GB2060937A - Fluid-flow restrictor assembly - Google Patents

Abstract

A lubrication system includes a duct (10) for the passage of lubricant a portion of which contains a cylindrical restrictor (12) so that an annular passage (14) is defined between them. The duct (10) portion and restrictor (12) areas wetted by the lubricant are of such a magnitude and are so spaced apart that together they impose viscous pressure losses but low dynamic pressure losses on lubricant passing in operation through the duct (10). The duct (10) and restrictor (12) may be formed of materials having differing coefficients of thermal expansion. <IMAGE>

Description

SPECIFICATION Lubrication system This invention relates to lubrication systems.

Lubrication systems suitable for mechanisms frequently comprise a series of passageways which interconnect items to be lubricated with a lubricant reservoir. Whilst such systems are designed to provide an adequate lubricant supply under normal operating conditions, there may be circumstances where the amount of lubricant supplied to the item to be lubricated is incompatible with the actual lubricant requirement of that item.

Thus for instance, certain bearings in gas turbine engines must be supplied with lubricant in order to function effectively. The amount of lubricant neces sary to ensure this is usually known and the engine lubrication system is accordingly designed to supply lubricant to the bearings at an adequate flow rate.

Whilst the flow rate chosen is one which is adequate at normal operating temperatures, it is sometimes found that it is not satisfactory when the engine is required to operate at lower temperatures. For example when a gas turbine engine is started up from cold, the high viscosity of the cold oil supplied to the engine bearings sometimes imposes such a high viscous drag on the bearings that engine starting times prove to be excessively long. This effect of course becomes more troublesome as the lubricant temperature decreases.

There is a requirement therefore for a lubrication system which provides an adequate supply of lubricant to an item to be lubricated under normal operating conditions but which provides a reduced supply of lubricant when the viscosity of that lubricant is increased. Thus by reducing the amount of lubricant supplied to the item to be lubricated when the lubricant viscosity is increased, lubrication is maintained but the high viscous drag of the lubricant on the item is reduced.

It is an object of the present invention to provide a lubrication system which satisfies this requirement.

According to the present invention, a lubrication system includes duct means adapted for the passage of lubricant therethrough and restrictor means posi tioned in at least a portion of said duct means, a major portion of said restrictor means being spaced apart from said duct means, the arrangement being such that the surface areas of said duct means portion and said restrictor means wetted by lubri cant passing in operation through said duct means are of sufficient magnitude and are so spaced apart that together they impose high viscous pressure losses but low dynamic pressure losses on said lubricant.

Said duct means portion and said restrictor means are preferably of circular cross-section and co-axially mounted so that together they define a generally annular lubricant passage.

Said restrictor means is preferably provided with a semi-spherical upstream end and a generally conical downstream end.

Said restrictor means and said duct means portion may be formed from materials having differing coefficients of thermal expansion.

Said lubrication system may be adapted for use on a gas turbine engine.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a sectioned side view of a portion of a lubrication system in accordance with the present invention.

Figures 2, 3 and 4 are graphs indicating the performance of the lubrication system shown in Figure 1.

With reference to Figure 1, a lubrication system includes a circular cross-section duct 10 which is adapted for the passage of a lubricant through it. The duct 10 may for instance interconnect a lubricant reservoir with a device such as a bearing which requires lubrication, suitable pumping means being provided to urge the lubricant through the duct 10.

A circular cross-section restrictor 12 is mounted coaxially within a portion 11 of the duct 10 by means of a plurality of cross-wires 13. The diameter of the restrictor 12 is less than that of the internal diameter of the duct 10 so that an annular passage 14 is defined between them.

Lubricant is adapted in operation to flow from left to right through the duct 10 and annular passage 14 as viewed in Figure 1. In order to minimise dynamic lubricant pressure losses, the upstream end 15 of the restrictor is semi-spherical in shape whilst its downstream end 16 is generally conical.

The annular passage 14 serves to present lubricant flowing through the duct 10 with an increased lubricant wetted surface area per unit duct length as compared with the remainder of the duct 10. This has the effect of imposing high viscous pressure losses on the lubricant whilst ensuring that dynamic pressure losses are low. Consequently if the lubricant is of high viscosity as a result of being at a low temperature, the lubricant flow rate through the annular passage will be low. However if the lubricant viscosity is reduced as a result of an increase in its temperature, the flow rate through the annular passage will increase. It will be seen therefore that the annular passage 14 constitutes a lubricant flow rate regulator, the output of which is dependent upon lubricant viscosity.

If the lubrication system in question is that of a gas turbine engine, then this lubricant flow regulator ensures that when the engine is starting up from cold, the lubricant flow to the engine's bearings will be reduced, thereby reducing the viscous drag on those bearings. When the lubricant increases in temperature as a result of engine operation, its viscosity will decrease, thereby reducing the high viscous pressure losses imposed upon it by the flow regulator. This in turn leads to an increased lubricant flow rate through the regulator and hence to the bearings to be lubricated.

