Warm turbulence in the Boltzmann equation

We study the single-particle distributions of three-dimensional hard sphere gas described by the Boltzmann equation. We focus on the steady homogeneous isotropic solutions in thermodynamically open conditions, i.e. in the presence of forcing and dissipation. We observe nonequilibrium steady state solution characterized by a warm turbulence, that is an energy and particle cascade superimposed on the Maxwell-Boltzmann distribution. We use a dimensional analysis approach to relate the thermodynamic quantities of the steady state with the characteristics of the forcing and dissipation terms. In particular, we present an analytical prediction for the temperature of the system which we show to be dependent only on the forcing and dissipative scales. Numerical simulations of the Boltzmann equation support our analytical predictions.Comment: 4 pages, 5 figure

Characterisation and modelling of water wicking and evaporation in capillary porous media for passive and energy-efficient applications

Passive devices based on water wicking and evaporation offer a robust, cheap, off-grid, energy-efficient and sustainable alternative to a wide variety of applications, ranging from personal thermal management to water treatment, from filtration to sustainable cooling technologies. Among the available, highly-engineered materials currently employed for these purposes, polyethylene-based fabrics offer a promising alternative thanks to the precise control of their fabrication parameters, their light-weight, thermal and mechanical properties, chemical stability and sustainability. As such, both woven and non-woven fabrics are commonly used in capillary-fed devices, and their wicking properties have been extensively modelled relying on analytical equations. However, a comprehensive and flexible modelling framework able to investigate and couple all the heat and mass transfer phenomena regulating the water dynamics in complex 2-D and 3-D porous components is currently missing. This work presents a comprehensive theoretical model aimed to investigate the wetting and drying performance of hydrophilic porous materials depending on their structural properties and on the external environmental conditions. The model is first validated against experiments (R2=0.99 for the wicking model; errors lower than 14% and 1% for the evaporation and radiative models, respectively), then employed in three application cases: the characterisation of the capillary properties of a novel textile; the assessment of the thermal performance of a known material for personal thermal management when used in different conditions; the model-assisted design of a porous hydrophilic component of passive devices for water desalination. The obtained results showed a deep interconnection between the different heat and mass transfer mechanisms, the porous structure and external working conditions. Thus, modelling their non-linear behaviour plays a crucial role in determining the optimal material characteristics to maximise the performance of porous materials for passive devices for the energy and water sector

On the Three-dimensional Central Moment Lattice Boltzmann Method

A three-dimensional (3D) lattice Boltzmann method based on central moments is derived. Two main elements are the local attractors in the collision term and the source terms representing the effect of external and/or self-consistent internal forces. For suitable choices of the orthogonal moment basis for the three-dimensional, twenty seven velocity (D3Q27), and, its subset, fifteen velocity (D3Q15) lattice models, attractors are expressed in terms of factorization of lower order moments as suggested in an earlier work; the corresponding source terms are specified to correctly influence lower order hydrodynamic fields, while avoiding aliasing effects for higher order moments. These are achieved by successively matching the corresponding continuous and discrete central moments at various orders, with the final expressions written in terms of raw moments via a transformation based on the binomial theorem. Furthermore, to alleviate the discrete effects with the source terms, they are treated to be temporally semi-implicit and second-order, with the implicitness subsequently removed by means of a transformation. As a result, the approach is frame-invariant by construction and its emergent dynamics describing fully 3D fluid motion in the presence of force fields is Galilean invariant. Numerical experiments for a set of benchmark problems demonstrate its accuracy.Comment: 55 pages, 8 figure

Deep-sea reverse osmosis desalination for energy efficient low salinity enhanced oil recovery

The decrease in the oil discoveries fuels the development of innovative and more efficient extraction processes. It has been demonstrated that Enhanced Oil Recovery (EOR, or tertiary recovery technique) offers prospects for producing 30 to 60% of the oil originally trapped in the reservoir. Interestingly, oil extraction is significantly enhanced by the injection of low salinity water into oilfields, which is known as one of the EOR techniques. Surface Reverse Osmosis (SRO) plants have been adopted to provide the large and continuous amount of low salinity water for this EOR technique, especially in offshore sites. In this article, we outline an original solution for producing low salinity water for offshore EOR processes, and we demonstrate its energy convenience. In fact, the installation of reverse osmosis plants under the sea level (Deep-Sea Reverse Osmosis, DSRO) is found to have significant potential energy savings (up to 50%) with respect to traditional SRO ones. This convenience mainly arises from the non-ideality of reverse osmosis membranes and hydraulic machines, and it is especially evident - from both energy and technological point of view - when the permeate is kept pressurized at the outlet of the reverse osmosis elements. In perspective, DSRO may be a good alternative to improve the sustainability of low salinity EOR

