Nicola Bianco

An Experimental Study of Radiative Effects on Natural Convection in Air in Convergent Channels

An experimental analysis on natural convection in air in a convergent channel uniformly heated at the principal walls has been carried out, in order to analyze the effects of the radiative heat transfer. Results in terms of wall temperature profiles as a function of the walls inclination angle, the interwall spacing, the heat flux are given for two values of the wall emissivity. Flow visualization has been carried out, in order to show the peculiar pattern of the flow between the plates in several configurations. The comparison between two wall emissivity values, 0.10 and 0.90, shows that the effect of thermal radiation is more marked for larger convergence angles. For a wall emissivity equal to 0.90 and for small values of the minimum channel spacing heat transfer in slightly convergent vertical channels is stronger than in a vertical parallel channel. Flow visualization points out a recirculating zone in the upper part of the channel for small values of the minimum channel spacing and for converging angles equal to 5° and 10°. Nusselt numbers and dimensionless maximum temperatures are then evaluated and correlated to the Rayleigh number, in the investigated range from 5 to 5×108 . A very good agreement between experimental data and correlations has been observed for the dimensionless parameters based on the maximum channel spacing.© 2003 ASME

Numerical Analysis of Opposing Mixed Convection in Air in a Vertical Channel With a Moving Plate

In this study a numerical investigation of mixed convection in air due to the interaction between a buoyancy flow and a moving plate induced flow in a vertical channel is carried out. The moving plate has a constant velocity and moves in the opposite direction with respect to the buoyancy force. The channel principal walls are heated at uniform heat flux. The numerical analysis is obtained by means of the commercial code Fluent. The effects of the channel spacing, heat transfer and moving plate velocity are investigated and results in terms of the channel wall and moving plate temperatures and Nusselt numbers are given. The wall temperature profiles allow to observe different behaviors of the flow motion inside the channel, a buoyancy flow, a forced flow and a transition flow related to the velocity of moving plate. The transition velocity increases as the heat flux and the channel gap increase. Dimensionless heat transfer results, Nu/Re0.68 as a function of Richardson number, Ri, present a good agreement with two correlations obtained for the buoyancy dominant flow, at Ri > 10, and forced dominant flow, at Ri < 10−3 .Copyright

Quasi-Steady State Numerical Model for a Multilayer Thin Film Irradiated by a Moving Laser Source at High Peclet Numbers

A numerical analysis of the conjugate optical-thermal fields in an amorphous silicon thin film deposited on a glass substrate and irradiated by a moving Gaussian laser source is carried out. The velocity of the laser is assumed constant and quasi-steady state conditions are considered. Optical and thermophysical properties of materials are assumed temperature dependent. The conjugate optical and thermal models are solved by means of a finite volume numerical technique and the heat conduction along the direction of motion has been neglected. The optical field is considered locally one-dimensional and it has been solved by means of the matrix method. Results are given in terms of radiative coefficients, absorption function distribution and temperature profiles and fields. The effect of the relative velocity and of the thin film depth are analyzed. For the considered model, radiative coefficients profiles show that for the largest thin film thickness, reflectance values do not depend very much on the Peclet number and on the x coordinate. Temperature profiles point out that the maximum temperature values are attained for an intermediate thin film thickness among the three considered values. This is due to the interference effects within the thin film that cause the maximum energy absorption for this thickness.Copyright

Effect of Impinging Jet on Heat Conduction in Workpieces Irradiated by a Moving Heat Source

A three dimensional conductive field is analyzed and solved by means of the COMSOL Multiphysics code. The investigated work-pieces are made up of a simple brick-type solid. A laser source with combined donut-Gaussian distributions is considered moving with a constant velocity along motion direction. The solid dimension along the motion direction is assumed to be infinite or semi-infinite, while finite width (2ly) and thickness (s) are considered. Thermal properties are considered temperature dependent and the materials are considered isotropic. Surface heat losses toward the ambient are taken into account. Several convective heat flux values on the upper surface, with corresponding Biot numbers, and Peclet numbers are considered with negligible radiative heat losses.Results are presented in terms of profile temperatures to evaluate the effect of impinging jet.

