Experimental Study of Diesel-Fuel Droplet Impact on a Similarly Sized Polished Spherical Heated Solid Particle

The head-to-head impact of diesel-fuel droplets on a polished spherical brass target has been investigated experimentally. High-speed imaging was employed to visualize the impact process for wall surface temperatures and Weber and Reynolds numbers in the ranges of 140–340 °C, 30–850, and 210–1135, respectively. The thermohydrodynamic outcome regimes occurring for the aforementioned ranges of parameters were mapped on a We–T diagram. Seven clearly distinguishable postimpact outcome regimes were identified, which are conventionally called the coating, splash, rebound, breakup–rebound, splash–breakup–coating, breakup–coating, and splash–breakup–rebound regimes. In addition, the effects of the Weber number and surface temperature on the wettability dynamics were examined; the temporal variations of the dynamic contact angle, dimensionless spreading diameter, and liquid film thickness forming on the solid particle were measured and are reported

Numerical Study of Cloud-Sized Droplet Impact and Freezing on Superhydrophobic Surfaces

In-flight icing is a serious meteorological hazard caused by supercooled cloud particles (with an average size of 20–50 µm) that turn into ice as an immediate consequence of impact with an aircraft, and it poses a serious risk to the safety of the aircraft and its passengers. Anti-icing surface treatment is a potential solution to mitigate ice accretion and maintain optimal flying conditions. Superhydrophobic coatings inspired by nature (e.g., lotus leaf) have attracted much attention in recent years due to their excellent water repellent properties. These coatings have been extensively applied on various substrates for self-cleaning, anti-fogging, and anti-corrosive applications. The performance of these coatings depends on the chemical composition and their rough hierarchical surface morphology composed of micron and sub-micron-sized structures. Recently, there has been an increased interest to fabricate superhydrophobic coatings that can repel droplets of cloud-relevant sizes (20–50 µm) before they freeze to the surface in practical flight conditions (i.e., icephobic surfaces). The main goal of this work was to numerically model the hydrodynamic and thermal behaviour of cloud-sized droplets on superhydrophobic surfaces when interacting with micron-sized surface features. Consequently, by correlating the hydrophobicity and the icephobicity of the surface, we found viable solutions to counteract icing and to prevent ice accumulation on critical aerodynamic surfaces. For this purpose, we developed a computational model to analyze the hydrodynamics of the impact of the micro-droplet on a micro-structured superhydrophobic surface under room temperature and freezing (including rapid-cooling and supercooling) conditions. All coding and implementations were carried out in the OpenFOAM platform, which is a collection of open-source C++ libraries for computational continuum mechanics and CFD analysis. Superhydrophobic surfaces were directly modelled as a series of fine, micro-structured arrays with defined cross sections and patterns. Surface chemistry was included in the simulations using a dynamic contact angle model that describes well the hydrodynamics of a micro-droplet on rough surfaces. A multi-region transient solver for incompressible, laminar, multi-phase flow of non-isothermal, non-Newtonian fluids with conjugate heat transfer boundary conditions between solid and fluid regions was developed to simulate both the dynamics of the micro-droplet impact on the substrate and the associated heat transfer inside the droplet and the solid bulk simultaneously. In addition, a phase change (freezing) model was added to capture the onset of ice formation and freezing front of the liquid micro-droplet. The computational model was validated using experimental data reported in the literature. In addition, an analytical model was derived using the balance of energy before impact and at the maximum spreading stage, which we found to be in good agreement with the data obtained from simulations. Since aluminum (Al) is the base material used in aerospace industries, the thermo-physical properties of aluminum were extensively used in our simulations. Comparing laser-patterned aluminum substrates with a ceramic base composite material that has a low thermal diffusivity (such as titanium-dioxide), we showed that the onset of icing was significantly delayed on the ceramic-based substrate, as the droplet detached before freezing to the surface. Finally, a freezing model for the supercooled water droplet based on classical nucleation theory was developed. The model is an approximation for a supercooled droplet of the recalescence step, which was assumed to be initiated by heterogeneous nucleation from the substrate. This research extended our knowledge about the hydrodynamic and freezing mechanisms of a micro-droplet on superhydrophobic surfaces. The developed solvers can serve as a design tool to engineer the roughness and thermo-physical properties of superhydrophobic coatings to prevent the freezing of cloud-sized droplets in practical flight conditions

Droplet impingement and wetting behavior on a chemically heterogeneous surface in the Beyond-Cassie-Baxter regime

Peer reviewedPostprin

Impingement and Mixing Dynamics of Micro-Droplets on a Solid Surface

Supported from National Natural Science Foundation of China (No.22078008) and the Fundamental Research Funds for the Central Universities (XK1802-1). Acknowledgement The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No.22078008) and the Fundamental Research Funds for the Central Universities (XK1802-1).Peer reviewedPostprin

Numerical investigation of high-speed droplet impact using a multiscale two-fluid approach

