The famous experiment of Nukiyama, carried out in 1934, was the first to classify the different regimes of pool boiling as shown in the figure below (Global Digital Central Encyclopedia, Thermal fluid central [13]). He observed that the bubble did not appear until the $\Delta T \approx 5°C $ in which $ T_{wall} = \Delta T + T_{sat} $ and this particular wall temperature is referred as $ T_{ONB} $ (the onset nucleate boiling, ONB). For $T_{wall}<T_{ONB}$, free convection boiling is expected to be the only mechanism for the system to evacuate heat to the surrounding domain.


Pool boiling curve for saturated water
Figure 10: Pool boiling curve for saturated water, according to Global Digital Central Encyclopedia, Thermal fluid central [13]

The boiling curve for propane, in logarithmic scale, is presented in Figure 3 (BIBLIOGRAPHIC RESEARCH - Boiling Regime section). The free convection boiling is said to exist for $ \Delta T < 7°C $, thus for heat fluxes lower than 2000W/m² before a region of transition regime that ranges between 2000 - 5000 W/m².

Therefore, single phase simulations were launched for various heat flux, from 200 to 5000 W/m². As discussed in the 'BIBLIOGRAPHIC RESEARCH - Boussinesq Approximation' section, the flow generated around the tube due to buoyancy force, as a result of variation of propane density, was estimated using Boussinesq Approximation. Results obtained from Neptune CFD were compared with correlations and experimental results in the 'Comparison' section to conclude on the reliability of Boussinesq Approximation and Neptune CFD to predict the fluid movement around the tube for natural convection boiling regime.