In general, rheological properties of gelled fuels change dramatically when temperature changes. In this work, we investigate flow and heat transfer of water-gel in a straight pipe and a tapered injector for non-isothermal conditions, which mimic the situations when gelled fuels are used in propulsion systems. The gel-fluid is modeled as a non-Newtonian fluid, where the viscosity depends on the shear rate and the temperature; a correlation fitted with experimental data is used. For the fully developed flow in a straight pipe with heating, the mean apparent viscosity at the cross section when the temperature is high is only 44% of the case with low temperature; this indicates that it is feasible to control the viscosity of gel fuel by proper thermal design of pipes. For the flow in the typical tapered injector, larger temperature gradients along the radial direction results in a more obvious plug flow; that is, when the fuel is heated the viscosity near the wall is significantly reduced, but the effect is not obvious in the area far away from the wall. Therefore, for the case of the tapered injector, as the temperature of the heating wall increases, the mean apparent viscosity at the outlet decreases first and increases then due to the high viscosity plug formed near the channel center, which encourages further proper design of the injector in future. Furthermore, the layer of low viscosity near the walls plays a role similar to lubrication, thus the supply pressure of the transport system is significantly reduced; the pressure drop for high temperature is only 62% of that of low temperature. It should be noticed that for a propellent system the heating source is almost free; therefore, by introducing a proper thermal design of the transport system, the viscosity of the gelled fuel can be greatly reduced, thus reducing the power input to the supply pressure at a lower cost.

Gelled fuels combine the main advantages of liquid fuels (throttle ability) and solid fuels (easy handling, etc.) due to their non-Newtonian characteristics. In this paper, we study the flow characteristics of water-hydroxypropylcellulose (HPC) gel in converging tubes and tapered injectors which mimic the flow and injection of kerosene gel in typical geometries of propulsion systems. The water-HPC gel is modeled as a non-linear fluid, where the shear viscosity is assumed to depend on the local shear rate and modeled by the Carreau–Yasuda model; the model parameters are fitted with our experimental measurements done by a rotational rheometer. The numerical simulations indicate that for the converging tubes, increasing the convergence angle, causes the mean apparent viscosity at tube exit to decrease while the mass flow rate reduces at a constant pressure drop. Therefore, there is a balance between the lowering of the pressure loss and reducing mean apparent viscosity. In the tapered injectors, the straight pipe after the converging part has a detrimental effect on the viscosity reduction.

The rheological behavior of gel propellants is crucial for their practical applications, especially in the rocket engine and ramjet fields. The thixotropic characteristics of gel propellants are an important component of their rheological properties and have a notable impact on their flow and injection process. However, most gel propellants contain rich, dynamic cross-linked network structures, which impart complex non-Newtonian fluid properties, and it is difficult to establish a unified mathematical model. In view of this, this study addresses the thixotropy of a prepared RP-3 kerosene gel and determines the mathematical model and model parameters describing its thixotropy. Experiments show that the kerosene gel exhibits shear-thinning properties as well as thixotropy. To describe the microstructural changes in the gel, three thixotropic constitutive models are introduced to analyze the rheological data, and the constitutive equation parameters are optimized. The three models are all structural dynamic models, which can be used to describe microstructural changes within the material. In addition, the fitting of the constitutive equation is a multiparameter optimization problem, and an appropriate optimization method must be used for parameter fitting. Therefore, the Bayesian optimization method combined with Gaussian process regression and the upper confidence bound (UCB) acquisition function is used in the multiparameter fitting of the constitutive models. Both experiments and numerical results show that the thixotropic model, which introduces a pre-factor with shear strain and assumes that the breakdown of the gel structure is related to energy dissipation rather than the shear rate, has a better fitting effect and prediction ability with regard to the gel. Combined with transient experiments at different shear rates, the model parameters of the constitutive law can be determined quickly by applying the Bayesian optimization method.

Many types of dense suspensions are complex materials exhibiting both solid-like and fluid-like behavior. These suspensions are usually considered to behave as non-Newtonian fluids and the rheological characteristics such as yield stress, thixotropy and shear-thinning/thickening can have significant impact on the flow and the engineering applications of these materials. Therefore, it is important to understand the rheological features of these fluids. In this paper, we study the flow of a nonlinear fluid which exhibits yield stress and shear-thinning effects. The geometries of interests are a straight channel, a channel with a crevice and a pipe with a contraction; we assume the fluid behaves as a Herschel-Bulkley fluid. The numerical simulations indicate that for flows with low Reynolds number and high Bingham number an unyielded plug may form in the center of the channel. In the case of a channel with a crevice, the fluid in the deep portion of the crevice is at an extremely high level of viscosity, forming a plug which is hard to yield. For the pipe with a contraction, near the pipe neck the unyielded region is smaller due to the enhanced flow disturbance.

Owing to their unique non-Newtonian characteristics, gel propellants have become a new type of fuel with promising applications in the aerospace industry. However, the complex rheological properties and high viscosity of gels pose significant challenges for flow control in gel engines, particularly in terms of fuel atomization and combustion efficiency. Focusing primarily on kerosene gel, this study investigates the secondary atomization characteristics of shear-thinning droplets, where the volume of fluid method is employed to capture interface dynamics on adaptive grids, and the viscosity of the gel is described by a power-law model. The accuracy of the numerical model is validated by comparing it with the experimental observations of the breakup process of 1% silica kerosene gel droplets. Numerical simulations are conducted to analyze the droplet breakup processes and formation mechanisms under bag, multimode, and shear breakup. Compared to kerosene, the apparent viscosity distribution inside the kerosene gel droplets is non-uniform, and the inhibition of viscous force delays the breakup time and reduces the droplet deformation, thereby affecting the transition of the breakup mode. By varying the Weber number, We, the rheological parameters, and the density ratio, the quantitative effects of these factors on droplet deformation, centroid velocity, average viscosity, the breakup time, etc. are revealed. The results indicate that the breakup becomes more severe with increasing We number, resulting in larger deformation and a greater number of smaller droplets; the rheological parameters significantly affect the droplet breakup by altering the fluid viscosity; moreover, under the same flow condition, the higher the density ratio, the more difficult it is for the droplet to break up; however, on dimensionless scales, the effect of density ratio appears weak. Additionally, a breakup regime diagram is constructed in the We–Oh (Ohnesorge) number space for low Weber numbers. This study validates the numerical method for simulating the secondary atomization of kerosene gel propellants, providing a thorough analysis of three breakup mechanisms and the impact of key parameters on kerosene gel droplets. These insights offer a theoretical foundation for the optimization of gel propellants, aiding in the development of efficient and stable propulsion systems.

To investigate the internal flow characteristics, pintle load, and performance of a pintle-controlled variable-thrust solid rocket motor, three-dimensional simulations with an overset grid method are conducted for different pintle geometries. The transient response during pintle motion is also analyzed. Results show that, as the pintle moves downstream, the equivalent throat area decreases and flow non-uniformity is intensified, with an oblique shock forming at the pintle head. Pintle geometry significantly affects shock structures and load distribution, while configurations with lower radial velocity exhibit higher specific impulse efficiency and better load reduction. Although geometric parameters have a limited impact on overall performance, the linearity of throat area variation governs flow stability. At high speeds, the pressure response shows a noticeable lag compared to pintle motion.