RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing from International Application No. PCT/GB2018/051136, filed Apr. 27, 2018, which claims priority to British Application No. 1706734.9, filed Apr. 27, 2017, the teachings of which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to techniques and apparatus for the measurement of fluid properties, and is concerned particularly with determining a yield stress of a non-Newtonian fluid.

In a Newtonian fluid, the shear stress is directly proportional to the shear rate, the constant of proportionality being the viscosity of the fluid. Therefore the viscosity as a single parameter can be used to model or define the relationship between shear stress and shear rate of the fluid, and therefore the flow behaviour of Newtonian fluids. Water is an example of a Newtonian fluid.

In a non-Newtonian fluid, the relationship between shear stress and shear rate is not so simple. The apparent viscosity of the fluid is found to vary with, for example, the shear stress or the shear rate. Fluids that exhibit non-Newtonian behaviour include tomato ketchup, mayonnaise and paint. The study of the flow of these types of fluids is the field of ‘rheology’.

Rheological models have been generated for non-Newtonian fluids, for which a small number of parameters can be obtained for a fluid, which may be used to determine the relationship between shear stress and shear rate in the fluid over a range of shear stresses or shear rates, or in other words the ‘apparent’ viscosity provided by the fluid under those particular conditions.

In the prior art, such parameters of rheological models are typically determined using a device that exerts shear forces in a fluid sample using a rotating or oscillating element. Complex viscoelastic parameters are usually determined by analysing the relationship between the shear rate and the developed shear stress. However, the devices use sensitive moving parts and their performance, and therefore the accuracy of measurement, can become affected by the environmental conditions in which they are used. Furthermore, the need to manage the fluid sample within a carefully defined volume, in order to calculate the shear rate accurately, generally makes these types of apparatus suitable only for the laboratory environment.

Of special interest is the real time measurement of the yield stress of a fluid. Yield stress is defined as the stress required in order for solid matter to flow and represents the point of plastic deformation of the solid. It is a feature of many fluids and has particular importance in the manufacture of foods, paints and petro-geological fluids, for example, but also in naturally occurring materials such as muds and sediments.

The details and origins of yield stress behaviour of fluids is a subject of interest to researchers. Traditional approaches to determining a yield stress are set out in “Yield Stress in Foods: Measurements and Applications” of Sun et al (International Journal of Food Properties, volume 12, pp 70-101, 2009), and in “Understanding Yield Stress Measurements”, a white paper produced by Malvern Instruments Limited of Grovewood Road, Malvern, Worcestershire, UK.

Traditional approaches measure yield stress of a static sample.

For example, in the extrapolation approach, there is extrapolation of the shear stress versus shear rate data obtained from conventional rheometers. The experimental data is extrapolated back to zero shear rate to obtain the yield stress value at the shear stress intercept.

For example, in the stress relaxation approach, the fluid material is first sheared at either constant shear rate or constant shear stress in a conventional rotational viscometer, followed by bringing the material to rest either gradually or suddenly. The yield stress is then measured as residual stress remaining in the fluid upon cessation of flow.

For example, in the creep/recovery response approach, a constant shear stress is applied in steps to the fluid material. If stresses applied are below the yield stress, the test material behaves as an elastic solid with a complete recovery upon removal of stress and it will not flow. The yielding point can be detected from a drastic change of slope in the time vs. shear strain curve.

For example, in the shear stress ramp approach, a gradual step increase is applied to the sample. The instantaneous (or apparent) viscosity is monitored for the presence of an inflexion point, which indicates onset of flow, and the yield stress.

For example, in the cone penetrometer approach, a metal cone of specific dimensions is forced into the surface of test specimen. The cone gradually slow down until it comes to rest. The yield stress is calculated as a function of the penetration depth when the cone stops.

For example, in the dynamic oscillation approach, the material is subjected to a sinusoidal strain and the resulting stress is measured as a function of both time and frequency. Depending on the strain amplitude, small deformations occur within the material. The presence of a low frequency plateau in the curve can be correlated to the yield stress.

For example, in the inclined plane approach, a uniform fluid layer is placed on an initially horizontal plane, followed by progressively increasing the angle of inclination of the plane until a critical value is reached, whereby the fluid starts to flow. The yield stress is correlated with the angle of inclination for the fluid to flow.

