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.     

Measurement and modeling of liquid film thickness evolution in stratified two-phase microchannel flows

Polymer electrolyte membrane (PEM) fuel cells incorporating microchannels (D < 500 μm) can benefit from improved fuel delivery and convective cooling. However, this requires a better understanding of two-phase microchannel transport phenomena, particularly liquid–gas interactions and liquid clogging in cathode air-delivery channels. This paper develops optical fluorescence imaging of water films in hydrophilic channels with varying air velocity and water injection rate. Micromachined silicon test structures with optical access and distributed water injection simulate the cathode channels of a PEM fuel cell. Film thickness data vary strongly with air velocity and are consistent with stratified flow modeling. This work facilitates the study of regime transitions in two-phase microchannel flows and the effects of flow regimes on heat and mass transfer and axial pressure gradients.     

Infrared micro-particle image velocimetry measurements and predictions of flow distribution in a microchannel heat sink

The flow distribution in a silicon microchannel heat sink was studied using infrared micro-particle image velocimetry (IR μPIV). The microchannel test piece consisted of seventy-six 110 μm wide × 371 μm deep channels etched into a silicon substrate. Inlet and outlet manifolds, also etched into the substrate, were fed by 1.4 mm inner-diameter tubing ports. An image-processing algorithm was developed that significantly improves the quality of IR μPIV recordings in low signal-to-noise ratio environments. A general expression for the PIV measurement depth is presented, which is valid for PIV images that have undergone a threshold image-processing operation. Experiments were performed at two different flow rates: 10 ml/min (Re = 10.2) and 100 ml/min (Re = 102). Little flow maldistribution was observed at the lower flow rate. However, significant flow maldistribution was observed at Re = 102, with the channels near the centerline having an approximately 30% greater mass flux than the channels near the lateral edges of the heat sink. Numerical simulations carried out for flow in the microchannel heat sink agreed very well with the experimental measurements, validating the use of a computational approach for studying the effect of manifold design on flow distribution in microchannel heat sinks.     

Two-phase flow pattern and frictional performance across small rectangular channels

This study presents flow visualizations and two-phase frictional pressure drop data for three rectangular channels with channel height of 3, 6 and 9 mm, and a fixed width of 3 mm. It is found that the stratified flow pattern still exists for an aspect ratio of unity at a low mass flux of 100 kg/m² s but it completely vanishes when G > 200 kg/m²s. For the same plug flow of intermittent flow pattern, the number of plug increases whereas its length decreases when the aspect ratio is increased. This is especially pronounced when the mass flux is further increased over 500 kg/m² s. The major departure of the observed flow pattern relative to the conventional Mandhane flow map is the transition boundary for slug/annular had been moved to a much lower superficial vapor velocity. The two-phase frictional pressure drop data are compared to homogeneous and Chisholm method, Wambsganss and Ide-Fukano correlations. It is found that none of the existing methods or correlations can satisfactorily predict the two-phase pressure gradient in rectangular channels. A modified C factor of Chisholm method considering the effect of aspect ratio was proposed from the empirical fit with the data sets of Wambsganss et al., Ide-Fukano, and this study. The corresponding mean deviations of the proposed correlation against the datasets are 24.99%, 10.83% and 10.73%, respectively. This correlation is applicable in wide rages of mass flux (50 < G < 700 kg/m² s), vapor quality (0.001 < x < 0.95), Martinelli parameter (0.05 < X < 20) and aspect ratio (0.1 < A < 1.0).     

