Majid Molki

Electrohydrodynamic Enhancement of Heat Transfer for Developing Air Flow in Square Ducts

Corona discharge in air was used to generate a secondary flow that significantly improved the convective heat transfer coefficient in the developing region of a square duct. A wire electrode, charged with a high voltage, was placed at the center of the duct. The electrode ionized the air and created a corona current, which was measured experimentally and used in the computations. The conservation differential equations for flow, energy, and electric field were considered for the 3D flow in the developing region of the duct, and they were solved numerically. The computational results indicated that the secondary flow gradually evolves into a fully-developed eight-cell vortical structure that creates flow mixing and turbulence and enhances the heat transfer coefficient. In this study, the ranges of Reynolds number and applied voltage were 500–10,000 and 0–11.7 kV, respectively. The normalized Nusselt number, Nu/Nuo, ranged from 1.06 (Re = 10,000 and 8.1 kV) to 13.58 (Re = 2000 and 11.7 kV), but the pressure drop penalty was relatively low. The normalized friction factor, f/fo, ranged from 1.01 (Re = 10,000 and 8.1 kV) to 7.28 (Re = 2000 and 11.7 kV). The computations were verified by comparison with the published literature.

EHD-Enhanced Condensation of Alternative Refrigerants in Smooth and Corrugated Tubes

An experimental study of EHD-enhanced in-tube condensation of alternative refrigerants is presented. The refrigerants tested were the single-component refrigerant R-134a, the zeotropic mixture R-407c, and the near-azeotrope R-404a. Tests for R-404a and R-407c were performed in smooth and corrugated tubes, whereas R-134a tests were performed only in corrugated tubes, with smooth tube data extracted from Singh (1995). The tests were performed with internally mounted cylindrical electrodes. It was found that, in general, all three refrigerants respond remarkably well to the EHD enhancement. The heat transfer performance of near-azeotrope R-404a is enhanced 18.8-fold in the smooth tube at the highest applied voltage of 18 kV, with a corresponding pressure drop penalty of 11.8-fold, and the maximum enhancement inside corrugated tube is 5.8-fold for the range of conditions tested in this study. R-134a has a maximum heat transfer enhancement of 8.3-fold, with a corresponding pressure drop penalty of 20.8-fold at an applied voltage of 18 kV inside the corrugated tube. Among the three refrigerants tested, R-407c shows the lowest heat transfer enhancement, with a 3.9-fold maximum enhancement at an applied EHD voltage of 18 kV inside the smooth tube, and a maximum enhancement of 2.9-fold inside the corrugated tube.

Patterns of Airflow in Circular Tubes Caused by a Corona Jet With Concentric and Eccentric Wire Electrodes

A computational investigation is conducted to study the patterns of airflow induced by corona discharge in the cross section of a circular tube. The secondary flow induced by corona wind in various flow passages has been the subject of numerous investigations. The flow patterns are often identified by multiple recirculation bubbles. Such flow patterns have also been anticipated for circular cross sections where the corona discharge is activated by an electrode situated at the center of the cross section. In this investigation, it is shown that, contrary to public perception, a symmetric corona discharge does not generate a secondary flow in circular cross sections. This investigation then proceeds to demonstrate that the flow responsible for thermal enhancements in circular tubes often reported in the published literature is induced only when there is a slight asymmetry in the position of the electrode. The present computations are performed in two parts. In part one, the electric field equations are solved using the method of characteristics. In part two, the flow equations are solved using a finite-volume method. It is shown that the method of characteristics effectively eliminates the dispersion errors observed in other numerical solutions. The present computations show that the flow in the eccentric configuration is characterized by a corona jet that is oriented along the eccentricity direction and two recirculation zones situated on either sides of the jet. In addition to the computational approach, a number of analytical solutions are presented and compared with the computational results.

An Improved Solution of Electrodynamics Equations for Corona Discharge Using Explicit Artificial Viscosity

A computational effort is undertaken to demonstrate how the numerical solution of electrodynamics equations may lead to a distorted solution. The computations are performed on geometries with circular and triangular cross sections using structured and unstructured grids. In addition, an analytical solution is presented for the circular geometry. It is demonstrated that the solution of the charge density equation suffers from dispersion errors. This leads to distorted values of charge density, which may distort the electric body forces and eventually affect the secondary flows generated by these forces. It is found that adding a judicious amount of artificial viscosity rectifies the problem.

Flow Characteristics of Rotating Disks Simulating a Computer Hard Drive

ABSTRACT A computational effort was undertaken to study the fluid flow inside a computer hard drive. Disk arrangements with multiple arms were considered inside a stationary enclosure. The results indicated large differences between circumferential velocities. In some cases, the slopes of the circumferential velocities were close to those represented by the solid-body rotation and the analytical solution for a thin fluid gap with one stationary and one rotating wall. The circumferential velocities were strongly affected by the arm length. The rotational speed and arm length had an increasing effect on the viscous power dissipation of the disks.

