summary

With the shrinking volume of electronic devices and the continuous increase of heat flux density, the microchannel heat sink can effectively deal with the heat dissipation problem of high-power devices in a small space with its excellent cooling performance and efficient heat transfer ability. In this paper, a new type of intersecting double-layer microchannel heat sink is designed to achieve efficient heat dissipation inside the chip package with high heat flux density, and the flow heat transfer characteristics of the double-layer microchannel heat sink with different interchange structure sizes are studied, and the results show that the maximum temperature of the structure is reduced by 8.25K and the thermal resistance is reduced by more than 20% compared with the traditional double-layer microchannel. In addition, the flow heat transfer characteristics of A1203 nanofluids in the intersecting bilayer microchannel were studied by numerical simulations, and it was found that the heat transfer performance of the nanofluids under the working fluid was significantly improved, and the temperature of the nanofluids was reduced by more than 3K compared with that of pure water.

Keyword:

introduction

In recent years, with the continuous improvement of manufacturing processes and integrated circuit technology, as well as the increasing demand for high-performance chip running speed, electronic chips are developing rapidly in the direction of miniaturization, high energy consumption and high integration. This trend has led to an increasing number of transistors, a gradual decrease in the actual size of electronic chips, and a sharp increase in the number of transistors integrated in the same area. The increase in density has significantly increased the heat flux density of chips, and microchannel heat sink cooling technology has been widely used due to its small size, light weight and high heat transfer efficiency, which can quickly and efficiently conduct heat from electronic devices and prevent failure caused by overheating.

As early as the 80s of the 20th century, Tuckerman and Pease first proposed the concept of a microchannel heat exchanger and demonstrated microchannel cooling by achieving a heat flux removal capability of up to 800 W/^{cm2 [6]. ],} and then a large number of scholars have studied it in more depth. Previous studies focused on the improvement of single-layer microchannels, focusing on the adjustment of internal microstructures, such as fins ^[11-13]. ^], and the cavity is added to enhance the degree of fluid chaos and the degree of secondary flow to enhance heat transfer ^[1, ^4-17]., to enhance heat transfer by setting porous structures in the microchannels to reduce thermal resistance ^[18-19], or to change the cross-sectional area of the microchannels^[7-10]In order to expand the contact range between the liquid and the solid surface, promote the flow disorder, interrupt the formation process of the thermal boundary layer, so as to achieve the effect of improving the heat conduction.

The fluid in the single-layer microchannel will continue to absorb heat during the flow process, resulting in a high temperature of the fluid at the end of the flow, which is quite different from the temperature of the inlet section of the flow channel, thereby reducing the temperature uniformity of the bottom heat source surface. In order to solve the heat transfer defects of single-layer microchannels, a large number of scholars have proposed double-layer microchannels on the basis of single-layer microchannels ^[21-24]. Kambiz Vafai ^[25] proposed rectangular double-layer microchannels, which have been shown to provide better heat transfer performance with the same heat transfer area and channel size compared to single-layer microchannel heat sinks. In order to obtain the law of the heat dissipation performance of double-layer microchannels with different structure size ratios, scholars have studied many double-layer microchannels with different structure size ^{ratios [2}, ^8,29,31].。 Lin ^[27] optimized the size of the bilayer microchannel structure and discussed the influence of different parameters on the global thermal characteristics and thermal resistance, and the results showed that the thickness of the middle rib was negatively correlated with the thermal resistance of the channel under the condition of parallel flow arrangement, that is, the thinner the middle rib, the smaller the thermal resistance of the channel. Han Shen ^[30] proposed different staggered proportional structures to study the specific effects of different flow arrangements, including parallel flow, upper inlet and lower inlet counterflow, on the heat transfer performance, and discussed the influence of the inlet direction and flow rate of the upper and lower fluids on the heat dissipation performance of the bilayer microchannel. Levac ^[32] performed a three-dimensional numerical analysis of laminar flow and conjugate heat transfer in single- and double-layer microchannel heat sinks. The results show that in terms of overall thermal resistance and temperature uniformity of the heat source surface, the double-layer radiator is better than the single-layer radiator under the same total coolant mass flow. Similarly, fins and ribs are used in bilayer microchannels to enhance the secondary flow of fluids in the flow channel to enhance heat transfer performance ^[36-43]. ^]。

