Optical Fiber Temperature Measurement Research Articles

Advances in the application of non-contact temperature measurement technology for aero-engine blade.

The advancement of the aviation sector has made the temperature measurement technology for aero-engine turbine blades essential for maintaining the engine's safe and steady performance. The non-contact temperature measurement technology is a trending research focus in turbine blade temperature measurement due to its benefits of not requiring direct touch with the object being measured and its suitability for high-temperature and high-speed conditions. This paper provides a concise overview of various key non-contact temperature measurement methods for aero-engines, such as fluorescence temperature measurement, fiber-optic temperature measurement, and radiation temperature measurement. It discusses the temperature measurement principle, technical characteristics, and the current research status both domestically and internationally. Based on this, this Review further discusses the main challenges faced by the non-contact temperature measurement technology and the development trend of the future.

The Impact of Liquids and Saturated Salt Solutions on Polymer-Coated Fiber Optic Sensors for Distributed Strain and Temperature Measurement.

The application of distributed fiber optic strain and temperature measurement can be utilized to address a multitude of measurement tasks across a diverse range of fields, particularly in the context of structural health monitoring in the domains of building construction, civil engineering, and special foundation engineering. However, a comprehensive understanding of the influences on the measurement method and the sensors is essential to prevent misinterpretations or measurement deviations. In this context, this study investigated the effects of moisture exposure, including various salt solutions and a high pH value, on a distributed strain measurement using Rayleigh backscattering. Three fiber optic sensors with different coating materials and one uncoated fiber were exposed to five different solutions for 24 h. The study revealed significant discrepancies (∼38%) in deformation between the three coating types depending on the surrounding solution. Furthermore, in contrast to the prevailing literature, which predominantly describes swelling effects, a negative deformation (∼-47 με) was observed in a magnesium chloride solution. The findings of this study indicate that corresponding effects can impact the precision of measurement, potentially leading to misinterpretations. Conversely, these effects could be used to conduct large-scale monitoring of chemical components using distributed fiber optic sensing.

Temperature Evaluation Considering Gradient Distribution for MV Cable XLPE Insulation Based on Wave Velocity

Temperature is an important factor for the service life of cable insulation. To ensure safety, the operating temperature of cables must be monitored. Since optical fiber temperature measurement technology is difficult to be used widely in medium voltage (MV) cables due to cost, this paper proposes a temperature evaluation method based on wave velocity. Firstly, the dielectric constant of cross-linked polyethylene (XLPE) cable insulation under different temperature is obtained through experiment. Based on the result, the relationship curve between wave velocity and temperature is established. The asymmetry effect due to temperature gradient in the cable insulation is discussed via finite element simulation. The effectiveness of obtaining the average insulation temperature of the cable based on wave velocity is validated. In addition, the mechanism of the temperature influence on the cable insulation material’s dielectric constant is analyzed by molecular dynamics simulation, which further deepens understanding of the characteristics of cable insulation materials.

Experimental Study on Spontaneous Combustion Monitoring in Coal Mine Airspace Based on Distributed Fiber Optic Temperature Measurement

ABSTRACT Spontaneous combustion fires in the mining airspace seriously affect safety at mines. It is crucial to accurately predict the location of fire sources and forecast spontaneous combustion disasters. This study, therefore, proposes an intuitive, accurate, and efficient distributed fiber optic temperature measurement technique for detecting spontaneous coal combustion. To overcome the limitations of using traditional optical fibers in the complex environment of the goaf, this study independently developed an enhanced armored distributed optical fiber for coal mines through experiments. Aiming at the possible influence of the armoring material on the accuracy of temperature measurement, the temperature of the armored distributed optical fiber was corrected through calibration experiments of the armored optical fiber and the heat transfer simulation study of the enhanced temperature measurement optical fiber, to improve the accuracy of the measurement. Based on COMSOL’s simulation study of the spontaneous combustion temperature field and fiber optic detection in the extraction zone, a method of laying optical fibers in parallel at 15-meter intervals to achieve accurate detection of the fire source location in the extraction zone. To explore the feasibility of this technology, a field test was carried out in the 22,117 working faces of Shanxi Lu’an Group Luning Coal Co. Ltd. in the hollow area. The temperature analysis results are consistent with the results of the beam pipe monitoring. The results show that the distributed fiber optic temperature measurement technology can produce a graded warning of spontaneous coal combustion and achieve an accurate detection of the fire source.

