Trazabilidad

Micro-scale pin–fins with different shapes such as circles, squares, rhombus, elliptical, eye- shaped, and sine-shaped cross sections that protrude out of the surface have shown significant improvements in removing heat. These usually made from materials have high thermal conductivity such as silicon, aluminium and copper. Using this type of pin-fins can increase the wall surface area, and interrupt the steady flow of the fluid allows better flow mixing and as a result, enhanced heat transfer. To improve the thermal heat transfer performance, micro pin-fins can take different shapes and sizes and be placed in different patterns (aligned and staggered).

Peles et al. (2005) investigated experimentally the convective heat transfer and pressure drop across a pin-fin micro heat sink by comparing its thermal resistance with that of a microchannel heat sink (see Fig. 2.6). They discovered that the thermo-hydraulic performance of a cylindrical micro-pin-fin heat sink is superior to that of a microchannel heat sink as very high heat fluxes can be dissipated with low wall temperature rises across the heat sink. Their results showed that for fin diameters larger than 50 μm, the thermal resistance is less sensitive to changes in the fin diameter and for increased efficiency short pins should be used.

Colgan et al. (2007) tested an offset strip fin silicon microchannel cooler for a single phase flow bonded to a high power chip with power density of 300 W/cm2 _{(see Fig. 2.7). In their}

study, the offset strip fin microchannels offer enhancements in heat transfer several times higher than a plain microchannel, depending on the fin length, but this was at the expense of increased pressure drop. To keep the pressure drop reasonable, shorter flow lengths were used by using multiple entry and exit ports.

Pyrex Flow in _Silicon block Pin fins 𝑞" 𝑤1 𝑤2 𝑡𝑤 Ab Ap t L D

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Fig. 2.7: 3-D rendering of assembled microchannel cooler (Colgan et al., 2007).

Experimental studies of single phase flow through micro pin-fin heat sinks have been conducted by Siu-Ho et al. (2007) and Qu and Siu-Ho (2008a and 2008b) to study the heat transfer and pressure drop characteristics. The heat sink model that they used in their work was composed of an array of 1950 staggered square micro-pin-fins with a cross section area of 0.2 × 0.2 mm2 _{and a height of 0.67 mm. They tested the model at various inlet water flow}

velocities with Reynolds number varying from 93 to 634 (Siu-Ho et al., 2007) and 46 to 180 (Qu and Siu-Ho, 2008a and 2008b) with two different temperature of 30 oC and 60 oC. According to their experimental results, Qu and Siu-Ho (2008a and 2008b) proposed two new heat transfer correlations for the low 𝑅𝑒 range (𝑅𝑒 < 1000) which showed much better accuracy as shown: 𝑁𝑢𝑎𝑣𝑔= 0.0285 𝑅𝑒𝑎𝑣𝑔0.932𝑃𝑟𝑓,𝑎𝑣𝑔 1 3⁄ (2.16) 𝑁𝑢𝑎𝑣𝑔= 0.0241 𝑅𝑒𝑎𝑣𝑔0.953𝑃𝑟𝑓,𝑎𝑣𝑔0.36 ( 𝑃𝑟𝑓,𝑎𝑣𝑔 𝑃𝑟𝑤,𝑎𝑣𝑔 ) 0.25 (2.17)

where 𝑅𝑒_𝑎𝑣𝑔 is the average Reynolds number, while 𝑃𝑟_{𝑓,𝑎𝑣𝑔} and 𝑃𝑟_{𝑤,𝑎𝑣𝑔} are respectively the water Prandtl numbers at average water bulk temperature and at average micro-pin-fin base temperature. The term 𝑃𝑟𝑓,𝑎𝑣𝑔

𝑃𝑟𝑤,𝑎𝑣𝑔 in Eq. (2.17) represents a property correction factor to account for the effect of fluid property variation on heat transfer. Local Nusselt numbers were predicted with a mean absolute error (MAE) of 13.7% and 13.0% respectively for the two new correlations. A new correlation was proposed for the friction factor in the micro-pin-fin array (𝑓_𝑓𝑖𝑛), and good agreement is achieved with MAE of 6.9% as:

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An experimental study was carried out by Nguyen et al. (2007) on the copper pin fin heat sink to investigate the behaviour and heat transfer enhancement of a nanofluid (namely, Al2O3

nanoparticle  distilled water mixture). The experiments were conducted within the single phase turbulent flow regime with two different particle average diameters and various volume concentrations. From the experimental data obtained, they observed that the heated block average temperature has considerably decreased with an increase of particle volume concentration. For example, at a given mass flow rate of 0.06 kg/s, the heated block average temperature has as approximate values, 40.9 oC, 39.3 oC, 38.7 oC and 37.3 oC, respectively for water and nanofluids with 1%, 3.1% and 6.8% particle concentration. Additionally, for a particle volume concentration of 6.8%, the heat transfer coefficient increased around 40% compared to that of the distilled water. The influence of particle size on nanofluid heat transfer behaviour was also studied, and the comparison was performed only for the particular particle volume concentration of 6.8%. The experimental results showed that a nanofluid with smaller particle size does provide a better heat transfer. For two different particle average diameters, a nanofluid with 36 nm particle size provides higher convective heat transfer coefficients than the ones given by nanofluid with 47 nm particles.

The efficacy of nanofluids containing copper oxide (CuO) nanoparticles in water as coolants has been investigated experimentally and numerically by Pantzali et al. (2009) in a similar miniature plate heat exchanger to that used experimentally by Nguyen et al. (2007). First, the thermophysical properties (i.e., thermal conductivity, heat capacity, viscosity, density and surface tension) of a typical nanofluid (CuO in water, 4% Particle volume fraction) were measured. The thermo-hydraulic behaviour of the miniature plate heat exchanger was also simulated using a CFD code and then compared with that of the flat plate (without pin fins), and the prediction results were in very good agreement with the experimental measurements. The results predicted showed that the presence of the nanoparticles greatly affects the properties of the base fluid (i.e., water). The measurements revealed that the increase in the thermal conductivity was accompanied by a significant decrease in specific heat capacity and an increase in viscosity. For example, with respect to the water at a temperature of 25 oC, the thermal conductivity of a nanofluid increases by about 10%, the specific heat capacity decreases by 20% and the viscosity is significantly higher (almost 100%). They found that at a given heat load, the nanofluid volumetric flow rate required for a given load is lower than that of water causing lower pressure drop. For example, at the specific heat load of 90 W, the water and nanofluid volumetric flow rates required were 11 and 3 ml/s, respectively, while the pressure drop produced was 240 Pa and 45 Pa, respectively. For all cases studied, it was observed that the nanofluid flow rate required is up to 4 times lower (compared to water)

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Fig. 2.8: Effect of nanofluid volume fraction on Nusselt number at different Reynolds number (Seyf and Feizbakhshi, 2012).

while the respective pressure drop is up to 6 times lower, and thus less pumping power required.

A numerical study was performed by Seyf and Feizbakhshi (2012) on single-phase heat transfer and pressure drop of two types of nanofluids (Al2O3water and CuOwater) in a

circular micro-pin-fin heat sink. The effects of Reynolds number, volume fraction, type and size of nanoparticles on thermal and hydrodynamic behaviour of system have been studied. The results showed that the transition from laminar to turbulent flow occurs very early (𝑅𝑒~100) compared with the flow in ordinary pin-fins arrays. For both types of nanofluids, it is found that the Nusselt number increases with increasing volume fraction, because the heat transfer coefficient is proportional to the thermal conductivity of coolant, which is increasing with volume fraction of the nanoparticles. At Reynolds number ranging from 25 to 75, it demonstrates that adding low volume fraction of nanoparticle (0.01-0.04) to the base fluid (deionized water) leads to significant increase in Nusselt number between 1.62%4.5% and 8.37%11.44% for Al2O3 with particle diameter of 47 nm and CuO with particle diameter

of 29 nm nanoparticles, respectively, as shown in Fig. 2.8. However, with increasing particle volume fraction the nanofluid viscosity increases, thus thicker boundary layer thicknesses on pin fins will be formed with a corresponding reduction in convection. With decreasing particle diameters the Nusselt number increases for Al2O3water nanofluid while the trend is

reverse for CuOwater nanofluid. This is due to the fact that for CuO nanoparticles decreasing the diameter of nanoparticles leads to decreasing thermal conductivity of nanofluids.