A single droplet impact onto solid surfaces remains a fundamental and challenging topic in both experimental and numerical studies with significant importance in a plethora of industrial applications, ranging from printing technologies to fuel injection in internal combustion engines. Under high-speed impact conditions, additional complexities arise as a result of the prompt droplet splashing and the subsequent violent fragmentation; thus, different flow regimes and a vast spectrum of sizes for the produced secondary flow structures coexist in the flow field. The present work introduces a numerical methodology to capture the multiscale processes involved with respect to local topological characteristics. The proposed methodology concerns a compressible Σ-Υ two-fluid model with dynamic interface sharpening based on an advanced flow topology detection algorithm. The model has been developed in OpenFOAM® and provides the flexibility of dealing with the multiscale character of droplet splashing, by switching between a sharp and a diffuse interface within the Eulerian-Eulerian framework in segregated and dispersed flow regions, respectively. An additional transport equation for the interface surface area density (Σ) introduces important information for the sub-grid scale phenomena, which is exploited in the dispersed flow regions to provide an insight into the extended cloud of secondary droplets after impact on the target. A high-speed water droplet impact case has been examined and evaluated against new experimental data; these refer to a millimetre size droplet impacting a solid dry smooth surface at velocity as high as 150m/s, which corresponds to a Weber number of ~7.6×10^5. At the investigated impact conditions compressibility effects dominate the early stages of droplet splashing. A strong shock wave forms and propagates inside the droplet, where transonic Mach numbers occur; local Mach numbers up to 2.5 are observed for the expelled surrounding gas outside the droplet. The proposed numerical approach is found to capture relatively accurately the phenomena and provide significant information regarding the produced flow structure dimensions, which is not available from the experiment

Numerical simulation of drop impingement and bouncing on a heated hydrophobic surface

The heat transfer of a single water droplet impacting on a heated hydrophobic surface is investigated numerically using a phase field method. The numerical results of the axisymmetric computations show good agreement with the dynamic spreading and subsequent bouncing of the drop observed in an experiment from literature. The influence of Weber number on heat transfer is studied by varying the drop impact velocity in the simulations. For large Weber numbers, good agreement with experimental values of the cooling effectiveness is obtained whereas for low Weber numbers no consistent trend can be identified in the simulations

Advances and challenges of ammonia delivery by urea-water sprays in SCR systems

Over the past decades, selective catalytic reduction (SCR) using aqueous urea sprays as ammonia precursor has become the prevalent technique for NO$_{X}$ emission control in mobile applications. Preparation of ammonia from urea water sprays still represents a challenge in aftertreatment engineering as complex interactions of multi-phase physics and chemical reactions have to be handled. Increasingly stringent emission legislations and the ongoing development of fuel-efficient engines and close-coupled aftertreatment systems raise high demands to SCR systems. Due to highly transient conditions and short mixing lengths, incomplete spray evaporation can result in liquid/wall contact and formation of solid urea deposits lowering ammonia selectivity and homogeneity. This article reviews the ongoing development of SCR systems with focus on the efficient evaporation and decomposition of the injected spray for a homogeneous ammonia distribution in front of the SCR catalyst. Critical aspects of spray evaporation and impingement, liquid film and deposit formation are pointed out and potentials for system optimization are discussed

We-T classification of diesel fuel droplet impact regimes

A combined experimental and computational investigation of micrometric diesel droplets impacting on a heated aluminium substrate is presented. Dual view high-speed imaging has been employed to visualize the evolution of the impact process at various conditions. The parameters investigated include wall-surface temperature ranging from 140 to 400°C, impact Weber and Reynolds numbers of 19–490 and 141–827, respectively, and ambient pressure of 1 and 2 bar. Six possible post-impact regimes were identified, termed as Stick, Splash, Partial-Rebound, Rebound, Breakup-Rebound and Breakup-Stick, and plotted on the We-T map. Additionally, the temporal variation of the apparent dynamic contact angle and spreading factor have been determined as a function of the impact Weber number and surface temperature. Numerical simulations have also been performed using a two-phase flow model with interface capturing, phase-change and variable physical properties. Increased surface temperature resulted to increased maximum spreading diameter and induced quicker and stronger recoiling behaviour, mostly attributed to the change of liquid viscosity

Coalescence and evaporation dynamics of multiple droplet impingement over a heated surface

The present work aims at unveiling the spread and heat transfer characteristics associated with droplet interactions over a heated wall. This information can be used to understand the overall process of spray cooling used for electronic cooling applications. Experimental investigation using high-speed photography and infrared thermography and numerical simulations implemented in the open-source CFD toolbox OpenFOAM are carried out to realise the underlying physics of the process

Droplet Dynamics Under Extreme Ambient Conditions

This open access book presents the main results of the Collaborative Research Center SFB-TRR 75, which spanned the period from 2010 to 2022. Scientists from a variety of disciplines, ranging from thermodynamics, fluid mechanics, and electrical engineering to chemistry, mathematics, computer science, and visualization, worked together toward the overarching goal of SFB-TRR 75, to gain a deep physical understanding of fundamental droplet processes, especially those that occur under extreme ambient conditions. These are, for example, near critical thermodynamic conditions, processes at very low temperatures, under the influence of strong electric fields, or in situations with extreme gradients of boundary conditions. The fundamental understanding is a prerequisite for the prediction and optimisation of engineering systems with droplets and sprays, as well as for the prediction of droplet-related phenomena in nature. The book includes results from experimental investigations as well as new analytical and numerical descriptions on different spatial and temporal scales. The contents of the book have been organised according to methodological fundamentals, phenomena associated with free single drops, drop clusters and sprays, and drop and spray phenomena involving wall interactions