Characterization of hygrothermal, gas pressure and stress characteristics for poplar wood during unilateral surface densification

Unilateral surface compression in wood presents notable benefits, including reduced wood volume loss and energy consumption, rendering a highly promising and extensive utilization. However, shortcomings in compressed wood arise due to unclear gas pressure, temperature, and moisture content distribution within the wood during hot pressing. In this paper, the temperature and pressure distribution were analyzed utilizing an optical fiber temperature and pressure measurement system within the wood during hot pressing. Moreover, the moisture content, stress and strain in each layer at the end of hot pressing were discussed extensively. The results showed that the wood surface layer (SL) and subsurface layer (SSL) maximum temperature values were similar even with different layer thicknesses. Under sealing conditions, SL and SSL temperatures were generally 6 ℃ to 7 ℃ higher compared to unsealing conditions. The gas pressure maximum value exhibited an opposite order, gradually transferring to the core layer (CL) and declining as the processing time extended. During compression treatment, moisture migrated from the hot end to the cold within the wood. The moisture content displayed significant differences between the middle and end parts of the wood under unsealed conditions, with the disparity becoming more pronounced further away from the hot end. The transition region of the compressed wood served as a stress concentration area, experiencing the highest levels of stress. The dense region followed with the second highest stress levels, while the un-densified region exhibited the least amount of stress. Under the same process parameters, the thickness had little influence on the distribution of dense layers.

Research on the recovery process and product performance of carbon fiber flexible sucker rod in microwave field

As the applications of carbon fiber-reinforced polymers (CFRP) expand, the structures and compositions of the functional products formed from them become more complicated, reducing the effectiveness of conventional pyrolysis in recovering carbon fibers. This research presented the recovery of carbon fiber flexible sucker rods (CFSR) by microwave pyrolysis, and the warming principle of CFSR in a microwave reactor is revealed by a dielectric test system and fiber-optic fluorescence temperature measurement technique. Utilizing a variety of characterization techniques, the structural and mechanical characteristics of the regenerated carbon fibers (RCF) were also assessed, and the influence of heat treatment temperature and duration on the RCF was fully examined. The results demonstrated that the regenerated carbon fibers (RCF) performed best when the heat treatment temperature was set to 550 °C, the reaction time was set to 20 min, and the RCF's outer layer was clean and flat. When compared to virgin carbon fibers (VCF), the RCF's tensile strength and tensile modulus reached 80 % and 91 %, respectively. Furthermore, the composition and structure of the liquid and solid byproducts of CFSR was examined. It was discovered that the liquids contained the highest proportion of hydrocarbons, including methylcyclohexene, and that the solid byproducts also contained cleaner inner glass fibers.

Shield tunnel leakage detection using distributed optical fiber

Leakage in operating tunnels is a serious problem during operating stage. In order to realize non-destructive detection of leakage in an operating shield tunnel, an optical fiber temperature measurement method based on Raman scattering and infrared thermal imaging technology is developed. The working principle of leakage monitoring with Distributed Fiber Optic Temperature Sensor (DTS) combined with time domain reflectometry (OTDR) technology are introduced. The distributed measurement of temperature field is realized in a typical tunnel. Infrared thermal imager is used to perform thermal imaging on the leakage area, and the temperature difference between the leakage area and the non-leakage area is 3.6 °C, which can meet the requirements of distributed optical fiber side leakage temperature accuracy. This illustrates the feasibility of quantitative monitoring of leaks in distributed optical fiber temperature sensing systems. The reasonable arrangement of leakage monitoring fiber in distributed fiber temperature sensing system is discussed in practical engineering.

