Convective Heat Transfer Studies in Mini-Channels Using Digital Interferometry

Abstract

Convective heat transfer in micro and mini channels has been recommended as an effective heat removal method for various electronic packages and systems. Experimental and theoretical investigations on the thermal performance of micro and mini channels have gained immense attention and hence, heat transfer studies in mini channels are of great importance. Some of the experimental results found in the literature on heat transfer in small-dimension channels are of contradicting nature even though some generally agreeing results are also found. One of the probable reasons for such deviations is the intrusive nature of the measurement techniques used. The traditional method of temperature measurement in channels uses the thermocouple probe, and for obtaining temperature distribution across the channel either a number of probes or a moving probe technique is required, both of which disturb the flow field and cause measurement errors. Hence a non intrusive measurement technique, such as an optical method is preferable for temperature measurement in small channels. In the present work, convective heat transfer studies have been performed on water flowing through a mini channel of hydraulic diameter 4 mm, using the non-intrusive technique of laser interferometry, coupled with digital image processing. The channel is fabricated using high quality optical glass and aluminum blocks. Mach Zehnder Interferometry is used for obtaining the temperature distribution in the channel. The experimental arrangement consists of two identical channels, one placed in the test section and the other in the reference section of the interferometric set up. As the test section is heated, a density variation is produced in the medium, which causes a refractive index variation, deforming interference fringes. This enables the calculation of the temperature distribution inside the channel. The interferograms are grabbed using a CCD camera and an AVT Fire package software. Digital image processing technique, using MATLAB software is used for locating the fringe-centers, and calculating the temperature distribution. The temperature profiles are obtained at different sections of the channel for various values of the average Reynolds number and various heating levels. The local and average heat flux values are obtained from the constructed temperature distributions. Variations of the local and average heat transfer coefficients and Nusselt number are determined and discussed. Results of parametric studies are compared and contrasted with relevant entry length solutions from the literature.

Measurement Techniques

“…38 The interferometric technique has been applied to minichannels as well. 39 Infrared thermography has been applied for surface temperature measurement in microchannel heat sinks [40][41][42] well as for localized temperature measurements near the triple line of an evaporating meniscus. 43,44 IR thermography can only be used to measure surface temperatures and an accurate value of the emissivity of the medium is essential to obtain meaningful measurements, which can be a limitation in some applications.…”

Digital Interferometric Measurement of Forced Convection Fields in Compact Channels

“…[26][27][28] In recent investigations, the MZI configuration has been utilized, along with digital image processing, to characterize convective heat dissipation in mini-channels and macro-channels were having dimensions suitable for integration into the substrates of electronic components. [29,30] Li et al [31] developed a MZI set up for measuring the pressure drop along a T shaped microchannel made of a polymer layer sandwiched between two quartz glass plates. The interfering beams were made to pass through two Acousto-Optic Modulators, and the interference patterns were captured using a CMOS camera.…”

Real Time Temperature Distribution Measurement of a Microheater by Using Off-Axis Digital Holography

“…In a group of optical methods which are called index of refraction methods, the temperature distribution is obtained by measuring the phase differences of light rays caused by variations in refractive index. In several relatively recent publications, such methods include classical interferometry [3] and holographic interferometry [4][5][6][7].…”