Adiabatic two-phase flow in rectangular microchannels with different aspect ratios: Part I – Flow pattern,pressure drop and void fraction

An aspect ratio is an important parameter for two-phase flow in a rectangular microchannel. To study the aspect ratio effect on the flow pattern, pressure drop and void fraction, experiments of adiabatic liquid water and nitrogen gas two-phase flow in rectangular microchannels were conducted. The widths and heights of rectangular microchannels are 510 μm × 470 μm, 608 μm × 410 μm, 501 μm × 237 μm and 503 μm × 85 μm. Therefore, the aspect ratios of the rectangular microchannels are 0.92, 0.67, 0.47 and 0.16; and the hydraulic diameters of the rectangular microchannels were 490, 490, 322 and 143 μm, respectively. Experimental ranges were liquid superficial velocities of 0.06–1.0 m/s and gas superficial velocities of 0.06–71 m/s. Visible rectangular microchannels were fabricated using a photosensitive glass. And pressure drop in microchannels was directly measured through embedded ports. The visualization of the flow pattern was carried out with a high-speed camera and a long distance microscope. Typical flow patterns in the rectangular microchannels observed in this study were bubble flow, transitional flow (multiple flow) and liquid ring flow. As the aspect ratio decreased, the bubble flow regime became dominant due to the confinement effect and the thickness of liquid film in corner was decreased. A void fraction in the rectangular microchannels has a linear relation with the volumetric quality. And the two-phase flow becomes homogeneous with decreasing aspect ratio owing to the reduction of the liquid film thickness. Like Zhang et al.’s [19] correlation, as the confinement number increased, the C-value in Lockhart and Martinelli correlation decreased. And a frictional pressure drop in the rectangular microchannels was highly related with the flow pattern.     

Experimental and numerical investigation of two-phase pressure drop in vertical cross-flow over a horizontal tube bundle

Experimental pressure drop data for vertical two-phase air–water flow across horizontal tubes is presented for gas mass fractions in the range 0.0005–0.6 and mass fluxes in the range 25–700 kg/m² s. The square in-line tube bundle had one column containing ten tubes and two columns of half tubes attached to the walls. The tubes had a diameter of 38 mm and a pitch to diameter ratio of 1.32. This data and air–water and R113 vapour–liquid data available in the literature are compared with the predictions from two kettle reboiler models, the one-dimensional model and a one-dimensional formulation of the two-fluid model. The one-dimensional model was implemented with three separate void fraction correlations and one two-phase friction multiplier correlation. The results show that the two-fluid model predicts air–water void fraction data well but R113 data poorly with pressure drop predictions for both being unsatisfactory. The one-dimensional model is shown to predict pressure drop and void fraction data reasonably well, provided a careful choice is made for the void fraction correlation.     

Determination of annular condensation heat transfer coefficient of steam in microchannels with trapezoidal cross sections

Experiments have been carried out to determine annular condensation heat transfer coefficient of steam in two silicon microchannels having trapezoidal cross sections with the same aspect ratio of 3.15 at 54 < G < 559 kg/m² s under 3-side cooling conditions. A semi-analytical method, based on turbulent flow boundary layer theory of liquid film with correlations of pressure drop and void fraction valid for microchannels, is used to derive the annular local condensation heat transfer coefficients. The predicted values based on the semi-analytical model are found within ±20% of 423 data points. It is shown that the annular condensation heat transfer coefficient in a microchannel increases with mass flux and quality and decreases with the hydraulic diameter.     

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.     

An experimental investigation on pressure drop of steam condensing in silicon microchannels

Experiments are carried out to study the two-phase pressure drop for water vapor condensation in four smooth trapezoidal silicon microchannels having hydraulic diameters of 109 μm, 142 μm, 151 μm, and 259 μm, respectively. It is found that two-phase frictional pressure drops in the microchannels are greatly influenced by the hydraulic diameter, mass flux and vapor quality. The two-phase pressure drop data in microchannels are compared with existing correlations for macro- and mini-channels based on the homogenous model and the separated flow model to determine their applicability to condensing flows in microchannels. A modified correlation for the Matinelli–Chisholm constant, taking into consideration of surface tension and diameter effects, is developed in the form of the Lockhart–Martinelli correlation for the pressure drop in steam condensation in microchannels. The resulting condensation pressure drop correlation equation is within ±15% of the experimental data.     

