Hydrodynamic and solid residence time distribution in a circulating fluidized bed: experimental and 3D computational study

Vertical profiles of local pressure, horizontal profiles of net vertical solid mass flux, and residence time distributions (RTD) of the solid phase are experimentally assessed in the riser of a small scale cold Circulating Fluidized Bed of 9 m high having a square cross section of 1111 cm. Air (density 1.2 kg/m3, dynamic viscosity 1.8×10-5 Pa.s) and typical FCC particles (density 1400 kg/m3, mean diameter 70 mm) are used. The superficial gas velocity is kept constant at 7 m/s while the solid mass flux ranges from 46 to 133 kg/m2/s. The axial dispersion of the solid phase is found to decrease when increasing the solid mass flux. Simultaneously, 3D transient CFD simulations are performed to conclude on the usability of the eulerian-eulerian approach for the prediction of the solid phase mixing in the riser. The numerical investigation of the solid mixing is deferred until later since the near-wall region where the solid phase downflow and mixing are predominant is not well predicted in spite of well-predicted vertical profiles of pressure

Expectations for the Deep Impact collision from cometary nuclei modelling

Using the cometary nucleus model developed by Espinasse et al. (1991), we calculate the thermodynamical evolution of Comet 9P/Tempel 1 over a period of 360 years. Starting from an initially amorphous cometary nucleus which incorporates an icy mixture of H2O and CO, we show that, at the time of Deep Impact collision, the crater is expected to form at depths where ice is in its crystalline form. Hence, the subsurface exposed to space should not be primordial. We also attempt an order-of-magnitude estimate of the heating and material ablation effects on the crater activity caused by the 370 Kg projectile released by the DI spacecraft. We thus show that heating effects play no role in the evolution of crater activity. We calculate that the CO production rate from the impacted region should be about 300-400 times higher from the crater resulting from the impact with a 35 m ablation than over the unperturbed nucleus in the immediate post-impact period. We also show that the H2O production rate is decreased by several orders of magnitude at the crater base just after ablation

Convergent readings of groundwater governance? Engaging exchanges between different research perspectives

In the last 50 years, the study of groundwater governance has given birth to rich and differentiated analytical approaches. This article proposes an exchange between the main heuristic viewpoints, results and theses of three approaches which studied groundwater governance: the study of groundwater governance as collective action initiated by E. Ostrom, the analysis made by T. Shah on informal groundwater economies and the works of A. Prakash, A. Mukherji and T. Birkenholtz drawing on political ecology. The aquifers on which each approach originally built its analysis played a key role in the design of analytical frameworks which, in the case of the first two approaches, were later applied to a much larger set of cases. The three approaches share many commonalities in their heuristic methods. However, they differ in the content of governance systems recommended to achieve sustainable groundwater use, and especially in the benefits of involving water users in the implementation of governance. Therefore, they also differ on what should be the focus of academic analyses. It is thus of interest to organize joint readings of cases of groundwater governance and to discuss the contents of research programs able to provide fruitful analyses of these cases, based on local specificities

Carbon-rich planet formation in a solar composition disk

The C--to--O ratio is a crucial determinant of the chemical properties of planets. The recent observation of WASP 12b, a giant planet with a C/O value larger than that estimated for its host star, poses a conundrum for understanding the origin of this elemental ratio in any given planetary system. In this paper, we propose a mechanism for enhancing the value of C/O in the disk through the transport and distribution of volatiles. We construct a model that computes the abundances of major C and O bearing volatiles under the influence of gas drag, sublimation, vapor diffusion, condensation and coagulation in a multi--iceline 1+1D protoplanetary disk. We find a gradual depletion in water and carbon monoxide vapors inside the water's iceline with carbon monoxide depleting slower than water. This effect increases the gaseous C/O and decreases the C/H ratio in this region to values similar to those found in WASP 12b's day side atmosphere. Giant planets whose envelopes were accreted inside the water's iceline should then display C/O values larger than those of their parent stars, making them members of the class of so-called ``carbon-rich planets''.Comment: 8 pages, 4 figures, accepted for publication Ap

