Plasma-based microwave power limitation in a printed transmission line: a self-consistent model compared with experimental data

Plasma-based microwave power limitation in a suspended microstrip transmission line integrating a micro hollow cathode discharge (MHCD) in its center is experimentally and numerically studied. Transient and steady state microwave power measurements exhibit a limitation threshold of 28 dBm and time responses of 25 microseconds. Intensified charge-coupled device (ICCD) imaging shows that microwave breakdown occurs at the top of the MHCD. The plasma then extends towards the microwave source within the suspended microstrip transmission line. Besides, a self-consistent model is proposed to simulate the non-linear interaction between microwave and plasma. It gives numerical results in great agreement with the measurements, and show that the plasma expansion during the transient response is related to a shift between the ionization source term and the electron density maximum. The propagation speed, under the tested conditions, depends mainly on the stepwise ionization from the excited states.

Physical Nature of The Density Maximum for Water at 4°C

A new approach to the physical nature of the water density maximum at 4°C is proposed. The main attention is focused on the role of H-bonds in the formation of the specific volume and thermal expansion coefficients for ordinary and heavy water. It is shown that the minimum of the specific volume for water is connected with the amplification of H-bonds (D-bonds) role at approaching their triple points.

Optimal preliminary design of variable section beams criterion

The present paper discusses about optimal shape solution for a non-prismatic planar beam. The proposed model is based on the standard Timoshenko kinematics hypothesis (i.e., planar cross-section remains planar in consequence of a deformation, but it is able to rotate with respect to the beam center-line). The analytical solution for this type of beam is thus used to obtain deformations and stresses of the beam, under different constraints, when load is assumed as the sum of a generic external variable vertical one and the self-weight. The solution is obtained by numerical integration of the beam equation and constraints are posed both on deflection and maximum stress under the hypothesis of an ideal material. The section variability is, thus, described assuming a rectangular cross section with constant base and variable height which can be described in general with a trigonometric series. Other types of empty functions could also be analyzed in order to find the best strategy to get the optimal solution. Optimization is thus performed by minimizing the beam volume considering the effects of non-prismatic geometry on the beam behavior. Finally, several analytical and numerical solutions are compared with results existing in literature, evaluating the solutions’ sensibility to some key parameters like beam span, material density, maximum allowable stress and load distribution. In conclusion, the study finds a critical threshold in terms of emptying function beyond which it is not possible to neglect the arch effect and the curvature of the actual axis for every different case study described in this work. In order to achieve this goal, the relevance of beam span, emptying function level and maximum allowable stress are investigated.

What determines the position of the transition zone between alpha and beta regions in the ocean? A model study.

<p>The stratification is primarily controlled by the temperature in subtropical regions (alpha ocean), and by salinity in subpolar regions (beta ocean). Between these two regions lies a transition zone where intense frontal systems are usually found, either in the Southern Ocean or in the North Atlantic and North Pacific basins. Transition zones are often characterized by deep mixed layers in winter responsible for the ventilation of intermediate layers. Here we want to investigate what determines the latitudinal position of the transition zone. It is generally assumed that this position is set by the wind stress pattern forcing Ekman downwelling, however the position of the transition zone does not match so well the wind stress convergence zone in the observations. Another possibility would be that it is controlled by the distribution of air-sea fluxes. The equation of state (EOS) for seawater determines the relative impact of heat and freshwater forcing on the buoyancy forcing. A key property of seawater is that the density becomes less sensitive to temperature at low temperatures (caused by an important nonlinearity of the EOS), increasing the effect of salinity on the stratification in polar region. We hypothesize that the decreasing of the relative influence of temperature on density is a major component in setting the position of the transition zone. To test this hypothesis, we developed an idealized triple-gyre configuration with the ocean global circulation model NEMO (Nucleus for European Modelling of the Ocean). A range of simplified EOS have been ran to test the effect of the buoyancy forcing on the position of the transition zone and the convective area. Under restoring conditions for the temperature and the salinity, augmenting or reducing the sensitivity of the density to the temperature is used as a way to modify the relative contribution of temperature and salinity to the buoyancy forcing. We show that the position of the convective area corresponds to a surface density maximum and is not directly related to the Ekman pumping zone. Moreover, alpha - beta ocean distinction becomes possible because the EOS is nonlinear. The first order influence of the forcing evolution on setting the localization of the transition zone and the associated deep water formation challenges the classical theories of thermocline ventilation by Ekman pumping.</p>

