Vol 25, No 4 (2016)

In the present study, the behavior and details of vapor–liquid contact on the dual-flow trays (DFTs) were investigated using a 3D CFD model within the two-phase Eulerian framework. The simulation provided good agreement with experimental results, which verified the reliability of the model. Firstly, the operation range was divided into four regimes having different characteristics of flow phenomena, and most attention was paid to the hydrodynamic behavior of froth regime and fluctuating regime due to its importance in operation and structure improvement. Then, the liquid flow characteristics of these two regimes were revealedwith the analysis of velocity profiles and liquid phase distribution contour. Meanwhile, some in-depth picturing of local events of hydrodynamics and mass-transfer performance was also presented through the discussion of cyclohexane liquid volume fraction distribution and vapor-phase mole fraction distribution. The froth regime was proved to have higher and homogeneous point efficiency at bulk zone around the plate center area than the fluctuating regime. Thus, the improvement of DFT should be focused on the restriction of “free-flow” to prolong the froth regime and delay the formation of vortex flow with circulation cells. Finally, an orthogonal wave tray (OWT) was designed and compared with DFT for evaluation of the effect of proposed modifications on the enhancement of tray hydraulics, mass-transfer efficiency and stability.

The paper represents results on investigation of methane oxidation in supercritical water (SCW) in autoclave and flow conditions. In the autoclave, oxidation is realized under uniform heating of a CH₄/O₂/H₂O and CH₄/O₂/N₂ mixture to 873 K (the water and nitrogen density ≈ 3.2 mmol/cm³, the molar ratio [O₂]₀/[CH₄]₀ ≈ 1 and 2). In the composition of the oxidation products we detected H₂ (only at [O₂]₀/[CH₄]₀ ≈ 1), CO and CO₂. Based on time dependences of the reaction mixture temperature we have found that temperature of the onset of self-heating of the CH₄/O₂/H₂O mixture is lower by 23 K than that of the CH₄/O₂/N₂ mixture and grows as the CH₄ concentration decreases. For comparable values of self-heating the average power in CH₄ combustion in the H₂O medium has appeared to be about two orders lower than in the N₂ medium, which evidences inhibition of SCWmethane oxidation. In the boiler-reactor, oxidation was realized while mixing CH₄ and O₂ in counter-propagating jets in the cocurrent upflow of SCW at 673–874 K, 30 MPa (molar ratio [O₂]/[CH₄] ≈ 2.2). Unsteady combustion was observed only at a reaction mixture temperature of 678 K, which became steady at 700 K after a series of flashes. The carbon-bearing methane oxidation products in the boiler-reactor contain only CO₂ (≥ 97.5%) and CO (≤ 2.5%mole).

The numerical stochastic Lagrangian modeling of the passive tracer in a convective atmospheric boundary layer (CABL) was performed based on the random walk and Langevinmodels of turbulent dispersion. The statistical structure of turbulence is modeled by the probability density function (PDF) of vertical velocity fluctuations, which is recovered by the calculated statistical moments of the vertical velocity fluctuations. Four models of the PDF reconstruction were tested and the results of simulations are compared with the experimental data in CABL. The superiority of Langevin model over the random-walk models is demonstrated.

The paper represents results on numerical investigation of flow and heat transfer between two isothermal vertical plates under laminar natural convection. A system of complete Navier–Stokes equations is solved for a two-dimensional gas flow between the plates along with additional rectangular regions (connected to inlet and outlet sections), whose characteristic sizes are much greater than the spacing between the plates. The calculations were performed over very wide ranges of Rayleigh number Ra = 10 ÷ 10⁵ and a relative channel length AR = L/w = 1 ÷ 500. The influence of the input parameters on the gas-dynamic and thermal structure of thermogravitational convection, the local and mean heat transfer, and also the gas flow rate between the plates (convective draft. We determined sizes of the regions and regime parameters when the local heat flux on the walls tends to zero due to the gas temperature approach to the surface temperature. It is shown that the mean heat transfer decreases as the relative channel length AR grows, whereas the integral gas flow rate (convective draft) and Reynolds number in the channel Re = 2wUm/ν increase. The use of a modified Rayleigh number Ra* = Ra · (w/L) (Elenbaas number) leads to generalization of calculation data on mean heat transfer. These data are in good agreement with the correlations for heat transfer [1, 2] and gas flow rate [3]. The reasons of variation of the data in the range of low Rayleigh numbers are discussed in detail.

A closedmathematical description of the fields of velocities, concentrations, and temperatures on a contact cross-flow tray was obtained via simultaneous application of a two-dimensional system of differential energy, mass, and momentum transfer equations (written for liquid phase of a turbulent bubble layer with regard to gas interaction) and equilibrium and balance equations. The paper presents results of numerical solving this system and comparison with experimental data on the velocity field, concentration and temperature fields in distillation of various mixtures. Results of calculating the heat and mass transfer efficiency of bubble trays are also presented.

This study investigates the peristaltic transport of magnetohydrodynamic (MHD) Carreau–Yasuda nanofluid through an asymmetric channel. Viscous dissipation, Joule heating and Hall effects are also included in the analysis. Velocity, thermal and concentration slip conditions are considered. The problem is modeled subject to long wavelength and low Reynolds number assumptions. Resulting nonlinear equations are numerically solved. Impact of embedded parameters on the fluid velocity, temperature, concentration of nanoparticles and heat and mass transfer rates at the wall are examined. Graphical results show that an escalation in the strength of appliedmagnetic field and increase in the value of Hall parameter reduce the velocity of nanofluid. Brownian motion and thermophoresis effects increase the temperature of the nanofluid. The present study shows an excellent agreement with the previously available studies in the limiting case.

Laminar natural convection plume of a microstructural non-Newtonian fluid along a vertical surface about a line heat source in a saturated high permeability porous medium is studied. The transformed non-linear boundary value problem is solved numerically using a rigorously tested SQLM algorithm, which combines a spectral collocationmethod with the quasilinearization method (QLM). The effects of Grashof number, Prandtl number, Darcy number and Eringen micropolar rheological material parameters are examined. Excellent stability and convergence of the spectral method is demonstrated. Validation of solutions with the Keller box finite difference is included. Applications of the study arise in geological (petroleum) fluid dynamics.