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This paper presents the experimental results of saturated two-phase flow ammonia in a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes. The tests were conducted for a wide range of parameters. The heat load was provided by hot ethylene glycol/water solution flowing on the shell side. Saturation temperature of ammonia was -20°C ≤ T sat ≤ 2°C, heat flux range 5 kW m−2 ≤ q ˙ ≤ 45 kW m−2, exit vapor quality range 0.1 ≤ x ≤ 0.9 and a mass flux range of 34.99 kg m−2 s−1 ≤ G ≤ 66.32 kg m−2 s−1. For each saturation temperature, the effects of heat flux and mass flux were studied for the saturated temperature range while the exit vapor quality at each mass flux was varied between 0.2 and 0.9. Two phase heat transfer coefficient increased with saturation temperature and heat flux. For all experimental conditions, performance showed initial increase and then a classic drop at the onset of dry out. The dimple tube showed better heat transfer coefficient compared to the previous studies performed on plain tubes with ammonia.

The present paper concerns an experimental investigation about the heat transfer coefficient during convective condensation of hydrocarbons and their zeotropic mixtures. Experiments were performed in a horizontal smooth channel, with internal diameter of 9.43 mm for R600a, R290, R1270, R600a/R290 (70/30 molar fraction) and R600a/R1270 (75/25 molar fraction), for mass velocities ranging from 50 to 250 kg m−2 s−1, vapor qualities from unity to zero, heat fluxes from 5 to 60 kW m−2 and a saturation temperature of 35°C. Experiments were also performed for R134a. In general, higher heat transfer coefficients were obtained for the hydrocarbons compared to R134a. The heat transfer process was dominated by shear effects for the hydrocarbons regardless of the mass velocity. At low mass velocities (below 100 kg m−2 s−1), both mixtures provided lower heat transfer coefficients than the pure fluids. At high mass velocities a reduction of the heat transfer coefficient compared to the pure fluid was noticed only for R600a/R1270 at high qualities. The experimental data is compared against prediction methods from literature and based on the pure fluids database, a new strictly empirical prediction method is proposed, which performs better for both pure fluids and mixtures database. For the mixtures, the performance of the methods was improved when considering a correction for the mass transfer effects associated to the concentration gradients near the liquid/vapor interface.

This paper presents results for the experimental vector velocity field obtained through spatial filter velocimetry in a triangular tube bundle with tubes of 20 mm O.D. and a transverse pitch of 25.2 mm. The experiments were conducted for single-phase water flows with Reynolds numbers ranging from 447 to 1842, and for air–water two-phase bubbly flow with liquid Reynolds number of 909 and gas Reynolds number of 42. The vector velocity field for each experimental condition was obtained using a variable meshing scheme to improve the spatial resolution in regions of low velocities. The results indicate that the variable meshing strategy is promising to increase the spatial resolution of the spatial filter velocimetry technique and to release from the constraint imposed by the uniform window sizes, especially for experimental data obtained with relatively low frame rates. Details of the flow structure of external flow across tube bundles, such as the near-wall velocity profiles, wake region and the effects of the gas buoyancy on vertical mean velocities are presented in this paper with high temporal and spatial resolution, which can be useful for validation of numerical models.

This paper presents a comparison of heat transfer and pressure drop during single-phase flows inside diverging, converging, and uniform microgaps using distilled water as the working fluid. The microgaps were created with a plain 10 mm × 10 mm heated copper surface and a polysulfone cover that was either uniform or tapered with an angle of 3.4 deg, and average gap heights of 400 and 800 μm. Experiments were conducted with single-phase water flow with an inlet temperature of 30 °C for flow rates varying from 57 to 498 mL/min and heat flux from 27 to 153 W/cm2 depending on the flowrate and microgap configuration. The uniform configuration resulted in the lowest pressure drop due to the less constricted flow. A slight decrease of pressure drop with heat flux was observed due to temperature dependent properties. The best heat transfer performance was obtained with the converging configuration, which was especially significant at low flow rates and shorter average gap. This behavior could be explained by an increase in the heat transfer coefficient due to flow acceleration in the converging gaps, which compensates for the decrease in temperature difference between the fluid and the surface along the flow length. Overall, the converging microgaps have better performance than uniform or diverging ones for single-phase flows, and this effect is more pronounced at lower flow rates, where the fluid experiences higher temperature changes.

