Scope of This Item Some examples of fluids that may exhibit non-Newtonian behaviour are listed in Table 2.1. They include polymer solutions and melts having high molecular weights@ suspensions of solids in liquids (which may be undissolved polymer particles)@ and emulsions of droplets of a denser or lighter liquid in a second immiscible liquid medium. The identification and classification of the fluids is carried out from two points of view: firstly in terms of the phase condition (Section 3.2.1) and secondly in terms of the rheological type (Section 3.2.2). Fluids are classified according to the phase condition as either homogeneous or pseudohomogeneous or heterogeneous. This Data Item is applicable to fluids that can be classified as homogeneous or pseudohomogeneous. The behaviour of such fluids can be complicated as they can show both Newtonian and non-Newtonian characteristics under different conditions of composition@ temperature and shear rate (see Section 3.1). It is difficult to predict whether a particular fluid will exhibit non-Newtonian behaviour under a given set of conditions@ and it must be recognised that a rheological test is the only sure method of ascertaining the rheological properties of a fluid. The layout of this Data Item is as follows. ? Section 3 summarises the classification and behaviour of Newtonian and non-Newtonian fluids. ? Section 4 introduces the concepts of viscosity and the flow curve and discusses the flow behaviour exhibited by fluids under steady-state shear. ? Section 5 outlines various flow curve models used for characterising the flow behaviour exhibited by fluids under steady-state shear. ? Section 6 summarises five types of viscometric geometry used for flow curve determination of non-Newtonian fluids. These are described in more detail in ESDU 9501222. ? Appendix A gives a glossary of rheological terms. Purpose and Scope of ESDU Data Items on Non-Newtonian Fluids Knowledge of fluid rheology is important in the design of process equipment such as pipelines@ pumps@ extruders@ mixers@ coaters and heat exchangers. Rheological behaviour can be divided into Newtonian and non-Newtonian categories and most fluids show non-Newtonian characteristics over at least part of the shear rate range that can be achieved in practice. There are many different forms of non-Newtonian behaviour@ and each of these forms has to be considered separately in the design process. In general@ fluids behave as Newtonian if they are composed of low molecular weight material (whether as a pure liquid or in the form of single-component molecular solutions)@ or when the temperature is well above the melting point. Newtonian behaviour is characterised by viscosity that is constant with respect to shear rate (see Section 3.1). Design data for process equipment carrying Newtonian fluids are well developed. For example@ data for pipeline systems carrying Newtonian fluids are provided in Volumes 1-4 of the ESDU Fluid Mechanics@ Internal Flow Sub-series. This Data Item@ the first in a group on the flow characteristics of non-Newtonian fluids@ provides an introductory guide to their identification@ classification and characterisation@ including measurement methods. A second Item in the group@ ESDU 95012@ provides more detailed guidance on obtaining the viscometric or flow curve data necessary for estimating frictional pressure drop for pipeflow of non-Newtonian fluids. ESDU 95012 also provides guidance on the selection and use of the different viscometric geometries available@ and advises on obtaining the viscometric data necessary for the generation of the flow curve. A third Item in the group@ ESDU 91025@ provides methods and computer programs for the estimation@ using the flow curve and by scale-up of small-scale pipeline flow data@ of frictional pressure drop in fully-developed@ isothermal flow in straight pipes of uniform circular cross section.