Online and offset micro pin-fin heat sinks arranged in variable fin density configurations were studied numerically by Rubio-Jimenez et al. (2013) to cool 10 × 10 mm2 integrated chip (IC).

𝜑 = 0.04 𝜑 = 0.01 𝜑 = 0.04 𝜑 = 0.01 Al2O3 CuO Water 𝑅𝑒 𝑁𝑢

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Each configuration consists of three zones (SI, SII, and SIII) with different fin densities as shown in Fig. 2.9. Water was used as the coolant in the single phase and laminar flow regime under nondeveloped flow conditions. The cooling system is formed by 4748 flat fins with rounded sides having a radius of 25 μm placed on a 200 μm thick silicon substrate, which are distributed in three different sections along the flow length. The fin width, length, and height are 50, 100, and 200 μm, respectively. These fins are formed on a microchannel heat sink has 33 rectangular channels with aspect ratio of 1.0 and space between channels 100 μm. The results revealed that the cooling system using the offset micro pin-fin configuration can achieve a thermal resistance as low as 0.1 K/W with a pumping power requirement of 0.45 W. Furthermore, it is observed that the offset fin configuration produces a more uniform substrate temperature profile compared with online fin configuration, and this belongs to that a large part of fluid in the online fin configurations passing through the straight gaps between the fins. Thus, the interaction between the coolant and the walls of the fins is significantly reduced, affecting the heat diffusion. From the results obtained, it is shown that the offset micro pin-fin configuration is not recommended for cooling systems that required thermal resistance < 0.1 K/W because of their large increase of the pressure drop.

An experimental and numerical investigation was conducted by Yu et al. (2016a) for single- phase fluid flow and heat transfer in a microchannel with Piranha Pin Fins (PPFs) as shown in Fig. 2.10. In their study, and depending on the channel hydraulic diameter, the 𝑅𝑒 varied from 508 to 2114. Numerically, a conjugate convection/conduction heat transfer model was

Fig. 2.9: Online and offset micro pin-fin heat sinks (Rubio-Jimenez et al., 2013). On-line fin configuration

Offset fin configuration Flow direction

Flow direction

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developed within COMSOL Multiphysics, which the pressure drop and 𝑁𝑢 predictions showed good agreement with experiments. Friction factor and Nusselt number obtained from the PPF heat sinks were compared with available correlations for conventional straight and pin fin channels.

From the results obtained, it is found that the friction factor for the PPFs heat sink is larger than for a plain channel. However, it is still much smaller than the friction factor predicted from most correlations developed for channels with pin fins (Kosar et al., 2005) and previous existing studies such as Chilton and Generaux (1933); Jacob (1938); and Metzger et al. (1982). Also, it is shown that the microchannel with PPFs enhances heat transfer due to the

Solid (Silicon) Symmetry Plane Outlet Heat flux Insulation Liquid Outlet Inlet

Fig. 2.10: Dimensions of the test device with enlarge for the PPF arrays and silicon-based microchannel with PPFs (half of the channel) (Yu et al., 2016a).

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larger area for heat spreading and interfacing with a cold fluid. Additionally, the extruded PPFs disturbed the velocity field, and the subsequent separation and mixing further enhances heat transfer.

An experimental and numerical study was carried out by Yang et al. (2017) for an array of microchannel heat sinks with five different staggered pin-fin configurations, namely triangle, square, pentagon, hexagon and circle geometries. A uniform heat flux was applied at the top surface of the microchannel heat sink, and deionized water was used as the coolant. The results showed that the lowest thermal resistance and uniformity of the chip’s top surface was found with a staggered hexagonal cross section pin fins and the lowest pressure drop with the staggered circular cross section pin fins because of the channels distribution that provides continuous and stable flow. For pin fins with staggered triangular cross section, it is found there are maximum blocking effects for the coolant flowing happened because of the narrowest channel between the adjacent triangle pin fins and the biggest back side area, which leads to an increase in the pressure drop.