WO2009070798A1 - Method and apparatus for three-dimensional digital particle image thermometry and velocimetry - Google Patents

Abstract

A 3D Digital Particle Image Thermometry and Velocimetry (3DDPIT/V) system and method is described. By combining Three-Dimensional Particle Tracking Velocimetry (3 DPTV) and Digital Particle Image Thermometry (DPIT) into one system, this technique provides simultaneous temperature and velocity data measurements using temperature-sensitive thermochromic liquid crystal particles (TLC) as flow sensors. An optional custom water-filled prism corrects for astigmatism caused by off-axis imaging. Six CCD cameras comprise the imaging system, with three allocated for velocity measurements and three, combined in one custom-made color camera, for temperature measurements. The cameras are optically aligned to sub-pixel accuracy using a precision grid and high-resolution translation stages. One or more high-intensity custom-designed Xenon flash lamps provide illumination. Temperature calibration of the TLC particle is then performed.

Description

METHOD AND APPARATUS FOR THREE-DIMENSIONAL DIGITAL PARTICLE IMAGE THERMOMETRY AND VELOCIMETRY

CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of U.S. Provisional Patent Application

No. 60/991171, filed November 29, 2007, the disclosure of which is hereby expressly incorporated by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under CTS-331140 awarded by the National Science Foundation. The Government has certain rights in the invention.

BACKGROUND

Turbulent convective heat and mass transfer is one of the most frequently encountered physical processes in applied engineering, and can be inherently difficult to study and to predict. These flows are a combination of turbulent fluid dynamics, heat or mass transfer, and their interaction as affected by the complex geometries in which they occur. While laminar convective heat transfer is well understood, turbulent convective heat transfer is not, making its prediction using numerical methods often difficult and inaccurate.

Over the years, science has made steady progress in describing and understanding turbulent phenomena. Each improvement has come largely as a result of better observational techniques: what can be described accurately, can also be modeled and better understood. Observational techniques began with single-point measurements, and progressed through 2D and (today) into simple 3D measurements.

Over the last several decades, particle image velocimetry ("PIV"), including digital particle image velocimetry ("DPIV"), has become a widely used quantitative flow visualization technique in fluid mechanics research due to its ability to provide non-intrusive, highly resolved measurement of planar velocity fields.

With the use of ever-advancing charge-coupled device (CCD) cameras, digital data acquisition systems, and sophisticated algorithms, DPIV continues to evolve and flourish. One of the key advancements is the development of Three-Dimensional Particle Tracking Velocimetry ("3DPTV"), which allows for three-dimensional velocity measurements within a volume. This technique uses triangulation to obtain a particle's position in three dimensions. However, off-axis imaging (as may be required by the hardware setup) through glass and water results in astigmatic images, which can be corrected by the use of a water-filled prism. The introduction of this prism into the optical path, while alleviating astigmatism, complicates the refractive effects.

With regard to techniques for measuring temperature, thermochromic liquid crystal particles (TLC particles), which change color as a function of temperature, may be used to track temperatures in a volume of the fluid. TLCs have been used for both surface temperature and flow temperature measurements. Particle Image Thermometry (PIT) for flow temperature measurement is performed in a manner similar to PIV. In this case, the flow is seeded with microencapsulated TLC particles and illuminated with a broad spectrum visible light source, instead of a laser, to get proper color display. Over time, this technique has evolved into an enhanced quantitative version, Digital Particle Image Thermometry (DPIT).

To properly study turbulent flows with heat transfer, both velocity and temperature measurement techniques are required, and it is desirable to use a single technique to provide both quantities at the same time. However, until now, there has been no method or system that would allow for a simultaneous measurement of velocity and temperature of a fluid. There is a need, therefore, for a solution that overcomes this deficiency and provides for a proper study of turbulent flows with heat transfer.

