Low Reynolds number thermo-hydraulic characterization of offset and diamond minichannel metal heat sinks

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Highlights

Abstract

Experimental investigation involving heat transfer and pressure drop studies in minichannel heat sinks has been performed at low Reynolds number in the range 17–450. In a steady state approach, heat supplied to the minichannel heat sinks has been carried away by the convective fluid, water. Continuous circulation of water through different minichannels, namely, straight, offset and diamond has been maintained with two syringe pumps connected in parallel and out of phase. Calibration of the experimental setup with straight minichannel is followed by the testing of offset and diamond minichannels. Thermo-hydraulic performances have been experimentally studied in terms of Nusselt number, thermal resistance and friction factor with experimental accuracies of 17%, 8% and 14% respectively. Nusselt number has been found varying linearly with the Reynolds number for all the samples.

Introduction

Miniaturization has become an inseparable part of the modern technology. About three decades ago, Tuckerman and Pease [1] introduced the concept of microchannel heat sinks. They pointed out that an increase in heat transfer coefficient can be achieved with decrease in the hydraulic diameter of the channels. This insight has given a new perspective for research on heat sinks. Subsequently, the concept of heat transfer augmentation by reducing channel hydraulic diameter to micron level has also been adopted for other engineering applications [2], [3], [4]. Different aspects of minichannel heat sinks study are under continuous exploration [5], [6], [7], [8], [9], [10]. However, there has always been debate on the validity of interpolation derived from the theory of conventional channels and use of existing thermo-hydraulic empirical correlations. The physical mechanisms distinguishing the flow through conventional channels and through microchannels have not been clearly established in spite of the diverse microchannel heat sink studies being carried out. Although the outcomes have been indecisive so far, the pool of knowledge has intensified enormously. Few researchers [11], [12], [13], [14], [15], [16], [17], [18], [19] have exquisitely summarized the literature of last three decades on micro and minichannel heat sinks. These literatures evidently illustrate the increase of heat dissipation by these non-conventional heat sinks.

In general, the dimensions of micro and minichannel heat sinks are optimized for specific thermo-hydraulic performance. Nevertheless, enhancement techniques must be employed for further increase of heat dissipation. Steinke and Kandlikar [20] gave an in-depth review of the different conventional enhancement techniques that can be employed to microchannels as well. These included passive techniques like flow obstruction, curved flow path, entrance effects, secondary flows, increase of surface roughness, and addition of phase change materials in the coolants. The active techniques involve vibration of the surface or coolant, variation of mass flow rate and exposing the flow to electrical field. The most common passive enhancement techniques employed in microchannel heat sinks are flow interrupting geometries such as offset-strip fin [21], [22], [23], [24], [25], [26], [27], diamond-shaped fin [28] and oblique-fins [29], [30] and curved flow path [31], [32]. Literatures related to offset and diamond microchannel heat sinks shall be discussed here. Kishimoto and Sasaki [21] at first suggested the use of diamond shaped micro-grooved cooling fins which are now generally referred as offset microchannel heat sinks. They demonstrated analytically the reduction in chip junction temperature with the adaptation of these enhanced heat sinks. Colgan et al. [22] fabricated different configurations of offset silicon microchannel heat sinks having an average hydraulic diameter of 100 μm. These heat sinks were bonded on single chip modules and tested experimentally for their cooling capacity under different flow conditions (Re = 26–282). They also illustrated, using computational fluid dynamic simulations, the increase in apparent heat transfer coefficient and a decrease of surface temperature in offset-type over continuous plain microchannel heat sink having same hydraulic diameters. This study was further extended by Colgan et al. [23] with improved channel geometries, use of different coolants and modification in the heat sink and chip module assembly. Kandlikar and Upadhye [24] developed a mathematical algorithm to analyze offset microchannel heat sinks. They concluded slightly-lower pressure drop and lesser mass flow rate in enhanced microchannels as compared to plain microchannels under same heat flux conditions. The use of split flow arrangements was suggested in enhanced microchannels to further increase the heat dissipation while lowering the flow pressure drop. Contrary to Kandlikar and Upadhye [24], Steinke and Kandlikar [25] concluded an increase in pressure drop for offset microchannel heat sinks. Four geometries having hydraulic diameter in the range of 76–98 μm were experimentally tested. The unit thermal resistance of enhanced microchannel heat sink was found to be approximately one-hundredth of that of plain microchannel heat sink under the same conditions of heat input and mass flow rate. Hong and Cheng [26] performed numerical study for optimizing the size of offset microchannel heat sink considering the offset-strip fin interval and fin length as variables. With the aid of computational fluid dynamics solver, FLUENT, they showed that local circulation of flow and breakage of thermal boundary layer periodically, due to the offset-strip fins, improved the heat transfer. Ndao et al. [27] compared plain, in-line circular pin–fin, staggered circular pin–fin and offset-strip fin microchannel heat sinks using multi-objective optimization technique with the aim of minimizing thermal resistance and pumping power. It was found that offset-strip fin performed the best over the other configurations, followed by staggered circular pin–fin and then by in-line circular pin–fin except with a penalty for high pressure drop. The increase in heat transfer was stipulated to be due to vortex shedding and interrupted growth of thermal boundary layer. Jiang and Xu [28] investigated the performance of staggered diamond and in-line square mini-fins along with mini-channel heat sinks by characterising these as porous media structures. Ten different geometries having different fin shape and arrangement, channel width and materials (copper, bronze) were tested for water and air as coolants. In-line square mini-fin heat sink was found to give the best overall thermal–hydraulic performance while staggered diamond copper mini-fin array gave the highest heat transfer coefficient when compared to an empty channel with water as coolant.

