Flow pattern transition instability during flow boiling in a single microchannel

We studied the unique characteristics of flow boiling in a single microchannel, including the periodic pressure drop, mass flow rate, and temperature fluctuations, in terms of a long time period. Experiments were conducted using a single horizontal microchannel and deionized water to study boiling instabilities at very small mass and heat flow rate conditions. A Polydimethylsiloxane (PDMS) rectangular single microchannel had a hydraulic diameter of 103.5 μm and a length of 40 mm. A series of piecewise serpentine platinum microheaters were fabricated on the inner bottom wall of the rectangular microchannel to supply thermal energy to the test fluid. Real-time flow visualizations of the flow pattern inside the microchannel were performed simultaneously with measurements of the experimental parameters. Tests were performed for mass fluxes of 170 and 360 kg/m² s and heat fluxes of 200–530 kW/m². The test results showed that the heated wall temperature, pressure drop, and mass flux all fluctuated with a long period and large amplitude. These periodic fluctuations exactly matched the transition of two alternating flow patterns inside the microchannel: a bubbly/slug flow and an elongated slug/semi-annular flow. Therefore, the flow pattern transition instability in the single microchannel caused a cyclic behavior of the wall temperature, pressure drop, and mass flux, and this behavior had a very long period (100–200 s) and large amplitude.     

Two-phase flow in converging and diverging microchannels with CO2 bubbles produced by chemical reactions

The present study investigates experimentally the evolution of two-phase flow pattern and pressure drop in the converging and diverging, silicon-based microchannels with mean hydraulic diameter of 128 μm and CO₂ bubbles produced by chemical reactions of sulfuric acid (H₂SO₄) and sodium bicarbonate (NaHCO₃). Three different concentrations of 0.2, 0.5 and 0.8 mol/L of each reactant at the inlet before mixing and 10 different flow rates from 1.60 × 10⁻⁹ m³/s to 16.0 × 10⁻⁹ m³/s are studied. Flow visualization is made possible by using a high-speed digital camera. It is found that the present design of the microchannel, with the inlet chamber, results in much more intensive chemical reactions in the diverging microchannel than that in the converging one. The void fractions at the entrance and exit regions and pressure drop through the channel are also measured. The results reveals that the presence of small void fraction, <0.1, at the inlet may promote CO₂ generation in the microchannel, irrespective of the channel is converging or diverging, indicating the agitation effects of bubbly flow in the microchannel. The increase of inlet concentration of reactants does not increase the pressure drop in the converging microchannel significantly, while the inlet concentration presents significant but mild effects on the pressure drop in the diverging microchannel. The two-phase frictional multiplier may be positively correlated with the mean void fraction in the channel linearly, and the data agree well with predictions from the correlations in the literature.     

Flow pattern maps and transition criteria for flow boiling of binary mixtures in a diverging microchannel

This paper presents a visualization study of flow boiling of binary mixtures (methanol–water and ethanol–water mixtures) in a diverging microchannel. The flow pattern and transition criteria are studied in terms of effects of mass flux, heat flux, and molar fraction of the more volatile component (i.e., methanol or ethanol). Four boiling regimes are identified: bubbly-elongated slug flow, annular flow, liquid film breakup, and dryout. Further, generalized flow pattern maps are constructed using coordinates of nondimensional parameter space (boiling number, Weber number, and Marangoni number), wherein relatively distinct boundaries between the flow patterns are identified. Criteria for transitions between flow patterns are proposed in the form of nondimensional groups and are successfully used to predict the experimental results. More than 92% of the data are correctly located within transition boundaries. The criterion for the onset of nucleate boiling—the boundary between single-phase flow and bubbly-elongated slug flow—is also determined for both methanol–water and ethanol–water mixtures on the basis of the same set of nondimensional parameters.     

