  Magnetic flux ropes are commonly observed throughout the heliosphere, and
recent studies suggest that interacting flux ropes are associated with some
energetic particle events. In this work, we carry out 2D particle-in-cell (PIC)
simulations to study the coalescence of two magnetic flux ropes (or magnetic
islands), and the subsequent plasma energization processes. The simulations are
initialized with two magnetic islands embedded in a reconnecting current sheet.
The two islands collide and eventually merge into a single island. Particles
are accelerated during this process as the magnetic energy is released and
converted to the plasma energy, including bulk kinetic energy increase by the
ideal electric field, and thermal energy increase by the fluid compression and
the non-ideal electric field. We find that contributions from these different
energization mechanisms are all important and comparable with each other. Fluid
shear and a non-gyrotropic pressure tensor also contribute to the energy
conversion process. For simulations with different box sizes ranging from $L_x
\sim 25$--$100 d_i$ and ion-to-electron mass ratios $m_i / m_e = 25$, 100 and
400, we find that the general evolution is qualitatively the same for all runs,
and the energization depends only weakly on either the system size or the mass
ratio. The results may help us understand plasma energization in solar and
heliospheric environments.
