Experimental study of rheology and impact behaviour of an ethanol gel droplet

Abstract

The present work focusses at the three major contributions to the ethanol gel propellants, namely, synthesis, rheological characterization and drop impact dynamics. This study is of particular importance in the droplet-engine wall interactions, which is promising aspect for the droplet dynamics under different conditions such as shear flow, impact behaviour, and heated substrate. In the synthesis of the ethanol gels (EG), an organic polymer Hydroxypropyl methylcellulose (HPMC) is used as a gelling agent. Synthesized gel fuels possess the maximum content of 85wt.% Synthesized EG fuels are subjected to rheology for understanding the shear sustained mechanisms. During the shear process, EG fuels exhibit the shear- thinning characteristics over the various shear rates and possess the definite yield stress values. The work proposes the sub-classification of the gel fuels based on the consistency and elasticity. The study shows the elastic effect in negligible shear regimes through the modified shear-thinning diameter. We find that the existence of secondary Normal stress stabilize the flow and the ratio of elastic Normal stresses are in good agreement with the existing molecular based theories. Drop Dynamics is found out through the impacting experiments on the targeted surface. We found that the spreading depend on the impacting Weber number ($W_e$), Weissenberg number ($W_i$) and Reynolds number ($R_e$). The initial spreading mechanism is modelled using the spring-dashpot system. We develop the new model based on $ \upsilon - \zata $ parameters that depend on elastic, shear stress and flow stress. It is observed that the high polymer concentration demonstrates the higher viscoelastic rheometric properties in which the splashing is very difficult to occur. The viscoelastic assisted lamella is quickly retracted by the elastic pull. Semi- empirical scaling agrees well in the lower impacting regimes, whereas the larger deviation occurs in the higher impacting conditions owing to the fact that rheology plays a key role. DI study on heated substrate shows that the spreading can be enhanced by heating the substrate to the higher temperatures. At low temperatures, the droplets do not bounce or recede and the deposition is the dominant phenomenon. In higher temperatures, at the low impacting conditions deposition occurs whereas in the higher impacting conditions spreading, bouncing happens. The distinct phenomena such as elastic recoiling, thermal footprint of the droplets, bouncing and somersaulting happen only in the high temperature conditions. This study briefly addresses the gel droplet dynamics subjected to various conditions and provides the insight into the gel fuel applications.