Abstract: The adaptive head of the fire water monitor can automatically adjust the muzzle opening according to the change of fluid pressure and flow rate. Thus, the jet performance of fire water monitor can be improved, and large fires can be put out quickly and efficiently. However, the adaptive mechanism at the end of the adaptive head makes the jet system have the characteristics of fluid-structure coupling. And under the action of high speed and high pressure jet, the vibration even resonance of the jet system may occur, which will affect the efficiency of fire extinguishing. Considering the fluid-structure coupling effect between the fluid and solid domains in the jet system and the discrete-continuum coupling effect between the core rod and end cap of the continuous body and other discrete structures in the adaptive head, the stiffness matrix of the mandrel and end cap structure is determined by the lumped mass method and the dynamic model and equation of the adaptive head jet system of fire water monitor are found by the lumped parameter method. The free vibration analysis of the jet system is carried out, and the natural frequencies and mode shapes of the system are determined. The influence of initial volumetric gas content and fluid pressure on the natural frequency of the system is analyzed. According to the modal experiment of the sample machine, the error between the theoretical and experimental result of first-order natural frequency is only 0.51%, which verifies the accuracy of the dynamic model of adaptive head jet system of fire water monitor. This research can provide the theoretical basis for the dynamic optimization of the adaptive head jet system of the fire water monitor.

Key words: fire water monitor, adaptive head, jet system, free vibration, modal experiment