GB2317025A

GB2317025A - Flow controller

Info

Publication number: GB2317025A
Application number: GB9618551A
Authority: GB
Inventors: Neil Charles Wallace
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1996-09-05
Filing date: 1996-09-05
Publication date: 1998-03-11
Also published as: GB9618551D0

Abstract

A valve for use in fine control of very low flow rates of fluids, such as xenon in an ion thruster, comprises a flow conduit 2,3 having flow restriction means 6 located within it, through which the fluid must pass, and means 7 for controlling the viscosity of the fluid passing through said valve. The viscosity is preferably controlled by changing the temperature of the fluid passing through the restriction. This is preferably be provided by a heating element located outside or within the conduit.

Description

Flow controller This invention relates to the fine control of very low flow rates of either gases or liquids. It has particular but not exclusive application on space vehicles for the control of propellant used in ion propulsion systems and as a thrust regulator in a cold gas propulsion system.

Ion propulsion operates on the principle of electrostatic acceleration of a propellant to exhaust velocities at least an order of magnitude greater than those achieved by conventional chemical systems. This allows the propellant requirements of a particular mission to be reduced to as little as one twentieth of the amount required by a chemical system. Ion propulsion also has the advantage that it allows a smooth throttling capability, permitting for example, precise drag compensation for satellites in low earth orbit.

A major difficulty of existing ion thrusters is the requirement to control the very small flow rates of propellant to the accuracies required. Current systems utilise a technique employing small solenoid valves. These valves are pulsed open to regulate the quantities of propellant gas flowing through the thruster. These systems have the disadvantage that they are relatively heavy, occupy a large volume, need to have their temperature maintained at a constant value.

In addition these valves have long time constants for changing flow rates and produce a flow rate with an inherent ripple. Further disadvantages are that they have moving parts and, as a result of the very small orifices employed, are prone to becoming blocked.

It is an object of the invention to provide a compact and easy to operate control system for fluid flow control.

According to the invention a valve for controlling flow of a fluid therethrough comprises a flow conduit having flow restriction means located within it, through which the fluid must pass, and means for controlling the viscosity of the fluid passing through said valve.

The viscosity is preferably controlled by changing the temperature of the fluid passing through the restriction. This may be provided by a heating element such as a heating coil wound around the fluid conduit in proximity to the restriction member or upstream thereof. Preferably the heating element is actually be located within the conduit.

The restriction member may take a number of forms but in essence is a structure in the flow conduit which offers restriction to fluid flow. Preferably the restriction member is a porous plug of suitable shape having a porous structure of cylindrical form located within a cylindrical conduit.

By way of example one embodiment of the invention will now be described with reference to figure 1 which is a schematic axial cross-section of a fluid control valve for controlling the flow of xenon.

With reference to Figure 1 a fluid control valve comprising two hollow, abutting, cylindrical parts 2 and 3, which may be part of the conduit or, as shown in the figure, formed as a separate unit. The parts have larger diameter recesses 4 and 5 to form a sleeve in which a cylindrical stainless steel, porous plug 6 is housed. During assembly the two parts 2 and 3 are welded to form a fluid tight seal. A heating coil 7 is wound in the vicinity of the porous plug on the outside surface of the valve.

When there is a pressure head upstream, fluid will flow through the porous plug. The ease with of which the fluid, such as xenon, can pass through the porous plug is dependent amongst other factors on the viscosity of the fluid. Consequently for a fixed pressure head the volume flow rate through the yalve is dependent on viscosity. The viscosity of most fluids is a function of temperature and in the case of xenon it is highly dependent. A change in temperature of the gas as a result of passing a current through the heating coil results in changes in the viscosity of the fluid passing through the plug and thus rate of fluid flow. In this manner the flow rate can be controlled by suitable temperature control of the gas.

In practice, flow measuring sensors would be utilised to measure flow rate and according to the flow rate demand a suitable control strategy may be implemented to vary the current through the coil.

The invention is not limited to the above mentioned example. The heating element may be provided within the flow conduit, for example upstream thereof. Alternatively the R.F heating may be utilised so that the necessity of a coil is dispensed with.

Claims

1. A valve for controlling flow of a fluid therethrough comprising a flow conduit having flow restriction means located within it, through which the fluid must pass, and means for controlling the viscosity of the fluid passing through said valve.

2. A valve as claimed in claim 1 wherein said means for controlling viscosity of the fluid passing through the valve are heating means.

3. A valve as claimed in claims 1 and 2 any of the above claims wherein the heating means comprises a heating element located within said conduit and upstream of said restriction member.

4. A valve as claimed in any of the above claims wherein said restriction member is a porous plug.

5. A valve as claimed in any of the above claims wherein said porous plug is stainless steel.

6. An ion thruster incorporating one or more valves as claimed in any of the above claims.

7. An ion thruster as claimed in claim 6 wherein said valve(s) are used to control the flow of xenon.

8. A method of controlling the flow of a fluid in a conduit by controlling its viscosity and thereby its flow rate through a restriction in the conduit.

9. A method of controlling the flow of a fluid as claimed in claim 8 wherein the viscosity of the fluid is controlled by heating said fluid.

10. A methods of controlling a fluid as claimed in claims 8 and 9 wherein said fluid is xenon.

11. A methods of controlling a fluid as claimed in claims 8 to 9 wherein said restriction is a porous plug.