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Thomas Povey - One of the best experts on this subject based on the ideXlab platform.

  • cooling optimization theory part i optimum wall temperature coolant exit temperature and the effect of wall film properties on performance
    Journal of Turbomachinery-transactions of The Asme, 2016
    Co-Authors: Benjamin Kirollos, Thomas Povey
    Abstract:

    Gas turbine cooling system design is constrained by a maximum allowable wall temperature (dictated by the material and the life requirements of the component), minimum coolant Mass flow rate (the requirement to minimize cycle-efficiency cost), and uniform wall temperature (to reduce thermal stresses). These three design requirements form the basis of an iterative design process. The relationship between the requirements has received little discussion in the literature, despite being of interest from both a theoretical and a practical viewpoint. In this paper, we consider the optimum cooling system for parts with both internal and film cooling. We show analytically that the coolant Mass flow rate is minimized when the wall temperature is uniform and equal to the maximum allowable wall temperature. Thus, we show that achieving uniform wall temperature achieves minimum coolant flow rate, and vice versa. The purpose is to clarify the interplay between two design requirements that are often discussed separately in the literature. The penalty (in terms of coolant Mass flow) associated with cooling nonisothermal components is quantified. We show that a typical high pressure nozzle guide vane (HPNGV) operating isothermally at the maximum allowable wall temperature requires two-thirds the coolant of a typical nonisothermal vane. The optimum coolant exit temperature is also considered. It is shown analytically that the optimum coolant exit temperature depends on the balance between the mean adiabatic film cooling effectiveness, the Nondimensional Mass flow rate, and the Biot number of the thermal barrier coating (TBC). For the large majority of gas turbine cooling systems (e.g., a typical HPNGV) it is shown that the optimum coolant exit temperature is equal to the local wall temperature at the point of injection. For a small minority of systems (e.g., long effusion cooling systems operating at low Mass flow rates), it is shown that the coolant exit temperature should be minimized. An approximation relating the wall/film properties, the Nondimensional Mass flow, and the overall cooling effectiveness is derived. It is used to estimate the effect of Biot number (TBC and metal), heat transfer coefficient (HTC) ratio, and film properties on the performance of a typical HPNGV and effusion cooling system. In Part II, we show that designs which achieve uniform wall temperature have a particular corresponding internal HTC distribution.

  • Cooling Optimization Theory—Part I: Optimum Wall Temperature, Coolant Exit Temperature, and the Effect of Wall/Film Properties on Performance
    Journal of Turbomachinery, 2016
    Co-Authors: Benjamin Kirollos, Thomas Povey
    Abstract:

    Gas turbine cooling system design is constrained by a maximum allowable wall temperature (dictated by the material and the life requirements of the component), minimum coolant Mass flow rate (the requirement to minimize cycle-efficiency cost), and uniform wall temperature (to reduce thermal stresses). These three design requirements form the basis of an iterative design process. The relationship between the requirements has received little discussion in the literature, despite being of interest from both a theoretical and a practical viewpoint. In this paper, we consider the optimum cooling system for parts with both internal and film cooling. We show analytically that the coolant Mass flow rate is minimized when the wall temperature is uniform and equal to the maximum allowable wall temperature. Thus, we show that achieving uniform wall temperature achieves minimum coolant flow rate, and vice versa. The purpose is to clarify the interplay between two design requirements that are often discussed separately in the literature. The penalty (in terms of coolant Mass flow) associated with cooling nonisothermal components is quantified. We show that a typical high pressure nozzle guide vane (HPNGV) operating isothermally at the maximum allowable wall temperature requires two-thirds the coolant of a typical nonisothermal vane. The optimum coolant exit temperature is also considered. It is shown analytically that the optimum coolant exit temperature depends on the balance between the mean adiabatic film cooling effectiveness, the Nondimensional Mass flow rate, and the Biot number of the thermal barrier coating (TBC). For the large majority of gas turbine cooling systems (e.g., a typical HPNGV) it is shown that the optimum coolant exit temperature is equal to the local wall temperature at the point of injection. For a small minority of systems (e.g., long effusion cooling systems operating at low Mass flow rates), it is shown that the coolant exit temperature should be minimized. An approximation relating the wall/film properties, the Nondimensional Mass flow, and the overall cooling effectiveness is derived. It is used to estimate the effect of Biot number (TBC and metal), heat transfer coefficient (HTC) ratio, and film properties on the performance of a typical HPNGV and effusion cooling system. In Part II, we show that designs which achieve uniform wall temperature have a particular corresponding internal HTC distribution.