Transient flow patterns and bubble slug lengths in parallel microchannels with oxygen gas bubbles produced by catalytic chemical reactions

Transient flow patterns and bubble slug lengths were investigated with oxygen gas (O₂) bubbles produced by catalytic chemical reactions using a high speed camera bonded with a microscope. The microreactor consists of an inlet liquid plenum, nine parallel rectangular microchannels followed by a micronozzle, using the MEMS fabrication technique. The etched surface was deposited by the thin platinum film, which is acted as the catalyst. Experiments were performed with the inlet mass concentration of the hydrogen peroxide from 50% to 90% and the pressure drop across the silicon chip from 2.5 to 20.0 kPa. The silicon chip is directly exposed in the environment thus the heat released via the catalytic chemical reactions is dissipated into the environment and the experiment was performed at the room temperature level. It is found that the two-phase flow with the catalytic chemical reactions display the cyclic behavior. A full cycle consists of a short fresh liquid refilling stage, a liquid decomposition stage followed by the bubble slug flow stage. At the beginning of the bubble slug flow stage, the liquid slug number reaches maximum, while at the end of the bubble slug flow stage the liquid slugs are quickly flushed out of the microchannels. Two or three large bubbles are observed in the inlet liquid plenum, affecting the two-phase distributions in microchannels. The bubble slug lengths, cycle periods as well as the mass flow rates are analyzed with different mass concentrations of hydrogen peroxide and pressure drops. The bubble slug length is helpful for the selection of the future microreactor length ensuring the complete hydrogen peroxide decomposition. Future studies on the temperature effect on the transient two-phase flow with chemical reactions are recommended.     

Development of a microchannel evaporator model for a CO2 air-conditioning system

This paper presents the development and verification of a heat exchanger model for evaluating the thermal performance of an evaporator for a CO₂ mobile air-conditioning system. The model has been developed, on the basis of the finite volume method, with emphasis placed on the air-side heat and mass transfer processes. The governing equations are derived from mass and energy balances using the newly developed air-side heat transfer and friction loss correlations for microchannel heat exchangers under both dry and wet conditions. The calculated air-side heat transfer and pressure drop data are in good agreement with measured data. However, the refrigerant-side pressure drop estimation for microchannel tubes usually underestimates the measured value. The simulation results and importance of selecting appropriate heat transfer and pressure drop correlations for the microchannel heat exchanger are addressed.     

CFD simulation of CO2 removal from hydrogen rich stream in a microchannel

The main object of this research is to perform computational fluid dynamics simulation of CO₂ capturing from hydrogen-rich streams by aqueous DEA solution in a T-Junction microchannel contactor with 250 μm diameter and 5 mm length at dynamic conditions. To develop a comprehensive mathematical framework to simulate the flow hydrodynamics and mass transfer characteristics of system, the continuity and Navier-Stokes equations, two phase transport, and reaction rate model are coupled in COMSOL Multiphysics software. The developed model is solved and the effects of gas and liquid velocities as well as amine concentration on the CO₂ absorption rate, hydrogen purification fraction, and flow hydrodynamic are investigated. The absorption process consists of CO₂ diffusion from bubble bulk toward the bubble boundary, CO₂ solubility in the liquid boundary, diffusion from the boundary into the liquid bulk, and reaction with the amine molecules. The results show that when the gas and liquid streams are mixed in the junction point to form a bubble, the gas cross-section area becomes narrow, and the fluid velocity increases due to the applied force on the bubble by the liquid layers. It appears that increasing the DEA concentration in the inlet from 5% to 20% increases hydrogen purification fraction from 42.3 to 66.4%, and up to 96.7% hydrogen purity is achieved by 20% aqueous solution of DEA.     

Two-phase flow instability for boiling in a microchannel heat sink

The present study explores experimentally the two-phase flow instability in a microchannel heat sink with 15 parallel microchannels. The hydraulic diameter for each channel is 86.3 μm. Flow boiling in the present microchannel heat sink demonstrates significantly different two-phase flow patterns under stable or unstable conditions. For the stable cases bubble nucleation, slug flow and slug or annular flows appear sequentially in the flow direction. On the other hand, forward or reversed slug/annular flows appear alternatively in every channel. Moreover, the length of bubble slug may oscillate for unstable cases with reversed flow demonstrating the suppressing effect of pressure field for bubble growth. It is found that the magnitude of pressure drop oscillations may be used as an index for the appearance of reversed flow. A stability map on the plane of inlet subcooling number versus phase change number is established. A very narrow region for stable two-phase flow or mild two-phase flow oscillations is present near the line of zero exit quality.     

