CN115144985A - Temperature-sensitive distributed optical cable and method for monitoring tunnel leakage - Google Patents
Temperature-sensitive distributed optical cable and method for monitoring tunnel leakage Download PDFInfo
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- 238000012544 monitoring process Methods 0.000 title claims abstract description 145
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- 239000013307 optical fiber Substances 0.000 claims abstract description 72
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- 239000000835 fiber Substances 0.000 claims description 10
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- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/44—Mechanical structures for providing tensile strength and external protection for fibres, e.g. optical transmission cables
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Abstract
The invention discloses a temperature-sensitive distributed optical cable and a method for monitoring tunnel leakage, wherein the optical cable comprises a multimode optical fiber layer and a yarn sleeve layer, and the multimode optical fiber layer is positioned in the yarn sleeve layer; the multimode optical fiber layer comprises a packaging sheath, a first optical fiber and a second optical fiber, wherein the first optical fiber and the second optical fiber are positioned in the packaging sheath, and the ratio of the core diameter to the outer diameter of the first optical fiber is different from that of the second optical fiber; the yarn sleeve layer is made of wet ball gauze. The monitoring method mainly comprises the steps of calculating the temperature difference between a certain monitoring point and an adjacent monitoring point, and judging that a leakage point exists near the monitoring point when the temperature of the certain monitoring point and the adjacent monitoring point reaches a set threshold value. The distributed optical cable and the monitoring method realize the distributed measurement of the tunnel leakage, improve the success rate and the accuracy of the leakage monitoring, and effectively improve the problem of the leakage detection of the FBG quasi-distributed measurement.
Description
Technical Field
The invention belongs to the technical field of distributed optical fiber sensing, and particularly relates to a temperature-sensitive distributed optical cable and a method for monitoring tunnel leakage.
Background
Water seepage is the most common disease in the operation process of the tunnel, and the wording of 'ten tunnels and nine leaks' is common in the tunnel field. The harm of the leakage water to the tunnel operation is mainly shown in the following aspects: firstly, driving safety in the tunnel, surface water accumulation caused by water leakage for a long time, deterioration of driving environment and easy occurrence of skidding of vehicles are realized; secondly, the service life of the facilities in the tunnel is influenced, the tunnel is generally provided with electromechanical systems such as lighting, ventilation, monitoring and the like, the facilities are easy to corrode when working in a humid environment, the service life is influenced, and the risks of electric leakage and short circuit exist; thirdly, the safety of tunnel lining is influenced, the lining structure is easy to peel off and weather due to long-term water leakage, the running reliability of the tunnel is reduced, and the strength and the stability of tunnel surrounding rocks are reduced under long-term erosion. Therefore, monitoring of leakage water during the operation period of the tunnel is of great significance to the health of the tunnel structure.
The traditional tunnel monitoring mode based on the point-type and electrical measurement sensing principle has the problems of low reliability, poor durability, easy omission and the like, the monitoring range is limited, the safety condition of the tunnel is difficult to master comprehensively, and the distributed optical fiber sensing technology which is developed rapidly in recent years can well make up for the defects.
The distributed optical fiber sensing technology can measure continuous distribution information of measured physical quantities at any position along the sensing optical cable, the maximum measurement length reaches dozens to hundreds of kilometers, the distributed optical fiber sensing technology is suitable for long-term real-time monitoring of various infrastructures, and distributed monitoring can be achieved for multiple parameters such as pressure, strain, displacement, humidity and seepage of the structure.
The strain monitoring by using the distributed optical cable is researched more, various types of sensing optical cables are developed according to engineering requirements, but the sensing optical cables specially applied to tunnel water damage monitoring are fewer. Chinese patent application 201711009400.9 discloses a device and a method for pipeline leakage quasi-distributed real-time monitoring, the method monitors pipeline leakage based on a Fiber Bragg Grating (FBG) quasi-distributed optical fiber monitoring technology, and the problem that the leakage amount is small and can not be reserved, so that the temperature change near an optical fiber is small and difficult to detect is solved by using an optical fiber sensing technology. However, the method is quasi-distributed monitoring, the monitoring range is limited, and the problem of missing detection exists. In addition, the method needs to arrange a fiber Bragg grating thermometer as a comparison group for judging the temperature difference caused by leakage, and when the leakage range is larger, the thermometer has the possibility of failure.