In addition to the structural design of the microchannel, the cooling working fluid also has an important impact on its heat dissipation. Compared with traditional dielectric water, nanofluids exhibit better flow heat transfer characteristics, and can be used as a cooling working fluid for microchannels to meet the heat dissipation needs of chips more effectively. Shakaker et al. ^[46] further extended the thermal conductivity model by introducing a tunable parameter for deriving the effective thermal conductivity of nanofluids. As a result, a new expression for effective thermal conductivity is constructed. Nguyen et al. ^[48] took an experimental approach and comprehensively explored the temperature and volume ratio of _Al2O3The action law of the viscosity properties of nanoparticle suspensions. It was revealed that with the gradual increase of the volume fraction of nanoparticles, the viscosity coefficient of nanofluids showed an obvious upward trend. At the same time, with the gradual increase of temperature, the viscosity coefficient of nanofluids shows a decreasing trend. Shuai Liu, et al. ^[51] constructed a nanofluid-based liquid cooling experimental system to study the cooling performance of different nanofluid concentrations and inlet flows at high flow rates and high temperatures. The results show that the cooling performance of nanofluids is limited and can easily lead to nanofluid failure under high flow rate and high temperature conditions, and the higher the concentration of nanofluids, the more obvious the effect.

In this paper, an intersecting double-layer microchannel is proposed, and the influence of different intersection structure sizes on the flow and heat transfer performance of the double-layer microchannel is analyzed and compared by the principle of field synergy and entropy field, and finally the double-layer microchannel with the best performance of the junction structure size is obtained. The heat transfer flow characteristics of the Al_2O3 nanofluid in the bilayer microchannel were analyzed by numerical simulation, and the different concentrations of Al were compared and analyzed₂Ø_Thermal resistance, maximum temperature, pressure drop and other heat transfer flow characteristics of 3 nanofluids in bilayer microchannels.

Model description

2.1 Physical model

The spatial location and size of the interchange are determined according to the maximum temperature distribution area of the traditional rectangular double-layer microchannel heat source surface ^[68], and the interchange double-layer microchannel radiator is fixed at Ly × Lx × H1 = 10 for the whole plate mm × 10 mm × 0.6 mm, a total of 33 intersection-type double-layer micropasses, as shown in Figure 3 1 (a). In order to save computing space and improve computing efficiency, a single double-layer microchannel is composed of multiple single double-layer microchannels with the same structure and symmetrical with each other Numerical studies are carried out as a computational simulation unit. Fig. 3-1(b) is a single-strip double-layer microchannel computing unit modelAs shown in Figure 3-2, the length of the microchannel is L=L_X=10mm, and the length of the microchannel is L=L X=10 mm The width is W₁ = 0.3mm and the runner width is W₂ = 0.1mm, the total height H₁ = 06mm，H₂=0.15mm，H₃=0.2mm，H₄=0.05mm specific detailed size parameters for each microchannel model

See Table 3-2 for the figures.

Figure 3-1 Schematic diagram of the model: (a) Schematic diagram of the double-layer microchannel heat sink model (b) Schematic diagram of the single-channel model

Figure 3-2: Single-strip double-layer microchannel model diagram (a) central cross-sectional plane (b), top view (c), front view (d), three-dimensional central cross-sectional view

Table 3-1 Dimensions of double-layer microchannel interchanges

The microchannel is made of siliceous material, and its physical properties are: density, specific heat, thermal conductivity,The fluid used as the working medium is pure water, and its physical properties change with temperature, and the specific object characteristics are shown in Table 32。

Table 3-2 Relationship between the physical properties of water and temperature ^[69].

Continued Table 32 Physical properties of water as a function of temperature

2.2 Numerical theoretical model

Channels are usually divided into different types according to the size of their hydraulic diameter. Kandlikar et al. ^[61] used the hydraulic diameter (Dh) of the channel as a criterion for dividing the channel size. Among them, the microchannel is 10μm, <, <, and 200μm, and the gas-liquid flow behavior is unstable at this scale, and it is difficult to predict the flow trajectory, so it is necessary to verify whether the continuum assumption and the Navier-Stokes equation are followed.

2.2.1 Continuum Assumptions

The magnitude of the Knudsen number (Kn) determines that the numerical simulation at the microscale can follow the numerical calculation model under the conventional size, and the gas-liquid fluid is divided into four categories: continuous medium flow, slip flow, transition flow and free molecular flow. According to the current research results of many scholars, when 0<Kn<0.001, the fluid is in continuous flow, and the Navier-Stokes equation and the non-slip boundary condition are applicable in this flow state

where is the mean free path of the fluid molecule, and Dh is the hydraulic diameter of the microchannel.