Visualization and Modeling of Flow Boiling of R-134A in Minichannels with Transverse Ribs

This research was undertaken to investigate the flow boiling of R-134a in minichannels with transverse ribs. The test channel consisted of two parallel plates; the lower plate was heated, and the upper plate was adiabatic. A number of transverse ribs were deployed along the lower plate. The hydraulic diameter of the channel was 1.93 mm. In these experiments, the main test parameters were mass flux, saturation temperature, heat flux, and average vapor quality. With these parameters, the convective and boiling numbers ranged from 0.04 to 0.90 and 7.49 × 10− 4 to 8.98 × 10− 4 , respectively. Of a total of 60 data points recorded, 78% of the data fell in the convective boiling region, while the remaining 22% were in the nucleate boiling region. A two-equation model was fit to a total of 60 experimental data, resulting in a correlation that predicted 58 data points to within ± 10%.

Patterns of Secondary Flow Field in a Circular Tube Caused by Corona Wind Using the Method of Characteristics

Corona discharge is widely known as an effective method for improving the characteristics of the flow field and enhancing heat transfer. Distribution of charge density and electric field form a Coulomb body force which acts on the charged particles within the fluid and generates a secondary flow field. The thermal enhancing effects of corona wind are normally dominant in low Reynolds numbers or free convection problems. Although the governing differential equations of corona discharge are relatively simple, solving these equations by conventional computational methods does not yield a smooth solution for charge density and electric field. In particular, the results obtained from finite-volume method suffer from dispersion errors and fluctuations which lead to distorted values of electric body force, and consequently a distorted secondary flow. In this study, the corona discharge in a circular tube with the electrode positioned at the tube centerline is considered. An exact solution for charge density, electric field, and potential distribution along the radius of the tube has been derived analytically using a Lagrangian formulation for the charge density and the Method of Characteristics. It was found that the results of this method do not show any fluctuations or dispersion effects on charge density and electric field. The solution of the electric field provided a body force which was used in the Navier-Stokes equations to obtain the secondary flow in the cross section of the tube. In this paper, the electric and fluid flow fields are presented. The results are compared with those obtained by other computational methods and the differences are discussed.Copyright

The Enhancement Effect of Corona Discharge on Natural Convection Heat Transfer in Triangular Channels

A numerical research was conducted to investigate the effect of corona discharge on natural convection heat transfer in a triangular channel. The channel side walls were isothermal, the lower wall was adiabatic, and the fluid was air at the atmospheric pressure. The electric field was generated by a positive corona discharge from a charged wire electrode located at the center of the channel. The governing equations of the electric and flow fields were solved by a finite-volume technique. Three cases were considered: (1) natural convection; (2) corona-induced convection; and (3) combined natural and corona-induced convection. The computations encompassed Rayleigh numbers from 3,737 to 37,377 and applied voltages from 7 to 9 kV in the laminar regime. Computations for the applied voltage of 7.5 kV indicated that the heat transfer was enhanced from Nu/Nu0 = 1.01 to 1.04, or 1–4%, for different values of Ra. The enhancement at Ra = 3,737 and the applied voltage of 7 to 9 kV was Nu/Nu0 = 1.01 to 1.40, or 1–40%.Copyright

Flow Boiling of R-134a in Minichannels With Transverse Ribs

An experimental effort was undertaken to investigate the flow boiling of R-134a in minichannels with transverse ribs. The test channel consisted of two parallel plates; the lower plate was heated while the upper plate was adiabatic and fabricated from a clear material to facilitate the visualization and photography of the boiling process. A number of transverse ribs were deployed along the lower plate to form a meandering flow passage inside the minichannel. The ribs blocked 3/4 of the flow cross-section in the transverse direction and the hydraulic diameter of the channel was 1.93 mm. In these experiments, the range of test parameters were: mass flux 100 to 225 kg/m2 s, saturation temperature 15 to 30°C, heat flux 14 kW/m2 , and average vapor quality 0.12 to 0.86. With these parameters, the Convective and Boiling numbers ranged from 0.04 to 0.90 and 7.49 × 10−4 to 8.98 × 10−4 , respectively. Of a total of 60 data points recorded, 78% of the data fell in the convective boiling region, while the remaining 22% were in the nucleate boiling region. In an attempt to model and correlate the data, it was found that the Kandlikar’s correlation proposed for augmented tubes was a suitable model for the present minichannel. Using the least-squares procedure, this two-equation model was fit to a total of 60 experimental data, resulting in a correlation that predicted 58 data points to within ±10%.Copyright

The Response of Framed Steel Structures to Fire

A combined computational fluid dynamics and structural stress investigation was conducted to study the response of steel structures to fire. The structure was a two-story building with six rooms. The fire was confined to one room on the first floor. The free convection of air and thermal radiation were both included in the analysis. The governing differential equations of flow and energy were solved by FLUENT. The air turbulence was modeled by the standard k-ϵ model, and the radiation was simulated by the Discrete Transfer Radiation Model. The temperature of the beams and columns were subsequently used in the thermal stress analysis to obtain the deflection and total strain using ANSYS. The result of FLUENT analysis indicates that the air velocity was high in the middle of the room above the fire and adjacent to the ceiling. The temperatures of the beams and columns generally decreased away from the center, with a more rapid decrease toward the ends. It was also observed that radiation played an important role in fire modeling (i.e., 20–40% of the heat transfer took place by radiation). The ANSYS analysis indicated a maximum deflection of 28.66 mm at a beam-column joint in the room with fire at the first level of the structure. The corresponding steel column design load capacity is exhausted after 34 minutes of standard ISO 834 fire due to a combined temperature and gravity loads. The maximum strain was observed at the center of the longitudinal beam, and the minimum was on the steel members of the second floor.