Analysis of vapor bubble diameter, departure frequency and dynamics in a single cavity

Vapor bubbles departure frequency at ferrofluid boiling on a single nucleation site in a uniform horizontal magnetic field

Lattice Boltzmann study of bubble dynamics and heat transfer on a hybrid rough surface with a cavity-pillar structure

The generation and departure of bubbles are closely related to the boiling heat transfer efficiency. To investigate the bubble boiling characteristics on a heated micro-wire, platinum wires with diameter of 0.1 and 0.2 mm were used as the experimental material. The bubble dynamics over a horizontal heated wire and an inclined heated wire were investigated by a high-speed camera and an electronic microscope. The influences of heat flux and subcooled condition on the bubble departure diameter and the departure frequency were analyzed. The results showed that three kinds of bubble motion track were observed over the heated micro-wires under subcooled condition, including jet flow, sweeping, and rotation. The multi-jet flow phenomenon could be seen over the bubble on the heated 0.2 mm wire under the conditions of heat flux of 1.7 × 105 W m−2 and mainstream temperature of 36 °C. As the diameter increased, the bubbles grown on the lower side of the heated wire would gradually rotate over the wire. There were three phenomena observed on the 0.1 mm wire when two bubbles collided under conditions of 2.9 × 105 W m−2 heat flux and 34 °C mainstream temperature. For an inclined micro-wire with the inclined angle of 25°, the movement of the bubble would only move along the wire to the tip and eventually break away. When the mainstream temperature was 40 °C, the bubbles departure diameter and the frequency increased with the increase in heat flux. When the heat flux was 3.0 × 105 W m−2, the bubble departure diameter and frequency increased with the increase in temperature.

Experiments to understand bubble base evaporation mechanisms and heat transfer on nano-coated surfaces of varying wettability under nucleate pool boiling regime

Bubble Dynamics and Heat Transfer During Pool Boiling on Wettability Patterned Surfaces

In this study, pool boiling heat transfer of de-ionized water was experimentally studied on a scored copper surface at a heat-flux range of 0 - 60 W/cm2. Bubble dynamics in an isolated bubble region were carefully investigated, including bubble departure diameters, bubble departure frequencies, and active nucleation site densities. The bubble dynamics were compared with available models, indicating the suitable models regarding the present experimental results. Then, based on the bubble dynamics, a mechanistic heat transfer model, developed in our previous studies, was employed to predict the present boiling curve. In the mechanistic model, heat fluxes from natural convection, transient heat conduction, and microlayer evaporation were incorporated.

Laser texturing of silicon surface to enhance nucleate pool boiling heat transfer

A critical review on bubble dynamics parameters influencing boiling heat transfer

Understanding the detailed dynamics of bubble ebullition cycle is central to effectively exploit coolant phase change in subcooled flow boiling. In the present study, the bubble growth rate and the bubble departure mechanism in subcooled flow boiling conditions are investigated. The bubble growth rate is estimated by employing an energy balance model, which ensures that the applied wall heat flux contribution to components, such as (i) microlayer evaporation, (ii) conduction through superheated layer region, and (iii) condensation heat transfer, are accounted. The bubble departure diameter and the type of departure (i.e., sliding or lift-off) are predicted using a simple force balance model on the vapor bubble. The implemented models are thoroughly validated against the benchmark experimental data. Furthermore, the influence of operating conditions, viz., pressure (1–3 bar), mass flux (200–1000 kgm−2s−1), heat flux (200–500 kWm−2), and the degree of subcooling (20–30 K) on the bubble dynamics, is investigated for subcooled flow boiling conditions. Based on the parameters considered, a flow regime map is developed to identify the bubble departure type and diameter for a given set of operating conditions. It was noticed that the bubble departure diameter was maximum at low pressure (1 bar), low mass flux (200 kgm−2s−1), high heat flux (500 kWm−2), and low degree of subcooling (20 °C). For all the values of pressure and degree of subcooling, the sliding mode of departure was noticed at low and high mass flux values, whereas bubble departs by lift-off for moderate values of mass flux.

