Slip-Flow and Heat Transfer in Isoflux Rectangular Microchannels with Thermal Creep Effects

A control-volume numerical approach has been used to study rarefaction effects in simultaneously hydrodynamically and thermally developing flow in rectangular microchannels with a prescribed uniform wall heat flux in the slip-flow regime (10-3 ≤ Kn ≤ 10-1). The effects of velocity slip and thermal creep on the key flow parameters are examined in detail. Low Reynolds number flows (Re ≤ 1) for different channel aspect ratios (0 ≤ α* ≤ 1) are considered. The effects of rarefaction on the global features of the flow and thermal development in the entrance region are examined. Dramatic reductions in the friction coefficient are observed in the entrance region due to rarefaction effects, which are enhanced by thermal creep. For the fluid heating cases considered here, thermal creep increases slip at the wall and thereby further reduces the friction coefficient and slightly enhances heat transfer at a given Reynolds number. For an identical heat flux applied to the microchannel walls, thermal creep effects become much more pronounced at lower Reynolds numbers since it results in higher axial temperature gradients

On the behavior of micro-spheres in a hydrogen pellet target

A pellet target produces micro-spheres of different materials, which are used as an internal target for nuclear and particle physics studies. We will describe the pellet hydrogen behavior by means of fluid dynamics and thermodynamics. In particular one aim is to theoretically understand the cooling effect in order to find an effective method to optimize the working conditions of a pellet target. During the droplet formation the evaporative cooling is best described by a multi-droplet diffusion-controlled model, while in vacuum, the evaporation follows the (revised) Hertz-Knudsen formula. Experimental observations compared with calculations clearly indicated the presence of supercooling, the effect of which is discussed as well.Comment: 22 pages, 8 figures (of which two are significantly compressed for easier download

Numerical investigation of aerodynamic droplet breakup in a high temperature gas environment

The Navier-Stokes equations, energy and vapor transport equations coupled with the VOF methodology and a vaporization rate model are numerically solved to predict aerodynamic droplet breakup in a high temperature gas environment. The numerical model accounts for variable properties and uses an adaptive local grid refinement technique on the gas-liquid interface to enhance the accuracy of the computations. The parameters examined include Weber (We) numbers in the range 15-90 and gas phase temperatures in the range 400-1000 K for a volatile n-heptane droplet. Initially isothermal flow conditions are examined in order to assess the effect of Weber (We) and Reynolds (Re) number. The latter was altered by varying the gas phase properties in the aforementioned temperature range. It is verified that the We number is the controlling parameter, while the Re number affects the droplet breakup at low We number conditions. The inclusion of droplet heating and evaporation mechanisms has revealed that heating effects have generally a small impact on the phenomenon due to its short duration except for low We number cases. Droplet deformation enhances heat transfer and droplet evaporation. An improved 0-D model is proposed, able to predict the droplet heating and vaporization of highly deformed droplets

A superposition approach to study slip-flow forced convection in straight microchannels of uniform but arbitrary cross-section

This work presents a superposition approach to investigate forced convection in microducts of arbitrary cross-section, subject to H1 and H2 boundary condition, in the slip-flow regime with further complication of a temperature jump condition assumption. It is shown that applying an average slip velocity and temperature jump definition, one can still use the no-slip/no-jump results with some minor modifications. Present results for slip flow in microchannels of parallel plate, circular, and rectangular cross-sections are found to be in complete agreement with those in the literature. Application of this methodology to microchannels of triangular cross-section is also verified by comparing the present results with those obtained numerically by undertaking the commercially available software CFD-ACE

Slip flow forced convection in a microporous duct of rectangular cross-section

Applying a Fourier series approach, closed form solutions for fully developed velocity and temperature distribution in a porous-saturated microduct of rectangular cross-section are presented in the slip-flow regime. The Brinkman flow model is applied. Invoking the temperature jump equation, the H1 thermal boundary condition is investigated. Expressions are presented for the local and average velocity and temperature profiles, the friction factor, and the slip coefficient in terms of the key parameters. The present results, which are applicable to microducts of rectangular cross-section filled with or without a porous medium, are found to be in complete agreement with those available in the literature for both no-slip and slip flow regime