Cooling Method for a High-Temperature Radial Gas Turbine Engine

Abstract

A method for cooling a high-temperature radial gas turbine engine increases turbine thermal efficiency and/or extends turbine operational lifetime. A bleed flow path enables cooling air to flow from a compressor outlet and along surfaces of the gas turbine rotors. The amount of cooling increases in proportion to a bleed fraction, which is defined as the ratio of mass flow in the bleed flow path to total mass flow in the compressor outlet. The heated air in the bleed flow path is mixed with the main mass flow into the turbine engine, so as to restore mass flow into the turbine, while maintaining a high turbine operating temperature and thermal efficiency. The thermal efficiency of a recuperator also increases in proportion to the bleed fraction.

Claims

1. A method for cooling hot components of a high-temperature radial gas turbine engine comprising: providing a radial gas turbine engine comprising a compressor outlet and at least one rotor comprising a rotor disc and a rotor blade; and providing a bleed flow path in which a fluid passes from the compressor outlet and flows along surfaces of the rotor disc and rotor blade; wherein a ratio of a mass flow in the bleed flow path to a mass flow in the compressor outlet is equal to a bleed fraction which is greater than or equal to zero and less than or equal to one.

2. The method of claim 1 wherein a root of the rotor blade is attached to the rotor disc.

3. The method of claim 1 wherein the mass flow in the bleed flow path cools the rotor disc and the rotor blade, and is mixed with a main mass flow along a surface of the rotor blade.

4. The method of claim 1 wherein an operational lifetime of the radial gas turbine engine increases in proportion to the bleed fraction, for a fixed engine thermal efficiency.

5. The method of claim 1 wherein a thermal efficiency of the radial gas turbine engine increases in proportion to the bleed fraction, for a fixed engine operational lifetime.

6. The method of claim 1 further comprising providing a recuperator heat exchanger.

7. The method of claim 6 wherein a thermal effectiveness of the recuperator heat exchanger increases in proportion to the bleed fraction.

8. The method of claim 1 wherein the bleed fraction is greater than or equal to 3 per cent.

9. The method of claim 7 wherein an increase in the bleed fraction causes an increase in a recuperator outlet temperature and substantially no change in a turbine inlet temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention is described herein, by way of example only, with reference to the accompanying drawings, wherein:

[0018] FIG. 1: An exemplary cross-sectional drawing of a bleed flow path for cooling a high temperature radial turbine, according to the principles of the invention.

[0019] FIGS. 2(a) and 2(b): Exemplary perspective drawings showing critical points on a rotor disc and a rotor blade, respectively.

[0020] FIG. 3: An exemplary graph of von Mises stress versus the Larson-Miller parameter (L) for creep rupture of an exemplary metal alloy.

[0021] FIG. 4: An exemplary graph showing the thermal effectiveness of a recuperator cross-flow heat exchanger.

DETAILED DESCRIPTION

[0022] FIG. 1 is an exemplary cross-sectional drawing of a bleed flow path for cooling a high temperature radial turbine according to the principles of invention. The bleed flow path 110 is indicated by a solid line with arrows. The bleed fluid (e.g. air) enters at a bleed flow inlet 110A from a compressor outlet (which is not shown), flows along exterior surfaces of rotor blade 150 and rotor disc 160, and then exits at bleed flow exit 110B. Region 130 is a main air flow exit region from the turbine rotors, and region 140 is a trapped air flow region, inside a static cone. FIGS. 2(a) and 2(b) are exemplary perspective drawings showing the locations of points P1 through P4, at which temperature and/or equivalent (von Mises) stress may reach critical values. Points P1 and P2 in FIG. 2(a) are on the rotor disc 160, and points P3 and P4 in FIG. 2(b) are on the root of rotor blade 150.

[0023] A bleed fraction is defined as the mass flow rate of the bleed flow path 110 divided by the total mass flow rate at the exit of the compressor outlet. Temperatures and equivalent stresses have been determined at points P1 through P4 for various simulation scenarios corresponding to different values of the bleed fraction.

