Inlet/Outlet Arrangements Effects

Flow maldistribution caused by improper channels and headers design can significantly effect on the thermal and hydraulic performance of the microchannel heat sinks. The fluid flow may enter and exit in different directions according to the inlet and outlet port locations. For example, it can enter in vertical direction and exits out to vertical or lateral direction or vice

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Fig. 2.2: Various inlet/outlet flow arrangements for: (a) I-type; (b) Z-type; (c) ]-type; (d) L-type; and (e) Γ-type (Lu and Wang, 2006).

versa. In this subsection, various types of flow arrangements in the heat sinks will be presented.

Five inlet flow configurations (namely I-, Z-, ]-, L-, and Γ-arrangement) are investigated numerically by Lu and Wang (2006) to explore the influence of inlet locations on the hydrothermal performance of the multichannel cold-plates (see Fig. 2.2). The size of the cold- plate is fixed for all models with width, length and depth of 60 mm, 64 mm, and 7.8 mm, respectively. The corresponding inlet and outlet port diameter is 5.8 mm. They observed that ]-flow arrangement displayed uniform flow distribution and lowest pressure drop with moderate heat transfer performance. In contrast, I- and Γ-flow configurations revealed the highest pressure drop and excellent heat transfer performance due to their impingement configurations. The Z- and L-flow arrangements showed deteriorate in heat transfer performance due to the flow maldistribution. These observations can be attributed to the flow- recirculation that eventually leads to a significant temperature difference along the surface of the cold-plate as well as flow maldistribution. These findings provoked the researchers to examine the effects of flow maldistribution on the microchannel heat sink performance.

(a) (b)

(c) (d)

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Fig. 2.3: Typical inlet/outlet configurations for S, D, N, U and V-arrangement of a straight rectangular microchannel heat sink (Chein and Chen, 2009).

Chein and Chen (2009) carried out three-dimensional numerical study on the microchannel heat sinks with various inlet/outlet arrangements. They considered six inlet/outlet flow arrangements to inspect the fluid flow and heat transfer in microchannel heat sink including D, I, N, S, U and V-types as shown in Fig. 2.3. The size of microchannel heat sink is fixed with width, length and depth of 6.2 mm, 18 mm, and 0.5 mm, respectively, with microchannels dimensions of 𝑊_𝑐ℎ = 200 μm, 𝐻_𝑐ℎ = 400 μm, and 𝐿_𝑐ℎ = 10 mm. For horizontal inlet/outlet flow arrangements (D, I, N and S-type), their results revealed that the flow maldistribution is more pronounced in the microchannel heat sink which leads to temperature non-uniformity. This finding is ascribed to the manifestation of recirculation at the corners of dividing and combining manifolds. In contrast, the vertical flow inlet (V-type and U-type) arrangement revealed lower flow maldistribution, and this due to the effect of jet impingement at the manifold surface that suppressed the appearance of flow recirculation. Overall, it is found that the V-type heat sink has the best hydrothermal performance among the heat sinks studied. For all of the heat sinks studied (U-, V-, I-, N-, D-, and S-types), the highest heat sink temperature took place at the edge of the heat sink since there is no heat dissipation by fluid convection.

Sehgal et al. (2011) experimentally explored the effect of different flow arrangements on the hydro-thermal performance of straight rectangular microchannel heat sink. Three different flow arrangements were considered (S-type, U-type, and P-type) as shown in Fig. 2.4. Three different heat fluxes of 125 W, 225 W, and 375 W are supplied at the bottom of the heat sink with Reynolds number ranging from 224 to 1121. The results indicated that the U-type flow arrangement demonstrated the maximum heat transfer followed by P-type and S-type. This is due to the fact that the U-type arrangement ensured the longest flow path and contributed to maximum heat absorption by the fluid. The S-type, on the other hand, is exposed to change in its direction in two bends which maximized the pressure drop more than other types. It is shown that the P-type flow arrangement is preferred in terms of having lowest pressure drop

D-type

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Fig. 2.4: Different flow arrangements in a straight rectangular microchannel heat sink: (a) U-type; (b) S-type; and (c) P-type (Sehgal et al., 2011).

with acceptable heat transfer performance, indeed it is the optimum one for practical applications.

Numerical studies were carried out by Vinodhan and Rajan (2014) on fluid flow and heat transfer characteristics in four new microchannel heat sink configurations consisting of four compartments with separate coolant inlet and outlet plenums for each compartment as shown in Fig. 2.5, to compare their performance with the conventional microchannel heat sink. At the same pumping power for a heat flux of 100 W/cm2_{, it is found that the thermal resistances}

and temperature gradient of the substrate in the four new designs are lower than the thermal resistances in conventional heat sink.

Fig. 2.5: Schematic diagram of heat sink configurations (Vinodhan and Rajan, 2014).

Configuration “A” Configuration “B”

Configuration “C” Configuration “D” Outlet Conventional heat sink Computational domain (a) (b) (c)

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Anbumeenakshi et al. (2014) experimentally examined the flow distribution over a multiple microchannel with two different types of header geometries namely rectangular and trapezoidal with inlet flow arrangement at three different flow rates. Their experimental investigation was carried out with a micro channel setup having 25 numbers of rectangular channels of 0.42 mm width, 4.2 mm depth and 100 mm length. In their study, the computational fluid dynamics (CFD) simulation is done for inline flow by using the CFD software Fluent 6.3.26. The results were compared with experiments and found to be in good agreement. From the experimental results, it was found that micro channel with trapezoidal header gives less flow maldistribution when compared to rectangular header.

It can be concluded from the above studies that the performance of the microchannel heat sinks can be severely deteriorated by the flow maldistribution caused by inappropriate manifold design.