| In these decades, the Complementary Metal Oxide Semiconductor (CMOS) industry has been developed so rapidly that the whole social life of human being is changed. However, the heat generation of the CMOS module in small space increases exponentially with the rapid development of the industry, which influences the performance and the operating life of the CMOS module if the heat can not be dissipated effectively. Therefore, the development of cooling technology in mini/micro space with high efficiency is in desperate need to maintain the normal operation of CMOS modules, and thus it has been a hot research topic in these decades. Micro pin fin array is one of the cooling structures with the highest cooling efficiency due to the large area-to-block ratio, and it gets more and more attention recently.Test sections of micro pin fin array were designed by an integrative design method to eliminate contact thermal resistance, and manufactured by precision machining to obtain test sections with different cross section shapes of circle, ellipse, diamond and triangle. The single phase flow and heat transfer was experimentally investigated in micro pin fins, and the results were compared with those in a plain rectangular micro channel. The comparisons illustrated that the single phase heat transfer was apparently enhanced by the micro pin fin array and the Nusselt number was 300% higher than that in plain micro channel with the same hydraulic diameter. Besides, the experimental results showed that the cross section shape was an important factor to the flow and heat transfer characteristics in micro pin fin array. At low flow rate, the deviation of pressure drop was not very large in micro pin fin arrays with different cross section shapes attributed to the end-wall effect. However, the end-wall effect was weakened by the increase of the flow rate and thus the deviation of pressure drop became large among the micro pin fin arrays with different cross section shapes. Although the axial conduction may influence the flow and heat transfer in micro scale, the effect was too slight to consider in the test sections and the operating cases of present research. Similar to the flow characteristics, the deviation of Nu was small among micro pin fin arrays with different cross section shapes due to the end-wall effect at low flow rate, and then it became large with the increase of the flow rate. Except for the cross section shape, the influence of heating load on the flow and heat transfer was also explored in micro pin fin arrays. It was found that the pressure drop became large with the increase of heating load at the same flow rate but the change rate of pressure drop with the flow rate was decreased. The friction factor became large as much as 110% at low Re with the increase of the heating load, however, the influence of heating load on the friction factor was diminished with the increase of Reynolds number. To circular and diamond micro pin fin array, the heating load effect could be neglected at Re>400 while Re>250 to triangular micro pin fin arrays attributed to the flow transition in the wake zones of micro pin fins. For the influence of heating load on heat transfer characteristics in micro pin fin arrays, the experimental results illustrated that the heat transfer was enhanced by increasing the heating load in circular and diamond micro pin fin arrays, but in triangular micro pin fin array the heat transfer was enhanced at first and then was weakened with the increase of Re. According to the experimental results, the Nu in circular and diamond micro pin fin array was more than 50% enhanced by the increase of heating load; however, the Nu became large with the increase of heating load at Re<250 in triangular micro pin fin array and then decreased when Re>250. In addition, the total thermal resistance was decreased with the increase of heating load in micro pin fin arrays with different cross section shapes at low Re, the effect of heating load on thermal resistance could be ignored at Re>600 in circular and diamond micro pin fin arrays and Re>250 in triangular micro pin fin array.Based on the comparison of friction factor, the flow resistance was much higher in micro pin fin array than that in plain micro channel which was a bottleneck problem in development of micro pin fin array cooling technology. In order reduce the flow resistance, the hydrophobic micro pin fin arrays with different contact angles were prepared by solidifying the hydrophobic layers containing nano-particles of different content. The impact of hydrophobic surfaces on flow and heat transfer characteristics was experimentally investigated in micro pin fin arrays with different contact angles. It was found that the flow resistance was reduced apparently in the hydrophobic micro pin fin arrays compared with the plain micro pin fin array, and the flow transition was delayed in the wake zones of micro pin fins. As a result, the flow resistance reduction was more significant in micro pin fin arrays with early flow transition and large differential