For example, in the vane technique, a vaned device is immersed in the material. The vane geometry consists of a vane spindle with typically four to eight thin blades arranged at equal angles, cantered on a narrow cylindrical shaft of a rheometer or viscometer. The vane can be operated under controlled shear-rate mode or controlled shear stress mode; in both cases the yield stress is the minimum stress required for continuous rotation of the vane.

For example, in the squeezing flow approach, a sample of fluid is placed between the two parallel test surfaces of a bench rheometer or viscometer and squeezed in either a ‘constant volume’ or ‘constant area’ arrangement. The determination of the yield stress from squeezing flow is based on the residual stress after shear and relaxation.

For example, in the plate method, a plate is immersed into a container filled with sample material. An attached balance records the force required to create plate movement and this is combined with the buoyancy force to determine yield stress.

For example, in the slump test, a cylindrical mould is filled with the test fluid and the mould is lifted off to allow the material to collapse and flow under its own weight. The slump height, which is the difference between the initial and final heights is measured and provides an inverse relationship with yield stress.

Unlike the above-described techniques, another traditional approach allows for yield stress measurement of a flowing sample, by way of the use of magnetic resonance imaging (MRI), whereby a material sample flowing at a constant velocity through a tube is flow imaged using an MRI scanner. The MRI scanner is able to measure the flow profile (flow velocity) of the fluid across the pipe. The flow profile is combined with an estimate of stress from the differential pressure along the pipe to provide a measurement of yield stress.

The disadvantage of almost all of the above traditional approaches to determining the yield stress of a fluid is the requirement for a captive sample in a static (non-flowing) environment. This makes these methods unsuitable for real-time measurement. The traditional methods requiring captive samples are largely confined to intermittent spot measurements and the laboratory environment.

It would be desirable to provide fast real-time rheology measurement of yield stress fluids under live conditions. This would allow detailed assessment of large material volumes and improved capability for their control and management. In particular, in process industries, in-line measurement is advantageous in that it allows for continuous product control. A feedback loop may allow parameters of the process to be adjusted based on in-line measurements of yield stress. While the traditional approaches using captive static samples may be used to take periodic measurements, there is inevitably a measurement lag. The provision of faster measurements in-line would reduce or eliminate the measurement lag and thus improving the controllability of the process, which can increase product consistency and reduce waste.

In other applications such as the measurement of rheological properties of drilling muds and lubricants, the fast real time rheological measurement of yield stress under live conditions may improve efficiency by allowing faster intervention to correct drift in fluid properties away from desired or permitted ranges.

In marine, civil engineering, mining and geotechnical fields it would be desirable to provide fast measurement of flowing materials with important rheology features such as poured concretes and cements, slurries and muds, in situ, without taking captive static samples, providing increased accuracy due to ease of taking multiple data points, while reducing or eliminating delays involved with spot testing and laboratory turnaround times.

SUMMARY

According to a first aspect of the invention, there is provided a method of estimating the yield stress of a yield stress fluid using one or more vibratory transducers, the method comprising: vibrating a vibratory transducer in the yield stress fluid and making a first measurement; providing a vibration to liquefy at least a portion of the yield stress fluid around the one or more vibratory transducers; while said portion of the yield stress material is liquefied, vibrating a vibratory transducer in the yield stress fluid and making a second measurement; estimating the yield stress of the yield stress fluid based on the first and second measurements, wherein, either: the vibratory transformer is vibrated in both cases at a first frequency and the measurements are made of the degree of damping in the yield stress fluid, or the vibratory transformer is vibrated in both cases at resonance in a first resonant mode and the first and second measurements are made of the resonant frequency. Vibration sources of an appropriate amplitude and frequency may be applied locally to the yield stress material surrounding the viscometer transducer to create liquefaction. This may be performed in flowing or static conditions. Yield stress is estimated by either the reduction in the apparent viscosity or a change in frequency due to a change in mass loading at the transducer surface. The vibration source may be the resonance of the transducer itself as used to make a viscosity measurement and in the context of the present disclosure, this is termed ‘intrinsic stimulation’. Alternatively it may be a separate vibration. The combination of high frequency and high amplitude of vibration may cause local liquefaction at and around the transducer surface, detected as a viscosity and frequency change.

Preferably, the vibration to liquefy a yield stress fluid around the one or more transducers is provided by the making of the second measurement at an increased amplitude of vibration relative to the first measurement.

More preferably, the yield stress is estimated based on the difference between the first and second measurements, the difference scaled by the difference in amplitude between the first and second measurements.

