Laminar Fully-Developed Flow in a Microchannel With Patterned Ultrahydrophobic Walls

2005 ◽  
Author(s):  
B. Woolford ◽  
K. Jeffs ◽  
D. Maynes ◽  
B. W. Webb
Keyword(s):  
Pressure Drop ◽  
Solid Wall ◽  
Relative Size ◽  
Frictional Resistance ◽  
Hydraulic Diameter ◽  
Fourth Power ◽  
Fully Developed Flow ◽  
Flowing Liquid ◽  
Frictional Pressure Drop ◽  
Liquid Vapor

Microfluidic transport is finding increasing application in a number of emerging technologies. At these scales, classical analysis shows that the required fluid driving pressure is inversely proportional to the hydraulic diameter to the fourth power. Consequently, generating fluid motion at these physical scales is a challenge. There is thus considerable incentive for developing strategies to reduce the frictional resistance to fluid flow. A novel approach recently proposed is fabrication of micro-ribs and cavities in the channel walls which are treated with a hydrophobic coating. This reduces the surface contact area between the flowing liquid and the solid wall, yielding walls with no-slip and shear-free regions at the microscale. The shear-free regions consist of a liquid-vapor meniscus above the cavities between micro-ribs. Reductions in the flow resistance are thus possible. This paper reports results of an analytical and experimental investigation of the laminar, fully-developed flow in a parallel plate microchannel whose walls are microengineered in this fashion. The micro-ribs and cavities are oriented parallel to the flow direction. The channel walls are modeled in an idealized fashion, with the shape of liquid-vapor meniscus approximated as flat and characterized by vanishing shear stress. Predictions are presented for the friction factor-Reynolds number product as a function of relevant governing dimensionless parameters. Comparisons are made between the smooth-wall classical channel flow results and predictions for the microengineered channel walls. Results show that significant reductions in the frictional pressure drop are possible. Reductions in frictional resistance increase as the channel hydraulic diameter and/or micro-rib width are reduced. The frictional pressure drop predictions are in good agreement with experimental measurements made at dynamically similar conditions, with greater deviation observed with increasing relative size of the shear-free regions.

Prediction of Laminar Flow in a Microchannel With Transverse Ultrahydrophobic Ribs

2005 ◽  
Author(s):  
J. Davies ◽  
B. Woolford ◽  
D. Maynes ◽  
B. W. Webb
Keyword(s):  
Friction Factor ◽  
Liquid Flow ◽  
Frictional Resistance ◽  
Vapor Flow ◽  
Flowing Liquid ◽  
Vapor Cavity ◽  
Vapor Interface ◽  
Smooth Channel ◽  
The Impact ◽  
Liquid Vapor

One approach recently proposed for reducing the frictional resistance to liquid flow in microchannels is the patterning of micro-ribs and cavities on the channel walls. When treated with a hydrophobic coating, the liquid flowing in the microchannel wets only the surfaces of the ribs, and does not penetrate the cavities, provided the pressure is not too high. The net result is a reduction in the surface contact area between channel walls and the flowing liquid. For micro-ribs and cavities that are aligned normal to the channel axis (principal flow direction), these micro-patterns form a repeating, periodic structure. This paper presents experimental and numerical results of a study exploring the momentum transport in a parallel plate microchannel with such microengineered walls. The liquid-vapor interface (meniscus) in the cavity regions is treated as ideal in the numerical analysis (flat). Two conditions are explored with regard to the cavity region: 1) The liquid flow at the liquid-vapor interface is treated as shear-free (vanishing viscosity in the vapor region), and 2) the liquid flow in the microchannel core and the vapor flow within the cavity are coupled through the velocity and shear stress matching at the interface. Predictions and measurements reveal that significant reductions in the frictional pressure drop can be achieved relative to the classical smooth channel Stokes flow. Reductions in the friction factor are greater as the cavity-to-rib length ratio is increased (increasing shear-free fraction) and as the channel hydraulic diameter is decreased. The results also show that the average friction factor – Reynolds number product exhibits a flow Reynolds dependence. Furthermore, the predictions reveal the impact of the vapor cavity regions on the overall frictional resistance.