Boiling two-phase flow and efficiency of co- and counter-current microchannel heat exchangers with gas heating

To learn how to utilize the exhaust heat from a high-temperature gas product of a methanol reformer, the present study experimentally investigates the boiling two-phase flow in co- and counter-current microchannel heat exchangers (MCHE) with gas heating. Boiling two-phase flow patterns, two-phase flow instability, and efficiency are explored. The working fluid on the hot and cold sides are helium and liquid methanol, respectively. The silicon-based MCHE, which has dimensions of 20 mm × 20 mm, is designed with 18 parallel microchannels on both sides and is prepared using microfabrication processes. Four types of two-phase flow patterns – bubbly-elongated slug flow, annular flow, annular flow with liquid film breakup, and dryout are identified in both types of MCHE that are studied. A flow pattern map is then constructed on the plane of the methanol mass flux versus heat flux for both types of MCHE. In the counter-current MCHE, the efficiency increases significantly with an increase in the mass flux in both the single- and two-phase flow regions, while the effect of mass flux is insignificant in the co-current MCHE. In the two-phase flow region, the efficiency of both types of MCHEs gradually increases with an increase in the hot-side thermal power until the CHF is approached. The highest efficiency obtained in the present study is about 0.85 and 0.90 for the co- and counter-current MCHEs, respectively.     

Flow pattern transition instability in a microchannel with CO2 bubbles produced by chemical reactions

The present study investigates experimentally the two-phase flow in a rectangular microchannel with CO₂ bubbles generated by chemical reactions of sulfuric acid (H₂SO₄) and sodium bicarbonate (NaHCO₃). The microchannel with a hydraulic diameter of 132.7 μm is prepared using bulk micromachining and anodic bonding process. Evolution of two-phase flow patterns in the microchannel was observed using a high speed video camera and the corresponding pressure drop was investigated. It is found that the inlet concentration and flow rate of reactants have a significant effect on the evolution of two-phase flow characteristics and slug flow is the dominant flow pattern. The flow pattern transition instability between bubbly-slug and slug flow takes place for the cases with highest inlet concentration, i.e., C = 0.8 mol/L, and low flow rates of this study. The oscillation frequency is from 0.024 to 0.041 Hz and the magnitude of oscillation in pressure drop is from 10 to 15 kPa. A mechanism based on the flow circulation in the liquid slug may reasonably explain the flow pattern transition instability. Small amplitude, high frequency oscillations with a frequency of about 45 Hz are superimposed on the low frequency flow pattern transition as well as prevail for other cases without the flow pattern transition instability. The two-phase flow pressure drop increases with increase in both flow rate and inlet concentration.     

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.     

Heat transfer of gas–liquid mixture in micro-channel heat sink

The main objective of the present investigation is to study heat transfer in parallel micro-channels of 0.1 mm in size. Comparison of the results of this study to the ones obtained for two-phase flow in “conventional” size channels provides information on the complex phenomena associated with heat transfer in micro-channel heat sinks. Two-phase flow in parallel micro-channels, feeding from a common manifold shows that different flow patterns occur simultaneously in the different micro-channels: liquid alone (or single-phase flow), bubbly flow, slug flow, and annular flow (gas core with a thin liquid film, and a gas core with a thick liquid film). Although the gas core may occupy almost the entire cross-section of the triangular channel, making the side walls partially dry, the liquid phase always remained continuous due to the liquid, which is drawn into the triangular corners by surface tension. With increasing superficial gas velocity, a gas core with a thin liquid film is observed. The visual observation showed that as the air velocity increased, the liquid droplets entrained in the gas core disappeared such that the flow became annular. The probability of appearance of different flow patterns should be taken into account for developing flow pattern maps. The dependence of the Nusselt number, on liquid and gas Reynolds numbers, based on liquid and gas superficial velocity, respectively, was determined in the range of Re_LS = 4–56 and Re_GS = 4.7–270. It was shown that an increase in the superficial liquid velocity involves an increase in heat transfer (Nu_L). This effect is reduced with increasing superficial gas velocity, in contrast to the results reported on two-phase heat transfer in “conventional size” channels.     

