Visualization of Low Heat and Mass Flux Boiling in a Small Metal Pipe Using Neutron Radiography

High speed visualization of boiling pentane in a circular steel tube (D_i = 1.2 mm, D_o = 2 mm) has been performed at the Neutrograph instrument at the Institut Laue-Langevin in Grenoble, France. The heat and mass flux were both very low and appropriate for cooling of PEM fuel cells. The spatial resolution of the images is approximately 0.15 mm and the maximum frequency is 154 Hz. In the images, the liquid-vapor differentiation is clearly visible. Time resolved measurements of the outer pipe wall temperature, synchronized with the images, show that at low mass flow rates, the pipe wall is high above the saturation temperature and the pipe filled with vapor and liquid slugs. At higher flow rates, the wall is superheated when filled with liquid and at saturation temperature during boiling when exposed to a liquid-vapor mixture. An irregular switching between these two states was observed. The superheated wall is shown to be consistent with superheated liquid in the pipe prior to boiling. Unfortunately the strong γ-radiation produced by the neutrons has a substantial effect on the onset of boiling, which is why comparisons with non-irradiated systems might be difficult.