Numerical analysis on a microchannel evaporator designed for CO2 air-conditioning systems

A microchannel heat exchanger was numerically analyzed using the finite volume method. The air and refrigerant-side heat transfer coefficients and pressure drops were calculated using the existing correlations that were developed for microchannel heat exchangers. To verify the present model, performance tests of the microchannel heat exchanger were conducted at various test conditions with R134a. The present model yielded a good correlation with the measured heat transfer rate, demonstrating a mean deviation of 6.8%. The performance of the microchannel evaporator for CO₂ systems can be improved by varying the refrigerant flow rate to each slab and changing fin space to increase the two-phase region in the microchannel. Based on the comparison of the performance of the microchannel heat exchanger with that of the fin-tube heat exchanger designed for CO₂ systems, it was proposed that the arrangement of the slabs and inlet air velocity in the microchannel heat exchanger need to be optimized by considering heat exchanger size, air outlet conditions and required capacity.     

An experimental study of flow boiling instability in a single microchannel

A simultaneous visualization and measurement study has been carried out to investigate stable and unstable flow boiling phenomena of deionized water in a single microchannel having a hydraulic diameter of 155 µm with a bottom Pyrex glass wall. Fifteen platinum serpentine microheaters, bonded on the Pyrex glass wall, were used to measure local instantaneous wall temperatures. At low mass flux, a syringe pump was used to drive the subcooled water passing through the microchannel. Stable and unstable flow boiling modes in the single microchannel are identified, and flow pattern maps in terms of heat flux and mass flux as well as in term of exit vapor quality are presented respectively. It was found that unstable flow boiling occurred in the single microchannel if the exit vapor quality x_e > 0.013.     

Preferential CO oxidation over a platinum ceria alumina catalyst in a microchannel reactor

We have demonstrated the application of a Pt/Al₂O₃/CeO₂ catalyst into a microreactor to reduce the CO content by preferential oxidation in a H₂ rich model gas mixture. At temperatures between 120 °C and 260 °C and CO:O₂ ratios smaller than 0.7 CO conversion of up to 98% for CO₂-selectivities of 20%-35% were reached. The maximum space time yield was about 0.35 mmol g⁻¹ min⁻¹.To increase the CO conversion and CO₂-selectivity the concept of adding air or an air + N₂ mixture through microholes to the CO + H₂ mixture has been investigated with the design of an SSMR (staged supply microreactor). It was found that the CO₂ selectivity is higher compared to the case of feeding of all gases in one passage, but only for lower CO conversion rates.The best results for the preferential oxidation reaction were obtained at low reactant partial pressures. We found that the highest CO conversion rates are reached at reactant partial pressures in the range between 1 and 5 mbar. These values can be locally reached for total flows with 1-2 vol.-% CO and O₂ by feeding N₂ into the coated microchannels and the reactant gases into the uncoated microchannels. In this case, CO and H₂ are equally lowered and the CO oxidation may be favoured over the H₂ oxidation steps. A strong argument supporting this assumption is our measurements with varying the H₂ inlet stream.     

An experimental investigation on the confined and elongated bubbles in subcooled flow boiling in a single microchannel

The characteristics of the confined bubble and elongated bubble in subcooled flow boiling in a single horizontal rectangular microchannel with hydraulic diameter Dh =1mm are studied experimentally.The channel with 1×1mm cross section is fabricated in a thin copper plate whose confinement number is Co=2.8 and the degassed deionized water is used as the working fluid.Visualization on the confined and elongated bubbles inside the microchannel is carried out by employing a high-speed CCD camera with a microlens.The recorded images are carefully analyzed to illustrate the behaviors of the confinement and elongation processes of the bubble.The boiling number is used as an adjustable parameter to regulate the operating conditions which is eventually found to take a vital role in the bubble elongation process.Two formation patterns of the confined and elongated bubble are identified and the interactions between the neighboring confined and elongated bubbles are elucidated.     