Alternatively, method further comprises obtaining third and fourth measurements at a lower frequency and without liquefaction, wherein the yield stress is estimated based on the difference between the first measurement scaled by the third measurement and the second measurement scaled by the fourth measurement, said difference being scaled by the difference in amplitude between the first and second measurements. The use of measurements obtained at the lower frequency without liquefaction allows temperature effects and other common mode fluctuations to be ratiometrically reduced or eliminated. The first, second, third and fourth measurements may be made using a single multi-mode vibratory transducer or using multiple transducers. For example, the first and second measurements may be made using a first vibratory transducer and the third and fourth measurements may be made using a second vibratory transducer.

In a preferred embodiment, a method of estimating the yield stress of a yield stress fluid using one or more vibratory transducers comprises the steps of: vibrating a vibratory transducer at resonance at a first resonant mode in the yield stress fluid and making a first measurement of the resonant frequency; providing a vibration to liquefy at least a portion of the yield stress fluid around the one or more vibratory transducers; while said portion of the yield stress material is liquefied, vibrating a vibratory transducer at resonance at the first resonant mode in the yield stress fluid and making a second measurement of the resonant frequency; estimating the yield stress of the yield stress fluid based on the first and second measurements. Preferably, the vibration to liquefy yield stress fluid around the one or more transducers is provided by the making of the second measurement at an increased amplitude of vibration relative to the first measurement. More preferably, the yield stress is estimated based on the difference between the first and second measurements of the resonant frequency scaled by the difference in amplitude between the first and second measurements. Alternatively, the method may further comprise: vibrating a vibratory transducer at resonance at a second resonant mode in the yield stress fluid and making a third measurement of the resonant frequency, the second resonant mode corresponding to a lower resonant frequency than the first resonant mode; and vibrating a vibratory transducer at resonance at the second resonant mode in the yield stress fluid and making a fourth measurement of the resonant frequency, wherein the yield stress is estimated based on difference between the first frequency measurement scaled by the third frequency measurement and the second frequency measurement scaled by the fourth frequency measurement, scaled by the difference in amplitude between the first and second measurements. Advantageously, the use of the measurements of the second resonant mode allows temperature effects and other common mode fluctuations to be ratiometrically reduced or eliminated.

In another preferred embodiment, a method of estimating the yield stress of a yield stress fluid using one or more vibratory transducers comprises the steps of vibrating a vibratory transducer in the yield stress fluid at a first frequency and making a first measurement of the degree of damping; providing a vibration to liquefy at least a portion of the yield stress fluid around the one or more vibratory transducers; while said portion of the yield stress material is liquefied, vibrating a vibratory transducer the first frequency and making a second measurement of the degree of damping; estimating the yield stress of the yield stress fluid based on the first and second measurements of degree of damping. Preferably, the vibration to liquefy yield stress fluid around the one or more transducers is provided by the making of the second measurement at an increased amplitude of vibration relative to the first measurement. More preferably, the yield stress is estimated based on the difference between the first and second measurements of the degree of damping scaled by the difference in amplitude between the first and second measurements. Alternatively, the method may further comprise: vibrating a vibratory transducer at a second frequency in the yield stress fluid and making a third measurement of the degree of damping, the second frequency being lower than the first frequency; and vibrating a vibratory transducer at the second frequency in the yield stress fluid and making a fourth measurement of the degree of damping, wherein the yield stress is estimated based on difference between the first degree of damping measurement scaled by the third degree of damping measurement and the second viscosity measurement scaled by the fourth degree of damping measurement, scaled by the difference in amplitude between the first and second measurements.

Preferably, the measurements of the degree of damping of the yield stress fluid comprise measurements of the viscosity of the yield stress fluid.

According to a further aspect of the invention, there is provided an apparatus for estimating the yield stress of a yield stress fluid using one or more vibratory transducers, the apparatus comprising: one or more vibratory transducers; a processing module configured to: vibrate a vibratory transducer at resonance at a first resonant mode in the yield stress fluid and make a first measurement of the resonant frequency; provide a vibration to liquefy at least a portion of the yield stress fluid around the one or more vibratory transducers; while said portion of the yield stress material is liquefied, vibrate a vibratory transducer at resonance at the first resonant mode in the yield stress fluid and make a second measurement of the resonant frequency; estimate the yield stress of the yield stress fluid based on the first and second measurements, wherein, either: the vibratory transformer is vibrated in both cases at a first frequency and the measurements are made of the degree of damping in the yield stress fluid, or the vibratory transformer is vibrated in both cases at resonance in a first resonant mode and the first and second measurements are made of the resonant frequency.