An estimation method for switching points of multimode spatiotemporal data based on SFSTAR

Various traditional methods, including statistical and spatial map-based approaches, are employed to model spatiotemporal data. However, the lack of explicit descriptions of spatial and temporal dependencies leads to unnecessarily intensive and uninterpretable relationships among the variables. To address this problem, space–time autoregressive moving average (STARMA)-type models that can depict temporal and spatial dependencies in a parametric manner are adopted in this study. However, such models may not be applicable to unlabeled multimode spatiotemporal data. In this study, we propose a switching flexible space–time autoregressive (SFSTAR) model to describe multimode spatiotemporal data. Considering the coupling between the parameter estimation of each mode and determination of the switching points, an estimation method is proposed to identify the mode switching points to partition the multimode data into different segments. First, the proposed estimation method identifies the number of segments by locating the transition regions that enclose the switching points based on the spatiotemporal feature extraction proposed herein. Second, with the estimated number of segments that is actually the number modes and the constraints on the locations of the switching points given by the transition regions, the proposed method jointly estimates the switching points and the SFSTAR model parameters in each mode through Bayesian optimization. The effectiveness of the proposed method is evaluated using a numerical example and a fiber optic temperature measurement dataset.

Testing the reproducibility of active-distributed temperature sensing for measuring groundwater specific discharge beneath a braided river

Braided rivers are a major contributor of groundwater recharge, yet little is known about how recharge rates vary in time. Existing methods for estimating groundwater recharge from rivers (i.e., river loss) are inadequate for studying highly heterogeneous braided river systems at a sufficient spatiotemporal resolution. To do so, active-distributed temperature sensing (A-DTS) is employed, which combines fibre optic temperature measurements with an active heat source, enabling high-resolution quantification of water fluxes. In this study, twelve successive A-DTS surveys were conducted during a 24-hour experiment on a 100 m horizontal subsurface hybrid fibre optic cable installed at 5 m depth beneath a braided river. The experiment was carried out under conditions where the river stage and flow were relatively stable to demonstrate the reproducibility and effectiveness of the A-DTS method for measuring groundwater specific discharge. This foundational work will provide a high level of confidence in the method for future studies aimed at evaluating temporal variations in groundwater recharge. The median groundwater specific discharge values calculated over the 24-hour period had a very narrow range from 3.5 to 4.0 m d−1 across the wetted footprint of the river, which is within the measurement error of the installation (6 %), indicating relatively stable groundwater recharge during the experiment. This provides confidence in the repeatability of the A-DTS method as an effective technology for quantifying river loss over longer time periods, to understand seasonal variability of groundwater recharge in braided river systems.

Determination of flow velocities using fiber-optic temperature measurements

A new flow measuring technique is introduced to measure liquid flow velocities under harsh circumstances in environments with dirt, high pressures and elevated temperatures as in boreholes within the earth’s crust. A glass fiber embedded in a cable with heating wires measures the temperature within the heated cable with fiber-optic temperature sensing. Similar to hot-wire anemometry (HWA), the velocity dependence of convective heat transfer is exploited to measure the velocity around the cable as a cylinder in crossflow. In the first experiment, a borehole-mimicking test rig and a realistic prototype of a borehole probe were built and the flow along the borehole axis was investigated. The concept of this new measurement technique was proven, since the expected Nusselt-Reynolds characteristic of a cylinder in crossflow has been successfully measured. Furthermore, a temperature profile model across the cables cross section has been developed to account for the unexpectedly low ranges of Nusselt number. The model accuracy has been addressed with a second experiment, where a straight segment of a custom-built heated cylinder was placed in a water channel perpendicular to the flow direction. The upstream flow speed during this set of measurements was recorded using particle image velocimetry (PIV), while multiple temperature sensors in the channel, on the probe’s sheath and within the probe delivered the information for the heat transfer model.

