Temperature-induced density variations within fluids drive gravity-driven flows. Such flows occur for example in geophysical processes such as differential cooling or stratification dynamics in lakes. Heat transfer through the boundaries, whether in a lake or in a laboratory flume, can impact flows. The objective of this study is to evaluate numerically and experimentally the effect of these boundary conditions on thermal convective mixing in two different configurations. The first configuration is the thermal convective mixing in a box of water with cooled boundaries. The second configuration is a lock exchange gravity flow. Numerical simulations were performed using OpenFOAM with a Large Eddy Simulation solver and the Boussinesq approximation for density effects. The sensitivity of the solution to different boundary conditions for heat transfer were analyzed. Experiments rely on an innovative phosphor thermometry technique able to measure spatial patterns of fluid temperature instead of common pointwise measurements. Notably, we introduce a novel approach that combines the use of a laser sheet and high-resolution CMOS sensors operated in a multi gate accumulation mode to extract the temperature pattern. The choice of appropriate parameters in the boundary condition enabled the accurate representation of the temperature evolution and convective flow patterns observed in the experiments.