SUMMARY This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

A novel 3D Digital Particle Image Thermometry and Velocimetry (3DDPIT/V) system and method is described. By combining 3D Particle Tracking Velocimetry (3 DPTV) and Digital Particle Image Thermometry (DPIT) into one system, the described technique provides simultaneous temperature and velocity data measurements using temperature-sensitive thermochromic liquid crystal particles (TLC) as flow sensors. An optional custom water-filled prism corrects for astigmatism caused by off-axis imaging. Six CCD cameras comprise the imaging system, with three allocated for velocity measurements and three, combined in one custom-made color camera, for temperature measurements. The cameras are optically aligned to sub-pixel accuracy using a precision grid and high-resolution translation stages. One or more high-intensity custom-designed Xenon flash lamps provide illumination. Temperature calibration of the TLC particle is then performed.

DESCRIPTION OF THE DRAWINGS The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a diagram illustrating schematically a 3DDPIT/V system; FIGURE 2 illustrates an optional assembly showing a water-filled prism attached to a test section, as used in the 3DDPIT/V system and method;

FIGURE 3 is a diagram illustrating a system for implementing the 3DDPIT/V method of simultaneous determination of both velocity and temperature of a fluid containing a plurality of thermochromic liquid crystals (TLC particles); FIGURE 4 illustrates the assembled 3DDPIT/V system;

FIGURE 5 is a flow diagram illustrating the method of simultaneous determination of both velocity and temperature of a fluid containing a plurality of thermochromic liquid crystals using the 3DDPIT/V system; and

FIGURE 6 shows exemplary spectral curves for the color filters for the system shown in FIGURE 4.

DETAILED DESCRIPTION

The system and method are described that provide the simultaneous determination of both velocity and temperature of a fluid containing a plurality of thermochromic liquid crystals (TLC particles). The system and method comprise a combination of Digital Particle Image Thermometry (DPIT) for measuring temperature of the fluid, and a 3D Particle Tracking Velocimetry (3 DPTV) technique for determining velocity of the fluid. The combined technique is hereinafter referred to as 3D Digital Particle Image Thermometry and Velocimetry (3DDPIT/V).

A system and method will now be described for conducting 3DDPIT/V, with reference to the figures wherein like numbers indicate like elements. In 3DDPIT/V, time-separated sequences of images of particles in the flow are used to determine the three-dimensional position of the particles, and the particle positions are tracked over time and used to determine the particle velocity. Simultaneously, color images of particles are used to determine the temperature of the flow.

Refer now to FIGURE 1, which illustrates schematically a 3DDPIT/V system 100 in accordance with the present invention. The system 100 includes a broad spectrum light, for example a white light source 102 directed toward a fluid test section 104, an optional water-filled prism assembly 106, and four imaging systems 108-114. In 3DDPITA'^', the test section 104 is provided containing an optically transparent liquid with a plurality of thermochromic liquid crystal particles suspended therein. The test section 104 is illuminated with the light source 102 to get proper color images of the TLC particles. The test section 104 comprises a container having at least two transparent, preferably glass, walls. The light source 102, in one embodiment, may comprise one or more Xenon flash lamps.

A velocimetry imaging system includes at least three monochrome imaging systems 108, 110, and 112 that are used to determine the three-dimensional position of TLC particles suspended in a volume of interest in the test section liquid. A thermometry imaging system 114 images the volume of interest in color, to determine the color of the TLC particles. A computer system 116 obtains the data from all four imaging systems and uses the image data to calculate time-dependent velocity and temperature profiles throughout a three-dimensional volume of interest in the test section 104. The system 100 is described below in greater detail in reference to FIGURES 2, 3, and 4.

3 DPTV typically images a volume within the fluid test section with at least 3 CCDs. However, off-axis imaging (which may occur due to various hardware setups) through mediums having different indexes of refraction, such as through glass and water, results in astigmatism, which can be corrected by the use of a water-filled prism. In practice, angles larger than approximately 6-8 degrees will cause noticeable astigmatism that must be accounted for or corrected. In such cases, a prism system can be used to decrease the amount of astigmatism to imperceptible levels by allowing the light to pass perpendicular to the interface with the largest index of refraction ratio (e.g., air to glass). By Snell's Law, light entering perpendicular to an interface does not bend, so the only refraction will occur at interfaces of smaller index of refraction ratios (e.g., glass to water). The calculation of refraction angles of a ray of light traveling through the water of the test section, the glass wall of the test section, the prism glass, and the prism water before entering a lens are within the skill of practitioners in the art.

