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An oil purge system for a mid turbine frame (MTF) of a gas turbine engine has an oil transfer tube surrounded by a heat shield tube. The oil transfer and heat shield tubes extend at their respective inner ends downwardly from an oil port of a bearing housing and terminate at their respective outer e

An oil purge system for a mid turbine frame (MTF) of a gas turbine engine has an oil transfer tube surrounded by a heat shield tube. The oil transfer and heat shield tubes extend at their respective inner ends downwardly from an oil port of a bearing housing and terminate at their respective outer ends projecting outwardly from an annular wall of an outer case of the MTF. Oil leaked from the oil port is purged by pressurized air through an annular cavity formed between the oil transfer and heat shield tubes, and is discharged out of the MTF.

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1. A gas turbine engine having a mid turbine frame, the mid turbine frame comprising: a bearing housing and an annular outer case connected together by a plurality of radial struts;an annular inter-turbine duct disposed radially between the annular bearing housing and the annular outer case for cond

1. A gas turbine engine having a mid turbine frame, the mid turbine frame comprising: a bearing housing and an annular outer case connected together by a plurality of radial struts;an annular inter-turbine duct disposed radially between the annular bearing housing and the annular outer case for conducting hot gases axially through the mid turbine frame, the annular inter-turbine duct defining a plurality of radial hollow strut fairings extending therethrough; andan oil transfer tube and a heat shield tube around the oil transfer tube defining an annular cavity therebetween, the heat shield tube being substantially surrounded by pressurized air, the oil transfer and heat shield tubes being received at a respective inner end thereof in an oil port of the bearing housing and extending radially downwardly through a radial passage of the annular inter-turbine duct and through an opening in the outer case, the oil transfer and heat shield tubes terminating at a respective outer end thereof projecting outwardly from an annular wall of the outer case, the outer end of the oil transfer tube being configured for fluid connection with an oil system, the annular cavity being in fluid communication with the pressurized air through a first passage located in a position adjacent the inner end of the heat shield tube, the annular cavity being in fluid communication with a second passage located outside the annular wall of the outer case, the second passage being configured for fluid connection with a leaked-oil dumping area located outside the outer case. 2. The gas turbine engine as defined in claim 1 wherein the oil port of the bearing housing comprises a first annular inner surface adjacent an entry end of the oil port for sealingly receiving the inner end of the heat shield tube, and a second annular inner surface adjacent the first annular inner surface for sealingly receiving the inner end of the oil transfer tube. 3. The gas turbine engine as defined in claim 1 wherein the oil port of the bearing housing comprises an annular inner surface adjacent an entry end of the oil port for sealingly receiving the inner end of the oil transfer tube and a telescoping nut threadingly engaging an annular outer surface adjacent the entry end of the oil port, the telescoping nut extending downwardly from the entry end of the oil port for sealingly receiving the outer end of the heat shield tube. 4. The gas turbine engine as defined in claim 1 wherein the mid turbine frame comprises a first annular seal positioned between the heat shield tube and the oil port of the bearing housing to provide a clearance between the first annular seal and the heat shield tube to form said first passage. 5. The gas turbine engine as defined in claim 1 wherein the oil port of the bearing housing comprises a cylindrical wall extending radially downwardly from the bearing housing, the cylindrical wall defining at least one hole extending therethrough and the at least one hole forming said first passage to restrictively allow the pressurized air surrounding the heat shield tube into the annular cavity. 6. The gas turbine engine as defined in claim 1 wherein the second passage is formed by an open end of the annular cavity at the outer end of the heat shield tube, the open end being in fluid communication with a location in an engine core cowl below the outer end of the heat shield tube. 7. The gas turbine engine as defined in claim 1 wherein the opening in the outer case extends through a boss radially outwardly projecting from the annular wall of the outer case, the opening of the boss sealingly receiving the outer end of the heat shield tube. 8. The gas turbine engine as defined in claim 7 wherein a second annular seal is positioned within the opening of the boss, between the oil transfer tube and the heat shield tube for sealing a low end of the annular cavity. 9. The gas turbine engine as defined in claim 8 wherein the second passage comprises at least one hole extending through the heat shield tube above the second annular seal, the at least one hole being in fluid communication with a hole extending through an annular wall of the boss. 10. The gas turbine engine as defined in claim 9 wherein the heat shield tube defines an annular groove on an outer surface adjacent the outer end, to form an annular space between the annular wall of the boss and the heat shield tube, the annular space being in fluid communication with the at least one hole extending through the heat shield tube and the hole extending through the annular wall of the boss. 11. The gas turbine engine as defined in claim 1 wherein the radial struts extend through respective radial hollow strut fairings of the inter-turbine duct and at least one of the struts is hollow, the oil transfer and heat shield tubes extending through the at least one of the hollow struts. 12. The gas turbine engine as defined in claim 1 comprising a leaked-oil line in fluid communication with the second passage and a bypass air duct of the engine. 13. The gas turbine engine as defined in claim 1 comprising a leaked-oil line in fluid communication with a turbine exhaust case. 14. A method for purging oil leakage from a mid turbine frame of a gas turbine engine, the mid turbine frame including at least a bearing housing and an annular outer case connected together by a plurality radial struts, and an annular inter-turbine duct disposed between the bearing housing and the outer case for conducting hot gases, the method comprising: a) providing a heat shield tube around an oil transfer tube to create an annular cavity therebetween, the cavity extending radially downwardly from an oil port of the bearing housing and terminating outside the outer case to allow oil leaked from between the bearing housing oil port and the oil transfer tube to drain into the cavity;b) introducing pressurized air into the cavity adjacent the oil port of the bearing housing for purging the leaked oil down along the cavity and out of the outer case of the mid turbine frame; andc) directing the leaked oil purged from the cavity into a leaked-oil dumping area of the engine. 15. The method as defined in claim 14 wherein step (b) is performed by providing a first passage to restrictively allow pressurized air surrounding the heat shield tube to enter the cavity at a predetermined velocity. 16. The method as defined in claim 14 wherein step (b) is performed by providing a second passage defined in a structure located outside an annular wall of the outer case, the second passage being in fluid communication with the cavity to allow the purged leaked oil from the cavity out of the mid turbine frame. 17. The method as defined in claim 14 wherein in step (c) the purged leaked oil is directed into a location of an engine core cowl below the heat shield tube. 18. The method as defined in claim 14 wherein in step (c) the purged leaked oil is directed into an engine bypass air duct. 19. The method as defined in claim 14 wherein in step (c) the purged leaked oil is directed into a turbine exhaust case.