Weilin Qu

Liquid Single-Phase Flow in an Array of Micro-Pin-Fins—Part I: Heat Transfer Characteristics

This is Paper I of a two-part study concerning thermal and hydrodynamic characteristics of liquid single-phase flow in an array of micro-pin-fins. This paper reports the heat transfer results of the study. An array of 1950 staggered square micro-pin-fins with 200X200 μm 2 cross-section by 670 μm height were fabricated into a copper test section. De-ionized water was used as the cooling liquid. Two coolant inlet temperatures of 30°C and 60°C and six maximum mass velocities for each inlet temperature ranging from 183 to 420 kg/m 2 s were tested. The corresponding inlet Reynolds number ranged from 45.9 to 179.6. General characteristics of average and local heat transfer were described. Six previous conventional long and intermediate pin-fin correlations and two micro-pin-fin correlations were examined and were found to overpredict the average Nusselt number data. Two new heat transfer correlations were proposed for the average heat transfer based on the present data, in which the average Nusselt number is correlated with the average Reynolds number by power law. Values of the exponent m of the Reynolds number for the two new correlations are fairly close to those for the two previous micro-pin-fin correlations but substantially higher than those for the previous conventional pin-fin correlations, indicating a stronger dependence of the Nusselt number on the Reynolds number in micro-pin-fin arrays. The correlations developed for the average Nusselt number can adequately predict the local Nusselt number data.

Experimental Study of Pressure Drop and Heat Transfer in a Single-Phase Micropin-Fin Heat Sink

The pressure drop and heat transfer characteristics of a single-phase micropin-fin heat sink were investigated experimentally. Fabricated from 110 copper, the heat sink contained an array of 1950 staggered square micropin fins with 200 X200 μm 2 cross section by 670 μm height. The ratios of longitudinal pitch and transverse pitch to pin-fin equivalent diameter are equal to 2. De-ionized water was employed as the cooling liquid. A coolant inlet temperature of 25°C, and two heat flux levels, q eff =50W/cm 2 and q eff =100 W/cm 2 , defined relative to the platform area of the heat sink, were tested. The inlet Reynolds number ranged from 93 to 634 for q eff = 50 W/cm 2 , and from 127 to 634 for q eff =100 W/cm 2 . The measured pressure drop and temperature distribution were used to evaluate average friction factor and local averaged heat transfer coefficient/ Nusselt number. Predictions of the previous friction factor and heat transfer correlations that were developed for low Reynolds number (Re < 1000) single-phase flow in short pin-fin arrays were compared to the present micropin-fin data. Moores and Joshis friction factor correlation (2003, Effect of Tip Clearance on the Thermal and Hydrodynamic Performance of a Shrouded Pin Fin Array, ASME J. Heat Transfer, 125, pp. 999-1006) was the only one that provided acceptable predictions. Predictions from the other friction factor and heat transfer correlations were significantly different from the experimental data collected in this study. These findings point to the need for further fundamental study of single-phase thermal/fluid transport process in micropin-fin arrays for electronic cooling applications.

Comparison of Micro-Pin-Fin and Microchannel Heat Sinks Considering Thermal-Hydraulic Performance and Manufacturability

This paper explores the potential of micro-pin-fin heat sinks as an effective alternative to microchannel heat sinks for dissipating high heat fluxes from small areas. The overall goal is to compare microchannel and micro-pin-fin heat sinks based on three metrics: thermal performance, hydraulic performance, and cost of manufacturing. The channels and pins of the microchannel and micro-pin-fin heat sinks, respectively, have a width of 200 ¿m and a height of 670 ¿m. A comparison of the thermal-hydraulic performance shows that the micro-pin-fin heat sink has a lower convection thermal resistance at liquid flow rates above approximately 60 g/min, though this is accompanied by a higher pressure drop. Methods that could feasibly fabricate the two heat sinks are reviewed, with references outlining current capabilities and limitations. A case study on micro-end-milling of the heat sinks is included. This paper includes equations that separate the fabrication cost into the independent variables that contribute to material cost, machining cost, and machining time. It is concluded that, with micro-end-milling, the machining time is the primary factor in determining cost, and, due to the additional machining time required, the micro-pin-fin heat sinks are roughly three times as expensive to make. It is also noted that improvements in the fabrication process, including spindle speed and tool coatings, will decrease the difference in cost.

