Introduction

Background

Objectives


Test facilities

Diagnostic techniques







 

 
Boiling flow in microchannels

micro nano engineering
   

  Diagnostic techniques

Experimental techniques include high speed visualization techniques to identify the nucleation and heat transfer regimes, synchronized with high-frequency pressure and thermometry to correlate the heat transfer mechanisms with the heat transfer rate and friction factor. A micro PIV system is used to quantify the velocity distributions inside the micro tubes.


The high speed digital imaging system comprises a PHANTOM v4.2 camera with a resolution of 512 x 512 pixels, 2100 full-resolution pictures-per-second (pps) and a maximum windowed speed of 90,000 pps.

Temperature rise and pressure drop along the micro channel length are measured by means of fast response k-thermocouples and ECO1 pressure transducers from Wika (0-4bar, 0-10V, Accuracy 0.5%) placed on the stagnation chambers. Five additional thermocouples allow measure the instantaneous temperature along the channel length.


       


The image acquisition system is synchronized with the fast response surface thermocouples and pressure transducers. The hardware allows simultaneous acquisition of images, thermocouple and pressure signals to be processed in Matlab to estimate the time-varying heat flux and fluid-dynamic characteristics of the boiling flow. The measurements allow calculation of the instantaneous heat fluxes, from boiling curves are obtained. The two characteristic parameters used in the thermal analysis of boiling flows, the critical heat flux (CHF) and Leidenfrost phenomenon, are then inferred in terms of the thermodynamic properties at the liquid-solid interface and correlated with the main design requirements of practical cooling systems (heat transfer rate and friction factor).

The mPIV system uses an inverted Leica DM ILM epi-fluorescent microscope and a Flowsense®_ 2M CCD (charge coupled device) camera from Dantec Dynamics to record 1mm average diameter nile red fluorescent tracing particles from Invitrogen® with an image field of 1600x1186 pixel2 (pixel size 7.41mm) at a maximum rate of 15Hz and 8/10-bit intensity resolution. The particles are illuminated via a dual Nd:YAG laser New Wave_Solo II-15 emitting at l=532nm with a repetition rate of 15Hz. The time delay between laser pulses is set between 100 and 500ms in which the particles have a maximum displacement of ¼ of the interrogation area length. An epi-fluorescent prism/filter cube is used to filter wavelengths lower than l=575nm (particles wavelength emission) to prevent background noise influence in the results. The field is magnified with an air-immersion N PLAN objective lens with magnification M=40 and numerical aperture NA=0.55


Schematic of the mPIV system

FlowManager® V4.0 from Dantec Dynamics is used for the acquisition, calculus and post processing of the data. Synchronization is assured by National Instruments hardware