Parameterization investigation on the microchannel heat sink with slant rectangular ribs by numerical simulation

TL;DR: In this paper, single and double-row slant rectangular ribs (SRRs) arranged in a microchannel can enhance heat transfer efficiency due to the flow disturbation, the interruption of boundary layer and increase of heat transfer area.

Abstract: The slant rectangular ribs (SRRs) arranged in a microchannel can enhance heat transfer efficiency due to the flow disturbation, the interruption of boundary layer and increase of heat transfer area. In order to fully understand the mechanism of heat transfer enhancement, the flow and heat transfer in the microchannel with single- and double-row slant rectangular ribs are numerically simulated at Reynolds number ranging from 62.5 to 625. Numerical results show us that, great difference of flow pattern between microchannel heat sinks (MCHS) with single- and double-row slant rectangular ribs exists, and enormous influence of parameters of SRRs on heat transfer presents in various conditions. The flow disturbation of SRRs is the crucial factor to enhance heat transfer. Thereafter, the parameters such as attack angle, height, length, width and pitch of SRRs are evaluated by the average Nusselts number with identical pump power. Solid foundation for design of MCHS with SRRs can be provided.

Summary

1 Introduction

• Microchannel heat sink has become most important cooling approach with the rapid increase in power density (even more than 100 W/cm2 ) owing to miniaturization of electronic packages.
• Another way to improve the heat transfer efficiency of MCHS is to interrupt the thermal boundary by various lugs or/and ruts periodic located on the side wall or bottom wall [14-20].
• Arslan et al [21] experimentally investigated heat transfer and pressure drop in rectangular channel located baffles with various attack angle on the bottom at high Reynolds number(Re=1000-10000) .
• The experimental results show us greater heat transfer enhancement and pressure drop with the increase of attack angle.

1.1 Geometric model

• Two MCHSs with single-row ribs and double-row ribs were designed for numerical investigation on the basis of analysis and reviews in last section.
• Single-row ribs channel (SRC) with rectangular slant ribs only on the bottom wall and double-row ribs channel (DRC) with rectangular slant ribs on both bottom and top walls are shown in Figure 1.
• Conjugate heat transfer of solid and fluid regions are under consideration.
• A constant heat flux q=100W/cm2 is loaded on the bottom wall, and q=0 on other walls (i.e. thermal insulation).

1.2 Numerical model

• Hence, the control equations for the fluid in MCHS read as: 1）the continuity equation： 0 V (1) where， V denotes flow velocity.
• 2）the momentum equations： VpVV t V 2])([ (2) where， ,p, μ are density of fluid, pressure and dynamic viscosity, respectively.
• The control equation for solid silicon reads as： 02 Tks (4) The physical property of silicon and water are listed in Table 1.
• Instead, the viscosity of water will decrease more than 50% when temperature increasing from 293K to 333K[32].
• Aq is heated area，Ac is conjugated area (i.e. solid-fluid interface area on bottom wall).

2 Results

• And discussion Chaotic advection in microchannel and unceasing interruption of thermal boundary are two effective approaches to improve the performance of MCHS.
• Periodic arranged transverse ribs or other similar structures in a microchannel can certainly interrupt boundary layer.
• Furthermore, the transverse ribs will influence flow pattern and increase heat transfer area.
• On the contrary, much higher Reynolds number can result in the disorder of flow pattern.
• It is sure after some trials that, stable flow pattern determinately exists only when Reynolds number less than ~700, because the motivation of this work is to clarify the mechanism of heat transfer enhancement in MCHS with SRRs.

2.1.2 Flow patterns in cross sections

• It is shown that, the contrarotations of majority of fluids lead to heat transfer enhancement because of the exchange between upgoing fluids and downgoing fluids.
• Temperature distributions and velocity vectors diagrams in the region of certain SRR can help us to understand the effect of SRRs on flow pattern.
• It can be seen from Figure 6 that, the vortex is right at the position of hot fluids region both in the front and rear of SRR.
• The fluids near the right of rib will flow into the vortex , and the fluids near the left of rib will flow into another small vortex newly generating in the region between wall and left side of rib.