Single optical channel based Raman distributed optical fiber temperature measurement system

Single optical channel based Raman distributed optical fiber temperature measurement system

Optimization of Pipeline Leakage Detection System in Utility Tunnel Based on Finite Element Method

In addressing the problems of delayed detection, inefficient identification, and coverage blind spots in optical fiber‐based pipeline leakage detection within pipe galleries, this study proposes a leakage detection strategy utilizing distributed optical fiber temperature measurement technology. Finite element method is employed to analyze the temperature influence radius around the pipeline leakage hole and a model test is conducted as a validation. The results show that the temperature field image at the leakage site is elliptical, influenced by temperature differences between ambient and liquid. An increase in this temperature difference accelerates changes in the temperature field’s range. Adjustments to the optical fiber’s winding angle and pitch demonstrated that an optimal pitch is 1/24 of the pipeline’s length, with a 45° winding angle. This configuration maximizes the optical fiber’s distribution in detection while maintaining its cost‐effectiveness. When the leakage site is constant, and only the winding mode is altered, it is observed that when the ambient temperature exceeds the liquid temperature in the pipeline, the temperature of the escaping liquid impacts the temperature‐measuring fiber due to gravity, registering approximately 2°C higher than the temperature measured directly at the leakage site. The temperature anomaly from field diffusion is significantly less than that caused by the water flow from the leakage impacting the fiber due to gravity. Conversely, when the ambient temperature is lower than the pipeline’s liquid temperature, the opposite occurs. These research findings offer a novel approach for distributed detection in water supply and drainage pipeline leakage.

Current Transformer Condition Online Monitoring Platform Based on Big Data

Abstract As an important power equipment indispensable to the normal operation of the power grid, the good or bad working condition of the current transformer directly affects the operational efficiency of the whole power grid. In this paper, an online monitoring platform based on big data technology has been developed to monitor the current status of transformers. First of all, for the temperature of the transformer, through the decoupling of the heat generation mode of the transformer, the state monitoring method based on fiber optic temperature measurement technology and optical multiplexing technology is proposed, and the overall scheme of transformer state monitoring is designed. The data signal of the transformer’s status is collected and processed, and a fault state diagnosis model is constructed based on the decision tree algorithm. Finally, according to the data requirements of the current transformer, an online monitoring platform for transformer status has been constructed using big data technology. Through the simulation test, the variation of ratio error of this paper’s monitoring method is less than 0.60%, and the variation of phase error is less than 0.75 percent, which is in line with international requirements. The average loading time of the monitoring platform is between 123.591s~143.911s, the average loading speed is less than 13s, the average throughput is between 241ops/sec ~257ops/sec, and the average latency of the read and upload operations is 24.225ms. The current transformer condition monitoring platform in this paper has good stability, the overall performance is excellent, and it provides a good solution for the research of the current transformer condition monitoring platform provides practical support for the research of current transformer condition monitoring platform.

Optimization of the Monitoring of Coal Spontaneous Combustion Degree Using a Distributed Fiber Optic Temperature Measurement System: Field Application and Evaluation

Coal spontaneous combustion (CSC) in gob not only leads to wasted resources and casualties, but also produces a lot of waste gas that pollutes the underground environment. Mastering the degree of CSC helps ensure that timely and effective control measures are taken. The real-time and accurate monitoring of temperature, as the primary indicator of the extent of CSC, is difficult due to the harsh and hidden environment of gob, resulting in a reduced ability to anticipate and prevent CSC. In this work, a complete distributed optical fiber temperature sensing system (DTSS) set with strong anti-damage ability was developed. The optical cable is protected by internal parallel steel cables and external protective pipes, which greatly improve the system’s reliability and integrity when used in gob. During its application in the Wangyun Coal Mine, an abnormal temperature rise in gob was discovered in time, and the effect of inhibitor spraying was monitored and evaluated. The degree of CSC in the gob was accurately identified, which shows that the work of coal mining can be targeted. This work is expected to improve early warning capability to prevent the risk of CSC in gob, and has promising applications.

