Prediction of the Onset of Flow Instability in a Single Horizontal Microtube With an Inlet Orifice

2013 ◽  
Vol 136 (2) ◽  
Author(s):  
YanFeng Fan ◽  
Ibrahim Hassan
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Flow Instability ◽  
Saturation Pressure ◽  
Absolute Error ◽  
Heating Condition ◽  
Two Phase ◽  
Mass Fluxes ◽  
Upstream Pressure ◽  
Phase Liquid

A methodology to predict the onset of flow instability (OFI) in a single horizontal microtube with an inlet orifice is developed based on the predication of pressure drop. The predictive methodology states, for the same flow rate, the flow instability occurs as the single-phase liquid pressure drop under no heating condition equals the two-phase pressure drop under heating condition in a single microtube. The addition of inlet orifice increases the heat flux at the onset of flow instability by increasing the upstream pressure. The present methodology is validated by comparing the predicted heat flux at the onset of flow instability with our previous experimental data in the microtubes with three sizes of inlet orifices. The results show that the present method can predict the heat flux at the onset of flow instability with a deviation of 30% and mean absolute error of 13% at mass fluxes from 700 to 3000 kg/m2 s. The effects of inlet orifice size and saturation pressure on the onset of flow instability are also studied based on the present methodology. It is found that, at mass fluxes from 100 to 2000 kg/m2 s, the area ratio less than 15% eliminates the flow instability completely before the critical heat flux occurs.

New Predictive Methodology for the Onset of Flow Instability in Single Horizontal Microtube With an Inlet Orifice

2013 ◽  
Author(s):  
Yanfeng Fan ◽  
Ibrahim Hassan
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Single Phase ◽  
Flow Instability ◽  
Absolute Error ◽  
Heating Condition ◽  
Pumping Power ◽  
Two Phase ◽  
Volume Rate ◽  
Phase Pressure

A new methodology to predict the onset of flow instability (OFI) in single horizontal microtube with inlet orifice is proposed. The predictive methodology states that OFI occurs as the pumping power under no heating condition is equal to the pumping power under heating condition in the microtube at the same volume rate. Since the pumping power can be simply described as the product of volume rate and pressure drop cross the microtube, the heat flux at OFI is determined as the two-phase pressure drop under heating condition is equal to the single-phase pressure drop under no heating condition at same flow rate. The addition of inlet orifice increases the pumping power under no heating condition. The increased pumping power by orifice delays the onset of flow instability. The predictive methodology is validated by comparing the predicted heat flux at OFI with our previous experimental data in the microtubes with three different inlet restriction ratios. The result shows that the proposed method is capable of prediction of heat flux at OFI with a deviation of 30% and mean absolute error of 13% at mass flux less than 2000 kg/m2·s.

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.

Critical Heat Flux in Microchannels With an Adjustable Inlet Orifice

2012 ◽  
Author(s):  
Brent A. Odom ◽  
Carlos A. Ortiz ◽  
Patrick E. Phelan
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Critical Heat Flux ◽  
Mass Flux ◽  
Cooling Capacity ◽  
Hydraulic Diameter ◽  
Low Flow ◽  
Two Phase ◽  
Mass Fluxes ◽  
Inlet Conditions

The benefits of eliminating instabilities in two-phase microchannel flow with inlet orifices come with costs. This study describes the tradeoffs between microchannels with and without inlet orifices, focusing on results from critical heat flux data obtained for various orifice sizes and mass fluxes. An adjustable inlet orifice controlled with a micrometer was placed in front of an array of 31 parallel microchannels each with a hydraulic diameter of 0.235 mm and a length of 1.33 cm. For mass fluxes ranging from 186 kg m−2 s−1 to 847 kg m−2 s−1, critical heat flux (CHF) data were obtained for 7 different orifice sizes. For low flow rates that provided a low quality saturated inlet condition, the difference in CHF values was found to be minimal between open and almost closed orifice conditions. The smallest orifice achieved a CHF value of 5 W cm−2 less than the largest orifice size for a mass flux of 186 kg m−2 s−1, and 7 W cm−2 less for a mass flux of 433 kg m−2 s−1. For mass fluxes higher than 433 kg m−2 s−1, subcooled conditions were present at the orifice inlet, and the highest CHF values occurred with an orifice hydraulic diameter of 35 percent of fully open. For the higher mass flux cases, orifice sizes in the range of 1.8 percent to 28 percent of fully open caused CHF to occur at lower values than less restrictive orifice sizes. This was due to loss of cooling capacity from rapid pressure drop through the orifice. Slightly higher average channel pressures also decrease the refrigerant’s latent heat of vaporization. For the orifice sizes from 35 to 70 percent of unrestricted flow, a very minimal increase in pressure drop over fully open inlet conditions occurred and the general trend was higher CHF values. Very small inlet orifices are beneficial for steady state conditions that do not approach CHF; however, overly restricting the flow at the inlet to microchannels reduces cooling capacity significantly and will cause early onset of CHF. A slightly restrictive inlet orifice will increase CHF.

