High Resolution 3-D Simulations of Venting in 18650 Lithium-Ion Cells

High temperature gases released through the safety vent of a lithium-ion cell during a thermal runaway event contain flammable components that, if ignited, can increase the risk of thermal runaway propagation to other cells in a multi-cell pack configuration. Computational fluid dynamics (CFD) simulations of flow through detailed geometric models of four vent-activated commercial 18650 lithium-ion cell caps were conducted using two turbulence modeling approaches: Reynolds-averaged Navier-Stokes (RANS) and scale-resolving simulations (SRS). The RANS method was compared with independent experiments of discharge coefficient through the cap across a range of pressure ratios and then used to investigate the ensemble-averaged flow field for the four caps. At high pressure ratios, choked flow occurs either at the current collector plate when flow through the current collector plate is more restrictive or the positive terminal vent holes when flow through the current collector plate is less restrictive. Turbulent mixing occurred within the vent cap assembly, in the jets emerging from the vent holes, and in recirculating zones directly above the vent cap assembly. The global maximum turbulent viscosity ratio (μT/μ) of the MTI, LG MJ1, K2, and LG M36 caps at pressure ratio of P1/P2 = 7 were 4,575, 3,360, 3,855, and 2,993, respectively. SRS and RANS simulations showed that both velocity magnitude and fluctuating velocity magnitude were lower for vent holes which are obstructed by the burst disk. SRS showed high levels of fluctuating velocity in the jets, up to 48.5% of the global maximum velocity. The present CFD models and the resulting insights provide the groundwork for future studies to investigate how jet structure and turbulence levels influence combustion and heat transfer in propagating thermal runaway scenarios.

Probing electrically driven nanojets by energy and mass analysis in vacuo

Time of flight (TOF) and energy analysis in vacuum are used in series to determine jet velocity Uj, diameter dj, electrical potential Vj and energy dissipated ΔV at the breakup point of electrified nanojets of the ionic liquid 1-Ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMI-FAP) (Ignat'ev et al., J. Fluorine Chem., vol. 126, issue 8, 2008, pp.1150–1159). The full spray is periodically gated by a grid held at a high voltage Vg, and received at a collector where the measured flight times provide the distribution of drop speeds u. Varying Vg provides the bivariate distribution of drop energies ξ and velocities. The collector plate, centred with the beam axis, is divided into eight concentric rings, yielding the angular distribution of the spray current, and high resolution (u,ξ) values in the whole spray. The energies of various particles of given u are all well defined, but depend uniquely on u, even though u and ξ are in principle independent experimental variables. Slow and fast particles have energies respectively well above and below the capillary voltage Ve (1.64 kV). As previously shown by Gamero-Castaño & Hruby (J. Fluid Mech., vol. 459, 2002, pp. 245–276), this behaviour is due to the 2-stage acceleration process, first jointly in the jet for all particles, and then separately for free flying drops or ions of different mass/charge. The measured two-dimensional distributions of u and ξ provide the jet velocity Uj (~0.44 km s−1) and electrical potential Vj (1.2 kV) at the breakup point. All molecular ions originate near the breakup point rather than the meniscus neck. A measurable fraction of anomalously fast drops is observed that must come from Coulomb fissions of the main drops.

Melt electrospun fibrous architectures with target geometries

In the melt electrospinning technique, the polymer melt is stretched under high voltage and the cooled to form microfibers structures with a fibre diameter in the tens of micrometres range, although some studies have reported values ranging from hundreds of nanometres to hundreds of micrometres. In this respect, this technique has significance in the biomedical field, where tissue engineering scaffolds with bimodal (nano and micro) fibrous structures are preferred in regard to cell adhesion, spreading and infiltration to final tissue reconstruction. This paper gives a review of recently reported melt electrospinning devices, especially those based on the direct writing principle, and of their comparison with the new melt Spraybase electrospinning device. The Spraybase device provides high precision melt jet deposition into 2D and 3D programmed architectures, with versatile translation speeds of the collector plate in the X-Y and the melt head in the Z direction. The melt spun fibrous architectures are designed depending on the types of tissue cells used in scaffold development.

