Viscous shear drives and methods using nanoparticles in the viscous medium

Abstract

A device such as a cooling fan drive for a motor vehicle engine has first and second members that are relatively moveable at a differential velocity. A liquid fills space separating relatively moving confronting surfaces of the respective members. The viscosity of the liquid enables the liquid to serve as a motion transmitting medium from one member to the other. Heat is generated within the liquid by shearing stress created by the relative movement of the members at a differential velocity with at least some of the generated heat being dissipated by conduction from at least one of the members through its surface. The viscous liquid comprises a nanoparticle-free base liquid to which have been added nanoparticles in an amount sufficient to measurably increase the thermal conductivity of the liquid filling the space over that of the base liquid.

Description

REFERENCE TO A RELATED APPLICATION AND PRIORITY CLAIM

This application claims the priority of provisional patent application No. 60/759,278, filed Jan. 17, 2006

TECHNICAL FIELD OF THE INVENTION

The present invention is directed to devices and methods in which a driving member transmits motion to a driven member through a viscous liquid medium that is subjected to shear stress due to relative motion between the members.

A speed differential between the two members causes the liquid medium to develop a frictional drag regardless of whether the particular device is functioning as a clutch, a brake, or a force transmission device. The continuous generation of shearing stress in the motion-transmitting medium continuously generates internal heat.

Although the present invention may be used advantageously in fluid coupling devices having various configurations and applications, it is especially advantageous in a coupling device of the type used to drive a cooling fan of an internal combustion engine, and will be described in connection therewith.

BACKGROUND OF THE INVENTION

Fluid coupling devices of the viscous shear type have been popular for many years for controlling operation of engine-driven cooling fans (“fan drives”) in motor vehicles. A typical viscous shear drive, when placed in an engaged condition, operates the fan at relatively higher speed to aid engine cooling by forcing more air through a radiator, and when cooling doesn't need to be aided, is placed in a disengaged condition where it transmits little or essentially no motion to the fan.

A viscous fan drive in a motor vehicle engine cooling system uses a liquid medium, such as silicone, in order to create a drag friction between confronting surfaces of respective rotary components of the fan drive. One component, the driving component, is rotated by the engine, typically by a crankshaft-driven belt drive turning a sheave to which the one component is direct coupled. Rotation of the one component is imparted to the other component (the driven component) through the viscous medium. The fan is direct coupled to the other component.

The amount of liquid permitted to flow between the confronting surfaces of the two components is controlled by a valve that normally is operated either closed or open, but may be operated to various intermediate positions by modulation or electronic control. At one limit of control, liquid is not allowed to flow between the surfaces whereas at an opposite limit liquid is allowed to flow. Presence of the liquid creates frictional forces related directly to the relative movement between the surfaces and sheer stresses in the fluid, consequently developing a torque or force that moves or drags the component to which the fan is direct coupled. Viscosity of the liquid determines the forces and speed of response.

Examples of viscous clutches are found in U.S. Pat. No. 3,893,555, by Elmer; U.S. Pat. No. 4,004,668, by Blair; U.S. Pat. No. 6,032,775, by Martin, et al.; U.S. Pat. No. 6,530,748, by Light, et al; and U.S. Pat. No. 6,752,251, by May, et al.

Fluid in a viscous drive heats up because of shear stress friction. That heat, sometimes called slip heat, is transferred to the relatively moving surfaces according to equations of heat transfer. One or both of the relatively moving surfaces may be coupled to a respective heat sink. Such heat sinks may have various shapes, textures, and/or fluid flow paths for dissipating the transferred heat.

Slip heat develops at all non-zero speed differentials, but attains a maximum at a specific ratio of the speed of one surface to that of the other.

Another type of fan drive uses a magnetorheological (MR) liquid as the coupling medium. While it too operates to create internal shear stresses, the shear strength, and hence the strength of coupling between the relatively moving surfaces is controlled by a magnetic field instead of by a valve controlling the amount of liquid available for contact between two surfaces.

When the magnetic field applied to the medium is zero, the relative friction between the two surfaces is determined by initial viscosity of the fluid and that viscosity can be made relatively small. When the magnetic field is turned on, the fluid exerts a frictional effect proportional to the magnetic field; the greater the magnetic field, the greater the shear stress in the liquid between the moving surfaces. This fluid does not follow a viscous response curve since its shear stress is not proportional to the velocity differential between the surface speeds. However, the liquid must still dissipate heat generated as a function of the speed difference between the relatively moving surfaces. Heat generated by this process can cause instability of the MR fluid, leading to fluid destruction, and loss of control and/or malfunctioning of the fan drive.

