A transient study on two phase adiabatic flow over micro circular pin heat sinks
Introduction
In the past two decades, the main focus of micro scale heat transfer research was concentrated on the heat dissipation in small devices. Amongst proffered methods to improve heat transfer rate in micro scale devices, enhanced surfaces manifest a superior capability [1], [2], [3], in spite of the fact that pressure drop is higher in them compared to ordinary micro channels.
Advances in micro fabrication in last two decades enhanced the use of micro pin heat sinks as a powerful cooling method in mini structures. A considerable amount of research was dedicated to this field that was aimed at the study of fluid flow and heat transfer characteristics dissipated from the hot surfaces experimentally. Amongst them, some studies dealt with a single-phase flow in the form of sub-cooled or saturated state. On the other hand, some researchers numerically studied the case of microchannels [4], [5], [6]. However, the number of studies and parameters studied by this method is still much lower than the number of experimental studies [7].
One of the interesting properties to observe and control in micro pin finned devices is the pressure drop. Reviews of predictive methods for pressure drop in mini and micro channels can be found extensively in references [8], [9], [10], [11]. As an example, Kuppusamy et al. [12] conducted research about a particular case of a micro channel where they employed secondary flow to reduce the pressure drop and increase the heat transfer. In fact, their case resembles an elongated trapezoid pin fin bank.
Other researchers investigated the two-phase flows passing through these micro structures; trying to find a methodology to formulate pressure drop or flow patterns. Some of them focused their studies on simple micro channels; for instance, to comprehend their characteristics in refrigeration. Lee and Mudawar [13] measured the pressure drop for two-phase flows in a micro-channel heat sink. They employed refrigerant R134a for their study. Qu and Mudawar [14] also discussed major flow patterns and instabilities in micro channels. They concluded that the instability in the cycle was completely suppressed by throttling a control valve situated upstream of the heat sink.
On the other hand, an abundance of studies about enhanced micro surface structures is available. Siu-Ho et al. [15], [16] conducted experiments to analyze the pressure drop and heat transfer of de-ionized water over square micro pins in the form of sub cooled fluid. Qu and Siu-Ho [17], [18], [19] surveyed adiabatic and diabatic pressure drops of several staggered micro pin fins banks. Konishi et al. [20] performed an experimental study on single-phase flow to evaluate the pressure drop across square micro-pin-fins having diameters of 200 . However, in all these studies, water played the role of working fluid, Koşar and Peles [21] considered both single-phase and two-phase heat transfer using R-123 as the working fluid.
A broad gamut of working fluids and configurations has been taken into account in previous researches [22]. In addition to the latter work, two-phase flows over enhanced surfaces are present in manifold works: Koşar and Peles [23] studied the flow of de-ionized water over a shrouded micro pin fin structure. Nitrogen–water mixture was used by Kirishnamurthy and Peles [24] to study the circular micro pins . Wei et al. [25] studied cooling of electronics devices with 30 and 50 thick fin pins. Furthermore, they employed FC-72 as the working fluid.
Koşar et al. [26] expanded their research for three different micro pin heat sinks utilizing two different working fluids, namely R134a and water. Konishi et al. [27] investigated the two-phase pressure drop of water liquid–vapor, excluding heat transfer from their study. Jasperson et al. [28] also performed a comparison between these two routes of heat dissipation from three different aspects: thermal performance, hydraulic performance, and cost of manufacturing. In recent years more researchers became interested in dielectric chemically inert HFE-7200 for cooling. As an example, Reeser et al. [29] implemented an investigation of this fluid over both staggered and inline configurations. They observed no major differences between the cooling ability of inline and staggered pin banks ;In addition to this work, in references [30], [31], [32], researchers employed HFE-7200 as working fluid, and a collection of similar studies were presented in their works.
Kirishnamurthy and Peles [33] studied a form of enhanced surface with a single row of micro pins under boiling conditions observing three major flow patterns: bubbly flow, multiple flows, and wavy-annular flow. Law et al. [34] analyzed another case of boiling fluid over the oblique-finned surface in order to extract heat transfer and pressure drop characteristics. Chang et al. [35] investigated sub-cooled flow as well as boiling heat transfer and bubble characteristics of FC-72 on a heated square micro-pin-finned. They concluded that micro pin effectively augments the flow boiling heat transfer coefficient.
In the last decade due to advances in computational tools and the need to characterize the flow with more detail, some researchers employed numerical methods to scrutinize pressure drop and heat transfer related phenomena in micro structures with enhanced surfaces.
John et al. [36] solved the case of single-phase flow over circular and square micro pins banks with the use of commercial software CoventorWareTM. Their results reveal better performance in circular pin fins for lower Re numbers. Lee et al. [37] employed direct numerical simulation to model the finned surfaces. Shafeie et al. [38] numerically studied the laminar flow in micro heat sinks with the micro pin-fin structure on a substrate. Izci et al. [39] modeled numerically a channel with different shape single micro pin-fins using software COMSOL Multiphysics 3.5a. Zhao et al. [40] performed a numerical investigation on square pin fins. Square pin angle and porosity were optimized in their study.
Two issues are mostly considered in both simple and micro channels with enhanced surfaces; improving the heat transfer and controlling the pressure drop. The primary goal of a micro heat sink is to enhance heat transfer and dissipation. In this regard, the optimum design of such a system is based on the increment of the device heat transfer. However, such improvement is accompanied by an increase in pressure drop of the device. Understanding the fluid flow in these chambers gives researchers a better tool to overcome the aforementioned difficulty. In the current study, the emphasis is on capturing the physical behavior of flow due to its transient nature in order to inspect the hydraulic behavior of the flow as well as the present mixing which correlates with the ability of the fluid to remove heat from the micro pins surfaces. Researchers benefited from many different numerical skims in their multiphase studies such as smoothed particle hydrodynamics (SPH) [41], [42], [43], [44], lattice Boltzman method (LBM) [45], [46], [47], [48] and volume of fluid (VOF) [49], [50], [51], [52]. Anyway, in current research a VOF approach accompanied by an adaptive meshing is used (see Section 2 for more details).
A glance over current literature in the field of micro enhanced surfaces reveals the usage of an extensive collection of working fluid as well as the need to formulate and studying the performance of this device in transition, especially in the presence of two fluids.
Section snippets
Problem description
The case of adiabatic two-phase flow over staggered and inline micro fin banks are considered in this study. Flow over a pin fin bank is studied for a time lapse of in order to capture the transient nature of the flow in this sort of devices. Initially, the microchannel was filled to its half by each fluid. Both fluids enter the micro fin bank having the same velocity in -direction which is indicated in figures by dimensionless Reynold’s number (). Density ratios between 1
Results and discussion
In the micro heat sink, the flow passes through the micro pillars that are constructed to improve the heat transfer with an escalation in the heat transfer area. The observed fluid flow demonstrates a transient behavior (look at Fig. 2 as an example).
In the present work, three different parameters are examined, including the density ratio of two flows, pin diameter, and surface tension.
In Fig. 2 the flow inside the micro pinned micro channel is illustrated ( and ). It depicts the
Conclusion
In the current study, two-phase flow over micro pinned surface microchannels is investigated from a hydrodynamical point of view. The flow behavior due to variation of three main parameters (i.e., density ratio, micro pin diameter, and surface tension) are scrutinized in the presence of two fluids. The flow solver Gerris with a tree-based and adaptive approach is utilized to solve the flow in a two-dimensional enhanced surface micro channel. Pressure drop, void fraction, and flow velocity field
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