The effect of psychrometry on the performance of a solar collector

The investigation of a solar collector is based on the thermal behaviour of a carrier fluid and the degradation of energy across a flat plate collector. The exergy analysis of a thermal system includes the change in the exergy function of a carrier fluid while transferring solar radiation across an air gap. The cell cast acrylic glass was used to transmit the incident solar radiation to the absorber plate, and to safeguard the absorber plate from the outside environment. With the help of the steady flow energy equation, the enthalpy of the carrier fluid (moist air) was calculated. The specific humidity of the incoming air was calculated at an average dry bulb temperature of 299.4 K. The stagnation temperature at a limiting condition was also estimated to find out the maximum permissible limit for a given thermal design. The mass flow rate of air was assumed to be 5.2 g-s−1. The efficiency of the solar collector was found to vary from 40 to 42%, whereas the thermal energy available for drying was 15–59% of the exergy of the carrier fluid. The net entropy generation rate due to the collector plate was calculated to be 0.12 W-K−1.

In Situ Collection of Nanoparticles during Femtosecond Laser Machining in Air

Nanoparticles generated during laser material processing are often seen as annoying side products, yet they might find useful application upon proper collection. We present a parametric study to identify the dominant factors in nanoparticle removal and collection with the goal of establishing an in situ removal method during femtosecond laser machining. Several target materials of different electrical resistivity, such as Cu, Ti, and Si were laser machined at a relatively high laser fluence. Machining was performed under three different charge conditions, i.e., machining without an externally applied charge (alike atmospheric pulsed laser deposition (PLD)) was compared to machining with a floating potential and with an applied field. Thereby, we investigated the influence of three different charge conditions on the behavior of laser-generated nanoparticles, in particular considering plume deflection, nanoparticle accumulation on a collector plate and their redeposition onto the target. We found that both strategies, machining under a floating potential or under an applied field, were effective for collecting laser-generated nanoparticles. The applied field condition led to the strongest confinement of the nanoparticle plume and tightest resulting nanoparticle collection pattern. Raster-scanning direction was found to influence the nanoparticle collection pattern and ablation depth. However, the laser-processed target surface remained unaffected by the chosen nanoparticle collection strategy. We conclude that machining under a floating potential or an applied field is a promising setup for removing and collecting nanoparticles during the machining process, and thus provides an outlook to circular waste-free laser process design.

A deeper insight into the influence of the electric field densities in Melt-Electrowriting: Printing in the 4th Dimension

Defined ordered structures for biomaterials and tissue engineering applications can be achieved by a variety of techniques, one of which includes the electrohydrodynamic (EHD)-based application of melt electrowriting (MEW), the extrusion of a molten polymer filament under pressure across a defined electric field. In this study, we investigate how to translate small fibremeshes which are usually formed on flat surfaces, to curved contours that would have more applicability to human anatomical structures. By modelling the electric field strength associated with the MEW process, we found that incorporation of a non conductive three-dimensional (3D) custom printed mould on the conductive collector plate offers the ability to accurately print patterns on non-flat surfaces successfully. Importantly, while the electric field strength is a constant in the MEW process; the electrostatic behaviour of the deposited polymer has the greatest impact on the accuracy of fibre patterning and stacking. Consequently, controlled fibre deposition was exhibited, provided that a constant electrical field strength and a continuous vertical distance between the nozzle and the mould is maintained.Overall, this study establishes the groundwork to support further developments in MEW technologies, from flat to anatomically relevant 3D structures in the fields of regenerative medicine and biofabrication.

