The Effects of Cross Links on Adiabatic Two-Phase Flow Characteristics in Scaled Microchannel Heat Sinks

2007 ◽  
Author(s):  
Minh N. Dang ◽  
Ibrahim G. Hassan
Keyword(s):  
Flow Regime ◽  
Single Channel ◽  
Flow Characteristics ◽  
Flow Distribution ◽  
Intermittent Flow ◽  
Straight Channel ◽  
Channel Design ◽  
Two Phase ◽  
Cross Links ◽  
The Cross

The effects of cross-links, introduced in the channel core of an array of parallel scaled microchannels, were investigated by comparison of the flow distribution in six different multichannel configurations. A standard straight channel test section and five other test sections, which incorporated cross-links were used. One case includes two cross-links located at 1/3 and 2/3’s of the channel length, with their width varied by one, two, and three times the channel width. Whereas, four and six cross-links were used for the other case. All test sections had 45 parallel rectangular channels, with a hydraulic diameter of 1.59 mm, and were fabricated from clear acrylic to enhance flow visualization. The flow distribution was monitored at four select channels. The working mixture was air and water with superficial velocities ranging from 0.03 to 9.93 m/s, and 0.04 to 0.83 m/s, respectively. This corresponds to an observed range of flow quality between 0 and 0.25, whereby the mass flux range is from 42 kg/m2s to 834 kg/m2s. The cross-linked designs permit fluid communication between channels, and the results showed that there is a significant impact on flow distribution when compared to the straight channel design. This is due to flow sharing between neighboring channels. Flow patterns were presented in terms of fractional time function, and provided further insight to flow characteristics. Comparing with a single channel flow regime map, the expected intermittent flow regime was observed 84% to 90% of the time for the cross-linked designs, whereas 65% to 80% of that for the straight channel design.

scholarly journals Numerically Investigating the Effects of Cross-Links in Scaled Microchannel Heat Sinks

2008 ◽  
Vol 130 (12) ◽  
Author(s):  
Minh Dang ◽  
Ibrahim Hassan ◽  
Sung In Kim
Keyword(s):  
Pressure Drop ◽  
Experimental Studies ◽  
Three Dimensional ◽  
Flow Distribution ◽  
Heat Sinks ◽  
Straight Channel ◽  
Two Phase ◽  
Microchannel Heat Sinks ◽  
Cross Links ◽  
Channel Configuration

Thermal management as a method of heightening performance in miniaturized electronic devices using microchannel heat sinks has recently become of interest to researchers and the industry. One of the current challenges is to design heat sinks with uniform flow distribution. A number of experimental studies have been conducted to seek appropriate designs for microchannel heat sinks. However, pursuing this goal experimentally can be an expensive endeavor. The present work investigates the effect of cross-links on adiabatic two-phase flow in an array of parallel channels. It is carried out using the three-dimensional mixture model from the computational fluid dynamics software, FLUENT 6.3. A straight channel and two cross-linked channel models were simulated. The cross-links were located at 1/3 and 2/3 of the channel length, and their widths were one and two times larger than the channel width. All test models had 45 parallel rectangular channels, with a hydraulic diameter of 1.59 mm. The results showed that the trend of flow distribution agrees with experimental results. A new design, with cross-links incorporated, was proposed and the results showed a significant improvement of up to 55% on flow distribution compared with the standard straight channel configuration without a penalty in the pressure drop. Further discussion about the effect of cross-links on flow distribution, flow structure, and pressure drop was also documented.