Studies have been largely carried out on enhanced microchannel heat sinks while the present study deals with enhanced minichannel heat sinks, referring to the classification of Kandlikar and Grande [33]. In anticipation that these enhanced minichannels can be of great importance in cryogenic heat transfer applications, low Reynolds number thermo-hydraulic characterisation has been performed. The enhancement geometries studied are offset-strip and diamond-shaped fins. The offset test samples are made of aluminium and four geometries, with different dimensions of offset-strip fin, have been fabricated. The diamond heat sinks are fabricated of stainless steel, copper and brass but have the same geometry. The performance of these offset and diamond minichannel heat sinks for single-phase, water-cooled convective heat transfer subjected to constant heat flux conditions has been experimentally determined in this article.

Section snippets

Theoretical background

Standard design and analysis procedures for laminar flow and convective heat transfer in conventional channels are available in abundance [34], [35], [36]. Similarly, researchers have been continuously attempting to channelize a standard design methodology for microchannel heat sinks using theoretical and experimental techniques. So far microchannel heat sinks have been generally analyzed either by considering the internal flow through channels [37], [38] or assuming the walls separating the

Test rig

The advancement of various methods in micro-fabrication technology has eased the manufacture of microchannel and minichannel heat sinks [11], [33]. The present test samples have been machined using wire electro-discharge machining (EDM) along with CNC milling. Wire EDM technique is appropriate for machining where straight channels are to be cut, for example, rectangular and diamond shape samples (refer to drawings in Table 2). However, it is not convenient to cut the intermediate portions of

Results and discussions

The fabricated offset-strip fin and diamond-shaped minichannel heat sink samples have been tested for three volumetric flow rates of 40 ml/min, 70 ml/min and 100 ml/min. Each sample has been tested subjecting them to two heat inputs, to be precise, 0.85 W/cm2 and 1.17 W/cm2. Performing tests at higher volumetric flow rates (and hence at higher Reynolds number) in the present set up (and size of the heat sink) has been limited because of the following reasons. The syringe pumps used in this

Conclusions

The thermal and hydraulic performance for enhanced offset and diamond minichannel heat sinks has been experimentally studied in terms of friction factor, Nusselt number and thermal resistance. The major conclusions derived have been enlisted below:

  • Nusselt number has been found varying almost linearly with the Reynolds number whereas it remains independent of heat flux (Fig. 7, Fig. 8). Thermal resistance, on the other hand, decreases with increase in fluid flow but the same is invariant to the

Acknowledgement

Financial support received from Council of Scientific and Industrial Research (CSIR), New Delhi, India (Project Sanction No. 22/500/10-EMR-II) is duly acknowledged.

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