FIGURE 2 illustrates a test section assembly 200 comprising the optional water-filled prism assembly 106 attached to the test section 104 as used in the 3DDPIT7V system 100 shown in FIGURE 1. The prism assembly 106 abuts an outer wall of the test section 104 as shown in FIGURE 2. Optical windows 202, 204, 206, and 208 are provided in the prism assembly 106 positioned to receive light coming from the light source 102 (not shown) and traveling through the test section 104 to the lenses 308, 316 (FIGURE 3) of the imaging systems 108-114 as described in relation to FIGURE 3. In this exemplary embodiment, the optical windows 204, 206, and 208 are arranged to define a triangle, e.g., an equilateral triangle, with the optical window 202 positioned generally at the center of the triangle. The optical windows 204, 206, and 208 are tapered, having a back surface parallel to the outer wall of the test section 104, and a front surface that is orthogonal to an optical path of a corresponding CCD camera of a velocimetry system 108, 110 or 112 (described below in greater detail). If the viewing optical path of the color CCD 314 is orthogonal to the test section, the optical window 202 need not be tapered.

If the lenses 308, 316 are sufficiently close to the prism assembly 106, the cone of the field of view for the associated camera is very narrow and the prism can, therefore, be very compact. If the optical off-axis angle is less than about 6-8 degrees, the prism may be neglected altogether.

FIGURE 3 illustrates a camera assembly 300 for the velocimetry and thermometry imaging systems 108-114 used in the 3DDPIT/V system 100. Each velocimetry imaging system comprises a monochrome CCD camera 302 mounted in a support structure 301 (shown in phantom) and a lens 308 that extends from the support structure 301 to engage the prism assembly 106 (FIGURE 2) and operable to receive light coming from the white light source 102 and traveling through the test section 104 and the prism assembly 106. The light that travels through each lens 308 is received at the corresponding CCD sensor of the camera 302. The CCD cameras 302 and corresponding lenses 308 are disposed to align with the prism assembly 106. In this embodiment, the lenses 308 are arranged such that they form an equilateral triangle. Suitable displacement of the cameras 302 allows a common reference area from the reference plane to be imaged by each of the three CCD cameras 302. Therefore, particles in the imaged volume will be imaged on each camera 302. However, because the cameras 302 view the volume from different positions, parallax produces an apparent shift with the position of the particle. Thus, each particle produces an image on each CCD camera 302 and when the images are superimposed, the three particle images approximate an equilateral triangle.

Ray tracing may be used to calculate the position of the particle in the direction perpendicular to the plane of the cameras, as is known in the art.

The thermometry imaging system 114 may be positioned laterally at any convenient position. If a prism assembly 106 is used, the thermometry imaging system 114 is positioned to align with the optional window 202, for example, at the geometrical center of the triangle formed by the velocimetry imaging systems 108, 110, and 112. The thermometry imaging system 114 comprises a color camera 314 and a corresponding lens 316. A conventional single CCD digital color camera makes use of a Bayer filter to generate a color image. This filter is a repeated pattern of red, green, and blue filters overlaid on the CCD pixel array, where each pixel is covered by one of these color filters. For each pixel, an algorithm is then used to generate a color value using its value as well as neighboring values. This is not preferred for use in a 3DDPIT/V system due to the fact that the TLC particles being imaged are extremely small (preferably 10-50 μm diameter), and the resulting particle images may only be several pixels in radius. Using a single CCD color camera with a Bayer filter to image particles this size would result in adjacent pixels having totally different color/hue and, thus, temperature, values. It is, of course, possible to simply average the differing colors/hues, but this could result in significant measurement error. A preferred solution is the use of a custom color camera, as shown in FIGURE 3.

This custom color camera 314 is an assembly of three individual CCD cameras 304 (two visible), an imaging prism 306, and three color filters (not shown). The imaging prism 306 splits the incoming light to the three CCD cameras 304. Each of the cameras 304 is then matched with a filter of a primary color, for example, a red, green, or blue filter. The filters are chosen not to reproduce colors as observed by humans, but to best measure the spectral reflectance from the TLC particles. An example of the spectral curves of these filters is shown in FIGURE 6. The individual CCD cameras 304 are also precisely mounted on the prism 306 such that each has the same field of view.