Liquid Single-Phase Flow in an Array of Micro-Pin-Fins—Part II: Pressure Drop Characteristics

This Technical Brief is Part II of a two-part study concerning water single-phase pressure drop and heat transfer in an array of staggered micro-pin-fins. This brief reports the pressure drop results. Both adiabatic and diabatic tests were conducted. Six previous friction factor correlations for low Reynolds number (Re < 1000) flow in conventional and micro-pin-fin arrays were examined and found underpredicting the adiabatic data except the correlation by Short et al. (2002, Performance of Pin Fin Cast Aluminum Coldwalls, Part 1: Friction Factor Correlation, J. Thermophys. Heat Transfer, 16(3), pp. 389-396), which overpredicts the data. A new power-law type of correlation was developed, which showed good agreement with both adiabatic and diabatic data.

Experimental Study of Water Liquid-Vapor Two-Phase Pressure Drop Across an Array of Staggered Micropin-Fins

This study concerns pressure drop of adiabatic water liquid-vapor two-phase flow across an array of 1950 staggered square micropin-fins having a 200 X 200 μm cross section by 670 μm height. The ratios of longitudinal pitch and transverse pitch to pin-fin equivalent diameter are equal to 2. An inline immersion heater upstream of the micropin-fin test module was employed to produce liquid-vapor two-phase mixture, which flowed across the micropin-fin array. The test module was well insulated to maintain adiabatic condition. Four maximum mass velocities of 184 kg/m 2 s, 235 kg/m 2 s, 337 kg/m 2 s, and 391 kg/m 2 s, and a range of vapor qualities for each maximum mass velocity were tested. Measured pressure drop increases drastically with increasing vapor quality. Nine existing two-phase pressure drop models and correlations were assessed. The Lockhart― Martinelli correlation for laminar liquid-laminar vapor combination in conjunction with a single-phase friction factor correlation proposed for the present micropin-fin array provided the best agreement with the data.

PRESSURE DROP AND HEAT TRANSFER IN A SINGLE-PHASE MICRO-PIN-FIN HEAT SINK

The pressure drop and heat transfer characteristics of a single-phase micro-pin-fin heat sink were investigated experimentally. Fabricated from 110 copper, the heat sink consisted of 1950 staggered micro-pins with 200×200 μm2 cross-section by 670 μm height. Deionized water was employed as the cooling liquid. A coolant inlet temperature of 25°C, and two heat flux levels, q eff = 50 W/cm2 and q eff = 100 W/cm2 , defined relative to the planform area of the heat sink, were tested. The inlet Reynolds number ranged from 93 to 634 for q eff = 50 W/cm2 , and 127 to 634 for q eff = 100 W/cm2 . The measured pressure drop and temperature distribution were used to evaluate average friction factor and local averaged heat transfer coefficient/Nusselt number. Predictions of the Moores and Joshi friction factor correlation and the Chyu et al. heat transfer correlation that were developed using macro-size pin-fin arrays were compared to micro-pin-fin heat sink data. While the Moores and Joshi correlation provide acceptable predictions, the Chyu et al. correlation overpredicted local Nusselt number data by a fairly large margin. These findings point to the need for further study of single-phase thermal/fluid transport process in micro-pin-fin heat sinks.Copyright

Experimental Study and Numerical Analysis of Water Single-Phase Pressure Drop Across an Array of Circular Micro-Pin-Fins

