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Free, publicly-accessible full text available June 1, 2025
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In the pursuit of enhanced engine performance and reduced emissions, the design of liquid-fueled propulsion systems is shifting towards much higher combustor pressures, surpassing the nominal critical pressure of the fuel and air. This trend leads to the adoption of supercritical conditions, wherein the liquid fuel is injected into the ambient air at supercritical pressure and temperature, causing the fuel temperature to exceed its nominal critical point. This transition from a liquid-like to a gas-like behavior, known as "transcritical behavior," is a crucial aspect governing the operation of modern high-pressure propulsion and energy conversion systems. In these systems, the primary liquid jet breakup and the subsequent break-up of the resulting droplets into smaller droplets, namely secondary breakup, significantly impact mixing and combustion processes. Despite its importance, there has been a limited focus on droplet breakup at supercritical conditions, particularly at higher flow speeds relevant to high-speed liquid-fuel propulsion systems. Surface tension effects are often neglected in the simulation of transcritical flow, assuming surface tension vanishes beyond the critical point, while recent experiments and molecular dynamics simulations suggest that surface tension effects persist at transcritical conditions. To gain insight into the effects of surface tension on transcritical flows, we have developed a fully compressible multiphaseDirect Numerical Simulation (DNS) approach that accounts for decaying surface effects. The diffuse interface method is employed to represent transcritical interfaces, accounting for surface tension effects calculated using molecular dynamics simulations. This approach is employed to investigate the behavior of subcritical n-dodecane droplets in a supercritical nitrogen environment interacting with a shockwave, aiming to identify the governing breakup regimes at transcritical conditions. The development of quantitative measures enables the generalization of droplet breakup modes for transcritical droplets. The insights gained from this study contribute to advancing the understanding of transcritical liquid breakup, providing valuable knowledge for designing and optimizing high-speed propulsion systemsmore » « less
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The phase transition from subcritical to supercritical conditions, referred to as transcritical behavior, significantly impacts the evaporation and fuel–air mixing in high-pressure liquid-fuel propulsion systems. Transcritical behavior is characterized as a transition from classical two-phase evaporation to a single-phase gas-like diffusion regime as surface tension and latent heat of vaporization reduce. However, the interfacial behavior represented by the surface tension coefficient and evaporation rate during this transition which are crucial inputs for Computational Fluid Dynamics (CFD) simulations of practical transcritical fuel spray is still missing. This study aims at developing new evaporation rate and surface tension models for transcritical n-dodecane droplets using molecular dynamics (MD) simulations irrespective of the droplet size. As MD simulations are primarily limited to the nanoscale, the new models can bridge the gap between MD and continuum simulations and enable the direct application of these findings to microscopic droplets. A new characteristic timescale, i.e., “undroplet time,” is defined which marks the transition from classical two-phase evaporation to single-phase gas-like diffusion behavior. The undroplet time indicates the onset of droplet core disintegration and penetration of nitrogen molecules into the droplet, which occurs after the vanishment of the surface tension. By normalizing the time with respect to the undroplet time, the rate of surface tension decay, evaporation rate, and the rate of droplet mass depletion become independent of the droplet size. Calculation of pairwise correlation coefficients for the entire MD results shows that both surface tension coefficient and evaporation rate are strongly correlated with the background temperature, while pressure and droplet size play a less significant role past the critical point. Therefore, new models for surface tension coefficient and evaporation rate spanning from sub- to supercritical conditions are developed as a function of background pressure and temperature, which can be used in continuum simulations. The identified phase change behavior based on the undroplet time shows a good agreement with the phase change regime maps obtained using microscale experiments and nanoscale MD predictions.more » « less
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Abstract Despite being a pillar of high‐performance materials in industry, manufacturing carbon fiber composites with simultaneously enhanced multifunctionality and structural properties has remained elusive due to the lack of practical bottom‐up approaches with control over nanoscale interactions. Guided by the droplet's internal currents and amphiphilicity of nanomaterials, herein, a programmable spray coating is introduced for the deposition of multiple nanomaterials with tailorable patterns in composite. It is shown that such patterns regulate the formation of interfaces, damage containment, and electrical‐thermal conductivity of the composites, which is absent in conventional manufacturing that primarily rely on incorporating nanomaterials to achieve specific functionalities. Molecular dynamics simulations show that increasing the hydrophilicity of the hybrid nanomaterials, which is synchronous with shifting patterns from disk to ring, improves the interactions between the carbon surfaces and epoxy at the interfaces,manifested in enhanced interlaminar and flexural performance. Transitioning from ring to disk creates a larger interconnected network leading to improved thermal and electrical properties without penalty in mechanical properties. This novel approach introduces a new design , where the mechanical and multifunctional performance is controlled by the shape of the deposited patterns, thus eliminating the trade‐off between properties that are considered paradoxical in today's manufacturing of hierarchical composites.
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Depositing carbon nanotubes (CNTs) into carbon fiber reinforced polymer composites (CFRPs) is challenging because of the need for complicated lab-scale processes and toxic chemical dispersants that makes conventional means of processing less compatible with existing industrial procedures for large-scale applications. In this work, a scalable supercritical CO2-assisted atomization technique is used to effectively deposit hybrid CNTs in CFRPs allowing them to boost their functionality and tailor the microstructure. Cellulose nanocrystals (CNCs) are utilized to create hybrid nanostructures with CNTs (CNC bonded CNT) that enables stabilization of CNTs in nontoxic media, i.e., water, and this promotes the scalability of the process. According to Zeta potential values, CNCs successfully stabilize CNTs in water suspension. Scanning electron microscopy (SEM) micrographs show hybrid CNC bonded CNTs are homogeneously dispersed on the carbon fiber surface. According to the in-situ bending test under the optical microscope, crack propagation is hindered by engineered hybrid CNT nanostructures in the modified CFRP whereas neat CFRP exhibits low crack growth resistance due to the uninterrupted crack propagation in the continuous epoxy matrix. Our results imply that this strategy probes the importance of new controlled manufacturing of hybrid nanostructures through evaporation‑induced self‑assembly of nanocolloidal droplets, and allows for tailoring of the desired properties of nanostructured composites.
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Abstract Achieving desired performance from self‐assembly of nanoparticles (NPs) is challenging due to the stochastic nature of interactions among the constituent building blocks. Self‐assembly of nano‐colloids through evaporation of particle‐laden droplets can be exploited to fabricate tailored nanostructures that add functionality and engineer the properties of manufactured components. The particle–particle and particle–solvent interactions, and delicate force balance among them are the main factors that define the pattern of the final 3D nanostructure. Here, a nanoparticle‐agnostic approach that allows the fabrication of nanostructures with precisely engineered patterns is introduced. Evaporative droplets of aqueous suspensions of pristine Carbon Nanotubes, Graphene Nanoplatelets, and Boron Nitride Nanotubes representing NPs of different elemental compositions, sizes, and shapes are investigated. Cellulose nanocrystals (CNCs) are used as a platform to make hybrid systems of CNC‐NP and utilize the repulsive‐attractive‐directional interactions in these multimaterial systems to enforce the desired final pattern between ring and disk. It is shown that irrespective of the type of NPs, the amphiphilicity of the hybrid system dictates the formation of deposited patterns. Finally, the effect of self‐assembled patterns on the functionality of multi‐material systems is demonstrated. The proposed method creates new capabilities in the precisely controlled nanostructures and facilitates smart self‐assembly systems.