1.

INTRODUCTION

In various industries, temperature monitoring is critical, such as food inspection, pharmaceuticals, oil/gas exploration, environmental and high-voltage power systems. In some applications, temperature sensors are required which are immune to electromagnetic interference, durability to harsh environments, remote sensing capability, multiplexing capability, wide operating range, and allow long-distance interrogation without the electrical interface. Fiber optic sensors provide a good solution for many of these challenges [1-4]. However, most of the fiber optic sensors are used for point measurement, whether it is single or multipoint measurement.

In some applications, such as temperature field monitoring in coal-fired boilers or exhaust gas temperature monitoring for turbine engines, the industry wants a 2D temperature distribution profiler rather than some temperature points. Traditionally, the industry is using electronic transducers as the acoustic pyrometers for the temperature field monitoring. However, these electronic transducers have some drawbacks. They cannot survive in the boiler high temperature environment, and also have electromagnetic interference.

In this paper, for the signal generator part, the electronic transducer has been replaced by fiber optic sensor [5-13]. Since it is fabricated by optical fibers, the fiber optic signal generator can survive in a higher temperature than the traditional electronic transducers. A carbon black-Polydimethylsiloxane (black PDMS) material was utilized as the photoacoustic material for the fiber optic ultrasonic generator. A test was performed outside of a lab furnace, the testing area temperature range was from 26°C to 70°C. A 2D temperature field was mapped. The 2D temperature field map matched with the reference thermocouple results.

This paper is organized as follows: Section 2 presents the methodology. Section 3 describes the experimental setup and results. Section 4 concludes the paper.

2.

METHODOLOGY

The fiber optic ultrasonic generator is based on the photoacoustic (PA) principle. This principle relates to a material that excited by optical irradiation. The energy of light radiation is absorbed in the material and is converted to local temperature rise. This temperature rise will result in ultrasonic waves through the thermoelastic effect. Ultrasonic waves can be generated by periodically exciting the material with optical energy.

In this paper, black PDMS (20% carbon black + 80% PDMS) material was used as the photoacoustic material and coated on a glass slide [14-17]. Light launched from a 532 nm Nd:YAG nanosecond laser (Surelite I-10, Continuum) traveling through a 1000/1035 μm optical fiber was shone onto the black PDMS and the ultrasonic signal was thus generated. Figure 1 shows an ultrasonic signal that was generated by the fiber optic ultrasonic generator.

Figure 1.

The ultrasonic signal generated by the fiber optic generator.

3.

EXPERIMENTS AND RESULTS

This test was to use the fiber optic ultrasonic sensing system to measure the temperature in air and reconstructed the 2D temperature field. The experimental setup is shown in figure 2. A fiber optic ultrasonic generator was used as the signal generator. Black PDMS material was coated on a glass slide. Light launched from a 532 nm Nd:YAG nanosecond laser (Surelite I-10, Continuum) travelled through a 1000/1035 μm optical fiber was shone onto the black PDMS and this generated the ultrasonic signal. A microphone (TMS 130C21, PCB) was used as the signal receivers. The generator and receiver were fixed on the two moving stages. The generator and the receiver were set on the same height. The test was performed outside of a lab furnace. A reference thermocouple (KHXL-116G-RSC-24, OMEGA) was used in this test.

Figure 2.

Experimental setup.

The testing area is shown in figure 3. It is a rectangle area (8cm * 6cm). The receiver (Microphone) was set on 5 different locations through the moving stage. The generator was set in 3 different locations. The blue points were the testing generator and receiver locations. We set the generator at the location 0, 0.5 and 1, and the receiver at the location 8, 7.5, 7, 6.5 and 6. Fifteen different paths were tested in the test. Data were collected at room temperature (26°C) and high temperature (furnace set temperature at 250°C) respectively. The reference thermocouple temperature map is shown in figure 4.

Figure 3.

Testing area.

Figure 4.

Reference temperature map recorded by the thermocouple.

As shown in figure 5, it is the location (0, 8) (generator at location 0 and receiver at location 8) ultrasonic signal data. The top one is the signal at room temperature (26°C), the bottom one is the signal at high temperature (furnace set temperature at 250°C). According to the website tool [18], the speed of sound v₁ at 26°C was calculated as 346.71 m/s. The travel time t₁ was 283 μs at room temperature from figure 5. The generator and receiver travel real distance s was calculated as 9.81 cm based on equation (1).

Figure 5.

Location (0, 8) ultrasound signal.

The travel time t₂ was 276 μs at high temperature from figure 5, so the speed of sound was calculated as 355.43 m/s based on equation (2).

Based on the website tool, the temperature is 41.24 °C when the speed of sound at 355.43 m/s. The average temperature for this path was 41.24 °C. 15 paths temperature data were calculated by using the ultrasonic signal traveling time as shown in table 1. The measurement error was ± 1°C. A 2D temperature field was mapped based on the 15 temperature paths as figure 6. The 2D temperature field map matched with the reference thermocouple results [19-21].

Table 1.

15 paths temperature information.

Figure 6.

A 2D temperature field reconstruction results.

4.

CONCLUSION

In this paper, we have designed, fabricated, and characterized the fiber optic ultrasonic sensing system to measure the temperature in air and reconstructed the 2D temperature field. This system is the active non-contact optical fiber sensing system using optically generated acoustic signals to operate in the air temperature environment. The 2D temperature field map matched with the reference thermocouple results. In summary, this fiber optic ultrasonic sensing system could lead to the development of a new generation temperature sensor for temperature field monitoring in coal-fired boilers or exhaust gas temperature monitoring for turbine engines.