Patent Application
20150059349
The application relates generally to gas turbine engines and, more particularly to cooling a combustor for such engines.
In aviation gas turbine engines, the amount of air supplied for combustion and dilution may be optimized for operability and to minimize emissions such as nitrous oxide (NOx), carbon monoxide (CO), hydrocarbons (HC), etc. Therefore, it is often desirable that the amount of air supplied for cooling combustor walls be minimized, which poses challenges to meeting the durability requirements of the combustor walls, because the reduction in combustion wall cooling air may lead to unwanted material oxidation, thermal mechanical fatigue and/or thermal wall buckling due to thermal gradients. Particularly in small aero gas turbine engines, the total amount of air available for combustor wall cooling within the gas turbine thermodynamic cycle can be limited, especially where rich-burn combustion is sought. Therefore it is a challenge to optimize the combustor wall cooling while still meeting the durability requirements of the combustor.
There is provided a combustor for a gas turbine engine comprising: inner and outer liners spaced apart from each other to define a chamber therebetween including a primary zone adjacent an upstream end of the chamber and a secondary zone downstream of the primary zone, the upstream end being closed by a dome, a dome heat shield disposed within the chamber, adjacent and spaced apart from the dome, an inner liner heat shield disposed within the chamber, adjacent and spaced apart from the inner liner, an outer liner heat shield disposed within the chamber, adjacent and spaced apart from the outer liner, the inner and outer liner heat shields extending from the upstream end of the chamber over the primary zone and terminating at the secondary zone, the inner and outer liners and the inner and outer liner heat shields defining a plurality of apertures therein for introducing dilution air jets into the chamber, the combustor including a plurality of impingement holes defined in the dome and inner and outer liners for directing impingement cooling air therethrough and into the combustor to impinge on a cold side of the respective heat shields, and a discharging apparatus to direct exhausted impingement cooling air discharged from the cold side of the respective heat shields to flow along and substantially parallel to a hot side of the respective inner and outer liner heat shields, the combustor further including a plurality of effusion holes defined in a section of the inner and outer liners free of coverage by the respective inner and outer liner heat shields, the effusion holes directing effusion cooling air therethrough and into the combustor to effusion cool said section of the inner and outer liners.
There is also provided a method for hybrid cooling a combustor of a gas turbine engine comprising steps of: a) providing heat shield panels in a chamber of the combustor to protect respective upstream sections of inner and outer liners of the combustor, the upstream sections of the inner and outer liners substantially covering a primary zone of the chamber; b) directing impingement cooling air for impingement cooling a cold side of the heat shield panels and then discharging a first portion of the impingement cooling air along and substantially parallel to a hot side of the heat shield panels and a second portion of the impingement cooling air into areas downstream of the heat shield panels, to thereby have the discharged cooling air substantially outside of the primary zone in order to reduce emission formation; and c) directing cooling air into a downstream area of the chamber for effusion cooling of respective downstream sections of the inner and outer liners free of coverage by the heat shield panels.
Reference is now made to the accompanying figures in which:
It will be noted that throughout the appended drawings, like features are identified with like reference numerals.
In small aero gas turbine engine combustors, the total amount of air available for combustor wall cooling is very limited especially with rich burn technologies. To improve small aero engine Specific Fuel Consumption (SFC), the pressure drop across the combustor wall has to be reduced below 3.0%, whereas in larger engines a pressure drop >3.0% across respective inner and outer liners and >4.5% pressure drop in a dome panel bulkhead due to optimal wall cooling may be possible. The SFC of larger engines can be improved by other means, such as ultra high pressure ratio and active turbine tip clearances etc., which are more affordable due to engine size.
In smaller engines, however, to improve emissions more mass flow is often desirable for combustion. Due to various constraints such as compressor mass flow, pressure ratios, combustor delta pressure, and a relatively larger combustor surface area for cooling, the wall cooling can be optimized in order to increase mass air flow for the combustion necessary to improve emissions.
To minimize NOx formation and to reduce the flame temperature in a primary zone of the combustor for high or medium pressure ratio engines, a high Fuel Air Ratio (FAR>0.1, Stoichiometric FAR=0.068) is desirable. As illustrated in
Therefore, to optimize the combustor cooling to improve emission reduction and durability of the combustor, the combustor may be provided with hybrid cooling techniques using heat shields in an upstream end of the combustor chamber with impingement cooling and having exhausted impingement cooling air substantially outside the primary zone in the chamber, meanwhile using effusion cooling for the single-skin wall of a downstream section of the combustor chamber which includes therein the secondary zone.
