| Summary: | 碩士 === 國立中興大學 === 機械工程學系 === 94 === Ring-type protrusions in circular tube would destruct flow stability and result in flow recirculation and turbulence. Thus this arrangement can be viewed as a device for turbulence enhancement. This work mainly investigates the effect of arc-to-diameter ratio (ra/d) on the heat transfer characteristics of tubes with the protrusions. Test tubes with various arc-to-diameter ratios, pitch-to-diameter ratios (p/d) and protrusion’s height-to-diameter ratios (e/d) were manufactured by a rolling process. The effects of p/d, e/d and ra/d on the heat transfer characteristics were experimentally determined. It intended to find an optimum range of the p/d value and e/d value for a large heat transfer coefficient and a small pressure drop. Totally 15 tubes in five categories were considered. In each category, three tubes with the same e/d value and p/d value, but with different ra/d values, were arranged. The three adopted p/d values individually are 0.75, 1.43 and 4.29；the five e/d values are 0.015, 0.04, 0.07, 0.075 and 0.143；the three ra/d values are 0.0725, 0.145 and 0.217. Another eleven previously made surface-roughened tubes, plus one smooth tube were also measured. These eleven tubes are with a ra/d value of 0.0725. In the measurement, an isothermal surface temperature condition was arranged. By passing air through every individual tube, the corresponding Nu value and f value were obtained. The result reveals that the effect of ra/d on the Nu is insignificant. For tubes with small p/d value and large e/d value, the f value would increase with a decrease of the ra/d value. The result also shows that, for the tubes with the ra/d value of 0.217, if e/d < 0.07 and p/d <1.43, an effective and efficient heat transfer can be achieved.
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