Hydraulic Resistance of Subcritical and Supercritical Water Flowing in a Rifled Tube

2016 ◽  
Author(s):  
Dong Yang ◽  
Zhi Shen ◽  
Xin Nie ◽  
Wanyu Liu ◽  
Fengjun Wang ◽  
...  
Keyword(s):  
Pressure Drop ◽  
Hydraulic Resistance ◽  
Mass Flux ◽  
Frictional Resistance ◽  
Resistance Coefficient ◽  
Vapor Quality ◽  
Two Phase ◽  
Bulk Fluid ◽  
Frictional Pressure Drop ◽  
Frictional Resistance Coefficient

Large capacity supercritical boiler is at the leading edge of efficiency boost for thermal power plant. Water wall design is a key issue for a supercritical boiler. To ensure successful design and safe operation of water wall, studying hydraulic resistance of water is significant. Considerable work on frictional pressure drop of gas-liquid two-phase flow in tubes has been done and various correlations have been proposed to predict it. However, these correlations are restricted to particular rib geometries and flow conditions. Because of significant variations in thermo physical properties near the critical and pseudo-critical points, pressure drop at supercritical pressures is different from that at subcritical pressures. However, limited studies have been devoted to estimate hydraulic resistance of supercritical water. More work need be conducted to develop prediction method for pressure drop at supercritical pressures. Therefore, to accumulate fundamental experimental data for the design of a supercritical boiler, an experiment on hydraulic resistance of water was performed in a vertical upward rifled tube. The experiment was carried out in the high-temperature and high-pressure steam-water test loop at Xi’an Jiaotong University. Based on the experimental data, the two-phase frictional multiplier was calculated to analyze the two-phase frictional pressure drop. At low to moderate vapor quality, the two-phase frictional multiplier increases rapidly and reaches a peak. When the vapor quality exceeds a certain value, the two-phase frictional multiplier starts to decrease with increasing vapor quality. It is because the tube wall is covered by liquid film at low to moderate vapor quality. Within the high vapor quality region, the high-speed vapor tears the liquid film and the flow pattern turns to mist flow with lower frictional pressure drop. Increasing pressure decreases the two-phase frictional multiplier and when the pressure approaches the critical pressure, the multiplier is close to 1. The effect of mass flux on the multiplier is so weak that it can be neglected. At supercritical pressures, the pressure drops due to frictional resistance and flow acceleration both increase with bulk fluid enthalpy. Increasing pressure decreases the frictional pressure drop. This result is mainly attributed to pressure approaching the critical point. Frictional pressure drop is significantly affected by fluid property variations; in particular, severe density decreases with increasing bulk fluid enthalpy. Acceleration pressure drop increases with decreasing pressure and increasing heat flux. When heat flux increases, the density difference between the inlet and the outlet increases with the same mass flux, which results in a considerable acceleration pressure drop. Decreasing pressure results in a similar acceleration pressure drop variation because of the same reason. The frictional resistance coefficient was calculated to analyze the supercritical frictional pressure drop. In the large specific heat region, the frictional resistance coefficient peaks at a certain enthalpy in the vicinity of the pseudo-critical point, and increasing mass flux reduces the magnitude of the peak value.