Correlation of critical heat flux and two-phase friction factor for subcooled convective boiling in structured surface microchannels

Critical heat flux (CHF) and pressure drop of subcooled flow boiling are measured for a microchannel heat sink containing 75 parallel 100 μm × 200 μm structured surface channels. The heated surface is made of a Cu metal sheet with/without 2 μm thickness diamond film. Tests and measurements are conducted with de-ionized water, de-ionized water +1 vol.% MCNT additive solution, and FC-72 fluids over a mass velocity range of 820–1600 kg/m² s, with inlet temperatures of 15(8.6)°C, 25(13.6)°C, 44(24.6)°C, and 64(36.6)°C for DI water (FC-72), and heat fluxes up to 600 W/cm². The CHF of subcooled flow boiling of the test fluids in the microchannels is measured parametrically. The two-phase pressure drop is also measured. Both CHF and the two-phase friction factor correlation for one-side heating with two other side-structured surface microchannels are proposed and developed in terms of the relevant parameters.     

Comparison of frictional pressure drop models during annular flow condensation of R600a in a horizontal tube at low mass flux and of R134a in a vertical tube at high mass flux

This study compares well-known two-phase pressure drop models with the experimental results of a condensation pressure drop of (i) R600a in a 1 m long horizontal smooth copper tube with an inner diameter of 4 mm, outer diameter of 6 mm and (ii) R134a in a 0.5 m vertical smooth copper tube with an inner diameter of 8.1 mm and outer diameter of 9.52 mm. Different vapour qualities (0.45–0.9 for R600a and 0.7–0.95 for R134a), various mass fluxes (75–115 kg m^?2 s^?1 for R600a and 300–400 for R134a kg m^?2 s^?1) and different condensing temperatures (30–43 °C for R600a and 40–50 °C for R134a) were tested under annular flow conditions. The quality of the refrigerant in the test section was calculated considering the temperature and pressure obtained from the experiment. The pressure drop across the test section was directly measured with a differential pressure transducer. The most agreeable correlations of various available options were then identified according to the results of analysis during annular flow regime.     

New prediction methods for CO2 evaporation inside tubes: Part I – A two-phase flow pattern map and a flow pattern based phenomenological model for two-phase flow frictional pressure drops

An updated flow pattern map was developed for CO₂ on the basis of the previous Cheng–Ribatski–Wojtan–Thome CO₂ flow pattern map [1], [2] to extend the flow pattern map to a wider range of conditions. A new annular flow to dryout transition (A–D) and a new dryout to mist flow transition (D–M) were proposed here. In addition, a bubbly flow region which generally occurs at high mass velocities and low vapor qualities was added to the updated flow pattern map. The updated flow pattern map is applicable to a much wider range of conditions: tube diameters from 0.6 to 10 mm, mass velocities from 50 to 1500 kg/m² s, heat fluxes from 1.8 to 46 kW/m² and saturation temperatures from ?28 to +25 °C (reduced pressures from 0.21 to 0.87). The updated flow pattern map was compared to independent experimental data of flow patterns for CO₂ in the literature and it predicts the flow patterns well. Then, a database of CO₂ two-phase flow pressure drop results from the literature was set up and the database was compared to the leading empirical pressure drop models: the correlations by Chisholm [3], Friedel [4], Grönnerud [5] and Müller-Steinhagen and Heck [6], a modified Chisholm correlation by Yoon et al. [7] and the flow pattern based model of Moreno Quibén and Thome [8], [9], [10]. None of these models was able to predict the CO₂ pressure drop data well. Therefore, a new flow pattern based phenomenological model of two-phase flow frictional pressure drop for CO₂ was developed by modifying the model of Moreno Quibén and Thome using the updated flow pattern map in this study and it predicts the CO₂ pressure drop database quite well overall.     

