Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs Shear-flow rheology and viscoelastic instabilities of ethanol gel fuels Purushothaman Nandagopalan, Jerin John, Seung Wook Baek ⁎ , Ankur Miglani, Kurniawan Ardhianto Department of Aerospace Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ARTICLE INFO Keywords: Ethanol gel fuel Shear-thinning fluid Viscoelastic instability Normal stress Yield stress fluid Polymeric 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-clas- sified 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 β , 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. 1. Introduction Previous research on polymer-based materials were primarily driven by the need for making them suitable for several applications ranging from food technology, where they can be utilized efficiently in the form of stabilizing agents and preservative coatings [1] to phar- maceutical drug delivery purposes [2–4]. Recently, another critical area that has attracted the attention is the utilization of polymer-additives to the fuel for rocket propulsion systems. However, the polymer-based fuel for the rocket engine that yields comparatively high performance re- quires a proper formulation methodology followed by the rigorous flow and rheological characterization [5]. These polymeric gellant based fuels or gel fuels were obtained by adding gelling agents to the base fuel, which changed the phase of the fuels from liquid to semi-solid (high viscous, solid like material) [6,7]. Commonly, the gel fuels pos- sess the enhanced rheo-physical property and contain the combined advantages of both liquid and solid rocket fuels. For example, at low or negligible applied shear force, the gel fuels behave like a solid or semi- solid, which allows easy handling and leakage-proof storage. Ad- ditionally, the gels in the semi-solid state are considered as the stable state at which the agglomeration of the gellants, aggregation and phase separation can be prevented. Conversely, a liquid-like behaviour at high shear enables easy pumping of the fuel at demand, thereby allowing thrust control and re-ignitability in rocket engines [8]. In this light, an initial step of the present study is to formulate organic gellant (HPMC: Hydroxypropyl methylcellulose) based ethanol gel (EG) fuels and characterize the shear rheological behaviour of EG using cone-plate rheometer (CPR). Prior studies on gel fuel rheology using the conventional rheometers and elongational viscometers investigated the effects of viscosity as a flow governing parameter. Rahimi et al. [8] explained how the exten- sional viscosity played a key role in governing the elastic nature of the viscoelastic gels, which is crucial in the spray process in controlling the breakup of the droplets. In addition, the effective concentration of the gelling agent, i.e., the gellant loading rate (GLR) was found to affect the rheological nature of the gel fuels. Yarin et al. [9] focused on the rheological elongational property of the single gel simulant droplet, which forms the sub-grid unit of spray. This study investigated the significance of viscosity in the process of droplet formation, detach- ment, and droplet breakup. Furthermore, it was concluded that the constitutive power law of the uniaxial elongational flow was analogous to the shear-thinning behaviour exhibited by the simple shear flow. Rahimi et al. [8] and Natan [10] carried out a detailed rheological characterization for large number of gel propellants ranging from https://doi.org/10.1016/j.expthermflusci.2018.07.024 Received 19 October 2017; Received in revised form 11 May 2018; Accepted 20 July 2018 ⁎ Corresponding author. E-mail address: swbaek@kaist.ac.kr (S.W. Baek). Experimental Thermal and Fluid Science 99 (2018) 181–189 Available online 21 July 2018 0894-1777/ © 2018 Elsevier Inc. All rights reserved. T