In order to investigate the performance of lubrication systems in accordance with the present invention more clearly, a series of tests were carried out.

A restrictor 12 similar to that shown in Figure 1 was constructed and mounted in a duct 10 by means of cross wires 12. The duct 10 was 10.8 mm internal diameter and 12 mm outside diameter, and restrictor 12 was 8.75 mm external diameter and 283.75 mm long. An oil having a kinematic viscosity of 1400 mm2 per sec. at 29"C was then passed through the duct 10 and its pressure drop across the restrictor 12 measured for varying flow rates. The tests was then repeated with the restrictor 12 removed, the pressure drop being measured across a portion of the duct 10 of the same length as the restrictor 14. The results of the test are shown in Figure 2 which is a graph of the lubricant flow rate in gallons per hour vs the pressure difference across the restrictor in pounds per square inch. During both tests it was attempted to maintain the temperature of the oil at 29"C. However due to the operating performance of the test equipment, the temperature of the oil varied between 23"C and 33"C during the test on the duct 10 with the restrictor 12 and was constant at 36"C during the test on the duct without the restrictor 12.

Since the oil used in the tests is very sensitive to viscosity changes with minor temperature variations, theoretical pressure drop vs. flow characteristics were matched to the test results and a comparative analysis was carried out for the duct 10 and restrictor 14 at the same oil temperature (290C). The results of this analysis are shown in Figure 3. In both Figures 2 and 3, A designates the results obtained for the duct 10 with the flow restrictor 14 and B designates the results obtained without the restrictor.

The oil chosen for the tests had a viscosity at 29"C which is similar to that of oil used in gas turbine engines at a temperature of -26 C. Thus the results shown in Figure 3 are indicative of the performance of the present invention in gas turbine engines at temperatures in the region of -26 C.

It is apparent from Figure 3, that when a high viscosity oil is passed through the duct 10, the restrictor 14 serves to considerably reduce the oil flow rate when compared with the flow rate through the duct 10 with the restrictor 14 removed.

The tests were then repeated using standard gas turbine engine oil at a temperature of 86"C, a temperature typical of oil temperatures encountered in gas turbine engines at normal running temperature. The results of the tests are shown in Figure 4.

Again A designates the results obtained with the flow restrictor 14 and B the results obtained without the restrictor 14.

Although, as is apparent from Figure 4, the pressure drop for a given oil flow through the duct 10 with the restrictor 14 is greater than the pressure drop across the duct 10 without the restrictor 14 this pressure drop penalty at normal operating temperatures is significantly smaller than that which would be imposed by a standard oil flow restrictor. Thus the flow rate through the duct 10 with the restrictor 14 is sufficiently great to provide gas turbine engine bearings with an adequate supply of oil under all running conditions.

The above tests relate a lubrication system which is designed to provide a particular type of mechanism with an adequate supply of lubricant over a certain temperature range. The present invention is not, however, limited to this mechanism and temperature range. Thus alterations could be made to the surface areas of the duct 10 and restrictor 14 wetted by the lubricant as well as the distance between the inner wall of the duct 10 and the outer wall of the restrictor 14 as required.

If it is required to alter the flow rate through the duct 10 in a manner which differs from that which results from changes in lubricant viscosity, it is envisaged that the duct 10 and the restrictor 14 could be formed from materials having differing rates of thermal expansion. Thus as the duct 10 and restrictor 14 alter in temperature over a range of differing operating conditions, the distance between them will vary. This in turn will vary the lubricant flow rate in addition to variations caused by alterations in lubricant viscosity.

Although the present invention has been described with reference to the requirements of a lubrication system suitable for a gas turbine engine which is started at low temperatures, it will be appreciated that it has other applications. Thus for instance improved lubricationsystem management could be achieved by applying the present invention to ducts which supply oil to bearing chambers in the hot regions of gas turbine engines. This would permit the matching of the oil flow rate to these chamber with the external heat input to the chambers under all engine operating conditions.

Claims (6)

1. A lubrication system including duct means adapted for the passage of lubricant therethrough and restrictor means postioned in at least a portion of said duct means, a major portion of said restrictor means being spaced apart from said duct means, the arrangement being such that the surface areas of said duct means portion and said restrictor means wetted by lubricant passing in operation through said duct means are of sufficient magnitude and are so spaced apart that together they impose high viscous pressure losses but low dynamic pressure losses on said lubricant.

2. A lubrication system as claimed in claim 1 wherein said duct means portion and said restrictor means are circular and co-axially mounted so that together they define a generally annular lubricant passage.

3. A lubrication system as claimed in claim 2 wherein said restrictor means is provided with a semi-spherical upstream end and a generally conical downstream end.

4. A lubrication system as claimed in any one preceding claim wherein said restrictor means and said duct means portions are formed from materials having differing coefficients of thermal expansion.

5. A lubrication system as claimed in any one preceding claim and adapted for use on a gas turbine engine.

6. A lubrication system substantially as hereinbefore described with reference to and as shown in Figure 1 of the accompanying drawings.