Julie E. Steinbrenner

Impact of wall hydrophobicity on condensation flow and heat transfer in silicon microchannels

While microchannel condensation has been the subject of several recent studies, the critical impact of wall hydrophobicity on the microchannel condensation flow has received very little attention. The paper experimentally studies steam condensation in a silicon microchannel 286 µm in hydraulic diameter with three different wall hydrophobicities. It is found that the channel surface wettability has a significant impact on the flow pattern, pressure drop and heat transfer characteristic. Spatial flow pattern transition is observed in both hydrophobic and hydrophilic channels. In the hydrophobic channel, the transition from dropwise/slugwise flow to plug flow is induced by the slug instability. In the hydrophilic channel, the flow transition is characterized by the periodic bubble detachment, a process in which pressure evolution is found important. Local temperature measurement is conducted and heat flux distribution in the microchannel is reconstructed. For the same inlet vapor flux and temperature, the hydrophobic microchannel yields higher heat transfer rate and pressure drop compared to the hydrophilic channel. The difference is attributed to the distinction in flow pattern and heat transfer mechanism dictated by the channel hydrophobicity. This study highlights the importance of the channel hydrophobicity control for the optimization of the microchannel condenser.

Two-phase microfluidics for semiconductor circuits and fuel cells

Industrial trends are presenting major challenges and opportunities for research on two-phase flows in microchannels. Semiconductor companies are developing 3D circuits for which multilevel microfluidic cooling is important. Gas delivery microchannels are promising for PEM fuel cells in portable electronics. However, data and modeling are needed for flow regime stability, liquid entrainment/clogging, and bubble inception/departure in complex 2D and 3D geometries. This paper provides an overview of the Stanford two-phase microfluidics program, with a focus on recent experimental and theoretical progress. Microfabrication technologies are used to distribute heaters, thermometers, pressure sensors, and liquid injection ports along the flow path. Liquid PIV quantifies forces on bubbles, and fluorescence imaging detects flow shapes and liquid volume fraction. Separated flow models account for conjugate conduction, liquid injection, evaporation, and a variety of flow regimes. This work benefits strongly from interactions with semiconductor and fuel cell companies seeking validated models for product design.

WATER SLUG DETACHMENT IN TWO-PHASE HYDROPHOBIC MICROCHANNEL FLOWS

In this paper we present a first order study of liquid water detachment and entrainment into air flows in hydrophobic microchannels. A silicon based microfabricated test structure was used for this purpose. It consists of a 500 μm wide by 45 μm deep U-shaped channel 23 mm in length through which air is flown. The structures are treated with a Molecular Vapor Deposition (MVD) process that renders them hydrophobic with a nominal contact angle of 108° (in situ contact angles inside the channels are measured directly during testing). Liquid water is injected through a single side slot located two-thirds of the way downstream from the air channel inlet. The side slot extends the whole depth of the air channel while its width is varied from sample to sample. Visualization of the water slugs that form as water is injected into the air channel was performed. Slug dimensions at detachment are correlated against superficial gas velocity. Proper dimensionless parameters are postulated and examined to compare hydrodynamics forces against surface tension. It is found that for Re below 200 slug detachment is dominated by pressure gradient drag arising from confinement of a viscous flow in the channel. On the other hand, for Re above 200 the predominant drag is inertial in nature with stagnation of the air due to flow obstruction by the slugs.Copyright

Experimental Investigation and Visualization of Two-Phase Flow and Water Transport in Microchannels

In this paper we present an overview of the experimental work carried out as part of research geared towards the understanding of two-phase flow in microchannels. The greater scope of the project is to use the knowledge gained towards the development of strategies to improve water management in fuel cell applications. We have conducted pressure versus flow rate experiments in microchannels with contrasting hydrophobic characteristics and under different liquid water injection conditions. These measurements have been complemented with flow visualization studies using white light and fluorescence. As expected, parameters associated to surface energy such as hydrophobicity have a big influence on the flow. Under hydrophobic conditions the formation of slugs or blobs of size comparable to that of the microchannel greatly impedes the flow of air, especially at low pressure drops. On the other hand liquid water effects under hydrophilic conditions are only noticeable at large injection rates (100 µL/min). In contrast to their hydrophobic counterparts, two-phase flow in hydrophilic microchannels is characterized by the formation of a thin film of liquid. Only when the thickness of the film becomes substantial does it have an effect on the air flow.

