Particle-fluid two-phase flows exist widely in energy, industry, environment, and other fields. Thus, it is important to simultaneously determine the parameters of two-phase flow, such as the particle size distribution, volume fraction, and flow velocity. For example, in a thermal power plant, the particle size distribution of pulverized coal and its feed flow are critical issues in ensuring combustion efficiency and reducing pollutant emissions. In mineral processing and feed manufacturing, real-time measurement of these parameters is important for maintaining stable conditions to ensure the quality of products. It is also important in other applications, such as pipeline transportation using air as the carrier medium, in which real-time monitoring of flow velocity is necessary. With the development of science and technology, different online measurement techniques have been proposed and used to measure the parameters of particles in a two-phase flow, including ultrasonic spectrometry, image methods, and digital holography.However, some techniques are limited in their applications because their measurements can be affected by poor environmental conditions. In principle, ultrasonic spectrometry requires several particle and dispersion parameters, which are usually unavailable. Digital holographic technology employs a complicated optical setup that can be easily contaminated by dust. In addition, this technique is suitable only for measuring low particle volume fractions. Therefore, there is an urgent need to develop a simple measurement setup for real-time and online (or inline) measurements of the particle parameters of two-phase flows.
Transmission fluctuation correlation spectrometry (TFCS) is an optical measurement technique that utilizes the autocorrelation characteristics of transmission fluctuation signals of a narrow light beam to obtain information on particle size distribution and volume fraction. In this study, a simple beam-splitting setup was used, which produced two parallel narrow beams, allowing the detection of two channels of transmission signals for further correlation of the transmission fluctuations. Compared with a method that uses only one light beam, this optical setup can measure two channels of transmission fluctuation signals; therefore, cross-correlation spectra of these signals may be obtained, from which particle velocities can be extracted. In addition, the auto-correlation spectra of the transmission fluctuations of either or both channels can be calculated to obtain information on the particle size distribution and volume fraction.
Experiments are performed using spherical/non-spherical particles dispersed in water. The nominal diameters of the samples range from 200 to 900 μm. In the first part of the measurements (Fig. 2), the suspension of the particles is driven by a pump (BT-800) and circulates in a closed system at a constant flow velocity of approximately 1.15 m/s in the measuring zone, which is measured using a laser velocimeter (TM680). The He-Ne laser beam is expanded and collimated at a diameter of approximately 8 mm. The light beam propagates along the direction normal to the particle flow, and a bi-element photodiode (S4204) is employed to measure the transmitted signals. Two pinholes are installed in front of the photodiode. The diameter of the pinholes is 850 μm and the distance between their centers is 1.4 mm. The signals are amplified and recorded using a multichannel A/D card (PCI-50612). The sampling time for each measurement is 4.096 s, and the sampling frequency is 125 kHz. The second part of the measurements uses the same optical setup; however, the particles are fed by a vibrator (Fig. 7). Under gravity, the particles pass through the measurement zone at a constant velocity. The cross-correlation spectra of the transmission signals of the channels and the autocorrelation spectra of the transmission signals of a single channel are calculated using the fast Fourier transform (FFT). The delay time of the cross-correlation spectrum corresponding to the peak of the spectrum is used to extract the velocity of the particles. The results (Table 1) agree with those obtained using a laser velocimeter. The relative errors range from -8.8% to 0.95%. The particle size distribution and volume fraction are extracted from the autocorrelation spectrum. A modified Chahine iteration algorithm is used for the inverse calculation. The mean particle sizes x50 (Table 2) of the measured samples agree with the nominal diameters. All samples are also measured using a laser particle analyzer (Bettersize2600). It is found that the mean particle sizes x50 measured using the proposed TFCS (Table 2) agree well with those of the laser particle analyzer (Table 3); the relative error range from 0.1% to 5%. However, the sizes of x10 and x90 obtained using the TFCS (Fig. 5 and Table 2) differ significantly from those of the laser particle analyzer (Table 3). This difference is caused mainly by the distribution of the velocity of the particles when they pass through the measurement zone owing to the flow condition and the pulse caused by the pump. The measured particle volume fractions are compared with those based on the known weights of the samples (Fig. 6). A good agreement is achieved between the two sets of values for spherical particles [Fig. 6 (a)]. However, for non-spherical particles, the volume fractions obtained using the TFCS are higher than those based on the weights of the samples [Fig. 6(b)], which means that shape correction is required. All measurements are repeated several times, and the results for particle velocity, particle size distribution, and volume fraction show good repeatability. Data processing, including computation of the spectra and inversion, can be completed within 3-5 s. Thus, the proposed measurement is useful for real-time applications.
This study introduces a simple optical setup for transmission fluctuation correlation spectrometry, which can be used to simultaneously measure particle velocity, particle size distribution, and volume fraction. Two parallel narrow light beams are produced to measure transmission fluctuations and obtain autocorrelation of the signals of a single beam and cross-correlation of the signals of two beams. The particle velocity is obtained from the transmission cross-correlation spectrum, and the particle size distribution and volume fraction are deconvoluted from the autocorrelation spectrum.
The measurements are implemented using both spherical and nonspherical particles, and the TFCS results are compared with those obtained using other methods. The results show good agreement and repeatability. The measurement and subsequent data processing can be completed within 10 s. Therefore, the proposed setup is promising for use in real-time and online applications in particle-fluid two-phase flows.
