Suppression of Boiling Flow Oscillations in Parallel Microchannels by Inlet Restrictors

2005 ◽  
Vol 128 (3) ◽  
pp. 251-260 ◽  
Author(s):  
Ali Koşar ◽  
Chih-Jung Kuo ◽  
Yoav Peles
Keyword(s):  
Dimensionless Parameter ◽  
Flow Instability ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Mass Fluxes ◽  
Boiling Flow ◽  
Flow Oscillations ◽  
Parallel Channels ◽  
Geometrical Effects ◽  
Parallel Microchannels

Geometrical effects of MEMS-based microfabricated inlet orifices on the suppression of parallel channel and upstream compressible volume instabilities commonly exhibited during flow boiling in parallel microchannels have been investigated. The heat fluxes at the onset of unstable boiling have been obtained over effective heat fluxes ranging from 9 to 614W∕cm2 and mass fluxes from 115to389kg∕m2s. A dimensionless parameter M, which accounts for the pressure drop increase imposed by the inlet restrictors, has been used to correlate the extent of flow instability suppression. It has been shown that the onset of unstable boiling asymptotically increases with M. At sufficiently high M values, parallel channels and upstream compressible volume instabilities are completely eradicated although it gives way to another instability to develop, namely, the critical heat flux conditions. A correlation has been developed in terms of M to predict the conditions leading to unstable boiling.

Experimental Investigation on Flow Instability in a Single Vertical Microtube

2015 ◽  
Author(s):  
Qian You ◽  
Ibrahim Hassan ◽  
Lyes Kadem
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Mass Flux ◽  
Flow Instability ◽  
Flow Boiling ◽  
Flow Direction ◽  
Density Wave ◽  
Mass Fluxes ◽  
Flow Oscillations ◽  
Flow Directions

The experiments are conducted to study the flow boiling instability in a single microtube with 0.889 mm hydraulic diameter in vertical upward and downward flow directions (VU and VD). The subcooled dielectric liquid FC-72 is driven at mass fluxes varying from 700 to 1400 kg/m2·s, and the heat flux uniformly applied on the microtube surface is up to 9.6 W/cm2. The onsets of flow oscillations (OFIs) in both flow directions are observed. Their oscillation types and characteristics are presented as well. The effects of mass flux and heat flux on flow instability in vertical flow directions are discussed. The results show that as the mass flux increases, the OFI occurrence is postponed, and the compounded oscillation types (Ledinegg, pressure drop and density wave oscillations) turn to pressure drop type dominant. At low mass fluxes, the OFI appears earlier in VD than in VU due to the buoyancy force impeded the bubble discharging. As the mass flux increases, the OFI appearance in VD is close to the ones in VU and its flow oscillations tend to be re-stabilized. After OFIs appeared at a given mass flux, with more heat flux added, the density wave oscillation type in VU becomes more active. However, at a constant mass flux, as the heat flux increases, the flow instability in VD becomes “stable” which may be due to the rapid flow pattern change, and this kind of “stable” is not expected because the local dryout may accompany. Hence, the microtube with vertical upward flow direction (VU) performs better from flow boiling instability point of view.

Further Experimental Studies on Flow Boiling Instabilities in Parallel Microchannels

2007 ◽  
Author(s):  
G. D. Wang ◽  
Ping Cheng ◽  
H. Y. Wu
Keyword(s):  
Mass Flux ◽  
Flow Boiling ◽  
Experimental Studies ◽  
Flux Ratio ◽  
Heat Fluxes ◽  
Unstable Flow ◽  
Boiling Regime ◽  
Mass Fluxes ◽  
Parallel Microchannels ◽  
Stable Flow

A simultaneous visualization and measurement study has been carried out to investigate flow boiling instabilities of water in parallel microchannels at various heat fluxes and mass fluxes. Eight parallel silicon microchannels, with an identical trapezoidal cross-section having a hydraulic diameter of 186 μm and a length of 30 mm, were used in this experiment. It was found that the flow boiling pattern depended on the heat to mass flux ratio q/G. Stable flow boiling regimes existed for q/G < 0.96 kJ/kg and q/G > 2.14 kJ/kg whereas unstable flow boiling existed in the range of 0.96 kJ/kg < q/G < 2.14 kJ/kg. At a given heat flux in the unstable flow boiling regime, the mass flux decreases continuously to a constant value after a sufficiently long time, and eventually reaches a stable flow boiling mode. It was found that the physical mechanism for the unstable boiling flow is owing to bubble expansion in both upstream and downstream direction. In particular, the expansion of bubble upstream will increase the upstream wall temperature, and the temperature of entire microchannel will eventually reach a steady state, i.e., a stable flow boiling regime.