1D Homogeneous Modeling of Microchannel Two-Phase Flow With Distributed Liquid Water Injection From Walls

This paper presents a theoretical model and a numerical simulation of a liquid-gas two-phase flow within a microchannel (50 μm × 500 μm × 2cm) equipped with distributed liquid water injection through the side walls. The modeling and solution of the conservation equations provide pressure drop as a function of inlet velocity. The influence of different parameters involving water injection is investigated, such as the quantity of water that is injected and the profile that is used to inject it. The numerical results show that for small water injection rates (1–10μL/min) the air flow velocity and pressure drop are not significantly perturbed by the presence of liquid water. But if water injection becomes important (10–100μL/min) larger pressure drops are observed. The influence of inlet pressure is also investigated. The model predictions are compared with experimental results obtained from testing a set of microchannels with a varying number of water injection slots on the side walls. Pressure drop distribution data from these experiments are consistent with model predictions.Copyright

Compact Model of Slug Flow in Microchannels

This paper presents a theoretical investigation of the movement of liquid droplets and slugs in hydrophobic microchannels and develops a compact model for this type of two-phase flow. This model is used in the prediction of pressure drop and liquid water coverage ratio, key parameters in the operation of Proton Exchange Membrane Fuel Cells (PEMFC), the primary motivation for this work. A semiempirical, periodic-steady two-phase separated flow compact model is formulated to characterize the slug flow behavior. The momentum equation includes the effects of acceleration, friction and surface tension on the pressure drop. The model considers spatial changes in slug velocity through the use of a force balance formulation. The model uses a departure scheme that computes slug size and shape at entrainment. The steady state slug flow compact model is capable of predicting liquid water coverage ratio and pressure drop using liquid and gas flow rates and advancing/receding triple point contact angles as its only inputs. The results indicate that the pressure drop increases as the droplet formation frequency increases.

Flow Regime Evolution in Long, Serpentine Microchannels With a Porous Carbon Paper Wall

An important function of the gas delivery channels in Proton Exchange Membrane (PEM) fuel cells is the evacuation of liquid water created at the cathode. The resulting two-phase flow can become an obstacle to reactant transport and a source of parasitic losses. The present work examines the behavior of two-phase flow in 500 �m x 500 �m x 60 cm channels with distributed water injection through a porous carbon paper wall to gain understanding of the physics of flows relevant to fuel cell water management challenges. Flow regime maps based on local gas and liquid flow rates are constructed for experimental conditions corresponding to current densities between 0.5 and 1 A/cm 2 and stoichiometric coefficients from 1 to 4. Flow structures are analyzed along the entire length of the channel. It is observed that slug flow is favored to plug flow at high air flow rates and low liquid flow rates. Stratified flow dominates at high liquid flow rates. Along the axial flow direction, the flow regime consistently transitions from intermittent to wavy to stable stratified flow. This progression is quantified using a parameter of flow progression which characterizes the degree of development of the two-phase flow toward the stable stratified condition. This parameter is discussed in relation to fuel cell operating conditions. It provides a metric for analyzing liquid water removal mechanisms in the cathode channels of PEM fuel cells.

3-D Numerical Simulation of Contact Angle Hysteresis for Slug Flow in Microchannel

Water management that ensures the effective removal of produced water in the microchannel at the cathode is critical for the performance of PEM fuel cells. The small dimension and confined space of channels leads to the importance of the surface force in determining the dynamics of inside liquid slugs. The present study focuses on the simulation of the slug detachment process in the micro-channel, using a contact angle hysteresis model within the framework of VOF approach. Based on solving the nonlinear equations accounting for the relationship among volume fraction, interface position, and contact angle, a special model is developed to replicate the hysteresis effect. In addition, a special algorithm is introduced to simulate the thin liquid/gas films. A systematic comparison between experiment and simulation has been conducted and the quantitative match in terms of slug dimensions is achieved for a wide range of flow conditions. The simulation reveals that the contact angle distribution along the slug profile could be approximated using piecewise linear function. The calculation also shows that the contact angle hysteresis might be responsible for several phenomena observed in experiment, such as slug instability.Copyright

Flow Structures and Frictional Characteristics on Two-Phase Flow in Microchannels in PEM Fuel Cells

This experimental paper presents a study of gas-liquid two phase flow in rectangular channels of 500μm × 45μm and 23.7mm long with different wall conditions of hydrophilic and hydrophobic surface, in order to investigate the flow structures and the corresponding friction factors of simulated microchannels of PEMFC. The main flow in the channel is air and liquid water is injected at a single or several discrete locations in one side wall of the channel. The flow structure of liquid water in hydrophilic wall conditioned channel starts from wavy flow, develops to stable stratified film flow, and then transits to unstable fluctuating film flow, as the pressure drop and the flow velocity of air increase from around 10 kPa to over 100 kPa. The flow structure in hydrophobic channel develops from the slug flow to slug-and-film flow with increasing pressure drop and flow velocity. The pressure drop for single phase flow is measured for a base line study, and the fRe product is in close agreement with the theoretical value (fRe = 85) of the conventional laminar flow of aspect ratio 1:11. At the low range of water injection rate, the gas phase fRe product of the two phase flow based on the whole channel area was not substantially affected by the water introduction. However, as the water injection rate increases up to 100 μL/min, the gas phase fRe product based on the whole channel area deviates highly from the single phase theoretical value. The gas phase fRe product with the actual gas phase area corrected by the liquid phase film thickness agrees with the single phase theoretical value.Copyright