A Color-Coded 3D Particle Tracking Velocimetry System for Velocity Measurement in Transient Evaporating Films

Technical Summary: A Color-Coded 3D Particle Tracking Velocimetry System for Velocity Measurement in Transient Evaporating Films

Problem Statement
Film evaporation is critical to various engineering applications, including coating processes, fuel vaporization, and electronic thermal management. While non-evaporating, heated, and unheated evaporating films have been extensively studied, the transient development of impulsively superheated evaporating films remains less understood. Specifically, the three-dimensional internal convective structure and fluid velocities within these transient films have not been previously observed or measured. Existing Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) techniques often require multiple carefully aligned cameras, which presents significant challenges in small-scale flows with limited view ports. Furthermore, single-camera defocusing methods, while simplifying optics, typically suffer from high noise levels in the out-of-plane (z-direction) velocity measurements.

Methodology
To address these challenges, the authors implemented a single-camera, color-coded 3D Particle Tracking Velocimetry (PTV) system. The core optical setup utilizes a 3-pinhole mask installed within a camera lens, where each pinhole is covered by a distinct color filter (Red, Green, Blue). This arrangement creates a color-coded triple exposure on the camera sensor for each tracer particle. The spatial separation of the color triplets corresponds to the particle's distance from the focal plane (z-location), while the centroid of the triplet indicates the in-plane (x, y) location.

• Experimental Setup: The system was tested using dichloromethane (DCM) films (100 mm diameter, 2–5 mm thick) subjected to an impulsive superheat via a rapid pressure drop. Tracer particles consisted of silver-coated hollow ceramic spheres (29–37 µm) with a density matched to DCM to minimize gravitational settling.
• Calibration: Due to DCM's solvent properties, a standard calibration plate could not be moved through the working fluid. Instead, a calibration plate was translated through a sucrose solution (52% concentration) with a refractive index matching that of DCM (1.42).
• Data Processing: A custom workflow combined MATLAB scripts and TSI Insight V3V 4g software. MATLAB pre-processed images to separate color triplets and remove background noise. The V3V software performed particle identification, matching, and initial vector calculation.
• Noise Reduction: To mitigate the inherent out-of-plane noise of defocusing systems, a pathline curve-fitting algorithm was developed. Particle trajectories were tracked across time steps to form pathlines. These pathlines were fitted with piece-wise polynomials to smooth the data, and velocity vectors were recalculated from the smoothed positions.

Key Contributions

1. System Implementation: The successful adaptation of a color-coded 3-pinhole mask system for transient evaporating films, allowing for 3D tracking with a single camera in a constrained volume (14 x 14 x 5 mm³).
2. Calibration Strategy: The use of a refractive-index-matched sucrose solution to calibrate the system for DCM, overcoming the solvent limitations of the working fluid.
3. Algorithm Development: The creation of a pathline curve-fitting method that significantly reduces out-of-plane velocity noise, a common limitation in single-camera defocusing PTV systems.
4. First 3D Velocity Measurements: The first experimental measurement of 3D velocity fields and convective structures within impulsively superheated evaporating films.

Results

• System Accuracy: The system demonstrated a location error of approximately 0.6% in the out-of-plane direction and 0.07% in the in-plane direction. For a simulated flow velocity of 6 mm/s, the out-of-plane velocity error was approximately 6.4%.
• Validation Tests:
  • Swirl Test: A stirred DCM film showed consistent circulation centered at a specific point, with average velocities of 2.6 mm/s, confirming the system's ability to track in-plane flow.
  • Syringe Test: An impinged jet flow demonstrated clear overturning in the z-direction with minimal vector oscillation, validating the out-of-plane tracking capability.
• Transient Evaporation Experiments: When applied to impulsively superheated DCM films, the system captured convective structures consistent with previous top-down schlieren imaging (Kimball et al., 2017; Gonzalez et al., 2019). The observed structures included polygonal cells and roller convective structures.
• Velocity Measurements: Measured particle velocities in the transient films ranged from 1 to 2 mm/s. The scale of the convective overturning was comparable to the film thickness.

Significance
The paper claims that this work provides the first observation and measurement of the three-dimensional internal convective structure and fluid velocities in transient evaporating films subjected to impulsive superheating. The developed system and pathline smoothing algorithm offer a viable method for investigating small-scale, transient flows where multi-camera setups are impractical. The measured velocities and structural scales align with predictions based on previous schlieren imaging, validating the technique for future studies of heat transfer and fluid dynamics in evaporating films. The authors note that while the system successfully measured previously unknown convective velocities, the primary contribution is the establishment of a measurement capability for these specific transient conditions.