Sanjay Mittal - One of the best experts on this subject based on the ideXlab platform.

  • Vortex-induced vibration of two circular cylinders at low Reynolds number
    Journal of Fluids and Structures, 2009
    Co-Authors: T. K. Prasanth, Sanjay Mittal
    Abstract:

    Abstract Vortex-induced vibration of a pair of equal-sized circular cylinders in tandem and staggered arrangements in laminar flow regime is investigated. A stabilized finite element method is utilized to carry out the computations in two dimensions. Both cylinders are free to oscillate in transverse as well as in-line directions. The Reynolds number, based on the free-stream speed, U, and the diameter, D, of the cylinders is 100. To encourage high amplitude of oscillation the structural damping is set to zero and cylinders of low Nondimensional Mass are considered ( m * = 10 ) . The computations are carried out for various values of reduced speed of the oscillator ( 2 ⩽ U * ⩽ 15 ) . The cylinders are separated by 5.5 D in the streamwise direction. They are separated by 0.7 D in the cross-flow direction to study the effect of stagger. The downstream cylinder lies in the wake of the upstream one and experiences an unsteady inflow. The upstream cylinder in both tandem and staggered arrangement responds qualitatively similarly to a single cylinder. Compared to an isolated cylinder, a small increase in transverse oscillation amplitude of the upstream cylinder is observed due to the presence of the downstream cylinder. In both arrangements, the downstream cylinder shows very large amplitude transverse oscillations comparable to that of a single cylinder at higher Re. In the staggered arrangement, very large streamwise oscillations of the downstream cylinder are observed. Compared to an isolated cylinder, the synchronization range for the two-cylinder arrangement is larger. The downstream cylinder in the staggered arrangement undergoes two types of motion: an orbital motion at most of the U * studied, and figure-of-eight motion for a small range of U * . In the tandem arrangement, only the figure-of-eight motion is observed. The stagger in the arrangement of the two cylinders is found to have a significant effect on the flow.

  • Flow-induced oscillation of two circular cylinders in tandem arrangement at low Re
    Journal of Fluids and Structures, 2009
    Co-Authors: T. K. Prasanth, Sanjay Mittal
    Abstract:

    Abstract Results are presented for flow-induced vibrations of a pair of equal-sized circular cylinders of low Nondimensional Mass ( m * = 10 ) in a tandem arrangement. The cylinders are free to oscillate both in streamwise and transverse directions. The Reynolds number, based on the free-stream speed and the diameter of the cylinders, D is 100 and the centre-to-centre distance between the cylinders is 5.5 D . The computations are carried out for reduced velocities in the range 2 ≤ U * ≤ 15 . The structural damping is set to zero for enabling maximum amplitudes of oscillation. A stabilized finite element method is utilized to carry out the computations in two dimensions. Even though the response of the upstream cylinder is found to be qualitatively similar to that of an isolated cylinder, the presence of a downstream cylinder is found to have significant effect on the behaviour of the upstream cylinder. The downstream cylinder undergoes very large amplitude of oscillations in both transverse and streamwise directions. The maximum amplitude of transverse response of the downstream cylinder is quite similar to that of a single cylinder at higher Re beyond the laminar regime. Lock-in and hysteresis are observed for both upstream and downstream cylinders. The downstream cylinder undergoes large amplitude oscillations even beyond the lock-in state. The phase between transverse oscillations and lift force suffers a 180 ∘ jump for both the cylinders almost in the middle of the synchronization regime. The phase between the transverse response of the two cylinders is also studied. Complex flow patterns are observed in the wake of the freely vibrating cylinders. Based on the phase difference and the flow patterns, the entire flow range is divided into five sub-regions.