One means of controlling the effect of shear stress heating of the liquid medium is to turn off the drive when it overheats. But turning the drive off at a time when engine cooling needs to be aided by operating the fan would obviously be undesirable. What is desirable is to increase the useful lifetime of an MR fluid, increase the range of operating time, and generate a more efficient heat transfer from the liquid to external heat sinks. An example of an MR clutch is found in U.S. Pat. No. 6,585,092 by Smith et al.

A relatively recent development in heat transfer technology is the use of nanofluids to increase the thermal conductivity of a liquid that transfers between a heat source and a heat sink.

U.S. Pat. No. 6,221,275 by Choi, et al. describes certain nanofluids that can provide increased thermal conductivity to a heat transfer medium that is used merely to transfer heat more efficiently from a one surface to an opposite surface along the heat transfer path.

U.S. Pat. No. 6,432,320 by Bonsignore, et al also teaches nanofluids for improved heat transfer, with the stated objective of transferring “heat from one body to another, typically from a heat source (e.g. a vehicle engine, boiler, computer chip, or refrigerator), to a heat sink, to effect cooling of the heat source, heating of the heat sink, or to remove unwanted heat generated by the heat source.”

U.S. Pat. No. 6,695,974 by Withers, et al. teaches a “novel fluid heat transfer agent suitable for use in a closed heat transfer system, for example, wherein energy is transferred between an evaporator and a condenser in heat exchange relationship”

U.S. Pat. No. 6,858,157 by Davidson, et al. teaches the use of nanofluids to transfer heat from the coils of a transformer wound on an iron core through oil to the outside, surrounding volume. The major objective is to transfer heat from a heat source to a heat sink. Within the liquid, temperatures are generally equal to, or intermediate between, those of the heat source and the heat sink, i.e. the internal temperature in the fluid is always both less than the temperature of the heat source and greater than that of the heat sink.

The common purpose in the various applications just summarized is to use an improved liquid to transfer heat from an external source to an external sink.

SUMMARY OF THE INVENTION

Unlike those various applications that are concerned with heat conduction properties of liquids for transferring heat through what amount to various forms of heat exchange structures or systems, a viscous shear drive requires a liquid that possesses the property of being able to transmit motion from a driving member to a driven member with some degree of efficiency. It has been accepted as inherent that heat will be generated internally of such a liquid. Consequently, liquids that are possess suitable motion-transmitting properties are essential to viscous shear drives.

When such a liquid also generates undesirable amounts of internal heat in a particular drive, traditional solutions have been to cool the liquid by designing the drive to have a larger surface area through which more heat can be dissipated to ambient and/or adding an external heat exchanger through which the liquid can circulate and transfer heat to ambient in the process.

The present invention provides a solution that is significantly more advantageous that those traditional solutions. The inventive solution can provide removal of heat that is inherently generated internally of liquid that is continually being sheared as it transmits motion from a driving member to a driven member without the need to circulate the liquid through an external heat exchanger in order to remove excessive heat. It also provides removal of heat without having to oversize the design of the drive in order to handle occasional peak temperatures caused by peaks in the internal generation of heat. Principles of the invention can be applied to any form of viscous drive including those that control the degree of coupling by controlling circulation of the liquid between driving and driven surfaces and by applying a magnetic field to MR liquid.

The inventor has observed that the internal molecular structure of liquid silicone is not conducive to efficiently conducting heat to nearby surfaces, i.e. its thermal constant is not very large. Internal temperature of silicone in an operating viscous fan drive is almost always higher than that of either confronting surface. As a consequence, slip heat is a factor in the size and weight of any fan drive. Improvements in dissipating heat generated internally of viscous liquid being sheared can lead to smaller, more efficient designs.

The present invention contemplates the inclusion of nanoparticles in a base liquid of a viscous drive to increase the heat transfer coefficient of the liquid. Although the inclusion of nanoparticles in low viscosity liquids is known, the inventor is unaware of any teaching of adding nanoparticles to a viscous fluid whose viscosity is greater than about 1,000 cp (centipois) and within which heat is generated by shear stress as occurs in a viscous drive. The inventor is also unaware of the addition of nanoparticles to MR fluid within which heat is generated by shear stress as occurs in a viscous drive.

