Thermal fatigue experiment of screw cooling tube under one-sided heating condition
Introduction
As part of development of high heat flux components (HHFCs) for fusion machines, JAERI has developed high performance cooling tubes with pressurized water flow. Along this line, a cooling tube with helical triangular fins on its inner surface has been proposed recently [1]. Since the fin can be machined by simple threading, this tube is called a screw tube. In our previous experiments, it was found that heat removal performance of such a screw tube is twice higher than that of a smooth tube. Although the screw tube has superior heat transfer properties, it is not presently considered as a reference concept for the ITER divertor because the screw geometry potentially acts as a crack initiator as pointed out by Raffray et al. [2]. To evaluate its practical application to HHFCs, its thermo-mechanical behavior, especially thermal fatigue, has been investigated under one-sided heating conditions relevant to fusion machines.
Thermal fatigue experiments of the screw tube are carried out by using the screw tube made of CuCrZr (a candidate material of the ITER divertor cooling tube). By using finite element analyses, thermo-mechanical behavior of the screw tube is investigated to predict lifetime. After the experiments, fractographic observations are made to examine crack propagation inside the tube wall.
Section snippets
Experimental study
Test samples are screw tubes with M10 of 1.5-mm-pitch, which is based on the ISO 261 standard screw thread design, as shown in Fig. 1. The screw tube with this geometry has the highest incident critical heat flux, one of indexes for heat transfer limit of a cooling tube, obtained in the previous experimental campaign compared with other tubes [1]. Two kinds of the test samples are provided for the present experiments; one is for measuring thermal response of the test sample to verify the
Numerical analysis
To simulate the stress–strain behavior of the test sample under the thermal fatigue experimental conditions, 3-D thermo-mechanical analyses are performed using a finite element method (FEM) code, ABAQUS [5]. The strain amplitudes obtained in the analyses are used to evaluate fatigue lifetime of the test samples.
A finite element (FE) model of the test sample and boundary conditions for the analyses are shown in Fig. 2. In this model, the fins are individually crossed at right angle to the flow
Thermo-mechanical analyses
To verify the numerical model of the test sample, Fig. 3 shows comparison of temperature histories measured by using the thermocouples with the numerically calculated steady-state values of 20 MW/m2. As shown, analytical results of the maximum temperatures are in good agreement with the experimental ones. This shows that the numerical model correctly predict thermal response of the test sample.
Based on the predicted thermal responses for six thermal cycles, the elasto-plastic stress analyses
Conclusion
To evaluate its practical application to HHFCs, thermo-mechanical behavior of a screw tube made of CuCrZr, especially thermal fatigue, is studied under high heat flux conditions relevant to fusion machines. In the test sample used here, the screw thread is directly shaped in a CuCrZr heat sink bar as the cooling channel. Slits of 1.5 mm wide are machined at the heated sides of the heat sink. The lower part of the heat sink remains monolithic, which prevents the cooling channel from expanding in
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