In this work, a new comprehensive dataset about bubble sizes in flow boiling of water is presented. The experimental setup basically consists of a lower copper heated plate embedded in a horizontal rectangular channel flow. Several experimental conditions, ranging bulk velocities: 0.1–0.9 [m·s−1]; mass fluxes: 96.9–871.8 [kg·m−2·s−1]; subcooling degrees: 16–36 [°C]; heat fluxes: 200–650 [kW·m−2] and pressures: 110–190 [kPa] were set for three cooper plates of different roughness, with Sa: 0.45, 1.23 and 7.43 [µm], respectively. Based on the current samples, the probability function for the normalized bubble diameter has been described by a lognormal pdf for any plate and experimental condition and a dimensionless correlation for the standard deviation of the bubble size distribution is presented. In order to completely describe the diameter distribution for bubbles in subcooled boiling systems, an improved correlation for the mean bubble diameter value is also presented here, after a deep review of previously published datasets. Further work is required to accommodate different channel configurations and surface morphologies. Agencia Estatal de Investigación | Ref. RTC2019-006955-4

This work concerns an experimental evaluation of the flow boiling critical heat flux of DI-water and nanofluids inside a 1.1 mm ID channel. Results were obtained for DI-water and solutions of Al2O3 (40–80 nm)/DI-water, Al2O3 (10 nm)/DI-water and SiO2 (80 nm)/DI-water for volumetric concentrations of 0.1% and 0.01%. Additional experiments were performed for pure DI-water on surfaces previously covered with nanoparticles through the flow boiling process. Experiments were performed for mass velocities ranging from 133 to 494 kg/m2s, saturation temperature of 130 °C and vapor qualities at the CHF location ranging from 0.65 to 0.84. The CHF results for DI-water in the tube before covering its surface with nanoparticles were compared with well reputed CHF prediction methods from literature and a reasonable agreement was found. Despite of the large augmentation of the wettability/wickability of the surfaces covered by a layer of nanoparticles, the differences between the CHF values for DI-water for the surfaces with and without the nanoporous covering were within the uncertainty of the CHF measurements. This behavior suggests a negligible effect of the nanoporous layer and, consequently, of wettability/wickability on the saturated CHF in microchannels for high vapor qualities and the range of conditions evaluated in present study. Based on an analysis of the CHF behaviors observed in this study, the saturated CHF mechanism was associated to periodical lumps of liquid that promotes intermittently the surface dryout and its re-wetting. The CHF is established when the time between two consecutives liquid lumps is low enough, so that the surface reaches a temperature for which its re-wetting is not possible.

This study concerns an experimental evaluation of the heat transfer coefficient for saturated flow boiling of DI water and nanofluids under conditions close to the critical heat flux. Experimental data were gathered for DI-water-based nanofluids (composed of Al2O3 40–80 nm, Al2O3 10 nm, and SiO2 80 nm in volumetric concentrations up to 0.1%) and pure DI water in microscale channels (1.1 mm internal diameter) as received from the manufacturer and with the surface covered with a nanoporous layer resulting from the boiling process. Results were obtained for a saturation temperature of approximately 130 °C, mass velocities ranging from 133 to 494 kg/m2s, vapor qualities up to 0.9, and heat fluxes up to 2.5 MW/m2. The DI-water experimental data on the bare surface were compared to prediction methods available in the literature. In general, the nanoporous layer characteristics play a major role in the effects of nanoparticles on the heat transfer coefficient.

