Abstract
We identify the physical mechanism through which newly developed quaternary ammonium salt (QAS) deposit control additives (DCAs) affect the rheological properties of cavitating turbulent flows, resulting in an increase in the volumetric efficiency of clean injectors fuelled with diesel or biodiesel fuels. Quaternary ammonium surfactants with appropriate counterions can be very effective in reducing the turbulent drag in aqueous solutions, however, less is known about the effect of such surfactants in oil-based solvents or in cavitating flow conditions. Small-angle neutron scattering (SANS) investigations show that in traditional DCA fuel compositions only reverse spherical micelles form, whereas reverse cylindrical micelles are detected by blending the fuel with the QAS additive. Moreover, experiments utilising X-ray micro computed tomography (micro-CT) in nozzle replicas, quantify that in cavitation regions the liquid fraction is increased in the presence of the QAS additive. Furthermore, high-flux X-ray phase contrast imaging (XPCI) measurements identify a flow stabilization effect in the region of vortex cavitation by the QAS additive. The effect of the formation of cylindrical micelles is reproduced with computational fluid dynamics (CFD) simulations by including viscoelastic characteristics for the flow. It is demonstrated that viscoelasticity can reduce turbulence and suppress cavitation, and subsequently increase the injector’s volumetric efficiency.
Highlights
Impurities in the fuel composition may lead to formation of deposit layers on injector parts, altering its internal geometry and significantly blocking the fuel flow
Surfactants can self-assemble to form a range of structures and there are many examples in aqueous solutions where the formation of elongated wormlike micelles can be promoted by the addition of salts or cosurfactants[21,22,23]
This study aims to provide an understanding about the effect of quaternary ammonium salts in deposit control additives on injector flow conditions and to suggest an explanation for the flowrate enhancement mechanism through the fuel injection systems
Summary
Impurities in the fuel composition may lead to formation of deposit layers on injector parts, altering its internal geometry and significantly blocking the fuel flow. Such effects degrade the injector’s performance, reduce the atomisation quality and result in excess and uncontrollable emissions, regardless of the legislation limits the engine met when new[2]. Polymers absorb elastic energy from the turbulent eddies and can transfer this energy back into large scales, resulting in truncation of the turbulent energy cascade[8] This mechanism effectively can supress the wall-normal turbulence while increasing the streamwise velocity and the mass flowrate. Shear-induced-structures are dynamic networks of entangled micelles that appear above a critical shear-rate and induce viscoelastic properties in the solution[14]
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