The viscosity of pseudo-plastic fluids

Abstract

In a previous paper [5] the authors have discussed the development of a correlation for the prediction of heat transfer coefficients of pseudo-plastic fluids. For the application of the correlation to any particular problem, it is necessary to have viscosity data over the range of conditions involved. The present paper discusses the method employed for obtaining the viscosity data used in the correlation of heat transfer coefficients. Three plastic (pseudo-) fluids were investigated covering a range of viscosities from one to thirteen centipoises. The pseudo-plastic liquids used were the aqueous solution of sodium carboxymethyl cellulose, polyvinyl alcohol, and sodium polymethacrylate. The modified Ostwald viscometer was designed and used in this work. Previous workers have used either pipe viscometers or the rotating cylinder type, both of which are cumbersome and expensive to install. The modified Ostwald viscometer maintains the condition of tubular flow and conducts the measurement at very low rates of shear. Another advantage of the modified Ostwald viscometer is that it enables viscosity measurements to be made at point values of the pressure drop across the capillary tube. This is important for pseudo-plastic fluids when the viscosity is dependent upon the rate of flow. The results of viscosity measurement can be directly applicable to the design of heat exchangers and pressure drop calculations for the specification of the pump handling the pseudoplastic fluids. It is probable that a similar treatment can be applied to those fluids exhibiting general non-Newtonian behaviour.