This study investigates pressure drop associated with water liquid single-phase flow across an array of staggered micro-pin-fins having circular cross-section. The micro-pin-fins are micro-end milled out of oxygen free copper and have the following dimensions: 180 micron diameter and 683 micron height. The longitudinal pitch and transverse pitch are equal to 400 microns. Seven water inlet temperatures from 22 to 80 °C, and seventeen maximum mass velocities for each inlet temperature, ranging from 159 to 1475 kg/m2 s, were tested. The test module was well insulated to maintain adiabatic conditions. The experimental results were compared to those from a micro-pin-fin array having similar size and geometrical arrangement but a square cross-section. The circular micro-pin-fins were seen to yield a significantly lower pressure drop than the square micro-pin-fins. The present experimental results were also compared with the predictions of several friction factor correlations as well as the results from a three-dimensional numerical analysis. Neither was able to accurately predict the experimental data.Copyright

Hydrodynamic and Thermal Characteristics of Single-Phase and Two-Phase Micro-Pin-Fin Heat Sinks

The pressure drop and heat transfer characteristics of single-phase and two-phase micro-pin-fin heat sinks were investigated experimentally. Fabricated from 110 copper, the heat sink contained an array of 1950 staggered square micro-pin-fins with 200×200 μm2 cross-section by 670 μm height. The ratios of longitudinal pitch and transverse pitch to pin-fin hydraulic diameter are equal to 2. Deionized water was employed as the cooling liquid. A coolant inlet temperature of 30 °C, and six maximum mass velocities, ranging from 183 to 420 kg/m2 s, were tested. The corresponding inlet Reynolds number ranged from 45.9 to 105.9. General hydrodynamic and thermal characteristics of the two flow regimes of single-phase flow and flow boiling were described. The measured temperature distribution was used to evaluate single-phase heat transfer coefficient and Nusselt number. Predictions of the previous friction factor and heat transfer correlations that were developed for low Reynolds number (Re<1000) single-phase flow in short pin-fin arrays were compared to the present micro-pin-fin single-phase pressure drop and Nusselt number data, respectively. The Short et al friction factor correlation and the Kosar et al. heat transfer correlation provided acceptable predictions.Copyright

Thin film heat flux sensors fabricated on copper substrates for thermal measurements in microfluidic environments

Micro-scale heat flux sensors are fabricated on bulk copper surfaces using a combination of lithography-based microfabrication and micro end milling. The heat flux sensors are designed to enable heat transfer measurements on an individual pin in a copper micro pin fin heat sink. Direct fabrication of the sensors on copper substrates minimizes the thermal resistance between the sensor and pin. To fabricate the devices, copper wafers were polished to a flatness and roughness suitable for microfabrication and standard processes, including photolithography, polyimide deposition via spinning, and metal deposition through physical vapor deposition were tailored for use on the unique copper substrates. Micro end milling was then used to create 3D pin features and segment the devices from the copper substrate. Temperature calibrations of the sensors were performed using a tube furnace and the heat flux sensing performance was assessed through laser-based tests. This paper describes the design, fabrication and calibration of these integrated heat flux sensors.

Experimental Study and Numerical Analysis of Water Single-Phase Pressure Drop Across a Micro-Pin-Fin Array

This study investigates the hydraulic performance of a copper micro-pin-fin array subjected to water liquid single-phase flow conditions. The test section contains an array of 1950 staggered square micro-pin-fins with 200 micron × 200 micron cross-section by 670 micron height. The ratios of longitudinal pitch and transverse pitch to pin-fin equivalent diameter are equal to 2. Seven water inlet temperatures from 22°C to 80°C, and seventeen maximum mass velocities for each inlet temperature, ranging from 181 to 1649 kg/m2 s, were tested. The test module was well insulated to maintain adiabatic conditions. Comparison of predictions of eleven existing friction factor correlations with the experimental data show relatively large discrepancies. The experimental study was complemented with a numerical analysis of single-phase flow in the micro-pin-fin array. Numerical results show excellent agreement with experimental data for Reynolds numbers below 700.Copyright