3.2 Optimization of geometric parameters of SRR

• It is known from the above analysis, the geometric parameters of the SRRs and Reynolds number influence significantly on the flow pattern and heat transfer.
• Active roles of the SRRs are summarized in the end of section 3.1, passive role of SRRs is much greater pressure drop.
• Therefore, pump power may be a suitable index to measure heat transfer enhancement, in other words, average Nusselts number under the identical pump power is a good option to evaluate thermal performance of MCHS.

3.2.1 Attack angle

• It is easily to understand, the pressure drop will increase with the increase of attack angle.
• The reason is that, one half of the high-speed flow is warded off when the rib height is H/2, while only two H/4 of low-speed flow near the wall are warded off in DRC.
• After the comprehensive consideration of heat transfer and pressure drop, pump power is prompted to measure the performance of MCHS.
• Figure 11c shows us that, the Nusselts number increases logarithmically with the pump power.
• Maximum Nusselts number for SRC at =52.5° means the optimization value of attack angle.

4 Conclusions

• Numerical simulations for the flow, pressure drop, temperature were carried out to analyze the flow pattern and temperature distribution in SRC and DRC.
• The following conclusions can be drawn: 1）Three important factors for enhancement of heat transfer in MCHS are interruption of thermal boundary, increase of heat transfer area, generation of chaotic advection by arranged SRRs.
• 2）Stable single-vortex pattern which can prompt to transport the fluids near the bottom and bottom-ribs upwards to the channel center exists in SRC, and every small vortex generating near the bottom-ribs by the SRRs will be combined by relative larger vortex from upstream.
• Blinking flow induced in this process by the shifts of the center of flow pattern can result in chaotic advection.

Figures

Table 1 Thermal physical property [32]

Figure 10 Nusselts number at various x

Figure 9 Temperature contours in SRC and DRC (Re=375，45°,h/H=0.5, l/W=0.8, d/W=1, t=30m)

Figure 4a-f show us stream-lines in whole region. It can be seen that, nearly all of the stream-lines in DRC are helical. Helical stream-lines indicate that, the fluids on the bottom and the fluids on the top can exchange each other to enhance heat transfer efficiency. However, some relatively flat stream-lines on the top of SRC result in recession of heat transfer enhancement. Moreover, the helix angle decreases with the increase of attack angle, that means the fluids on the bottom and the fluids on the top will exchange more frequently, hence more stretching and folding of fluids lead to more heat transfer enhancement.

Figure 8 Temperature and velocity distributions in the region of SRRs at various x in DRC (Re=625, =45°,h/H=0.5, l/W=0.8, d/W=1, t=30μm)

Figure 14 Effect of rib length on heat transfer Figure 15 Effect of rib width on heat transfer

Figure 12c shows us, the highest Nusselts numbers exist both in SRC and DRC when h/H=0.3. Moreover, the Nusselts number in SRC and DRC is much bigger than that in straight channel (h/H=0).

Figure 12c shows us, the highest Nusselts numbers exist both in SRC and DRC when h/H=0.3. Moreover, the Nusselts number in SRC and DRC is much bigger than that in straight channel (h/H=0).

Figure 5 Flow patterns in cross section at various x in SRC(Re=625, 45°,h/H=0.5, l/W=0.8, d/W=1, t=30μm)

Figure 11 Influence of attacks on the heat transfer（h/H=0.5, l/W=0.8, d/W=1, t=30μm）

Figure 7 are flow patterns in cross section at various x when Re=625, =45°,h/H=0.5, l/W=

Table 2 The influence of viscous dissipation on numerical results

Frequently asked questions

Q1. What have the authors contributed in "Parameterization investigation on the microchannel heat sink with slant rectangular ribs by numerical simulation" ?

In this paper, the performance of a single-and double-row slant rectangular ribs ( SRRs ) in a microchannel is simulated at Reynolds number ranging from 62.5-625, and the parameters such as attack angle, height, length, width and pitch of SRRs are evaluated by the average Nusselts number.