Experimental Investigation and Flow Visualization of the Two-Phase Flow Instability at Low Vapor Quality in a Vertical Narrow Channel

2018 ◽  
Author(s):  
Liqiang Pan ◽  
Yang Liu ◽  
Weihua Li ◽  
Yefei Liu
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Test Section ◽  
Two Phase Flow ◽  
Flow Instability ◽  
Narrow Channel ◽  
Bubble Nucleation ◽  
Phase Flow ◽  
Vapor Quality ◽  
Two Phase

The two-phase flow instability of forced convection has been experimentally investigated in a vertical narrow channel with the hydraulic diameter of 2.857mm and aspect ratio of 20. Transparent, metallic and conductive films on external surfaces of the test section can provide visualization and uniform heating for deionized water. The heat flux is 6–18.2 kW · m−2. When the instability occurs at low vapor quality, a series of parameters are measured and visualized images are obtained by a high-speed camera. The results show that the large amplitude of pressure drop between the inlet and outlet in the test section is due to the elongated bubble, and the value of pressure drop is positively correlated with the volume of the bubble. The oscillation period of pressure drop decreases with the increase of heat flux, and the period can be determined by the method of the Fast Fourier Transform. The backflow phenomenon is analyzed, which has a greater effect on the oscillation of pressure drop than bubble nucleation, bubble growth, bubble coalescence and recoiling of bubble boundary.

Flow Boiling of R134a in Circular Microtubes—Part II: Study of Critical Heat Flux Condition

2011 ◽  
Vol 133 (5) ◽  
Author(s):  
Saptarshi Basu ◽  
Sidy Ndao ◽  
Gregory J. Michna ◽  
Yoav Peles ◽  
Michael K. Jensen
Keyword(s):  
Experimental Data ◽  
Experimental Study ◽  
Heat Flux ◽  
Flow Pattern ◽  
Flow Regime ◽  
Critical Heat Flux ◽  
Flow Boiling ◽  
Saturation Pressure ◽  
Absolute Error ◽  
Two Phase

A detailed experimental study was carried out on the critical heat flux (CHF) condition for flow boiling of R134a in single circular microtubes. The test sections had inner diameters (ID) of 0.50 mm, 0.96 mm, and 1.60 mm. Experiments were conducted over a large range of mass flux, inlet subcooling, saturation pressure, and vapor quality. CHF occurred under saturated conditions at high qualities and increased with increasing mass fluxes, tube diameters, and inlet subcoolings. CHF generally, but not always, decreases with increasing saturation pressures and vapor qualities. The experimental data were mapped to the flow pattern maps developed by Hasan [2005, “Two-Phase Flow Regime Transitions in Microchannels: A Comparative Experimental Study,” Nanoscale Microscale Thermophys. Eng., 9, pp. 165–182] and Revellin and Thome [2007, “A New Type of Diabatic Flow Pattern Map for Boiling Heat Transfer in Microchannels,” J. Micromech. Microeng., 17, pp. 788–796]. Based on these maps, CHF mainly occurred in the annular flow regime in the larger tubes. The flow pattern for the 0.50 mm ID tube was not conclusively identified. Four correlations—the Bowring correlation, the Katto-Ohno correlation, the Thome correlation, and the Zhang correlation—were used to predict the experimental data. The correlations predicted the correct experimental trend, but the mean absolute error (MAE) was high (>15%) A new correlation was developed to fit the experimental data with a MAE of 10%.