Prediction of two-phase condensation in horizontal tubes using probabilistic flow regime maps

A flow regime based condensation model is developed for refrigerants in single, smooth, horizontal tubes utilizing a generalized probabilistic two-phase flow map. Flow map time fraction information is used to provide a physically based weighting of heat transfer models developed for different flow regimes. The developed model is compared with other models in the literature, with experimentally obtained condensation data of R134a in 8.92 mm diameter tubes, and with data found in the literature for 3.14 mm, 7.04 mm, and 9.58 mm tubes with R11, R12, R134a, R22, R410A, and R32/R125 (60/40% by weight) refrigerants and a wide range of mass fluxes and qualities.     

Experimental study of convective condensation in an inclined smooth tube. Part II: Inclination effect on pressure drops and void fractions

This article is the second part of a two-part paper, dealing with an experimental study of convective condensation of R134a at a saturation temperature of 40 °C in an 8.38 mm inner diameter smooth tube in inclined orientations. The first part concentrates on the flow pattern and the heat transfer coefficients. This second part presents the pressures drops in the test condenser for different mass fluxes and different vapour qualities for the whole range of inclination angles (downwards and upwards). Pressures drops in a horizontal orientation were compared with correlations available in literature. In a vertical orientation, the experimental results were compared with pressure drop correlations associated with void fraction correlations available in literature. A good agreement was found for vertical upward flows but no correlation predicted correctly the measurements for downward flows. An apparent gravitational pressure drop and an apparent void fraction were defined in order to study the inclination effect on the flow. For upward flows, it seems as if the void fraction and the frictional pressure drop are independent of the inclination angle. Apparent void fractions were successfully compared with correlations in literature. This was not the case for downward flows. The experimental results for stratified downward flows were also successfully compared with the model of Taitel and Dukler.     

Pressure drop studies on two-phase flow in a uniformly heated vertical tube at pressures up to the critical point

Measurements of two-phase flow pressure drop have been made during a phase-change heat transfer process with refrigerant (R-134a) as a working fluid for a wide range of pressures right up to the critical pressure. The experiments were conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length over a heat flux range of 35–80 kW/m², mass flux range of 1200–2000 kg/m² s, exit quality range of 0.19–0.81 and for reduced pressures ranging from 0.24 to 1 with a fixed inlet subcooling of 3 °C. The measurements were compared with the predictions from the homogeneous flow model, a separated flow model using correlations drawn from the literature for void fraction and frictional pressure drop, and finally, using a flow pattern-based predictive method accounting specifically for bubbly, slug and annular flow regimes. It was found that the best results were obtained with the flow pattern-based approach with a mean deviation of ±20% over the entire pressure range.     

Ammonia two-phase flow in a horizontal smooth tube: Flow pattern observations,diabatic and adiabatic frictional pressure drops and assessment of prediction methods

