Haruo Miura

Evaluation of thermal degradation of unidirectional CFRP rings

We have studied the thermal degradation of unidirectional carbon-FRP (CFRP) rings. First, we developed a method based on the ring tensile test for evaluating hoop directional strength. And we calculated the distribution of hoop stress by a finite-element method considering friction. Then, we measured the thermal reduction of fatigue strength of unidirectional CFRP rings. To evaluate the degradation of fatigue strength, we applied the Arrhenius equation to confirm the relation between temperature and time. Using this evaluation method, we predicted the thermal reduction of fatigue strength under high temperature and long exposure time.

Effect of Gas Environment on Friction Behavior and Tribofilm Formation of PEEK/Carbon Fiber Composite

This study investigated the friction and wear behaviors of polyetheretherketone composites containing carbon fibers (PEEK-CF) sliding against stainless steel in terms of their dependencies on gas environments. Sliding tests with a block-on-ring apparatus were conducted in humid/dry air and dry nitrogen at normal temperature and nitrogen at cryogenic temperature of 113 K. Friction and wear significantly decreased when sliding occurred in low-humidity, low-oxygen, and low-temperature environments. The friction coefficient, initially 0.25 in ambient humid air, decreased to below 0.10 in the dry nitrogen environment and 0.03 in nitrogen at 113 K. Microscopic observations, surface roughness measurements, and surface composition analysis of worn surfaces were conducted after the sliding tests. After sliding in humid air, the surface roughness of the PEEK-CF block was increased from its initial value by adhesions of large wear particles. However, the surface roughness of the block decreased due to the formations of tribofilms that were micrometers thick and composed of fine wear particles of polymer and carbon fibers after sliding in dry nitrogen. X-ray photoelectron spectroscopy analysis and the Raman spectroscopy analysis indicated increased carbon content with graphite structures on the worn surface after sliding in dry nitrogen. Consequently, it was considered that PEEK-CF changed its wear process in the dry nitrogen environment and formed a smooth surface with carbon particles containing graphite, which is thought to be advantageous in reducing friction.

Natural Frequency Shift in a Centrifugal Compressor Impeller for High-Density Gas Applications

In designing an impeller for centrifugal compressors, it is important to predict the natural frequencies accurately in order to avoid resonance caused by pressure fluctuations due to rotor-stator interaction. However, the natural frequencies of an impeller change under high-density fluid conditions. The natural frequencies of pump impellers are lower in water than in air because of the added mass effect of water, and in high-pressure compressors the mass density of the discharge gas can be about one-third that of water. So to predict the natural frequencies of centrifugal compressor impellers, the influence of the gas must be considered. We previously found in the nonrotating case that some natural frequencies of an impeller decreased under high-density gas conditions but others increased and that the increase of natural frequencies is caused by fluid-structure interaction, not only the added mass effect but also effect of the stiffness of the gas. In order to develop a method for predicting natural frequencies of centrifugal compressor impellers for high-density gas applications, this paper presents experimental results obtained using a variable-speed centrifugal compressor with vaned diffusers. The maximum mass density of its discharge gas is approximately 300 kg/m3. The vibration stress on an impeller when the compressor was speeding up or slowing down was measured by strain gauges, and the natural frequencies were determined by resonance frequencies. The results indicate that for high-density centrifugal compressors, some natural frequencies of an impeller increased in high-density gas. To predict this behavior, we developed a calculation method based on the theoretical analysis of a rotating disk. Its predictions are in good agreement with experimental results.

Rotordynamic Evaluation of Centrifugal Compressor Using Electromagnetic Exciter

Since heavier gases exert larger effects on rotordynamic stability, stability evaluation is important in developing or designing high-pressure compressors. To evaluate the rotor stability during operation, an excitation test using a magnetic bearing is the most practical method. In stability analysis, labyrinth seals can produce significant cross coupling forces, which particularly reduce the damping ratio of the first forward mode. Therefore, forward modes should be distinguished from backward modes in the excitation test. One method that excites only the forward modes, not the backward modes (and vice versa), is the use of a rotating excitation. In this method, the force is simultaneously applied to two axes to excite the rotor in circular orbits. Two trigonometric functions, i.e., cosine and sine functions, are used to generate this rotation force. Another method is the use of a unidirectional excitation and a mathematical operation to distinguish the forward whirl from the backward whirl. In this method, a directional frequency response function that separates the two modes in the frequency domain is obtained from four frequency response functions by using a complex number expression for the rotor motion. In this study, the latter method was employed to evaluate the rotor stability of a high-pressure compressor. To obtain the frequencies and damping ratios of the eigenvalues, the curve fitting based on system identification methods, such as the prediction error method, was introduced for the derived frequency response functions. Firstly, these methods were applied to a base evaluation under a low-pressure gas operation, in which the stability mainly depends on the bearing property. Using the obtained results, the bearing coefficients were estimated. Next, the same methods were applied to stability evaluations under high-pressure gas operations. The destabilizing forces were also estimated from the test results and compared with the calculation results.

