermal performance. The results from the conjugate heat transfer problem suggest using steam as coolant if it is available as it requires less mass flow rate to get similar wall temperature values as compared to air at similar thermal and flow conditions. It is also found that staggered arrangement of ribs is the best option compared to others to enhance heat transfer in trailing edge of the gas turbine blade with the pressure drop in the cooling duct in the acceptable range.
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| 2. |
- Siddique, Waseem, et al.
(författare)
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Design of internally cooled trailing edge at engine similar conditions- A conjugate heat transfer problem
- 2012
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Ingår i: ASME Turbo Expo 2012. - NEW YORK : ASME Press. - 9780791844700 ; , s. 1357-1372, s. 1357-1372
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Konferensbidrag (refereegranskat)abstract
- Gas turbines are operated at elevated temperatures as the thermal efficiency of the gas turbine is directly linked to the turbine inlet gas temperature. The different regions of the turbine blade require different means of cooling. This paper presents different designs of the two-pass trapezoidal channel which represents the trailing edge of a real engine. Engine similar boundary conditions are applied and conjugate heat transfer method is used to predict the wall temperatures. The aim is to design a cooling channel that through use of steam can reduce wall temperatures to below a target value while maintaining minimal pressure drop. The variations in design of a smooth two-pass channel were made to achieve the design target. These variations included installation of ribs at the walls, tapered divider wall, tilted divider wall and L-shaped divider wall to promote fluid impingement on the trailing wall. The results suggest that a channel with staggered ribs at the outlet pass, a tilted divider wall and impingement at the corner is the best arrangement for reducing wall temperatures below the target value; however, it has low overall aerothermal performance due to high pressure losses. A similar channel without impingement can yield acceptable results if a thermal barrier coating is applied or if a small corner of the tip-trailing edge is truncated to reduce material volume. This modification though can improve the thermal performance of the channel, is to result in higher profile and aerodynamics losses.
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| 3. |
- Siddique, Waseem, et al.
(författare)
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Flow structure, heat transfer and pressure drop in varying aspect ratio two-pass rectangular smooth channels
- 2012
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Ingår i: Heat and Mass Transfer. - : Springer Science and Business Media LLC. - 0947-7411 .- 1432-1181. ; 48:5, s. 735-748
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Tidskriftsartikel (refereegranskat)abstract
- Two-pass channels are used for internal cooling in a number of engineering systems e.g., gas turbines. Fluid travelling through the curved path, experiences pressure and centrifugal forces, that result in pressure driven secondary motion. This motion helps in moving the cold high momentum fluid from the channel core to the side walls and plays a significant role in the heat transfer in the channel bend and outlet pass. The present study investigates using Computational Fluid Dynamics (CFD), the flow structure, heat transfer enhancement and pressure drop in a smooth channel with varying aspect ratio channel at different divider-to-tip wall distances. Numerical simulations are performed in two-pass smooth channel with aspect ratio W-in/H = 1:3 at inlet pass and W-out/H = 1:1 at outlet pass for a variety of divider-to-tip wall distances. The results show that with a decrease in aspect ratio of inlet pass of the channel, pressure loss decreases. The divider-to-tip wall distance (W-el) not only influences the pressure drop, but also the heat transfer enhancement at the bend and outlet pass. With an increase in the divider-to-tip wall distance, the areas of enhanced heat transfer shifts from side walls of outlet pass towards the inlet pass. To compromise between heat transfer and pressure drop in the channel, W-el/H = 0.88 is found to be optimum for the channel under study.
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| 4. |
- Siddique, Waseem, et al.
(författare)
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On flow structure, heat transfer and pressure drop in varying aspect ratio two-pass rectangular channel with ribs at 45 degrees
- 2013
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Ingår i: Heat and Mass Transfer. - : Springer Science and Business Media LLC. - 0947-7411 .- 1432-1181. ; 49:5, s. 679-694
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Tidskriftsartikel (refereegranskat)abstract
- To increase the thermal efficiency of gas turbines, inlet temperature of gas is increased. This results in the requirement of cooling of gas turbine blades and vanes. Internal cooling of gas turbine blades and vanes is one of several options. Two-pass channels are provided with ribs to enhance heat transfer at the expense of an increased pressure drop. The space in the blade is limited and requires channels with small aspect ratios. Numerical simulations have been performed to investigate heat transfer, flow field and pressure loss in a two-pass channel equipped with 45A degrees ribs with aspect ratio (W-in/H) equal to 1:3 in the inlet pass and 1:1 in the outlet pass with both connected together with a 180A degrees bend. The results are compared with a higher aspect ratio channel (W-in/H = 1:2, inlet pass). In the ribbed channel, a decrease in pressure drop was observed with a decrease in the aspect ratio of the channel. The smaller aspect ratio channel not only allows using more cooling channels in the blade, but also results in more heat transfer enhancement. The divider-to-tip wall distance (W-el) has influence on the pressure drop, as well as on the heat transfer enhancement at the bend and outlet pass. Heat transfer decreases with decrease in aspect ratio of the inlet pass of the two-pass channel. With increase in divider-to-tip wall distance, heat transfer tries to attain a constant value.