The present study illustrates new experimental two-phase flow pattern observations together with diabatic boiling and adiabatic two-phase frictional pressure drop results for ammonia (R717) flowing inside a 14-mm internal diameter, smooth horizontal stainless steel tube. The flow pattern observations were made for mass velocities of 50, 100 and 160 kg s^?1 m^?2 and saturation temperatures of ?14, ?2 and 12 °C for vapor qualities ranging from 0.05 to 0.6. The flow patterns observed during the study included: stratified-wavy, slug-stratified-wavy, slug, intermittent and annular. For all the experimental conditions, the flow structure observations were compared against the predictions of the flow pattern map model of Wojtan et al. [L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: part I – a new diabatic two-phase flow pattern map, Int. J. Heat Mass Transfer 48 (2005) 2955–2969] and showed very good correspondence. The frictional pressure drop measurements were obtained for vapor qualities from 0.05 to 0.6, saturation temperatures from ?14 to 14 °C, mass velocities from 50 to 160 kg s^?1 m^?2 and heat fluxes from 12 to 25 kW m^?2. The experimental results show the traditional pressure drop trends: the frictional pressure drop increases with vapor quality and mass velocity. Moreover, the results also show that both diabatic and adiabatic frictional pressure drop values are similar, that is, the boiling process in itself does not affect the frictional pressure drop. The correlations of Friedel [L. Friedel, Improved friction drop correlations for horizontal and vertical two-phase pipe flow, in: European Two-Phase Flow Group Meeting, paper E2, Ispra, Italy, 1979], Lockhart and Martinelli [R.W. Lockhart, R.C. Martinelli, Proposed correlation of data for isothermal two-phase two-component in pipes, Chem. Eng. Process 45 (1949) 39–48] and Müller-Steinhagen and Heck [H. Müller-Steinhagen, K. Heck, A simple friction pressure correlation for two-phase flow in pipes, Chem. Eng. Process 20 (1986) 297–308] predicted only 54%, 52% and 60% of the experimental data within ±30%, respectively. The correlation of Grönnerud [R. Grönnerud, Investigation of liquid hold-up, flow-resistance and heat transfer in circulation type of evaporators, part iv: two-phase flow resistance in boiling refrigerans, in: Annexe 1972-1, Bull. de l’Inst. Froid, 1979] predicted 93% of the data and the flow pattern based method of Moreno Quibén and Thome [J. Moreno Quibén, J.R. Thome, Flow pattern based two-phase frictional pressure drop model for horizontal tubes. Part II: new phenomenological model, Int. J. Heat Fluid Flow 28 (2007) 1060–1072] predicted more than 97% of the experimental data within the same error band, while the latter method captures almost 89% of the data within ±20%.     

Saturated flow boiling heat transfer and pressure drop in silicon microchannel arrays

Flow boiling in arrays of parallel microchannels is investigated using a silicon test piece with imbedded discrete heat sources and integrated local temperature sensors. The microchannels considered range in width from 102 μm to 997 μm, with the channel depth being nominally 400 μm in each case. Each test piece has a footprint of 1.27 cm by 1.27 cm with parallel microchannels diced into one surface. Twenty five microsensors integrated into the microchannel heat sinks allow for accurate local temperature measurements over the entire test piece. The experiments are conducted with deionized water which enters the channels in a purely liquid state. Results are presented in terms of temperatures and pressure drop as a function of imposed heat flux. The experimental results allow a critical assessment of the applicability of existing models and correlations in predicting the heat transfer rates and pressure drops in microchannel arrays, and lead to the development of models for predicting the two-phase pressure drop and saturated boiling heat transfer coefficient.     

A complete evaluation method for the experimental data of flow boiling in smooth tubes

A complete solution for boiling phenomena in smooth tubes has been giving as a procedure regarding with the calculation of convective heat transfer coefficient and pressure drop using accurate experimental data validated by flow regime maps and sight glasses on the experimental facility. The experimental study is conducted in order to investigate the effect of operating parameters on flow boiling convective heat transfer coefficient and pressure drop of R134a. The smooth tube having 8.62 mm inner diameter and 1100 mm length is used in the experiments. The effect of mass flux, saturation temperature and heat flux is researched in the range of 290–381 kg/m² s, 15–22 °C and 10–15 kW/m², respectively. The experiments revealed that the heat transfer coefficient and pressure drop are significantly affected by mass flux for all tested conditions. Moreover, the experimental results are compared with well-known heat transfer coefficient and frictional pressure drop correlations given in the literature. In addition, 122 number of heat transfer and pressure drop raw experimental data is given for researchers to validate their theoretical models.     

