GB2241536A

GB2241536A - Intakes of aerospace propulsors

Info

Publication number: GB2241536A
Application number: GB8806883A
Authority: GB
Inventors: John Lanfear Scott-Scott
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1988-03-23
Filing date: 1988-03-23
Publication date: 1991-09-04
Also published as: FR2664649A1; DE3904851A1

Abstract

An air intake (10) suitable for an aerospace propulsor comprises a duct (11) which contains, in flow series, a liquid oxygen injector (16) and two heat exchangers (17, 18). At low altitude, liquid oxygen directed into the air flow through the duct (11) via the injector (16), by reducing the air temperature to -50 DEG C or lower, causes water therein to be converted into small dry ice crystals which pass through the heat exchangers (17, 18) without any tendency to melt. Ice accretion on the heat exchangers (17, 18) is thereby avoided. At high altitude and high speed, the liquid oxygen injection is adjusted to reduce the air temperature to a level at which heat damage to the heat exchangers (17, 18) is substantially avoided. <IMAGE>

Description

1 IMPROVEMENTS IN OR RELATING TO THE INTAXES OF AEROSPACE PROPULSORS 1

This invention relates to the intakes of aerospace propulsors and has particular reference to intakes suitable for use with the type of aerospace propulsor disclosed in UK patent application number 8430157.

A common method of preventing icing in the region of the intake of a gas turbine engine is to take hot air f rom a suitable stage of the engine compressor, and to use that air to preheat the appropriate components in the intake region eg struts and inlet guide vanes. This technique in used for engine operation at low altitudes where the ice-forming water is in the form of supercooled droplets or very cold vapour which cause coalescence.

At high altitudes, ice can still be encountered but in a hazardous form. Thus in such situations, crystallisation has already occurred and the ice is in the form of a "dusC of very dry, very cold crystals. The crystals can collide with each other and the metal surfaces within the intake without coalescence and hence pass through the intake region of the engine.

in the case of a conventional gas turbine engine compressor, there is a point in the compressor process where the crystals evaporate and become water vapour. As far as is known, this process is not harmful to compressor components.

In thermodynamic terms, the process is beneficial since its effects are similar to those of a.compressor intercooler by virtue of the associated latent heat transfer process. Thus if the water vapour can be turned into fine, dry ice crystals, then the passage of these crystals through an engine is possible with accretion or coalescence.

The propulsor described and claimed in UK patent application number 8430157 includes a low pressure compressor which receives air via two heat exchangers arranged in series. Both heat exchangers receive coolant in he form of high pressure liquid hydrogen. The coolant flows are used to cool the low pressure compressor inlet air to a temperature 2 which is appropriate for the efficient operation of the engine.

In certain low altitude regions of the flight envelope of the propulsor outlined above, warm, moisture laden air can be cooled in the heat exchangers to a level at which there is a considerable accretion of ice on the heat exchangers, particularly the upstream heat exchanger. indeed in a very short time interval, the ice accretion can be so severe that the effective operation of the propulsor is compromised.

The air compression system of the propulsor cannot be used to provide any means of heating since, even though the compression process increases the air temperature, that temperature is still likely to be below 0 0 C, and at the outlet from the low pressure compressor, the air temperature will be considerably below Oc C.

It is an object of the present invention to provide an air intake suitable for an aerospace propulsor which air. intakes has at least one heat exchanger in which means are provided to inhibit the accretion ice in the heat exchanger.

According to the present invention, an air intake suitable for an aerospace propulsor comprises a duct having an upstream end for receiving an air flow and a downstream end for delivering said air flow to said propulsor, said duct containing at least one heat e...Ichanger adapted to place said air flow through said duct in heat exchange relationship with a fluid, the temperature of which is arranged to be lower than that of said air flow entering said heat exchanger, and a cryogen injector situated in said duct upstream of said heat exchanger and downstream of said upstream duct end, said cryogen injector operationally injecting a cryogen into said air flow which cryogen in a first low altitude mode of operation is arranged to reduce the temperature of said air flow to a level at which any water therein is converted into ice crystals which are sufficiently small and dry to pass through said heat ex.changer without any tendency to melt therein and in a second, high altitude mode of operation is arranged to reduce the temperature of said air flow to a t 1 i 1 1 1 3 1 level at which any heat damage to said at least one heat exchanger by said air flow is substantially avoided.

The invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic sectional side View of an aerospace propulsor air intake in accordance with the present invention.

With reference to the drawing, an air intake generally indicated at 10 is suitable for use with an aerospace propulsor of the type described in UK patent application number 8430157. The intake 10 comprises a duct 11 having an upstream end 12 at which is located in air inlet 13 and a downstream end 14 which delivers the air flow through the intake lO to an aerospace propulsor (not shown).

In operation, at low altitude moisture containing ambient air enters the duct 11 through the air inlet 13 as indicated by the arrows 15 and flows over a cryogen injector 16. The cryogen injector 16 is adapted to spray liquid oxygen into the air flow through the duct 11 so as to reduce the temperature of the air to a temperature of -50 0 C or lower. This rapid reduction in air temperature causes water in the air flow to rapidly crystallise into small dry ice crystals.

The air flow carrying the the resultant small dry ice crystals then passes in series through two heat exchangers 17 and 18 which are located within the duct 11. The heat exchangers 17 and 18 correspond with the two heat exchangers situated in the air inlet of' the propulsor described in UK patent application number 8430157. Each heat exchanger 17 and 18 is provided with a flow of liquid hydrogen to provide further reductions in the temperature of the air flow through the duct 11.

Since the heat exchangers 17 and 18 are colder than the air flow cooled by the cryogen injector 16, the small, dry ice crystals carried by the air flow pass straight through the heat exchangers 17 and 18 to enter the air i'nlet of the aerospace propulsor (not shown) with, as described earlier, no detrimental effect upon the operation of the engine.

4 There is, therefore, no undesirable build-up of ice upon the heat exchangers 17 and 18.

A valve 19 regulates the f low of liquid oxygen to the cryogen injector 16 so as to ensure that the air flow temperature is always maintained at a value of -50 OC or lower. This ensures in turn that the ice crystals formed by the liquid oxygen injection are sufficiently small and dry to pass through the heat exchangers 17 and 18 without melting upon impact within the heat exchangers 17 and 18. Melting on impact is termed "regelation" and occurs if the kinetic energy of the ice crystals is greater than the heat required to raise the temperature of the ice to 0 0 C and also cause some melting at the points of impact of the ice crystals. We have found that if the air temperature resulting from the liquid oxygen injection is above -50 0 C, some regelation is still possible and therefore there remains the danger that particularly the upstream heat exchanger 17 could become blocked with ice. At temperatures below -50 0 C, there is a satisfactory degree of formation of the desirable small dry ice crystals.

Liquid oxygen is the preferred cryogen for injection into the air flow through the duct 11 in view of the enrichment which it provides to the air supply to the aerospace propulsor. It will be appreciated however that other suitable cryogens could be utilised if so desired.

It is inevitable that some of the small dry ice crystals passing through the heat ex6hangers 17 and 18 will enter stagnation zones with the heat exchangers. However as long as the heat exchangers 17 and 18 are functioning normally, that is with liquid hydrogen passing through them, then the ice crystals which do settle within the heat exchangers 17 and 18 suffer from a form of thermal stress and aerodynamic drag and are eventually shed in a periodic manner.

As stated previously, both of the heat exchangers 17 and 18 are fed with liquid hydrogen. it will be appreciated however that other suitable coolants such as liquid methane could be used of so desired. Moveover although the heat i exchangers are described as being provided with separate supplies of liquid hydrogen, it may be convenient in certain circumstances to direct the hydrogen exhausted from the downstream heat exchanger 18 in to the inlet of the upstream heat exchanger 18, the downstream. heat exchanger 18 normally operating at a lower temperature than the upstream heat exchanger 17.