In summary, how to overcome the defects of the existing tunnel leakage monitoring technology is still an urgent task to be solved in the field.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides the temperature-sensitive distributed optical cable and the method for monitoring the tunnel leakage, and the distributed optical cable and the monitoring method have the outstanding advantages of distributed measurement, easiness in installation, high sensitivity, strong reliability, electromagnetic interference resistance, good safety and the like.
The invention discloses a temperature-sensitive distributed optical cable and a method for monitoring tunnel leakage, the temperature difference caused by water evaporation is monitored by means of distributed optical fibre temperature sensing technology (DTS), the technology has the advantages that the temperature sensor is only sensitive to temperature and is not influenced by other physical quantities, interference factors in the temperature measurement process are avoided, and the accuracy is high.
In order to realize the purpose, the invention adopts the following technical scheme:
a temperature-sensitive distributed optical cable for monitoring tunnel leakage, comprising a multimode optical fiber layer and a yarn cover layer, the multimode optical fiber layer being located within the yarn cover layer; the multimode optical fiber layer comprises a packaging sheath, a first optical fiber and a second optical fiber, wherein the first optical fiber and the second optical fiber are positioned in the packaging sheath, and the ratio of the core diameter to the outer diameter of the first optical fiber is different from that of the second optical fiber; the yarn sleeve layer is made of wet ball gauze. The wet ball gauze has good water absorption, and can effectively collect the leakage water under the condition of small leakage amount. When the leakage stops, the wet ball gauze can still be used for leakage monitoring after being air-dried, and the repeatability is good.
Further, the ratio of the diameter of the first optical fiber core to the outer diameter is 50/125 μm, and the ratio of the diameter of the second optical fiber core to the outer diameter is 62.5/125 μm.
Furthermore, a metal armor pipe is arranged between the multimode optical fiber layer and the yarn sleeve layer, and the metal armor pipe is sleeved on the multimode optical fiber layer.
Furthermore, a Kevlar fiber net is arranged between the metal armor pipe and the yarn sleeve layer, and the Kevlar fiber net is fixed on the surface of the metal armor pipe through a metal wire winding net. The multimode optical fiber layer is arranged in the armor tube, and the metal wire winding net and the Kevlar fiber net wrap the multimode optical fiber layer, so that the metal material is favorable for heat transfer while the protection of the optical fiber is improved.
Further, the weaving mode on yarn jacket layer is for in the tight package type or the winding type, tight package type weaving mode for make wet ball gauze shaping sheath tightly wrap in a temperature sensitive type distributed optical cable for monitoring the tunnel seepage is outmost, the winding type weaving mode for closely twine strip wet ball gauze in a temperature sensitive type distributed optical cable for monitoring the tunnel seepage is outmost.
A method for monitoring by adopting the temperature-sensitive distributed optical cable for monitoring tunnel leakage comprises the following steps:
1) Installing the temperature-sensitive distributed optical cable for monitoring the leakage of the tunnel on the inner surface of the tunnel along the length direction of the tunnel;
2) Determining a plurality of monitoring points on the temperature-sensitive distributed optical cable for monitoring the tunnel leakage by using a distributed optical fiber temperature measuring system (DTS), and acquiring and calculating the temperature at the monitoring points in real time;
3) And calculating the temperature difference between a certain monitoring point and an adjacent monitoring point, and judging that leakage exists near the monitoring point when the temperature of the certain monitoring point and the adjacent monitoring point reaches a set threshold value.
Further, the process of collecting and calculating the temperature at the monitoring point is to obtain the temperature by analyzing the light intensity of the optical cable at the monitoring point, and the calculation formula is as follows:
wherein R (T) is a function of the temperature at the collection point, I F To an anti-Stokes light intensity, I S Is the Stokes light intensity, v F Is the Stokes light center frequency, c is the light velocity in vacuum, v is the Raman frequency shift, h is the Planckian constant, K is the Boltzmann constant, and T is the absolute temperature.