The color camera 314 is mounted in a support structure 301. In the preferred embodiment, the color camera 314 and its associated lens 316 are placed directly on the optical axis of the camera assembly 300, as shown in FIGURE 3. The optical path distance between the lens 316 and the cameras 304 is adjusted such that the effective optical path distance for all three cameras 304 is the same.

In the preferred embodiment, the camera assembly 300 consists of six Illunis XMV-11000 11 -megapixel CCD cameras with 4008x2672 resolution. Three cameras 302 are allocated for the velocity measurements, and the remaining three cameras 304 are assembled into the custom color camera 314 described above. Four Makro-Symmar £5.6/120mm lenses from Schneider Optics are used due to their excellent spherical and chromatic aberration properties. Each of the three individual CCDs, in addition to the four-lens assembly, is mounted on a Thorlabs APT high accuracy six-axis translation stage, to allow for extremely accurate positioning during calibration.

FIGURE 4 illustrates the exemplary 3DDPIT/V system setup used for performing simultaneous measurements of velocity and temperature of a fluid. The camera assembly 300 is designed such that the lenses 316 and 308 are aligned with the respective optical windows 202, 204, 206, and 208 of the water-filled prism assembly 106. In operation, the camera assembly 300 is positioned adjacent to the test section assembly 200, with an appropriate imaging distance such that the lenses 308 and 316 view the flow within the test section through the corresponding optical windows 202, 204, 206, and 208 of the optional water-filled prism assembly 106, as indicated in FIGURE 4. Referring now to FIGURE 5, a method of basic operation of the 3DDPIT/V system in accordance with the present invention will be described. As discussed above, the method provides for simultaneous measurement of velocity and temperature of the flow in a volume. At block 502, the test section 104 populated with TLC particles suspended in the test liquid in accordance with the current invention is provided. At block 504, a white light source is directed at a transparent wall of the test section. At block 506, the optional prism assembly 106 is attached to a transparent wall of the test section 104. At block 508, the camera assembly is assembled as described above in regard to FIGURE 3. At block 510, the camera assembly 300 is positioned to image the volume of interest in the test section, so as to provide the path for the light traveling through the test section 104, the water prism assembly 106, the lenses 308 and 316 to the cameras 302 and 314. At block 512, the images of the particles are taken in sequences by the cameras 302 and 314 of the camera assembly 300. At block 514, velocity field and temperature field of the fluid are calculated by the computer system 116 based on the particle images taken at block 512. Such calculations are known in the art.

3D Digital particle tracking velocimetry is a measurement system capable of measuring three-component velocities within a volume. To do this, sub-pixel coordinates are determined for the center of each particle in the images. Once identified, these coordinates are used to trace rays and determine their intersection in physical space. The pixel coordinates are first converted to physical locations on each CCD, and then into the global coordinate system. The locations on each CCD are then connected to the center of their respective lenses. This gives the starting angle for the ray, which then propagates for bounding the imaging volume. The ray is refracted at each interface of the optional water-filled prism, the test-section wall, and then into the test section.

In theory, each of the three rays should intersect at a single point in space, which will be the physical coordinates of the particle imaged onto each CCD. A mathematically perfect intersection of three lines in three dimensions, from an experimental standpoint, is unlikely. For three rays there may be three minimum distances between the rays instead of a single point of intersection, and six points at which these minimum distances occur. In order to assure the equations were robust enough to handle slight experimental deviations from perfect imaging, the physical coordinate determination of the particle is based on a weighted average of the coordinates based on the distance between each set of points. The inverse of the minimum distance between each ray is used to weight the coordinate points along those rays at which the minimum occurs. The closest two points get the highest weight, the next two closest the next highest weighting, and the furthest the lowest weighting. This helps accuracy if two of the three particles are imaged correctly while the other may be off slightly. The result of the weighted average is then defined to be the physical coordinate of the imaged particle. Based on the found physical coordinates of the particle, a velocity field of the fluid may be calculated. As described above, the temperature field of the fluid is determined by the color of TLC particles measured from the colored particle image taken by the color camera 314 by performing temperature calibration of the particle color. This procedure is well-known in the art.

