To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Infrared thermography (IRT) applications for building diagnostics: A review

A technique is described that measures the instantaneous three-dimensional temperature distribution in water using two-color laser-induced fluorescence (LIF). Two fluorescent dyes, Rhodamine B and Rhodamine 110, are used as temperature indicators. A laser light sheet scanned across the entire measurement volume excites the fluorescent dye, and an optical system involving a color beam splitter gives the intensity distribution of the individual fluorescent dyes on two separate monochrome CCD cameras. The ratio of these fluorescence intensities at each point of the image is calibrated against the temperature to eliminate the effect of the fluctuation of illuminating light intensity. A stable thermally stratified layer was measured by this system to evaluate the total accuracy of the measurement system. The random error of the measurement was ±1.4 K with 95% confidence. Measurements of thermal convection over a heated horizontal surface show temperature iso-surfaces having typical structures such as plumes, ridges and thermals.

Whole Field Measurement of Temperature in Water Using Two-Color Laser Induced Fluorescence

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1142&context=coolingpubs

A microscale model for thin-film evaporation in capillary wick structures

As the field of colloidal science continues to expand, tools for rapid and accurate physiochemical characterization of colloidal particles will become increasingly important. Here, we present Particle Scattering Diffusometry (PSD), a method that utilizes dark field microscopy and the principles of particle image velocimetry to measure the diffusivity of particles undergoing Brownian motion. PSD measures the diffusion coefficient of particles as small as 30 nm in diameter and is used to characterize changes in particle size and distribution as a function of small, label-free, surface modifications of particles. We demonstrate the rapid sizing of particles using three orders-of-magnitude less sample volume than current standard techniques and use PSD to quantify particle uniformity. Furthermore, PSD is sensitive enough to detect biomolecular surface modifications of nanometer thickness. With these capabilities, PSD can reliably aid in a wide variety of applications, including colloid sizing, particle corona characterization, protein footprinting, and quantifying biomolecule activity.

/pdf/physical-characterization-of-nanoparticle-size-and-surface-nqyujzu068.pdf

Physical characterization of nanoparticle size and surface modification using particle scattering diffusometry

Micro-particle image velocimetry measurements of the three-dimensional (3D) convection patterns generated near an evaporating meniscus in horizontally oriented capillary tubes are presented. Analysis of the vapor diffusion away from the meniscus reveals a zone of intense heat flux near the solid-liquid-vapor junction that creates a temperature gradient along the meniscus. This results in a surface tension gradient which, coupled with buoyancy effects, causes buoyant-thermocapillary convection in the liquid film. The relative influence of buoyancy and thermocapillarity on the flow was investigated for tube diameters ranging from 75 to 1575μm. A transition from a pure two-dimensional thermocapillary flow to a 3D buoyant-thermocapillary flow is observed with an increase in tube diameter. For the 75μm tube, a symmetrical toroidal vortex is observed near the meniscus. For larger tubes, buoyancy effects become apparent as they dominate the flow field. The high mass fluxes in smaller-diameter tubes drive stronge...

/pdf/experimental-investigation-of-steady-buoyant-thermocapillary-1tkzhq6dmk.pdf

Experimental investigation of steady buoyant-thermocapillary convection near an evaporating meniscus

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1136&context=coolingpubs

Measurement of the temperature non-uniformity in a microchannel heat sink using microscale laser-induced fluorescence

Experimental visualizations of the three dimensional (3D) convection patterns generated near an evaporating meniscus in horizontally oriented capillary tubes are presented. These patterns are caused due to the differential evaporation along the meniscus. In this study, transparent capillary tubes with refractive index close to that of the evaporating liquid were used to minimize refraction effects and obtain velocity vectors near the walls. Polystyrene fluorescent particles of 0.5 μm diameter suspended in methanol were used to make the measurements in tubes of 75, 200 and 400 μm diameter. For the 75 μm tube, gravity was observed to have no effect on the flow patterns and an axisymmetric counter-rotating vortex pair was present along the horizontal and vertical center planes, suggesting the presence of a toroidal vortex near the meniscus. With an increase in tube size, buoyancy effects became apparent as the axisymmetric pattern broke down. A counter-rotating symmetric vortex pair was observed in the horizontal center plane, whereas in the vertical center plane, a single vortex dominated the flow and pushed the secondary vortex to a corner. Particle streak and μPIV images were obtained in multiple horizontal planes and a vertical center plane to understand this 3D flow behavior.

/pdf/visualization-of-convection-patterns-near-an-evaporating-506sm5l7bn.pdf

Visualization of convection patterns near an evaporating meniscus using μPIV

μPIV is a widely accepted tool for making accurate measurements in microscale flows. The particles that are used to seed the flow, due to their small size, undergo Brownian motion which adds a random noise component to the measurements. Brownian motion introduces an undesirable error in the velocity measurements, but also contains valuable temperature information. A PIV algorithm which detects both the location and broadening of the correlation peak can measure velocity as well as temperature simultaneously using the same set of images. The approach presented in this work eliminates the use of the calibration constant used in the literature (Hohreiter et al. in Meas Sci Technol 13(7):1072–1078, 2002), making the method system-independent, and reducing the uncertainty involved in the technique. The temperature in a stationary fluid was experimentally measured using this technique and compared to that obtained using the particle tracking thermometry method and a novel method, low image density PIV. The method of cross-correlation PIV was modified to measure the temperature of a moving fluid. A standard epi-fluorescence μPIV system was used for all the measurements. The experiments were conducted using spherical fluorescent polystyrene-latex particles suspended in water. Temperatures ranging from 20 to 80°C were measured. This method allows simultaneous non-intrusive temperature and velocity measurements in integrated cooling systems and lab-on-a-chip devices.

/pdf/non-intrusive-temperature-measurement-using-microscale-1a33qgm3wt.pdf

Non-intrusive temperature measurement using microscale visualization techniques

In recent years, electronics have significantly reduced in size at maintained or increased functionality. This trend has led to an increased demand for more capable thermal management solutions at smaller scales. However, miniaturization of conventional fan and heat sink cooling systems introduces significant size, weight and efficiency challenges. In this study the flow performance of a novel thin form-factor cooling solution, the advanced dual piezoelectric cooling jet(DCJ), is evaluated. A DCJ is a micro-fluidic device that disturbs the boundary layer over a hot component and hence increases heat transfer. The design of an equivalent fan-curve experiment is described in detail. A first ever fan curve for a bimorph DCJ device is presented. This is coupled to a thermal performance analysis using an experiment simulating thin profile consumer electronics.

Pramod Chamarthy

Papers

Experimental investigation of steady buoyant-thermocapillary convection near an evaporating meniscus

Measurement of the temperature non-uniformity in a microchannel heat sink using microscale laser-induced fluorescence

Visualization of convection patterns near an evaporating meniscus using μPIV

Non-intrusive temperature measurement using microscale visualization techniques

Investigation and application of an advanced dual piezoelectric cooling jet to a typical electronics cooling configuration