A Czochralski melt is simulated in a model experiment and by numerical analysis. We use a transparent fluid (0.65 cSt silicone oil with Pr=6.8) which is contained in a transparent sapphire crucible (40 mm inner diameter, 40 mm height, crystalline material). A copper rod (e.g. 14 mm diameter) is used to simulate the growing crystal. An imposed temperature difference between the crucible and the crystal dummy gives rise to convective flow in the fluid. Forced convection by crystal rotation has also been applied but will be treated in part 2 of this paper. Our experiment models to a good degree the growth of oxide and fluoride crystals from noble metal crucibles. Flow visualization and flow velocity measurements are performed. We report the flow patterns observed in the fluid for different parameters (temperature difference, fluid depth, meniscus, free surface cooling). We observe a separated surface tension driven convection roll at the free surface and a "cold jet" beneath the crystal dummy. The Row is strongly affected by thermocapillary forces. The experimental results can be used as benchmarks to test numerical simulation programs for Czochralski growth. Numerical simulations are performed (2D, steady, axisymmetric). The simulation results compare favorably with the experimental data. In a parametric numerical study the relative importance of thermocapillary and buoyant forces is investigated.