Effect of circular pin-fins geometry and their arrangement on heat transfer performance for laminar flow in microchannel heat sink

Highlights

• For MCHS with h_p = 0.5 mm Nu is 4 times higher than for the MCHS without pins fins.

• The recommended pin placement step for h_p = 0.1–0.4 mm is s_p/d_p = 6.

• The effectiveness factor of MCHS with h_p = 0.5 mm has maximum at s_p/d_p = 3.

Abstract

A computational simulation of seventeen types of microchannel heat sink (MCHS) models with the channels equipped with circular pin fins having various diameters (0.25 and 0.5 mm), spacing (1.5, 3.0 and 6.0 mm) and height (0.1, 0.25, 0.4 and 0.5 mm) in order to compare their performance with the conventional MCHS was performed. Mesh independence validation and experiments on MCHS without any pin fins were used to assess the reliability of computer simulation results. The simulation was carried out for five different Reynolds numbers in the range from 100 to 1000. Hydraulic and thermal parameters including pressure drop, Nusselt number and thermal resistance were considered for performance evaluations. To evaluate comprehensive performance, the heat sink effectiveness factor which represents a combination of the hydraulic and thermal performance was defined and corresponding figures were provided to define an optimum geometry. It was observed that thermal resistance substantially depends on the height of the pins: the higher are the pins, the better is the heat flux. The optimal pin placement step s_p was revealed as six diameters d_p of the pin: s_p = 6 d_p for the height of the pins h_p = 0.1–0.25 mm; for h_p = 0.5 mm the effectiveness factor of MCHS has the maximum value at s_p = 3 d_p.

Introduction

Semiconductor devices are widely used in many applications such as microelectronics, processors, solar cells, etc. At the same time, with the rapid development of such devices, the packing density of the elements and their thermal power increase significantly. This fact leads to high operating temperatures, which can significantly reduce the reliability of components and shorten their life. Today, one processor has billions of transistors in a crystal of approximately 1 cm², and the heat flux exceeds 100 W/cm² [1]. It is predicted that by 2026 the heat flux dissipated in next generation electronic devices (such as three-dimensional integrated circuits) will exceed 1000 W/cm² [2] with local hotspots ranging from 1200 to 4500 W/cm² [3]. Consequently, efficient cooling methods are needed that can dissipate large amounts of heat load down to surface temperatures of less than 125 °C for defense use [4] and less than 85–100 °C for general microelectronics [5]. Thus, the reliability of the microcircuit decreases by 5%, and the service life is significantly reduced, with an increase in temperature for every 1 °C above the limiting temperature [6]. Thermal management of microcircuits will be one of the biggest obstacles to computing technology in the near future.

Cooling electronics is not the only application of highly efficient heat dissipation methods, as all important sectors related to the production and consumption of electricity are struggling with the problem of thermal management. Since Tuckerman and Pease [7] first proposed the microchannel heat sink (MCHS) in 1981, rapid advances in technology have made it possible to adapt this technology to industries such as gas turbine manufacturing, laser diode arrays, rocket engines, hybrid vehicles, hydrogen storage and refrigeration [8].

MCHS had higher heat transfer characteristics compared to conventional or macro-channel heatsinks due to the larger heat transfer area. However, previously developed MCHS usually have the problem of uneven temperature distribution over the base surface, which is unfavorable for local cooling and resulted in hot spots.

There are three main methods to improve the heat transfer performance of MCHS [9]. (1) MCHS geometry optimization to better coolant distribution. By using various types of channel configurations [[9], [10], [11]], double-layer or multilayer microchannel heat sink [12,13], fractal structures MCHS [14,15], arrangement of coolant inlets and outlets [[16], [17], [18]], etc. (2) Better transport properties of coolants, including the use of two-phase flow [[19], [20], [21]], ‘nanofluids’ [22,23], etc. (3) Distribution of flux, mainly due to increase of heat transfer area and/or improving the convective heat transfer coefficient. There have been a lot of investigations in this area in the past two decades and examples will be discussed further.