The hydraulic diameter of the bilayer microchannel heat sink in this paper is 133μm, and the average free path of the liquid is much smaller than 0.1μm, KnMuch less than 0.001. Based on the above analysis, the Navier-Stokes equation and the slip-free boundary condition can be used for the double-layer microchannel structure used in this paper. In other words, this study treats fluids as a continuum and follows the fundamental laws of mechanics and thermodynamics.

2.2.2 Governing Equations

In this paper, we study computational fluid dynamics and heat transfer, and the computational equations are based on: continuity equations, momentum equations, and energy equations. In order to simplify the calculation model, a series of assumptions are proposed for the initial conditions of the calculation: (1) the cooling working fluid used in this paper is incompressible and the flow is in a laminar state; (2) Mechanical influences such as gravity and thermal stress are not considered; (3) Ignore the relative slip between the fluid and the runner wall. The governing equation is:

Continuity Equation:

Momentum Equation:

Energy Equation:

In the above equation ~, , and respectively correspond to the fluid in spatial coordinatesand on the flow velocity,,,,The density, viscosity, pressure, specific heat capacity, and thermal conductivity of the working fluid are respectively the thermal conductivity of microchannel solid materials.

2.3 Grid independence verification and reliability analysis

THE ICEM 21 MESHING SOFTWARE WAS USED TO STRUCTURE THE 3D PHYSICAL MODEL BUILT IN SOLIDEWORKS. In order to verify the correctness of meshing, it is necessary to verify the quality of the mesh, and the traditional rectangular double-layer microchannel is divided into four computational grid models with different mesh numbers under the method of structured mesh, and the antagonism of the four meshes is checked, and the average temperature, maximum temperature and pressure drop characteristics of the bottom surface of the rectangular double-layer microchannel solved with an inlet velocity of 1 m/s under different grid conditions are shown in Table 3-2.

Table 3-2 Grid independence verification

As can be seen from Table 3-2, the deviation of the temperature characteristics and pressure drop characteristics of each mesh is within 0.5%, indicating that the calculation accuracy of each mesh is suitable for the simulation calculation of the model, but considering the efficiency of the calculation and the required computing space, the meshing form of Mesh III. was chosen to mesh the traditional rectangular double-layer microchannel and the intersecting rectangular double-layer microchannel, as shown in Figure 3-3.

Figure 3-3 Schematic diagram of the meshing and local encryption mesh of the double-layer microchannel model

In order to verify the reliability of the double-layer microchannel numerical model, the temperature value of the centerline of the heat source surface in the numerical simulation results was compared with the previous experiments. According to the experimental rectangular double-layer microchannel three-dimensional physical model in Ref. [70], the corresponding three-dimensional model and numerical simulation model were established, and the numerical simulation boundary conditions were set according to the initial conditions of the literature experiment: the microchannel material was silicone grease, the working fluid was pure water, the bottom heat flux density, and the inlet velocities of the two experiments were 83, respectively，147。 As shown in Figure 3-4, the maximum error between the simulation results and the experimental results is 6.4%, indicating that the numerical calculation method adopted in this paper is reliable.

Figure 3-4: Comparison of the simulated temperature values of the centerline temperature of the heat source surface with the experimental values

Data simplification

The thermal resistance is calculated as:

where is the maximum temperature of the bottom heat source surface, in the unit of , is the temperature of the inlet coolant of the microchannel, in the unit is,is the heat flux density of the heat source surface, in the unit of , is the area of the microchannel heat source surface, in the unit is.

The hydraulic diameter is calculated as:

Wherein, , are the width and height of the cross-section of the fluid flow channel, respectively, in the units of; is the cross-sectional area of the fluid flow channel, in the unit.

The average Nussel number is calculated as:

where is the average convective heat transfer coefficient, which is the thermal conductivity of the coolant fluid, in .

The average convective heat transfer coefficient is calculated as:

where is the total area of the coupling wall, in the unit; is the channel wall temperature in units; is the average temperature of the fluid in units.

The average temperature of the fluid is calculated as:

Among them, the coolant fluid inlet and outlet temperatures are respectively, and the unit is.

The formula for calculating the power of the pump is:

where is the velocity of the coolant fluid entering the microchannel, in units; is the pressure drop of the fluid in units.

The formula for the comprehensive heat transfer coefficient ^[62] is as follows;

The sum is the average Nussel number and the voltage drop of the traditional rectangular bilayer microchannel, respectively. It is a ratio relation, and a value greater than 1 represents an improved comprehensive performance of the flow heat transfer of the improved bilayer microchannel.

