Ahmet Cansiz

Vertical and lateral forces between a permanent magnet and a high-temperature superconductor

The vertical and horizontal forces and associated stiffnesses on a permanent magnet (PM) above a high-temperature superconductor (HTS) were measured during vertical and horizontal traverses in zero-field cooling (ZFC) and in field cooling (FC). In ZFC, the vertical stiffness was greater in the first descent than in the first ascent and second descent, and the stiffness in the second descent was between those of the first descent and the first ascent. At the FC position, the vertical stiffness was two times greater than the lateral stiffness at each height, to within 1% of the vertical stiffness value. The cross stiffness of vertical force with respect to lateral position was positive for FC, but negative for ZFC. Free-spin-down experiments of a PM levitated above a HTS were also performed. These results showed that the coefficient of friction is double valued at frequencies just below the rotor resonance, a result attributed to cross stiffness in the PM/HTS interaction. A frozen-image model was used to ca...

Stable load-carrying and rotational loss Characteristics of diamagnetic bearings

We investigated the use of diamagnetic materials in magnetic bearings using an experimental apparatus to study the static equilibrium, load-carrying capacity, and dynamic characteristics of the bearings. The apparatus consisted of a 0.79-g NdFeB disk-shaped permanent-magnet rotor levitated by a ferrite magnet with a diamagnetic stabilizer made of either bismuth or graphite, all in a bell-jar vacuum chamber to eliminate air friction on the rotor. The equilibrium position of the rotor was statically and dynamically stable, according to theoretical calculations based on the magnetic-image method. In dynamic tests, the rotor of the diamagnetic bearing was spun up to 100 Hz by a tangential jet of nitrogen gas and then was allowed to spin down freely to measure the rotational losses. The rotational losses varied with the frequency of the rotation, indicating that most of the losses were mainly due to eddy currents. Minor losses due to magnetic inhomogeneities in the permanent magnets and diamagnetic parts were also observed over a small frequency range.

Correlation between free oscillation frequency and stiffness in high temperature superconducting bearings

A superconducting magnetic bearing is a dynamic system, which undergoes vibrations at various frequencies during its operation. In this study, we investigated the free vibration frequency modes of a permanent magnet (PM) levitated over a high temperature superconductor (HTS) where the vibration was provided by the seismic activities of the earth. The amplitude of the vibration was less than 1 μm as measured by a vibrometer. A disk shaped PM was levitated over a melt-textured HTS YBCO (yttrium barium copper oxide). The experimental setup was adopted to do the fast Fourier transform analysis of the vibration characteristics of the levitated PM. A cross-coupling between the vibration frequency modes of vertical, lateral and angular is observed in all respective directions for any particular vibration frequency measurement. The results indicate that all the vibration modes are actually the combination of the pure vibration frequency modes. The theoretical predictions based on the frozen-image concept show that the ratio of the vertical to lateral stiffness should be higher than 2 in the dynamic case, which is observed experimentally.

Static and dynamic analysis of a diamagnetic bearing system

A unique numerical calculation method was constructed in order to analyze the static and dynamic characteristics of the diamagnetic bearing. An experimental setup consisting of a permanent magnet rotor, levitated with the arrangement of ferrite magnet and bismuth blocks, was used to confirm theoretical calculations. The stability analysis of the bearing relating to the vertical and lateral forces was incorporated with the dynamic model. The interaction force between the rotor magnet and the diamagnetic materials was modeled with the Amperian current approach via the diamagnetic-mirror-image concept. The magnetic potential of the system, which provides the equation of motion, was stated in terms of diamagnetic and gravitational interactions. The equations of motion of the dynamic part of the system were stated as a function of lateral, vertical, and tilt angles. Correlation of the vibrations such as vertical, radial, and tilt was determined with a numerical calculation obtained by the Runge–Kutta method. T...

Vertical, radial and drag force analysis of superconducting magnetic bearings

The behavior of the force between a permanent magnet (PM) and a high temperature superconductor (HTS) was tested with the frozen-image model based on flux pinning. It was found that the associated dipole moment assumptions of the method of the frozen image underestimate the force somewhat; thus a quadrupole moment analysis is proposed. The radial and drag forces associated with the rotation of the PM levitated above the HTS were measured by using a force transducer and by means of a cantilevered beam technique. The radial force was found not to be dependent on the radial direction, and the least radial force was found to be periodic with an angular displacement during the slow rotation of the PM relative to the HTS. The periodicity behavior of the force is attributed to the geometric eccentricity from the magnetization distribution of the PM and HTS. The drag force associated with the torsional stiffness of the levitated PM during the low and high rotational speeds was incorporated with the data from the literature.

