Computational Dynamics of a Thermally Decomposable Viscoelastic Lubricant Under Shear

Abstract

The effect of viscoelasticity on the thermodynamic performance of a thermally decomposable lubricant subjected to shear and Arrhenius kinetics is investigated with direct numerical simulations. A numerical algorithm based on the finite difference method is implemented in time and space with the Oldroyd-B constitutive equation as the model for the viscoelastic liquids. We report enhanced efficiency in the case of a polymeric lubricant as compared with the purely viscous lubricant. In particular, it is demonstrated that the use of polymeric liquids helps to delay the onset of thermal runaway as compared with progressively Newtonian liquids.

Analysis of Entropy Generation Rate in an Unsteady Porous Channel Flow with Navier Slip and Convective Cooling

“…For ξ = 1, these are first-order accurate in time but second order in space. The schemes in Chinyoka [19] have ξ = 1/2, which improves the accuracy in time to second order. As in Chinyoka [8,9,20] we, however, use ξ = 1 here so that we are free to choose larger time steps and still converge to the steady solutions.…”

“…Our numerical algorithm is based on the semi-implicit finite difference scheme [8,9,[17][18][19][20]. Implicit terms are taken at the intermediate time level…”

Effects of Fluid Viscoelasticity in Non-Isothermal Flows

“…In either case the fluid viscosity will be considered temperature dependent and our investigations will focus on the fluids' heat transfer characteristics in simple flows. As in Chinyoka (2008;2009a;b;2010;2011) the viscoelastic fluid behavior is compared to that for corresponding inelastic (Newtonian and/or Generalized Newtonian) fluids and it is demonstrated that depending on the physical application, viscoelasticity may or may not be favorable. For a comprehensive overview of non-Newtonian flows in general and viscoelastic fluid phenomena in particular, we refer to the excellent treatises of Bird et al (1987);Ferry (1981).…”

“…Our current investigations with Johnson-Segalman fluids show that the answer is negative. The viscoelastic models used in Chinyoka (2008;2009a;b;2010;2011) exhibit a monotonic shear stress/shear rate relationship in simple shear flows. The Johnson-Segalman fluid, allows for a non-monotonic relationship between the shear stress and rate of shear.…”

“…Similarly it would be important to understand and be able to anticipate and hopefully delay the onset of such thermal runaway phenomena in the pipeline transport of reactive petroleum products in order that the end-products remain usable. Investigations into reactive lubricants subjected to shear flow were carried out, say, in Chinyoka (2008) where it was demonstrated that viscoelastic (Oldroyd-B) liquids can withstand higher values of the reaction parameter without undergoing thermal runaway as opposed to corresponding Newtonian lubricants. A similar investigation in Chinyoka (2010) also showed that the linear Phan-Thien-Tanner fluids also display better thermal loading properties compared to corresponding Newtonian fluids.…”