Some researchers have studied the influence of the cross-sectional shape of the MCHSs (for example, circular, triangular, and trapezoidal) [[24], [25], [26], [27], [28], [29]] for the performance enhancement. To promote heat transfer performance authors [[29], [30], [31]] developed several techniques such as increase of surface area and heat transfer coefficient using interrupted strip-fin design. Some authors have studied the characteristics of flow passage in serpentine configuration are for microfluidic cooling applications including wavy [32,33], zig-zag [33] and convergent-divergent [34] MCHS. Xia et al. [35,36] studied and optimized the structural parameters of the microchannel with offset fan-shaped reentrant cavities, triangular reentrant cavities and aligned fan-shaped reentrant cavities by numerical simulation of flow and heat transfer mechanism.

A large number of previous studies on micro pin-fin heat sinks have also been devoted to the optimization of the fin cross section shapes, fin geometry (width, height), as well as their arrangement. Shaeri and Yaghoubi [37], Maji et al. [38] numerically studied perforated fins, which gave better performance and also had weight reduction. Ricci et al. [39] experimentally investigated the pin fin heat sink with three pins of different shapes of fins (circular, square, triangular and rhomboidal) arranged in line under constant heat flux boundary conditions. Zhou and Catton [40] have numerically evaluated the 22 different plate-pin fin heat sinks with various shapes of pin cross-sections sections (circular, square, elliptic, NACA profile and drop form) and different ratios of pin widths to plate fin spacing. Joshi et al. [41,42] conducted a parametric numerical study to investigate the effect of circular and square cross section micro pin fins dimensions including diameter, transversal and longitudinal spacing and height on pressure drop under conditions of different friction factors and heat transfer coefficients. Selvarasu et al. [43] numerically studied the density of cylindrical fins with a fixed diameter and height in microchannel. When measuring heat capacity and pressure drop, channels with fins of lower density provided the best performance in the laminar flow however the effects of the increase pressure drop greatly outweighed the increase of heat removed. Xie J. et al. [44] studied the effect of sidewalls on the flow and thermal characteristics of a microchannel with cylindrical pin-fins. The results of numerical simulations show that for design of a pin-finned microchannel, the gap between the sidewall and the pin should be in the range from 0.9 to 1.1. These clearances have relatively better overall thermal performance. Yang et al. [45] numerically examined the effect of the un-uniform fin height design on the plate pin fin heat sink, and showed that the plate circular pin-fin heat sink has better overall performance than the plate fin heat sink. It should be noted that turbulent flow exists in traditional air-cooled strip-fin structure because of the high Reynolds number (at least several thousands) while the laminar flow is typical for micro structures of the strip-fin or pin-fin because of low flow rate in liquid-cooled microchannel heat sink with Reynolds numbers below 1000 [31,46,47].

The aim of this work is a qualitative, comprehensive study of the improving heat transfer characteristics and efficiency of the MCHS by using circular pin-fins with different diameters, heights and steps of their arrangement (pitch). While there has been much research carried out on this subject, there are a number of limitations in the published works. Most of these studies are just numerical simulations [37,38,[40], [41], [42], [43], [44], [45]]. At the same time, in order to save computational resources, numerical models often use from one [37,43] to three or four [[38], [39], [40]] pin-fins in a row along streamlines. Such studies do not make it possible to assess the degree of influence of adjoining pins on hydrodynamics or heat transfer in a real microchannel which has much more pins. Some works are also characterized by a narrow research range of variables, including flow rates. It's limited to the Reynolds number ranges: 100 < Re < 350 [37], 22 < Re < 100 [41], 13 < Re < 202 [44]. In contrast to similar studies, in this work, along with extended numerical study the theoretical and experimental investigations were performed as a proof of some numerical case studies. After a multi-stage confirmation of the adequacy and reliability of the numerical simulation, seventeen types of heat sink models were simulated.

The present article describes the efforts aimed to improve the performance of MCHS by employing circular pin-fins with various diameter (0.25, 0.5 mm), height (0.1, 0.25, 0.4 and 0.5 mm), and different ratios of pin spacing to pin diameter (3, 6, 12 and 24) in laminar fluid flow for five different Reynolds numbers (from 100 to 1000). The thermal and hydraulic performance was compared with MCHS without any pins in terms of pressure drop, Nusselt number, overall thermal resistance. In addition, an efficiency factor was used to comprehensively compare the heat transfer efficiency of different types of MCHS. Heat transfer effectiveness factor takes into account both the improvement in heat transfer and energy consumption.