Design of cryogenic flow meter using fiber Bragg grating sensors

Abstract

This paper describes a novel technique to measure cryogenic fluid flow rate using fiber Bragg grating (FBG) sensors. The proposed design utilizes the drag force caused by fluid flow on the FBG sensor. The strain thus induced in the sensor will cause a Bragg wavelength shift, which can be utilized to estimate the flow rate. Theoretical calculations are done using mathematical models and the design is found to be feasible for a wide range of flow rates and cryogenic temperatures. Calibration experiments are performed for Helium at ambient conditions and the results obtained are regressed to obtain its calibration equation. The sensor shows an overall sensitivity of 20 pm/(g/s) in the range of operation between 0 and 2 g/s and 50 pm/(g/s) in the range of 2 to 5 g/s. Keywords-fiber Bragg gratings (FBG); cryogenic; flow measurement; drag force I. MOTIVATIONBreakthroughs in the area of super conductivity and space technology demand minimal invasive flow sensors to measure cryogenic fluids, which are used in superconductor magnet cooling channel, regenerative cooling in rockets, etc. Conventional flow meters are mostly based on differential pressure methods or electronic methods. Differential pressure based flow meters cause a lasting pressure loss in the flow which turns out to be an expensive affair while handling cryogens. On the other hand, electronic methods based flow meters are influenced by the highly corrosive nature of cryogenic fluids, strong electromagnetic fields, etc., causing high measuring errors. To overcome such disadvantages, fiber Bragg gratings (FBG) based flow meters are being preferred over the conventional ones [1,2]. FBG sensors are considered to be viable candidates for such applications due to their miniature size, high sensitivity, corrosion resistance, wide operational range, multiplexing capabilities and immunity to electrical and magnetic fields [3,4]. As the current study is focused on cryogenic flow, the sensors have to withstand extremely low temperatures. FBG sensors, made of Germanium doped silica and coated with acrylate, have been reported to be good candidates for accurate measurement at cryogenic temperatures and can withstand up to 2.2 K [5]. II. FIBER BRAGG GRATINGSAs documented in many literatures [6,7], FBG is a short segment in the core of an optical fiber which has a periodic variation of refractive index, inscribed using UV interferometer. Due to the varying refractive index of the FBG, one particular wavelength of the incident light is reflected and the others are transmitted, as depicted in Fig. 1. This reflected wavelength (central wavelength of the reflected band) is called the Bragg wavelength (λ B ) and it satisfies the following condition [8]:where is the grating period and n eff is the effective refractive index of the gratings. FBG is sensitive to mechanical strain which causes linear expansion of the gratings, leading to a change in the grating period and refractive index due to photo-elastic effect. The gratings are also sensitive to temperature due to thermal ex...

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