Straight Microchannel Heat Sink Research Articles

Thermal-hydraulic performance of interrupted microchannel heat sinks with different rib geometries in transverse microchambers

The thermal-hydraulic performance of microchannel heat sinks with ribs in the interrupted transverse microchambers is studied using a three-dimensional conjugated heat transfer model and considering entrance effect, viscous heating and temperature-dependent thermophysical properties. Five different configurations of ribs and four lengths along the flow direction for every rib configuration are selected to analyze the effects of rib geometry on the thermal-hydraulic performance. The five rib configurations are rectangular, backward triangular, diamond, forward triangular and ellipsoidal, and the rib geometry parameters include expansion-constriction profile, ratio and length. The effects of rib geometry on thermal-hydraulic performance are firstly examined by the variations of friction factor and Nusselt number with Reynolds number, and corresponding correlations are proposed. Then, the conductive, convective and fluid capacitive thermal resistances are analyzed to obtain some insight into the basic heat transfer mechanism. Next, the entropy generation rates due to heat transfer and fluid friction are investigated for the analysis of the lost available work and irreversibility in the heat transfer process. Finally, the performance evaluation criteria is calculated to comprehensively assess the performance of such interrupted microchannel heat sinks with different rib geometry. For the studied operation parameters and rib geometries, the interrupted microchannel heat sinks with ribs in the transverse microchambers show a 4–31% decrease in the total thermal resistance, a 4–26% decrease in the total entropy generation rates, the maximum value 1.39 in performance evaluation criteria, compared with the straight microchannel heat sink.

NUMERICAL STUDY OF PRESSURE DROP AND HEAT TRANSFER IN A STRAIGHT RECTANGULAR AND SEMI CYLINDRICAL PROJECTIONS MICROCHANNEL HEAT SINK

The heat transfer and pressure drop characteristics of a straight rectangular and semi cylindrical projections microchannel heat sink were investigated. The heat sinks, made of copper, were aligned in row as a cluster of 21 microchannels, 231 µm wide and 713 µm deep. Water was used as cooling fluid and was made to flow through the channels. A three dimensional analysis was done for Reynolds number ranging between 200 to 1000 with constant heat flux of 10 6 W/m 2 defined relative to the platform area of the heat sink. The temperature rise and pressure drop of the fluid in straight microchannel heat sink was evaluated using ANSYS-CFX package and was validated using experimental data. Similar analysis was done for semi cylindrical projections microchannel heat sink by solving the conjugate heat transfer problem. A comparison of heat transfer and pressure drop between rectangular and semi cylindrical projections microchannel was done and the results were laid out. Results shows that heat transfer increases with the use of semi cylindrical projections microchannel heat sink.

Laminar flow and heat transfer characteristics of interrupted microchannel heat sink with ribs in the transverse microchambers

The paper is focused on the investigation of the laminar flow and heat transfer characteristics in the interrupted microchannel heat sink with ribs in the transverse microchambers. A three-dimensional model based on finite volume approach and SIMPLEC algorithm is performed considering entrance effect, conjugate heat transfer, viscous heating and temperature-dependent thermo-physical properties. Five different rib configurations are considered, including rectangular, backward triangular, diamond, forward triangular and ellipsoidal. The present study firstly examines the effect of such ribs on velocity contour, pressure distribution and temperature distribution, further the local pressure drop and heat transfer characteristics in such microchannel heat sinks. Result shows that the ribs in the transverse microchambers can effectively prevent the decline of local heat transfer coefficient along the flow direction. Secondly, the average friction factor and Nusselt number for such microchannel heat sinks have been studied. Results show that for Reynolds number ranging from 187 to 715, there is a 24–57% increase in the Nusselt number for the interrupted microchannel heat sinks with ribs, while the friction factor increased by 3–70%, comparing with the straight microchannel heat sink. Finally, based on the performance evaluation criteria, the performance of such microchannel heat sink has been comprehensively evaluated. Results shows that for the studied Reynolds number range and microchannel geometries in this paper, the interrupted microchannel heat sink with ellipsoidal ribs in the transverse microchambers shows the best heat transfer performance, with maximum 1.39 of performance evaluation criteria.

