16 research outputs found
Experimental Investigation of Bubble Dynamics and Flow Patterns during Flow Boiling in High Aspect Ratio Microchannels with the Effect of Flow Orientation
Multiphase flow and boiling phase-change within microchannels are of paramount importance to thermal management and electronics cooling, among others. The present experimental study investigated the bubble dynamics and flow patterns during flow boiling in a high aspect ratio microchannel at different channel orientations. Hydrofluoroether (HFEâ��7000) was used as the working fluid within a transparent glass microchannel with a width-to-height aspect ratio of 10 and a hydraulic diameter of 909 μm. A tantalum layer coating was applied to the channel, allowing visual observation to be conducted whilst imposing uniform heating along the channel. Under horizontal and vertical upward flow, the mass fluxes were varied between 14 kg mâ��2 sâ��1 and 42 kg mâ��2 sâ��1 for Reynolds numbers between 28.4 and 85.2, while the heat fluxes were set from 2.8 kW mâ��2 to 18.7 kW mâ��2. The results show that single bubble growth can be divided into three stages, namely: bubble-free growth, partially confined bubble growth, and fully confined growth. The effects of mass flux and heat flux on bubble behaviour and transition points between the defined stages are discussed. Equally important, the channel orientation influences the bubble behaviour in terms of bubble evolution, bubble shape, and bubble nose velocity, with no significant differences observed for the flow patterns when comparing horizontal and vertical upward cases. The comparison of the present work with previous flow pattern map results from the literature shows that earlier flow pattern transition models cannot accurately predict the observed flow patterns in the present configuration. Two types of instabilities are observed, which are associated with the dominant flow pattern during flow boiling and hence a function of the mass flux to heat flux ratio. The first type of instability, high amplitude and long period oscillation, is found during slug flow which is characterised by the presence of bubble rapid expansion along the channel. In addition, a second instability ensues when churn and annular flow create short period oscillations due to the high number of nucleation events, bubble coalescence, and the breakage of liquid slug or liquid film
Experimental Investigation of Bubble Oscillation and Leaping Driven by Thermocapillary Effects with Non-condensable Gas
Boiling phase-change plays a crucial role in heat transfer as it can dissipate higher heat fluxes than single phase. Bubble nucleation, growth, motion (oscillation or leaping), coalescence and departure, govern the boiling and the heat transfer performance on wires. Many factors affect the bubble mechanisms and interactions taking place, which include the nature of the fluid, experimental conditions, thermocapillary effects, etc. This work investigates the bubble oscillation and leaping phenomena caused by thermocapillary effects in the presence of oxygen and air as non-condensable gases during boiling on a platinum micro-wire. More in particular, the bubble oscillation performance is compared under various bulk temperatures and heat fluxes for two different non-condensable gases. It is observed that for a similar fluid bulk temperature, the lower the heat flux the longer the bubble displacement. Moreover, bubble oscillation phenomenon is influenced by the concentration of non-condensable gas dissolved in the liquid showing larger harmonic periods and shorter waiting times with decrease in the contact line pinning force by approximately 7–44% in the presence of air when compared to nitrogen. Last during oscillations, bubble leaping phenomenon was observed as a consequence of the interaction between the jet flows above the oscillating bubble
Silicone Oil Grafted Low Hysteresis Water Repellent Surfaces
Wetting plays a major role in the close interactions
between liquids
and solid surfaces, which can be tailored by modifying the chemistry
as well as the structures of the surfaces’ outermost layer.
Several methodologies, such as chemical vapor deposition, physical
vapor deposition, electroplating, and chemical reactions, among others,
have been adopted for the alteration/modification of such interactions
suitable for various applications. However, the fabrication of low-contact
line-pinning hydrophobic surfaces via simple and easy methods remains
an open challenge. In this work, we exploit one-step and multiple-step
silicone oil (5–100 cSt) grafting on smooth silicon substrates
(although the technique is suitable for other substrates), looking
closely at the effect of viscosity as well as the volume and layers
(one to five) of oil grafted as a function of the deposition method.
Remarkably, the optimization of grafting of silicone oil fabrication
results in non-wetting surfaces with extremely low contact angle
hysteresis (CAH) below 1° and high contact angles (CAs) of ∼108°
after a single grafting step, which is an order of magnitude smaller
than the reported values of previous works on silicone oil-grafted
surfaces. Moreover, the different droplet–surface interactions
and pinning behavior can additionally be tailored to the specific
application with CAH ranging from 1 to 20° and sliding angles
between 1.5 and 60° (for droplet volumes of 3 μL), depending
on the fabrication parameters adopted. In terms of roughness, all
the samples (independent of the grafting parameters) showed small
changes in the root-mean-square roughness below 20 nm. Lastly, stability
analysis of the grafting method reported here under various conditions
shows that the coating is quite stable under mechanical vibrations
(bath ultrasonication) and in a chemical environment (ultrasonication
in a bath of ethanol) but loses its low-pinning characteristics when
exposed to saturated steam at T ∼ 99 °C.
The findings presented here provide a basis for selecting the most
appropriate and suitable method and parameters for silicone oil grafting
aimed at low pinning and low hysteresis surfaces for specific applications