Calculation of the temperature uniformity coefficient ^[63].For:

  (2-15)

where ,,,, represent the maximum temperature of the bottom heat source surface of the microchannel, and the minimum temperature of the bottom heat source surface, respectively. The average temperature of the bottom heat source surface, the temperature at which the coolant fluid enters the microchannel. The size represents the degree of change in the temperature of the bottom heat source surface, and the smaller the temperature, the more uniform the temperature.

Results and discussions

4.1 Flow characteristics

Figure 3-5 shows the variation curve of the flow pressure drop characteristics of the double-layer microchannel under different intersection size configurations, and it can be seen from the figure that the pressure drop of all double-layer microchannels increases with the increase of inlet velocity, and the pressure drop of the traditional rectangular double-layer microchannel case1 changes the least, indicating that the introduction of the junction structure in the double-layer channel will increase the flow resistance of the cooled working fluid and thus increase the pressure drop, and the traditional rectangular double-layer microchannel case1 has the advantage of low flow resistance at the same flow velocity. Compared with case2~case4, with the increase of the width of the intersection of the double-layer microchannel at the same speed, the flow rate of the upper and lower layers of fluid exchange increases, but the flow velocity of the upper and lower layers is opposite, which will form a certain degree of convection in the intersection area, and with the increase of the flow rate of the fluid exchange, the degree of flow will also increase, resulting in an increase in pressure drop. Comparing case3 and case4, it can be seen that when the width of the double-layer microchannel intersection increases to a certain level, and when the width of the double-layer microchannel intersection increases, the change of the mutual flow resistance of the upper and lower layers of fluids does not change much, resulting in a less obvious difference in pressure drop.

Figure 3-5 Trend of pressure drop in bilayer microchannel as a function of velocity

In order to further illustrate the variation of pressure drop under different junction sizes, Figure 3-6 visually shows the velocity distribution and fluid path line diagram of the fluid region in different junction structure sizes on the Y-axis center plane. The results show that when the fluid inside the traditional rectangular double-layer microchannel case1 just enters the microchannel, the velocity of the fluid at the inlet changes due to the friction with the internal wall of the microchannel, and the velocity of the fluid in the upper fluid domain decreases compared with the fluid in the lower fluid domain because the fluid in the lower fluid domain is subjected to friction of one less surface in the Z direction.

Figure 3-6 Velocity contour and streamline diagram of the central section of the double-layer microchannel fluid domain (y=0.15 mm), =1.5 m/s

In case2~case4, due to the opposite inlet velocity of the upper and lower fluids, the convection of the upper and lower fluids at the junction causes energy loss and makes part of the flow of the lower fluid domain flow to the upper fluid area, and part of the flow of the upper fluid region flows to the lower fluid area, and there is a process of mixing and exchange between the upper and lower fluid domains. In the flow channel, the beginning and end of the junction are abruptly structurally abrupt, resulting in the sudden convection of the upper and lower fluids at this position, so that the fluid velocity also changes suddenly, thereby destroying the heat transfer boundary layer and increasing the heat transfer efficiency. Among them, the flow trajectory of a large part of the fluid exchanged between the top and bottom of case3 and case4 is to impact the inner wall surface of the microchannel, which destroys the heat transfer boundary layer and increases the heat exchange efficiency, resulting in energy loss and an increase in pressure drop. In case 3 and case 4, the degree of convection of the upper and lower fluids is similar, so that the energy loss is close and the pressure drop loss caused by the energy loss is close. However, the area and degree of impact of the flow flow between the upper and lower layers in case 3 are larger than those in case 4, so that the heat exchange efficiency of case 3 is higher than that of case 4. Although the fluid in case2 has two fluid domains, the upper and lower fluid domains hedge each other to exchange the upper and lower layers of fluid, but the degree of hedging and exchange is small, and most of the fluid still follows the original trajectory, so the degree of damage to the heat transfer boundary layer is small, so the heat exchange efficiency is improved slightly.

4.2 Heat transfer characteristics

The goal of this study is to reduce the maximum temperature on the chip surface and optimize the uniformity of its temperature distribution, in order to solve the two major failures of electronic components caused by high temperatures. Figure 3-8 shows the variation curve of the heat source surface temperature at the bottom of the double-layer microchannel with the inlet velocity of different intersection structure sizes, for the average temperature and maximum temperature of the heat source surface as shown in the figure, the traditional rectangular double-layer microchannel with zero intersection size is higher than the double-layer microchannel with the intersection, indicating that the temperature index of the double-layer microchannel can be greatly improved after adding the intersection.

