1,721,097 research outputs found
The response of a rarefied gas in a slab due to various time-dependent boundary heating configurations
No full textThere is both theoretical and practical interest in the
investigation of time dependent heat flows which is motivated
by their applications in several technological fields including
micro-electromechanical systems and microelectronics
ranging from microprocessor chip heating to ultrafast
temperature variations encountered in the laser industry as
well as in the design of microsensors and microactuators. The
response of a rarefied gas between two parallel plates due to
instantaneous changes in the wall temperature of one plate
has been studied first in [1] and more recently in [2], while the
periodic behaviour of a rarefied gas caused by the oscillatory
heating of one plate has been recently investigated based on
the linearized Boltzmann equation in [3].
In the present work, the response of a gas in a slab due to
various time-dependent boundary heating configurations is
investigated in the whole range of the Knudsen number based
on the linear Shakhov kinetic model. In particular, in addition
to the sudden change of the wall temperature the cases of
linear and arctangential variation of the temperature at the
boundary with some cut-off is investigated. In all three cases
the time response of the gas including the evolution of the
heat flow field in terms of the Knudsen number is examined.
The total response time to reach steady-state conditions is
estimated deducing a non-monotonic behaviour in terms of
the gas rarefaction. This type of information may be important
in the design and optimization of the Pirani sensors enlarging
the range of their applicability. The case of oscillatory heating
of one boundary is also studied. The solution is given in
terms of two parameters namely the Knudsen number and
the dimensionless oscillation frequency. The results based
on the Shakhov model is in very good agreement with the
corresponding ones in [3] both in the mass and energy flow
fields demonstrating the validity of the implementation of the
kinetic model equations instead of the Boltzmann equation in
order to reduce the involved computational effort
Experimental analysis of heat transfer between a heated wire and a rarefied gas in an annular gap with high diameter ratio
No full textIn this paper a first experimental attempt is performed to measure heat conduction through rarefied air at rest contained between two concentric cylinders. The heat transfer between a heated platinum wire having a diameter (d) of 0.15 mm, disposed along the axis of a cylindrical shell in stainless steel having an inner diameter (D) of 100 mm, and a surrounded rarefied gas has been studied experimentally and numerically. The ratio between the outer and inner diameter of the annular region filled by the gas is large (D/d=667). In the annular region filled with air the pressure was varied by using a vacuum pump from atmospheric value down to 10-3 mbar. Temperature differences between the wire and the external stainless steel wall in the range 50-125 K were imposed and the heat power transferred from the wire to the surround was measured as a function of the gas pressure starting from air at atmospheric conditions down to 10-3 mbar. The experimental results obtained in these tests were compared with the numerical results obtained by using the linear and nonlinear Shakhov kinetic models. © Published under licence by IOP Publishing Ltd
Design of gas micro distribution systems consisting of long tubes
No full textThis paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.A novel algorithm is developed for the design of gaseous micro distribution systems consisting of long tubes based on linear kinetic theory. Provided that the geometry of the pipe network is fixed the algorithm is capable of estimating the mass flow rates through the pipes as well as the pressure heads at the nodes of the network. The pressure distribution along each pipe element may also be provided. The analysis is valid and the results are accurate in the whole range of the Knudsen number, while the involved computational effort is very small. This is achieved by successfully integrating the well known kinetic results for single tubes into a typical solver for designing gas pipe networks.The European Communities under the contract of Association EURATOM / Hellenic Republic
Switching criteria for hybrid rarefied gas flow solvers
Full text linkA set of local Knudsen numbers are defined, which are demonstrated to be more appropriate than conventional ones for the purposes of identifying gas flow non-equilibrium. The problematic area of choosing an appropriate switching criteria is addressed by adopting a local Knudsen number definition based on higher-order constitutive relations; the R13 equations are chosen. A procedure is then described that allows the estimation of the R13 local Knudsen number within a Navier-Stokes solver, and the efficacy of this as a switching criterion is tested within an illustrative hybrid BGK/Navier-Stokes procedure. For the test case investigated, the results from the hybrid procedure compare very well with the full BGK solution, and are obtained at a fraction (depending on the global Kn) of the computational cost
Gas flow through a micro-orifice due to small pressure difference
No full textThis paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Rarefied gas flow through a micro-orifice connecting two reservoirs at small pressure differences is considered in the whole range of rarefaction by the linearized BGK kinetic model equation. The problem is computationally challenging due to the five dimensional nature of the distribution function and techniques such as parallelization and numerical schemes of low memory requirements have been applied. Results include the distributions of density, velocity, temperature, as well as flow rates. The independence of flow rate in terms of the wall surface accommodation properties is confirmed.The European Community under the contract of Association EURATOM/Hellenic Republic
Flow rate measurement of rarefied binary gases in long rectangular microchannels
No full textThis paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.The flow rate of binary gas mixtures through rectangular long microchannels is measured and compared to the numerical solution of the McCormack kinetic model. The microchannels are etched in silicon, and each individual channel has width=21, height=1.15, length=5000 μm. The measurement refers to He/Ar and Kr gas mixtures are based on the constant volume method. The microchannel is placed between an upstream and a downstream reservoir having different pressures. The flow through the microsystem is maintained by the pressure drop between the containers and the flow rate is determined from the pressure variations in reservoirs. In the case of He/Ar, measurements have been performed for several values of its concentration varying between zero and one, while in the case He/Kr only a concentration equal to 0.5 is considered. The pressure ratio between the two containers is in range of 3-7 and the corresponding average Knudsen numbers are in the range of 0.12-0.98. The results of the flow rate measurement are compared to the discrete velocity solution of the McCormack kinetic model and very good agreement between experiment and simulation has been obtained for all flow configurations. The relative discrepancy between the experimental and numerical results is in the range of the experimental uncertainty.The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement no 21550
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
Study of the thermomolecular pressure difference phenomenon in thermal creep flows through microchannels of triangular and trapezoidal cross sections
No full textThis paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.A detailed study of pressure and temperature driven flows through long channels of triangular and trapezoidal cross sections is carried out. The solution is based on the linearized Shakhov model subject to Maxwell boundary conditions and it is valid in the whole range of the Knudsen number. In addition to the dimensionless flow rates, a methodology is presented to estimate the pressure distribution along the channel, as well as the coefficient of the thermomolecular pressure difference
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
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