Single-phase Free Convection Research Articles

Effect of the vertical magnetic field on the bubble dynamic behaviors during photoelectrochemical water splitting

The bubble dynamic behaviors on the electrode surface are significantly affected by the magnetohydrodynamic (MHD) flow induced by the Lorentz force resulting from the applied magnetic field. However, the mechanism of the impact of a magnetic field perpendicular to the electrode on the dynamic behaviors of bubble remain unclear. In this study, the evolution law of a single oxygen bubble on the surface of a TiO2 photoelectrode under the influence of a vertical magnetic field with different magnetic induction intensities was investigated. The presence of a vertical magnetic field effectively promoted the rapid detachment of bubbles from the electrode surface and greatly improved the gas evolution efficiency. The mass transfer was dominated by single-phase free convection during bubble evolution. Under the influence of the vertical magnetic field, the mass transfer coefficient near the electrode was enhanced, and the adverse effects of bubbles on mass transfer were counteracted when they entered the reaction-controlled growth phase. The further multiphysics simulation found that the MHD convection decreased the local supersaturation of reaction product oxygen molecules near the electrode, thereby reducing concentration overpotential and increasing the reaction rate. This study provides an experimental and theoretical basis for promoting the bubble detachment and the reaction rate on the gas evolving electrode surface during photoelectrochemical water splitting.

Mass transfer mechanism during bubble evolution on the surface of photoelectrode

The limitation of the transfer of gas products within the photocatalytic system is the key to the efficiency of the water splitting, which is directly influenced by the bubble evolution process on the catalyst surface. In this study, the entire process of bubble evolution on the surface of a superhydrophilic TiO2 photoelectrode was simultaneously measured visually and electrochemically. The bubble growth and detachment characteristics were investigated under different laser powers and constant bias voltages. An expression for the bubble coverage characterized by the bubble radius and photocurrent, which is applicable to the photoelectrocatalytic water splitting, was proposed to accurately obtain the mass transfer coefficient during bubble evolution. The mass transfer mechanism of gas product transfer in the low current density range dominated by single-phase free convection was revealed. The mass transfer coefficient was found to increase in the inertia-controlled phase and decrease in the chemical reaction-controlled phase of bubble growth. Furthermore, a method to accelerate bubble detachment from the photoelectrode surface by periodic illumination was proposed, which could significantly enhance the gas product transfer process.

Heat Transfer Enhancement Using Ultrasonic Waves in Presence of Liquid: A Basic Research for Cooling Electronic

This work collects the experimental results obtained in the Thermal Fluid Dynamics Lab at the Department of Energy, Systems, Land and Constructions Engineering at the University of Pisa, concerning a basic physics research on the influence of ultrasounds in single phase free convection and in subcooled boiling, at atmospheric pressure. The ultrasounds are applied at the set frequency of 40 kHz, with a transducer output changing from 300 to 500W, on a circular horizontal cylinder heated by Joule effect, immersed in distilled water. The tests in single phase free convection, without ultrasonic waves, are validated by means of the classical correlations reported in literature, but they do not produce distinctive augmentation of the heat transfer. The enhancement of the heat transfer coefficient is maximum in subcooled boiling conditions (about 57%). In this regime a detailed investigation was performed to optimize the variables involved, such as the ultrasound generator power, the position of the cylinder and, especially, the subcooling degree. This paper, makes clear systematically the effects of ultrasounds on the heat transfer and shows as they could be very useful as cooling system for the last generation electronic components.

A Numerical Study of Single Phase Dielectric Fluid Immersion Cooling of Multichip Modules

Immersion, single phase free convection cooling of multichip modules on a printed circuit board in a pool of dielectric fluid was examined numerically, with experimental verification of baseline cases. A multi-chip module with multiple thermal test cells with temperature sensing capability was simulated. The commercially available computational fluid dynamics program from ANSYS, Fluent, was used with the electronics packaging front end, Icepak, employed to create the models and compact conduction modules. Simulations were first performed of an experimental test vehicle which had five 18 mm by 18 mm die, arranged in a cross pattern, equally spaced die, 25 mm between them. Two of the die were aligned vertically with the center die, two aligned horizontally with it. The board was suspended vertically in a large pool of dielectric fluid. Heat was dissipated in the die at a flux of up to 2 W/cm2, based on the die surface area. Simulation results were compared with experimentally measured die temperature values and excellent agreement was seen for the cases of one die heated and all five die uniformly heated with the board cooled by FC-72. A numerical parametric study was performed to examine the effect of die size and spacing on temperature rise. In addition to FC-72, immersion cooling in Novec 649 and HFE 7100 were modeled. Design guidelines are suggested for dielectric fluid immersion cooled multichip modules.