Streamline Modeling of Manifold Microchannels in Thin Film Evaporation

2013 ◽  
Author(s):  
Raphael Mandel ◽  
Amir Shooshtari ◽  
Serguei Dessiatoun ◽  
Michael Ohadi
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Coefficient ◽  
Pressure Drop ◽  
Transfer Coefficient ◽  
Wall Heat Transfer ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Mass Fluxes ◽  
Flow Length

Manifold microchannels utilize a system of manifolds to divide long microchannels into an array of parallel ones, resulting in reduced flow length and more localized liquid feeding. Reducing flow length is desirable because it enables the simultaneous enhancement of heat transfer rate and reduction of pressure drop. Furthermore, localized feeding reduces potential for localized dryout, increasing the operational heat flux. Because of the failure of the available conventional heat transfer correlations to predict the thermal performance of manifold microchannels operating in two phase mode, a “streamline” model was created. The heat transfer surface area was divided into parallel, non-interacting streamlines, and the quality, void fraction, film thickness, heat transfer coefficient, heat flux, and pressure drop was calculated sequentially along the streamline. The mass flow rate through each streamline was adjusted in order to obtain the specified pressure drop, and the value of this pressure drop was adjusted in order to obtain the desired microchannel mass flux. Finally, the average wall heat transfer coefficient was calculated, and temperature profile in the fin was adjusted to correspond with the analytical 1-D temperature distribution of a thin fin with an average wall heat transfer coefficient and specified base superheat. The average wall heat transfer coefficients predicted by the model was then compared to the available experimental data with sufficiently good agreement with a wide variety of geometries and working fluids at low mass fluxes.

Embedded Two-Phase Cooling of High Flux Electronics via Micro-Enabled Surfaces and Fluid Delivery Systems (FEEDS)

2015 ◽  
Author(s):  
Raphael Mandel ◽  
Serguei Dessiatoun ◽  
Patrick McCluskey ◽  
Michael Ohadi
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Saturation Temperature ◽  
Absolute Pressure ◽  
High Flux ◽  
Vapor Quality ◽  
Two Phase ◽  
Fluid Delivery ◽  
Future Work

This work presents the experimental design and testing of a two-phase, embedded manifold-microchannel cooler for cooling of high flux electronics. The ultimate goal of this work is to achieve 0.025 cm2-K/W thermal resistance at 1 kW/cm2 heat flux and evaporator exit vapor qualities at or exceeding 90% at less than 10% absolute pressure drop. While the ultimate goal is to obtain a working two-phase embedded cooler, the system was first tested in single-phase mode to validate system performance via comparison of experimentally measured heat transfer coefficient and pressure drop to the values predicted by CFD simulations. Upon validation, the system was tested in two phase mode using R245fa at 30°C saturation temperature and achieved in excess of 1 kW/cm2 heat flux at 45% vapor quality. Future work will focus on increasing the exit vapor quality as well as use of SiC for the heat transfer surface upon completion of current experiments with Si.

Effects of Parameters on the Two-Phase Flow Instability in a Microchannel

2018 ◽  
Author(s):  
Yefei Liu ◽  
Yang Liu ◽  
Xingtuan Yang ◽  
Liqiang Pan
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Flow Instability ◽  
Working Fluid ◽  
Two Phase ◽  
Short Period ◽  
Period Oscillation ◽  
Data Acquisition Card ◽  
Series Of Experiments ◽  
Vertical Microchannel

Series of experiments are conducted in a single microchannel, where subcooled water flows upward inside a transparent and vertical microchannel. The cross section of the channel is rectangle with the hydraulic diameter of 2.8mm and the aspect ratio of 20. The working fluid is 3–15K subcooled and surface heat flux on the channel is between 0–3.64 kW/m2, among which two-phase instability at low vapor quantity may occur. By using a novel transparent heating technique and a high-speed camera, visualization results are obtained. The parameters are acquired with a National Instruments Data Acquisition card. In the experiments, long-period oscillation and short-period oscillation are observed as the primary types of instability in a microchannel. Instability characteristics represented from signals correspond well with the flow pattern. Moreover, effects of several parameters are investigated. The results indicate that the oscillating period generally increases with the heat flux density and decreases with inlet subcooling, while the effects of inlet resistance are more complex.

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