According to a further aspect of the invention, there is provided an apparatus for estimating the yield stress of a yield stress fluid using one or more vibratory transducers, the apparatus comprising: means for vibrating a vibratory transducer in the yield stress fluid and making a first measurement; means for providing a vibration to liquefy at least a portion of the yield stress fluid around the one or more vibratory transducers; means for while said portion of the yield stress material is liquefied, vibrating a vibratory transducer in the yield stress fluid and making a second measurement; means for estimating the yield stress of the yield stress fluid based on the first and second measurements, wherein, either: the vibratory transformer is vibrated in both cases at a first frequency and the measurements are made of the degree of damping in the yield stress fluid, or the vibratory transformer is vibrated in both cases at resonance in a first resonant mode and the first and second measurements are made of the resonant frequency.

According to a further aspect of the invention, there is provided a non-transitory computer-readable medium having stored thereon instructions that, when executed by one or more processors, cause the one or more processors to: vibrate a vibratory transducer in the yield stress fluid and making a first measurement; provide a vibration to liquefy at least a portion of the yield stress fluid around the one or more vibratory transducers; while said portion of the yield stress material is liquefied, vibrate a vibratory transducer in the yield stress fluid and make a second measurement; estimate the yield stress of the yield stress fluid based on the first and second measurements, wherein, either: the vibratory transformer is vibrated in both cases at a first frequency and the measurements are made of the degree of damping in the yield stress fluid, or the vibratory transformer is vibrated in both cases at resonance in a first resonant mode and the first and second measurements are made of the resonant frequency.

In preferred embodiments, the apparatus comprises means for carrying out one or more of the above-described methods or indeed any of the methods described herein.

According to a further aspect, there is provided a non-transitory computer-readable medium having stored thereon instructions that, when executed by one or more processors, cause the one or more processors to: vibrate a vibratory transducer in the yield stress fluid at a first frequency and make first measurement of the viscosity; provide a vibration to liquefy at least a portion of the yield stress fluid around the one or more vibratory transducers; while said portion of the yield stress material is liquefied, vibrate a vibratory transducer the first frequency and make a second measurement of the viscosity; estimate the yield stress of the yield stress fluid based on the first and second measurements of viscosity.

Aspects of the invention relate to the measurement of one or more of the following properties of Newtonian and non-Newtonian fluids: yield stress, viscosity (at one or more equivalent shear rates), power law index n, consistency factor K, flow velocity, and the detection of onset of flow from initially static conditions. In addition, aspects of the invention may be applicable in both static and flowing conditions.

Aspects of the invention may be used in a wide range of settings, for example: as an in-line harsh process-tolerant device in pipes, vessels, open channels; for the taking of field measurements in marine and earth science environments; in the laboratory with continuous, automated or discrete sample measurements; as a portable device for both laboratory, field and at-line use; and as a component of a system incorporating measurement of these fluid properties including resonators based on micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS) scale architectures.

Aspects of the invention are based on the use of one or more viscosity measurement transducers and the modulation of the rheological state of the fluid at the transducer measurement surface through one or more of the formation of a boundary layer as a result of fluid flow and the liquefaction of non-yielded solid material by vibrational agitation.

In some cases, aspects of the invention can be performed using a conventional viscometer design, such as rotational viscometers including rotational viscometers of the cone and plate type and the rotational cylinder type. Other conventional viscometer designs with which aspects of the invention may be performed include differential pressure viscometers and falling-ball-type viscometers. However, compared with traditional approaches to measuring yield stress, aspects of the invention may have particular application for the surface-loaded attributes of resonant viscometer transducers, especially such types which work by the formation and dissipation of a pure shear wave.

Compared with traditional approaches to measuring yield stress that for the most part require a captive static sample of fluid, aspects of the invention may have particular application in the continuous rheological measurement of materials in the flowing condition, while also being capable of use with static materials.

In particular, aspects of the invention may have application in the monitoring of yield stress or other fluid properties in-line in a process in which the yield stress and its variation is a value of interest, and further substances may be added in greater or lesser amounts according to the estimated yield stress of the fluid and any particular desired yield stress or other fluid property of the yield stress fluid.

