雙 U 管科氏質量流量計的流固耦合分析

Abstract

因應產業對於質量流量的量測精度之需求，本文的目的是藉由COMSOL的有限元素模擬，進行雙U型管科氏質量流量計的流固耦合分析。雙U型管科氏質量流量計由二支相同的U型管所組成；在U型管的中點裝置驅動器以驅動該二支U型管在垂直於其平面的方向振動，且具有相同的頻率和振幅但相反的相位；在相對於U型管的中點，對稱地裝置二個運動感測器，以感測U型管在感測器位置的位移。當U型管中的流體不流動時，因對稱的振動模態，該二個運動感測器輸出的信號應為相同；當U型管中的流體流動時，流體和振動管的相對運動會產生科氏力，導致振動模態的扭曲，使得該二個運動感測器輸出的信號產生相位差；藉由該二個運動感測器信號的相位差，便可得到質量流量。 本文採用COMSOL軟體來模擬雙U型管科氏質量流量計的流固耦合效應，探討U型管的共振頻率、流體的密度和黏度、運動感測器訊號的相位差、和流體的質量流量等之間的關係。本文並運用COMSOL的應用程式開發平台，針對雙U型管科氏質量流量計，開發了一套設計界面，以利產產品的研發。此外，本文還探討了運動感測器之安裝位置和重力對輸出信號的影響，分流結構的設計對壓降(pressure drop)的影響，和結構不平衡對於輸出訊號和質流量量測的影響等。 前述的模擬結果以實驗驗證，實驗方法則採購Yogokawa公司所生產的雙U型管科氏質量流量計，借用桓達科技股份有限公司所提供的標準測試流廠，來進行實驗驗證。實驗證實本文所建立的模型和模擬結果與實驗非常吻合。此外，本文初步探討科氏質量流量計量測流體黏度的方法，並且透過有限元素模擬證實，可利用管流壓降與驅動電流來量測流體黏度，管流壓降的模擬結果與經驗公式所計算之結果有相同的趨勢，而驅動力與流體黏度之關係是由模擬與曲線擬合的方法來建立。

In response to the demand for high-accuracy mass flowrate measurement in industry, this thesis aims to analyze the operation of dual U-tube Coriolis mass flowmeter by using the commercial software COMSOL that is a finite element simulation package. The dual U-tube Coriolis mass flowmeter contains two identical U-tube connected in parallel. An actuator mounted on the midpoint of the U-tube is used to drive the U-tubes to oscillate normal to the plane of the U-tube. The two U-tubes oscillate symmetrically with the same frequency and amplitude. Two motion sensors mounted symmetrically with respect to the midpoint of the U-tube are used to detect the displacements of the U-tube at the positions of the motion sensors. Due to the symmetrical vibrational mode of the U-tube, when the fluid does not flow, the output signals of the two motion sensors should be the same. When the fluid flows, the Coriolis force induced by the relative motion of the fluid and the oscillatory U-tube will distort the vibration mode and thereby results in the phase difference of the signals output by the two motion sensors. Therefore, one can obtain the mass flow rate through the phase difference of the signals output by the two motion sensors. The author adopts COMSOL to simulate the fluid-structure coupling effects of a dual U-tube Coriolis mass flowmeter and investigates the relationships among the resonant frequencies of the U-tube, the density and viscosity of the fluid, the phase difference of the signals output by the motion sensors, and the fluid flow rate. The author also uses COMSOL’s application development platform to develop a design interface for dual U-tube Coriolis mass flowmeter to facilitate the product development. Furthermore, the influence of the positions of the motion sensors and gravity on the output signals, the influence of the flow splitter on the pressure drop, and the influence of the structure unbalance on the output signals and mass flowrate measurement are discussed as well. The aforementioned simulation results are verified by experiments. A dual U-tube Coriolis mass flowmeter produced by Yokogawa company is used and the experiment is conducted in the standard flow testing factory provided by FineTek Technology Co., Ltd. The experiment shows that the simulation results of the present model agree well with the experiment. Furthermore, to investigate the feasibility of measuring the fluid viscosity by Coriolis mass flowmeter, the author also simulates the effect of the fluid viscosity on the pressure loss of flow tube and on the drive current of the actuator. The result about the pressure loss has the same trend with empirical formula while the relationship of the viscosity and driving force of the actuator is obtained by curve fitting on the simulation results.