While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims

CLAIMSThe embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A system for simultaneously determining temperature and velocity of a fluid containing a plurality of thermochromic liquid crystal particles suspended in the fluid, comprising: a light source emitting white light; a test section containing the fluid, the test section having a first transparent wall and a second transparent wall, wherein the white light is directed toward the first transparent wall of the test section; and at least three velocimetry imaging systems, each velocimetry imaging system comprising: i) a lens adapted to receive a portion of the white light that has passed through the test section, and ii) a monochrome CCD camera adapted to receive light from the associated lens, the monochrome CCD camera producing an image of the particles; and a thermometry imaging system comprising: i) a fourth lens adapted to receive a portion of the white light that has passed through the test section, and ii) a 3 -CCD color camera adapted to receive light from the fourth lens.

2. The system of Claim 1, further comprising a prism assembly having at least three peripheral water-filled prisms, and a fourth water-filled prism, the prism assembly abutting the second transparent wall of the test section.

3. The system of Claim 2, wherein the at least three peripheral prisms comprise three prisms positioned to define a triangle.

4. The system of Claim 3, wherein the fourth water-filled prism is placed within the triangle defined by the three peripheral prisms.

5. The system of Claim 1 , wherein the 3 -CCD color camera comprises :

(1) an imaging prism operable to split the received light into three images;

(2) three monochrome CCD cameras, each CCD camera receiving one of the images passing from the imaging prism, the three monochrome CCD cameras producing images that are superimposed to generate a color image of the particles; and

(3) three color filters, each color filter disposed to filter the images received by one of the three monochrome CCD cameras.

6. The system of Claim 5, wherein the monochrome CCD cameras of the thermometry imaging system are mounted such that each CCD camera has the same field of view.

7. The system of Claim 5, wherein the three color filters comprise primary color filters.

8. A method for simultaneously determining temperature and velocity in a fluid, comprising: providing a test section containing a fluid with a plurality of thermochromic liquid crystal particles, the test section having a first transparent wall and a second transparent wall; placing a white light source emitting such that white light illuminates a volume of interest within the test section; directing at least three velocimetry imaging systems at the volume of interest within the test section, the at least three velocimetry imaging systems configured to receive light scattered by the liquid crystal particles, and that has passed through the test section, each imaging system comprising: i) a lens, and ii) a monochrome CCD camera adapted to receive light from the associated lens, such that the monochrome CCD camera produces an image of the particles; directing a thermometry imaging system at the volume of interest within the test section, the thermometry imaging system configured to receive light scattered by the liquid crystal particles that has passed through the test section, the thermometry imaging system comprising: i) a fourth lens, and ii) a 3-CCD color camera adapted to receive light from the fourth lens; determining a three dimensional position of the particles based on the images obtained by the monochrome CCD cameras; calculating a velocity field of the fluid based on the determined three dimensional position of the thermochromic liquid crystal particles in successive images; and determining a temperature field of the fluid based on the color image of the thermochromic liquid crystal particle such that the temperature field and velocity field are determined for the same point in time.

9. The method of Claim 8, further comprising: placing a prism assembly having at least three peripheral water-filled prisms and a fourth water-filled prism such that the prism assembly abuts the second transparent wall, such that each of the at least three velocimetry imaging systems receives light from one of the at least three peripheral water-filled prisms, and such that the thermometry imaging system receives light from the fourth prism.

10. The method of Claim 9, further comprising disposing the at least three peripheral prisms such that they define a triangle, wherein the at least three peripheral prisms comprise three peripheral prisms.

11. The method of Claim 10, further comprising placing the fourth water-filled prism within the triangle defined by the three peripheral prisms.

12. The method of Claim 8, wherein the color camera comprises:

(1) an imaging prism operable to split the received light into three images;

(2) three monochrome CCD cameras, each CCD camera receiving one of the images from the imaging prism; and

(3) three color filters, each color filter disposed to filter the images received by one of the three monochrome CCD cameras.

13. The method of Claim 12, further comprising mounting the monochrome CCD cameras of the thermometry imaging system such that each monochrome CCD camera has the same field of view.

14. The method of Claim 12, wherein the three color filters comprise primary color filters.