Patent Application
20160161189
The present invention relates to apparatus and methods for exchanging heat between various fluids and, more particularly, apparatus and methods for exchanging heat between fuel, fan-air, and bleed-air in an integrated heat exchanger.
With development of de-oxygenated jet fuels, which is burned in engines, the fuel can be used for additional heat sink capacity by taking the fuel above normal coking limits of about 350° F. Using additional heat sink capacity of the fuel in lieu of fan-air can reduce required cooling air flow and reduce aircraft drag. Additionally, delivering higher temperature fuel to the combustors can improve engine specific fuel consumption (SFC).
One system within aircraft, which needs cooling and operates at higher temperatures, is an engine bleed air system. Typically ram air or engine fan air is used to cool the bleed air from around 1000° F. to under 400° F. By using engine fuel flow to cool the bleed air, the amount of ram or fan air used in cruise can be reduced with engine and aircraft performance benefits. A bleed air pre-cooler is normally sized for a ground or hold case where fuel flow is low such that there is still a need for air cooling of the bleed air.
Stainless steel or Inconel plate fin heat exchangers are typically used for bleed air pre-coolers because of the need for very compact designs. To reduce the risk of fuel leakage into the hot bleed air, double header bars plus a buffer passage have been used in other liquid to air plate fin designs. This imposes a large weight and/or size penalty due to the poor thermal conductance of the buffer passage.
In one aspect of the present invention, a heat exchanger includes a first bleed air passage; a second bleed air passage; a fan-air passage between the first and second bleed air passages; and a fuel tube in the fan-air passage.
In another aspect of the present invention, a heat exchanger includes a first bleed air passage; a second bleed air passage; a fan-air passage between the first and second bleed air passages; a serpentine-shaped fuel tube in the fan-air passage; and a tube sheet between each of the first and second bleed air passages and the fan-air passage.
In a further aspect of the present invention, a method of cooling bleed-air in a heat exchanger includes flowing bleed air through a first bleed air passage and a second bleed air passage; flowing fan air through a fan-air passage to enable heat exchange between the fan air and the bleed air; and flowing fuel through tubes within the fan-air passage to enable heat exchange between the fuel and the bleed air.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.
The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.
Various inventive features are described below that can each be used independently of one another or in combination with other features. However, any single inventive feature may not address any of the problems discussed above or may only address one of the problems discussed above. Further, one or more of the problems discussed above may not be fully addressed by any of the features described below.
Broadly, embodiments of the present invention provide integrating air and fuel cooling functions into one heat exchanging unit for cooling bleed air. Thereby, an associated bleed air system can be simplified with weight and/or size savings compared to using separate air cooled and/or fuel cooled pre-coolers.
The apparatus and methods of the present disclosure integrate air cooling passages and fuel cooling passages with bleed air passages into one heat exchanging unit. The fan air cooling circuit may be used when, for example, fuel flow is low such as when aircraft engines are at idle on the ground or when an aircraft is in a hold or descent condition. The fuel cooling circuit can be used, for example, when there is sufficient fuel flow and when the parasitic loss of fan air extraction or ram air usage should be avoided (e.g., as in long range aircraft cruise).
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If a serpentine configuration is used, a fuel tube 106c may include turning elbows 107c proximate a first end 101c and/or turning elbows (not shown in
Additionally, turning elbows (not shown in
A fuel tube 106c may be welded to a fuel inlet manifold 105c and/or a fuel outlet manifold 110c (not shown in
A fuel tube 106c may be slightly flattened, for example, in a press to provide sufficient land for brazing the fuel tube 106c to corresponding tube sheets 135c. This increased tube land, in comparison to fuel tubes 106c without a tube land, may provide additional thermal contact area to reduce conduction resistance from a hot bleed-air passage 136c to a fuel tube 106c.
Any fan-air passage 116c may be filled with fin material 126c to improve heat transfer conductance between the fan-air passages 116c and the bleed-air passages 136c.
While the fuel tubes 106c and the fin material 126c are shown in
A heat exchanger of the present disclosure may use multiplassed fuel tubes 106c to contain fuel, and the fuel tubes 106c may be directly connected thermally to hot bleed-air passages 136c by, for example, welding or brazing the fuel tube 106c to tube sheets 135c. The fuel tubes 106c may be directly integrated into the fan-air passages 116c by, for example, co-molding or extruding the fuel tubes 106c along with associated tube sheets 135c, such that two cold fluids (e.g., fuel and fan-air) transfer heat from the hot bleed-air passages 136c.
A fuel tube 106c within a fan-air passage 116c may be formed, from one continuous tube, into a serpentine arrangement such that the only joints in the fuel tubes 106c are the connections to the inlet and outlet fuel manifolds 105b, 110b. Thereby, potential fuel leakage is minimized. Connections between the fuel tubes 106c and the inlet and outlet fuel manifolds 105b, 110b may be defined by welding the fuel tubes 106c to the inlet and outlet fuel manifolds 105b, 110b.
Dimples 111c may be formed on a fuel tube 106c, either ring dimples or side dimples, to disturb the fuel boundary layer and promote wall to bulk mixing of the fluid in order to enhance the fuel side heat transfer.
In a non-limiting application, an integrated fuel-fan air-bleed air heat exchanger 100a may, for example, be incorporated into a business jet bleed-air pre-cooler. In such a non-limiting specific example, a pre-cooler core fan-air fin height may be increased from 0.178 inches, for a heat exchanger that does not have fuel tubes incorporated with fan-air passages, to 0.25 inches, for a heat exchanger that has fuel tubes incorporated with fan-air passages. Fin count may be increased from twelve, for a heat exchanger that does not have fuel tubes incorporated with fan-air passages, to thirteen or more for a heat exchanger that has fuel tubes incorporated with fan-air passages. Fin thickness may be increased from 0.003 inches, for a heat exchanger that does not have fuel tubes incorporated with fan-air passages, to 0.004 inches for a heat exchanger that has fuel tubes incorporated with fan-air passages. Fin height, fin count and/or fin thickness may be varied to meet performance requirements as hot bleed-air flow length and/or fuel tubes 106a are varied.
In a specific non-limiting example, by integrating fuel tubes 106a within fan-air passages 117a, an integrated heat exchanger 100a may include two hundred and twenty (end-to-end fuel tube passes), with 0.25 inch diameter fuel tubes 106a, within a space similar to a space associated with a non-integrated heat exchanger. In another specific non-limiting example, an integrated heat exchanger 100a may deliver bleed-air temperature output, Thot_out=250 F, in a fuel cooled case with fuel temperature output, Tfuel out=311 F. In a further non-limiting example, an integrated heat exchanger may include a length 145a, Lhot=12.9 inches, a width 150a, Lcold=9.0 inches, and a height 140a, Lno=8.12 inches. In yet another specific non-limiting example, a core weight of an integrated heat exchanger 100a-c may be, wt=52 lb., and a unit weight may be, Unit wt=74 lb.
This disclosure is generally intended for describing a three-fluid heat exchanger such as fuel/bleed air/fan air. The design may overcome the usual problem with three-fluid heat exchangers by integrating tubes for high-temperature/high-pressure fuel with fan-air passages thus eliminating a need for buffer passages that result in significant size and/or weight penalties. The intended application enables bleed-air cooling by fan-air and/or fuel as appropriate. Fuel cooling harvests the heat instead of wasting it. The design may also provide additional separation between the fuel and hot bleed air circuits.
It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.
