A critical review on bubble dynamics parameters influencing boiling heat transfer

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

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.

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

Pool boiling heat transfer is an aggressive and complex phenomenon which needs to be simplified for a better understanding of the mechanism of bubble growth and departure and how boiling heat transfer can be enhanced. Single bubble boiling heat transfer is a simple version of boiling phenomenon which has been used to study the effective elements on pool boiling heat transfer. The purpose of the present review paper is to understand how to produce single bubble pool boiling on a heated substrate and investigate, how single bubble boiling phenomenon can be affected by geometry of cavities, cavity size, wettability, roughness, working fluid, subcooling, wall superheat, heat flux, gravity, etc. It was demonstrated that cylindrical cavities are capable to generate stable and continuous bubbling, small temperature fluctuation, low superheat with short waiting period. The cylindrical cavities can be manufactured very easily in small sizes which can be a good candidate to produce single bubble pool boiling. As heat flux increases, smaller cavities start becoming active. For a given depth, as cavity size increases, the bubble growth rate and departure volume increase. Surface wettability is another complex and important factor to modify the single bubble boiling heat transfer. Wettability depends mainly on force balance at the triple contact line which relies on solid–liquid–gas materials. In case of hydrophobic surfaces, the triple line has tendency to move toward liquid phase and expand the radius of triple line, so the initiation of nucleation is easier, the waiting time is shorter, the downward surface tension force becomes bigger since radius of triple line is larger, the bubble departure volume is higher and bubble growth period is longer. The effects of the rest of main parameters on single bubble boiling are discussed in this paper in details. In addition, a theoretical model is developed to predict the liquid-vapor interface for the single bubble boiling. The theoretical model is compared with single bubble boiling experimental data and good results observed.

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

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

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.

Numerical simulation of bubble dynamics and heat transfer in the 2D saturated pool boiling from a circular surface

A building block type approach is used to investigate mechanisms associated with nucleate boiling heat transfer and vapor removal under microgravity conditions. In this approach, starting with a single bubble, the complexity of the experiments is increased to include merger of neighboring bubbles. Key processes of interest are the heat transfer rates associated with micro/macrolayer evaporation and bubble dynamics. The bubble dynamics includes bubble growth and departure. The bubble growth is influenced by heat transfer, whereas, the forces acting on the bubble determine the bubble diameter at departure and bubble growth period. In microgravity, buoyancy force is practically non-existent. As such, forces (e.g. lift) that are insignificant at earth normal gravity become important at zero gravity and determine the bubble departure diameter. Experiments using single and multiple cavities microfabricated in silicon wafers have been performed at normal gravity and in the parabolic flight of the KC135. The experimental results of bubble dynamics and heat transfer have been validated with 2-D and 3-D numerical simulations of the process. Based on the scaling of the effect of gravitational acceleration, the requirements for space flight experiments are developed. The hardware used in the experiments will

Bubble departure frequency and active nucleation site density are two main factors that affect the nucleate boiling heat transfer. The potential enhancement of boiling heat transfer can be accomplished by surface modification. This treatment can be realized with changing parameters such as porosity, tilting angle and cavity radius. In this study, effects of different nanostructured Aluminum-Alloy (Al-Alloy) 2024 sheets on subcooled boiling heat transfer are investigated. A simple and environmentally friendly technique is used in order to produce these plates that are immersed into boiling deionized water for 20, 60 and 120 minutes. To examine boiling heat transfer characteristics, nanostructured plates are placed inside a rectangular channel. The channel is heated through four cartridge heaters connected to a DC power supply while deionized water is pumped inside using a micro gear pump at constant mass fluxes of 50 kg/m2s, 75 kg/m2s and 125 kg/m2s. It was found that an increase in nano-structure height leads to higher boiling heat transfer coefficients. Furthermore, a high speed camera system was used to investigate flow patterns in the microchannel. Visualization results indicated that bubbles movde faster the nano-structure height increased.

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.

Bubble growth, departure and the following flow pattern evolution during flow boiling in a mini-tube

Using deionized water as working medium, studies on visualization of the behavior of subcooled boiling bubbles in narrow rectangular channels have been conducted at atmospheric pressure, with the help of a high-speed digital camera. The effects of wall superheat, fluid subcooling and mass flux on the bubble dynamic model of nucleation site density, bubble departure frequency and bubble departure diameter were investigated. This paper analyzed the visual graphic results and obtained quantitative values about bubble nucleation site density, bubble departure frequency and bubble departure diameter under different conditions. Based on the results, wall nucleate boiling models in the conventional channel put forward by Nilanjana Basu have been corrected. The new correlation agrees reasonably well with existing experimental data in 2mm narrow rectangular channel.

Numerical investigation on bubble dynamics during pool nucleate boiling in presence of a non-uniform electric field by LBM

The trends of decrease in size and increase in power dissipation for micro-electronic systems present a significant challenge for thermal management of modern electronics. The preferable cooling solution could be micro heat exchangers based on forced flow boiling. Nanoparticle deposition can affect nucleate boiling heat transfer coefficient via alteration of surface thermal conductivity, roughness, capillary wicking, wettability, and nucleation site density. It can also affect heat transfer by changing bubble departure diameter, bubble departure frequency, and the evaporation of the micro and macrolayer beneath the growing bubbles. In this study, flow boiling was investigated for 0.001 vol% aluminum oxide nanofluids in a brass microchannel and compared to results for regular water. For the case of nanofluid flow boiling, high speed images were taken after boiling durations of 25, 75, 125, and 150 min. Bubble growth rates were measured and compared for each case. Flow regime oscillation was observed and regime duration was split into two periods: single-phase liquid and two-phase. The change in regime timing revealed the effect of nanoparticle suspension and deposition on the Onset of Nucelate Boiling (ONB) and the Onset of Bubble Elongation (OBE). The addition of nanoparticles was shown to stabilize bubble growth as well as the transition of flow regimes between liquid, two-phase, and vapor.