Development of Scallop Cut Type Damper Seal for Centrifugal Compressors

This paper deals with a new type of damper seal developed for a high-pressure centrifugal compressor. Honeycomb seals and hole-pattern seals are popularly used as damper seals and provide superior rotordynamic damping characteristics. Honeycomb seals are expensive because the manufacturing process is complex. Hole-pattern seals are easier to manufacture, but they are still expensive. Use of a scallop pattern is one way to reduce manufacturing cost and time. A new seal that has a scallop pattern and small teeth on the stator surface is proposed. This pattern is cut on the stator surface using a disc type tool. To estimate the rotordynamic coefficients of this new seal, a bulk flow model code that is based on a two-control-volume model developed by Matsuda for labyrinth seals was newly developed. This model uses the Hirs model for the viscous shear stresses. The friction factor coefficients for the rotor surface, the stator surface and the surface between the two control volumes were determined by CFD steady analysis. The rotordynamic coefficients can also be obtained by using CFD perturbation analysis. The high accuracy of the bulk flow model was demonstrated by comparing its results with CFD perturbation analysis results. In the perturbation analysis, the whirling motion was treated as a steady state problem by using a rotating frame of reference. For the damper seal, the rotor surface and its neighboring region were treated with a rotating frame of reference and the neighboring region of the stator was treated with a stationary frame of reference. The damping property of the new seal was evaluated by conducting rotor stability tests using a high-pressure compressor with an electromagnetic exciter. The new seal equipped with swirl brakes was used for the balance piston of the compressor. Stability was evaluated by exciting the rotor during operation and identifying the eigenvalues of the rotor. The experimental results showed that the new seal increases damping. Comparison of the damping effect with calculations based on the bulk flow analysis showed good agreement.© 2014 ASME

Verification of Performance and Operating Characteristics of High Pressure Gas Injection Centrifugal Compressor

The development process of a 70-MPa high pressure compressor for oil and gas applications is presented in this paper. Great attention was paid to any relevant technical challenges due to the high design pressures, such as the material selection, deformation casing structure, stability of the rotordynamics, and the aerodynamic performance. Among the technical considerations, some technical findings for the operational tests are proposed in this paper as well.Copyright

Use of Design-by-Analysis for Designing Shear Key Structure

In many vessels with container, such as those involving rotating machinery, barrel casings are closed with disc shaped end covers on both axial sides. The end covers are fixed to the barrel casing with bolted joints or retaining devices. A shear key structure is often applied to pressurized vessels as a retaining device, because it can endure higher pressure than bolted joints can. Stress distribution in the shear key and the casing depends on the contact condition between the shear key and the casing. Thus, it is difficult to design a shear key structure merely by using Design-by-Formula procedures. We designed the shear key structure according to Design-by-Analysis procedures based on finite element (FE) analysis. By taking the contact condition between shear key and casing into consideration in an FE model and carrying out elastic and inelastic analyses, we developed a new shear key structure configuration. We will confirm the validity of the new shear key structure by means of pressure tests that will be conducted in May of 2009.Copyright

Thermally Induced Vibration in an Active Magnetic Bearing Supported Flexible Rotor

This paper describes a synchronous vibration instability problem that occurred on a. two-stage overhung centrifugal compressor. The authors encountered an unbalance vibration that increased spirally in a polar plot at/near the first bending critical speed. An iron loss concentration and a thermal bend by heat have been identified as the cause of the phenomenon, because the vibration stops increasing, when an unbalance force rejection control (UFRC) is applied. Iron loss is generated intensively in a higher flux density part that produces larger magnetic force than the others. If an unbalance force is exerted on a shaft, iron loss is concentrated on a certain part of a rotating core and the core is deformed by heat, because the bearing reaction force to the unbalance force is stationary on the rotating core. In order to exceed the first bending critical speed stably, rotor balancing under UFRC and a rapid speed acceleration/deceleration are carried out. The vibration behaviors around the critical speed are measured and the results verify the instability mechanism induced by interaction between iron loss and unbalance vibration control. The mechanism is also modeled by using a transfer function and the stability limit depending on rotational speed is presented. The measurement method for the most important parameter |H/R| that determines the stability is proposed and some measured results are presented.