A new method for measuring wetness of flowing steam based on surface plasmon resonance
© Li et al.; licensee Springer. 2014
Received: 8 December 2013
Accepted: 5 January 2014
Published: 13 January 2014
A novel method for real-time and inline wetness measurement based on surface plasmon resonance (SPR) is presented in this paper. The Kretschmann geometry is adopted to excite surface plasmon waves in our measurement system. In order to prevent water coating, an ultrathin layer of hydrophobic coating is formed on the surface of Au layer. The experimental results show that the level of steam wetness can be obtained via the area ratio of water and air on the prism, which is determined by analyzing the SPR spectrum of wet steam based on a Gaussian model. In addition, during the online measurement of flowing wet steam wetness, significant shifts in the resonant position of the SPR spectrum occurred, which can be attributed to the strong interaction of the adjacent water droplets.
KeywordsStream wetness Surface plasmon resonance Kretschmann configuration
Interest in wet steam research was sparked by the need for efficient steam turbines used in power generation. The subject has become increasingly important in the current decade with the steep increase in fuel cost. Since the 1970s, wetness measurement technology has made a great progress. Although with a simple principle, thermodynamic method has its disadvantages, such as a long measuring period and large error[1, 2]. Optical method, primarily based on light scattering techniques and microwave resonant cavities, has a high measuring precision, however, with the estimation of steam quality strongly depending on the droplet size classification[3–5]. Electrostatic charge and capacitance methods are new with rare studies on electrostatic charge of droplets in wet steam flow in low-pressure steam turbines[6–8]. For its high precision, tracer determination method is popular in nuclear power plants, but there are several adverse aspects such as complicated operating process, intricate data processing, and costly instruments[9, 10]. Therefore, up to now, online measurement of wetness in steam turbines as accessibility is still a major challenge.
where Sw and Sg are the area ratio of water and air on SPR surface, respectively. Here, Sw and Sg can be measured by SPR.
The transmitter unit is configured to convert the light source into a parallel light beam with transverse magnetic polarization (i.e., the magnetic field direction parallel to the metal/prism surface in Kretschmann configuration). It comprises the DH-2000 Deuterium Tungsten Halogen Light Source (Ocean Optics, Dunedin, FL, USA), optical fiber, lens, and polarizer.
The core component of measuring space is the Kretschmann configuration, also referred to as attenuated total reflection, in which a 45-nm Au layer is evaporated on top of a SF2 prism. In order to prevent water coating, a 2- to 3-nm ultrathin layer of hydrophobic thiol coating is formed on the surface of the Au layer. In our experiments, the special container on top of the Kretschmann configuration is designed to hold water.
The receiver, which is composed of lens, optical fiber, and spectrometer, accepts the reflected light and couples it to our spectrometers.
The sensing application of the SPR system can be realized by modulating either the wavelength or incident angle. The controlling of light injection angle requires a fine adjustment of the physical configuration precisely; therefore, we choose to implement such a wetness sensing through controlling and analyzing the reflection spectrum under SPR, i.e., wavelength modulation surface plasmon resonance. Since under different incident angles, SPRs occur in different wavelengths, we fix the incident angle to be 69.3° which simplifies the system as well as provides high enough sensitivity.
Results and discussion
According to the dispersion relation of SPR, the effective permittivity of air droplet (two phases) composition can be obtained without a doubt. There exist several theories which can calculate the effective permittivity of such mixtures. One of the most widely used formulations is the Maxwell Garnett (MG) theory. Unfortunately, MG theory and other dielectric mixture theories are useful only for the case when the gap size between the droplets is far less than the effective wavelength. Notice here that the ratio of gap size of the adjacent droplets to effective wavelength of SP is between 101 and 102; therefore, the steam wetness cannot be simply derived from the summation of the two-phase behavior.
We demonstrate a novel method for wetness measurement based on surface plasmon resonance. The obtained SPR spectrum of wet steam is analyzed by a Gaussian model. From this analysis, the area ratio of water and air via the reflectivity of SPR spectrum of wet steam is determined, and the wetness of wet steam can be obtained. Moreover, a clear shift in the resonant position of SPR with continuously spraying wet steam is observed and has been tentatively ascribed to interaction between adjacent droplets.
Surface plasmon resonance.
The work is supported in part by the National Natural Science Foundation of China (61378057, 91233119, and 61204066), the National 863 High-Tech Project of China (2013AA031901), the National Research Foundation for the Doctoral Program of Higher Education of China (20103201110015), and the project of the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.
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