Effects of channel dimension,heat flux,and mass flux on flow boiling regimes in microchannels

Experiments are conducted with a perfluorinated dielectric fluid, Fluorinert FC-77, to investigate the effects of channel size and mass flux (225–1420 kg/m²s) on microchannel flow boiling regimes by means of high-speed photography. Seven different silicon test pieces with parallel microchannels of widths ranging from 100 to 5850 μm, all with a depth of 400 μm, are considered. Flow visualizations are performed with a high-speed digital video camera while local measurements of the heat transfer coefficient are simultaneously obtained. The visualizations and the heat transfer data show that flow regimes in the microchannels of width 400 μm and larger are similar, with nucleate boiling being dominant in these channels over a wide range of heat flux. In contrast, flow regimes in the smaller microchannels are different and bubble nucleation at the walls is suppressed at a relatively low heat flux for these sizes. Two types of flow regime maps are developed and the effects of channel width on the flow regime transitions are discussed.     

Measurements and high-speed visualizations of flow boiling of a dielectric fluid in a silicon microchannel heat sink

Experiments were conducted to investigate flow boiling heat transfer to a dielectric fluid in a silicon chip-integrated microchannel heat sink. Twenty-four microchannels, each 389 μm × 389 μm in cross-section, were fabricated into the 12.7 mm × 12.7 mm silicon substrate. High-speed visualizations (at 12,500 frames per second) were performed simultaneously with heat transfer and pressure drop measurements to investigate the physics of flow boiling in parallel microchannel arrays. At low heat fluxes, bubbly flow is dominant, with the bubbles coalescing to form vapor slugs as the heat flux is increased. At high heat fluxes, the flow regimes in the downstream portion of the microchannels are characteristic of alternating wispy-annular flow and churn flow, while flow reversal is observed in the upstream region near the microchannel inlet. Local heat transfer measurements, obtained at three flow rates ranging from 35 to 60 ml/min, show that at lower heat fluxes, the heat transfer coefficient increases with increasing heat flux. The heat transfer coefficient in fully developed boiling is seen to be independent of flow rate in this range. At higher heat fluxes (exceeding 542, 673, 730 kW/m², respectively, for flow rates of 35, 47 and 60 ml/min), this trend is reversed, and the heat transfer coefficient decreases with further increases in heat flux due to partial dryout in some of the microchannels. Heat fluxes at which fully developed boiling is achieved depend on the flow rate. The pressure drop in fully developed boiling increases with increasing heat flux and is independent of flow rate for the test conditions considered in this work.     

Macro-to-microchannel transition in two-phase flow: Part 2 - Flow boiling heat transfer and critical heat flux

This part of the paper presents the current experimental flow boiling heat transfer and CHF data acquired for R134a, R236fa and R245fa in single, horizontal channels of 1.03, 2.20 and 3.04 mm diameters over a range of experimental conditions. The aim of this study is to investigate the effects of channel confinement, heat flux, flow pattern, saturation temperature, subcooling and working fluid properties on the two-phase heat transfer and CHF. Experimentally, it was observed that the flow boiling heat transfer coefficients are a significant function of the type of two-phase flow pattern. Furthermore, the monotonically increasing heat transfer coefficients at higher vapor qualities, corresponding to annular flow, signifies convective boiling as the dominant heat transfer mechanism in these small scale channels. The decreasing heat transfer trend at low vapor qualities in the slug flow (coalescing bubble dominated regime) was indicative of thin film evaporation with intermittent dry patch formation and rewetting at these conditions. The coalescing bubble flow heat transfer data were well predicted by the three-zone model when setting the dryout thickness to the measured surface roughness, indicating for the first time a roughness effect on the flow boiling heat transfer coefficient in this regime. The CHF data acquired during the experimental campaign indicated the influence of saturation temperature, mass velocity, channel confinement and fluid properties on CHF but no influence of inlet subcooling for the conditions tested. When globally comparing the CHF values for R134a in the 0.51-3.04 mm diameter channels, a peak in CHF peak was observed lying in between the 0.79 (Co ≈ 0.99) and 1.03 (Co ≈ 0.78) mm channels. A new CHF correlation has been proposed involving the confinement number, Co that is able to predict CHF for R134a, R236fa and R245fa in single-circular channels, rectangular multichannels and split flow rectangular multichannels. In summary, the present flow boiling and CHF trends point to a macro-to-microscale transition as indicated by the results presented in Ong and Thome (2011) [1].     

Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (∼10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (∼100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000–2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.     

Elongated bubbles in microchannels. Part II: Experimental study and modeling of bubble collisions

The collision of elongated bubbles has been studied along adiabatic glass microchannels of 509 and 790 μm internal diameters for refrigerant R-134a. The slug flow regime obtained here comes from the nucleation process inside a micro-evaporator located upstream. Using an optical measurement technique based on two lasers and two photodiodes, it was possible to determine the vapor bubble length distributions at the exit of the micro-evaporator and 70 mm downstream and thus characterize both diabatic and adiabatic bubble collisions. The database includes 412 coupled sets of distributions involving thousands of bubbles. Half of the database has been obtained under diabatic conditions and the second half under adiabatic conditions.     

Adiabatic and diabatic two-phase venting flow in a microchannel

The growth and advection of the vapor phase in two-phase microchannel heat exchangers increase the system pressure and cause flow instabilities. One solution is to locally vent the vapor formed by capping the microchannels with a porous, hydrophobic membrane. In this paper we visualize this venting process in a single 124 μm by 98 μm copper microchannel with a 65 μm thick, 220 nm pore diameter hydrophobic Teflon membrane wall to determine the impact of varying flow conditions on the flow structures and venting process during adiabatic and diabatic operation. We characterize liquid velocities of 0.14, 0.36 and 0.65 m/s with superficial air velocities varying from 0.3 to 8 m/s. Wavy-stratified and stratified flow dominated low liquid velocities while annular type flows dominated at the higher velocities. Gas/vapor venting can be improved by increasing the venting area, increasing the trans-membrane pressure or using thinner, high permeability membranes. Diabatic experiments with mass flux velocities of 140 and 340 kg/s/m² and exit qualities up to 20% found that stratified type flows dominate at lower mass fluxes while churn-annular flow became more prevalent at the higher mass-flux and quality. The diabatic flow regimes are believed to significantly influence the pressure-drop and heat transfer coefficient in vapor venting heat exchangers.     

Artificial boiling heat transfer in the free convection to carbonic acid solution

Free convection phenomenon has been experimentally investigated around a horizontal rod heater in carbonic acid solution. Because of the tendency of the solution to desorb carbon dioxide gas when temperature is increased, bubbles appear when cylinder surface is heated. The bubbles consists mainly carbon dioxide and also a negligible amount of water vapor. The results present that dissolved carbon dioxide in water significantly enhances the heat transfer coefficient in compare to pure free convection regime. This is mainly due to the microscale mixing on the heat transfer surface, which is induced by bubble formation. In this investigation, experiments are performed at different bulk temperatures between 288 and 333 K and heat fluxes up to 400 kW m⁻² at atmospheric pressure. Bubble departure diameter, nucleation site density and heat transfer coefficient have been experimentally measured. A model has been proposed to predict the heat transfer coefficient.     

Bubble dynamics and interactions with a pair of micro pillars in tandem

This study investigates flow patterns and bubble dynamics of two-phase flow around two 100 μm diameter circular pillars in tandem, which were entrenched inside a horizontal micro channel. Bubble velocity, trajectory, size, and void fraction were measured using a high speed camera and analyzed using a particle tracking velocimetry method. A range of gas and liquid superficial velocities were tested, resulting in different bubbly flow patterns, which were consistent with previous studies. These flow patterns were altered as they interacted with the pillars. Depending on the relative transverse location of bubbles to the pillars, and through bubble–bubble interaction, the flow sometimes returned to its original state. It was also determined that the pillars altered both the bubble trajectory and void fraction, especially in the pillars region.     

Flow boiling heat transfer and pressure drop of R-134a in a mini tube: an experimental investigation