Flow boiling heat transfer is investigated in a single silicon microchannel with microstructured sidewalls using R134a (at 0.19 reduced pressure) and HFE-7000 (at 0.035 reduced pressure) as working fluids. The channel is 51 mm long with a 400 μm x 400 μm square cross section. Sidewall microstructures in the form of triangular cavities for the entire height of the channel were introduced during the fabrication process. By including the cavity pattern in the photolithography mask design, high accuracy and reproducibility is ensured during the fabrication of the test sample. A smooth channel with the same geometry but without sidewall microstructures was also fabricated. Flow boiling experimental results are presented for both channel configurations. For R134a at mass velocity G = 800 kg m−2 s−1, the comparison of the boiling curves shows that: in the channel with the microstructures, the wall superheat at the onset of nucleate boiling (ONB) is reduced from 21 K to 6 K as compared to the smooth channel. An increase of the flow boiling heat transfer coefficient up to 40% is found in the microstructured channel, which must be due to the increased number of active nucleation sites. Heat transfer results, flow visualizations and comparison between R134a and HFE-7000 data provide a clearly evidence of the active role of bubble nucleation during flow boiling in microchannels. These results are of fundamental importance in the design of next generation thermal management systems for electronics.

The critical heat flux (CHF) was briefly introduced in Chap. 5, and in the present chapter, the analyses of studies concerning CHF is deepened.

The entire liquid-film dryout process in a vertical two-phase annular flow is characterized experimentally, from inception to completion. Experiments are conducted using saturated R245fa at high vapor qualities in a heated rectangular channel with a hydraulic diameter of 18 mm and an aspect ratio of 1/3. The walls of the test section are made of glass coated with fluorine-doped tin oxide (FTO). Heat fluxes up to 50 kW/m 2 are generated at the inner surface of the window by passing an electrical current through the FTO coating. Instantaneous pressure and temperature in the test section, temperature on the outer wall of the test section, liquid-film thickness, and high-speed videos are recorded simultaneously during the dryout events. In addition, the state (wet or dry) of the heated surface is measured using a non-invasive laser reflectance technique at high sampling rate (2000 Hz) and over long periods of time ( > 1000 s). The laser reflectance measurement is used to calculate the time-averaged dry fraction, f d r y , which is the fraction of time that the wall is dry during intermittent cycles of dryout and rewet. Data show that cyclic dryout starts before the critical heat flux (CHF) is reached. The dryout heat flux (DHF), which marks the onset of dryout, is typically 90% of the CHF, except at very high quality ( x > 0.95 ), where it can be as low as 50% of CHF. For all the investigated mass fluxes, CHF, where the heat transfer coefficient peaks, occurs consistently at f d r y ≈ 0.05. Further insight into the liquid-film behavior at the onset of dryout is obtained by combining analyses of high-speed videos, time-resolved liquid-film thickness signals, and statistics about the duration of and time between dryout events. The rewetting process is driven by disturbance waves. In the wake of disturbance waves, the liquid film is almost stationary. Calculations of the characteristic time it takes for this stationary film to evaporate predict well the characteristic time during dryout events measured with the laser reflectance method.

This study consists of a broad and critical literature review concerning in-tube convective boiling and condensation of hydrocarbons and their mixtures. A summary of observed experimental behaviors observed in literature for heat transfer coefficient, pressure drop, flow pattern and void fraction is presented. In general, it was found that hydrocarbons and HFCs presents similar heat transfer and pressure drop trends. An assessment of the accuracy of prediction methods for the heat transfer coefficient and pressure drop is also performed by comparing them with data gathered from literature. Reasonable agreement between the data and prediction methods was obtained only for convective condensation heat transfer coefficient of pure hydrocarbons inside microchannels and hydrocarbons mixtures inside conventional-sized and microchannels with the Silver-Bell and Ghally correction. No experimental study was found concerning flow boiling heat transfer coefficient and/or pressure drop of hydrocarbons mixtures inside microchannels. It is worth to highlight that a restricted number of data sources (independent laboratories) was found, highlighting the importance of further studies.