Development of a thermoelectric refrigerator with two-phase thermosyphons and capillary lift

A thermoelectric domestic refrigerator has been developed, with a single compartment of 0.225 m³, for food preservation at 5 °C. The cooling system is made up of two equal thermoelectric devices, each composed of a Peltier module (50 W) with its hot side in contact with a two-phase and natural convection thermosyphon (TSV) and a two-phase and capillary lift thermosyphon (TPM), in contact with the cold side.The entire refrigerator has been simulated and designed using a computational model, based on the finite difference method. Subsequently an experimental optimization phase of the thermosyphons was carried out, until thermal resistance values of R_TSV = 0.256 K/W and R_TPM = 0.323 K/W were obtained. These values were lower than those obtained with finned heat sinks.Finally, a functional prototype of a thermoelectric refrigerator was built, and the results which were obtained demonstrate that it is able to maintain a thermal drop (Ambient Temperature–Inside Temperature) of 19 °C. The electric power consumption at nominal conditions was 45 W, reaching a COP value of 0.45. The study demonstrated that by incorporating these two-phase devices into thermoelectric refrigeration increases the COP by 66%, compared with those which use finned heat sinks.     

Flow condensation in parallel micro-channels – Part 1: Experimental results and assessment of pressure drop correlations

In this first part of a two-part study, experiments were performed to investigate condensation of FC-72 along parallel, square micro-channels with a hydraulic diameter of 1 mm and a length of 29.9 cm, which were formed in the top surface of a solid copper plate. The condensation was achieved by rejecting heat to a counter flow of water through channels brazed to the underside of the copper plate. The FC-72 entered the micro-channels slightly superheated, and operating conditions included FC-72 mass velocities of 68–367 kg/m² s, FC-72 saturation temperatures of 57.2–62.3 °C, and water mass flow rates of 3–6 g/s. Using high-speed video imaging and photomicrographic techniques, five distinct flow regimes were identified: smooth-annular, wavy-annular, transition, slug, and bubbly, with the smooth-annular and wavy-annular regimes being most prevalent. A detailed pressure model is presented which includes all components of pressure drop across the micro-channel. Different sub-models for the frictional and accelerational pressure gradients are examined using the homogenous equilibrium model (with different two-phase friction factor relations) as well as previous macro-channel and mini/micro-channel separated flow correlations. Unexpectedly, the homogenous flow model provided far more accurate predictions of pressure drop than the separated flow models. Among the separated flow models, better predictions were achieved with those for adiabatic and mini/micro-channels than those for flow boiling and macro-channels.     

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.     

Analysis of flow patterns emerging during evaporation in parallel microchannels

The evaporation processes of 2-propanol and water in cyclo olefin polymer (COP) and silicon microchannels of square cross-section are studied with a high-speed camera. The COP channels with a cross-section of 50 μm × 50 μm are rather smooth, whereas the 30 μm × 30 μm silicon channels have comparatively rough surfaces. For the COP channels, two different evaporation modes are identified, both with oscillating liquid–vapor menisci. One of these modes is characterized by an extremely rapid evaporation and a corresponding discontinuous shift of the meniscus. In the silicon channels four different evaporation modes are observed. Oscillatory motion of the liquid fronts also dominates here, and depending on the total mass flow and the wall temperature the oscillations in different channels are synchronized or desynchronized. Besides the flow patterns also the velocity trajectories of the evaporating liquid fronts are analyzed in detail and show a rather good reproducibility over different channels and different cycles. Compared to most other studies reported in this field, bubble nucleation is found to be of secondary importance for the evaporation processes.     

Detection of two-phase flow patterns using the recurrence network analysis of pressure drop fluctuations

The two-phase flow (water–air) occurring in the square minichannel (3 × 3 mm) has been analysed. In the minichannel it has been observed: flow of grouped isolated bubbles, flow of confined bubbles, flow of elongated bubbles, slugs flow and semi-annular flow. The time series of pressure drop fluctuations was analysed using the analyses of traditional recurrence quantification and recurrence network. The two coefficients: recurrence period density entropy and transitivity have been used for identification of differences between the dynamics of two-phase flow patterns. The algorithm which has been used normalizes the analysed time series before calculating the recurrence plots. Despite the neglect of quantitative signal characteristics the analysis of its dynamics allows us to identify the two-phase flow patterns. This confirms that this type of analysis can be used to identify the two-phase flow patterns in minichannels.     