Influence of the ultrasounds on the heat transfer in single phase free convection and in saturated pool boiling

This paper collects the experimental results obtained in the Thermal Fluid Dynamics Lab at the Department of Energy and Engineering Systems at the University of Pisa, concerning the influence of ultrasounds in single phase free convection and in pool saturated boiling, at atmospheric pressure. The authors have just reported in previous papers their results about the enhancement induced by the presence of ultrasonic waves in subcooled boiling conditions. The ultrasounds are applied at the set frequency of 40 kHz, with a transducer output changing from 300 to 500 W, on a circular horizontal cylinder heated by Joule effect, immersed in distilled water. The tests in single phase free convection, without ultrasonic waves, are validated by means of the classical correlations reported in literature. Moreover, the authors managed to correlate the data also in presence of ultrasounds. The enhancement of the heat transfer coefficient is lower than the one obtained in subcooled boiling and with regards to the saturated pool boiling conditions, as far as for subcooling degrees very close to the saturation, the ultrasounds cause a decrease of the heat transfer. At the same time, the ultrasounds induce the critical heat flux (CHF) at higher heat flux compared with the case without ultrasounds. Also in saturated pool boiling, without ultrasonic waves, the experimental value of the CHF is compared with the one obtained by equations reported in the literature. This paper, joined to the others published by the authors, makes clear systematically the effects of ultrasounds on the heat transfer and shows as they could be very useful in the last generation electronic components cooling.

Nucleate pool boiling, film boiling and single-phase free convection at pressures up to the critical state. Part II: Circumferential variation of the wall superheat for a horizontal 25 mm copper cylinder

Transcritical working cycles of refrigerants led to increased interest in heat transfer near the Critical State. In general, experimental results for this region differ significantly from those far from it because some fluid properties vary much more there than at a greater distance. In this paper, two-phase and single-phase free convective heat transfer to refrigerant R125 is discussed for fluid states very close to the Critical Point and far from it. In Part II, circumferential variation of the wall superheat is related to the motion of the fluid within the superheated boundary layer. In nucleate boiling, a minimum in the superheat develops on the lower parts of the wall that can be related to additional evaporation into the bubbles sliding upwards in close contact with the heated wall. In film boiling and supercritical free convection, a slight maximum superheat is discovered on the lower parts of the wall, the relative size of which increases close to the Critical Point, while it does not exist in far sub-critical free convection.

Nucleate pool boiling, film boiling and single-phase free convection at pressures up to the critical state. Part I: Integral heat transfer for horizontal copper cylinders

Transcritical working cycles for refrigerants have led to increased interest in heat transfer near the Critical State. In general, experimental results for this region differ significantly from those far from it because some fluid properties vary much more there than at a greater distance. In this paper, measurements for two-phase and single-phase free convective heat transfer from an electrically heated copper tube with 25 mm O.D. to refrigerant R125 are discussed for fluid states very close to the Critical Point and far from it. It is shown that heat transfer for film boiling slightly below and for free convection slightly above the critical pressure is very similar. The new – and also previous – experimental data for nucleate boiling, film boiling, and single-phase free convection are compared with calculated results between atmospheric and critical pressure. It can be concluded that the Principle of Corresponding States in its simplest form is very well suited to transfer the results to other refrigerants. In Part II, particular attention will be given to a minimum superheat for nucleate boiling and a maximum superheat for film boiling and single-phase free convection within the circumferential variation of the isobaric wall superheat on the lower parts of the tube.

Single bubble growth during water electrolysis under microgravity

Single bubble evolution on a micro-electrode (Pt, φ = 0.2 mm) was observed in 0.36 M KOH solution under terrestrial (1-G) and microgravity (μ-G) environments. The bubble size during galvanostatic electrolysis ( j = −2.6 × 10 3 A m −2) was measured by CCD images, which allowed us to calculate the gas evolution efficiency, f G, by comparison with the consumed charge. The efficiency under μ-G increased until 1 s after starting electrolysis and then reached constant value ( f G = 0.85), while, under 1-G, it showed a lower value and remarkably decreased 2 s after the beginning of the measurement. Such differences between μ-G and 1-G were explained by the mass transfer rate of the dissolved gas. Bubble-induced microconvection dominated the mass transfer under μ-G without any buoyancy force, on the other hand, the single-phase free convection (microscopic natural convection) influenced the bubble growth under 1-G.

