Bakhtier Farouk

A numerical study of three-dimensional natural convection in a differentially heated cubical enclosure

A high-resolution, finite difference numerical study is reported on three-dimensional steady-state natural convection of air, for the Rayleigh number range 103 ⩽ Ra ⩽ 106, in a cubical enclosure, which is heated differentially at two vertical side walls. The details of the three-dimensional flow and thermal characteristics are described. Extensive use is made of state-of-the-art numerical flow visualizations. The existence of the transverse z-component velocity, although small in magnitude, is clearly shown. Comparison of the present three-dimensional results with the two-dimensional solutions is conducted. The three-dimensional data demonstrate reasonable agreement with the experimental measurements.

Characterization of a dc atmospheric pressure normal glow discharge

Atmospheric pressure dc glow discharges were generated between a thin cylindrical anode and a flat cathode. Voltage?current characteristics, visualization of the discharge and estimations of the current density indicate that the discharge is operating in the normal glow regime. Emission spectroscopy and gas temperature measurements using the 2nd positive band of N2 indicate that the discharge forms a non-equilibirum plasma. Rotational temperatures are 700?K and 1550?K and vibrational temperatures are 5000?K and 4500?K for a 0.4?mA and 10?mA discharge, respectively. The discharge was studied for inter-electrode gap spacing in the range of 20??m?1.5?cm. It is possible to distinguish a negative glow, Faraday dark space and positive column regions of the discharge. The radius of the primary column is about 50??m and is relatively constant with changes in electrode spacing and discharge current. Estimations show that this radial size is important in balancing heat generation and diffusion and in preventing thermal instabilities and the transition to an arc.

DC normal glow discharges in atmospheric pressure atomic and molecular gases

DC glow discharges were experimentally investigated in atmospheric pressure helium, argon, hydrogen, nitrogen and air. The discharges were characterized by visualization of the discharges and voltage and current measurements for current of up to several milliamperes. Significant differences are seen in the gas temperature; however all the discharges appear to operate as temperature and pressure scaled versions of low pressure discharges. In the normal glow discharges, features such as negative glow, Faraday dark space and positive column regions are clearly observable. In hydrogen and to a lesser degree in helium and argon standing striations of the positive column were visible in the normal glow regime. Normal glow characteristics such as normal current density at the cathode and constant electric field in the positive column are observed although there are some unexplained effects. The emission spectra for each of the discharges were studied. Also the rotational and vibrational temperature of the discharges were measured by adding trace amounts of N2 to the discharge gas and comparing modeled optical emission spectra of the N2 2nd positive system with spectroscopic measurements from the discharge. The gas temperatures for a 3.5 mA normal glow discharge were around 420 K, 680 K, 750 K, 890 K and 1320 K in helium, argon, hydrogen, nitrogen and air, respectively. Measured vibrational and excitation temperatures indicate non-thermal discharge operation. Mixtures of gases achieved intermediate temperatures.

Spectroscopic studies and rotational and vibrational temperature measurements of atmospheric pressure normal glow plasma discharges in air

DC normal glow (NG) discharges were created in atmospheric pressure air for a pin to plate type geometry. The rotational and vibrational temperatures of the discharge were measured by comparing modelled optical emission spectra with spectroscopic measurements from the discharge. The temperatures were measured as a function of discharge current, ranging from 50 µA to 30 mA, and discharge length, ranging from 50 µm to 1 mm. Rotational temperatures from 400 to 2000 K were measured over this range. Vibrational temperatures vary from 2000 K to as high as 5000 K indicating a non-equilibrium plasma discharge. Spectroscopic measurements were compared using several different vibrational bands of the 2nd positive system of N2, the 1st negative system of and the UV transitions of NO. NO and transitions were also used to determine the electronic temperature and density. The discharge temperature appears to be controlled by two cooling mechanisms: (1) radial conductive cooling which results in an increase in temperature with increasing discharge current and (2) axial cooling to the electrodes which results in a temperature saturation with increase in discharge current. The measured discharge temperature initially increases rapidly with discharge current then becomes nearly constant at a higher discharge current. Thus, radial cooling appears to dominate at lower discharge currents and the axial cooling at higher discharge currents. The vibrational temperature decreases with increasing rotational temperature due to increased vibrational to translation relaxation but the discharge remains non-thermal and stable over the range studied. The discharge appears to have a maximum vibrational temperature at the low current limit of the NG regime.