Further, the method for calculating the temperature difference between a certain monitoring point and an adjacent monitoring point comprises the following steps:
31 Calculating the mass of water evaporation at a certain monitoring point, and the calculation formula is as follows:
wherein M is the mass of water evaporation, E is the saturated vapor pressure (hPa) corresponding to the temperature when the optical cable is wetted, E is the actual vapor pressure (hPa) in the air, c is the water exchange coefficient between the air and the wet-bulb gauze, and s is the evaporation area (cm) 2 ) And p is atmospheric pressure (hPa).
The heat consumed by this evaporation process is:
in the formula, Q 1 L is the latent heat of vaporization, the amount of heat consumed for vaporization.
If the certain monitoring point is wetted, transferring heat to the wetted optical cable through air, wherein the transferred heat is expressed as:
Q 2 ＝hs(T-T w ) (4)
in the formula, Q 2 Heat transferred by air to a wetted cable, h is the heat exchange coefficient, T is the air temperature (i.e., the temperature of the adjacent un-wetted monitoring point), T w The temperature of the certain wetted monitor point.
Q1 and Q when the temperature of the wetted optical cable is stable 2 In equilibrium, having Q 1 ＝Q 2 Simultaneous equations (3) and (4) yield:
according to the principle of measuring air humidity by using dry-wet balls, the dry-wet ball coefficient is introduced
The (5) is abbreviated as:
e＝E-Ap(T-T w ) (6)
the air humidity U is expressed as:
in the formula, e w The saturated water vapor pressure (hPa) of air.
Obtaining the temperature difference delta t between the adjacent non-wetted monitoring point and the certain wetted monitoring point by the formula (7):
and (3) calculating a dry-wet bulb coefficient A by adopting a fitting formula (9):
therefore, the temperature difference Δ t is obtained:
further, the temperature-sensitive distributed optical cable for monitoring tunnel leakage is connected with a distributed optical fiber temperature measurement system (DTS), the real-time collection and calculation of the temperature at a monitoring point and the calculation of the temperature difference between a monitoring point and an adjacent monitoring point are completed by the distributed optical fiber temperature measurement system, and the result is displayed by the distributed optical fiber temperature measurement system.
Furthermore, the temperature-sensitive distributed optical cable for monitoring the tunnel leakage is horizontally arranged along the length direction of the tunnel and is tightly attached to the inner wall of the tunnel.
The temperature-sensitive distributed optical cable and the method for monitoring tunnel leakage, disclosed by the invention, realize distributed measurement, effectively solve the problem of easy leak detection in FBG quasi-distributed measurement, and obtain data with better continuity in space. By adopting the distributed optical cable, a larger wetting range section can be obtained, the requirement of DTS sensing on the spatial resolution can be met, the success rate of leakage monitoring can be effectively improved, and leakage points can be accurately positioned according to the change of the temperature gradient of the sensing optical cable. The DTS has a real-time data acquisition function, can realize all-weather automatic monitoring, obtains a continuous time sequence, and further improves the accuracy of leakage point identification according to the change process, the maximum change amount and the temperature change rate of the temperature of each sampling point. The distributed optical cable also has the advantages of light weight, convenient carrying and simple arrangement mode, and simultaneously avoids using a mercury thermometer, thereby being more environment-friendly.
Drawings
FIG. 1 is a schematic structural view of a temperature-sensitive distributed optical cable (with a tight-buffered yarn jacket layer) for monitoring tunnel leakage according to the present invention;
FIG. 2 is a schematic structural diagram of a temperature-sensitive distributed optical cable (with a winding type yarn cover layer) for monitoring tunnel leakage according to the present invention;
FIG. 3 is a schematic view of the cable arrangement in a tunnel according to the monitoring method of the present invention;
FIG. 4 is a schematic diagram of fixed-point cable routing in the monitoring method according to the present invention;
FIG. 5 is a graph showing the monitoring results in the example of the present invention;
FIG. 6 is a schematic view of a penetration region in an example of the present invention.