Thermal Transport in a Microchannel Exhibiting Transverse Ultrahydrophobic Micro-Ribs Maintained at Constant Temperature

2006 ◽  
Author(s):  
J. Davies ◽  
D. Maynes ◽  
B. W. Webb
Keyword(s):  
Nusselt Number ◽  
Pressure Drop ◽  
Laminar Flow ◽  
Constant Temperature ◽  
Thermal Transport ◽  
Flow Direction ◽  
Relative Size ◽  
Cavity Depth ◽  
Frictional Pressure Drop ◽  
Smooth Channel

There exists considerable incentive for reducing the required pumping power in microscale heat exchanger applications. One approach recently proposed is the use of super ultrahydrophobic channel walls. The influence such walls exert on the overall thermal transport has not been previously addressed and is the focus of this paper. Specifically, this paper presents numerical results exploring the periodically repeating thermal transport in a parallel plate microchannel with ultrahydrophobic walls maintained at constant temperature. The walls considered here exhibit alternating microribs and cavities positioned perpendicular to the flow direction. Results describing the thermally periodically repeating dynamics far from the inlet of the channel have been obtained over a range of flow Reynolds numbers and relative microrib/cavity widths and depths in the laminar flow regime. Previously it has been shown that significant reductions in the overall frictional pressure drop can be achieved relative to the classical smooth channel laminar flow. The present predictions reveal that the overall thermal transport is also reduced as the relative size of the cavity region is increased. The overall Nusselt number behavior is presented and discussed in conjunction with the frictional pressure drop behavior for the parameter range explored. In summary the following conclusions can be made regarding thermal transport for a constant temperature channel exhibiting ultrahydrophobic surfaces: 1) Increases in the shear free fraction (relative cavity length) yields decreases in the Nusselt number 2) increasing the relative rib/cavity module length yields a decrease in the Nusselt number 3) decreases in the Reynolds number result in smaller values of the Nusselt number and 4) the relative cavity depth exhibits negligible influence on the magnitude of the Nusselt number.

Thermal Transport in a Microchannel Exhibiting Ultrahydrophobic Microribs Maintained at Constant Temperature

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
D. Maynes ◽  
B. W. Webb ◽  
J. Davies
Keyword(s):  
Nusselt Number ◽  
Pressure Drop ◽  
Laminar Flow ◽  
Constant Temperature ◽  
Thermal Transport ◽  
Flow Direction ◽  
Relative Size ◽  
Reynolds Numbers ◽  
Frictional Pressure Drop ◽  
Smooth Channel

This paper presents numerical results exploring the periodically repeating laminar flow thermal transport in a parallel-plate microchannel with ultrahydrophobic walls maintained at constant temperature. The walls considered here exhibit alternating microribs and cavities positioned perpendicular to the flow direction. Results describing the thermally periodically repeating dynamics far from the inlet of the channel have been obtained over a range of laminar flow Reynolds numbers and relative microrib/cavity module lengths and depths in the laminar flow regime. Previously, it has been shown that significant reductions in the overall frictional pressure drop can be achieved relative to the classical smooth channel laminar flow. The present predictions reveal that the overall thermal transport is also reduced as the relative size of the cavity region is increased. The overall Nusselt number behavior is presented and discussed in conjunction with the frictional pressure drop behavior for the parameter range explored. The following conclusions can be made regarding thermal transport for a constant temperature channel exhibiting ultrahydrophobic surfaces: (1) Increases in the relative cavity length yield decreases in the Nusselt number, (2) increasing the relative rib/cavity module length yields a decrease in the Nusselt number, and (3) decreases in the Reynolds number result in smaller values of the Nusselt number.

Condensation of Refrigerant in a Multi-Port Channel

2003 ◽  
Author(s):  
Shigeru Koyama ◽  
Ken Kuwahara ◽  
Koichi Nakashita
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Surface Tension ◽  
Pressure Drop ◽  
Large Diameter ◽  
Hydraulic Diameter ◽  
Enhancement Effect ◽  
Frictional Pressure Drop ◽  
New Correlation ◽  
Rectangular Channels