When the aerospace propulsor reaches high altitude where the atmospher is very dry, no liquid oxygen injection is necessary to provide further drying of the air. However at high altitude, when the propulsor is flying at very high speed, the temperature entering the propulsor intake will be high. In fact the air flow temperature can be so high as to cause heat damage to at least the upstream heat exchanger 17. In order to avoid such damage under these conditions, the injection of liquid oxygen is continued at a rate which is sufficient to reduce the temperature of the air flow to a level at which no heat damage occurs.

There is an added advantage in providing such liquid oxygen injection at high altitude by way of the enrichment it provides to the engine air supply. Such enrichment is particularly beneficial at high altitude where additional speed may be required to achieve exit from the atmosphere.

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Claims

Claims:-

1. An air intake suitable for an aerospace propulsor comprising a duct having an upstream end for receiving an air f low and a downstream end for delivering said air f low to said propulsor, said duct containing at least are heat exchanger adapted to place said air flow through said duct in heat exchange relationship with a f luid, the temperature of which is arranged to be lower than that of said air f low entering said heat exchanger, and a cryogen injector situated in said duct upstream of said heat exchanger and downstream of said upstream duct end, said cryogen injector operationally injecting a cryogen into said air flow which cryogen in a first, low altitude mode of operation is arranged to reduce the temperature of said air flow to a level at which any water therein is converted into ice crystals which are sufficiently small and dry to pass through said heat exchanger without any tendency to melt therein and in a second, high altitude mode of operation is arranged to reduce the temperature of said air flow to a level at which any heat damage to said at least one heat exchanger by said air flow is substantially avoided.

2. An air intake as claimed in claim 1 wherein in said first low altitude mode of operation said cryogen is so injected into said air flow as to reduce the temperature of said air flow to -50 OC or lower.

3. An air intake as claimed in claim 1 or claim 2 wherein said duct contains two of said heat exchangers arranged in series relationship within said duct and downstream of said cryogen injector.

4. An air intake as claimed in claim 3 wherein said heat exchangers are connected in flow series relationship so that the low temperature fluid exhausted from the downstream heat exchanger is directed into the inlet of the upstream heat exchanger so that said low temperature fluid is twice placed in heat eXchange relationship with the air flow operationally flowing through said duct.

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5. An air intake as claimed in any one preceding claim wherein means are provided to modulate the flow of cryogen to said cryogenic injector.

6. An air intake as claimed in any one preceding claim wherein said cryogen operationally injected into said air flow is liquid oxygen.

7. An air intake as claimed in any one preceding claim wherein said heat exchanger fluid is hydrogen.

8. A method of preventing the accretion of ice in the intake of an aerospace propulsor containing at least one heat exchanger adapted to reduce the temperature of the air flow immediately upstream thereof comprising injecting a cryogen into said air flow upstream of said at least one heat exchanger to reduce the air temperature to a level at which any water in said air flow is converted into ice crystals which are sufficiently small and dry to pass through said heat exchanger without any tendency to melt therein.

9. A method of operating an aerospace propulsor having an intake containing at least one heat exchanger at high altitude and at high speed comprising injecting a cryogen into said air flow upstream of said at least one heat exchanger to reduce the air temperature to a level at which any heat damage to said at least one heat exchanger by said air flow is substantially avoided.

10. An air intake substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

11. A method of preventing the accretion of ice in the intake of an aerospace propulsor substantially as hereinbefore described with reference to the accompanying drawing.

Published 3991 at The Patent Office. Concept House. Cardiff Road. Nm-port. G.Aent NP9 I PH. Further copies maybe obtained from Sales Branch. Unit 6. \me Mile Point. Cu-mfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techWques lid. St Man Crav. Kent.