Multi-Scale Modelling of Aggregation of TiO2 Nanoparticle Suspensions in Water

Titanium dioxide nanoparticles have risen concerns about their possible toxicity and the European Food Safety Authority recently banned the use of TiO2 nano-additive in food products. Following the intent of relating nanomaterials atomic structure with their toxicity without having to conduct large-scale experiments on living organisms, we investigate the aggregation of titanium dioxide nanoparticles using a multi-scale technique: starting from ab initio Density Functional Theory to get an accurate determination of the energetics and electronic structure, we switch to classical Molecular Dynamics simulations to calculate the Potential of Mean Force for the connection of two identical nanoparticles in water; the fitting of the latter by a set of mathematical equations is the key for the upscale. Lastly, we perform Brownian Dynamics simulations where each nanoparticle is a spherical bead. This coarsening strategy allows studying the aggregation of a few thousand nanoparticles. Applying this novel procedure, we find three new molecular descriptors, namely, the aggregation free energy and two numerical parameters used to correct the observed deviation from the aggregation kinetics described by the Smoluchowski theory. Ultimately, molecular descriptors can be fed into QSAR models to predict the toxicity of a material knowing its physicochemical properties, enabling safe design strategies

3D printed lattice metal structures for enhanced heat transfer in latent heat storage systems

The low thermal conductivity of Phase Change Materials (PCMs), e.g., paraffin waxes, is one of the main drawbacks of latent heat storage, especially when fast charging and discharging cycles are required. The introduction of highly conductive fillers in the PCM matrix may be an effective solution; however, it is difficult to grant their stable and homogeneous dispersion, which therefore limits the resulting enhancement of the overall thermal conductivity. Metal 3D printing or additive manufacturing, instead, allows to manufacture complex geometries with precise patterns, therefore allowing the design of optimal paths for heat conduction within the PCM. In this work, a device-scale latent heat storage system operating at medium temperatures (similar to 90 celcius) was manufactured and characterized. Its innovative design relies on a 3D Cartesian metal lattice, fabricated via laser powder bed fusion, to achieve higher specific power densities. Numerical and experimental tests demonstrated remarkable specific power (approximately 714 +/- 17 W kg-1 and 1310 +/- 48 W kg-1 during heat charge and discharge, respectively). Moreover, the device performance remained stable over multiple charging and discharging cycles. Finally, simulation results were used to infer general design guidelines to further enhance the device performance. This work aims at promoting the use of metal additive manufacturing to design efficient and responsive thermal energy storage units for medium-sized applications, such as in the automotive sector (e.g. speed up of the engine warm up or as an auxiliary for other enhanced thermal management strategies)

Techno-economic analysis of a solar thermal plant for large-scale water pasteurization

Water pasteurization has the potential to overcome some of the drawbacks of more conventional disinfection techniques such as chlorination, ozonation and ultraviolet radiation treatment. However, the high throughput of community water systems requires energy-intensive processes, and renewable energy sources have the potential to improve the sustainability of water pasteurization plants. In case of water pasteurization by solar thermal treatment, the continuity of operation is limited by the intermittent availability of the solar irradiance. Here we show that this problem can be addressed by a proper design of the plant layout, which includes a thermal energy storage system and an auxiliary gas boiler. Based on a target pasteurization protocol validated by experiments, a complete lumped-component model of the plant is developed and used to determine the operating parameters and size of the components for a given delivery flow rate. Finally, we report an economic analysis of the proposed plant layout, which allows its optimization for different scenarios based on two design variables, namely the solar multiple and the duration of the thermal energy storage. Based on the analyzed cases, it is found that the proposed plant layouts may yield a unit cost of water treatment ranging from ≈32 EUR-cents m−3 to ≈25 EUR-cents m−3

Numerical simulation of droplet impact on wettability-patterned surfaces

© 2020 American Physical Society. Numerical simulations have unexplored potential in the study of droplet impact on nonuniform wettability surfaces. In this paper, we compare numerical and experimental results to investigate the application potential of a volume-of-fluid method utilized in OpenFOAM. The approach implements the Kistler model for the dynamic contact angle of impacting droplets. We begin with an investigation into the influence of the most important solver parameters to optimize the computational setup and reach the best compromise between computational cost and solution errors, as assessed in comparison to experimental results. Next, we verify the accuracy of the predictions for droplet impact on uniformly hydrophilic or superhydrophobic surfaces. Benchmarking the maximal spreading factor, contact, and spreading times, as well as contact-line behavior, we show strong agreement between the present numerical results and the models of Pasandideh-Fard, Phys. Fluids 8, 650 (1996)PHFLE61070-663110.1063/1.868850 and Clanét, J. Fluid Mech. 517, 199 (2004)JFLSA70022-112010.1017/S0022112004000904. Lastly, we demonstrate the capability of the model to accurately predict outcome behaviors of droplets striking distributed-wettability surfaces, which introduce 3D outcome characteristics, even in orthogonal impact. The model successfully predicts droplet splitting and vectoring, as reported in the experiments of Schutzius, Sci. Rep. 4, 7029 (2014)2045-232210.1038/srep07029. Finally, we demonstrate a configuration wherein a droplet centrally strikes a circular disk of different wettability than its surrounding domain. The main contribution of the present paper is a numerical model capable of accurately simulating droplet impact on spatially nonuniform wettability patterns of any foreseeable design