Following a loss of coolant accident in a nuclear reactor, the materials of the core can crumble and form a porous, hot debris bed that must be efficiently cooled. This is done by reflooding the debris bed with water. This operation involves complex thermal-hydraulic phenomena that are better understood by studying forced convection boiling in heating porous media. To that end, we use a heat-generating model porous medium made of 392 cylindrical temperature probes transversely placed between two ceramic plates. The probes are used both as temperature sensors and as heating elements. The front ceramic plate is transparent, allowing visualization of the flow inside the test section. Boiling curves are obtained by applying an increasing heating power to one specific element, while a constant global heating power is applied to the other elements. On the one hand, thermal measurements show that the critical heat flux on a specific element decreases as the global thermal power released by the porous medium increases. On the other hand, visualizations show that the void fraction in the test section increases with the global heating power. An image processing program was developed to measure the void fraction and characterize the bubble dynamics, in order to investigate their influence on the critical heat flux. Such measurements have been validated using a second test section where air is injected at a known flow rate inside the device, at isothermal conditions, and where flow regimes comparable to those observed in the boiling experiment are obtained. In this talk, we will present the experimental techniques allowing to measure the void fraction in our experiment, before discussing the influence of this parameter on the critical heat flux.