In the present paper, the local characteristics of pressure drop and heat transfer are investigated experimentally for the condensation of pure refrigerant R134a in four kinds of multi-port extruded aluminum tubes of about 1 mm in hydraulic diameter. Two tubes are composed of plane rectangular channels, while remaining two tubes are composed of rectangular channels with straight micro-fins. The experimental data of frictional pressure drop (FPD) and heat transfer coefficient (HTC) in plane tubes are compared with previous correlations, most of which are proposed for the condensation of pure refrigerant in a relatively large diameter tube. It is confirmed that parameters such as tube diameter, surface tension, free convection in FPD and HTC correlations should be taken into account more precisely. Considering the effects of surface tension and kinematic viscosity, new correlation of FPD is developed based on the Mishima-Hibiki correlation. New correlation of HTC is also developed modifying the effect of diameter in the correlation of Haraguchi et al. Both new correlations are compared with experimental data for tubes with micro-fins. Satisfactory agreement between experimental and predicted results is obtained. This means that the micro-fin effect is taken into account by using hydraulic diameter and the heat transfer enhancement effect of micro-fins is mainly due to the enlargement of heat transfer area.

Experimental Study on Frictional Resistance Characteristics of CO2-Emulsified Viscoelastic Surfactant Fracturing Fluid

2014 ◽  
Vol 577 ◽  
pp. 35-38 ◽  
Author(s):  
Ze Feng Jing ◽  
Shu Zhong Wang ◽  
Xiang Rong Luo ◽  
Zhi Guo Wang
Keyword(s):  
Pressure Drop ◽  
Frictional Resistance ◽  
Resistance Coefficient ◽  
Fracturing Fluid ◽  
Frictional Pressure Drop ◽  
Test Loop ◽  
Friction Pressure ◽  
Friction Pressure Drop ◽  
Viscoelastic Surfactant ◽  
Frictional Resistance Coefficient

Rheological properties and friction resistance properties of CO2-emulsified viscoelastic surfactant fracturing fluid were investigated on the large-scale test loop of foam fracturing fluid. When the velocity is below 2.8 m·s-1, the friction pressure drop gradient gradually decreases with the increase of foam quality, however, when the velocity is above 2.8 m·s-1, the friction pressure drop gradient of VES-CO2with higher foam quality is higher than lower foam quality. In addition, frictional pressure drop gradient of the pure CO2is higher than the VES-CO2fracturing fluid system when the velocity is more than 1.6 m·s-1. And frictional resistance coefficient decreases with the increase of foam quality and increases slightly with the increase of temperature. Experimental correlation between frictional resistance coefficient and Reynolds number is obtained and has high precision.

Experiments on Liquid Pressure-Drop in Rectangular Microchannels, Subject to Non-Unity Aspect Ratio and Finite Roughness

2006 ◽  
Author(s):  
Wolf Wibel ◽  
Peter Ehrhard
Keyword(s):  
Pressure Drop ◽  
Aspect Ratio ◽  
Hydraulic Diameter ◽  
Turbulent Regime ◽  
Dimensionless Group ◽  
Flow Conditions ◽  
Liquid Pressure ◽  
Fully Developed Flow ◽  
Turbulent Transition ◽  
Aspect Ratios

We concentrate in this contribution onto the pressure-drop in rectangular stainless steel microchannels with a hydraulic diameter of dh ≈ 133 μm. Three aspect ratios are engaged, namely 1:1, 1:2, 1:5, whereas the hydraulic diameter is maintained constant. The roughness of the channel walls is around r ≈ 1.3 μm, the Reynolds number is up to Re ≈ 4000. We investigate all microchannels in two different lengths to infer highly-accurate correlations for fully-developed flow conditions from pressure difference measurements between the inlet- and outlet plenum. We find with this new technique pressure drop correlations for the fully-developed flow, which agree, both in the laminar and the turbulent regime, reasonably with conventional (macroscopic) correlations. In all cases the laminar/turbulent transition is in the range Rec ≈ 1800–2300, consistent with findings in macroscopic channels. The influence of roughness is found to be particularly strong for the microchannel of aspect ratio 1:5. This raises the question, whether the dimensionless group (r/dh) remains the responsible parameter for roughness at extreme aspect ratios.

Sign in / Sign up

Export Citation Format

Share Document