  • Effect of blockage on vortex-induced vibrations at low Reynolds numbers
    Journal of Fluids and Structures, 2006
    Co-Authors: T. K. Prasanth, Suresh Behara, Saurav Singh, Rahul Kumar, Sanjay Mittal
    Abstract:

    Abstract There have been quite a few studies in the past to investigate the effect of blockage on flow past a stationary cylinder, but very few for the case when the cylinder is vibrating. Compared to a stationary cylinder, a vibrating cylinder is associated with a wider wake and therefore the blockage is expected to play an even more significant role. The effect of blockage on the vortex-induced vibrations of a cylinder at low Re ( Re ⩽ 150 ) is investigated numerically via a stabilized space–time finite element formulation. The cylinder of low Nondimensional Mass ( m * = 10 ) is free to vibrate in both transverse and in-line directions. Two sets of computations are carried out for each of the cases with 1% and 5% blockage. In the first set of computations the reduced velocity, U * ( = U / f n D , where f n is the natural frequency of the oscillator, U the free-stream speed, and D the cylinder diameter) is fixed to 4.92 and the effect of Re is studied. In the second set of computations, both Re and U * are varied. Lock-in is observed for a range of Re. A hysteretic behavior of the cylinder response close to the lower and upper limits of the synchronization/lock-in region is observed for the case with 5% blockage. The flow is associated with a different arrangement of vortices in the wake depending on whether one is on the “increasing Re” or “decreasing Re” branch. However, for the case with 1% blockage, the hysteretic behavior is completely eliminated near the lower Re range of the lock-in. The solutions for the decreasing as well as increasing Re branch are very similar. They are both associated with intermittent switching of the vortex shedding frequency between the structural frequency and the vortex shedding frequency for stationary cylinder. The hysteretic behavior for a range of Re close to the upper limit of the lock-in region is observed for both the low and high blockage.

  • vortex induced oscillations at low reynolds numbers hysteresis and vortex shedding modes
    Journal of Fluids and Structures, 2005
    Co-Authors: Suryabhan Singh, Sanjay Mittal
    Abstract:

    Abstract Results are presented for the numerical simulation of vortex-induced vibrations (VIVs) of a cylinder at low Reynolds numbers (Re). A stabilized space–time finite-element formulation is utilized to solve the incompressible flow equations in primitive variables. The cylinder, of low Nondimensional Mass ( m * = 10 ), is free to vibrate in, both, the transverse and in-line directions. To investigate the effect of Re and reduced natural frequency, F n , two sets of computations are carried out. In the first set of computations the Reynolds number is fixed ( = 100 ) and the reduced velocity ( U * = 1 / F n ) is varied. Hysteresis, in the response of the cylinder, is observed at the low- as well as high-end of the range of reduced velocity for synchronization/lock-in. In the second set of computations, the effect of Reynolds number ( 50 ⩽ Re ⩽ 500 ) is investigated for a fixed reduced velocity ( U * = 4.92 ). The effect of the Reynolds number is found to be very significant for VIVs. While the vortex-shedding mode at low Re is 2S (two single vortices shed per cycle), at Re ∼ 300 and larger, the P + S mode of vortex shedding (a single vortex and one pair of counter-rotating vortices are released in each cycle of shedding) is observed. This is the first time that the P + S mode has been observed for a cylinder undergoing free vibrations. This change of vortex-shedding mode is hysteretic in nature and results in a very large increase in the amplitude of in-line oscillations. Since the flow ceases to remain two-dimensional beyond Re ∼ 200 , it remains to be seen whether the P + S mode of shedding can actually be observed in reality for free vibrations.

Benjamin Kirollos - One of the best experts on this subject based on the ideXlab platform.

  • cooling optimization theory part i optimum wall temperature coolant exit temperature and the effect of wall film properties on performance
    Journal of Turbomachinery-transactions of The Asme, 2016
    Co-Authors: Benjamin Kirollos, Thomas Povey
    Abstract:

    Gas turbine cooling system design is constrained by a maximum allowable wall temperature (dictated by the material and the life requirements of the component), minimum coolant Mass flow rate (the requirement to minimize cycle-efficiency cost), and uniform wall temperature (to reduce thermal stresses). These three design requirements form the basis of an iterative design process. The relationship between the requirements has received little discussion in the literature, despite being of interest from both a theoretical and a practical viewpoint. In this paper, we consider the optimum cooling system for parts with both internal and film cooling. We show analytically that the coolant Mass flow rate is minimized when the wall temperature is uniform and equal to the maximum allowable wall temperature. Thus, we show that achieving uniform wall temperature achieves minimum coolant flow rate, and vice versa. The purpose is to clarify the interplay between two design requirements that are often discussed separately in the literature. The penalty (in terms of coolant Mass flow) associated with cooling nonisothermal components is quantified. We show that a typical high pressure nozzle guide vane (HPNGV) operating isothermally at the maximum allowable wall temperature requires two-thirds the coolant of a typical nonisothermal vane. The optimum coolant exit temperature is also considered. It is shown analytically that the optimum coolant exit temperature depends on the balance between the mean adiabatic film cooling effectiveness, the Nondimensional Mass flow rate, and the Biot number of the thermal barrier coating (TBC). For the large majority of gas turbine cooling systems (e.g., a typical HPNGV) it is shown that the optimum coolant exit temperature is equal to the local wall temperature at the point of injection. For a small minority of systems (e.g., long effusion cooling systems operating at low Mass flow rates), it is shown that the coolant exit temperature should be minimized. An approximation relating the wall/film properties, the Nondimensional Mass flow, and the overall cooling effectiveness is derived. It is used to estimate the effect of Biot number (TBC and metal), heat transfer coefficient (HTC) ratio, and film properties on the performance of a typical HPNGV and effusion cooling system. In Part II, we show that designs which achieve uniform wall temperature have a particular corresponding internal HTC distribution.