Detailed Description
The following describes in detail a temperature-sensitive distributed optical cable and a method for monitoring tunnel leakage according to the present invention with reference to the accompanying drawings; in the description of the present invention, it is to be understood that the terms "left side", "right side", "upper", "lower", "bottom", etc. indicate orientations or positional relationships based on those shown in the drawings only for convenience of describing the present invention and simplifying the description, but do not indicate or imply that the device or element referred to must have a specific orientation, be constructed in a specific orientation, and be operated, and that "a", "B", "C", etc. do not represent an important degree of the component parts and thus are not to be construed as limiting the present invention; the specific dimensions used in the present example are only for illustrating the technical solution and do not limit the scope of protection of the present invention.
As shown in fig. 1, a temperature-sensitive distributed optical cable for monitoring tunnel leakage includes a multimode optical fiber layer and a yarn cover layer 1, the multimode optical fiber layer being located in the yarn cover layer 1. The multimode optical fiber layer comprises a packaging sheath 5, a first optical fiber 6 and a second optical fiber 7, wherein the first optical fiber 6 and the second optical fiber 7 are positioned in the packaging sheath 5, the ratio of the core diameter to the outer diameter of the first optical fiber 6 is 50/125 μm, and the ratio of the core diameter to the outer diameter of the second optical fiber 7 is 62.5/125 μm, so that the requirements of different types of DTS demodulation equipment are met. The yarn sleeve layer 1 is made of wet ball gauze.
The knitting mode of the yarn cover layer 1 is one of a tight-wrapping type or a winding type. As shown in fig. 1, the tight-wrapping type weaving method is to make a forming sheath from wet-ball gauze and tightly wrap the outermost layer of the temperature-sensitive type distribution optical cable for monitoring the tunnel leakage. As shown in fig. 2, the winding type weaving manner is to tightly wind a strip-shaped wet-bulb gauze on the outermost layer of the temperature-sensitive distribution type optical cable for monitoring the tunnel leakage. Through test measurement, the two compiling modes have no obvious influence on the testing effect of the invention.
Preferably, a metal armor tube 4 is arranged between the multimode optical fiber layer and the yarn sleeve layer 1, and the metal armor tube 4 is sleeved on the multimode optical fiber layer. A Kevlar fiber net 3 is arranged between the metal armor tube 4 and the yarn sleeve layer 1, and the Kevlar fiber net 3 is spirally wound through a metal wire winding net 2 and fixed on the surface of the metal armor tube 4.
A method for monitoring by adopting the temperature-sensitive distributed optical cable for monitoring tunnel leakage comprises the following steps:
1) As shown in fig. 3 and 4, a temperature-sensitive distribution optical cable for monitoring tunnel leakage is installed on the inner surface of the tunnel along the length direction of the tunnel; considering the movement track of tunnel leakage water, the temperature-sensitive distributed optical cable for monitoring the tunnel leakage is arranged along the horizontal direction and is tightly attached to the inner wall of the tunnel. The distributed optical cables are arranged on tunnel arch springing or side walls along the length direction of the tunnel, so that leakage monitoring can be carried out on the whole optical cable, and sensing optical cables can be arranged in an easily-leaking area, such as a tunnel segment joint, along the joint, so that leakage monitoring on key parts of the tunnel is realized. When the device is installed, the yarn sleeve layer made of wet ball gauze is in a dry state.
2) Selecting a plurality of monitoring points on the temperature-sensitive distributed optical cable for monitoring the tunnel leakage, and acquiring and calculating the temperature at the monitoring points in real time.
3) And calculating the temperature difference between a certain monitoring point and an adjacent monitoring point, and judging that a leakage point exists near the monitoring point when the temperature of the certain monitoring point and the adjacent monitoring point reaches a set threshold value.
4) The temperature-sensitive distributed optical cable for monitoring tunnel leakage is connected with a distributed optical fiber temperature measuring system (DTS), the temperature at a monitoring point is collected and calculated in real time, the temperature difference between a monitoring point and an adjacent monitoring point is calculated by the DTS, and the result is displayed by the DTS. The distributed optical cable has no bending, the optical path is smooth, the connection with a distributed optical fiber temperature measurement system (DTS) is reliable, and the test signal has good signal-to-noise ratio.