Saturated flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct

Among the different ways of achieving heat transfer enhancement in pool boiling, modification of surface roughness and the associated change in the surface wettability are two widely studied methods. It is well established that microstructure can play an important role in varying surface roughness and improving interfacial wettability. In the present study, pool boiling heat transfer on a number of micro grooved and flat (brass and copper) surfaces with water as the working fluid is investigated experimentally for enhancement of critical heat flux. The micro structured sample surfaces feature parallel grooves having dimensions of tens of microns and are fabricated using a mechanical micromachining process. Classical pool boiling experiments are conducted for three different values of heat flux (75 kw/m2, 100 kw/m2, 125 kw/m2). The bubble nucleation process is visualized and the bubble departure diameter is measured with the help of a high speed camera. It is observed that the nucleation site density and departing bubble diameter increases, while the bubble departure frequency decreases with an increasing heat flux for both kind of surfaces and materials. Comparison of the results among the flat and micro-grooved surfaces reveals that the boiling on micro-grooved surfaces results in higher heat transfer coefficient, more nucleation sites, lower bubble departure frequency and larger bubble diameter under similar operating conditions.Among the different ways of achieving heat transfer enhancement in pool boiling, modification of surface roughness and the associated change in the surface wettability are two widely studied methods. It is well established that microstructure can play an important role in varying surface roughness and improving interfacial wettability. In the present study, pool boiling heat transfer on a number of micro grooved and flat (brass and copper) surfaces with water as the working fluid is investigated experimentally for enhancement of critical heat flux. The micro structured sample surfaces feature parallel grooves having dimensions of tens of microns and are fabricated using a mechanical micromachining process. Classical pool boiling experiments are conducted for three different values of heat flux (75 kw/m2, 100 kw/m2, 125 kw/m2). The bubble nucleation process is visualized and the bubble departure diameter is measured with the help of a high speed camera. It is observed that the nucleation site density and d...

Saturated flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip

Numerical investigation of flow boiling in manifold microchannel-based heat exchangers

A model of dynamics of the vapor bubbles that emerge on solid surfaces of porous structures and the steam generating wall (bottom layer) is presented in this work. The model was filmed and photographed by a high-speed camera SKS-1М. The discharge of high heat flows (up to 2·106 W/m2) was maintained by the joint action of capillary and mass forces with the help of intensifiers. An analytical model was developed based on the theory of thermoelasticity. The limit state of the porous coating with poor thermal conductivity and the metal bottom layer was determined. Heat flows were calculated from the spontaneous birth of the vapour nucleus (10–8) to the material destruction (102–103 s), thus the interval from the process of relaxation to the maximum process (destruction) was described. The size of the pullout particles determined in the model at the moment of porous coating destruction showed good congruence with the experimental data obtained at the optic stand. The destruction of coating under the compression forces occurs much earlier than the tension forces. It is probable that the destruction will happen under the impact of the compression and shear forces. The intervals of the heat flow when such destruction takes place are different for quartz and granite coating. Each thickness of the pullout particles under the impact of compression forces has its limit values of the heat flows, which are located within the mentioned intervals. As the specific heat flow in the heated layer increases and, therefore, the heating time decreases, the impact of the compression stresses increases as well. Despite the high resistance to compression, destruction from the compressive heat tension occurs in more favorable conditions immediately, and in diminutive volumes. Experimental testing units, test conditions, the outcome of the heat exchange crisis, the limit state of the surface and the calculation of critical heat flows are presented. The capillary porous system that works under the joint action of capillary and mass forces is studied. The system has advantages compared to pool boiling, thin-film evaporators and heat pipes.

This study investigates the bubble dynamics of a single bubble during pool boiling using deionized water at sub-atmospheric pressures. Experiments were conducted on a mirror-polished copper surface with an artificial nucleation site, at pressures of 1 bar, 0.35 bar, and 0.15 bar, and heat fluxes of 2 and 5 W/cm2. Key parameters like bubble base diameter, equivalent diameter, departure frequency, waiting time, and heat transfer rate were analyzed. Using the rainbow-schlieren technique for visualization, results showed that lower pressures led to larger, more elongated bubbles with decreased departure frequency, increased waiting time, and higher instantaneous heat transfer rates during bubble growth.