Abstract: The wide range of industrial applications involved with boiling flows promotes the necessity of establishing fundamental knowledge in boiling flow phenomena. For this purpose, a number of experimental and numerical researches have been performed to elucidate the underlying physics of this flow. In this paper, the improved wall boiling models, implemented on ANSYS CFX 14.5, were introduced to study subcooled boiling flow at elevated pressure. At the heated wall boundary, the Fractal model, Force balance approach and Mechanistic frequency model are given for predicting the nucleation site density, bubble departure diameter, and bubble departure frequency. The presented wall heat flux partitioning closures were modified to consider the influence of bubble sliding along the wall before the lift-off, which usually happens in the flow boiling. The simulation was performed based on the Two-fluid model, where the standard k-ω SST model was selected for turbulence modelling. Existing experimental data at around 5 bars were chosen to evaluate the accuracy of the presented mechanistic approach. The void fraction and Interfacial Area Concentration (IAC) are in good agreement with the experimental data. However, the predicted bubble velocity and Sauter Mean Diameter (SMD) are over-predicted. This over-prediction may be caused by consideration of only dispersed and spherical bubbles in the simulations. In the future work, the important physical mechanisms of bubbles, such as merging and shrinking during sliding on the heated wall will be incorporated into this mechanistic model to enhance its capability for a wider range of flow prediction.
Abstract: Bubble generation was observed using a high-speed
camera in subcooled flow boiling at low void fraction. Constant heat
flux was applied on one side of an upward rectangular channel to
make heated test channel. Water as a working fluid from high
subcooling to near saturation temperature was injected step by step to
investigate bubble behavior during void development. Experiments
were performed in two different pressures condition close to 2bar and
4bar. It was observed that in high subcooling when boiling was
commenced, bubble after nucleation departed its origin and slid
beside heated surface. In an observation window mean release
frequency of bubble fb,mean, nucleation site Ns and mean bubble
volume Vb,mean in each step of experiments were measured to
investigate wall vaporization rate. It was found that in proximity of
PNVG vaporization rate was increased significantly in compare with
condensation rate which remained in low value.
Abstract: Vibration characteristics of subcooled flow boiling on
thin and long structures such as a heating rod were recently
investigated by the author. The results show that the intensity of the
subcooled boiling-induced vibration (SBIV) was influenced strongly
by the conditions of the subcooling temperature, linear power density
and flow velocity. Implosive bubble formation and collapse are the
main nature of subcooled boiling, and their behaviors are the only
sources to originate from SBIV. Therefore, in order to explain the
phenomenon of SBIV, it is essential to obtain reliable information
about bubble behavior in subcooled boiling conditions. This was
investigated at different conditions of coolant subcooling
temperatures of 25 to 75°C, coolant flow velocities of 0.16 to
0.53m/s, and linear power densities of 100 to 600 W/cm. High speed
photography at 13,500 frames per second was performed at these
conditions. The results show that even at the highest subcooling
condition, the absolute majority of bubbles collapse very close to the
surface after detaching from the heating surface. Based on these
observations, a simple model of surface tension and momentum
change is introduced to offer a rough quantitative estimate of the
force exerted on the heating surface during the bubble ebullition. The
formation of a typical bubble in subcooled boiling is predicted to
exert an excitation force in the order of 10-4 N.
Abstract: Molecular dynamics simulation of annular flow
boiling in a nanochannel with 70000 particles is numerically
investigated. In this research, an annular flow model is developed to
predict the superheated flow boiling heat transfer characteristics in a
nanochannel. To characterize the forced annular boiling flow in a
nanochannel, an external driving force F ext ranging from 1to12PN
(PN= Pico Newton) is applied along the flow direction to inlet fluid
particles during the simulation. Based on an annular flow model
analysis, it is found that saturation condition and superheat degree
have great influences on the liquid-vapor interface. Also, the results
show that due to the relatively strong influence of surface tension in
small channel, the interface between the liquid film and vapor core is
fairly smooth, and the mean velocity along the stream-wise direction
does not change anymore.
Abstract: Evaporator is an important and widely used heat
exchanger in air conditioning and refrigeration industries. Different
methods have been used by investigators to increase the heat transfer
rates in evaporators. One of the passive techniques to enhance heat
transfer coefficient is the application of microfin tubes. The
mechanism of heat transfer augmentation in microfin tubes is
dependent on the flow regime of two-phase flow. Therefore many
investigations of the flow patterns for in-tube evaporation have been
reported in literatures. The gravitational force, surface tension and
the vapor-liquid interfacial shear stress are known as three dominant
factors controlling the vapor and liquid distribution inside the tube. A
review of the existing literature reveals that the previous
investigations were concerned with the two-phase flow pattern for
flow boiling in horizontal tubes [12], [9]. Therefore, the objective of
the present investigation is to obtain information about the two-phase
flow patterns for evaporation of R-134a inside horizontal smooth and
microfin tubes. Also Investigation of heat transfer during flow
boiling of R-134a inside horizontal microfin and smooth tube have
been carried out experimentally The heat transfer coefficients for
annular flow in the smooth tube is shown to agree well with Gungor
and Winterton-s correlation [4]. All the flow patterns occurred in the
test can be divided into three dominant regimes, i.e., stratified-wavy
flow, wavy-annular flow and annular flow. Experimental data are
plotted in two kinds of flow maps, i.e., Weber number for the vapor
versus weber number for the liquid flow map and mass flux versus
vapor quality flow map. The transition from wavy-annular flow to
annular or stratified-wavy flow is identified in the flow maps.
Abstract: Equipment miniaturisation offers several opportunities such as an increased surface-to-volume ratio and higher heat transfer coefficients. However, moving towards small-diameter channels demands extra attention to fouling, reliability and stable operation of the system. The present investigation explores possibilities to enhance the stability of the once-through micro evaporator by reducing its flow boiling induced pressure fluctuations. Experimental comparison shows that the measured reduction factor approaches a theoretically derived value. Pressure fluctuations are reduced by a factor of ten in the solid conical channel and a factor of 15 in the porous conical channel. This presumably leads to less backflow and therefore to a better flow control.