Pool boiling enhancement through contact line augmentation

Abstract

In addition to transient conduction, microconvection, and microlayer evaporation, contact line region heat transfer has been identified as an important mode of heat transfer during boiling. In this work, we demonstrate that generating additional contact line regions within the base of a nucleating bubble leads to critical heat flux (CHF) enhancement. The creation of a liquid meniscus adjacent to 10–20 μm deep microgrooves in the bubble base area was responsible for the generation of the additional contact line regions. The depth of the microgrooves was determined such that a sufficient reservoir of liquid is present in the meniscus to sustain evaporation in the contact line region throughout the bubble cycle. The effective contact line length at the base of the bubble was seen to be a good indicator of the CHF (wetted area) over the surface. The microgroove geometry played a significant role in influencing the bubble dynamics and bubble departure diameter during boiling. It was seen that the bubbles were able to bridge and grow over the shallow microgrooves of 10–20 μm depth and generate additional contact line regions but were pinned and constrained within the grooves in the case of the microgrooves deeper than 100 μm. For shallow grooves, narrow grooves resulted in smaller bubbles, which in turn reduced the contact line length. The findings of this work could be used to design heat transfer surfaces that significantly enhance the contact line region contribution and CHF by placing shallow enhancement features on the surface of the heater.

Endoscopic Visualization of Contact Line Dynamics during Pool Boiling on Capillary‐Activated Copper Microchannels

“…[ 56 ] The local heat flux near the contact line has been observed to be one or more orders of magnitude higher than the average heat flux. [ 10 ] A similar linear relationship emphasizing the importance of contact line length for boiling heat transfer was obtained in the past, [ 35 ] as shown in Figure 5b. However, due to the lack of capillary wicking consideration and visualization limitations, high uncertainty in the contact line length estimation is present in past studies.…”

“…As shown in Figure 1g, the width of the deposited fins between two channels ( t ) and the depth of the channels ( H ) was fixed at 40 ± 2 µm to create opportunities for bubbles to traverse over multiple channels during bubble expansion without being pinned. [ 35 ] The channel width ( W ) was varied from 40 to 200 µm in 80 µm increments. The selected channel widths were rationally chosen to obtain a variety of wicking capability.…”

“…The base of the bubble traverses multiple channels during bubble growth, resulting in enhanced macro‐convection between the vapor bubble and surrounding liquid. [ 35 ] Additionally, due to the existence of channels underneath the bubble, surrounding liquid can replenish the dry area through capillary pumping, [ 31 ] thus separating liquid–vapor pathways. The benefits of enhanced macro‐convection and separation of liquid–vapor pathways synergistically enhances boiling inversion.…”

“…The CHF enhancement on wider‐microchannel‐patterned surfaces is generally considered to be partly due to the creation of additional menisci at microfin corners underneath the bubble base during expansion. [ 35 ]…”

Liquid film–induced critical heat flux enhancement on structured surfaces

“…At higher heat fluxes, the density of nucleation sites increased, raising the possibility of bubble coalescence during growth and additional complexity for interface analysis. Given that the main contributor to increased heat flux during pool boiling is the activation of nucleation sites and that the average bubble departure diameter and frequency for a particular surface structure have a relatively weak sensitivity to surface heat flux, analysis of bubble dynamics at reduced heat fluxes reflects the heat transfer mechanisms occurring at higher heat fluxes ( 17 , 23 , 37 ).…”