A preferred nanofluid particle should be asymmetric and have a molecular size less than 100 nm (nanometers) in one of its dimensions. The other dimension should be at least 10 times larger if the particle is a prolate spheroid or rod shaped (aspect ratio P≧10), or 50 times larger if the particle is an oblate spheroid or disc shaped (aspect ratio P≦0.02). The preferred selection of asymmetric particles is pointed out in a paper, “Effective thermal conductivity in nanofluids of non-spherical particles with interfacial thermal resistance: Differential effective medium theory”, Journal of Applied physics 100, 024913 (2006), by Xiao Feng Zhou and Lei Gao. Although the volume fraction in this paper has been chosen as 0.05 (5%), it's not necessary to limit this parameter to a specific range.

The interfacial thermal resistance described in the above paper should be minimized for the specific viscous liquid used in a fan drive. Silicone is the preferred liquid but other liquids such as hydrocarbons can be used. MR liquids are often made with hydrocarbon base liquids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a snapshot of a nanoliquid between two surfaces that are relatively moving at a relative speed differential.

DETAILED DESCRIPTION OF THE INVENTION

Components 1 and 2 in FIG. 1 have respective confronting surfaces 6 and 7. Surface 6 is shown as fixed and component 2 is in motion with velocity v, shown as reference numeral 8. This velocity v is the relative velocity between surfaces 6 and 7. Instead of one surface being stationary, both surfaces may be in motion with a differential velocity of v. The confronting surfaces need not be perfectly flat as schematically portrayed by FIG. 1, but may alternately be grooved or undulating, or otherwise contain features that interrupt surface flatness.

Although surfaces 6 and 7 are shown as straight sections, a viscous rotary fan drive has circular surfaces, and has a ribbed flow path developing a large, drag surface area, as is well known to those skilled-in-the-art. Particles of a nanoliquid 3 are at rest on surface 6 with no horizontal or lateral motion with respect to FIG. 1. Particles of the liquid on surface 7 are also at rest with that surface, moving at the same speed v relative to surface 6. Between the two surfaces, liquid particles move at varying horizontal speeds dependent on their distance from surfaces 6 and 7.

Nanoliquid 3 comprises a base liquid that can be either a viscous liquid or an MR liquid. The latter type of base liquid is under the selective influence of a magnetic field (not shown) serving to control the shear stress generated within the liquid. A preferred embodiment of a nanoliquid 3 for use in a viscous drive is a mixture of liquid silicone and nanoparticles. The silicone viscosity is chosen to satisfy specifications for a specific viscous drive design. The nanoparticles are chosen to increase the heat conduction coefficient of the liquid.

The nanoparticles may be selected from a group consisting of metals, metal compounds, and combinations thereof. Also, the nanoparticles may be fullerenes, including carbon nanotubes with an aspect ratio of at least 10. The nanoparticles are dispersed in a silicone base liquid with an appropriate surfactant or coating, if necessary, to reduce the thermal resistance between the nanoparticles and the base liquid and to prevent the nanoparticles from agglomerating within the base liquid.

A preferred nanoliquid comprises a loading of nanoparticles forming a volume fraction of no less than 1% (0.01), but possibly much higher if the thermal coefficient of heat transfer is significantly affected by volume fraction. A volume fraction is related to a weight fraction by a well-known equation. A preferred embodiment uses a molecular weight as low as possible to achieve a desired volume fraction. Aluminum discs with an aspect ratio of P≦0.02 or carbon nanotubes with an aspect ratio of P≧10 are preferred nanoparticles for volume fractions of less than 10%.

The inclusion of these nanoparticles in either liquid silicone or in MR liquid should cause no significant change in characteristics of the base liquid that are detrimental to a viscous drive. The invention can improve the useful life of an MR-type drive by slowing the rate of deterioration of the MR liquid. Additive nanoparticles will allow the body of a viscous drive to be smaller and lighter, leading to greater fuel efficiency in application to a motor vehicle for both MR-type and silicone-type drives.

In order to show what effect a change in thermal conductivity has on the temperatures involved, we consider the simple heat transfer equation between two parallel surfaces shown with equal areas 4 and 5 (A) separated by an infinitesimal small distance dy between them in the vertical, or y-axis. We define dQ/dt as the heat transfer per unit time and k as the thermal conductivity of the material. The temperature gradient is defined as dT/dy where T is in degrees Kelvin (degK). Then,

dQ/dt=−k·A·dT/dy.

The minus sign is necessary with heat flow in the positive y-axis direction if the temperature is lower (smaller) as y increases. A higher temperature is assumed in the plane of area 4 and heat flows from area 4 toward surface 6. No attempt is made to make the model of FIG. 1 absolutely accurate, but those skilled-in-the-art will recognize the principle.

