Numerical modeling of adjacent bubble interactions under the influence of induced vibrations in liquid pool using lattice Boltzmann method (LBM)

Abstract

This paper reports our numerical investigation on the bubble dynamics of two adjacent bubbles formed on the heated surface as the liquid pool is subjected to induced vibrations caused by oscillating solid bodies in periodic motion. The modeling involves 2D simulations of the entire ebullition cycle comprising of bubble nucleation, growth, coalescence, and departure by employing a combination of multiple relaxation time based lattice Boltzmann method with the finite difference method based thermal model. The numerical results throw insight into the different processes pertaining to bubble growth in the two systems, viz., the quiescent system (QS) and the system with oscillating solid bodies (OSBS). These include the bubble growth rate, vapor bridge formation, subsequent coalescence, and movement of three-phase contact lines. It is observed that the induced vibrations in the liquid pool leads to earlier nucleation and growth of the bubbles, and higher bubble departure frequency (f∗) due to additional forces acting on the bubble, which at one instance helps the two adjacent bubbles to coalesce and at the following instance pulls the coalesced bubble off the solid surface. A force balance analysis is presented to explain the evolution of the adjacent bubbles and their interactions. A sensitivity study is conducted to investigate the effects of unequal sizes of nucleation sites, unequal surface superheat (Ja), and distance between the nucleation sites. In all these cases, multiple bubbles are seen to form on the heated elements, which subsequently coalesce with each other and depart in a single ebullition cycle in OSBS, whereas only two initially formed bubbles are seen to merge and depart in a single ebullition cycle in QS. Subsequently, a sensitivity study is conducted to investigate the effects of surface wettability, and it is found that for a given surface superheat (Ja) and configuration of nucleation sites, f* reduces after a threshold value of wetting angle (θ∗) in QS while it reaches a maximum in OSBS before coming down. It is further observed that if the hydrophobicity of the surface is increased from θ* = 1.0 to 1.1 in OSBS, f∗ remains high until a threshold Ja, beyond which it reduces drastically due to a higher rate of bubble generation compared to detachment.