Experimental investigation on flame pattern formations of DME-air mixtures in a radial microchannel

Flame pattern formations of premixed DME-air mixture in a heated radial channel with a gap distance of 2.5 mm were experimentally investigated. The DME-air mixture was introduced into the radial channel through a delivery tube which connected with the center of the top disk. With an image-intensified high-speed video camera, rich flame pattern formations were identified in this configuration. Regime diagram of all these flame patterns was drawn based on the experimental findings in the equivalence ratio range of 0.6-2.0 and inlet velocity range of 1.0-5.0 m/s. Compared with our previous study on premixed methane-air flames, there are several distinct characteristics for the present study. First, Pelton-wheel-like rotary flames and traveling flames with kink-like structures were observed for the first time. Second, in most cases, flames can be stabilized near the inlet port of the channel, exhibiting a conical or cup-like shape, while the conventional circular flame was only observed under limited conditions. Thirdly, an oscillating flame phenomenon occurred under certain conditions. During the oscillation process, a target appearance was seen at some instance. These pattern formation characteristics are considered to be associated with the low-temperature oxidation of DME.     

Bubble growth with chemical reactions in microchannels

This work investigates the nucleation and growth of CO₂ bubbles due to chemical reactions of sulfuric acid and sodium bicarbonate in three types of microchannels: one with uniform cross-section, one converging, and another one diverging. The Y-shaped test section, composed of main and two front microchannels, was made of P-type 〈1 0 0〉 orientation SOI (silicon on insulator) wafer. Bubble nucleation and growth in microchannels under various conditions were observed using a high-speed digital camera. The theoretical model for bubble dynamics with a chemical reaction is reviewed or developed. In the present study, no bubble was nucleated at the given inlet concentration and in the range of flow rate in the converging microchannel while the nucleation and growth of bubbles were observed in the diverging and uniform cross-section microchannels. Bubbles are nucleated at the channel wall and the equivalent bubble radius increases linearly during the initial period of the bubble growth. The bubble growth behavior for a particular case, without relative motion between the bubble and liquid, shows that the mass diffusion controls the bubble growth; consequently, the bubble radius grows as a square root of the time and agrees very well with the model in the literature. On the other hand, for other cases the bubbles stay almost at the nucleation site while growing with a constant gas product generation rate resulting in the instant bubble radius following the one-third power of the time.     

Flow pattern transition for vertical downward two-phase flow in capillary tubes. Inlet mixing effects

Experimental results of flow pattern for vertical downward two-phase flow in capillary tubes are reported and flow pattern regime maps are presented. In addition theoretically based transition criteria for the flow pattern are presented. The experimental results and theory seem to match each other fairly well. When results given in this paper are compared with empirical ones presented in literature stratified flow has not been reported in this study. Additional experiments were undertaken and inlet mixing effects have been found to be extremely important for the existence of fully developed flow.     

Measurement of heat transfer enhancement in forced convection due to hydrogen bubbles produced by electrolysis

Measurements of heat transfer were performed for a circular cylinder in cross-flow with hydrogen bubbles uniformly generated by electrolysis. The heat transfer coefficient as a function of velocity and temperature differences between the wall and the fluid is presented.Measurements in the low-velocity range show a heat transfer increase due to the stirring action of the bubbles.The use of this technique to investigate bubble dynamics and boiling heat transfer is discussed.     