This paper presents a comparison of heat transfer and pressure drop during single-phase flows inside diverging, converging, and uniform microgaps using distilled water as the working fluid. The microgaps were created on a plain heated copper surface with a polysulfone cover that was either uniform or tapered with an angle of 3.4°. The average gap height was 400 microns and the length and width dimensions were 10 mm × 10 mm, resulting in an average hydraulic diameter of approximately 800 microns for all configurations. Experiments were conducted at atmospheric pressure and the inlet temperature was set to 30 °C. Heat transfer and pressure drop data were acquired for flow rates varying from 57 to 485 ml/min and the surface temperature was monitored not to exceed 90 °C to avoid bubble nucleation, so the heat flux varied from 35 to 153 W/cm2 depending on the flow rate. The uniform configuration resulted in the lowest pressure drop, and the diverging one showed slightly higher pressure drop values than the converging configuration, possibly because the flow is most constrained at the inlet section, where the fluid is colder and presents higher viscosity. In addition, a minor dependence of pressure drop with heat flux was observed due to temperature dependent properties. The best heat transfer performance was obtained with the converging configuration, which was especially significant at low flow rates. This behavior could be explained by an increase in the heat transfer coefficient due to flow acceleration in converging gaps, which compensates the decrease in temperature difference between the fluid and the surface due to fluid heating along the gap. Overall, the comparison between the three configurations shows that converging microgaps have better performance than uniform or diverging ones for single-phase flows, and such effect is more pronounced at lower flow rates, when the fluid experiences higher temperature changes.

Flow boiling heat transfer in microchannels can remove high heat loads from restricted spaces with high heat transfer coefficients and minimum temperature gradients. However, many works still report problems with instabilities, high pressure drop and early critical heat flux, which hinder its possible applications as thermal management solutions. Much comprehension on the phenomena concerning flow boiling heat transfer is still missing, therefore many investigations rely on empirical methods and parametric studies to develop novel configurations of more efficient heat sinks. Nevertheless, investigations involving vapor extraction have successfully addressed all these previously reported issues while also increasing the heat transfer of heat sinks employing flow boiling in microchannels. In this sense, the objective of this review is to identify the main techniques employed for vapor extraction in microchannels-based heat sinks and analyze the physical mechanisms underneath the observed improvements during flow boiling, such that some design guidelines can be drawn. Three main strategies can be identified: passive vapor extraction, active vapor extraction, and membrane-based vapor extraction. All these strategies were able to dissipate heatfluxes higher than 1 kW/cm2, with the best performance achieved by a membrane-based heat sink, followed by active and passive designs. According to the present experimental and numerical data available in the literature, there is still room for improvement.

This paper presents an analysis of the performance of the Spatial Filter Velocimetry (SFV) technique applied to external flows across tube bundles to evaluate the benefits of using low particle concentration in a complex geometry. The experimental vector velocity fields were obtained in a tube bundle test section made of acrylic with 20 rows of 4 tubes of 20mm O.D. mounted in a normal triangular configuration and transversal pitch per diameter ratio of 1.25. The SFV results reliability was evaluated by comparing the volumetric flow rates estimated by the experimental vector velocity fields with the volumetric flow rates measured by the flowmeter, showing good agreement. Additionally, the experimental results are used to check the validity of numerical results obtained through CFD simulation. The experiments were conducted for Reynolds numbers of 909, 1842 and 3902. The velocity profiles obtained through the SFV technique agree with the CFD simulations results, which indicate consistent results for both analyzes. Therefore, the numerical and experimental results indicate that the SFV technique provides accurate velocity data and is suitable to be applied to complex geometries, especially in regions away of the test section wall due to the good visibility provided by low particle concentration.