Monitoring heat transfer for seepage in earth-rock dams with clay cores considering damaged areas: Laboratory experiments and numerical modeling

This study conducted a laboratory test and two-dimensional numerical simulation on seepage heat monitoring of a core rockfill dam. A fully automatic laboratory test system for dam seepage monitoring has been developed, which can achieve automatic control of water flow and temperature, as well as automatic data collection. A flow-heat coupling model based on [Johansen, O., (1975). Thermal conductivity of soils. P. D. thesis. University of Trondheim, Norway, Trondheim] empirical thermal conductivity model was established through secondary development using COMSOL Multiphysics finite element software, and its accuracy was verified. The influence of the location of damaged leakage in the core wall of the dam on flow and heat migration was analyzed. Among the investigated conditions, a damaged channel with a curved cross-section causes the formation of the prominent arc in the saturation line of the dam body downstream, induces the largest increase in the average seepage discharge, and makes the dam body most unstable. The effects of damage on the temperature and seepage fields are positively correlated for different damage types. The presence of a damaged area induces sudden changes in the pressure, velocity, and temperature at each monitoring point. The pressure, velocity and temperature are correlated, and there is a lag in temperature transfer. This study provides a reference for the application of distributed fiber optic temperature measurement technology in core rockfill dam engineering, especially in terms of fiber optic placement position. The established numerical model can provide an accurate and effective reference method for identifying abnormal temperature data and locating dam leakage. Our next step will be to study the flow and heat transfer characteristics throughout the entire process from undamaged to dam failure.

Leakage Monitoring and Quantitative Prediction Model of Injection–Production String in an Underground Gas Storage Salt Cavern

The leakage of the injection–production string is one of the important hidden dangers for the safe and efficient operation of underground salt cavern gas storage. Although distributed fiber optic temperature measurement system (DTS) can accurately locate the position of the string leakage port, how to establish the quantitative relationship between the temperature difference and leakage rate of the leakage port still needs further exploration. This paper proposes a new quantitative prediction model based on a DTS for the leakage monitoring of the injection–production string of salt cavern gas storage. The model takes into account the gas’s physical parameters, unstable temperature conditions, and the Joule–Thomson effect. In order to verify the accuracy of the model, a simulation experiment of string leakage based on a DTS was carried out. The test results show that the relative deviation between the predicted leakage rate and the measured value is less than 5% compared with the calculated value. When the leakage rate drops to 0.16 m3/h and the temperature range is less than 0.5 °C, it is difficult to accurately monitor the DTS. The results of this study help to improve the early warning time of underground salt cavern gas storage string leakage.

A real time monitoring method for well-kick and lost circulation based on distributed fiber optic temperature measurement

In the process of oil drilling, if the formation pressure does not match the pressure in the borehole, it may cause well-kick or lost circulation. The existing methods of mud level monitoring and micro-flow monitoring have some shortcomings. In these methods, information feedback is untimely and inaccurate. Well-kick or lost circulation position cannot be located in time. In order to monitor the well-kick and lost circulation situation more accurately and find the location of well-kick or lost circulation in time, the borehole temperature distribution models under well-kick and lost circulation conditions were established and solved. The distribution law of wellbore temperature when well-kick and lost circulation occur during drilling was analyzed. Based on the above research results, this work proposes a method for real-time monitoring of well-kick and lost circulation based on distributed optical fiber temperature measurement. This method compares and analyzes the real-time measured temperature with the theoretically calculated temperature, promptly determines whether well-kick or lost circulation has occurred, identifies the location of well-kick or lost circulation, and issues an alarm. This method effectively solves the problems of low monitoring accuracy and information delay, shortens the time required to search for the locations of well-kick or lost circulation, and has broad application prospects.