Local heat transfer distribution and effect of instabilities during flow boiling in a silicon microchannel heat sink

Flow boiling of the perfluorinated dielectric fluid FC-77 in a silicon microchannel heat sink is investigated. The heat sink contains 60 parallel microchannels each of 100 μm width and 389 μm depth. Twenty-five evenly distributed temperature sensors in the substrate yield local heat transfer coefficients. The pressure drop across the channels is also measured. Experiments are conducted at five flow rates through the heat sink in the range of 20–80 ml/min with the inlet subcooling held at 26 K in all the tests. At each flow rate, the uniform heat input to the substrate is increased in steps so that the fluid experiences flow regimes from single-phase liquid flow to the occurrence of critical heat flux (CHF). In the upstream region of the channels, the flow develops from single-phase liquid flow at low heat fluxes to pulsating two-phase flow at high heat fluxes during flow instability that commences at a threshold heat flux in the range of 30.5–62.3 W/cm² depending on the flow rate. In the downstream region, progressive flow patterns from bubbly flow, slug flow, elongated bubbles or annular flow, alternating wispy-annular and churn flow, and wall dryout at highest heat fluxes are observed. As a result, the heat transfer coefficients in the downstream region experience substantial variations over the entire heat flux range, based on which five distinct boiling regimes are identified. In contrast, the heat transfer coefficient midway along the channels remains relatively constant over the heat flux range tested. Due to changes in flow patterns during flow instability, the heat transfer is enhanced both in the downstream region (prior to extended wall dryout) and in the upstream region. A previous study by the authors found no effect of instabilities during flow boiling in a heat sink with larger microchannels (each 300 μm wide and 389 μm deep); it appears therefore that the effect of instabilities on heat transfer is amplified in smaller-sized channels. While CHF increases with increasing flow rate, the pressure drop across the channels has only a minimal dependence on flow rate once boiling is initiated in the microchannels, and varies almost linearly with increasing heat flux.     

An experimental study of pressure loss and heat transfer in the pin fin-dimple channels with various dimple depths

An experimental study was conducted to investigate the effects of dimple depth on the pressure loss and heat transfer characteristics in a pin fin-dimple channel, where dimples are located on the endwall transversely between the pin fins. The pin fin-dimple channels considered consist of ten rows of pin fin-dimple combined structure. The pin fin transverse spacing-to-diameter ratio S/D = 2.5, the streamwise spacing-to-diameter ratio X/D = 2.5, the pin fin height-to-diameter ratio H/D = 1.0. The dimples have a print diameter the same with the pin fins, but have three different dimple depth-to-diameter ratios, i.e. δ/D = 0.1, 0.2 and 0.3. The experimental results, mainly the average Nusselt number and friction factor, for the pin fin-dimple channels with various dimple depths have been obtained and compared with each other for the Reynolds number range of 8200–50,500. The study showed that, compared to the baseline pin fin channel, the pin fin-dimple channels have further improved convective heat transfer performance by up to 19.0%, and the pin fin-dimple channel with deeper dimples shows relatively higher Nusselt number values. The study still showed dimple depth-dependent pressure loss behaviors for the pin fin-dimple channels compared to the pin fin channel, and the pin fin-dimple channel with shallower dimples shows relatively lower friction factors by up to 17.6% over the studied Reynolds number range. Furthermore, three-dimensional conjugate computations have been carried out for similar experimental conditions, and the computations showed the detailed characteristics in the distribution of the velocity and turbulence level in the flow, which revealed the underlying mechanisms for the associated dimple depth-dependent pressure loss and heat transfer characteristics in the pin fin-dimple channels.     

Two-phase pressure drops in a helically coiled steam generator

An experimental investigation regarding two-phase diabatic pressure drops inside a helically coiled heat exchanger have been carried out at SIET thermo-hydraulics labs in Piacenza (Italy). The experimental campaign is part of a wide program of study of the IRIS innovative reactor steam generator. The test section consists of an AISI 316 stainless steel tube, 32 m length, 12.53 mm inner diameter, curved in helical shape with a bend radius of 0.5 m and a helix pitch of 0.8 m, resulting in a total height of the steam generator tube of 8 m. The explored operating conditions for two-phase flow experiences range from 192 to 824 kg/m² s for the mass flux, from 0 to 1 for the quality, from 1.1 to 6.3 MPa for the pressure, from 50 to 200 kW/m² for the heat fluxes. A frictional two-phase pressure drops correlation, based on an energy balance of the two-phase mixture and including the 940 experimental points, is proposed. Comparison with existing correlations shows the difficulty in predicting two-phase pressure drops in helical coil steam generators.     

