This research presents a numerical study of drag force and mass transfer in gravity-driven bubbly flows on inclined channels. Direct Numerical Simulations (DNS) of single bubbles and bubble swarms are performed using the multi-marker Unstructured Conservative Level-Set (UCLS) method. In this framework, the multiple marker approach avoids the so-called numerical coalescence of fluid interfaces in bubble swarms, whereas interface capturing and surface tension computation are accurately performed by the UCLS method. Furthermore, the finite-volume method discretizes transport equations on 3D collocated unstructured meshes. The fractional-step projection method solves the pressure-velocity coupling. The central difference scheme discretizes the diffusive term. The convective term within the momentum transport equation, level-set advection equations, and mass transfer equation are discretized by unstructured flux-limiter schemes to avoid numerical oscillations around the interface and minimize numerical diffusion. Numerical simulations show that the Reynolds number initially increases with the inclination angle ( \(\theta \) ); however, beyond a transition point at approximately \(\theta \approx 50^{\circ }\) , \(\textrm{Re}\) decreases. On the other hand, the Sherwood number ( \(\textrm{Sh}\) ) shows a sharp reduction after \(\theta \approx 50^{\circ }\) .