Enhanced Flow Boiling of HFE 7000 by Chaotic Mixers in Microchannels

2013 ◽  
Author(s):  
Fanghao Yang ◽  
Mohammad Alwazzan ◽  
Chen Li
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Operating Conditions ◽  
Smooth Wall ◽  
Microchannel Array ◽  
Mass Fluxes ◽  
Center Position ◽  
Parallel Channels ◽  
Herringbone Grooves

Improving mixing is an effective method to enhance flow boiling in microchannels. However, it is challenging to induce since the flow in microchannels is laminar under typical operating conditions. We report that flow boiling of 1-methoxyheptafluoropropane (HFE 7000) in a parallel microchannel array was significantly enhanced by chaotic mixers patterned on the bottom walls. The microchannel array consists of five parallel channels (height, width, length: 250 μm × 220 μm × 10 mm). The chaotic mixers consist of seven cycles with 12 staggered herringbone grooves (50 μm depth and width with 90° between two asymmetric arms) in each cycle. Its asymmetry is defined by the off center position of the apex of the herringbone groove. Compared with a smooth-wall microchannel array with identical channel dimensions, heat transfer coefficient and critical heat flux of flow boiling on HFE 7000 were enhanced up to 45 % and 61 % using chaotic mixer pairs in microchannels. Mass fluxes range from 1000 to 2200 kg/m2-s and wall heat fluxes from 10 to 198 W/cm2.

Flow Boiling of Coolant (HFE-7000) Inside Structured and Plain Wall Microchannels

2009 ◽  
Vol 131 (12) ◽  
Author(s):  
C.-J. Kuo ◽  
Y. Peles
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Mass Fluxes ◽  
Side Walls ◽  
Parallel Microchannels ◽  
The Heat Transfer Coefficient

Flow boiling was experimentally studied using coolant HFE-7000 for two types of parallel microchannels: a plain-wall microchannel and a microchannel with structured reentrant cavities on the side walls. Flow morphologies, boiling inceptions, heat transfer coefficients, and critical heat fluxes were obtained and studied for mass fluxes ranging from G=164 kg/m2 s to G=3025 kg/m2 s and mass qualities (energy definition) ranging from x=−0.25 to x=1. Comparisons of the performance of the enhanced and plain-wall microchannels were carried out. It was found that reentrant cavities were effective in reducing the superheat at the onset of nucleate boiling and increasing the heat transfer coefficient. However, they did not seem to increase the critical heat flux.

An Experimental Study of Bubble Nucleation Frequency in Subcooled Flow Boiling

2010 ◽  
Author(s):  
Guodong Wang
Keyword(s):  
Flow Boiling ◽  
Rectangular Cross Section ◽  
Bubble Nucleation ◽  
Heat Fluxes ◽  
Dimensionless Frequency ◽  
Micro Channel ◽  
Subcooled Flow Boiling ◽  
Nucleation Frequency ◽  
Mass Fluxes ◽  
Subcooled Flow

In this paper, a simultaneous visualization and measurement study have been carried out to investigate bubble nucleation frequency of water in micro-channel at various heat fluxes and mass fluxes. A single micro-channel with an identical rectangular cross-section having a hydraulic of 137 μm and a heating length of 30 mm was used in this experiment. It is shown that the frequency of bubble nucleation increased drastically with the increase of heat flux and was also strongly dependent on the mass flux. A dimensionless frequency of bubble nucleation was correlated in terms of the Boiling number. The predictions of bubble nucleation frequency in the microchannel are found in good agreement with experimental data with a MAE of 10.4%.

Confined Bubble and Heat Transfer during Flow Boiling in a High Aspect Ratio Mini-Channel

2011 ◽  
Vol 312-315 ◽  
pp. 548-553 ◽  
Author(s):  
Yuan Wang ◽  
Khellil Sefiane
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Bubble Growth ◽  
Flow Boiling ◽  
Critical Time ◽  
Heat Fluxes ◽  
Equivalent Radius ◽  
Mass Fluxes ◽  
Transfer Mechanisms ◽  
Working Liquid

Single vapour bubble growth and heat transfer mechanism during flow boiling in a rectangular horizontal mini-channel were experimentally investigated. The hydraulic diameter of the channel was 1454 μm, with an aspect ratio (Win/din) of 10. Degassed FC-72 was used as the working liquid. In this paper, bubble equivalent radius was found to increase linearly till a critical time, beyond which the growth turned into exponential. Bubble growth rate increases with increasing heat flux. Heat transfer mechanisms of the bubble growth at different heat fluxes and mass fluxes were discussed. In addition, the relation between thermal and flow conditions with bubble temporal geometry was explored.

scholarly journals A Study of Critical Heat Flux During Flow Boiling in Microchannel Heat Sinks

2011 ◽  
Vol 134 (1) ◽  
Author(s):  
Tailian Chen ◽  
Suresh V. Garimella
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Flow Rate ◽  
Heat Input ◽  
Heat Sink ◽  
Flow Boiling ◽  
Strong Dependence ◽  
Heat Fluxes ◽  
Abrupt Decrease ◽  
Parallel Microchannels