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| 5. |
- Siddique, Waseem, et al.
(författare)
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Validation and Analysis of Numerical Results for a Two-Pass Trapezoidal Channel With Different Cooling Configurations of Trailing Edge
- 2012
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Ingår i: Journal of turbomachinery. - : ASME International. - 0889-504X .- 1528-8900. ; 135:1, s. 011027-
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Tidskriftsartikel (refereegranskat)abstract
- High inlet temperatures in a gas turbine lead to an increase in the thermal efficiency of the gas turbine. This results in the requirement of cooling of gas turbine blades/vanes. Internal cooling of the gas turbine blade/vanes with the help of two-pass channels is one of the effective methods to reduce the metal temperatures. In particular, the trailing edge of a turbine vane is a critical area, where effective cooling is required. The trailing edge can be modeled as a trapezoidal channel. This paper describes the numerical validation of the heat transfer and pressure drop in a trapezoidal channel with and without orthogonal ribs at the bottom surface. A new concept of ribbed trailing edge has been introduced in this paper which presents a numerical study of several trailing edge cooling configurations based on the placement of ribs at different walls. The baseline geometries are two-pass trapezoidal channels with and without orthogonal ribs at the bottom surface of the channel. Ribs induce secondary flow which results in enhancement of heat transfer; therefore, for enhancement of heat transfer at the trailing edge, ribs are placed at the trailing edge surface in three different configurations: first without ribs at the bottom surface, then ribs at the trailing edge surface in-line with the ribs at the bottom surface, and finally staggered ribs. Heat transfer and pressure drop is calculated at Reynolds number equal to 9400 for all configurations. Different turbulent models are used for the validation of the numerical results. For the smooth channel low-Re k-e model, realizable k-e model, the RNG k-ω model, low-Re k-ω model, and SST k-ω models are compared, whereas for ribbed channel, low-Re k-e model and SST k-ω models are compared. The results show that the low-Re k-e model, which predicts the heat transfer in outlet pass of the smooth channels with difference of +7%, underpredicts the heat transfer by -17% in case of ribbed channel compared to experimental data. Using the same turbulence model shows that the height of ribs used in the study is not suitable for inducing secondary flow. Also, the orthogonal rib does not strengthen the secondary flow rotational momentum. The comparison between the new designs for trailing edge shows that if pressure drop is acceptable, staggered arrangement is suitable for the outlet pass heat transfer. For the trailing edge wall, the thermal performance for the ribbed trailing edge only was found about 8% better than other configurations.
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| 6. |
- Siddique, Waseem, et al.
(författare)
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Validation and analysis of numerical results for a two-pass trapezoidal channel with different cooling configurations of trailing edge
- 2012
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Ingår i: Proceedings of the ASME Turbo Expo 2011, Vol 5, Parts A And B. - : American Society Of Mechanical Engineers. - 9780791854655 ; , s. 1571-1581
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Konferensbidrag (refereegranskat)abstract
- High inlet temperatures in a gas turbine lead to an increase in the thermal efficiency of the gas turbine. This results in the requirement of cooling of gas turbine blades/vanes. Internal cooling of the gas turbine blade/vanes with the help of two-pass channels is one of the effective methods to reduce the metal temperatures. Especially the trailing edge of a turbine vane is a critical area, where effective cooling is required. The trailing edge can be modeled as a trapezoidal channel. This paper describes the numerical validation of the heat transfer and pressure drop in a trapezoidal channel with and without orthogonal ribs at the bottom surface. A new concept of ribbed trailing edge has been introduced in this paper which presents a numerical study of several trailing edge cooling configurations based on the placement of ribs at different walls. The baseline geometries are two-pass trapezoidal channels with and without orthogonal ribs at the bottom surface of the channel. Ribs induce secondary flow which results in enhancement of heat transfer therefore for enhancement of heat transfer at the trailing edge, ribs are placed at the trailing edge surface in three different configurations: first without ribs at the bottom surface, then ribs at trailing edge surface in-line with the ribs at bottom surface and finally staggered ribs. Heat transfer and pressure drop is calculated at Reynolds number equal to 9400 for all configurations. Different turbulent models are used for the validation of the numerical results. For the smooth channel low-Re kappa-epsilon model, realizable kappa-epsilon model, the RNG kappa-omega model, low-Re kappa-omega model and SST kappa-omega models are compared, whereas for ribbed channel low-Re kappa-omega model and SST kappa-omega models are compared. The results show that the low-Re k-epsilon model, which predicts the heat transfer in outlet pass of the smooth channels with difference of +7%, underpredicts the heat transfer by -17% in case of ribbed channel compared to experimental data. Using the same turbulence model shows that the height of ribs used in the study is not suitable for inducing secondary flow. Also, the orthogonal rib does not strengthen the secondary flow rotational momentum. The comparison between the new designs for trailing edge shows that if pressure drop is acceptable, staggered arrangement is suitable for the outlet pass heat transfer. For the trailing edge wall, the thermal performancefor ribbed trailing edge only, was found about 8% better than other configurations.
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