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FLIR Technical Series

Application Note for Research & Science

Synchronized Thermographyand PIV (Particle ImagingVelocimetry) Fluid FlowMeasurementsJohn S. Allen, In Mei Sou, Christopher N. LaymanCollege o Engineering, University o Hawaii-Manoa

Abstract

For a variety o engineering applications, the simultaneous real-timemeasurements o the velocities and temperature changes in a ow feld

are needed. This is particularly important in the design and optimization

o ultrasound systems used or mixing and sonochemistry. In this case,

the ultrasound vibrations o an acoustic horn induce localized heating andstreaming in the uid inside a reactor. We present novel experimental

results o the synchronized measurements o streaming velocities using

a PIV (Particle Imaging Velocimetry) system (TSI, Shoreview, MN) withthermal measurements o the surace temperatures using a FLIR Titanium

Thermal Camera (FLIR, Wilsonville, OR). The convective heat transer is

quantifed with respect to the development o vortices in the ow feld.Spatial correlation analysis is conducted between the temperature and uid

velocity felds. The combined eects o heating and streaming are examined

or a range o uids with di erent rates o thermal dissipation.

Introduction

Inrared thermography has been used to obtain heat transer

measurements or uids engineering applications. For example, convectiveheat uxes have been investigated as well as the thermal boundary

layer behavior associated with ow over complicated-shaped objects.Typically, thermocouples are devices used in engineering heat transermeasurements, which prove the temperature and heat ux at a specifc

single point. Though these devices have proven to be both robust and

useul or a variety studies, many limitations exist with respect tothe quantifcation o high spatial temperature gradients. The surace

temperature maps obtained by IR thermography provide signifcantly better

spatial data on convective heat transer processes at uid interaces.

Instantaneous measurement o the ow feld velocity is also important in

the quantifcation o convective heat transer. Flow marker techniques

have been developed or visualization and through recording the markersdisplacement in time, the velocity can be determined. This allows or

the direct determination o the velocity vector without need or prior

calibration. In particular, Particle Imaging Velocimetry (PIV) tracks the

position o groups o particles in a plane allowing or measurement othe instantaneous velocity feld. A sufciently low particle density is used

such that particles behave as non-interacting tracers. A laser light sheetilluminates the particles and a camera captures two exposures at the

same time intervals or the same set o particles. The method o cross

correlation can be used to determine the distance the particles havemoved during the time between subsequent images and hence compute the

local ow velocity.

In a pioneering study, Volino and Smith reported on the simultaneoususe o IR and PIV to invest igate natural convection. They used a digital IR

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camera together with a PIV system to study the ree convection o thermal

layers o thermal plumes in a uid layer cooled rom above. The cool, dense

uid sinks and is replaced by warmer rising uid rom below. The eectso viscosity and buoyancy govern the heat transer with the dominating

mechanism depending on the relative thickness o the layer. New eatures

on the uid dynamic behavior and thermal response o the layer werereported. The hydrodynamics associated with the convection process

was observed to have previously unreported structures that developed

both parallel and normal to the uid interace. Moreover, Volino and Smithconcluded that the simultaneous, combination measurement system

provided more detailed and comprehensive inormation than could be

obtained by the use o each respective individual system. However, despitethe advances in both PIV technology and thermal imaging cameras over

the past ten years, this combined method has received little attention. One

recent related study was o a thermal boundary layer in a ume by Hetsroni

and co-workers. While natural or ree convection was the ocus o theseprevious works, we examine orced convection rom an ultrasound source.

Ultrasound Application Heating and Streaming Fields

In this study, we examine the orced convection due to an acoustic source

which is that o a high power ultrasonic horn. The horn tip vibrates atultrasonic requencies propagating sound into the surrounding uid mediumo a reactor. Associated with this sound propagation is the transer o i ts

momentum to the surrounding uid resulting in an induced temperature

feld (acoustic heating) and a ow feld (acoustic streaming). The processeso heating and streaming (along with cavitation bubbles) are the underlying

physical mechanisms needed or ultrasound applications. Ultrasonic

cleaners rely on streaming or abrasive movement and transport. Sono-

reactors are used to enhance mixing and are capable o degradingdeleterious chemicals and biological waste products.

