Objective Combustion mechanism research and new burner design rely on the measurement and analysis of important parameters such as temperature, velocity, and composition of the combustion process. Combustion laser diagnostic technology provides the advantages of being nonintrusive and highly precise, providing both real-time results as well as high temporal and spatial resolutions. In recent decades, combustion diagnosis methods based on laser and optical technology have been developed vigorously. Combustion diagnosis has always emphasized the importance of scalar fields such as temperature, component concentration, density, and soot volume fraction to understand the combustion mechanism. However, with the emphasis on combustion dynamics in complex and rapidly evolving flow fields in the recent years, the demand for the measurement of transport characteristics such as velocity, vorticity, and diffusion is increasing. As two important parameters that reflect chemical reactions and flow field transport, velocity and temperature affect combustion mechanism and combustion dynamics. Therefore, simultaneous measurement and visualization of the two parameters become crucial to characterize and understand complex combustion.

Methods Herein, a measurement technique coupled with deflection tomography and particle image velocimetry (PIV) is proposed to obtain the three-dimensional (3D) temperature and velocity fields of swirling combustion. PIV overcomes the limitation of single-point velocity measurement and enables two-dimensional (2D) or even 3D velocity field measurements. PIV records velocity distributions from numerous spatial points under certain transient conditions and provides abundant flow field spatial structures and flow characteristics. Deflection tomography using Moiré technology is known for its simple devices, wide temperature-measurement range, and a low requirement for mechanical stability. Deflection tomography is considered suitable for the measurements of high temperature and high flow-velocity fields.

A combustion system is developed to generate a nonpremixed swirling flame. A hybrid imaging system is designed for the simultaneous sampling of Moiré fringes and particle images (Fig. 2). Particle image acquisition and particle recognition were realized under a flame high brightness background. The effects of the particle size and concentration of tracer particles on deflection tomography and PIV imaging are studied, and the optimum particle parameters are determined.

Results and Discussions The PIV and deflection tomography systems are experimentally calibrated to obtain the correspondence between the actual size and the camera pixels. Direct measurement is performed prior to the laser measurement. A thermocouple and a hot-wire anemometer are used for point measurements of the combustion temperature and velocity. Then, the laser beams emitted from the Nd:YAG for PIV and He-Ne lasers for deflection tomography are passed through the flame. Fringe patterns are obtained in four view angles, and the deflection angles are extracted. The deflection angle revision reconstruction technique is used to reconstruct the temperature distributions on different cross sections. Visualization technology is then employed to generate a 3D temperature field (Fig. 6). Four CCD cameras capture the particle images illuminated by a volume light source. The tracer particles in four particle images obtained are then screened, identified, and matched. A 3D cross-correlation calculation is performed on the particle distributions of the A- and B-frame images generated through the double pulses to obtain a 3D velocity field. Then, the isovelocity surface, constant vorticity surface, and velocity streamlines are visualized simultaneously (Fig. 7 and Fig. 8). The relative error of the temperature between the reconstruction value and the thermocouple measurement is 6.9%, and the relative error of the velocity between the reconstruction value and the anemometer measurement is 3.3%. The factors affecting the measurement results are analyzed. The swirling combustion characteristics are analyzed based on the quantitative measurement and visualization of parameter distributions. The evolution of the flow field and the combustion reaction process are explained.

Conclusions A multitechnology integrated method for the multiparameter measurement of swirling combustion was constructed herein. PIV and deflection tomography are combined for the simultaneous measurement and visualization of 3D temperatures and velocity fields. Fringe patterns are obtained in four view angles using a Moiré deflectometer to reconstruct the temperature distributions in different cross sections. Visualization technology is used to generate a 3D temperature field. The particle images illuminated by a volume light source are captured using four CCD cameras to reconstruct the velocity distribution. Furthermore, the isovelocity surface, isosurface of vorticity magnitude, and velocity streamlines are visualized simultaneously. The swirling combustion characteristics are analyzed on the bases of quantitative measurement and visualization of the parameter distributions. The validity of the experimental results is verified via direct point measurements, and the measurement errors are then discussed.