  • Cooling Optimization Theory—Part I: Optimum Wall Temperature, Coolant Exit Temperature, and the Effect of Wall/Film Properties on Performance
    Journal of Turbomachinery, 2016
    Co-Authors: Benjamin Kirollos, Thomas Povey
    Abstract:

    Gas turbine cooling system design is constrained by a maximum allowable wall temperature (dictated by the material and the life requirements of the component), minimum coolant Mass flow rate (the requirement to minimize cycle-efficiency cost), and uniform wall temperature (to reduce thermal stresses). These three design requirements form the basis of an iterative design process. The relationship between the requirements has received little discussion in the literature, despite being of interest from both a theoretical and a practical viewpoint. In this paper, we consider the optimum cooling system for parts with both internal and film cooling. We show analytically that the coolant Mass flow rate is minimized when the wall temperature is uniform and equal to the maximum allowable wall temperature. Thus, we show that achieving uniform wall temperature achieves minimum coolant flow rate, and vice versa. The purpose is to clarify the interplay between two design requirements that are often discussed separately in the literature. The penalty (in terms of coolant Mass flow) associated with cooling nonisothermal components is quantified. We show that a typical high pressure nozzle guide vane (HPNGV) operating isothermally at the maximum allowable wall temperature requires two-thirds the coolant of a typical nonisothermal vane. The optimum coolant exit temperature is also considered. It is shown analytically that the optimum coolant exit temperature depends on the balance between the mean adiabatic film cooling effectiveness, the Nondimensional Mass flow rate, and the Biot number of the thermal barrier coating (TBC). For the large majority of gas turbine cooling systems (e.g., a typical HPNGV) it is shown that the optimum coolant exit temperature is equal to the local wall temperature at the point of injection. For a small minority of systems (e.g., long effusion cooling systems operating at low Mass flow rates), it is shown that the coolant exit temperature should be minimized. An approximation relating the wall/film properties, the Nondimensional Mass flow, and the overall cooling effectiveness is derived. It is used to estimate the effect of Biot number (TBC and metal), heat transfer coefficient (HTC) ratio, and film properties on the performance of a typical HPNGV and effusion cooling system. In Part II, we show that designs which achieve uniform wall temperature have a particular corresponding internal HTC distribution.

Mahmood Farzaneh-gord - One of the best experts on this subject based on the ideXlab platform.

  • Effects of Swirl and Flow Rate on the Flow and Heat Transfer in a Pre-swirl Rotating-disc System
    2015
    Co-Authors: Mahmood Farzaneh-gord, Michael Wilson, Michael J. Owen
    Abstract:

    Pre-swirl is often used in the internal cooling-air systems of gas turbines to reduce the temperature of the cooling air relative to the rotating turbine blades. In a "direct-transfer " system, the air passes axially across the wheel-space from stationary pre-swirl nozzles to receiver holes in the rotating turbine disc. This paper investigates the effects of inlet flow conditions and rotational speed on the flow and heat transfer in such a system, using a simplified computational model for 3D steady, incompressible turbulent flow. Computed results are compared with measurements of tangential velocity and Nusselt number made in a complementary experimental study. The results show that there are significant mixing losses near the inlet nozzles, resulting in a reduced “effective ” pre-swirl ratio at inlet. The computed tangential velocity distributions suggest free-vortex-type behaviour for the flow between the pre-swirl nozzle radius and that of the receiver holes. There is mainly good agreement between the computed and measured velocities, although the measured values are generally lower than the computed values and follow free-vortex behaviour less closely. There is less good agreement between computed and measured values of Nusselt number, although the correct trends are obtained for the effects of the main flow parameters. NOMENCLATURE a, b inner, outer radius of disc cp specific heat of air at constant pressure Cw Nondimensional Mass flow rate ( = m /µb) d pre-swirl nozzle diameter G gap ratio ( = s/b) k turbulence kinetic energy, thermal conductivity of air m Mass flow rate N number of pre-swirl nozzles Nu local Nusselt number (=qr/k(Taw – Tw)