According to one embodiment of the present disclosure as shown in
A dome heat shield 44, which may be formed with a plurality of dome heat shield panels, is disposed within the chamber 28, adjacent and spaced apart from the dome 34, attached to the dome 34 to protect the dome from exposure to the hot combustion gases in the chamber 28. An inner liner heat shield 46 which may be formed with a plurality of inner liner heat shield panels, is disposed within the chamber 28, adjacent and spaced apart from the inner liner 30, attached to the inner liner 30 to protect the inner liner 30 from being exposed to the hot combustion gases within the chamber 28. An outer liner heat shield 48 which may be formed with a plurality of outer liner heat shield panels, is disposed within the chamber 28, adjacent and spaced apart from the outer liner 32, and is attached to the outer liner 32 to protect the outer liner from being exposed to the hot combustion gases within the chamber 28. The inner and outer liner heat shields 46 and 48 extend from the upstream end of the chamber 28 over the primary zone 40 and terminate at the secondary zone 42 and therefore are used only to protect the upstream sections of the inner and outer liners 30 and 32. Each of the dome heat shield 44 and inner and outer liner heat shields 46, 48 may include a plurality of pin fins 49 projecting from a cold side thereof (facing the respective dome 34, inner and outer liners 30, 32) to increase contact areas with cooling air to increase heat transfer in convection cooling.
The inner and outer liners 30, 32 and the inner and outer liner heat shields 46, 48 define a plurality of apertures therein which are in communication with the chamber 28, for introducing dilution air jets 52 into the chamber 28 between the primary zone 40 and the secondary zone 42. The dilution air helps to reduce flame temperature by quenching in the secondary zone 42 and provides for combustor exit temperature distribution acceptable for turbines 24, 18.
A plurality of impingement cooling holes 54 (see
A discharging apparatus may be provided for directing exhausted impingement cooling air discharged from the cold side of the respective heat shields to flow substantially along and parallel to a hot side (facing the hot combustion gases in the chamber) of the respective heat shields. For example, the dome shield 44 is positioned with respect to the inner and outer liner heat shields 46, 48 to provide respective gaps 56, 58 between the dome heat shield 44 and the inner liner heat shield 46, and between the dome heat shield 44 and the outer liner heat shield 48. The dome heat shield 44 is configured such that the impingement cooling air introduced to the space between the dome heat shield 44 and the dome 34 is forced to flow towards the respective gaps 56, 58 and is discharged therefrom to form a cooling film along and substantially parallel to the hot side to cool the hot side of the respective inner and outer liner heat shields 46, 48. The exhausted impingement cooling air discharged from the respective gaps 56, 58 in the form of a cooling air film along the hot side of the respective inner and outer liner heat shields 46, 48 is substantially outside the primary zone 40 to reduce the NOx, CO and HC emission formation in the combustion gases. Thus, there is no low momentum effusion to be discharged from the exhausted impingement cooling air into the pocket of high fuel air ratio as indicated in circle A of
A portion of exhausted impingement cooling air in the respective spaces between the inner liner 30 and the inner liner heat shield 46, and between the outer liner 32 and the outer liner heat shield 48 flows towards the upstream end of the chamber 28 (as indicated by the arrows shown in
Optionally, the discharging apparatus may include a plurality of splash louvers on the cold side at the downstream end of the respective inner and outer liner heat shields 46, 48, acting as a film starter for the portion of the exhausted impingement cooling air discharged from the downstream end of the inner and outer liner heat shields 46, 48 to flow along a downstream section of the respective inner and outer liners 30, 32.
Optionally, the inner and outer liner heat shields 46, 48 may be provided with two rows of small effusion hose 60, located downstream of the apertures 50 for introduction of dilution jet air, near the downstream end of the inner and outer liner heat shields 46, 48. Therefore, a further portion of the exhausted impingement cooling air is discharged from the effusion hose 60 and may enter the secondary zone 42 (see
The swirlers 38 are provided with swirl air passages 62 for introduction of air flows in a swirling flow into the chamber 28 to mix with the fuel ejected by the fuel nozzles 36 for combustion. In this embodiment, the swirlers 38 each may include cooling air passages 64 for introducing cooling air into the combustor independent from the air flow introduced by the swirl air passages 62 of the swirlers 38. Each swirler 38 is configured as to provide a cooling air director 66 for directing the cooling air introduced from the cooling air passages 64 to generate a cooling air film along and substantially parallel to a hot side of the dome heat shield 44 to cool the same.
In
It is noted that in the combustor 26 of the reverse flow type, the downstream section of the inner and outer liners 30, 32 are made of curved wall of large exit duct (LED) and small exit duct (SED) to turn the flow. It is very cumbersome to manufacture curved heat shields and difficult to minimize leakage between heat shields and the outer impingement skin of the combustor inner and outer liners. The downstream section of the inner and outer liners substantially define the secondary zone therebetween in which most of the fuel is already oxidized and there is very low fuel air ratio, and therefore the effusion flow entering the effusion holes 68 and into the secondary zone will not help to form NOx, CO and HC emission.
The hybrid cooling techniques for a gas turbine engine combustor, such as using rich burn techniques, minimize cooling air entering in the primary zone and thus high FAR can be maintained therein. Lack of effusion air in the primary zone with high FAR flame will not be stabilized locally near the combustor liners. This also helps to minimize thermal gradients in the structure members of the combustor liners by eliminating direct exposure to the flame in the primary zone, thus minimizing combustor liners buckling. The hybrid cooling techniques improve the reduction of NOx at high engine power conditions and CO/HC emission at low engine power conditions.
The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the described subject matter. Modifications which fall within the scope of the described subject matter will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.