The distributed optical fiber temperature sensing technology (DTS) is an optical fiber monitoring technology based on a Raman scattering principle, the temperature of any point along an optical fiber is continuously measured by combining an Optical Time Domain Reflectometer (OTDR), and the distributed temperature measurement is realized by detecting Raman scattering light in the optical fiber. When pulse pump light with certain energy is injected into the optical fiber, photons collide with optical fiber molecules to generate sound waves, and the inelastic collision of the photons and the phonons generates Raman scattering to generate two components with different wavelengths, namely Stokes light and anti-Stokes light, wherein the wavelength of the former is greater than that of incident light, and the wavelength of the latter is less than that of the incident light. The DTS demodulator obtains the temperature information of the measuring point by analyzing the light intensity.
Specifically, the process of collecting and calculating the temperature at the monitoring point is to obtain the temperature by analyzing the light intensity of the optical cable at the monitoring point, and the calculation formula is as follows:
wherein R (T) is a function of the temperature at the collection point, I F To an anti-Stokes light intensity, I S Is the Stokes light intensity, v F Is the Stokes light center frequency, c is the light velocity in vacuum, v is the Raman frequency shift, h is the Planckian constant, K is the Boltzmann constant, and T is the absolute temperature.
The calculation process of the temperature difference between a certain monitoring point and the adjacent monitoring point is as follows:
31 When the seepage water contacts with the sensing optical cable, the surface of the soaked wet ball gauze is evaporated to take away heat to form a temperature difference, and the evaporation water quality of a certain monitoring point can be expressed as follows according to the Dalton law:
wherein M is the mass of water evaporation, E is the saturated vapor pressure (hPa) corresponding to the temperature when the optical cable is wetted, E is the actual vapor pressure (hPa) in the air, c is the water exchange coefficient between the air and the wet-bulb gauze, and s is the evaporation area (cm) 2 ) And p is atmospheric pressure (hPa).
The heat consumed by this evaporation process is:
in the formula, Q 1 L is the latent heat of vaporization, the amount of heat consumed for vaporization.
If the certain monitoring point is wetted, transferring heat to the wetted optical cable through air, wherein the transferred heat is expressed as:
Q 2 ＝hs(T-T w ) (4)
in the formula, Q 2 Heat transferred by air to a wetted cable, h is the heat exchange coefficient, T is the air temperature (i.e., the temperature of the adjacent un-wetted monitoring point), T w The temperature of the certain wetted monitoring point.
Q1 and Q when the temperature of the wetted optical cable is stable 2 In equilibrium, having Q 1 ＝Q 2 Simultaneous equations (3) and (4) yield:
according to the principle of measuring air humidity by using dry-wet balls, the dry-wet ball coefficient is introduced
The (5) is abbreviated as:
e＝E-Ap(T-T w ) (6)
the air humidity U is expressed as:
in the formula, e w The saturated water vapor pressure (hPa) of air.
Obtaining the temperature difference delta t between the adjacent non-wetted monitoring point and the certain wetted monitoring point by the formula (7):
and (3) calculating a dry-wet bulb coefficient A by adopting a fitting formula (9):
therefore, the temperature difference Δ t is obtained:
formula (9) only considers the air velocity that flows through the partial surface of soaking, i.e. wet bulb gauze surface, does not consider the influence of ambient temperature (not soak optical cable temperature T), has certain error, synthesizes under ambient temperature and the air velocity condition, and the partial actual measurement result of wet-dry bulb coefficient A is as shown in table 1:
TABLE 1 relationship of wind speed, temperature and wet-dry bulb coefficient
The effect of the temperature-sensitive distributed optical cable and the method for monitoring tunnel leakage disclosed by the invention is tested through experiments, and the specific process is as follows:
the distributed optical cable of the invention was tested separately for its performance at different leak rates and reusability.
Setting two working conditions of quick leakage and slow leakage on the leakage speed respectively, and defining the testing process: the flow rate during fast leakage is 90-100ml/min, the flow rate during slow leakage is 5-20ml/min, and the maximum temperature difference and the cooling rate under two working conditions are shown in table 2:
table 2 maximum temperature difference and cooling rate generated by temperature-sensitive distributed optical cable for monitoring tunnel leakage at different leakage speeds
For the distributed optical cable, the leakage speed has little influence on the monitoring performance, and in the same test, the maximum temperature difference and the cooling rate are not obviously influenced by the rapid leakage and the slow leakage, so that the accuracy of the optical cable in the small leakage amount monitoring is proved.