An Evershed type superconducting flywheel bearing

The objective of this work is to develop a bearing using high temperature superconductors (HTSs) for use in an energy storage flywheel. The experimental apparatus includes a cylindrical rotor levitated with the Evershed design in which the majority of the levitation force is provided by a permanent magnet arrangement and the stabilization of the system is achieved by HTS elements. The design characteristics and dynamics of the bearing associated with the rotor part are presented. The instrumentation measures the out of balance force and magnetomechanical stiffness associated with the rotor. A study of the rotational losses was performed using free spin down experiments associated with magnetic field variation measurements. The results are consistent with the loss being caused by hysteresis in the superconductor due to magnet inhomogeneity.

Application of S-UTD-CH Model into Multiple Diffraction Scenarios

In this study, propagation prediction models based on ray tracing in coverage estimation for broadcasting systems are compared with respect to computation time and accuracy in the case of close building heights. Uniform Theory of Diffraction (UTD), Slope Diffraction (S-UTD) and Slope UTD with Convex Hull (S-UTD-CH) models are compared for computation time and propagation path loss. Moreover in this study, effects of transmitter height to relative path loss at the receiver are analyzed. Furthermore, contribution of S-UTD-CH model to UTD model increases in the transition zone. As a conclusion, S-UTD-CH model is optimum model with respect to computation time and relative path loss in transition zone.

An Effective Noncontact Torque Mechanism and Design Considerations for an Evershed-Type Superconducting Magnetic Bearing System

An Evershed type of superconducting magnetic bearing is designed and fabricated. An alternative torque mechanism is proposed to drive the rotor of the designed bearing. The design consideration for the rotor consists of a permanent magnet with 7.5-kg mass and its force interaction with the superconductor defined in terms of a semi-analytical frozen image model. The driving mechanism is based on eddy-current induction on a conducting disk placed on the rotor of the bearing. The driving system is intended not only to sustain stable levitation during the variation of the rotor speed but also to rotate the rotor with a speed higher than that of the torque provider. Levitation, drag, and lift forces are discussed via considering various conditions such as the rotor configuration and conducting material. The thickness of the conductor strongly affects the optimum working conditions of the magnetic bearing. In order to compensate the repulsive force due to lift force on the rotor, the top of the rotor was covered with iron sheet. The results indicate that the designed rotor and driving system have potential solutions for more advanced bearing applications for superconducting flywheel energy storage.

Heat Transfer Properties and Energy-Exergy Efficiency in a Finned Cross-Flow Heat Recovery Unit

In this study, a cross-flow heat recovery-exchanger system operating with unmixed fluids was manufactured and tested. The thermodynamic analysis of the system was presented via determining the variations of exergy loss with Reynolds number. The analysis also included the effects of convergent and divergent plate-type winglets on heat transfer properties of the designed system. Experimental tests consisted of temperature and pressure drop measurements for the Reynolds numbers (3000 and 12,000) according to the working conditions of the boiler, fan, preheater, and reheater. The effects of inlet conditions of the working fluids on the heat transfer characteristics were correlated via entropy generation number and exergy loss. The variation of the effectiveness with the number of transfer unit of heat recovery unit was calculated and discussed in terms of literature.

Driving stability of superconducting magnetic bearing system

A superconducting bearing system including a 7.5 kg disc shaped rotor is stabilized by using superconducting and magnetic levitation combination. The rotor is stabilized with the Evershed type bearing where the majority of the rotor mass is lifted via attractive permanent magnets configuration, and the stability of the rotor is provided by superconductors. The stability tests are performed via obtaining levitation force characteristic. Preliminary levitation force results show that the system is stable and robust; indicating rotor mass can be increased more without losing the efficiency. Dynamic tests are performed during spin of the designed bearing system. The resonance behavior of the system indicates that the bearing is not vulnerable to higher speeds.