Figure 3-7 Variation of heat source surface temperature at the bottom of the bilayer microchannel with inlet velocity (a) average temperature (b) maximum temperature

As shown in Figure 3-7(a), for the average temperature, case 3 has a minimum temperature of 20.6 K lower than case 1 at an inlet velocity of 0.5 m/s, which is also a great advantage compared with other intersection size double-layer microchannels. When the velocity is between 1~2.5m/s, the average temperature of the heat source surface of the double-layer microchannel with different intersection sizes is close, but the average temperature of the heat source surface is 6~7K lower than that of the traditional rectangular double-layer microchannel.

As shown in Figure 3-7(b), for the maximum temperature of the bottom heat source surface, with the increase of velocity, the temperature performance of the double-layer microchannel with intersection is further improved compared with the traditional rectangular double-layer microchannel case1, and the maximum temperature of the heat source surface of case3 at each speed is lower than that of the double-layer microchannel of other structural forms, indicating that case3 not only performs well at the average temperature of the heat source surface, but also performs the best performance at the maximum temperature of the heat source surface. At a speed of 2m/s, the case3 has the lowest average temperature and the highest temperature. For the average temperature of the bottom surface, case3 is reduced by 5.89K, 0.69K and 0.96K respectively compared with case1, case2 and case4; For the maximum temperature of the bottom surface, case3 is reduced by 8.05K, 1.76K, and 4.25K compared to case1, case2, and case4, respectively.

In order to further explore the temperature distribution characteristics of the heat source surface at the bottom of the double-layer microchannel with different intersection structure sizes, Figure 3-8 shows the temperature distribution cloud of the bottom surface of the double-layer microchannel with different intersection structure sizes when the inlet velocity of the upper and lower microchannels is 1.5 m/s. It can be seen from the figure that the maximum temperature of the bottom surface of case1 is the largest under the inlet velocity, and the maximum temperature is distributed in the middle of the bottom surface, and the temperature of the inlet end of the upper and lower fluids is lower, especially the temperature of the inlet end of the lower fluid domain is lower than the temperature of the outlet end of the lower fluid. This is due to the continuous increase in the heat absorption temperature of the lower fluid in the process of flowing from the inlet along the lower microchannel to the outlet, and the ability of heat exchange with the bottom heat source surface gradually decreases, so that the temperature of the upper fluid inlet section area is lower, and the temperature of the bottom heat source surface increases with the temperature of the lower fluid flow direction. The upper fluid continuously absorbs heat during the flow of the upper microchannel from the inlet, and the temperature continues to increase, so thatThe temperature at the end of the heat source surface decreases, but the heat transfer capacity of the upper fluid also decreases, resulting in the formation of the highest temperature area in the middle of the heat source surface. Case3 has the lowest heat source surface temperature and its highest temperature area is divided into two parts. Compared with case1, because the junction position is set in the middle of the whole flow channel, and the thermal boundary layer is destroyed by the hedging and mixing of the upper and lower fluids at the junction, the heat exchange efficiency is increased, and the high-temperature region originally formed in the middle of the microchannel is divided into two pieces, and the temperature of the high-temperature region of each piece is 9K lower than that of case1.

Figure 3-8 Surface temperature distribution of the heat source of the double-layer microchannel, =1.5 m/s

Figure 3-9 shows the temperature curve on the center line of the double-layer microchannel heat source surface. It can be seen from the diagram that the area with the larger temperature of the heat source surface of each structure is distributed in the middle, and the temperature at the front and end of the heat source surface along the flow direction is lower. The maximum temperature of the double-layer microchannel with the junction structure is at the middle position, and the maximum temperature value is much higher than that of the double-layer microchannel with the junction structure, indicating that the double-layer microchannel with the junction has obvious advantages in improving the heat transfer and reducing the temperature of the heat source surface.

Figure 3-9 Temperature change on the centerline of the heat source surface of each microchannel, = 1.5 m/s

The temperature change curves on the centerline of the heat source surface of the microchannel in case2~case4 are similar, all of them increase with the temperature of the fluid flow direction at the front end, fluctuate within a certain range near the middle junction, and decrease with the temperature of the fluid flow direction at the end, but each structure has different hedging and mixing of the upper and lower fluids at the intersection due to the different size of the junction structure, and the trajectory of the fluid change and the position of the impact on the inner wall of the microchannel are different, so the temperature changes at and near the junction.

