1,721,063 research outputs found
Multiphase Rayleigh-Bénard convection
No full textNumerical simulations of two-phase Rayleigh-Bénard convection in a cylindrical cell with particles or vapor bubbles suspended in the fluid are described. The particles or bubbles are modeled as points, the Rayleigh number is 2×106 and the fluids considered are air, for the particle case, and saturated water for bubbles. It is shown that the presence of a second phase has a profound effect on the flow and heat transfer in the cell. The heat capacity of the particles and the latent heat of the liquid are used, in dimensionless form, as control parameters to modulate these effects. It is shown that, as these parameters are varied, the nature of the flow in the cell changes substantially, in some cases with adverse and in others beneficial effects on the Nusselt number. By the analysis of several aspects of the numerical results, a physical discussion of several mechanisms is provided
Heat Transfer Mechanisms in Bubbly Rayleigh-Bénard Convection
No full textThe heat transfer mechanism in Rayleigh-Beacutenard convection in a liquid with a mean temperature close to its boiling point is studied through numerical simulations with pointlike vapor bubbles, which are allowed to grow or shrink through evaporation and condensation and which act back on the flow both thermally and mechanically. It is shown that the effect of the bubbles is strongly dependent on the ratio of the sensible heat to the latent heat as embodied in the Jakob number Ja. For very small Ja the bubbles stabilize the flow by absorbing heat in the warmer regions and releasing it in the colder regions. With an increase in Ja, the added buoyancy due to the bubble growth destabilizes the flow with respect to single-phase convection and considerably increases the Nusselt number
Effects of momentum and thermal energy exchange on the Rayleigh-Bénard convection of disperse flows
No full textThis paper reports some preliminary results of a computational study of the effect of suspended solid particles in a gas ow driven by natural convection. The study is conducted in the point-particle approximation including two-way coupling. It differs from other models of this type in that the particles do not only exert a force on the gas, but also exchange energy with it. Their ability to do so is measured by the ratio J of the gas heat capacity to the particle heat capacity, both per unit volume. This parameter is shown to exert a powerful in uence on the ow. When J = 0 the particles maintain their initial temperature although they heat up the surrounding uid. In spite of this, if they are not too small, they increase the natural convection as, being swept up by the cold descending currents, they increase their weight and accelerate them. As J is increased above 0 the particle-gas temperature difference decereases and a maximum heat transfer is reached when the gas can absorb all the heat carried by the particles. A simple argument suggests that this condition is reached when J equals 1/2 of the particle volume fraction
Effective velocity boundary condition at a mixed slip surface
Full text linkThis paper studies the nature of the effective velocity boundary condition for liquid flow over a plane boundary on which small free-slip islands are randomly distributed. It is found that an effective Navier partial-slip condition for the velocity emerges from a statistical analysis valid for arbitrary fractional area coverage β. As an example, the general theory is applied to the low-β limit and this result is extended heuristically to finite β with a resulting slip length proportional to aβ/(1 − β), where a is a characteristic size of the islands. A specification of the nature of the free-slip islands is not required in the analysis. They could be nano-bubbles, as suggested by recent experiments, or hydrophobic surface patches. The results are also relevant for ultra-hydrophobic surfaces exploiting the so-called ‘lotus effect
Heat transfer mechanisms in bubbly Rayleigh-Benard convection
No full textThis Proceeding contribution summarizes our results to be published in ref. [1]:
The effectiveness of boiling as a heat transfer mechanism has been known for centuries and the process has formed the object of a very large number of studies [2]. The emerging vapor bubbles introduce a new parameter to the classical Rayleigh-Bénard convection [3], namely the Jacob number
Ja=ρcp(Th−Tsat)/ρVL (1) in which L is the latent heat, ρV and ρ the vapor and liquid density, respectively, c p the liquid specific heat and T sat the saturation temperature of the liquid. Physically, Ja represents the ratio of the sensible heat to the latent heat. A very small Jacob number may be thought of as a very large value of the latent heat, which will tend to limit the volume change of the bubbles due to evaporation or condensation. For Ja = 0 the latent heat is effectively infinite and bubbles cannot grow or shrink
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