Flow Distribution and Slug Flow Characteristics for Hydraulically Equilibrium Air-Water Two-Phase Flows in a Vertical 2×3 Rod Channel

2003 ◽  
Author(s):  
Keiko Kano ◽  
Michio Sadatomi ◽  
Akimaro Kawahara ◽  
Tsukasa Kuno
Keyword(s):  
Flow Characteristics ◽  
Prediction Method ◽  
Flow Distribution ◽  
Bubble Velocity ◽  
Accurate Estimation ◽  
Two Phase ◽  
Taylor Bubble ◽  
Rod Bundle ◽  
Superficial Gas Velocity ◽  
Churn Flow

To complete subchannel analysis for predicting thermal-hydraulic behavior of coolant in a BWR rod bundle channel, accurate estimation of fluid transfer between subchannels is necessary. In order to validate a prediction method, flow distributions data of gas and liquid phases are essential. But, such data reported so far are limited to those in a two-subchannel system alone. Then we have measured flow distributions of both phases and Taylor bubble velocity in a multi-subchannel system as called 2×3 rod bundle channel. It has been found that flow distributions of gas and liquid in bubble and annular flows under a hydraulically equilibrium flow condition are close to those of single-phase flow, but in slug-churn flow the distributions are different. In slug-churn flow, both superficial gas velocity and Taylor bubble velocity are higher in larger subchannel. These experimental data are presented and discussed in this paper.

Computer Programming Simulation of Two-Phase Flow Pipelines Using Mechanistic and Semi-Empirical Models

2006 ◽  
Author(s):  
Ahmad Fazeli ◽  
Ali Vatani
Keyword(s):  
Pressure Drop ◽  
Flow Regime ◽  
Two Phase Flow ◽  
Empirical Models ◽  
Intermittent Flow ◽  
Phase Flow ◽  
Liquid Holdup ◽  
Two Phase ◽  
Segregated Flow ◽  
Semi Empirical

Two-phase flow pipelines are utilized in simultaneous transferring of liquid and gas from reservoir fields to production units and refineries. In order to obtain the hydraulic design of pipelines, pressure drop and liquid holdup were calculated following pipeline flow regime determination. Two semi-empirical and mechanistical models were used. Empirical models e.g. Beggs & Brill, 1973, are only applicable in certain situations were pipeline conditions are adaptable to the model; therefore we used the Taitel & Dukler, 1976, Baker et al., 1988, Petalas & Aziz, 1998, and Gomez et al., 1999, mechanistical models which are practical in more extensive conditions. The FLOPAT code was designed and utilized which is capable of the determining the physical properties of the fluid by either compositional or non-compositional (black oil) fluid models. It was challenged in various pipeline positions e. g. horizontal, vertical and inclined. Specification of the flow regime and also pressure drop and liquid holdup could precisely be calculated by mechanistical models. The flow regimes considered in the pipeline were: stratified, wavy & annular (Segregated Flow), plug & slug (Intermittent Flow) and bubble & mist (Distributive Flow). We also compared output results against the Stanford Multiphase Flow Database which were used by Petalas & Aziz, 1998, and the effect of the flow rate, pipeline diameter, inclination, temperature and pressure on the flow regime, liquid holdup and pressure drop were studied. The outputs (flow regime, pressure drop and liquid holdup) were comparable with the existing pipeline data. Moreover, by this comparison one may possibly suggest the more suitable model for usage in a certain pipeline.

Two-Phase Flow Regime Transitions in Microchannel Tubes: The Effect of Hydraulic Diameter

2000 ◽  
Author(s):  
John W. Coleman ◽  
Srinivas Garimella
Keyword(s):  
Flow Regime ◽  
Mass Flux ◽  
Two Phase Flow ◽  
Small Diameter ◽  
Flow Patterns ◽  
Hydraulic Diameter ◽  
Intermittent Flow ◽  
Phase Flow ◽  
Two Phase ◽  
Flow Mechanisms