This paper presents the results of an experimental study carried out with R-134a during flow boiling in a horizontal tube of 2.6 mm ID. The experimental tests included (i) heat fluxes in the range from 10 to 100 kW/m², (ii) the refrigerant mass velocities set to the discrete values in the range of 240-930 kg/(m² s) and (iii) saturation temperature of 12 and 22 °C. The study analyzed the heat transfer, through the local heat transfer coefficient along of flow, and pressure drop, under the variation of these different parameters. It was possible to observe the significant influence of heat flux in the heat transfer coefficient and mass velocity in the pressure drop, besides the effects of saturation temperature. In the low quality region, it was possible to observe a significant influence of heat flux on the heat transfer coefficient. In the high vapor quality region, for high mass velocities, this influence tended to vanish, and the coefficient decreased. The influence of mass velocity in the heat transfer coefficient was detected in most tests for a threshold value of vapor quality, which was higher as the heat flux increased. For higher heat flux the heat transfer coefficient was nearly independent of mass velocity. The frictional pressure drop increased with the increase in vapor quality and mass velocity. Predictive models for heat transfer coefficient in mini channels were evaluated and the calculated coefficient agreed well with measured data within a range 35% for saturation temperature of 22 °C. These results extend the ranges of heat fluxes and mass velocities beyond values available in literature, and add a substantial contribution to the comprehension of boiling heat transfer phenomena inside mini channels.     

Influence of film thickness and cross-sectional geometry on hydrophilic microchannel condensation

Condensation in hydrophilic microchannel is strongly influenced by the channel cross-sectional geometry and the condensing surfaces hydrophobicity, which govern the evolution of the liquid film. This work makes progress on studying the relationship between channel geometry and condensation through flow regime visualizations, film-thickness measurements with optical interferometery, and temperature profile measurements with heat flux distribution construction. The hydrophilic microchannels have aspect ratios ranging from 1 to 5 and hydraulic diameters from 100 μm through 300 μm. The experimental measurement qualitatively matches the prediction of previous theoretical models accounting for the surface tension effect, which highlights the importance of surface tension force and channel geometry in the microchannel condensation. Pressure drop and mean heat flux measurements show that a larger channel is favorable for minimizing the pressure drop, while a smaller channel size and higher aspect ratio are desirable for maximizing the mean heat flux. The optimization of the channel geometry for a given application lies in the trade-off between these two factors.     

Experimental investigation of flow boiling characteristics in a cross-linked microchannel heat sink

This paper experimentally investigates flow boiling characteristics in a cross-linked microchannel heat sink at low mass fluxes and high heat fluxes. The heat sink consists of 45 straight microchannels each with a hydraulic diameter of 248 μm and heated length of 16 mm. Three cross-links, of width 500 μm, are introduced in the present microchannel heat sink to achieve better temperature uniformity and to avoid flow mal-distribution. Flow visualization, flow instability, two-phase pressure drop, and two-phase heat transfer measurements are conducted using the dielectric coolant FC-72 over a range of heat flux from 7.2 to 104.2 kW/m², mass flux from 99 to 290 kg/m² s, and exit quality from 0.01 to 0.71. Thermochromic liquid crystals are used in the present study as full-field surface temperature sensors to map the temperature distribution on the heat sink surface. Flow visualization studies indicate that the observed flow regime is primarily slug. Visual observations of flow patterns in the cross-links demonstrate that bubbles nucleate and grow rapidly on the surface of the cross-links and in the tangential direction at the microchannels’ entrance due to the effect of circulations generated in those regions. The two-phase pressure drop strongly increases with the exit quality, at x_e,o < 0.3, and the two-phase frictional pressure drop increases by a factor of 1.6–2 compared to the straight microchannel heat sink. The flow boiling heat transfer coefficient increases with increasing exit quality at a constant mass flux, which is caused by the dominance of the nucleation boiling mechanism in the cross-link region.     

Flow condensation heat transfer characteristics of hydrocarbon refrigerants and dimethyl ether inside a horizontal plain tube

Flow condensation heat transfer coefficients (HTCs) and pressure drop of R22, propylene, propane, DME and isobutane are measured on a horizontal plain tube. The main test section in the experimental flow loop is made of a plain copper tube of 8.8 mm inner diameter and 530 mm length. The refrigerant is cooled by passing cold water through the annulus surrounding the test section. Tests are performed at a fixed refrigerant saturation temperature of 40 ± 0.2 °C with mass fluxes of 100, 200, and 300 kg/m² s and heat flux of 7.3–7.7 kW/m². The heat transfer and pressure drop data are obtained in the vapor quality range of 10–90%. Test results show that for a given mass flux the flow condensation HTCs of propylene, propane, DME and isobutane are higher than those of R22 by up to 46.8%, 53.3%, 93.5% and 61.6%, respectively. Also well-known correlations developed based upon conventional fluorocarbon refrigerants predict the present data within a mean deviation of 33%. Finally, the pressure drop increases as the mass flux and quality increase and isobutane shows the highest pressure drop due to its lowest vapor pressure among the fluids tested.     

