Heat and mass transfer in a cylindrical grain silo submitted to a periodical wall heat flux

1994 ◽  
Vol 37 (12) ◽  
pp. 1699-1712 ◽  
Author(s):  
Jean-Paul Fohr ◽  
Hocine Ben Moussa
Keyword(s):  
Mass Transfer ◽  
Heat Flux ◽  
Heat And Mass Transfer ◽  
Wall Heat Flux

Using Direct Simulation Monte Carlo With Improved Boundary Conditions for Heat and Mass Transfer in Microchannels

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
J. Yang ◽  
J. J. Ye ◽  
J. Y. Zheng ◽  
I. Wong ◽  
C. K. Lam ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat Flux ◽  
Aspect Ratio ◽  
Heat And Mass Transfer ◽  
Wall Temperature ◽  
Gas Flow ◽  
Wall Heat Flux ◽  
Entrance Region ◽  
Mass Flowrate

Micro-electromechanical systems and nano-electromechanical systems have attracted a great deal of attention in recent years. The flow and heat transfer behaviors of micromachines for separation applications are usually different from that of macro counterparts. In this paper, heat and mass transfer characteristics of rarefied nitrogen gas flows in microchannels are investigated using direct simulation Monte Carlo with improved pressure boundary conditions. The influence of aspect ratio and wall temperature on mass flowrate and wall heat flux in microchannels are studied parametrically. In order to examine the aspect ratio effect on heat and mass transfer behaviors, the wall temperature is set constant at 350 K and the aspect ratio of the microchannel varies from 5 to 20. The results show that as the aspect ratio increases, the velocity of the flow decreases, so does the mass flowrate. In a small aspect ratio channel, the heat transfer occurs throughout the microchannel; as the aspect ratio of the microchannel increases, the region of thermal equilibrium extends. To investigate the effects of wall temperature (Tw) on the mass flowrate and wall heat flux in a microchannel, the temperature of the incoming gas flow (Tin) is set constant at 300 K and the wall temperature varies from 200 K to 800 K while the aspect ratio is remained unchanged. Results show that majority of the wall heat flux stays within the channel entrance region and drops to nearly zero at the halfway in the channel. When Tw<Tin, under the restriction of pressure-driven condition and continuity of pressure, the molecular number density of the flow decreases along the flow direction after a short increase at the entrance region. When Tw>Tin, the molecular number density of the flow drops rapidly near the inlet and the temperature of the gas flow increases along the channel. As Tw increases, the flow becomes more rarefied, the mass flowrate decreases, and the resistance at the entrance region increases. Furthermore, when Tw>Tin, a sudden jump of heat transfer flux and temperature are observed at the exit region of the channel.

Heat Transfer and Air Diffusion Phenomena in a Bed of Polymer Powder Using Apparent Heat Capacity Method: Application to the Rotational Molding Process

2007 ◽  
Author(s):  
M. Boutaous ◽  
E. Pe´rot ◽  
A. Maazouz ◽  
P. Bourgin ◽  
P. Chantrenne
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat Flux ◽  
Phase Change ◽  
Heat And Mass Transfer ◽  
Granular Media ◽  
Transfer Model ◽  
Melting Front ◽  
Polymer Powder ◽  
Air Diffusion

The process of rotational moulding consists in manufacturing plastic parts by heating a polymer powder in a biaxial rotating mould. In order to optimise the production cycle of this process, a complete simulation model has to be used. This model should describe the phenomena of heat and mass transfer in a moving granular media with phase change, coalescence, sintering, air evacuation and crystallization during the cooling stage. This paper focus on the study of heat and mass transfer in a quiescent polymer powder during the heating stage. An experimental device has been built. It consists in an open plane static mold on which an initial thickness, e, of a polymer powder is deposited. This powder is then heated until it melts. An inverse heat conduction method is used to determine the heat flux and temperature at the interface between the mold and the powder. This interfacial heat flux is taken as a boundary condition in a numerical heat transfer model witch takes into account the heat transfer in granular media with phase change, coalescence, sintering, air bubbles evacuation and rheological behaviour of the polymer. For the numerical simulation of the heat transfer, the apparent specific heat method is used. This approach allows to solve the same energy equation for all the material phases, so one do not have to calculate the melting front evolution. This fine modelling, close to the real physical phenomena makes it possible to estimate the temperature profile and the evolution of the polymer powder characteristics (phase change, air diffusion, viscosity, evolution of the thermophysical properties of the equivalent homogeneous medium, thickness reduction, air volume fraction...). Several results are then presented, and the influence of different parameters, like the thermal contact resistance, the process initial conditions and the polymer’s rheological characteristics are studied and commented. Indeed the predictions of the temperature rises in the polymer bed, agree well with the experimental measurements.

