Fabrication and Validation of Rotational Viscometer

Abstract

An efficient rotational viscometer capable of determining viscosity of Non-Newtonian fluids has been developed and the design of this viscometer is described in detail in this paper. The Equations to find the viscosity of fluids is described in this paper. Viscosity of a Non-Newtonian fluid (ketchup) is found. The instrument is calibrated using standard fluids and a correction coefficient is obtained. An efficient method to find the viscosity of Non-Newtonian fluids is introduce in this paper.

Introduction

I. INTRODUCTION

This paper reviews the theory of co-axial concentric rotational viscometer. Viscosity is one of the important properties that must be dealt with in many liquid flow problems. Efficient method are developed for measuring the viscosity of liquids. These methods generally have been developed around Newton’s law of viscosity and it’s assumptions that viscosity is independent of rate of shear. The viscosity of Newtonian fluids are easy to calculate as there are many instruments which provide direct value of viscosity. The viscosity of Non Newtonian fluids is difficult to determine as the shear rate is not proportional to the strain rate i.e. they do not follow newton’s law of viscosity. In nature the fluids found are mainly non Newtonian fluids and therefore it becomes very essential to find their viscosity.

The interest to design and fabricate a rotational viscometer with locally available materials that could be used in the measurement of viscosity of different fluids were due to the listed reasons below:

1. Unavailability in the local market.
2. High cost for the instrument.

A coaxial cylindrical rotational viscometer works on the principle that a solid (inner) cylinder is rotated inside a hollow cylinder which contains test sample of liquid. The torque experienced by the solid cylinder due to viscous effect of the test sample is measured and hence viscosity can be calculated. Rotational viscometers measure the amount of torque needed to rotate an object moving through fluid at a known RPM. Using the measured torque, RPM, and dimensions of the device, the viscosity can be calculated [2]. To find the viscosity of the fluids one must obtain torque-angular velocity data.

 By rotating the spindle or inner cylinder at different speeds, shear dependent behavior can be analyzed, which are applicable to measure the viscosity of various fluids such as engine oil, water, kerosene, petroleum oil, paint, printing ink.

II. METHODOLOGY

A. Equation

An equation for viscosity measurement for a sample has been developed.

Viscosity = (Shear stress)/ (shear rate)

Shear stress = force/ (area in contact with fluid)

The torque is offered by curved surface area and the base circular area of inner cylinder.

B. Design of the components

The main parts of a co-axial concentric rotational viscometer are an outer cylinder (hollow), an inner cylinder (solid) and a motor. The other parts are a base and a driving shaft for inner cylinder. The diameter of the inner cylinder is 50mm and the height is 20 mm. Due to its low inertia, wider temperature range and non-reactivity with the tested liquid aluminum is chosen for inner cylinder. In case of outer cylinder, stainless steel is selected as the material for the outer cylinder.

C. Design parameters

There are three general categories for rotational viscometer based on their design configuration 1.Coaxial-Cylinder Viscometer. Cone and Plate Viscometer 3. Conic-Cylinder Viscometer. The major disadvantages of third and second type are the involvement of complexity of construction and more friction. In this project the inner cylinder is rotating and outer cylinder remains stationary. The gap between the inner cylinder and Outer cylinder plays an important role for obtaining torque. More accurate measurements are obtained if the gap between the inner cylinder and outer cylinder is reduced. In our instrument there is a gap of 50 mm between the outer and inner cylinder [2]. The length of the shaft should be small to avoid off-axial rotation of the cylinder. For proper rotation of the cylinder along its axis a bearing was fixed at half length of the shaft.

Conclusion

From the table and graph it is clear that the viscosity of non-newtonian fluids like ketchup can be measured. Shear thinning can be seen from the above graph i.e. the viscosity increases with the shear strain rate. The ketchup has a tendency to show shear thinning [3]. Thus the viscometer correctly shows the physics of fluid flow of ketchup. Thus the viscometer successfully measures viscosity of fluids.

References

[1] Malcolm M. Cross, “Rheology of Non-Newtonian fluids: A new flow equation for pseudo plastic systems”, Journal of colloid science 20, 417-437 (1965). [2] Brian Cherrington and Jack Rothstein, “Building and validating a Rotational Viscometer” [3] Marco Berta, Johan Wiklund, Reinhardt Kotze, Mats Stading, “Correlation between in-line measurements of tomato ketchup shear viscosity and extensional viscosity”, Journal of Food Engineering 173 (2016) 8-14

Copyright

Copyright © 2022 Mayank Udayakumar Shetty, Dhananjay Vijay Kapse. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.