Tourisme de croisière et industrie de la croisière

International audienceThis paper proposes a reflection on the cruise industry, the importance and scope of this industry. One of the objectives is to examine in particular the relationship between cruise lines and ports that receive cruises and excursionists. The cruise product built from the 1970s engendred a specific mass tourism. And the boat is both the means of transport and the destination. The stops have a secondary role in this logic that favors gigantism, standardization and uniformity. While gigantism known some limits, cruise products reconstructs itself spatially and innovative segments of the high-end and luxury.Ce texte pose les bases d’une réflexion sur les fondements de l’industrie de la croisière et sur l’importance et la portée de ce secteur d’activité. Il s’interroge notamment sur les relations entre les compagnies de croisières et le port récepteur des croisiéristes lors des excursions. Le produit croisière construit à partir des années 1970 a donné naissance à un tourisme de masse spécifique ; le bateau est à la fois le moyen de transport et la destination. Les escales n’ont qu’un rôle secondaire dans cette logique qui privilégie le gigantisme, la standardisation et l’uniformisation. Alors que le gigantisme touche certaines limites, l’offre croisière se recompose spatialement et innove sur les segments du haut de gamme et du luxe

When economists and ecologists meet on Ecological Economics: two science paths around two interdisciplinary concepts

International audienceEcological economics essentially grew out of economists working in the environmental field and growing dissatisfied with the way that standard economics saw interactions between nature and societies and ecologists anxious to take human activities (including economic) into account in a much more direct way, within the dynamic of the ecosystems on which they depend. This clearly inscribed the new field of ecological economics within an interdisciplinary and even transdisciplinary perspective. In order to try to provide some thoughts on the evolution of this trend and the relationship between economists and ecologists, we have chosen to focus on two items 1 that are undoubtedly among the achievements of ecological economics, although their mobilization is far from uniform among the authors who make use of them: coevolution and ecosystem services. In order to do so, the itinerary of two authors recognized in the field of ecological economics will be examined: Richard B. Norgaard, whose work on the coevolutionary paradigm (Norgaard, 1994) is recognized as one of the foundations of ecological economics (Munda, 1997); Robert Costanza, who initiated work on the monetary valuation of ecosystem services 1 For the moment, we would like to use the generic term for the subject. We shall see later that these subjects may be alternatively (and often also simultaneously) used as metaphors, concepts, and instruments of public policy. 2 (Costanza et al., 1997) that is a marker in the field of ecological economics. What unites these two authors is a manifest interest in the work coming out of systems analysis in the 1970s-as we shall see, that interest ultimately led to fairly contrasting visions of the field of ecological economics

KBO binaries: are they really primordial ?

Given the large orbital separation and high satellite-to-primary mass ratio of all known Kuiper Belt Object (KBO) binaries, it is important to reassess their stability as bound pairs with respect to several disruptive mechanisms. Besides the classical shattering and dispersing of the secondary due to a high-velocity impact, we considered the possibility that the secondary is kicked off its orbit by a direct collision of a small impactor, or that it is gravitationally perturbed due to the close approach of a somewhat larger TNO. Depending on the values for the size/mass/separation of the binaries that we used, 2 or 3 of the 8 pairs can be dispersed in a timescale shorter than the age of the solar system in the current rarefied environment. A contemporary formation scenario could explain why we still observe these binaries, but no convincing mechanism has been proposed to date. The primordial formation scenarios, which seem to be the only viable ones, must be revised to increase the formation efficiency in order to account for this high dispersal rate. Objects like the large-separatioKBO binary n2001 QW$_{322}$ must have been initially an order of magnitude more numerous. If the KBO binaries are indeed primordial, then we show that the mass depletion of the Kuiper belt cannot result from collisional grinding, but must rather be due to dynamical ejection.Comment: Submitted to Icarus, Mai 9th, 200

Theoretical evaluation of MTF and charge collection efficiency in CCD and CMOS image sensor

Classical models used to calculate the Modulation Transfer function (MTF) of a solid-state image sensor generally use a sinusoidal type of illumination. The approach, described in this paper, consists in considering a point-source illumination to built a theoretical three dimensional model of the diffusion and the collection of photo-carriers created within the image sensor array. Fourier transform formalism is used for this type of illumination. Solutions allow to evaluate the spatial repartition of the charge density collected in the space charge region, i.e. to get the Pixel Response Function (PRF) formulation. PRF enables to calculate analytically both MTF and crosstalk at every needed wavelengths. The model can take into account a uniformly doped substrate and an epitaxial layer grown on a highly doped substrate. The built-in electric field induced by the EPI/Substrate doping gradient is also taken into account. For these configurations, MTF, charge collection efficiency and crosstalk proportion are calculated. The study is established in the case of photodiode pixel but it can be easily extended to pinned photodiode pixels and photogate pixels