Hydrodynamic structure of the flow around a stationary gas bubble in an annular channel

Characteristics of the slug gas liquid flow in an annular channel were studied experimentally. The channel had the diameters of outer and inner tubes of 32 and 10 mm. The liquid flow was downward. The stationary bubble (gas slug) was produced by injecting air through a capillary tube. Wall shear stress measurements were performed by electrodiffusional technique. The measured circumferential distributions of wall shear stress demonstrated a strong non-uniformity across the channel. The highest liquid velocity was in the region of bridge streamining the bubble. The highest values of wall shear stress fluctuations are in the transition region between gas bubble and liquid bridge.

Numerical Study of Marangoni: Thermocapillary Convection Influence During Boiling Heat Transfer in Minichannels

Marangoni thermocapillary convection and its contribution to heat transfer during boiling has been the subject of some debate in the open literature. Currently, for certain conditions, such as microgravity boiling, is being shown that has a significant contribution to heat transfer [1]. Typically, this phenomenon is investigated for the idealized case of an isolated and stationary bubble resting atop a heated solid which is immersed in a semi-infinite quiescent fluid or within a two-dimensional cavity. However, little information is available with regard to Marangoni heat transfer in miniature confined channels in the presence of a cross flow. As a result, this paper presents a numerical study that investigates the influence of steady thermal Marangoni convection on the fluid dynamics and heat transfer around a bubble during laminar flow of water in a minichannel with the view of developing a refined understanding of boiling heat transfer for such a configuration. This mixed convection problem is investigated for channel Reynolds numbers in the range of 0 ≤Re ≤500 and Marangoni numbers in the range of 0 ≤ Ma ≤ 17114. The influence of the thermocapillary flow is most pronounced for low Re and high Ma numbers showing an average of 40% increase in heat transfer. For low Ma and high Re inertial effects dominate and the thermocapillary effect is not as noticeable. However, the disruption of the fully developed flow does tend to enhance the heat transfer at the expense of additional pressure drop.