pressure resistance. At the same Re, the reduction ratio of friction factor became large with the increase of contact angle of the micro pin fin array. At the same contact angle, the reduction ratio of friction factor was reduced by increasing Re in elliptical micro pin fin array, however, the reduction ratio was reduced at first and then became a constant in diamond and circular micro pin fin arrays, and the minimal value of the friction factor reduction ratio was 50.81% and 58.68% in diamond and circular micro pin fin arrays with contact angle of 151.5°, respectively. When the contact angle of micro pin fin was relatively small, the reduction ratio of friction factor in elliptical micro pin fin array was larger than that in micro pin fin arrays with cross section shapes of diamond and circle, but the former became smaller than the latter when Re>600. Besides, the friction factor reduction ratio in circle micro pin fin array were larger than those in elliptical and diamond micro pin fin arrays with the increase of contact angle; the friction factor reduction ratio in elliptical micro pin fin array was close to than that in diamond micro pin fin array at low Re, but the latter became larger than the former with the increase of Re. Except for the flow resistance characteristic, the heat transfer was also influenced by the hydrophobic surfaces in micro pin fin arrays in present research. It was found that the Nu was reduced in hydrophobic micro pin fin arrays compared with those without hydrophobic surfaces at high Re attributed to the small thermal conductivity of hydrophobic layers. In order to evaluate the comprehensive effect of hydrophobic surfaces on flow and heat transfer in micro pin fin arrays, the performance and power characteristics was explored in present research. The results illustrated that the heat transfer was enhanced in hydrophobic micro pin fin arrays considering the flow resistance reduction and thermal resistance increase comprehensively; however, this enhancement is diminished with the increase of Re except the super-hydrophobic micro pin fin array with contact angle larger than 150° at Re>400.The fluid-solid coupled numerical model was employed to investigate the effect of heat flux, hydraulic diameter, aspect ratio and block ratio on the flow separation across the single micro cylinder and micro pin fins. For the single micro cylinder, the separation angle and the recirculation length both became large with the increase of heat flux, and the profiles of recirculation length along axial direction were asymmetrical, which was different from the heating cylinders with dimensions of millimeter-level. Besides, the recirculation lengths on planes near the heating wall were apparently longer than those on planes far away from the heating wall. For the single micro cylinder, both of recirculation length and the separation angle became large with the increase of aspect ratio and block ratio; however, the recirculation length and the separation angle in micro pin fin arrays was related to the location of the pin fin, and the separation angle of the micro pin fin in the middle row became small along the flowing direction and the separation angle of the same micro pin fin became large with increase of Re. Different from the separation angle, the recirculation length was decided by the pitches among the micro pin fins. Through the numerical calculation on flow and the temperature field in micro pin fins with different arrangement, pitches, heights and hydraulic diameters, it was found that the number of the vortexes was less in micro pin fin arrays with small pitches than those with large pitches and the flow rate distribution in different zones of micro pin fin array was changed by increasing the pitches among the pin fins. Besides, the heat transfer was enhanced by decreasing the hydraulic diameter, the pin fin pitch and the pin fin number along transverse direction, or increasing the micro pin fin height.Based on the numerical results, new correlations of friction factor and Nu in micro pin fin arrays were proposed to predict the flow and heat transfer characteristics. In addition, the mechanism of heating load effect on the flow and heat transfer in micro pin fins was analyzed by numerical method, and the results illustrated that the boundary layer thickness was over 30% reduced by increasing the heating load due to the change of thermal physical properties, due to which the end-wall effect was weakened. As a result, the heat transfer is enhanced and the Nu was over 20% increased in micro pin fin arrays with the increase of heating load. Due to the weakness of end-wall effect with the increase of heating load, the discrepancy of Nu was reduced by the increase of heating load in micro pin fin arrays with different pin fin heights. In the thermal physical properties of the working fluid, the effect of dynamic viscosity change on the heat transfer with the heating load was the most apparent and the impact factor on Nu reached 20%. |
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