Design of a Microscale Breather for Two-Phase Thermal Management Devices

2008 ◽  
Author(s):  
B. R. Alexander ◽  
E. N. Wang
Keyword(s):  
Phase Change ◽  
Removal Rate ◽  
Heat Removal ◽  
Design Guidelines ◽  
Heat Sinks ◽  
Two Phase ◽  
Cross Sectional ◽  
Optical Visualization ◽  
Gas Removal ◽  
Microchannel Heat Sinks

Two-phase microchannels promise an efficient method to dissipate heat from high performance electronic systems by utilizing the latent heat of vaporization during the phase-change process. However, phase-change in microchannel heat sinks leads to challenges that are not present in macroscale systems due to the increasing importance of surface tension and viscous forces. In particular, flow instabilities often occur during the boiling process, which lead to liquid dry-out in the microchannels and severely limits the heat removal capabilities of the system. We propose a microscale breather device consisting of an array of hydrophobic breather ports which allow vapor bubbles to escape from the microchannels to improve flow stability. In this study, we use the combination of microfabricated structures and surface chemistry to separate vapor from the liquid flow. We designed test devices that allow for cross-sectional optical visualization to better understand the governing parameters of a breather design with high vapor removal efficiencies and minimal liquid leakage. We examined breather devices with average liquid velocities ranging from 0.5 cm/s to 4 cm/s and breather vacuum levels between 1 kPa and 9 kPa on the maximum gas removal rate through the breather. We demonstrated successful breather performance. In addition, a model was developed that offers design guidelines for future integrated breathers in microchannel heat sinks. The breathers also have significant promise for other microscale systems, such as micro-fuel cells, where liquid-vapor separation can significantly enhance system performance.

Control of Instabilities in Two-Phase Microchannel Flow Using Artificial Nucleation Sites

2007 ◽  
Author(s):  
Rory J. Jones ◽  
Daniel T. Pate ◽  
Sushil H. Bhavnani
Keyword(s):  
Phase Change ◽  
Flow Regime ◽  
Flow Instability ◽  
Heat Removal ◽  
Nucleate Boiling ◽  
Heat Sinks ◽  
Dielectric Fluid ◽  
High Speed Photography ◽  
Unstable Flow ◽  
Microchannel Heat Sinks

Microchannel heat sinks offer the promise of an effective and compact technique for heat dissipation from high-powered microelectronics. When combined with phase-change mechanisms, it is possible to extend heat removal capabilities over several future technology nodes on electronics roadmaps. Many studies have documented instabilities in phase-change flows associated with uncontrolled transition from bubbly flows to slug flows at fairly low void fraction. The phase-change thermal transport reported here was carried out on microchannel heat sinks etched in silicon and cooled by the dielectric fluid FC72. Microchannels with a hydraulic diameter of 253 microns were studied. The base of each microchannel is augmented with 20-micron cavities to trigger controlled nucleation activity and help control large-scale instabilities reported in the literature. The cavities significantly impact the flow regime by promoting stable nucleate boiling. Temperature and pressure measurements show stable flow regimes under all combinations leading to saturated channel exit conditions and even for subcooled exit conditions at low-to-moderate heat fluxes. These measurements are confirmed by high-speed photography. Instabilities observed were confined to a sub-set of the subcooled exit flow regime. Within this region, an analysis of the data shows that the frequency of the flow instability is between 8–14 Hz. Instability maps demarcating regions of stable and unstable flow are presented as functions of mass flux (535–2140 kg/m2–s), and inlet subcooling (5–25°C).

Modeling and Numerical Simulations of Vapor-Liquid Flow and Heat Transfer Within Microchannel Heat Sinks

2012 ◽  
Author(s):  
Chengyun Xin ◽  
Jianhua Wang ◽  
Jianheng Xie ◽  
Yuee Song
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Solid Wall ◽  
Heat Fluxes ◽  
Heat Sinks ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Flowing Fluid ◽  
Liquid Coolant ◽  
Microchannel Heat Sinks

Microchannel heat sinks have demonstrated the ability to dissipate large amounts of heat flux. This ability can be strongly enhanced by phase change of a liquid coolant. This paper numerically simulates the processes of liquid coolant flow, heat absorption and phase change within a microchannel, which is heated at one side by given heat fluxes. The two-phase flow model widely used in the investigations on heat and mass transfer within porous media is firstly introduced into microchannnel heat sinks by this paper. Experiential equations of the heat transfer coefficients in single phase and boiling region within microchannels are employed to calculate the convective heat exchange between solid wall and flowing fluid by an iterative process. The numerical results of pressure and temperature distributions obtained at different conditions are exhibited and analyzed. The results indicated that the trends predicted by this approach agree well with the previous references. Therefore the modeling is validated in some sense. At the same time, two phenomena, countercurrent flow in two-phase region and special pressure variations near the transition point, are exhibited.