Analysis of Vapor Blanket Formation in the Porous Wick of a Loop Heat Pipe With High Heat Load

2013 ◽  
Author(s):  
X. M. Huang ◽  
X. Jin ◽  
B. B. Chen ◽  
W. Liu
Keyword(s):  
Mass Transfer ◽  
Heat Flux ◽  
Heat Pipe ◽  
Heat And Mass Transfer ◽  
Heat Load ◽  
High Heat ◽  
Loop Heat Pipe ◽  
Porous Wick ◽  
High Heat Load ◽  
Theoretical Results

A loop heat pipe has different transport mechanisms depending on heat flux. The interface of liquid and vapor cannot maintain at the surface of the wick when heat flux is high, and a vapor blanket will form in the wick. To investigate when the vapor blanket appears and how it affects heat and mass transfer in the system is very import to minimize the device. A mathematical model of heat and mass transfer in the evaporator, coupled with analysis of fluid flow in the loop, is developed in the paper. The model is applied to calculate the critical heat load that the vapor blanket forms, and to analyze how the blanket delays. A comparison of theoretical results and experimental measurements is further presented. The consistence of the results validates the model and the mechanisms.

scholarly journals Heat and Mass Transfer of Unsteady Hydromagnetic Free Convection Flow Through Porous Medium Past a Vertical Plate with Uniform Surface Heat Flux

2017 ◽  
Vol 47 (3) ◽  
pp. 25-58 ◽  
Author(s):  
Mohamed Abd El-Aziz ◽  
Aishah S. Yahya
Keyword(s):  
Mass Transfer ◽  
Heat Flux ◽  
Porous Medium ◽  
Free Convection ◽  
Heat And Mass Transfer ◽  
Transport Model ◽  
Convection Flow ◽  
Physical Parameters ◽  
Free Convection Flow ◽  
Laplace Transform Technique

AbstractSimultaneous effects of thermal and concentration diffusions in unsteady magnetohydrodynamic free convection flow past a moving plate maintained at constant heat flux and embedded in a viscous fluid saturated porous medium is presented. The transport model employed includes the effects of thermal radiation, heat sink, Soret and chemical reaction. The fluid is considered as a gray absorbing-emitting but non-scattering medium and the Rosseland approximation in the energy equations is used to describe the radiative heat flux for optically thick fluid. The dimensionless coupled linear partial differential equations are solved by using Laplace transform technique. Numerical results for the velocity, temperature, concentration as well as the skin friction coefficient and the rates of heat and mass transfer are shown graphically for different values of physical parameters involved.

Heat and Mass Transfer in Wet Fibroporous Material Considering Moisture Evaporation

2015 ◽  
Vol 1097 ◽  
pp. 51-55
Author(s):  
V.Yu. Polovnikov ◽  
E.V. Gubina
Keyword(s):  
Thermal Conductivity ◽  
Numerical Simulation ◽  
Mass Transfer ◽  
Heat Flux ◽  
Effective Thermal Conductivity ◽  
Heat And Mass Transfer ◽  
Total Heat Flux ◽  
Moisture Evaporation ◽  
Volume Fractions ◽  
Mass Fluxes

Results of numerical simulation of heat and mass transfer in a wet fibroporous material in conditions of evaporation and steam diffusion were completed. Values of heat and mass fluxes were established. The contribution of evaporation effect to total heat flux and need to consider volume fractions of water and steam into the structure of fibroporous material in calculation of effective thermal conductivity were shown. Nonstationarity of heat and mass transfer in conditions of considered problem can be ignored.

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