Flow pattern of boiling in micro-channel by numerical simulation

Boiling flows of R-134a and R-22 fluids in a 0.50 mm circular channel have been simulated to analyze bubbly flow, bubbly/slug flow, slug flow and slug/semi-annular flow depending on bubble evolution. The vapor–liquid interface was captured using VOF method. We studied the behavior of bubble growth and coalescence related to flow pattern transitions (bubbly/slug flow to slug flow, slug flow to slug/semi-annular flow) and analyzed the effect of fluid properties on transition lines. Some parameters, including heat flux, mass velocity, ONB point, vapor velocity, bubble lifting diameter, growth rate and generation frequency, have been analyzed in detail. The results show that bubble growth and coalescence are important factors for flow pattern transitions. The flow patterns at the micro-channel outlet predicted by simulation were in agreement with phenomena observed in experiments for bubbly/slug flow, slug flow and slug/semi-annular flow. In addition, the peak bubble frequency at the outlet was predicted and the general shape of the bubble frequency distribution at the outlet from simulation was found to be consistent with the achieved experimental results.     

Photo-induced CO2 reduction by hydrogen for selective CO evolution in a dynamic monolith photoreactor loaded with Ag-modified TiO2 nanocatalyst

Ag-promoted TiO₂ nanoparticles immobilized over the cordierite monolithic support for dynamic and selective photo-reduction of CO₂ to CO by the use of hydrogen has been investigated. Ag-loaded TiO₂ NPs synthesized by a facile sol–gel method were coated over the monolith channels by dip-coating method. The samples were characterized by XRD, Raman, FTIR, SEM, TEM, XPS, N₂ adsorption–desorption, UV–Vis and PL spectroscopy. The photo-activity test of Ag-modified TiO₂ NPs was conducted for dynamic photocatalytic CO₂ reduction with H₂ as a reductant via a reverse water gas shift (RWGS) reaction in a cell type and monolith photo-reactors. Using 5 wt. % Ag/TO₂ NPs, CO₂ was energetically converted to CO with a yield rate 1335 μmole g-catal.^?1 h^?1, a 111 fold-higher than the amount of CO produced over the pure TiO₂ catalyst. More importantly, photo-activity of Ag/TiO₂ catalyst for CO evolution can be improved by 209 fold using monolith photo-reactor than the cell type reactor under the same operating conditions. This enactment was evidently due to the efficient light harvesting with larger illuminated surface area inside monolith micro-channels and efficient charges separation in the presence of Ag-metal. The reusability of Ag/TiO₂ NPs loaded over the monolithic support showed favorable recycling capability than the catalyst dispersed in a cell reactor. A possible reaction mechanism for this observation has been discussed in detail.     

Capturing and using CO2 as feedstock with chemical looping and hydrothermal technologies

Chemical looping technology for capturing and hydrothermal processes for conversion of carbon are discussed with focused and critical assessments. The fluidized and stationary reactor systems using solid, including biomass, and gaseous fuels are considered in chemical looping combustion, gasification, and reforming processes. Sustainability is emphasized generally in energy technology and in two chemical looping simulation case studies using coal and natural gas. Conversion of captured carbon to formic acid, methanol, and other chemicals is also discussed in circulating and stationary reactors in hydrothermal processes. This review provides analyses of the major chemical looping technologies for CO₂ capture and hydrothermal processes for carbon conversion so that the appropriate clean energy technology can be selected for a particular process. Combined chemical looping and hydrothermal processes may be feasible and sustainable in carbon capture and conversion and may lead to clean energy technologies using coal, natural gas, and biomass. Copyright © 2014 John Wiley & Sons, Ltd.     

水平微细管内CO2流动沸腾流态研究

针对CO_2水平微细管内流动沸腾换热流态及流态转变特性进行实验研究。实验工况:热流密度(5~35 k W/m~2)、质量流率(50~600 kg/(m~2·s))、饱和温度(-40~0℃)、管径(0.5~1.5 mm)。实验表明:CO_2在微细管内实际流态分别是泡状流、弹状流、间歇流、层流、波状流、混状流、环状流和雾状流;干涸过程中的流态主要为环状流-雾状流、波状流-雾状流的过程及不稳定的环状流;通过理论计算获得CO_2微细管内流动沸腾换热流态图,流态图显示热流密度对高干度区域流态转变有显著影响,质量流率大小直接决定了换热过程所经历的流态;不同饱和温度工质热物性不同改变了流型;理论分析所采用的流态形式与实际CO2在微细管内所具有的流态类型基本一致。