Laminar and turbulent natural convection. Radiation interactions in a square enclosure filled with a nongray gas

A numerical investigation of interactions oflaminar and turbulent natural convection and radiation in a differentially heated square enclosure was performed. Songray gas radiation was analyzed with the P^-approximation method for the radiative transfer equation and the weighted sum of gray gases model, A desirable compatibility was achieved in the resultant formulation for radiative transfer with the governing equations for natural convection flows. The Favre-averaged formulation was employed for analyzing turbulent flows together with a k-e model. In the analysis, a two-dimensional enclosure filled with carbon dioxide gas was considered. Solutions were obtained for a range of Grashof numbers and Prandtl numbers varying from 104 to 1010 Characteristics of flow and temperature fields were compared with predictions in the literature and were found to be in good agreement in general.

Numerical simulation of acoustic streaming generated by finite-amplitude resonant oscillations in an enclosure

Acoustic streaming motion in a compressible gas filled two-dimensional rectangular enclosure is simulated and the effects of the sound field intensity on the formation process of streaming structures are investigated numerically. The oscillatory flow field in the enclosure is created by the vibration of the left wall of the enclosure. The frequency of the wall vibration is chosen such that the lowest acoustic mode propagates along the enclosure. The fully compressible form of the Navier–Stokes equations is considered and an explicit time-marching algorithm is used to track the acoustic waves. The formation of the wave field in the enclosure is computed and fully described and the acoustic boundary layer development is predicted. The interaction of the wave field with viscous effects and the formation of streaming structures are revealed by time-averaging the solutions over a given period. The strength of the pressure waves associated with the acoustic effect and the resulting streaming flow pattern is fou...

An experimental study of heat transfer in a vertical annulus with a rotating inner cylinder

Abstract The results of an experimental study of the convective flows engendered within the annular gap between concentric vertical cylinders are presented. The inner cylinder is rotating and heated, while the outer cylinder is stationary and cooled. Stationary horizontal endplates are used to seal the annulus, forming an enclosure. The working fluid is air. Of particular interest is the accurate prediction of the heat transfer rates, which are intimately linked to the structure of the flow field. In rotating systems, the existence of hydrodynamic instabilities may lead to a variety of secondary flows as the parameters describing the system are varied. Along with each transition in a flow, the transport mechanisms are altered, and usually result in markedly changed rates of heat and momentum transport. Experiments are conducted to determine the interdependence between the heat transfer mechanism and the structure of the secondary flows. Specifically, a parametric study of the mean heat transfer rate across the annular gap is performed, as well as a qualitative study (using smoke visualization techniques) of the secondary flow characteristics of the rotating system. The results provide a qualitative description of the transition from a buoyancy-dominated flow regime to one dominated by rotation. A correlation for the heat transfer rate as a function of the rotational Reynolds number and radius ratio is obtained in the forced convection limit.

NATURAL AND MIXED CONVECTION HEAT TRANSFER AROUND A HORIZONTAL CYLINDER WITHIN CONFINING WALLS

Laminar natural and mixed convection around a heated cylinder placed within confining walls is investigated numerically. The numerical scheme involves the use of a cylindrical network of nodes in the vicinity of the cylinder with a Cartesian mesh covering the remainder of the flow domain. Results are obtained for the streamlines, isotherms, and heat transfer coefficients. Effects of varying the ratio of width across the walls to cylinder diameter are also investigated. The results obtained are of direct use, for instance, in the design of certain thermal energy storage devices and ocean thermal energy conversion units under investigation.

Simulation of dc atmospheric pressure argon micro glow-discharge

A hybrid model was used to simulate a dc argon micro glow-discharge at atmospheric pressure. The simulations were carried out for a pin-plate electrode configuration with inter-electrode gap spacing of 200??m together with an external circuit. The predicted voltage?current characteristics and current density profiles identify the discharge to be a normal glow-discharge. The neutral gas temperature predictions indicate that the discharge forms a non-thermal, non-equilibrium plasma. Experimental studies were conducted to validate the numerical model. Predictions from the numerical model compare favourably with the experimental measurements.

Low dielectric fluorinated amorphous carbon thin films grown from C6F6 and Ar plasma

Fluorinated amorphous carbon (a-C:F) thin films were synthesized for applications in low dielectric constant intermetal dielectric materials. The deposition was carried out in a capacitively coupled, asymmetric plasma reactor using hexafluorobenzene (C6F6) as the source gas and argon as the carrier gas. The effects of applied rf power on the electric, optical and mechanical properties of the a-C:F films were investigated. The bonding structures and properties of the films were evaluated by FTIR spectrometry, UV-Vis spectrophotometry and stress measurements. The films exhibit a dielectric constant as low as 2.0 and have high transparency in the visible range. The deposition rate of the a-C:F films increases, reaches a maximum, and then gradually decreases with increasing rf power. High rf power raises the negative self-bias voltage at the substrate and leads to an increase in the content of conjugated CC bonds in the a-C:F films. As the negative self-bias voltage at the substrate is increased, the dielectric constant, residual stress, and graphitization of the a-C:F films are increased while the optical energy gap is decreased.