The temperature of the centerline of the heat source surface will change abruptly at 3.5mm, 5mm, 6mm, and 7.5mm of the flow channel, and these abrupt changes the flow state of the upper and lower fluids due to the existence of the junction structure. For example, the above fluid, At 3.5mm is the beginning section of the lower fluid flowing into the intersection, because there is no barrier of the flow channel wall, the lower fluid has a tendency to go up the flow channel at this time, but under the action of inertia, only a part of the fluid flows into the upper flow channel and hedges and mixes with the upper fluid to improve the heat transfer performance to a certain extent, the temperature decreases, but the main body still flows along the lower flow channel, at 3.5mm to 5mm, because the upper and lower fluids are almost hedged and mixed, the heat transfer performance decreases and the temperature increases, the lower fluid is fully developed at 5mm to 6mm, and the fluid body flows to the upper flow channel and the upper flow channel fluid is fully hedged and mixed, which increases the mixing degree of the fluid in the area, which improves the heat transfer efficiency and lowers the temperature of the area. Since the upper and lower fluid inlets and outlets are set in the opposite direction, 6 mm to 7.5 mm is the beginning of the upper fluid flowing into the junction structure, and the flow situation is similar to that of the lower fluid 3.5 to 5 mm.

In order to better study the temperature distribution in the double-layer microchannel, the temperature analysis of the central section of the fluid domain with an inlet velocity of 1.5m/s (y=0.15mm) was used to explore the influence of the existence of the junction on the heat transfer process of the microchannel. As can be seen from Figure 3-10, the fluid temperature of all double-layer microchannels will be exchanged with the flow channel wall when flowing along the channel, so that the coolant fluid temperature increases, and the fluid outlet temperature of the double-layer microchannel with the intersection is higher than that of the traditional rectangular double-layer microchannel, indicating that the fluid can bring out, absorb and take away more heat from the heat source surface due to the existence of the intersection, and reduce the temperature of the heat source surface to have better heat dissipation performance. It can be clearly seen that the bottom heat source surface of the traditional rectangular double-layer microchannel in CASE1 is not only lower at the upper and lower fluid inlets, but also has a higher temperature in the middle and a large high temperature area. This is because the temperature of the upper and lower fluids at the inlet is low, and the temperature difference between the upper and lower fluids and the heat source surface is large, and the ability to absorb heat is strong, but as the temperature increases, the temperature difference between the heat source surface and the heat source surface decreases, and the heat exchange capacity decreases, so the temperature in the middle of the heat source surface is high and the high temperature area is large. The temperature change trend of the upper and lower fluids at the inlet of the double-layer microchannel with the junction is close to that of the traditional rectangle, but when it flows through the intersection, it causes the upper and lower fluids to hedge and mix, destroys the thermal boundary layer, enhances the heat transfer, and makes the upper and lower fluids absorb more heat, resulting in the temperature of the upper and lower fluids increasing compared with the traditional rectangular double-layer microchannels, taking away more heat and reducing the temperature of the heat source surface.

Figure 3-10 Temperature contour of the central section of the fluid domain (y=0.15 mm), =1.5 m/s

In the previous paper, the temperature characteristics of the bottom heat source surface of the double-layer microchannel with different intersection sizes with the inlet velocity are expressed. It is also necessary to understand the overall heat transfer performance of the microchannel by judging the size of the thermal resistance value, and the larger the thermal resistance value, the worse the heat transfer performance of the microchannel. Figure 3-11(a) shows the thermal resistance curves of four bilayer microchannels with different intersection sizes. As can be seen from the figure, the thermal resistance of the double-layer microchannel with the junction structure is significantly lower than that of the traditional rectangular double-layer microchannel, which indicates that the thermal resistance of the microchannel can be effectively reduced by using the junction structure, which indicates that the junction structure is in the overall situationThe heat transfer performance of the microchannel is enhanced. As can be seen from the diagram, the thermal resistance of each structure gradually decreases as the inlet velocity increases, the heat transfer capacity increases.