Boiling of water and FC-72 in microchannels enhanced with novel features

A microchannel test section comprised of parallel square microchannels with a 25 × 25 μm and 50 × 50 μm cross section was manufactured. Boiling of perfluorinated dielectric fluid FC-72 and water in microchannels was studied. Troublesome occurrences associated with flow boiling in microchannels were reduced or eliminated with inlet/outlet restrictors, inlet/outlet manifolds and potential nucleation cavities incorporated in the array of microchannels. The gradual reduction of channel cross section in the manifolds ensured a uniform distribution of the working fluid among the microchannels. The flow restrictors provided a higher upstream pressure drop in comparison with the downstream pressure drop which favors vapor flow in the downstream direction and consequentially suppresses the vapor backflow present in flow boiling. The superheat of the microchannel wall necessary for the onset of boiling was decreased significantly with the incorporation of properly sized artificial cavities. Experimental results confirmed the benefits of the etched features, as there was (i) an even working fluid distribution (ii) without dominating backflows of vapor (iii) at a low temperature of the onset of boiling. Bubble growths as well as other events in the microchannels were visualized with a high-speed imaging system which captured images at over 87,000 frames per second. Results exhibit boiling hysteresis dependence of the working fluid and its mass flux through the microchannels. The temperature of the onset of boiling is highly dependent on the working fluid, microchannel size and its roughness.     

Experimental investigation of fluid flow and heat transfer in a single-phase liquid flow micro-heat exchanger

This work presents an experimental analysis of the hydrodynamic and thermal performance of micro-heat exchangers. Two micro-heat exchangers, characterized by microchannels of 100 × 100 and 200 × 200 μm square cross-sections, were designed for that purpose. The fluid used was deionized water and there was no phase change along the fluid circuit. The fluid pressure drop along the heat exchanger and the heat transfer were measured and corrections were made to isolate the contribution of the microchannels. The results were compared with the predictions of the classical viscous flow and heat transfer theory. The main conclusions show that the experimental results fit well with these theories. No effects of heat transfer enhancement or pressure drop increase were observed as a consequence of the small scale of the microchannels.     

High heat flux flow boiling in silicon multi-microchannels – Part III: Saturated critical heat flux of R236fa and two-phase pressure drops

New experimental critical heat flux results for saturated boiling conditions have been obtained for R236fa flowing in a silicon multi-microchannel heat sink composed of 67 parallel channels, 223 μm wide, 680 μm high and with 80 μm thick fins separating the channels. The microchannel length was 20 mm. The footprint critical heat fluxes measured varied from 112 to 250 W/cm² and the wall critical heat fluxes from 21.9 to 52.2 W/cm² for mass velocities from 276 to 992 kg/m²s. When increasing the mass velocity, the wall critical heat flux was observed to increase. The inlet saturation temperatures (20.31 ? T_sat,in ? 34.27 °C) and the inlet subcoolings (0.4 ? Δ T_sub ? 15.3 K) were found to have a negligible influence on the saturated CHF. The best methods for predicting the data were those of Wojtan et al. [L. Wojtan, R. Revellin, J. R. Thome, Investigation of critical heat flux in single, uniformly heated microchannels, Exp. Therm. Fluid Sci. 30 (2006) 765–774] and Revellin and Thome [R. Revellin, J. R. Thome, A theoretical model for the prediction of the critical heat flux in heated microchannels, Int. J. Heat Mass Transfer 50 (in press)]. They both predict the experimental CHF results with a mean absolute error of around 9%. Using the critical vapour quality, an annular-to-dryout transition is also proposed as a limit in a diabatic microscale flow pattern map. Pressure drop measurements were measured and analysed, showing that the homogeneous model could correctly predict the observed trends.     

Investigation of flow phenomena in a kettle reboiler

Two-phase flow phenomena were investigated while boiling R113 and n-pentane in a 241-tube thin slice kettle reboiler. For heat fluxes between 10 and 40 kW/m², row pressure drop measurements were made in three columns and visual observations of the flow patterns were recorded by a video camera. The height of the two-phase mixture above the tube bundle was also varied. The results revealed that the height of the mixture had little effect on the row pressure drop distribution in each column. At heat fluxes below 10 kW/m², the pressure drops were reasonably constant. However, at heat fluxes greater than this, the row pressure drop continuously declined.Two one-point-five-dimensional models were developed, one to aid the investigation of static liquid driven lateral flow in the tube bundle, and another to aid the investigation of the cause of the change from reasonably constant to continually declining row pressure drop. The data and the analysis showed that the flow within the tube bundle was always two-dimensional and that the flow pattern was dominated by the static liquid at the tube bundle edge when the heat flux was less than 10 kW/m². This corresponded to the bubbly flow regime. At larger heat fluxes, the flow pattern changed to intermittent flow. The change occurred when the Kutateladze number was 1.09. Declining row pressure drops occurred in this latter flow regime.     

