Numerical investigation of laminar heat transfer performance of various cooling channel designs

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Abstract

This study addresses the heat transfer performance of various cooling channel designs e.g., parallel, serpentine, wavy, coiled and novel hybrid channels. The cooling channel is designed to be placed on top of an electronic chip which dissipates heat at a constant flux. Laminar flow of a Newtonian fluid in a square cross-section channel is investigated using a three-dimensional computational fluid dynamic approach. Five channels Reynolds number are investigated to quantify the effect of Reynolds number on the performance of the cooling channel designs. Advantages and limitations of each design are discussed in the light of numerical results. Figures of merit, viz. heat transferred per unit pumping power are compared for the wide variety of channels examined.

Research highlights

► A computational study has been conducted to investigate the heat transfer performance of various cooling channel designs. ► It is found that coiled-base channel designs give higher and more uniform heat transfer rate; however, they also impose a significantly higher pressure drop penalty. ► The coiled-base channel can be a desired choice especially in critical applications where cooling performance is of paramount importance.

Introduction

In recent years electronic devices have become indispensable part in every aspect of our daily life. In operating these devices, it is essential to maintain the temperature of electronic components below the recommended upper limit level to achieve optimum performance, maximum efficiency and reliability of the components. Inability in maintaining recommended temperature range will reduce the performance, efficiency and life span of the system and may even lead to catastrophic system failure [1]. With the rapid improvement in microprocessors, this problem has become more serious, not only for the electronic components but also for the power systems that supply energy to the electronic components. In attempts to overcome this problem, various cooling strategies have been proposed and developed [2], [3].

Currently, there are five cooling strategies available [2]: (i) liquid cooling [4], [5], (ii) forced convection cooling [6], [7], (iii) natural convection cooling [8], [9], (iv) edge cooling [10], [11], and (v) phase change cooling [12], [13]. Among these methods, the liquid cooling systems offer considerably higher heat transfer rates due to the superior heat dissipation rate offered by a high Prandtl number fluid such as water. Liquid cooling systems can be generally classified into two main categories, i.e. direct (immersion) cooling and indirect cooling [14], [15]. In direct cooling, the processor chip is basically immersed on a coolant chamber. This cooling strategy allows the on-conductive liquid coolant to make a direct contact with the processor chip, which results in the elimination of most of the internal thermal resistance. As a result, it generally offers higher heat transfer rate compared to indirect cooling. However, the heat transfer performance of direct cooling depends upon the thermo-physical properties of the coolant which are sometimes lower than that of water. Moreover, cost of all liquid coolants is higher than that of water. In indirect cooling, on the other hand, water can be used as the coolant since it does not make direct contact with the processor chip. Instead, it flows inside a microchannel which is attached to or inserted within the processor chip. As such, the channel walls behave as a separator which increases the thermal resistance. Therefore careful considerations are required in designing a cooling channel which can provide high heat transfer performance.

Numerous studies have been conducted to investigate and enhance the heat transfer performance of various cooling channels, e.g. parallel [16], [17], [18], [19], [20], serpentine [20], [21], [22], tree-shaped [15], [23], [24], [25], and wavy [26], [27]. Recently, Lee et al. [28], [29] proposed use of oblique fins in cooling channels to enhance heat transfer performance. In this case, the flow in the channel is always in the developing stage. This results in thinner boundary layers and hence better heat transfer rates. Despite of the wide-ranging studies that have been conducted on the heat transfer performance of the cooling channels, none has arrived at a definitive conclusion yet. Therefore, there is still room for further improvement of heat transfer performance of cooling channels by evaluating some novel configurations which is the theme of this work.

This paper reports results of numerical modeling of several new cooling channel designs, as illustrated in Fig. 1. They are: conventional parallel (Fig. 1a) and serpentine channels (Fig. 1b), wavy (Fig. 1c) and the recently proposed oblique fin (Fig. 1d) channels, as well as rectangular coils (Fig. 1e) and novel hybrid channels (see Fig. 1f–h) which are proposed for the first time in this study. The aim of this study is to determine an optimum cooling channel design that has the highest heat transfer performance. To compare the heat transfer performance of different cooling channels, a figure of merit is defined. Essentially, it is the ratio of heat transferred from the processor chip to the fluid per unit of required pumping power. Aside from figure of merit, uniformity of processor chip temperature needs to be taken into consideration in determining heat transfer performance. Five Reynolds numbers and three heat flux conditions are simulated to evaluate the cooling rate and the heat transfer performance of each cooling channel. Note that the results presented are also relevant to thermal management of Polymer Electrolyte Membrane (PEM) fuel cell stacks and battery stacks as well.

Section snippets

Mathematical model

The mathematical model (see Fig. 2) comprises two components, viz., the solid separator and cooling channel, which allows for a conjugate heat transfer between solid separator and cooling fluid. A constant heat flux, which represents the heat from electronic component, is prescribed at the bottom of the solid separator. The heat is transferred through separator by conduction and then it is taken away by cooling fluid. The solid separator is assumed to have isotropic thermal conductivity;

Numerics

The computational domains (see Fig. 2) were created in AutoCAD 2010; the commercial pre-processor software GAMBIT 2.3.16 was used for meshing, labeling boundary conditions and determines the computational domain. Three different amount of mesh − 2.5 × 105, 5 × 105 and 1 × 106 – were implemented and compared in terms of local pressure, velocities, and temperatures to ensure a mesh independent solution. We found that the mesh amount of around 5 × 105 gives about 1% deviation compared to the mesh

Results and discussion

The numerical simulations were carried out for typical conditions found in electronic cooling; the base-case conditions together with the physical parameters are listed in Table 1, while the geometry details can be found in Table 2. In the following, eight different channel designs, five different coolant flow rates, and three different heat flux values are simulated to study the impact of these factors on thermal management. The Figure of Merit (FoM) concept was implemented to investigate and

Concluding remarks

A computational study has been conducted to investigate the heat transfer performance of various cooling channel designs. Eight channel configurations, − parallel, wavy, oblique-fin, serpentine, and coiled-base channel design, i.e., coil with outer inlet/outlet, coil with outer inlet/outlet, hybrid coil with serpentine channel, and hybrid coil with double serpentine – were investigated and their performance are compared with each other in terms of the figure of merit. It is found that even

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