Numerical Insight into the Kelvin-Helmholtz Instability Appearance in Cavitating Flow

Recently the development of Kelvin-Helmholtz instability in cavitating flow in Venturi microchannels was discovered. Its importance is not negligible, as it destabilizes the shear layer and promotes instabilities and turbulent eddies formation in the vapor region, having low density and momentum. In the present paper, we give a very brief summary of the experimental findings and in the following, we use a computational fluid dynamics (CFD) study to peek deeper into the onset of the Kelvin-Helmholtz instability and its effect on the dynamics of the cavitation cloud shedding. Finally, it is shown that Kelvin-Helmholtz instability is beside the re-entrant jet and the condensation shock wave the third mechanism of cavitation cloud shedding in Venturi microchannels. The shedding process is quasi-periodic.

Molecular dynamics simulation of evaporation of R32 on the solid surface

The evaporation of fluid on solid surface is an important process in nature and industry. The high-efficiency heat transport between the working fluid and the solid surface can enhance the energy conversion and utilization. Thus, it is of great significance to study the mechanism of the evaporation phenomenon at the liquid–solid interface. In this study, the evaporation of refrigerant R32 on Pt surface is investigated by molecular dynamics (MD) method. The effects of the substrate temperature on the evaporation behavior of R32 are discussed in detail. It is found that R32 molecules mainly enter the vapor region by evaporation when the substrate temperature is no larger than 300 K. The evaporation rate increases with the increase of substrate temperature. The nucleate boiling and film boiling clearly occur when the substrate temperature is 350 K. The nanobubble formation, growth and coalesce is observed in the simulation. The heat flux changes rapidly when the system is boiling. As time goes on, a vapor film forms and then it leads to the heat transfer deterioration.

Effects of the Thickness of Boundary Layer on Droplet’s Evaporation Rate

The evaporation rate of a droplet was explained in relation to the thickness of the boundary layer and the condition near the droplet’s surface. However, the number of results obtained from experiments is very limited. This study aims to investigate the thickness of the boundary layer of an ethanol‐water mixture droplet and its effect on the evaporation rate by Z-type Schlieren visualization. Single and double droplets are tested and compared to identify the effect of the second droplet on the average and instantaneous evaporation rate. The double droplet’s lifetime is found to be longer than the single droplet’s lifetime. The formation of a larger vapor region on the top of the droplet indicates a higher instantaneous evaporation rate. The thickness of the boundary layer is found to increase with increase in ethanol concentration. Furthermore, a larger vapor distribution area is found in the case of higher ethanol concentration, which explains the faster evaporation rate at higher ethanol concentration.

Structural studies on fluid Hg and fluid Se at high temperatures and high pressures by means of X-ray diffraction and small angle X-ray scattering

X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) measurements for fluid Hg and fluid Se up to the supercritical region have been carried out using synchrotron radiation at SPring-8. We obtained the structure factor, $S\left(Q\right)$, including a small angle region, and the pair distribution function, $g\left(r\right)$, for both fluids from the liquid to the dense vapor region. Change of the local structure and medium-range correlations at the metal-insulator transition in fluid Hg were revealed. On the other, the average coordination number of two was preserved at the semiconductor-metal transition in fluid Se. From a SAXS experiment of fluid Se in 2012, SAXS spectra near the semiconductor-metal transition region show the Ornstein–Zernike profile and the SAXS intensity is reduced with increasing pressure. These results indicate difficulties of separating fluctuations intrinsic to the semiconductor-metal transition from those arising from the liquid-vapor critical point in fluid Se, although fluctuations intrinsic to the electronic transitions are largely expected in both fluids.

Optimization of the Width of Vapor Chamber Regions by Using Particle Swarm Optimization Method

An optimization of vapor chamber (VC) region's width and its wick porosity to achieve the minimum temperature rise of the vapor chamber is studied in this paper. The optimization process is carried out by particle swarm optimization (PSO) method. The study is performed at various widths of the vapor chamber, cooling rates, and input powers. The vapor chamber includes two solid copper plates, two wick regions, and vapor region between them. The required chamber characteristics for the optimization process are obtained by solving a complete VC mathematical model, which couples the thermal and hydrodynamic models. The optimum vapor chamber regions' thicknesses and the fluid flow through the vapor chamber regions are studied. The results illustrate that to minimize the chamber temperature, the wick region width must be minimized. They also show that increasing the total width of the chamber from 3 to 7 mm does not have a great impact on the chamber optimized temperature. Moreover, the vapor chamber width does not have a great impact on the optimum wick region width. The optimum width of the vapor region and the chamber walls augments with increasing the total vapor chamber width. Additionally, the form of temperature, streamlines, and velocity distributions at liquid and vapor regions at optimum conditions are not greatly influenced by increasing vapor chamber width.