Analysis of micro-channel heat sinks with rectangular-shaped flow obstructions

ABSTRACTIt is recognized that cooling methods incorporating micro-channel heat sinks with high capacities of heat removal are necessary for cooling of electronic devices. In this paper, based on a water-cooled smooth micro-channel heat sink, a series of rectangular-shaped flow obstructions are designed into a heat sink, and then the corresponding laminar flow and heat transfer have been analyzed numerically by computational fluid dynamics. Five different configurations of the flow obstructions are considered by adjusting the length of the flow obstructions. The influence of the length of the flow obstructions on heat transfer, pressure drop, and thermal resistance is also observed and compared to that of the traditional smooth micro-channel heat sink without flow obstructions. The overall resistance versus inlet Reynolds number and pumping power are also compared for whole micro-channel heat sinks (including smooth micro-channel heat sink). The results show that the capacity of heat removal of micro-channel heat sinks with flow obstructions is much better than that of the corresponding straight micro-channel heat sink. It is found that the length of the flow obstructions has a major influence on thermal performance. In other words, the flow obstructions placed in the micro-channel heat sink can improve thermal performance.

English

Investigation of heat transfer characteristics has been carried out by theoretically and experimentally for a straight microchannel heat sink of rectangular cross section of 507µm hydraulic diameter with water as the circulating fluid. The temperature distribution, pressure drop, heat transfer coefficient and total thermal resistance of the heat sink are selected as criteria for their cooling performance. Parameters that changes in this study are Reynolds number in ranges from 200 to 700. The theoretical results for heat transfer coefficient and thermal resistance are compared with the experimental results. The obtained theoretical results are having the good agreement with experimental results. It is found that the temperature of water in the microchannels increases with channel length. The present work proposes an approach to improve the heat transfer enhancement of microchannel heat sinks

Laminar thermal performance of microchannel heat sinks with constructal vertical Y-shaped bifurcation plates

With increasing output power of integrated chips, the involved heat flux is accordingly increased. Traditional cooling methods failed to satisfy such a situation, and thus new cooling methods incorporating microchannel heat sinks with high capabilities of heat removal are necessary. In this paper, on the basis of a water-cooled smooth microchannel heat sink, vertical Y-shaped bifurcation plates are designed into the heat sink, and then the corresponding laminar flow and heat transfer are investigated numerically. Four different configurations of Y-shaped plates are considered by adjusting the angle between the two arms of the Y profile. The effects of the angle on heat transfer, pressure drop, and the thermal resistance are also observed and compared with those of the traditional straight microchannel heat sink without bifurcation flow. The overall resistances subjected to inlet Reynolds number and pumping power are also compared for the five microchannel heat sinks. The results show that the thermal performance of the microchannel heat sinks with Y-shaped bifurcation plates is much better than that of the corresponding straight channel. It is suggested that the Y-shaped bifurcation plates placed in water-cooled microchannel heat sinks could improve the overall thermal performance when the angle between the two arms of the Y-shaped plates is designed properly.

Numerical Analysis of Flow and Thermal Performance of Liquid-Cooling Microchannel Heat Sinks with Bifurcation

Single-phase liquid-cooling microchannels have received great attention to remove the gradually increased heat loads of heat sinks. Proper changes of the flow path and/or heat transfer surface can result in much better thermal performance of microchannel heat sinks. In this study, a kind of rectangular straight microchannel heat sink with bifurcation flow arrangement has been designed, and the corresponding laminar flow and heat transfer have been investigated numerically. Four different configurations are considered. The effects of the bifurcation ratio (the initial channel number over the bifurcating channel number) and length ratio (the channel length before bifurcation over the bifurcation channel length) on laminar heat transfer, pressure drop, and thermal resistance are considered and compared with those of the traditional straight microchannel heat sink without bifurcation flow. The overall thermal resistances subjected to inlet Reynolds number and pumping power are compared for the five microchannel heat sinks. Results show that the thermal performance of the microchannel heat sink with bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sinks with larger bifurcation ratio and length ratio provide much better thermal performance. It is suggested to employ bifurcation flow path in the liquid-cooling microchannel heat sinks to improve the overall thermal performance by proper design of the bifurcation position and number of channels.