A diagram o the experimental set-up with synchronized PIV andthermography is shown in Figure 1. A model sono-reactor consists o a

clear acrylic tank (50 30 30 cm3) flled with degassed water with a 20

KHz acoustic horn mounted on the side. The horn was driven at 32 Watts

or several di erent time intervals. The uid velocity measurements in thesurace plane were done using a TSI (Shoreview, MN) PIV system composed

o two pulsed Nd:YAG lasers, a PIV digital camera synchronizer, and adedicated computer or image processing. PIV images were obtained at a

rate o 60 Hz. A calibrated mid-wave IR camera (Titanium SC7000, FLIR,

Wilsonville, OR) was synchronized through a pulse generator with a 30 Hz

trigger to the PIV camera.

Acoustic streaming-induced vortices play signifcant role in the convective

heat transer. In this study, the main vortices develop in the tank withrespect to the centerline o the horn. In this case, a vortex ow feld

develops ater seconds o ultrasound exposure with a vortex pair visible

rom the velocity feld measurements. The strength o the vortex cores wasdetermined rom the PIV data o the instantaneous velocity felds. In this

calculation, we ollow the ormulation given by Chong et. al. (1990) or the

swirling (vortex) strength, which is expressed in terms o the local velocitygradient tensor. The streamlines o the vortices are shown in Figure 2 with

the iso-regions o swirling strength indicated by the color portions in red.

A corresponding thermal image is captured by the synchronized IRcamera. The actual PIV imaging plane and IR camera plan are 1.5 cm

apart; however, repeated measurements with a thermocouple array reveal

negligible temperature dierences between these two planes. The vortexquantifcation o the strength and location can be assumed to be same or

the two planes. Shown in Figure 3 is the corresponding thermal camera

image with the respective swirling strength shown with a black outline.

Figure 1. A diagram o the experiment set-up with PIVand IR camera. The thermocouple array was used toexamine measurements with respect t o the tank depth.

Figure 2. The vortex pair measured by the PIV systemwith the swirling strength iso-regions shown with the redportions.

Figure 3. Shown is the corresponding instantaneoustempera ture feld or the uid veloci ty feld in Figure 2.

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The uid temperatures increase near the vortex structures. The counter-

rotating vortices drive the convective heat transer process. We record

the greatest temperature in the region between the two vortices.Moreover, a more rigorous relationship between the vortex strength and

convective heat transer can be determined with this technique. Such

spatial resolution is not possible with thermocouple measurements. Asnoted in previous studies, the process is highly coupled and nonlinear so

the higher heat uxes do not always correlate directly with the velocity feld.

Correlation techniques can be used or urther quantifcation since the owfeld and its thermal response can evolve at dierent rates. For industrial

ultrasound applications, an understanding o the temperature changes with

respect to reactor design and acoustic parameters is sought. This methodyields novel visualization into the underlying physical processes o acoustic-

induced heating and streaming.

Summary

Acoustic streaming and heating rom an ultrasonic horn were investigated

using synchronized PIV and IR thermography. The convective heat transer

process can be examined more extensively and comprehensively bycombining the two techniques. Understanding the evolution o the uid

velocity and temperature felds is key in making improvements in the designo sono-reactors or a variety o industrial and research applications. Thecombined technique o IR thermography and PIV should also be useul to

other as yet unexplored thermo-uids engineering applications.

References

Gurka, R., Liberzon A., Hetsroni G.; Detecting coherent patterns in a ume

by using PIV and IR imaging techniques; p. 230-236; Experiments in Fluids,37, 2004.

Volino, R.J. and Smith G.B.; Use o simultaneous IR temperature

measurements and DPIV to investigate thermal plumes in a thick layercooled rom above; p. 70-78; Experiments in Fluids; 27,1999.

Chong, M.S. Perry A.E. Cantwell B.J.; A general classifcation o three-

dimensional ow felds; p. 765-777, Physics o Fluids, A2, 1990.

Acknowledgements

The authors wish to thank Chris Bainter o FLIR Systems or many helpul

suggestions and acknowledge support the CIMES Center at the University

o Hawaii.

About the Authors

John S. Allen is an Associate Proessor in the Mechanical EngineeringDepartment at the University o Hawaii-Manoa, Honolulu, HI. Chris

Layman and In Mei Sou are postdoctoral research associates with the

Departments o Mechanical and Civil Engineering at the University o Hawaii

at Manoa, respectively.