Flow boiling performance in horizontal microfinned copper tubes with the same geometric characteristics

This article reports an experimental investigation on flow boiling heat transfer and pressure drop of refrigerant R-134a in a smooth horizontal and two microfinned tubes from different manufacturers with the same geometric characteristics. Experiments have been carried out in an experimental facility developed for change of phase studies with a test section made with 9.52 mm external diameter, 1.5 m long copper tubes, electrically heated by tape resistors wrapped on the external surface. Tests have been performed under the following conditions: inlet saturation temperature of 5 °C, vapor qualities from 5% to 90%, mass velocity from 100 to 500 kg/s m², and a heat flux of 5 kW/m². Experimental results indicated that the heat transfer performance was basically the same for both microfin tubes. The pressure drop is higher in the microfinned tubes in comparison to the smooth tube over the whole range of mass velocities and vapor qualities. The enhancement factor, used to evaluate the combination of heat transfer and pressure drop, is higher than one for both tubes for mass velocities lower than 300 kg/s m². Values lower than one have been obtained for both tubes in the mass velocity upper range as a result of a significant pressure drop increment not followed by a correspondent increment in the heat transfer coefficient. Some images, illustrating the flow patterns, were obtained from the visualization section, located in the exit of the test section with the same internal diameter of the tested tube.     

Two-phase flow of R-134a refrigerant during flow boiling through a horizontal circular mini-channel

This research focuses on heat transfer to R-134a during flow boiling in a 1.75 mm internal diameter tube. Flow visualisation and heat transfer experiments are conducted to obtain heat transfer coefficients for different flow patterns. The measured data in each flow regime are compared with predictions from a three-zone flow boiling model. The calculations are in fair agreement with the experimental results which correspond in particular to slug flow, throat-annular flow and churn flow regimes under conditions of low heat flux.     

Flow structure and bubble dynamics in supercritical water fluidized bed and gas fluidized bed: A comparative study

Supercritical water (SCW) fluidized bed is a new reactor concept for hydrogen production from biomass or coal gasification. In this paper, a comparative study on flow structure and bubble dynamics in a supercritical water fluidized bed and a gas fluidized bed was carried out using the discrete element method (DEM). The results show that supercritical water condition reduces the incipient fluidization velocity, changes regime transitions, i.e. a homogeneous fluidization was observed when the superficial velocity is in the range of the minimum fluidization velocity and minimum bubbling velocity even the solids behave as Geldart B powders in the gas fluidized bed. Bubbling fluidization in the supercritical water fluidized bed was formed after superficial velocity exceeds the minimum bubbling velocity, as in the gas fluidized bed. Bubble is one of the most important features in fluidized bed, which is also the emphasis in this paper. Bubble growth was effectively suppressed in the supercritical water fluidized bed, which resulted in a more uniform flow structure. By analyzing a large number of bubbles, bubble dynamic characteristics such as diameter distribution, frequency, rising path and so on, were obtained. It is found that bubble dynamic characteristics in the supercritical water fluidized bed differ a lot from that in the gas fluidized bed, and there is a better fluidization quality induced by the bubble dynamics in the supercritical water fluidized bed.     

Characterization of diabatic two-phase flows in microchannels: Flow parameter results for R-134a in a 0.5 mm channel

An optical measurement method for two-phase flow pattern characterization in microtubes has been utilized to determine the frequency of bubbles generated in a microevaporator, the coalescence rates of these bubbles and their length distribution as well as their mean velocity. The tests were run in a 0.5 mm glass channel using saturated R-134a at 30 °C (7.7 bar). The optical technique uses two laser diodes and photodiodes to measure these parameters and to also identify the flow regimes and their transitions. Four flow patterns (bubbly flow, slug flow, semi-annular flow and annular flow) with their transitions were detected and observed also by high speed video. It was also possible to characterize bubble coalescence rates, which were observed here to be an important phenomena controlling the flow pattern transition in microchannels. Two types of coalescence occurred depending on the presence of small bubbles or not. The two-phase flow pattern transitions observed did not compare well to a leading macroscale flow map for refrigerants nor to a microscale map for air–water flows. Time averaged cross-sectional void fractions were also calculated indirectly from the mean two-phase vapor velocities and compared reasonably well to homogeneous values.     