  • Effects of Swirl and Flow Rate on the Flow and Heat Transfer in a Pre-swirl Rotating-disc System
    2003
    Co-Authors: Mahmood Farzaneh-gord, Michael T. Wilson, J. Michael Owen
    Abstract:

    Pre-swirl is often used in the internal cooling-air systems of gas turbines to reduce the temperature of the cooling air relative to the rotating turbine blades. In a "direct-transfer" system, the air passes axially across the wheel-space from stationary pre-swirl nozzles to receiver holes in the rotating turbine disc. This paper investigates the effects of inlet flow conditions and rotational speed on the flow and heat transfer in such a system, using a simplified computational model for 3D steady, incompressible turbulent flow. Computed results are compared with measurements of tangential velocity and Nusselt number made in a complementary experimental study. The results show that there are significant mixing losses near the inlet nozzles, resulting in a reduced “effective” pre-swirl ratio at inlet. The computed tangential velocity distributions suggest free-vortex-type behaviour for the flow between the pre-swirl nozzle radius and that of the receiver holes. There is mainly good agreement between the computed and measured velocities, although the measured values are generally lower than the computed values and follow free-vortex behaviour less closely. There is less good agreement between computed and measured values of Nusselt number, although the correct trends are obtained for the effects of the main flow parameters. NOMENCLATURE a , b inner, outer radius of disc cp specific heat of air at constant pressure Cw Nondimensional Mass flow rate (= m /µb) d pre-swirl nozzle diameter G gap ratio (= s/b) k turbulence kinetic energy, thermal conductivity of air m Mass flow rate N number of pre-swirl nozzles Nu local Nusselt number (=qr/k(Taw –T w )) Pr Prandtl number (=µcp /k) q heat flux (from air to disc) r, φ, z radial, tangential and axial coordinates

Povey T - One of the best experts on this subject based on the ideXlab platform.

  • Cooling optimization theory - Part I: Optimum wall temperature, coolant exit temperature, and the effect of wall/film properties on performance
    'ASME International', 2016
    Co-Authors: Kirollos B, Povey T
    Abstract:

    Gas turbine cooling system design is constrained by a maximum allowable wall temperature (dictated by the material and the life requirements of the component), minimum coolant Mass flow rate (the requirement to minimize cycle-efficiency cost), and uniform wall temperature (to reduce thermal stresses). These three design requirements form the basis of an iterative design process. The relationship between the requirements has received little discussion in the literature, despite being of interest from both a theoretical and a practical viewpoint. In this paper, we consider the optimum cooling system for parts with both internal and film cooling. We show analytically that the coolant Mass flow rate is minimized when the wall temperature is uniform and equal to the maximum allowable wall temperature. Thus, we show that achieving uniform wall temperature achieves minimum coolant flow rate, and vice versa. The purpose is to clarify the interplay between two design requirements that are often discussed separately in the literature. The penalty (in terms of coolant Mass flow) associated with cooling nonisothermal components is quantified. We show that a typical high pressure nozzle guide vane (HPNGV) operating isothermally at the maximum allowable wall temperature requires two-thirds the coolant of a typical nonisothermal vane. The optimum coolant exit temperature is also considered. It is shown analytically that the optimum coolant exit temperature depends on the balance between the mean adiabatic film cooling effectiveness, the Nondimensional Mass flow rate, and the Biot number of the thermal barrier coating (TBC). For the large majority of gas turbine cooling systems (e.g., a typical HPNGV) it is shown that the optimum coolant exit temperature is equal to the local wall temperature at the point of injection. For a small minority of systems (e.g., long effusion cooling systems operating at low Mass flow rates), it is shown that the coolant exit temperature should be minimized. An approximation relating the wall/film properties, the Nondimensional Mass flow, and the overall cooling effectiveness is derived. It is used to estimate the effect of Biot number (TBC and metal), heat transfer coefficient (HTC) ratio, and film properties on the performance of a typical HPNGV and effusion cooling system. In Part II, we show that designs which achieve uniform wall temperature have a particular corresponding internal HTC distribution