Effects of helical strakes on the vortex-induced vibration over a thermowell in a pipe flow
A thermowell is a device to protect a thermometer used for measuring temperature in many industrial application. The thermowell is exposed to vortex-induced vibration when the flow speed in a pipe is large so that shedding frequency due to Karman vortex streets downstream of the thermowell corresponds with its natural frequency. The vortex-induced vibration can cause fracture of the thermowell structure by cyclic loading. This phenomenon has long been a research topic in the field of marine engineering. Now, it can be a concern in metrology area. The present study investigates the suppressing effect of the vortex-induced vibrations of the thermowell by means of helical strakes. Flow visualizations can show that the helical strakes can suppress the vortex-induced vibration by generating three-dimensional shedding vortices with short spanwise length. The three-dimensional shedding vortices does not give resonance to the thermowell effectively because the vortices generate asynchronous forces along the thermowell. In this study, flow visualizations of the thermowell vibrations and resonant frequency measurements were attempted to investigate the suppressing effect of the helical strakes for safely using the thermowell in operating conditions with high flow speeds.
Circular cylinders fitted with small-scale straight and helical wires: A comparative study on the wire-induced critical effects
Abstract An experimental investigation is carried out to compare critical flow effects induced by a single straight wire and a system of three-start helical wires, protruding from the surface of a circular cylinder in uniform cross flow. The cinema technique of Particle Image Velocimetry (PIV) is employed. The Reynolds number is 10,000 and the diameter of the wires is 1.2% of the diameter of the cylinder. This size wire is a small-scale wire on the basis of a comparison with the boundary-layer thickness forming around a smooth cylinder. The cylinder fitted with the single straight wire was tested for the specific case when the wire is at θ = 60° on the cylinder surface. This is a critical location, where the spanwise wire yields: (i) the greatest extension in the time-averaged near-wake bubble, (ii) a bistable instability in the shear layer separating at the wire, and (iii) early onset of shear-layer instability on the wire-side shear layer. Spectral analysis shows that spanwise application of this wire on the cylinder at the critical location exerts no significant influence on the strength and coherence of the Karman instability. As for the cylinder-helical wire model, the three wires were arranged to pass, in the plane of visualization, from the critical angle and the base of the cylinder (i.e., θ = +60°, −60° and 180°). Distinct similarities are identified in the flow structure induced by the single spanwise wire and the helical wires in the plane where the wire(s) is/are at the critical location (θ = ±60°). Similar to the same-size spanwise wire, the helical wires under consideration do not alter the coherence of the Karman instability; however, while passing from the critical angle, they induce several intriguing effects to the shear layer and the overall near-wake structure. In the plane where the helical wires pass from the critical location: (i) a bistable phenomenon develops in the shear layer, (ii) significant extension in the near-wake is accompanied by a reduction in the peak levels of Reynolds stress and rms velocity fluctuations, and (iii) the onset of shear-layer instability advances to upstream locations.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.