2708 水蒸気-空気混合気の細い鉛直管外面への自由対流層流膜状凝縮

An experimental study is presented on free convection laminar film condensation of steam-air mixture on a vertical tube. The test tube used is of about 220mm height and about 6mm outer diameter. The range of concentration of air is 0.1∿0.9. The average heat flux values or average heat transfer coefficients obtained have been about 2.5 times larger than the corresponding values from the two-phase boundary layer theory for a vertical flat surface. The enhanced values are quantitatively explained by the curvature effect upon the vapor boundary layer, which is analogically derived from single phase free convection heat transfer.

The role of single-phase free convection in mass transfer at gas evolving electrodes—I. Theoretical

The catalogue of known mass transfer mechanisms acting at gas evolving electrodes has to be extended by a mechanism which has obviously been ignored so far. Under certain conditions, free convection owing to density gradients within the liquid phase (single-phase convection) exerts the controlling effect on mass transfer. Generally, the mechanism competes with other mass transfer mechanisms which have been investigated for a long time and are rather well understood. Available mass transfer equations for these mechanisms are extended to fit the special conditions of gas evolving electrodes. A general mass transfer equation for single-phase free convection is derived.

The role of single-phase free convection in mass transfer at gas evolving electrodes—II. Experimental verification

An equation for single-phase mass transfer at gas evolving electrodes derived in part I of this paper [ Electrochim. Acta 38, 1421 (1993).] is tested in comparison with experimental data for ions transferred to the electrode and for dissolved gas transferred from the electrode. The agreement between experiment and theory not only shows the reliability of the new equation but evidences the action of a hitherto disregarded mass transfer mechanism effective in a low current density range in absence of forced flow.

The gravity controlled film condensation of saturated and superheated binary vapour mixtures on a vertical plate.

The gravity controlled film condensation of saturated and superheated binary vapour mixtures on a vertical plate is theoretically treated. Similarity solutions are numerically obtained for ethanol-water and air-water mixtures. It is revealed that he buoyancy force due to temperature difference is important as well as due to concentration difference and convective heat transfer is appreciable for superheated vapour and that the temperature distribution in the vapour boundary layer is markedly affected by the diffusion flux toward the vapour-liquid interface. New correlation equation of heat and mass transfer are derived from the numerical results. These equations can be reduced to the equations of condensation of saturated and superheated pure vapours, simultaneous heat and mass transfer for very small diffusion velocity, and single-phase free convection.

Incipient Boiling Characteristics at Atmospheric and Subatmospheric Pressures

Experimental data are reported for incipient boiling of water, refrigerant R-113, and methanol from a flat horizontal stainless steel surface of known surface roughness. The work was carried out over a pressure range of 1, 0.5, and 0.25 atm. Overshoot temperatures at incipience for R-113 and methanol were systematically investigated and found to exist for all pressures employed. The overshoot in temperature at incipience did not occur for water within the experimental range covered. Surface temperature fluctuations were found to increase in amplitude with decreasing pressure, reaching a maximum at incipience. In general, these temperature fluctuations became less pronounced as fully developed nucleate boiling became established. An abnormal occurrence of transition from single-phase free convection to film boiling is also reported.

Stability characteristics of a single-phase free convection loop

Stability characteristics of a single-phase free convection loop are presented. In the experiments, water was placed inside a toroidal glass loop oriented in a vertical plane. The lower half of the loop was heated and the upper half was cooled. At low heat-transfer rates and also a t high heat-transfer rates the free convection flow was observed to be steady. For the intermediate range, however, the flow was found to be highly oscillatory. Stability predictions are also developed. The comparison between theory and experiment yields favourable agreement.Observations of unstable behaviour have been reported previously for single-phase fluids in the vicinity of the thermodynamic critical point. In these situations it has been assumed that the unusual behaviour of the fluid properties in the near-critical region necessarily constitutes the underlying cause of such instabilities. In contrast t o this view, analyses by Keller (1966) and Welander (1967) indicate that instabilities can occur for ordinary fluids as well. Results of the present study confirm this contention, since instabilities were clearly observed for water at atmospheric pressure and moderate temperatures.