Based on lattice Boltzmann thermal model and large density ratio model

a phase transition source term was introduced into two-phase density distribution control function to describe effect of temperature field on gas-liquid phase transition. A composite lattice Boltzmann thermal model

which can describe heat transfer and phase transition at gas-liquid ratio up to 2778

was proposed to study two-phase heat transfer mechanism. Numerical instability caused by extremely large gas-liquid density ratio was resolved by regress correction of pressure velocity distribution function. Double bubble rising movement and the surrounding temperature field distribution in lithium bromide solution was simulated using the model. Upper bubble temperature was higher than that of lower bubble prior to collision during moving up. Upon coalescence

liquid bridge between the two bubbles was open

heat transfer started and temperature inside the bubble became uniform. The volume of the double bubbles decreased first and then increased with maximum at time of two bubbles collision. After bubble coalescence

the volume gradually decreased and eventually stabilized to certain value. The larger the initial bubble volume is

the greater the velocity of the bubble rises.