The cooling capacity of two-phase transport in microchannels is limited by the occurrence of critical heat flux (CHF). Due to the nature of the phenomenon, it is challenging to obtain reliable CHF data without causing damage to the device under test. In this work, the critical heat fluxes for flow boiling of FC-77 in a silicon thermal test die containing 60 parallel microchannels were measured at five total flow rates through the microchannels in the range of 20–80 ml/min. CHF is caused by dryout at the wall near the exit of the microchannels, which in turn is attributed to the flow reversal upstream of the microchannels. The bubbles pushed back into the inlet plenum agglomerate; the resulting flow blockage is a likely cause for the occurrence of CHF which is marked by an abrupt increase in wall temperature near the exit and an abrupt decrease in pressure drop across the microchannels. A database of 49 data points obtained from five experiments in four independent studies with water, R-113, and FC-77 as coolants was compiled and analyzed. It is found that the CHF has a strong dependence on the coolant, the flow rate, and the area upon which the heat flux definition is based. However, at a given flow rate, the critical heat input (total heat transfer rate to the coolant when CHF occurs) depends only on the coolant and has minimal dependence on the details of the microchannel heat sink (channel size, number of channels, substrate material, and base area). The critical heat input for flow boiling in multiple parallel microchannels follows a well-defined trend with the product of mass flow rate and latent heat of vaporization. A power-law correlation is proposed which offers a simple, yet accurate method for predicting the CHF. The thermodynamic exit quality at CHF is also analyzed and discussed to provide insights into the CHF phenomenon in a heat sink containing multiple parallel microchannels.

Flow Boiling Heat Transfer on Micro Pin Fins Entrenched in a Microchannel

2010 ◽  
Vol 132 (4) ◽  
Author(s):  
Santosh Krishnamurthy ◽  
Yoav Peles
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Boiling ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Heat Fluxes ◽  
Axial Distance ◽  
Mass Fluxes ◽  
Pin Fins

Flow boiling of 1-methoxyheptafluoropropane (HFE 7000) in 222 μm hydraulic diameter channels containing a single row of 24 inline 100 μm pin fins was studied for mass fluxes from 350 kg/m2 s to 827 kg/m2 s and wall heat fluxes from 10 W/cm2 to 110 W/cm2. Flow visualization revealed the existence of isolated bubbles, bubbles interacting, multiple flow, and annular flow. The observed flow patterns were mapped as a function of the boiling number and the normalized axial distance. The local heat transfer coefficient during subcooled boiling was measured and found to be considerably higher than the corresponding single-phase flow. Furthermore, a thermal performance evaluation comparison with a plain microchannel revealed that the presence of pin fins considerably enhanced the heat transfer coefficient.

Characteristics of Flow Boiling Oscillations in Silicon Microchannel Heat Sinks

2007 ◽  
Vol 129 (10) ◽  
pp. 1341-1351 ◽  
Author(s):  
R. Muwanga ◽  
I. Hassan ◽  
R. MacDonald
Keyword(s):  
Heat Sink ◽  
Flow Boiling ◽  
Heat Fluxes ◽  
Heat Sinks ◽  
Working Fluid ◽  
Inlet Temperature ◽  
Outlet Temperature ◽  
Standard Heat ◽  
Flow Oscillations ◽  
Footprint Area

Flow boiling oscillation characteristics in two silicon microchannel heat sink configurations are presented. One is a standard heat sink with 45 straight parallel channels, whereas the second is similar except with cross-linked paths at three locations. Data are presented over a flow range of 20–50ml∕min(91–228kg∕(m2s)) using distilled water as the working fluid. The heat sinks have a footprint area of 3.5cm2 and contain 269μm wide by 283μm deep reactive ion etching channels. Flow oscillations are found to be similar in characteristic trends between the two configurations, showing a decreasing frequency with increasing heat flux. The oscillation amplitudes are relatively large and identical in frequency for the inlet temperature, outlet temperature, inlet pressure, and pressure drop. Oscillation properties for the standard heat sink at two different inlet temperatures and various flow rates are correlated for different heat fluxes. This work additionally presents a first glimpse of the cross-linked heat sink performance under flow boiling instability conditions.

Local Heat Transfer Coefficient of Flow Boiling in Microchannels With Artificial Reentrant Cavities

2007 ◽  
Author(s):  
Chih-Jung Kuo ◽  
Yoav Peles
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Flow Boiling ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Heat Fluxes ◽  
Transfer Coefficients ◽  
Bubble Generation ◽  
Parallel Microchannels

Flow boiling in parallel microchannels with structured reentrant cavities was experimental studied. Flow patterns, boiling inceptions and heat transfer coefficients were obtained and studied for G = 83 kg/m2-s to G = 303 kg/m2-s and heat fluxes up to 643 W/cm2. The heat transfer coefficient-mass velocity and quality relations had been analyzed to identify boiling mechanism. Comparisons of the performance of the enhanced and plain-wall microchannels had also been made. The microchannels with reentrant cavities were shown to promote nucleation of bubbles and to support significantly better reproducibility and uniformity of bubble generation.

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