Influence of Inlet/Outlet Configurations

To suppress flow instability, to reduce the severe reduction in the critical heat flux, and to enhance heat transfer during two phase flow boiling, Kandlikar et al. (2006) experimentally introduced two modifications in a set of six 1054 µm wide, 197 µm high and 63.5 mm long parallel rectangular microchannels. They fabricated inlet area restriction (pressure drop elements) and artificial nucleation sites (cavities), these are studied alone and in conjunction with each other. The microchannel heat sinks used were made from copper, and water was employed as coolant. Artificial cavities of diameters 5–30 µm are drilled on the bottom surface of the microchannel using a laser beam, these are spaced at a regular interval of 762 µm. Two different areas of restrictive inlet header are machined on the Lexan cover, the first and second sets consisting of circular holes with a cross sectional area 4% and 51% of a single microchannel flow area, and both sets of restrictors had a length of 1.6 mm. These pressure restrictors are expected to eradicate the backflow by forcing an expanding vapor bubble in the downstream direction and not allowing the liquid-vapor mixture to enter the inlet manifold.

In their work five cases have studied and flow stability is determined through high-speed digital video camera and measurement of pressure drop fluctuations across the microchannels. These are carried out under the same mass flow and heat flux conditions. They observed that introducing the 51% area pressure drop elements (restrictors) in the inlet manifold alone seem to reduce the severity of backflow partially with lower pressure drop fluctuations, while using the artificial cavities alone actually increased the instabilities. The presence of both the 51% area pressure drop elements and the artificial cavities partially reduced the vapour backflow and significantly reduced the pressure drop fluctuations. However, introduction of artificial cavities in conjunction with the 4% area pressure drop

[64]

Fig. 2.31: (a) Schematic illustration of the heat sink device used by Koşar et al. (2006); (b) flow distributive pillars; (c) geometry of the inlet orifices (dimensions in µm).

elements completely eliminated the instabilities, but this was at the expense of high pressure drop.

Koşar et al. (2006) investigated experimentally the effects of introducing inlet restrictors (orifices) on the suppression of boiling flow instabilities in parallel microchannels. They used a microchannel device consisting of five 1 cm long, 200 µm wide, and 264 µm high, parallel microchannels, spaced 200 µm apart. Five 20 µm wide orifices with different lengths varying from 50 µm to 400 µm were installed at the entrance of each microchannel, see Fig. 2.31. To get homogeneous distribution of flow in the inlet, flow distributive pillars have been introduced, these are arranged in 2 columns of 12 circular pillars having a diameter of 100 µm. Once boiling occurs, they observed severe flow oscillations in the device without the restrictors. However, with increasing restrictor length, they found a decrease in the instabilities and larger heat fluxes could be obtained before reaching critical heat flux.

Experimental study has been carried out by Wang et al. (2008) to investigate effects of inlet/outlet configurations on the flow boiling instabilities in eight parallel trapezoidal microchannels etched in a silicon substrate, having the same length of 30 mm and a hydraulic diameter of 186 µm. In their study, water is employed as coolant in three types of inlet/outlet connections, these are classified as Type-A, Type-B and Type-C connections, see Fig. 2.32. In the Type-A connection, the water flow in both inlet and outlet plenums were restricted by the conduits that being perpendicular to the parallel microchannels. In the Type-B connection,

(c) Pressure port

Exit

(a)

Inlet Air gap

10000

(b) Flow distributors

[65]

Fig. 2.32: Different inlet/outlet configurations investigated by Wang et al. (2008).

the fluid flow could enter to and exit from the microchannels without any restriction. In the Type-C connection, the entrance of each microchannel was restricted whereas fluid flow could freely discharge from the outlet, this type of connection was used previously by both Kandlikar et al. (2006) and Koşar et al. (2006) in their experiments on flow boiling in microchannels. In their study, they found that the Type-B connection had the largest oscillations followed by the Type-A connection and finally the Type-C connection had nearly complete steady flow boiling.

Further experiment study was carried out by Harirchian and Garimella (2008) to investigate the effect of four mass fluxes of a perfluorinated dielectric fluid (Fluorinert FC-77) ranging from 250 to 1600 kg/m2.s and microchannel size on the local flow boiling heat transfer and pressure drops. Seven silicon microchannel heat sink test sections with cross-sectional area of 12.7 mm × 12.7 mm and thickness of 0.65 mm are fabricated for experiments, and each one was mounted on a printed circuit board. These heat sinks consisting of parallel rectangular microchannels having a constant depth of 400 μm and variable widths ranging from 100 to 5850 μm, with the channel length held constant at 12.7 mm. The heat transfer coefficients obtained are compared to values predicted using a number of existing correlations for pool boiling and saturated flow boiling in different channel size. For a fixed wall heat flux, their experiments revealed that the microchannel width has a modest effect on both of the heat transfer coefficient and boiling curves, whilst the opposite was shown when a base heat flux is fixed, where the heat transfer coefficient and wall temperature were increased as (a) Type–A connection: flow entering and exiting from

parallel microchannels with restrictions because inlet/outlet conduits perpendicular to microchannels.

(b) Type–B connection: flow entering to and exiting from microchannels freely without restriction.

(i) Arrangement of parallel microchannels with restrictive inlet.

(ii) Sketch of the microchannel with a restrictive inlet.

[66]

the microchannels width increases. In the nucleate boiling region, the heat transfer coefficient and boiling curves are found to be independent of mass flux when channel size is fixed. However, when convective boiling dominates, the boiling curves diverge and more heat is dissipated as the mass flux increases. Also, a strong dependence of pressure drop on both channel size and mass flux was observed at a fixed wall heat flux.

Harirchian and Garimella (2009) then extended their work to include a study of the effect of changing channel depth as well. Their experiments are performed with five additional microchannel test sections with channel depths of 100 and 250 μm and widths ranging from 100 to 1000 μm. They presented the variation in heat transfer coefficient graphically as a function of channel cross-sectional area at different heat fluxes. They observed that the heat transfer coefficient remained constant without any noticeable change with microchannels with a cross-sectional area of 0.089 mm2 _{and larger, due to vapour confinement as they affect}

the heat transfer mechanisms in flow boiling.