Constructal design and thermal analysis of microchannel heat sinks with multistage bifurcations in single-phase liquid flow

Based on constructal theory, five different cases with multistage bifurcations are designed as well as one case without bifurcations, and the corresponding laminar fluid flow and thermal performance have been investigated numerically. All laminar fluid flow and heat transfer results are obtained using computation fluid dynamics, and a uniform wall heat flux thermal boundary condition is applied all heated surfaces. The inlet velocity ranges from 0.66 m/s to 1.6 m/s with the corresponding Reynolds number ranging from 230 to 560. The pressure, velocity, temperature distributions and averaged Nusselt number are presented. The overall thermal resistances versus inlet Reynolds number or pumping power are evaluated and compared for the six microchannel heat sinks. Numerical results show that the thermal performance of the microchannel heat sink with multistage bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sink with a long bifurcation length in the first stage (Case 1A) is superior. The usage of multistage bifurcated plates in microchannel heat sink can reduce the overall thermal resistance and make the temperature of the heated surface more uniform (Case 3). It is suggested that proper design of the multistage bifurcations could be employed to improve the overall thermal performance of microchannel heat sinks and the maximum number of stages of bifurcations is recommended to be two. The study complements and extends previous works.

A Numerical Study of the Thermal Performance of Microchannel Heat Sinks with Multiple Length Bifurcation in Laminar Liquid Flow

A rectangular, straight microchannel heat sink with multiple length bifurcation has been designed, and the corresponding laminar flow and thermal performance have been investigated, numerically. Five different cases with multiple length bifurcation and various arrangements are investigated, as well as one case without bifurcations. All laminar fluid flow and surface heat transfer results are obtained using computation fluid dynamics, and a constant heat flux thermal boundary condition is applied to the heated surfaces. The inlet velocity ranges from 0.6 m/s to 1.4 m/s, with the corresponding Reynolds number from 220 to 520. The local pressure, velocity, temperature distributions, and averaged heat transfer coefficient are presented. The overall thermal resistances subjected to inlet Reynolds number are evaluated and compared for six different microchannel heat sinks. Numerical results show that the thermal performance of the microchannel heat sink with bifurcation flow is better than that of the corresponding straight microchannel heat sink, especially the heat sink with longer length bifurcation. The usage of multiple length bifurcated plates in microchannel heat sink can reduce the overall thermal resistance by a factor of up to 2, and make the temperature of a heated surface more uniform, especially for the heat sink having a short bifurcation in the center and two bifurcations near the sides (case 4). It is suggested that proper design of the multiple length bifurcation could be employed to improve the overall thermal performance of microchannel heat sinks.

Comparative Study of the Flow and Thermal Performance of Liquid-Cooling Parallel-Flow and Counter-Flow Double-Layer Wavy Microchannel Heat Sinks

Applications of microchannel heat sinks for dissipating heat loads have received great attention. Wavy channels are recognized to be an alternative cooling technology to enhance the heat transfer, and are successfully applied in heat exchangers. In this article, three kinds of liquid-cooling double-layer microchannel heat sinks, such as a rectangular straight microchannel heat sink, a parallel-flow wavy microchannel heat sink, and a counter-flow double-layer wavy microchannel heat sink, have been designed and the corresponding laminar flow and heat transfer have been investigated numerically. The effects of the wave amplitude and volumetric flow ratio on heat transfer, pressure drop, and thermal resistance are also observed. Results show that the counter-flow double-layer wavy microchannel heat sink is superior at a larger flow rate, and a more uniform temperature rise is achieved. For a slightly larger flow rate, the parallel flow layout shows better performance. In addition to the overall thermal resistance, other criteria for evaluation of the overall thermal performance, e.g., (Nu/Nu0)/(f/f0) and (Nu/Nu0)/(f/f0)1/3, are applied and similar results are obtained.