Effect of Channel Spacing on Mixed Convection in Vertical Convergent Channels

Mixed convection in air in a convergent channel with the two principal flat plates at uniform heat flux is analyzed numerically by Fluent code. In the considered system two parallel adiabatic extensions are placed downstream of the convergent channel. The forced flow is obtained by imposing a pressure drop between the inlet and the outlet of the channel. The flow in the channel is assumed to be two-dimensional, turbulent and incompressible. A k-e turbulent model is employed. Results in terms of dimensionless wall temperature distribution as a function of the walls converging angle, the Grashof number, the pressure drop and the channel aspect ratio are presented in the ranges: 0° ≤ θ ≤ 10°; 4.10 102 ≤ Gr ≤ 32.1 105 , 0 ≤ ΔP ≤ 8.82·107 , 10.15 < Lw /bmin < 58.0. Results show that Reynolds number, and then the mass flow rate flowing in the channel, increases at decreasing aspect ratios, Lw /bmin . The converging angle that optimizes the fluid-dynamic within the channel is equal to 5°. Dimensionless maximum wall temperature values decreases at increasing Reynolds number and the larger the aspect ratio, the larger the decrease. The Reynolds number over which natural convection become negligible, with respect to forced convection, increases at increasing converging angle and at decreasing aspect ratio.Copyright

Mixed Convection Heat Transfer in a Convergent Vertical Channel With a Moving Plate

A numerical investigation of mixed convection in air in a convergent vertical channel, due to the interaction between a buoyancy flow and a moving plate induced flow, is presented. The plate moves at a constant velocity along the buoyancy force direction and the principal inclined walls of the channel are heated at uniform heat flux. The numerical analysis is carried out by means of the finite volume method, using the commercial code Fluent. The effects of the channel spacing, wall heat flux, moving plate velocity and converging angle are investigated. Heated wall temperature increases at increasing converging angle, except for natural convection in a 10 mm minimum channel gap. The effect of the converging angle on the wall temperatures is less marked at the larger channel spacing. Maximum temperature of the moving plate is attained in the parallel wall channel for a 30 W m−2 wall heat flux, both in the 10 mm and 40 mm channel, whereas for a 220 W m−2 wall heat flux in the 40 mm channel in mixed convection, maximum wall temperatures are exhibited for a 10° angle. Nusselt, Reynolds and Richardson numbers are correlated by a monomial equation for each converging angle and a unique monomial correlation for all investigated angles in the 2.1·10−2 – 5.1·105 Richardson number range is presented.Copyright

Mixed Convection in Air in an Open Ended Cavity With a Moving Plate Parallel to the Cavity Open Surface

In this study, a numerical investigation of mixed convection in air in an open ended cavity, with a moving plate parallel to the cavity open surface, is carried out. The moving plate has a constant velocity, whereas a vertical plate of the open cavity is heated at uniform heat flux. All the other walls are adiabatic. The numerical analysis is obtained by means of the commercial code FLUENT. Two configurations, assisting and opposing, are analyzed. In the assisting configuration, natural convection is supported by the plate motion, whereas, in the opposing configuration, natural convection and plate motion have opposing effects. The effect of different geometrical parameters, heat flux and moving plate velocity are analyzed. Results in terms of heated plate and moving plate temperature profiles are presented and simple monomial correlation equations for both the configurations are proposed between the terms Nu/Re0.6 and Ri.Copyright

Radiation Effects on Natural Convection in Air in a Divergent Channel with Uniformly Heated Plates

Nowadays trends in natural convection heat transfer are oriented toward either the seeking of new configurations to enhance the heat transfer parameters or the optimization of standard configurations. An experimental investigation on air natural convection in divergent channels with uniform heat flux at both the principal walls is presented in this paper to analyze the effect of radiative heat transfer. Results in terms of wall temperature profiles as a function of the walls diverging angle, the interwall spacing, the heat flux are given for two value of the wall emissivity. Flow visualization is carried out in order to show the peculiar pattern of the flow between the plates in several configurations. Nusselt numbers are then evaluated and correlated to the Rayleigh number. The investigated Rayleigh number ranges from 7.0 × 102 to 4.5 × 108 . The maximum wall temperature decreases at increasing divergence angles. This effect is more evident when the minimum channel spacing decrease. A significant decrease in the maximum wall temperature occurs passing from e = 0.10 to e = 0.90, except in the inlet region. Flow visualization shows a separation of the fluid flow for bmin = 40 mm and θ = 10°. Correlations between Nusselt and Rayleigh numbers show that data are better correlated when the maximum channel spacing is chosen as the characteristic length.Copyright

Numerical Analysis of Heat Conduction in Cooling of Aluminum Extrusion

A thermal analysis of the cooling of an extruded aluminum alloy by means of water sprays is carried out. The heat conduction problem has been solved numerically by means of a finite volume method. The heat transfer coefficients used in the boundary conditions has been evaluated by means of spray heat transfer correlations, which relate these coefficients to the spray hydrodynamic parameters. The influence of the number of sprays and of the solid velocity has been investigated. Results show that the efficiency of the cooling system decreases as the number of jets increases. The efficiency of each spray increases with the velocity for the same number of sprays. As the workpiece velocity increases it needs to increase the number of sprays to obtain the same temperature difference between the entry and the exit of the cooling system. The greater the number of sprays related to the case with lower velocity, the smaller the increase of the number of sprays.Copyright