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In this subsection, the experimental and numerical studies that carried out to investigate the heat transfer enhancement and fluid flow characteristics for microchannels with various aspect ratios will be discussed.

Wang and Peng (1994) studied experimentally the single-phase forced convective heat transfer characteristics by using six microchannel heat sinks made of stainless steel plate, 18 mm wide and 125 mm long, each microchannel cross-section was rectangular with different widths and identical channel height of 0.7 mm. The tested length of the microchannel was 45 mm with a thickness of 2 mm, with microchannels characteristics of (Width (mm), height (mm) and number of channels) (0.8, 0.7, 4), (0.6, 0.7, 4), (0.4, 0.7, 4), (0.4, 0.7, 6), (0.2, 0.7, 4) and (0.2, 0.7, 6). There were 4 or 6 microchannels with identical geometries evenly distributed on each test plate. Methanol and deionized water were employed as the working fluids in their experiments, and the liquid velocities evaluated varied from 0.2 to 2.1 m/s for water and 0.2 to 1.5 m/s for methanol. The working liquid temperature varied from 10 to 35 °C for deionized water and 14 to 19 °C for methanol.

The results provide significant data and considerable insight into the behaviour of the forced- flow convection in microchannels. They observed three different trends for the variation of

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single phase heat transfer coefficient in the above microchannels. In first the trend on (0.8, 0.7, 4) heat sink, heat transfer coefficient smoothly increased with wall temperature. In the second trend on (0.6, 0.7, 4), (0.4, 0.7, 4) and (0.4, 0.7, 6) heat sink, a steep increase in heat transfer coefficient at low wall temperature was observed. This was followed by moderate increase in heat transfer coefficient at high wall temperature. In third trend on (0.2, 0.7, 4) and (0.2, 0.7, 6) heat sink, the heat transfer coefficient decreased first and then moderately increased as the wall temperature was increased. They further concluded that heat transfer characteristics in laminar and transition region of microchannels are highly complicated when compared to a conventional channel. This was attributed to the considerable change in thermo-physical properties of the flowing fluid because of large variation in liquid temperature along the length of microchannel. Compared with conventional theory, they found that fully turbulent flow is induced much earlier with 𝑅𝑒 of about 1000-1500 for liquid flow in microchannels.

Peng et al. (1994a and 1994b) experimentally studied friction flow and heat transfer characteristics of water flowing through rectangular stainless steel microchannels with hydraulic diameters (𝐷ℎ) varying from 0.133 mm to 0.367 mm and with aspect ratios (∝) ranging from 0.33 to 1.0. Under steady-state and fully developed flow conditions they conducted measurements for the liquid temperatures, the liquid flow rates, inlet and outlet pressures, wall surface temperatures and heat input to the substrate. To calculate the heat transfer coefficient (ℎ) at the downstream end of the microchannel, two thermocouples were used for recording of the fluid temperature at the inlet of the microchannel (𝑇_{𝑓,𝑖𝑛}), and three thermocouples were used for measuring the local wall temperature (𝑇_𝑤) at the downstream end. In their experimental studies, they showed that their results of both flow friction and heat transfer in microchannels deviated from the value predicted by the classical correlations (see Eqs. (2.5-2.11)), and also they observed that the laminar heat transfer occurred at a 𝑅𝑒 of 200 to 700, and that the fully turbulent convective heat transfer was achieved at a value of 𝑅𝑒 ranging from 400 to 1500. Their data analysis revealed that the flow was found to be most strongly affected by the 𝐷ℎ and ∝. For the laminar heat transfer regime, experimental Nusselt number (𝑁𝑢) values were smaller than the predicted ones and they were dependent on the Reynolds number, 𝑁𝑢 is being to 𝑅𝑒0.62_{. For the turbulent flow, the experimental Nusselt} number were higher than those predicted although trend correctly captured by correlation. The heat transfer coefficient values for both laminar and turbulent flow regimes were changed significantly for different values of 𝐷_ℎ and ∝. They reported an optimum value of ∝ as 0.75 for laminar and 0.50.75 for turbulent flows. The friction factor was found to be proportional to 𝑅𝑒−1.98 _{under laminar conditions and 𝑅𝑒}−1.72 _{for turbulent flow.}

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Peng and Peterson (1996) also investigated experimentally the heat transfer coefficient (ℎ) and Nusselt numbers (𝑁𝑢) for single-phase forced convective heat transfer microchannel structures with small rectangular straight channels having hydraulic diameters between 0.133 mm and 0.367 mm. Water was employed as a working fluid with a relatively high velocity, ranging from 0.2 to 12 m/s, the Reynolds number spanned a range of pure laminar to highly turbulent cases, 504000. They created empirical heat transfer correlations for both the laminar and turbulent flow regimes which indicated that the geometric configuration of the microchannel heat sink (specifically the ∝, 𝐷ℎ, and the ratio of hydraulic diameter and microchannel centre to centre distance (𝐷ℎ⁄𝑊𝑐)) were very influential.

Poh and Ng (1998) carried out a numerical analysis of fluid flow and thermal resistance with a uniform wall heat flux and uniform inlet velocity in manifold microchannels. They used a commercial CFD package, ANSYS, to simulate the manifold microchannels of sixteen cases with different geometric parameters of the heat sink. Fluorocarbon liquid FX-3250 was used as the coolant and the flow was laminar. In their study the effects of channel geometries (width, height and length), wall heat flux and inlet velocity of the coolant were investigated and compared to an analytical model. The numerical results showed a very strong relationship between thermal resistance and inlet velocity. It was found that the thermal resistance of the heat sink decreased with increasing inlet velocity and channel depth. The numerical results were compared with available analytical results and found to be in good agreement.

A three-dimensional numerical study was conducted by Zhang et al. (2008) on the microchannel heat sink cooler. The commercial Fluent package was used to simulate the temperature distribution in MCHS under uniform and non-uniform heating source. In their study, water was chosen as the coolant with velocity varying from 0.01 m/s to 10 m/s. A comparison between two heating conditions was carried out, and the results showed that the heat sink has better heat dissipation character under uniform heating conditions. A new type of micro channel heat sink with various ranges in channel width was adopted to enhance the heat transfer under non-uniform heating conditions. They observed that the heat dissipation rate increased by about 10% with the narrower width channel at the same inlet velocity.