Bubble Dynamics and Enhancement of Pool Boiling in Presence of an Idealized Porous Medium - a Numerical Study using Lattice Boltzmann Method

This paper reports our numerical investigations on enhancement of pool boiling heat transfer inside an array of solid cylinders of square cross section using lattice Boltzmann Method. The entire saturated pool boiling curve for the flat surface comprising of different nucleate boiling regimes has been obtained numerically. The effect of solid cylinder array has been quantitatively evaluated and expressed in the form of its corresponding boiling curve. It is found that the boiling incipience in presence of the cylinder array occurs at a lower surface superheat compared to that of a plane surface. Further, the solid array effectively delays the onset of film boiling. The bubble dynamics in such solid structure array including bubble nucleation, coalescence, growth, entrapment, splitting and escape is found to be very different compared to a flat surface. Based on the heat flux values and trends, the entire boiling curve could be classified into 4 distinct zones. To the best of our knowledge, this is the first instance where LBM could predict the entire pool boiling curve for any porous medium. Finally, two different cylinder arrays of porosity 90% and 98% are studied to examine the effect of porosity. It is found that the sensitivity of the heat transfer rates to porosity is significant especially at higher values of surface superheat.

Drag on a Square-Cylinder Array Placed in the Mixing Layer of a Compound Channel

There are no studies specifically aimed at characterizing and quantifying drag forces on finite cylinder arrays in the mixing layer of compound channel flows. Addressing this research gap, the current study is aimed at characterizing experimentally drag forces and drag coefficients on a square-cylinder array placed near the main-channel/floodplain interface, where a mixing layer develops. Testing conditions comprise two values of relative submergence of the floodplain and similar ranges of Froude and bulk Reynolds numbers. Time-averaged hydrodynamic drag forces are calculated from an integral analysis: the Reynolds-averaged integral momentum (RAIM) conservation equations are applied to a control volume to compute the drag force, with all other terms in the RAIM equations directly estimated from velocity or depth measurements. This investigation revealed that, for both tested conditions, the values of the array-averaged drag coefficient are smaller than those of cylinders in tandem or side by side. It is argued that momentum exchanges between the flow in the main channel and the flow in front of the array contributes to reduce the pressure difference on cylinders closer to the interface. The observed drag reduction does not scale with the normalized shear rate or the relative submersion. It is proposed that the value of the drag coefficient is inversely proportional to a Reynolds number based on the velocity difference between the main-channel and the array and on cylinder spacing.

Heat Transfer in Flow Past Two Cylinders in Tandem and Enhancement with a Slit

This study aimed to determine the flow structures and heat transfer for flow past a tandem cylinder array and the effect of a slit on the enhancement of heat transfer. Different distances between cylinders and inclination angles of the slit were simulated to determine the effects on the flow pattern and heat transfer. Overall, the Nusselt number of the array is increased by 6–15% with applying a slit on a cylinder. However, in some special conditions, the slit induces two kinds of flow pattern transforms which are Suppression and Revival. The suppression mode inhibits the vortex shedding and reduces the heat transfer. In contrast, the revival mode triggers the vortex shedding and increases heat transfer.