Abstract An experimental investigation of two-phase flow mechanisms during condensation of refrigerant R134a in small diameter round and rectangular tubes was conducted. A 4.91 mm round tube, and four round tubes with hydraulic diameters ranging from 1 mm – 4 mm were studied to characterize the influence of tube miniaturization on the flow mechanisms. For each tube under consideration, flow mechanisms were recorded over the entire range of qualities 0 < x < 1, and for five different mass fluxes between 150 kg/m2-s and 750 kg/m2-s. Approximately 50 data points were recorded for each tube to obtain a comprehensive understanding of the effects of geometry, mass flux and quality on the phase-change flow mechanisms. The flow mechanisms were categorized into four different flow regimes: intermittent flow, wavy flow, annular flow, and dispersed flow. In addition, the large amount of data over a wide range of test conditions enabled the delineation of several different flow patterns within each flow regime, which provides a clearer understanding of the different modes of two-phase flow. Transition lines between the respective flow patterns and regimes on these maps were established based on the experimental data. It was found that the intermittent flow regime becomes larger as the tube hydraulic diameter is decreased. Also, the size of the wavy flow regime decreases for the small diameter tubes, and disappears completely for the 1 × 1 mm square tube. These maps and transition lines can be used to predict the flow pattern or regime that will be established for a given mass flux, quality and tube geometry.

Analysis of Fluid Flow and Heat Transfer in Corrugated Perforated Plate Fin Heat Sinks

2021 ◽  
pp. 1-30
Author(s):  
Shripad A Upalkar ◽  
Saksham Gakhar ◽  
Shankar Krishnan
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Heat Dissipation ◽  
Single Channel ◽  
Perforated Plate ◽  
Flow Characteristics ◽  
Flow Distribution ◽  
Optimum Number ◽  
Perforated Plates ◽  
Optimum Diameter

Abstract This paper reports a mathematical model for predicting the fluid and heat flow characteristics of a Z-shaped corrugated perforated plate heat sink. Experiments were carried out to validate overall pressure drop as well as heat transfer predictions. A two-pronged approach was undertaken to design a corrugated perforated fin geometry: (a) macroscopic packaging, where the flow is distributed into conduits before being fed into perforated plates, and (b) microscopic design, where the pores are sized to maximize heat dissipation. A methodology typically used for predicting flow maldistribution is extended for packaging porous perforated plates in the macroscopic approach. An illustrative study is carried that estimates the optimum number of porous perforated plate fins that can be packaged within a given volume under fixed pressure drop constraint. In the microscopic approach, an order of magnitude analysis was carried out to decide the optimum diameter to maximize the heat transfer rate and expression for optimum diameter, and maximum achievable heat flux is proposed. Numerical simulations were carried out by considering full perforated plate porous fin geometry and single-channel geometry, and good agreement in their results was found. Finally, this study elaborates on the importance of achieving uniform flow distribution across the porous perforated plate fins.

The Effects of Inlet Geometry and Gas-Liquid Mixing on Two-Phase Flow in Microchannels

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
M. Kawaji ◽  
K. Mori ◽  
D. Bolintineanu
Keyword(s):  
Void Fraction ◽  
Two Phase Flow ◽  
Flow Characteristics ◽  
Flow Patterns ◽  
Intermittent Flow ◽  
Internal Diameter ◽  
Inlet Section ◽  
Phase Flow ◽  
Two Phase ◽  
Inlet Geometry

The effects of gas-liquid inlet geometry and mixing method on adiabatic gas-liquid two-phase flow in a microchannel of 100 μm diameter have been investigated using a T-junction inlet with the same internal diameter as the microchannel. Two-phase flow patterns, void fraction, and friction pressure drop data obtained with the T-junction inlet were found to be significantly different from those obtained previously with a reducer inlet. For the T-junction inlet, the two-phase flow patterns in the microchannel were predominantly intermittent flows with short gas and liquid plugs/slugs flowing with nearly equal velocities. The void fraction data then conformed nearly to that of a homogeneous flow model, and the two-phase friction multiplier data could be described by the Lockhart–Martinelli correlation applicable to larger channels. However, when a reducer inlet was used previously and the diameter of the inlet section was much larger than that of the microchannel, an intermittent flow of long gas slugs separated by long liquid slugs became prevalent and the void fraction decreased to values far below the homogeneous void fraction. The differences in the two-phase flow characteristics between a T-junction inlet and reducer inlet were attributed to the differences in the gas bubble/slug generation mechanisms.