Physical processes behind interactions of microplastic particles with ice

<p>Microplastic particles (MPs) are found in marine ice in larger quantities than in seawater, indicating that the ice is an important link in the chain of spreading of this contaminant. Some studies indicate larger MPs abundance near the ice surface, while others did not find any consistent pattern in the vertical distribution of MPs within sea ice cores. We discuss physical mechanisms of incorporation of MPs in the ice and present the results of laboratory tests, underpinning our conclusions.</p><p>First, plastic hydrophobicity is shown to cause the effect of pushing the floating MPs further up of the newly-forming ice. This leads to a concentration of MPs at the ice surface in the laboratory, while in the field the particles at the surface may by covered by snow and become a part of the upper ice layer. Under open-air test conditions, the bubbles of foamed polystyrene (density 0.04 g/cm<sup>3</sup>), initially floating at the water surface, were gone by weak wind when the firm ice was formed.</p><p>Second, the difference between freshwater and marine ice is considered. Since fresh water has its temperature of the density maximum (Tmd=3.98 C) well above the freezing point (Tfr=0 C), the freshwater ice is formed when the water column is stably stratified for a relatively long period of cooling from the Tmd down to the Tfr. Under such steady conditions, even just slightly positively/negatively buoyant MPs have enough time to rise to the surface / to settle to the bottom. In contrast, the ice in the ocean freezes when thermal convection is at work, further enhanced by the brine release. Thus, strong convection beneath the forming marine ice keeps slightly positively/negatively buoyant MPs in suspension and maintains the contact between the MPs and the forming ice. Laboratory tests show both the difference between the solid-and-transparent freshwater ice and the layered, filled with brine marine ice, and the difference in the level of their contamination.</p><p>Lastly, it is demonstrated that MPs tend to be incorporated in the ice together with air bubbles and in-between the ice plates (in brine channels). This is most probably due t plastics’ hydrophobicity.</p><p>Investigations are supported by the Russian Science Foundation, grant No 19-17-00041.</p>

Determination of Zero Dimensional, Apparent Devolatilization Kinetics for Biomass Particles at Suspension Firing Conditions

As part of the strive for a carbon neutral energy production, biomass combustion has been widely implemented in retrofitted coal burners. Modeling aids substantially in prediction of biomass flame behavior and thus in boiler chamber conditions. In this work, a simple model for devolatilization of biomass at conditions relevant for suspension firing is presented. It employs Arrhenius parameters in a single first order (SFOR) devolatilization reaction, where the effects of kinetics and heat transfer limitations are lumped together. In this way, a biomass particle can be modeled as a zero dimensional, isothermal particle, facilitating computational fluid dynamic calculations of boiler chambers. The zero dimensional model includes the effects of particle aspect ratio, particle density, maximum gas temperature, and particle radius. It is developed using the multivariate data analysis method, partial least squares regression, and is validated against a more rigorous semi-2D devolatilization model. The model has the capability to predict devolatilization time for conditions in the parameter ranges; radius (39–1569 μμm), density (700–1300 kg/m3), gas temperature (1300–1900 K), aspect ratio (1.01–8). Results show that the particle radius and gas phase temperature have a large influence on the devolatilization rate, and the aspect ratio has a comparatively smaller effect, which, however, cannot be neglected. The impact of aspect ratio levels off as it increases. The model is suitable for use as stand alone or as a submodel for biomass particle devolatilization in CFD models.