The distribution cable of the present invention still exhibited good monitoring performance after five wet-dry cycles. The temperature difference at the leakage point is obvious, as shown in table 3, the maximum temperature difference and the cooling rate monitored by the optical cable are obvious, and the novel sensing optical cable is proved to have good reusability.
TABLE 3 temperature-sensitive distributed monitoring of tunnel leakage maximum temperature difference and cooling rate under optical cable recycling
When a leakage event occurs, the temperature appears to be relatively low relative to the non-wetted portion of the cable wetted by the leakage, and the temperature distribution at each point on the cable is shown in figure 5. According to the temperature difference calculation formula (8), the temperature difference of any monitoring point of the sampling sensing optical cable at a certain time is calculated according to a formula (11):
in the formula, i is the number of a monitoring point; t is sampling time;
is the average value of the temperature change within the range of 5 times DTS spatial resolution length before and after the monitoring point i.
The typical distribution of temperature differences between adjacent monitoring points is shown in fig. 6. When the temperature difference reaches a certain threshold value or more, for this embodiment, the temperature difference threshold value is-0.5 ℃, and it can be determined that the monitoring point is the most or a leakage area.
And extracting the time sequence of the temperature of each sampling point in the most or leakage region, calculating the cooling rate, and considering that the tunnel leaks when the cooling rate reaches a certain threshold value, wherein the threshold value of the cooling rate is-0.4 ℃/h for the embodiment. The test data are shown in tables 2 and 3.
Repeated tests prove that the distributed optical fiber and the monitoring method of the invention have good monitoring effect.
The test of this example was carried out under the conditions of an ambient humidity of 90% and an indoor temperature of about 21 ℃, and the saturated steam pressure at the corresponding temperature was pressed into formula (8), and the calculation was carried out in combination with the actually measured dry-wet bulb coefficient a in table 1, and the results are shown in table 4:
TABLE 4 comparison of theoretical and actual values
In the experiment, the actual temperature difference is 1 ℃, the corresponding theoretical calculated value and the actual temperature difference have higher fitting degree under the conditions of 4, 5 and 6 which are close to the experimental temperature, and the distributed optical fiber and the monitoring method have very good practicability.
Based upon the foregoing description of the preferred embodiment of the invention, it should be apparent that the invention defined by the appended claims is not limited solely to the specific details set forth in the foregoing description, as many apparent variations thereof are possible without departing from the spirit or scope thereof.
Claims (10)
1. A temperature-sensitive distributed optical cable for monitoring tunnel leakage is characterized by comprising a multimode optical fiber layer and a yarn sleeve layer, wherein the multimode optical fiber layer is positioned in the yarn sleeve layer; the multimode optical fiber layer comprises a packaging sheath, a first optical fiber and a second optical fiber, wherein the first optical fiber and the second optical fiber are positioned in the packaging sheath, and the ratio of the core diameter to the outer diameter of the first optical fiber is different from that of the second optical fiber; the yarn sleeve layer is made of wet ball gauze.
2. The temperature-sensitive distribution optical cable for monitoring tunnel leaks of claim 1, wherein the first fiber core to outer diameter ratio is 50/125 μ ι η and the second fiber core to outer diameter ratio is 62.5/125 μ ι η.
3. The temperature-sensitive distributed optical cable for monitoring tunnel leakage according to claim 1, wherein a metal armor tube is disposed between the multimode optical fiber layer and the yarn sheath layer, and the metal armor tube is sleeved on the multimode optical fiber layer.
4. The temperature-sensitive distributed optical cable for monitoring tunnel leakage according to claim 3, wherein a Kevlar fiber mesh is arranged between the metal armor tube and the yarn sleeve layer, and the Kevlar fiber mesh is fixed on the surface of the metal armor tube through a metal wire winding mesh.