Modeling of Pressure Drop During Condensation in Circular and Noncircular Microchannels

2008 ◽  
Vol 131 (1) ◽  
Author(s):  
Akhil Agarwal ◽  
Srinivas Garimella
Keyword(s):  
Pressure Drop ◽  
Flow Regime ◽  
Intermittent Flow ◽  
Condensation Process ◽  
International Institute ◽  
Two Phase ◽  
Pressure Drops ◽  
Smooth Transitions ◽  
Phase Pressure ◽  
Circular Microchannels

This paper presents a multiple flow-regime model for pressure drop during condensation of refrigerant R134a in horizontal microchannels. Condensation pressure drops measured in two circular and six noncircular channels ranging in hydraulic diameter from 0.42mmto0.8mm are considered here. For each tube under consideration, pressure drop measurements were taken over the entire range of qualities from 100% vapor to 100% liquid for five different refrigerant mass fluxes between 150kg∕m2s and 750kg∕m2s. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to assign the applicable flow regime to the data points. Garimella et al. (2005, “Condensation Pressure Drop in Circular Microchannels,” Heat Transfer Eng., 26(3) pp. 1–8) reported a comprehensive model for circular tubes that addresses the progression of the condensation process from the vapor phase to the liquid phase by modifying and combining the pressure drop models for intermittent (Garimella et al., 2002, “An Experimentally Validated Model for Two-Phase Pressure Drop in the Intermittent Flow Regime for Circular Microchannels,” ASME J. Fluids Eng., 124(1), pp. 205–214) and annular (Garimella et al., 2003, “Two-Phase Pressure Drops in the Annular Flow Regime in Circular Microchannels,” 21st IIR International Congress of Refrigeration, International Institute of Refrigeration, p. ICR0360) flows reported earlier by them. This paper presents new condensation pressure drop data on six noncircular channels over the same flow conditions as the previous work on circular channels. In addition, a multiple flow-regime model similar to that developed earlier by Garimella et al. for circular microchannels is developed here for these new cross sections. This combined model accurately predicts condensation pressure drops in the annular, disperse-wave, mist, discrete-wave, and intermittent flow regimes for both circular and noncircular microchannels of similar hydraulic diameters. Overlap and transition regions between the respective regimes are also addressed to yield relatively smooth transitions between the predicted pressure drops. The resulting model predicts 80% of the data within ±25%. The effect of tube shape on pressure drop is also demonstrated.

Void Fraction, Pressure Drop, and Flow Pattern Behavior for Two-Phase Non-Newtonian Slug Flow

2021 ◽  
Author(s):  
Faraj Ben Rajeb ◽  
Syed Imtiaz ◽  
Yan Zhang ◽  
Amer Aborig ◽  
Mohamed M. Awad ◽  
...  
Keyword(s):  
Pressure Drop ◽  
Flow Pattern ◽  
Flow Regime ◽  
Void Fraction ◽  
Slug Flow ◽  
Two Phase Flow ◽  
Flow Patterns ◽  
Intermittent Flow ◽  
Phase Flow ◽  
Two Phase

Abstract Slug flow is one of the most common flow patterns in non-Newtonian two-phase flow in pipes. It is a very common occurrence in gas-liquid two-phase flow in the pipe. Usually, it is an unfavorable flow pattern due to its unsteady nature, intermittency as well as high pressure drop. The differences between slug flow and elongated bubble flow are not clear because usually these two types of flow combined under one flow category. In general, these two-phase flow regimes are commonly defined as intermittent flow. In the present study, pressure gradient, and wave behavior in slug flow have been investigated depending on experimental work. In addition, void fraction has been estimated regarding available superficial liquid and gas velocities. The experimental records of superficial velocities of gas and liquid for slug flow and other flow patterns is used to create flow regime map for the gas non-Newtonian flow system. The effect of investigated flow regime velocities for non-Newtonian/gas flow on pressure drop and void fraction is reported. Pressure drop has been discovered to be reduced in slug flow more than other flow patterns due to high shear thinning behavior.

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