Parametric Study of the Effect of the Vapor Chamber Characteristics on Its Performance

In this work, the effect of vapor chamber characteristics, the properties of its working fluid and the operating parameters on the vapor chamber performance are studied. Also, the effects of these parameters on the cooling process are considered. A three dimensional hydrodynamic model is used for solving the fluid flow through the liquid and vapor regions of the vapor chamber. The hydrodynamic model is coupled with a three dimensional thermal model to calculate the model temperature. The hydrodynamic model takes into consideration the circulation of liquid between the two wick regions. An implicit finite difference method is used to solve the numerical model and a validation of the numerical model is presented. The effect of porosity of the wick material, wick structure, solid wall material, working fluid, wick region thickness, vapor region thickness, power input, and heat transfer coefficient of the cooling fluid are taken in the study. Their effects on the heat pipe temperature, pressure difference of the heat pipe, liquid and vapor velocities and mass evaporated are studied. The results show that, to increase the cooling performance of the heat pipe, the porosity, wick thickness, power input, and vapor region thickness should be decreased and the heat transfer coefficient should be increased. To minimize the maximum pressure difference of the heat pipe, increase porosity, wick thickness, and vapor thickness and decrease heat transfer coefficient and power input. The study shows that the increase of wick thickness by a factor of four decreases the maximum pressure difference by about 75% and increases the maximum vapor chamber temperature 30%. It also shows that the vapor region thickness has an insignificant effect on the vapor chamber temperature and pressure. The increase of the heat transfer coefficient of the cooling liquid decreases its effect on heat pipe performance.

Phase-Change Heat Transfer of Ethanol-Water Mixtures: Towards Development of a Distributed Hydrogen Generator

Hydrogen fuel from renewable bio-ethanol is a potentially strong contender as an energy carrier. Its distributed production by steam reforming of ethanol on microscale platforms is an efficient upcoming method. Such systems require (a) a pre-heater for liquid to vapor conversion of ethanol water mixtures (b) a gas-phase catalytic reactor. We focus on the fundamental experimental heat transfer studies (pool and flow boiling of ethanol-water mixtures) required for the primary pre-heater boiler design. Flow boiling results (in a 256 μm square channel) clearly show the influence of mixture composition. Heat transfer coefficient remains almost constant in the single-phase region and rapidly increases as the two-phase region starts. On further increasing the wall superheat, heat transfer starts to decrease. At higher applied heat flux, the channel is subjected to axial back conduction from the single-phase vapor region to the two-phase liquid-vapor region, thus raising local wall temperatures. Simultaneously, to gain understanding of phase-change mechanisms in binary mixtures and to generate data for the modeling of flow boiling process, pool-boiling of ethanol-water mixtures has also been initiated. After benchmarking the setup against pure fluids, variation of heat transfer coefficient, bubble growth, contact angles, are compared at different operating conditions. Results show strong degradation in heat transfer in mixtures, which increases with operating temperature.

Numerical Simulation of Combustion for Freely Falling Gelled Fuel Droplets under Normal Gravity Conditions

Understanding the evaporation and combustion mechanisms of single droplets of gel propellant is the first stage to predict the burning characteristics in the combustion chamber. This paper, taking into account convection heat for freely falling gelled fuel droplets under normal gravity conditions, as well unsteady mass diffusion and thermal diffusion inside droplet, a theoretical model was developed to understand mass and heat transport mechanisms, and bubble growth within the gel droplet during processes of droplet combustion. The results show that at the first stage, shrinkage of the radius obeys the d2-law; steep temperature gradient and fuel mass concentration gradient appear within droplet, especially region near droplet surface. At the second stage, liquid fuel near the gellant layer within droplet starts to boiling, gellant layer formation resist the vaporizing fuel gas flow to extent; the vapor region appears between gellant layer and vaporizing surface within the droplet, and the droplet expands, swells, the layer thickness decreases until it ruptures.