5. The temperature-sensitive distributed optical cable for monitoring tunnel leakage according to any one of claims 1 to 4, wherein the yarn cover layer is woven in one of a tight-wrapping type and a winding type, the tight-wrapping type is formed by making a wet-ball gauze into a formed sheath and tightly wrapping the formed sheath on the outermost layer of the temperature-sensitive distributed optical cable for monitoring tunnel leakage, and the winding type is formed by tightly winding a strip-shaped wet-ball gauze on the outermost layer of the temperature-sensitive distributed optical cable for monitoring tunnel leakage.
6. A method of monitoring using the temperature sensitive distributed optical cable for monitoring tunnel leakage of claim 5, comprising the steps of:
1) Installing the temperature-sensitive distributed optical cable for monitoring the leakage of the tunnel on the inner surface of the tunnel along the length direction of the tunnel;
2) Selecting a plurality of monitoring points on the temperature-sensitive distributed optical cable for monitoring the tunnel leakage, and collecting and calculating the temperature at the monitoring points in real time;
3) And calculating the temperature difference between a certain monitoring point and the adjacent monitoring point, and judging that leakage exists nearby the monitoring point when the temperature of the certain monitoring point and the adjacent monitoring point reaches a set threshold value.
7. The detection method according to claim 6, wherein the process of collecting and calculating the temperature at the monitoring point is to obtain the temperature by analyzing the light intensity of the optical cable at the monitoring point, and the calculation formula is as follows:
wherein R (T) is a function of the temperature at the collection point, I F To an anti-Stokes light intensity, I S Is the Stokes light intensity, v F Is the Stokes light center frequency, c is the light velocity in vacuum, v is the Raman frequency shift, h is the Planckian constant, K is the Boltzmann constant, and T is the absolute temperature.
8. The detection method according to claim 7, wherein the temperature difference between a certain monitoring point and an adjacent monitoring point is obtained by:
31 Calculating the mass of water evaporation at a certain monitoring point, and the calculation formula is as follows:
in the formula, M is the mass of water evaporation, E is the saturated water vapor pressure corresponding to the temperature under the condition that the optical cable is soaked, E is the actual water vapor pressure in the air, c is the water exchange coefficient of the air and the wet ball gauze, s is the evaporation area, and p is the atmospheric pressure;
the heat consumed by this evaporation process is:
in the formula, Q 1 L is the latent heat of evaporation, the heat consumed for evaporation;
if the certain monitoring point is wetted, heat is transferred to the wetted optical cable through air, and the transferred heat is expressed as:
Q 2 ＝hs(T-T w ) (4)
in the formula, Q 2 The heat transferred from the air to the wetted optical cable, h is the heat exchange coefficient, and T is the air temperature, namely the temperature of the adjacent non-wetted monitoring point; t is w The temperature of the certain wetted monitoring point;
q1 and Q when the temperature of the wetted optical cable is stable 2 In equilibrium, having Q 1 ＝Q 2 Simultaneous equations (3) and (4) yield:
according to the principle of measuring air humidity by using dry-wet balls, the dry-wet ball coefficient is introduced
The (5) is abbreviated as:
e＝E-Ap(T-T w ) (6)
the air humidity U is expressed as:
in the formula, e w Saturated water vapor pressure of air;
obtaining the temperature difference delta t between the adjacent non-wetted monitoring point and the certain wetted monitoring point by the formula (7):
and (3) calculating a dry-wet bulb coefficient A by adopting a fitting formula (9):
therefore, the temperature difference Δ t is obtained:
9. the detection method according to claim 6, wherein the temperature-sensitive distributed optical cable for monitoring tunnel leakage is connected to a distributed optical fiber temperature measurement system, the real-time acquisition and calculation of the temperature at a monitoring point and the calculation of the temperature difference between a monitoring point and an adjacent monitoring point are completed by the distributed optical fiber temperature measurement system, and the result is displayed by the distributed optical fiber temperature measurement system.
10. The detection method as claimed in claim 6, wherein the temperature-sensitive distribution optical cable for monitoring the tunnel leakage is horizontally arranged along the length direction of the tunnel and is tightly attached to the inner wall of the tunnel.
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