Method and Application of Spatial Positioning for Valid Temperature-measuring Optical Fibers in Concrete Dams

Spatial positioning of Valid Temperature-measuring Optical Fiber (VTOF) is of great significance for accurately grasping the real temperature distribution of a concrete dam. However, there is barely an effective way to accurately locate the measuring points of optical fiber in concrete dams. A comprehensive data symmetry analysis technique is presented for determining the optimal spatial positioning of valid temperature-measuring optical fibers (VTOFs) in concrete dams. This approach includes a mathematical model to confirm the center of symmetry in monitoring data, which is required to reconstruct the real spatial temperature distribution. This process can identify VTOF locations and verify the reliability of temperature predictions. Experimental validation was conducted using findings from research on a new mathematical model, site measurements from a concrete dam, and corresponding data from distributed optical fibers. These results were successfully applied to the construction of the Baihetan dam; the concrete temperature from 405 pouring blocks in 5 typical dam monoliths was accurately monitored, and the temperature-measuring points of VTOFs were more than 24711. Meanwhile, the optical fiber temperature measurement and temperature field analysis system of Baihetan dam located in southwest China was developed based on accurately acquired temperature measurement data, which provides data basis and engineering application platform support for temperature evolution and temperature field analysis and precise temperature control management of pouring blocks.

Temperature tracing test and numerical simulation study during leakage of earth-rock dam

The leakage of earth rock dams exhibits characteristics of space-time randomness, concealment, and significant destructiveness. Prompt detection and identification of dam leakage are critical to ensuring the structural safety of these constructions. The distributed optical fiber temperature measurement method, which is based on optical frequency domain reflection (OFDR) technology, possesses attributes such as high spatial resolution, minimal temperature error, short response time, and real-time monitoring. As a result, it is an ideal technique for experimental research on leakage monitoring of earth rock dams. In this study, a model test was conducted on a clay core rockfill dam to investigate the feasibility of utilizing distributed optical fiber for identifying dam leakage. The variation pattern of partial distributed optical fiber temperature measurements in the dam body under the leakage state was compared and analyzed using the gradient method and the thermal pulse method. Furthermore, the heat flow coupled numerical simulation method was employed to verify and expand the findings of the indoor tests. The results of this study indicate that the OFDR distributed optical fiber leakage monitoring method using the thermal pulse approach can entirely reflect the temperature changes that occur during leakage, thereby providing strong reliability and exhibiting considerable potential for leakage location. The study offers valuable insights and a reference for the practical use of distributed optical fiber to monitor earth rock dam leakage.

Toward local core outlet temperature monitoring in gas-cooled nuclear reactors using distributed fiber-optic temperature sensors

Gas-cooled nuclear reactors operate at temperatures up to 950 ℃ with significant spatial variations in power generation and coolant flow through hundreds of parallel flow channels, resulting in complex mixing at the core outlet. The high temperatures and complex mixing can damage downstream components, challenge reactor calorimetry for power determination, and result in significant conservatism in calculated peak fuel temperatures, which ultimately limits the total power output. Directly measuring each gas stream individually is unrealistic using single point thermocouples, requiring more robust sensing techniques. Fiber-optic sensors are resilient to high temperatures (up to 1,000 ℃) and radiation damage. Moreover, distributed measurements can be made along the length of one fiber, making them potential candidates for monitoring local core outlet temperatures to improve core calorimetry and identify hot channels. To the authors’ knowledge, this manuscript is the first to report spatially distributed fiber optic temperature measurements to quantify mixing at the outlet of a relevant orifice plate under prototypic temperature and flow regimes to assess the feasibility of using fiber-optic sensors for distributed measurements of coolant temperature in gas-cooled reactors. A single optical fiber captured dynamic changes in local temperatures, whereas the mixed outlet temperature exhibited a muted response and could not identify which channels were responsible for the change in the mixed outlet temperature. The discussion focuses on potential challenges for deploying distributed optical fibers in gas-cooled reactors, including the effects of vibrations, radiation-induced signal attenuation and drift, and routing of the fiber while minimizing flow obstructions.