Convective Boiling in Parallel Micro-Channels

2003 ◽  
Author(s):  
G. Hetsroni ◽  
D. Klein ◽  
A. Mosyak ◽  
Z. Segal ◽  
E. Pogrebnyak
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Heat Sinks ◽  
Micro Channel ◽  
Clear Water ◽  
Two Phase ◽  
Convective Boiling ◽  
Flow Rates ◽  
Micro Channels ◽  
Infrared Radiometry

Experiments were performed with clear water and with surfactant flowing in parallel triangular micro-channels. The study is based on systematic measurements of temperature and flow pattern by infrared radiometry and high-speed digital video imaging. Different flow patterns were observed simultaneously in various micro-channels at a fixed value of water or surfactant flow rates. Depending on flow and heat flux, pressure and temperature instabilities in the heated micro-channels were studied. This work develops a practical modeling approach for two-phase micro-channel heat sinks and considers also effect of surfactant on convective boiling in micro-channels.

Transport Phenomena in Two-Phase Micro-Channel Heat Sinks

2002 ◽  
Author(s):  
Weilin Qu ◽  
Issam Mudawar
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Hydrodynamic Instability ◽  
Transport Phenomena ◽  
Flow Patterns ◽  
Heat Sinks ◽  
Micro Channel ◽  
Two Phase ◽  
Convective Boiling ◽  
Micro Channels

The design and reliable operation of a two-phase micro-channel heat sink require a fundamental understanding of the complex transport phenomena associated with convective boiling in small, parallel coolant passages. This understanding is the primary goal of this paper. This goal is realized by exploring the following aspects of boiling in micro-channels: hydrodynamic instability, two-phase flow patterns, pressure drop, and convective boiling heat transfer. High-speed photographic methods were used to determine dominant flow patterns and explore as well as characterize hydrodynamic instabilities. Two types of dynamic instability were identified, a severe pressure drop oscillation and a mild parallel channel instability, and a simple method is recommended to completely suppress the former. Predictions of three popular two-phase pressure drop models and correlations were compared to micro-channel water data, and only a separated flow (Lockhart-Martinelli) correlation based on the assumption of laminar flow in both phases gave acceptable predictions. Several popular heat transfer correlations were also examined and deemed unsuitable for micro-channel heat sinks because all these correlations are based on turbulent flow assumptions, and do not capture the unique features of micro-channel flow such as abrupt transition to slug flow, hydrodynamic instability, and high droplet entrainment in the annular regime. These findings point to the need for further study of boiling behavior and new predictive tools specifically tailored to micro-channel heat sinks.

Transport Phenomena in Two-Phase Micro-Channel Heat Sinks

2004 ◽  
Vol 126 (2) ◽  
pp. 213-224 ◽  
Author(s):  
Weilin Qu ◽  
Issam Mudawar
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Hydrodynamic Instability ◽  
Transport Phenomena ◽  
Flow Patterns ◽  
Heat Sinks ◽  
Micro Channel ◽  
Two Phase ◽  
Convective Boiling ◽  
Micro Channels

The design and reliable operation of a two-phase micro-channel heat sink require a fundamental understanding of the complex transport phenomena associated with convective boiling in small, parallel coolant passages. This understanding is the primary goal of this paper. This goal is realized by exploring the following aspects of boiling in micro-channels: hydrodynamic instability, two-phase flow patterns, pressure drop, and convective boiling heat transfer. High-speed photographic methods were used to determine dominant flow patterns and explore as well as characterize hydrodynamic instabilities. Two types of dynamic instability were identified, a severe pressure drop oscillation and a mild parallel channel instability, and a simple method is recommended to completely suppress the former. Predictions of three popular two-phase pressure drop models and correlations were compared to micro-channel water data, and only a separated flow (Lockhart-Martinelli) correlation based on the assumption of laminar flow in both phases gave acceptable predictions. Several popular heat transfer correlations were also examined and deemed unsuitable for micro-channel heat sinks because all these correlations are based on turbulent flow assumptions, and do not capture the unique features of micro-channel flow such as abrupt transition to slug flow, hydrodynamic instability, and high droplet entrainment in the annular regime. These findings point to the need for further study of boiling behavior and new predictive tools specifically tailored to micro-channel heat sinks.

