Vol 26, No 1 (2017)

An autothermal mode of tar conversion was implemented in a vertical tubular reactor. The pressure was 30 MPa; the tar and water flows were 4.9 and 6.9 g/min, respectively; the oxygen flow varied: 0, 2.9, 4.5 and 5.8 g/min. The tar was supplied from above, at the first stage into a counter-flow of supercritical water (SCW) and at the subsequent ones into a flow of SCW/O₂ fluid. The high-molecular sedimentary layer (HSL) that formed at the first stage was replenished continuously at the subsequent stages, consisted of tar components depositing on the bottom of the reactor and was suppressing formation of soot. The autothermal mode of the process was achieved due to heat release in the combustion of the lower part of the HSL in the SCW/O₂ fluid. With increased O₂ flow, the power of ohmic heaters was reduced to zero and the reactor wall temperature increased from the initial 723 K to 818 K. Elemental and mass spectrometric analyses of the liquid and volatile conversion products collected at the outlet of the reactor, as well as of the solid conversion residue taken from the reactor, enabled determination of their amount and composition. This in turn allowed us to write down the gross reaction of tar conversion in the SCW/O₂ fluid and determine the characteristics of the equivalent fuel and the thermal effects of its oxidation

Mathematical and computermodeling of thermal processes, applied presently in thermal design of electronic systems, is based on the assumption that the factors determining the thermal processes are completely known and uniquely determined, that is, they are deterministic.Meanwhile, practice shows that the determining factors are of indeterminate interval-stochastic character. Moreover, thermal processes in electronic systems are stochastic and nonlinearly depend on both the stochastic determining factors and on the temperatures of electronics elements and environment. At present, the literature does not present methods of mathematical modeling of nonstationary, stochastic, nonlinear, interval-stochastic thermal processes in electronic systems to model thermal processes, which satisfy all the above-listed requirements to modeling adequacy. The present paper develops a method of mathematical and computer modeling of the nonstationary intervalstochastic nonlinear thermal processes in electronic systems. The method is based on obtaining equations describing the dynamics of time variation of statistical measures (expectations, variances, covariances) of temperature of electronic system elements with given statistical measures of the initial interval-stochastic determining factors.

The present study investigates a Casson fluid flow in the presence of free convection of combined heat and mass transfer toward an unsteady permeable stretching sheet with thermal radiation, viscous dissipation and chemical reaction. The governing partial differential equations are reduced to a system of nonlinear ordinary differential equations and then solved by an efficient Runge–Kutta–Fehlberg method. The dimensionless velocity is decreased by increasing values of the chemical reaction and magnetic parameter while fluid temperature is significantly reduced by increasing values of the Prandtl number. The heat transfer rate is reduced with increasing values of thermal radiation and magnetic parameters.

We study heat transfer in the incompressible flow of a conducting third-grade fluid subject to a uniform magnetic field past an oscillating porous vertical plate.We obtain the analytical form of the boundary-layer velocity profile, the temperature profile, and the skin friction coefficient for small deviations from the Newtonian rheology. We examine the dependence of these quantities on the Prandtl number, the mixed convection parameter, the Hartmann number, and the suction parameter.

Two-dimensional steady-state laminar natural convection was studied numerically for differentially heated air-filled closed cavity with adiabatic top and bottom walls. The temperature of the left heated wall and cooled right wall was assumed to be constant. The governing equations were iteratively solved using the control volume approach. In this paper, the effects of the Rayleigh number and the aspect ratio were examined. Flow and thermal fields were exhibited by means of streamlines and isotherms, respectively.Variations of the maximum stream function and the average heat transfer coefficient were also shown. The average Nusselt number and was correlated to the Rayleigh number based on curve fitting for each aspect ratio. The investigation covered the range 10⁴ ≤ RA ≤ 10⁷ and is done at Prandtl number equal to 0.693. The result shows the average Nusselt number is the increasing function of Rayleigh number. As the aspect ratio increases, Nusselt number decreases along the hot wall of the cavity. As Rayleigh number increases, Nusselt number increases. Result indicates that at constant aspect ratio, with increase in Rayleigh number the heat transfer rate increases.

A steady two-dimensional free convective flow of a viscous incompressible fluid along a vertical stretching sheet with the effect of magnetic field, radiation and variable thermal conductivity in porous media is analyzed. The nonlinear partial differential equations, governing the flow field under consideration, have been transformed by a similarity transformation into a systemof nonlinear ordinary differential equations and then solved numerically. Resulting non-dimensional velocity and temperature profiles are then presented graphically for different values of the parameters. Finally, the effects of the pertinent parameters, which are of physical and engineering interest, are examined both in graphical and tabular form.

An unsteady MHD laminar viscous dissipative fluid flow past a semi-infinite vertical plate with variable surface temperature in the presence of heat source is considered in the present analysis. The present approach transforms the governing boundary layer equations into nondimensional form using the appropriate nondimensional quantities, which is valid in the free convection region. The resulting governing equations are solved numerically using the Crank–Nicolson method, an efficient implicit finite-difference scheme. Numerical results are obtained and presented in the form of local as well as average shearing stress, local and average heat transfer rate, velocity and temperature during the transient period. The present results are compared with the available results in the literature and are found to be in an excellent agreement.