Experimental investigations of the anode flow field of a micro direct methanol fuel cell

The effect of the anode flow field design on the performance of an in-house fabricated micro direct methanol fuel cell (μDMFC) with an active area 1.0 cm × 1.0 cm was investigated experimentally. Single serpentine and parallel flow fields consisting of micro channels were tested. The experimental results indicated that the serpentine flow field exhibited significantly higher cell voltages than did the parallel flow field, particularly at high current densities. The study of the effect of channel depth of the serpentine flow field suggested that there exists an optimal channel depth for the same channel width and the same open ratio when the same methanol flow rate is supplied; either shallower or deeper channels will lead to a reduction in the cell performance. Finally, it was demonstrated that performance of the μDMFC with the reactants fed by an active means was insensitive to the cell orientations, which is different from conventional DMFCs with larger flow channels reported in the literature.     

Visualization of flow patterns and bubble behavior during flow boiling in open microchannels

Flow boiling in microchannels is favored by the heat transfer community due to the high heat transfer rates that can be obtained with lower mass flow rates. However, the heat transfer rates for flow boiling in microchannels are impacted by flow reversals and flow instabilities. An open microchannel structure was recently proposed to reduce the impact of the flow boiling instabilities. Subcooled flow boiling experiments were conducted in open microchannels using deionized water. The open microchannels had 6 parallel channels with a 0.3 mm uniform thickness gap above them The channels were fabricated on a 6 mm × 40 mm copper block. The channels were 0.5 mm wide and 0.3 mm deep with 0.43 mm wide fins between them. Flow visualizations were performed with a high-speed CCD camera with the results showing that the flow regimes in the open microchannels differ from those in closed microchannels with stratified flow and no flow instability. Two types of confined bubbles were observed with characterizations of the effects of the bubbles on each other. The heat transfer mechanisms for flow boiling in open microchannels are also described.     

Two-phase refrigerant distribution in a parallel flow minichannel heat exchanger having horizontal headers

The refrigerant R-134a flow distributions are experimentally studied for a round header-ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet orientations (parallel, normal, vertical) are investigated. Tests are conducted with downward flow for the mass flux from 70 to 130 kg/m² s and quality from 0.2 to 0.6. Tubes are protruded to the center of the header. It is shown that the preferred inlet configuration is dependent on mass flux or quality. At low mass flux or quality, vertical inlet yields the best flow distribution, followed by normal and then parallel inlet. As mass flux or quality increases, the optimum configuration switches to normal inlet configuration. As compared with the flush-mounted configuration, more uniform flow distribution is obtained for the present protruded configuration. Possible explanation is provided based on flow visualization results. Correlations are developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream.     

Study of carbon dioxide condensation in chevron plate exchangers; heat transfer analysis

The experimental investigation of carbon dioxide condensation in brazed plate heat exchangers is the main objective of this study. The current level of concern for the environment is at an all time high, therefore, it is important to look into methods and resources that lead to a cleaner and healthier future for the planet. This study details one such effort to reach this goal, focusing on condensation of carbon dioxide as a natural refrigerant in refrigeration systems. Three brazed plate heat exchangers with different geometry, each consisting of three channels, are tested. This paper focuses on the two-phase analysis, where carbon dioxide was the working fluid, flowing through the middle channel, and dynalene brine, the cooling fluid, flowed through the side channels of each geometry. Condensation of carbon dioxide occurred at saturation temperatures ranging from ?17.8 °C to ?34.4 °C and heat fluxes spanning 2.5–15.7 kW/m². An in-depth dimensional analysis was completed on the two-phase data yielding heat transfer correlations. Relationships of the two-phase heat transfer characteristics are presented, the data are compared with related studies, and conclusions are made from the two-phase data.