Influence of coflow on the flow interaction and thermal characteristics of two plane parallel turbulent jets under non-buoyant and buoyant conditions

A numerical investigation is performed to examine the influence of coflow on the flow interaction and thermal characteristics of two plane parallel turbulent jets under non-buoyant and buoyant conditions. The study is conducted within the Reynolds-averaged Navier–Stokes (RANS) framework using the standard k − ϵ turbulence model with the Boussinesq approximation for a fixed jet spacing ratio s / d = 9, where s denotes the centre-to-centre jet spacing and d is the jet width. The coflow velocity ratio is varied over 0 ≤ V r ≤ 0.4 , while the Richardson number ranges from 0 ≤ R i ≤ 1 / 2 . The flow evolution is characterized using the merging point (MP), combined point (CP), and, for buoyant jets, the maximum velocity point (MVP). For non-buoyant jets ( R i = 0 ), increasing coflow weakens jet–jet interaction, reduces entrainment and lateral spreading, and shifts the MP and CP downstream. For buoyant jets, buoyancy promotes jet interaction, entrainment, and inward deflection, resulting in upstream displacement of the MP and CP relative to the non-buoyant case. In the downstream combined region, increasing coflow suppresses entrainment and lateral spreading and modifies the evolution of normalized excess maximum mean streamwise velocity and temperature. Quantitative comparison shows that coflow suppresses the increase in entrainment more strongly than jet spreading, particularly for buoyant jets, while lateral spreading continues even when entrainment increases weakly. Empirical correlations are developed for the locations of the MP, CP, and MVP as functions of V r and R i , providing simple predictive formulae for the characteristic locations of two plane parallel turbulent jets under non-buoyant and buoyant coflowing conditions.