A reasonable assumption for an equilibrium system is that dQ/dt is constant, i.e. the temperature at any surface within the liquid is established by the frictional shear stress, and for a constant velocity differential it keeps generating internal energy from the viscous shear stress process. The heat flows from the liquid, through the separation, and into the components 1 and 2 which also serve as heat sinks or heat transfer elements to ambient. We only consider one side of the heat sink, knowing that the actual heat flow can be twice the calculated result. Integrating from area 4 in the liquid to surface 6, we have

Q/t=−(k·A·ΔT/s)

where s is the distance from the plane of the heat source in the fluid to the heat sink surface, and t is the time over which heat, Q, flows. Distance s is the distance from the plane of area 4 to surface 6. The important observation is that Q/t is proportional to k, the thermal conductivity coefficient! We assume that Q/t is a constant to 1^st order. Whatever value ΔT is, doubling the thermal conductivity k causes ΔT to be cut in half. If the temperature inside the fluid at plane 4 is 200 degK higher than the heat sink 1, doubling k will reduce the difference to only 100 degK. Although this calculation is good only to 1^st order, it shows the importance of increasing the thermal conductivity of a shearing material for heat transfer and temperature reduction. If we decide to keep the same temperature differential, twice as much heat can be transferred into the heat sinks per unit time, allowing for smaller, lighter, more fuel efficient fan drive designs.

MR fluids can have much longer operational lifetimes at half the interior temperature in the fluids. In fact, the nanoparticles can be made magnetic, allowing for greater stresses and strain and greater force control than currently available.

Although asymmetric nanoparticles loaded into a base liquid is a preferred embodiment, other nanoparticles may produce a sufficiently large increase in the heat transfer coefficient to allow practice of the invention.

Claims

1. A device comprising a first member and a second member that are relatively moveable at differential velocity, and a viscous liquid which fills space separating relatively moving confronting surfaces of the respective members and internally of which heat is generated by shearing stress created in the liquid by the relative movement of the members at differential velocity with at least some of the generated heat being dissipated into at least one of the members through its confronting surface, wherein the viscous liquid comprises a nanoparticle-free base liquid to which have been added nanoparticles in an amount sufficient to measurably increase the thermal conductivity of the liquid filling the space over that of the base liquid.

2. A device as set forth in claim 1 in which the viscosity of the base liquid is greater than about 1000 cp.

3. A device as set forth in claim 2 in which the base liquid comprises liquid silicone.

4. A device as set forth in claim 1 in which the base liquid comprises a magnetorheological (MR) liquid.

5. A device as set forth in claim 1 in which at least a majority of the nanoparticles are each asymmetric and have a molecular size less than 100 nm (nanometers) in one of their dimensions.

6. A device as set forth in claim 1 in which the volume fraction of nanoparticles is no less than 1% of the space.

7. A device as set forth in claim 1 in which one of the members comprises a driving member and the other member comprises a driven member that is driven by the driving member via the liquid in the space.

8. A device as set forth in claim 7 including a control for controlling the degree to which the liquid is effective to transmit motion of the driving member to the driven member.

9. A device as set forth in claim 8 in which the base liquid comprises a magnetorheological (MR) liquid and the control comprises a magnetic field that is selectively effective on the magnetorheological (MR) liquid.

10. A device as set forth in claim 8 in which the base liquid comprises a liquid silicone and the control comprises a liquid flow path including a control valve for controlling flow of liquid through the space between the confronting surfaces.

11. A device as set forth in claim 1 in which the nanoparticles are selected from a group consisting of metals, metal compounds, and combinations thereof.

12. A method for removing heat generated internally of viscous liquid within space separating relatively movable confronting surfaces of respective members that are movable relative to each other at a velocity differential and that generate the heat by shearing the liquid when relatively moving, the method comprising making the liquid by mixing a nanoparticle-free base liquid and nanoparticles in an amount sufficient to measurably increase the thermal conductivity of the resulting mixture in comparison to that of the base liquid, and filling the space with the mixture.

13. A method as set forth in claim 12 including selecting the nanoparticles from a group consisting of metals, metal compounds, and combinations thereof.

14. A method as set forth in claim 12 in which the mixing step comprises mixing a sufficient amount of nanoparticles to create a volume fraction of nanoparticles no less than about 1% of the mixture.

15. A method as set forth in claim 12 in which the mixing step comprises mixing the nanoparticles with a base liquid having viscosity greater than about 1000 cp.

16. A method as set forth in claim 12 in which the mixing step comprises mixing the nanoparticles with a base liquid comprising liquid silicone.

17. A method as set forth in claim 12 in which the mixing step comprises mixing the nanoparticles with a base liquid comprising magnetorheological (MR) liquid.