When the inlet velocity is 0.5m/s, due to the large size and small inlet velocity of the junction in case4, the hedging pressure of the upper and lower fluids at the junction is small, so that the mixing degree of the upper and lower fluids at the intersection is low, and there is a gap in the thermal conductivity matrix compared with the complete channel of the traditional rectangular double-layer microchannel, so that the thermal conductivity of case4 is reduced and the thermal resistance is increased compared with case1. However, with the increase of the inlet velocity, the hedging pressure of the upper and lower fluids at the intersection of case4 increases, which increases the mixing degree of the upper and lower fluids there, which makes up for the low heat exchange capacity caused by the gap in the thermal conductive matrix, so that the thermal resistance of case4 is lower than that of case1. When the inlet velocity increases to 2.5m/s, the degree of convection mixing of the upper and lower layers of the double-layer microchannel at the junction and the ability to destroy the heat exchange boundary layer are close to the degree of convection and mixing of the upper and lower layers of the double-layer microchannel at the junction, so that the thermal resistance of the three double-layer microchannels with the junction is close but the thermal resistance is quite different from that of the traditional rectangular double-layer microchannel. The thermal resistance of case3 is the smallest under the same inlet velocity, which indicates that the heat exchange capacity of case3 is the best, so that the average temperature of the bottom heat source surface of case3 has the best maximum temperature and lowest temperature performance. When the inlet velocity is 1.5m/s, the thermal resistance of case3 is reduced by 20.5%, 5.8% and 13.1% compared with case1, case2 and case4, respectively. When the inlet velocity is 2.5m/s, the thermal resistance of case3 is reduced by 19.5%, 4.4% and 7.4% compared with case1, case2 and case4, respectively.

Figure 3-11 Variation of thermal performance of double-layer microchannel with inlet velocity (a) Thermal resistance; (b) Average Nussel number

The average Nusselt number can reflect the heat dissipation capacity of the microchannel, and the higher the value, the stronger the heat dissipation capacity of the microchannel. Figure 3-11(b) shows the average Nusselt number of the four bilayer microchannels as a function of the inlet velocityCurves. As can be seen from the figure, the average Nussel number of the double-layer microchannel of the junction structure is significantly higher than that of the traditional rectangular double-layer microchannel, and the heat transfer performance of the microchannel is improved by the junction structure. It can also be seen from the figure that with the increase of inlet velocity, the average Nussel number of each structure gradually increases, but the difference between the double-layer microchannel with the intersection and the traditional rectangular double-layer microchannel is larger, indicating that with the increase of the inlet velocity, the heat transfer performance advantage of the double-layer microchannel with the intersection is more obvious. When the inlet velocity increases to 2.5m/s, the average Nussel number of the three double-layer microchannels with intersections is close but is quite different from the average Nussel number of the traditional rectangular double-layer microchannels, and the average Nussel number of case3 is the largest under the same inlet velocity, indicating that case3 has the best heat transfer capacity. When the inlet velocity is 1.5m/s, the average Nussel number of case3 increases by 17.7%, 2.9% and 5.6% compared with case1, case2 and case4, respectively. When the inlet velocity is 2.5m/s, the average Nussel number of case3 increases by 25.3%, 3.5% and 3.1% compared with case1, case2 and case4, respectively.

Based on the above analysis, the intersection structure in the traditional rectangular double-layer microchannel can improve the heat dissipation performance of the microchannel and reduce the maximum and average temperature of the chip surface. As the inlet velocity increases, the overall temperature performance improves. The heat absorbed by the upper fluid in the traditional rectangular double-layer microchannel mainly comes from the middle microchannel wall between the upper and lower layers and cannot carry out heat exchange with the lower fluid, and the addition of the junction can make the upper and lower fluids that are not in contact with each other realize convective mixing to a certain extent, carry out the heat exchange between the upper and lower fluids, make the upper fluid take away a part of the heat in the lower fluid, and increase the heat exchange efficiency between the fluid in the lower fluid domain and the bottom heat source surface. The size of the junction affects the degree of convective mixing heat transfer between the upper and lower fluids and the flow rate participating in the convective heat transfer of the upper and lower fluids, thus affecting the efficiency of the heat exchange of the double-layer microchannel. The size of the intersection of case3 structure in case2~4 is the best, and its temperature index is the best.

4.3 Comprehensive characteristic analysis

The temperature uniformity factor is one of the key parameters to measure the temperature equilibrium of the bottom surface of the heat source inside the double channel, which reveals the range of temperature variation of the bottom surface of the heat source. The smaller the uniformity factor, the smaller the temperature fluctuation of the heat source surface, and the more uniform the temperature distribution. Figure 3-11 shows the temperature uniformity factor of the double-layer microchannel heat source surface with the inlet velocity of each structure. From the figure, it can be seen that the temperature uniformity factor values of case2 and case3 are lower than those of case1 at each velocity, indicating that case2 and case3 have good temperature uniformity compared with case1. This is because the intersection structure is arranged in the middle of the microchannel, so that the upper and lower fluids hedge and mix with each other there, change the original trajectory of the flow along the flow channel, guide the upper and lower fluids to collide with the walls of the upper and lower flow channels, destroy the boundary layer of the flow channel, make more low-temperature fluids contact with the wall, increase the heat exchange efficiency of the upper and lower fluids and the wall, make the temperature distribution of the bottom heat source surface more uniform, and improve the temperature uniformity. However, the temperature uniformity factor of case4 is only advantageous over case1 at a high inlet speed of 2.5 m/s.