[0024] The bleed flow temperature and velocity fields are calculated using computational fluid dynamics (CFD) software (e.g. Fluent version 6.3), which is available, for example, from ANSYS Corporation. See hyperlink https://www.ansys.com/products/fluids/ansys-fluent, the contents of which are attached hereto as Appendix A. The temperature and stress fields in the turbine metal parts are calculated using finite element analysis (FEA) software (e.g. ANSYS Mechanical Enterprise) which is available, for example, from ANSYS Corporation. See hyperlink https://www.ansys.com/products/structures/ansys-mechanical-enterprise, the contents of which are attached hereto as Appendix B.

[0025] The calculated results for a bleed fraction of 3% are summarized in the table below.

TABLE-US-00001 TABLE 1 Calculated Temperature and Stress Results (Bleed Fraction = 3%) Temperature Stress Point Location (° K.) (MPa) P1 on rotor disc 735 612 P2 on rotor disc 660 746 P3 on rotor blade 1000 574 P4 on rotor blade 815 670

[0026] In Table 1, point P3 on the root of the rotor blade is seen to have the highest temperature. When the bleed fraction is reduced from 3% to zero (i.e. no bleed flow at all), the temperature calculated at point P3 is found to increase from 1000° K to 1100° K.

[0027] Rotor lifetime depends strongly on material composition and temperature. FIG. 3 shows an exemplary graph of von Mises stress versus the Larson-Miller parameter (L) for creep rupture of a metal alloy, known as Mar-M-247, which consists primarily of Nickel, Cobalt, and Tungsten. The parameter on the horizontal axis is the Larson-Miller parameter, L=0.001×T×[20+log.sub.10(t)], where T denotes temperature in degrees Rankine (° R), and (t) denotes the operational lifetime in hours. The value on the vertical axis is the equivalent (von Mises) stress, in units of kilo-pound force per square inch (ksi).

[0028] The operational lifetime (t) of the rotor is calculated by the equation:

log.sub.10(t)=1000L/T(° R)−20  (eqn. 1)

where the Larson-Miller parameter, L, is determined from the calculated stress level at point P3. According to Table 1, the stress at P3 is equal to 574 MPa, which is approximately equal to 83 ksi (using the conversion 1 ksi=6.895 MPa). From FIG. 3, the corresponding value of L is approximately equal to 43. The following table shows the operational lifetime (t) in hours for bleed fractions of 0% and 3%.

TABLE-US-00002 TABLE 2 Operational Lifetime (L = 43) Operational Bleed Temperature Lifetime, t Fraction (° K.) (° R) log.sub.10(t) (hrs) 0% 1100 1980 1.717 52 3% 1000 1800 3.889 7743
According to Table 2, the cooling provided by a bleed fraction of just 3% increases the rotor lifetime from 52 hours to 7743 hours. Note also that the energy of the bleed flow is not lost, insofar as the bleed air is heated by the hot rotors and then mixed with the main air flow into the turbine, so that it too contributes to the total turbine energy efficiency.

[0029] A side benefit of the bleed flow is an increase in recuperation thermal effectivity, in the case of a recuperated gas turbine. FIG. 4 shows the thermal effectivity (Eff) of an exemplary cross-flow recuperator heat exchanger in terms of the number of transfer units (NTU) and the hot-to-cold flow ratio (Rhc) in the heat exchanger. For a fixed value of NTU, Eff increases with decreasing Rhc. Since the bleed flow reduces the mass flow rate of fluid flowing into the heat exchanger by an amount equal to the mass flow rate of the bleed flow path 110, the value of Rhc is reduced. As seen in FIG. 4, reducing Rhc has the effect of increasing Eff and, with it, the overall energy efficiency of the turbine engine. For example, for a typical recuperator and bleed flow design, it has been found that Eff increases from 83% to 88% when the bleed fraction increases from 0% to 10%.

[0030] Although the invention has been described in conjunction with specific embodiments, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of this disclosure.