Two-Phase Flow and Heat Transfer in Rectangular Micro-Channels

2003 ◽  
Author(s):  
Weilin Qu ◽  
Seok-Mann Yoon ◽  
Issam Mudawar
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Flow Pattern ◽  
Two Phase Flow ◽  
Flow Patterns ◽  
Heat Sinks ◽  
Phase Flow ◽  
Micro Channel ◽  
Two Phase ◽  
Micro Channels

Knowledge of flow pattern and flow pattern transitions is essential to the development of reliable predictive tools for pressure drop and heat transfer in two-phase micro-channel heat sinks. In the present study, experiments were conducted with adiabatic nitrogen-water two-phase flow in a rectangular micro-channel having a 0.406 × 2.032 mm cross-section. Superficial velocities of nitrogen and water ranged from 0.08 to 81.92 m/s and 0.04 to 10.24 m/s, respectively. Flow patterns were first identified using high-speed video imaging, and still photos were then taken for representative patterns. Results reveal that the dominant flow patterns are slug and annular, with bubbly flow occurring only occasionally; stratified and churn flow were never observed. A flow pattern map was constructed and compared with previous maps and predictions of flow pattern transition models. Annual flow is identified as the dominant flow pattern for conditions relevant to two-phase micro-channel heat sinks, and forms the basis for development of a theoretical model for both pressure drop and heat transfer in micro-channels. Features unique to two-phase micro-channel flow, such as laminar liquid and gas flows, smooth liquid-gas interface, and strong entrainment and deposition effects are incorporated into the model. The model shows good agreement with experimental data for water-cooled heat sinks.

Film Thickness for Two Phase Flow in a Microchannel

2006 ◽  
Author(s):  
Ronan Grimes ◽  
Colin King ◽  
Edmond Walsh
Keyword(s):  
High Speed ◽  
Channel Wall ◽  
Experimental Program ◽  
Successful Implementation ◽  
Phase Flow ◽  
High Speed Camera ◽  
Micro Channel ◽  
Two Phase ◽  
Flow Rates ◽  
Carrier Fluid

The issue of contamination of micro channel surfaces by bio fluids is a significant impediment to the development of many biomedical devices. A solution to this problem is the use of a carrier fluid, which segments the bio fluid and forms a thin film between the bio fluid and the channel wall. A number of issues need to be addressed for the successful implementation of such a solution. Amongst these is the prediction of the thickness of the film of carrier fluid which forms between the bio sample and the channel wall. The Bretherton and Taylor laws relate the capillary number to the thickness of this film. This paper investigates the validity of these laws through an extensive experimental program in which a number of potential carrier fluids were used to segment aqueous droplets over a range of flow rates. The aqueous plugs were imaged using a high speed camera and their velocities were measured. Film thicknesses were calculated from the ratio of the velocity of the carrier fluid to the velocity of the aqueous plug. The paper concludes that significant discrepancies exist between measured film thicknesses and those predicted by the Bretherton and Taylor laws.

Porous Media Modeling of Two-Phase Micro-Channel Cooling of Electronic Chips With Non-Uniform Power Distribution

2013 ◽  
Author(s):  
Hua Zhang ◽  
Jun Jie Liu ◽  
Yubai Li ◽  
S. C. Yao
Keyword(s):  
Porous Media ◽  
Mass Flow ◽  
Flow Rate ◽  
Power Distribution ◽  
Heat Load ◽  
Computation Time ◽  
Micro Channel ◽  
Two Phase ◽  
Flow Rates ◽  
Micro Channels

Compared to single phase heat transfer, two-phase micro-channel heat sinks utilize latent heat to reduce the needed flow rate and maintaining a rather uniform temperature close to the boiling temperature. The challenge in the application of cooling for electronic chips is the necessity of modeling a large number of micro channels using large number of meshes and extensive computation time. In the present study, a modified porous media method modeling of two phase flow in micro-channels is performed. Compared with conjugate CFD method, it saves computation effort and provides a more convenient means to perform optimization of channel geometry. The porous media simulation is applied to a real chip. The channels of high heat load will have higher qualities, larger flow resistances and lower flow rates. At a constant available pressure drop over the channels, the low heat load channels show much higher mass flow rates than needed. To avoid this flow mal-distribution, the channel widths on a chip are adjusted to ensure the exit qualities and mass flow rate of channels are more uniform. As a result, the total flow rate on the chip is drastically reduced, and the temperature gradient is also minimized. However, it only gives a relatively small reduction on the maximum surface temperature of chip.

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