Next Generation Devices for Electronic Cooling

2003 ◽  
Author(s):  
Ralph L. Webb
Keyword(s):  
Heat Sink ◽  
New Technology ◽  
Heat Removal ◽  
Cost Effective ◽  
Heat Sinks ◽  
Working Fluid ◽  
Two Phase ◽  
Conventional Technology ◽  
Removal Technology ◽  
R Value

Conventional technology to cool desktop computers and servers is that of the “direct heat removal” heat sink, which consists of a heat sink/fan mounted on the CPU. Although this is a very cost effective solution, it is nearing its end of life. This is because future higher power CPUs will require a lower R-value than can be provided by this technology, within current size and fan limits. This paper discusses new technology that uses “indirect heat removal” technology, which involves use of a single or two-phase working fluid to transfer heat from the hot source to an ambient heat sink. This technology will support greater heat rejection than is possible with the “direct heat removal” method. Further, it will allow use of higher performance air-cooled ambient heat sinks than are possible with the “direct heat removal” heat sink. A concern of the indirect heat removal technology is the possibility that it may be orientation sensitive. This paper identifies preferred options and discusses the degree to which they are (or or not) orientation sensitive. It should be possible to attain an R-value of 0.12K/W at the balance point on the fan curve.

Microchannel Size Effects on Two-Phase Local Heat Transfer and Pressure Drop in Silicon Microchannel Heat Sinks With a Dielectric Fluid

2007 ◽  
Author(s):  
Tannaz Harirchian ◽  
Suresh V. Garimella
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Flow Boiling ◽  
Heat Sinks ◽  
Dielectric Fluid ◽  
Two Phase ◽  
Microchannel Heat Sinks ◽  
The Heat Transfer Coefficient

Two-phase heat transfer in microchannels can support very high heat fluxes for use in high-performance electronics-cooling applications. However, the effects of microchannel cross-sectional dimensions on the heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer in microchannel heat sinks. The effect of channel size on the heat transfer coefficient and pressure drop is studied for mass fluxes ranging from 250 to 1600 kg/m2s. The test sections consist of parallel microchannels with nominal widths of 100, 250, 400, 700, and 1000 μm, all with a depth of 400 μm, cut into 12.7 mm × 12.7 mm silicon substrates. Twenty-five microheaters embedded in the substrate allow local control of the imposed heat flux, while twenty-five temperature microsensors integrated into the back of the substrates enable local measurements of temperature. The dielectric fluid Fluorinert FC-77 is used as the working fluid. The results of this study serve to quantify the effectiveness of microchannel heat transport while simultaneously assessing the pressure drop trade-offs.

Numerical Investigation on Pool Boiling Over a Vertical Tube Coupled With In-Tube Condensation

2019 ◽  
Author(s):  
Shuai Ren ◽  
Wenzhong Zhou
Keyword(s):  
Heat Transfer ◽  
Phase Change ◽  
Boiling Heat Transfer ◽  
Power Plants ◽  
Pool Boiling ◽  
Heat Removal ◽  
Vertical Tube ◽  
Two Phase ◽  
Maximum Heat ◽  
Maximum Heat Transfer

Abstract Pool boiling and in-tube condensation phenomena have been investigated intensively during the past decades, due to the superior heat transfer capacity of the phase change process. In passive heat removal heat exchangers of nuclear power plants, the two phase-change phenomena usually occur simultaneously on both sides of the tube wall to achieve the maximum heat transfer efficiency. However, the studies on the effects of in-tube condensation on external pool boiling heat transfer are very limited, especially in numerical computation aspect. In the present study, the saturated pooling boiling over a vertical tube under the influences of in-tube steam condensation is investigated numerically. The Volume of Fluid (VOF) interface tracking method is employed based on the 2D axisymmetric Euler-Euler multiphase frame. The phase change model combining with a mathematical smoothing algorithm and a temporal relaxation procedure has been implemented into CFD platform by user defined functions (UDFs). The two-phase flow pattern and bubble behavior have been analyzed. The effects of inlet steam mass flow rate on boiling heat transfer are discussed.

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