Study of novel solar assisted heating system

The potential for energy, carbon dioxide equivalent (CO2e) and cost savings when using low emissivity (low-ε) transpired solar collectors (TSCs), combined with heat pumps in a range of configurations, has been investigated using computer modelling. Low-ε TSCs consist of metal solar collector plates with a spectrally sensitive surface, perforated with holes. Ambient air is drawn through the holes and heated by convection from the solar collector plate, increasing the air temperature by up to 25 K. The heated air can be used for e.g. space heating, or pre-heating water in buildings. The models developed have been used to compare the performance of low-ε TSC/heat pump heating systems in small and large buildings, at a range of locations. The model results showed savings in energy, CO2e and costs of up to 16.4% when using low-ε TSCs combined with an exhaust air heat pump compared with using the exhaust air heat pump alone. Practical application: If the UK is to meet its target of reaching net zero greenhouse gas emissions by 2050, it will be necessary to adopt low or zero carbon heating technologies. The novel low emissivity transpired solar collector device investigated can contribute to this. Its advantages include: (i) utilising solar radiation; (ii) readily integrated with existing heating systems e.g. heat pumps; (iii) significant energy, CO2e emissions and cost savings; (iv) low cost device; (v) minimal energy input i.e. one small fan; (vi) can be retrofitted to existing buildings; (vii) its benefits were applicable at all of the (wide range of) locations tested.

Improvement of the Solar-Thermal Characteristics of the Flat-Plate Collector Using a Composite Coating

A commercially available matt black paint is used as the primary component of a composite coating for low-temperature flat collector systems. The composite coating is composed of the paint, carbon black powder (CBP) and lithium metal oxide (LMO) powder. The CBP and LMO powders are added on the top of the primary paint to improve the solar-thermal conversion characteristics of the collector. The optical and solar-thermal conversion characteristics of these coatings are investigated by means of optical microscopy, optical spectrophotometry, total reflectance measurement, and the measurement of the maximum collector temperature. Results show that the solar-thermal conversion response (maximum temperature) of the collector plate can be improved with the addition of the CBP and LMO powder at low concentrations. This is been demonstrated and discussed in this report. The coatings prepared in the present work can easily be applied on the collector surface and exhibit competing characteristics to other coatings.

Thermal behavior of natural convection flow in an inclined solar air heater

The thermal behavior of hot air in a natural convection mode on a solar absorber-plate has not been, so far, modeled experimentally. The present work aimed to assess the performance of the inclined solar air heater [SAH] experimentally by investigating the temperature distribution field in the natural convection flow. The solar plate collector is designed based on the aspect ratio of length to height, L / H, of 6 and 12. The measurements are carried out for the collector tilt angles of 30°, 45°, 60° and 75°. The present investigation demonstrates the temperature distribution of hot air floated in an inclined channel of the SAH. The investigation showed 2D thermal stratification increases when increasing the distance along the collector plate, which looks clear in the SAH with a height of 10 cm. The results of the study show that the thickness of the thermal layers increases with increasing the tilt angle from 30˚ to 75˚. The reason dates back to increasing the buoyancy force of the hot air over the absorber. The results demonstrated that the air temperatures for the height of 0.1 m and 45˚ tilt angle are higher than that for the height of 0.2 m by 23%.

Electrostatic Self-Assembly of Composite Nanofiber Yarn

Electrospinning polymer fibers is a well-understood process primarily resulting in random mats or single strands. More recent systems and methods have produced nanofiber yarns (NFY) for ease of use in textiles. This paper presents a method of NFY manufacture using a simplified dry electrospinning system to produce self-assembling functional NFY capable of conducting electrical charge. The polymer is a mixture of cellulose nanocrystals (CNC), polyvinyl acrylate (PVA) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). When treated with ethylene glycol (EG) to enhance conductivity, fibers touching the collector plate align to the applied electrostatic field and grow by twisting additional nanofiber polymers injected by the jet into the NFY bundle. The longer the electrospinning continues, the longer and more uniformly twisted the NFY becomes. This process has the added benefit of reducing the electric field required for NFY production from >2.43 kV cm−1 to 1.875 kV cm−1.