Cooling of microprocessors using flow boiling of CO2 in a micro-evaporator: Preliminary analysis and performance comparison

The flow pattern based flow boiling heat transfer and two-phase pressure drop models for CO₂, recently developed by Cheng et al. [L. Cheng, G. Ribatski, J. Moreno Quibén, J.R. Thome, New prediction methods for CO₂ evaporation inside tubes: Part I – A two-phase flow pattern map and a flow pattern based phenomenological model for two-phase flow frictional pressure drops, Int. J. Heat Mass transfer 51 (2008) 111–124; L. Cheng, G. Ribatski, J.R. Thome, New prediction methods for CO₂ evaporation inside tubes: Part II – An updated general flow boiling heat transfer model based on flow patterns, Int. J. Heat Mass transfer 51 (2008) 125–135], have been used to predict the thermal performance of CO₂ in a silicon multi-microchannel evaporator (67 parallel channels with a width of 0.223 mm, a height of 0.68 mm and a length of 20 mm) for cooling of a microprocessor. First, some simulation results of CO₂ flow boiling heat transfer and two-phase pressure drops in microscale channels are presented. The effects of channel diameter, mass flux, saturation temperature and heat flux on flow boiling heat transfer coefficients and two-phase pressure drops are next addressed. Then, simulations of the base temperatures of the silicon multi-microchannel evaporator using R236fa and CO₂ were performed for the following conditions: base heat fluxes from 20 to 100 W/cm², a mass flux of 987.6 kg/m²s and a saturation temperature of 25 °C. These show that the base temperatures using CO₂ are much lower than those using R236fa. Compared to R236fa, CO₂ has much higher heat transfer coefficients and lower pressure drops in the multi-microchannel evaporator. However, the operation pressure of CO₂ is much higher than that of R236fa. Based on the analysis and comparison, CO₂ appears to be a promising coolant for microprocessors at low operating temperatures but also presents a great technological challenge like other new cooling technologies.     

Flow boiling in horizontal flattened tubes: Part I – Two-phase frictional pressure drop results and model

Experiments of diabatic two-phase pressure drops in flow boiling were conducted in four horizontal flattened smooth copper tubes with two different heights of 2 and 3 mm. The equivalent diameters of the flat tubes are 8.6, 7.17, 6.25, and 5.3 mm. The working fluids are R22 and R410A, respectively. The test conditions are: mass velocities from 150 to 500 kg/m² s, heat fluxes from 6 to 40 kW/m² and saturation temperature of 5 °C (reduced pressures p_r are 0.12 for R22 and 0.19 for R410A). The experimental results of two-phase pressure drops are presented and analyzed. Furthermore, the predicted two-phase frictional pressure drops by the flow pattern based two-phase pressure drop model of Moreno Quibén and Thome [J. Moreno Quibén, J.R. Thome, Flow pattern based two-phase frictional pressure drop model for horizontal tubes, Part I: Diabatic and adiabatic experimental study, Int. J. Heat Fluid Flow 28 (2007) 1049–1059; J. Moreno Quibén, J.R. Thome, Flow pattern based two-phase frictional pressure drop model for horizontal tubes, Part II: New phenomenological model, Int. J. Heat Fluid Flow 28 (2007) 1060–1072] using the equivalent diameters were compared to the experimental data. The model, however, underpredicts the flattened tube two-phase frictional pressure drop data. Therefore, correction to the annular flow friction factor was proposed for the flattened tubes and now the method predicts 83.7% of the flattened tube pressure drop data within ±30%. The model is applicable to the flattened tubes in the test condition range in the present study. Extension of the model to other conditions should be verified with experimental data.