Characterization and prediction of two-phase flow regimes in miniature tubes

Visual observations of two-phase regimes for R134a and R245fa flowing in 0.509 mm and 0.790 mm horizontal tubes are documented and compared to the predictions of the analytical flow regime models available in the literature. Annular flow was found to dominate the behavior of these two miniature channels, with a significant Slug flow regime at intermediate qualities. Despite the horizontal orientation of these tubes, there were no observations of Stratified flow and a very limited region of Bubble flow. A comparison of the more than 2200 flow regime observations to the predictions of the Taitel–Dukler flow regime methodology revealed that 67% of the empirically observed flow pattern data were correctly identified. The Ullmann–Brauner model, based on an air–water database, correctly predicted the appropriate flow regime for 81% of the reported data. Proposed modifications in the Bubble-to-Slug and Slug-to-Annular transition criteria, respectively, were shown to provide a modest further improvement in the overall predictability to 90% of the observed data for the two refrigerants studied.     

Flow regime development analysis in adiabatic upward two-phase flow in a vertical annulus

In this work radial and axial flow regime development in adiabatic upward air-water two-phase flow in a vertical annulus has been investigated. Local flow regimes have been identified using conductivity probes and neural networks techniques. The inner and outer diameters of the annulus are 19.1 mm and 38.1 mm, respectively. The equivalent hydraulic diameter of the flow channel, D_H, is 19.0 mm and the total length is 4.37 m. The flow regime map includes 1080 local flow regimes identifications in 72 flow conditions within a range of 0.01 m/s < 〈j_g〉 < 30 m/s and 0.2 m/s < 〈j_f〉 < 3.5 m/s where 〈j_g〉 and 〈j_f〉 are, respectively, superficial gas and liquid velocities. The local flow regime has been classified into four categories: bubbly, cap-slug, churn-turbulent and annular flows. In order to study the radial and axial development of flow regime the measurements have been performed at five radial locations. The three axial positions correspond to z/D_H = 52, 149 and 230, where z represents the axial position. The flow regime indicator has been chosen as some statistical parameters of local bubble chord length distributions and self-organized neural networks have been used as mapping system. This information has been also used to compare the results given by the existing flow regime transition models. The local flow regime is characterized basically by the void fraction and bubble chord length. The radial development of flow regime shows partial and complete local flow regime combinations. The radial development is controlled by axial location and superficial liquid velocity. The radial flow regime transition is always initiated in the center of the flow channel and it is propagated towards the channel boundaries. The axial development of flow regime is observed in all the flow maps and it is governed by superficial liquid velocity and radial location. The prediction results of the models are compared for each flow regime transition.     

Natural convection boiling of water and surfactants in narrow horizontal annular channels

Natural convection boiling of water and surfactants at atmospheric pressure in narrow horizontal annular channels was studied experimentally in the range of Bond numbers Bo = 0.185–1.52. The flow pattern was visualized by high-speed video recording to identify the different regimes of boiling of water and surfactants. The channel length was 24 mm and 36 mm, the gap size was 0.45, 1.2, 2.2, and 3.7 mm. The heat flux was in the range of 20–500 kW/m², the concentration of surfactant solutions was varied from 10 to 600 ppm. For water boiling at Bond numbers Bo < 1 the CHF in restricted space is lower than that in unconfined space. This effect increases with increasing the channel length. For water at Bond number Bo = 1.52, boiling can almost be considered as unconfined. Additive of surfactant led to enhancement of heat transfer compared to water boiling in the same gap size, however, this effect decreased with decreasing gap size. For the same gap size, CHF in surfactant solutions was significantly lower than that in water. Hysteresis was observed for boiling in degraded surfactant solutions.     

Flow pattern and boiling heat transfer of CO2 in horizontal small-bore tubes

Increasing attention has been focused on carbon dioxide (CO₂) heat pump system where the temperature level is rather low, while the operating pressure is rather high. In this system, the density difference between vapor and liquid becomes rather small, which significantly affects flow patterns. Low surface tension and latent heat also have significant influence on two-phase flow patterns and heat transfer. This paper describes experimental and numerical investigation on flow patterns and heat transfer characteristics of boiling flow CO₂ at high pressure in horizontal small-bore tubes ranging from 1.0 mm to 3.0 mm I.D. Even though the density difference is rather small at high pressure, phase stratification takes place, which leads to the intermittent dryout at the upper wall. So far developed discrete bubble model by the authors for vertical flows is modified so as to include horizontal flow mechanisms. The predicted flow patterns with this new model agree on the whole with the experimental observation.