Compared with case2, case3 and case4, the temperature uniformity factor of case3 is significantly lower than that of case2 and case4 at each speed, indicating that the structural size of the junction affects the temperature uniformity of the bottom heat source surface, which is because the structural size of the junction affects the degree of convection mixing of the upper and lower fluids and the degree of change of the original flow trajectory of the upper and lower fluids, which affects the heat exchange efficiency of the whole microchannel. For case3, when the velocity is less than 1.5 m/s, the temperature uniformity factor decreases with the increase of velocity, but increases with the inlet velocity greater than 1.5 m/s. Case1 and case2 also have a tendency to decrease the temperature uniformity factor at low velocity and increase at high speed, reflecting that the temperature uniformity factor is not larger than the inlet velocity, the smaller the temperature uniformity factor, the more uniform the temperature, indicating that the optimal temperature uniformity factor of each structure corresponds to different optimal inlet velocities.

Figure 3-11 The temperature uniformity factor of the heat source surface of the double-layer microchannel of each structure varies with the inlet velocity

Figure 3-12 Variation curve of the comprehensive evaluation factor of the bilayer microchannel with the inlet velocity

In order to verify whether the intersection double-layer microchannel can obtain higher heat transfer performance with lower pressure drop loss than the traditional rectangular double-layer microchannel, the comprehensive evaluation factor was introduced to more effectively explain the role of the intersection in the traditional rectangular double-layer microchannel. The comprehensive evaluation factor is greater than 1, which indicates that the intersection double-layer microchannel can effectively enhance the heat transfer. Figure 3-12 shows the result. The comprehensive evaluation factor increases with the increase of inlet velocity, and the comprehensive evaluation factors of the double-layer microchannel with the intersection are higher than those of the traditional rectangular double-layer microchannel, indicating that the method of adding the junction to the traditional rectangular double-layer microchannel can improve the thermal performance of the microchannel radiator and improve the energy utilization caused by voltage drop loss. When the inlet velocity is less than 1.5m/s, the comprehensive evaluation factor of case3 is the largest, and when the inlet velocity is greater than 1.5m/s, the comprehensive evaluation factor of case2 is the largest. This is due to the fact that the improvement of heat transfer performance at low speed is greater than the pressure drop loss accompanied by the increase of inlet velocity, but with the continuous increase of inlet velocity, the improvement of heat transfer performance tends to be less than the pressure drop loss accompanied by the increase of inlet velocity, resulting in the optimal value of the comprehensive evaluation factor of case3 at low velocity.

conclusion

In this chapter, on the basis of the traditional rectangular double-layer microchannel case1, a new type of intersection double-layer microchannel radiator is designed with reference to the previous research results, and the correctness of the numerical calculation model is verified by comparing the simulation results with the experimental results of the predecessors, and the flow heat transfer characteristics in the double-layer microchannel with different intersection sizes are studied, and the main conclusions are as follows:

The analysis of the flow characteristics of the double-layer microchannel shows that the flow state of the fluid is changed due to the collision and mixing of the upper and lower fluids, resulting in a large pressure drop, and the pressure drop loss caused by the double-layer microchannel is greater with the increase of the inlet velocity.

According to the analysis of the heat transfer characteristics of the double-layer microchannel, the interchange double-layer microchannel can increase the heat dissipation efficiency and improve the temperature performance. When the inlet velocity is 2m/s, the average temperature of the bottom surface is reduced by 5.89K, 0.69K and 0.96K respectively for case3 compared with case1, case2 and case4. For the maximum temperature of the bottom surface, case3 is reduced by 8.05K, 1.76K, and 4.25K compared to case1, case2, and case4, respectively. For thermal resistance, when the inlet velocity is 1.5m/s, case3 is reduced by 20.5%, 5.8%, and 13.1% compared to case1, case2, and case4, respectively. When the inlet velocity is 2.5m/s, case3 is reduced by 19.5%, 4.4% and 7.4% compared with case1, case2 and case4, respectively