The purpose of this paper is to study about effects of natural frequency for a variation of design variables each other within restrictive range of helical tube design in terms of decreasing fluidelastic instability. In order to study for natural frequency, parameters are selected from design variables such as a coil diameter of helical tube, thickness of helical tube and number of turns.
To investigate effects of a variation of design variables each other, other design variable are changed within value of analysis range when one design variable is constant. also a number of tube supports are changed to investigate effects of a number of tube supports in same design variables.
Calculating a natural frequency in helical tube is preformed by finite element analysis method. To calculate a critical velocity for instability ratio, tightly supported condition in damping ratio, triangular patten condition in tube array patten is used.
This study results shows belows.
1) When coil diameter of tube is constant, a variation of thickness (15.5 %) more effective than a variation of number of turns (5.4 %) in terms of decreasing instability ratio. When thickness is more thick, a effect of decreasing instability ratio more decrease.
2) When thickness of tube is constant, a variation of coil diameter (13.5 %) more effective than a variation of number of turns (0.01 %) in terms of decreasing instability ratio. When coil diameter is more increase, a effect of decreasing instability ratio more decrease.
3) When number of turns is constant, a variation of coil diameter (44.0 %) more effective than a variation of thickness (8 %) in terms of decreasing instability ratio. When coil diameter is more increase, effects of a variation of thickness is more increase.
4) Effects of a variation of a number of tube supports in terms of decreasing natural frequency is most effective with coil diameter (654.0 Hz) more than thickness (20.8 Hz) and number of turns (0.3 Hz).
As a result, it is most sensitive to a variation of coil diameter of tube rather thickness and number of turns in terms of decreasing fluidelastic instability.