Shear-flow rheology and viscoelastic instabilities of ethanol gel fuels

Abstract

This study presents a rheological investigation to understand the instability mechanism in newly formulated ethanol gel (EG) fuels when subjected to simple shear flow in a cone-plate rheometer (CPR). Shear-thinning EG fuels are formulated using an organic gellant, Hydroxypropyl methylcellulose (HPMC). EG fuels are sub-classified into concentrated, semi-dilute or dilute gels based on the consistency and elasticity number (El) that describes an interplay between the elastic effects and the viscous effects. The extent of shear-thinning effect in the zero-shear regime is found out by comparing shear-thinning parameter beta, with an elastic parameter . The relation proves the existence of minimal elastic component even in the dilute gels. Ratio of normal stress to shear stress variation with Re, shows the existence of critical Re only for the semi-dilute and dilute gels (EG1-EG4) that marks the onset of elastic instability, whereas this transition is absent in concentrated EG5. During the shear process, two distinct phenomena are identified. First, in the low Wi regime, the presence of Normal stress N-1 induces the secondary flow based elastic instability, which is counterbalanced by the presence of another Normal stress N-2 to stabilize the flow. Secondly, in the high Wi regime, the polymer shows the maximum extension beyond which polymer breaks down with increase in the applied shear rate.