Numerical Predictions of the Flow and Thermal Performance of Water-Cooled Single-Layer and Double-Layer Wavy Microchannel Heat Sinks

Various microchannel heat sinks for dissipating heat loads have received great attention. Wavy channels are recognized to be suitable to enhance the heat transfer, and are successfully applied in heat-exchange devices. In this article, three kinds of water-cooled microchannel heat sinks, such as a rectangular straight microchannel heat sink, a single-layer wavy microchannel heat sink and a double-layer wavy microchannel heat sink, have been designed and the corresponding laminar flow and heat transfer have been investigated numerically. The effects of the wave amplitude on heat transfer, pressure drop, and thermal resistance are also observed. Results show that for removing an identical heat load, the overall thermal resistance of the single-layer wavy microchannel heat sink decreases with increasing volumetric flow rate, but the pressure drop is increased greatly. At the same flow rate, the double-layer wavy microchannel heat sinks can reduce not only the pressure drop but also the overall thermal resistance compared to the single-layer wavy microchannel heat sinks. In addition to the overall thermal resistance, other criteria for evaluation of the overall thermal performance, e.g., (Nu/Nu0 )/(f/f 0 ), and (Nu/Nu0 )/(f/f 0 ) 1/3 , are applied and some controversial results are obtained.

Experimental investigations of flow boiling heat transfer and pressure drop in straight and expanding microchannels – A comparative study

Flow boiling experiments were conducted in straight and expanding microchannels with similar dimensions and operating conditions. Deionized water was used as the coolant. The test vehicles were made from copper with a footprint area of 25 mm × 25 mm. Microchannels having nominal width of 300 μm and a nominal aspect ratio of 4 were formed by wire cut Electro Discharge Machining process. The measured surface roughness (Ra) was about 2.0 μm. To facilitate easier comparison with the straight microchannels and also to simplify the method of fabrication, the expanding channels were formed with the removal of fins at selected location from the straight microchannel design, instead of using a diverging channel. Tests were performed on both the microchannels over a range of mass fluxes, heat fluxes and an inlet temperature of 90 °C. It was observed that the two-phase pressure drop across the expanding microchannel heat sink was significantly lower as compared to its straight counterpart. The pressure drop and wall temperature fluctuations were seen reduced in the expanding microchannel heat sink. It was also noted that the expanding microchannel heat sink had a better heat transfer performance than the straight microchannel heat sink, under similar operating conditions. This phenomenon in expanding microchannel heat sink, which was observed in spite of it having a lower convective heat transfer area, is explained based on its improved flow boiling stability that reduces the pressure drop oscillations, temperature oscillations and hence partial dry out.

Experimental investigation of flow boiling characteristics in a cross-linked microchannel heat sink

This paper experimentally investigates flow boiling characteristics in a cross-linked microchannel heat sink at low mass fluxes and high heat fluxes. The heat sink consists of 45 straight microchannels each with a hydraulic diameter of 248 μm and heated length of 16 mm. Three cross-links, of width 500 μm, are introduced in the present microchannel heat sink to achieve better temperature uniformity and to avoid flow mal-distribution. Flow visualization, flow instability, two-phase pressure drop, and two-phase heat transfer measurements are conducted using the dielectric coolant FC-72 over a range of heat flux from 7.2 to 104.2 kW/m 2, mass flux from 99 to 290 kg/m 2 s, and exit quality from 0.01 to 0.71. Thermochromic liquid crystals are used in the present study as full-field surface temperature sensors to map the temperature distribution on the heat sink surface. Flow visualization studies indicate that the observed flow regime is primarily slug. Visual observations of flow patterns in the cross-links demonstrate that bubbles nucleate and grow rapidly on the surface of the cross-links and in the tangential direction at the microchannels’ entrance due to the effect of circulations generated in those regions. The two-phase pressure drop strongly increases with the exit quality, at x e,o < 0.3, and the two-phase frictional pressure drop increases by a factor of 1.6–2 compared to the straight microchannel heat sink. The flow boiling heat transfer coefficient increases with increasing exit quality at a constant mass flux, which is caused by the dominance of the nucleation boiling mechanism in the cross-link region.