Experimental study of ammonia nucleation boiling onset in dead-end tube segments at different boundary conditions

The nucleation boiling onset of liquid ammonia in stainless steel, aluminium, and titanium tube segments of ID 4mm, 6 mm, and 8 mm without flow conditions has been experimentally studied at saturation pressures of 12.5–19.8 bar. It has been shown that heterogeneous nucleation is the determining mechanism for the considered cases. Maximal achievable liquid superheating at 19 K has been observed in the tests at low saturation pressure (12.5 bar), decreasing to 6 K at 19.8 bar. The heating rate in the range of 0.2–60 K·min−1 (average heat fluxes of ≈0–4 kW m−2) does not impact much on boiling onset. Internal diameter (4, 6 and 8 mm), roughness (1 and 10 μm) and wettability (10°–50°) do not drastically change the level of maximum achievable superheating before boiling onset. However, quantitatively, the titanium surface with the poorest wettability (≈50°) among considered samples leads to higher overshooting of ammonia bulk temperature by several K. A stainless steel surface with artificial scratches shows a bit higher superheating than a smooth surface. The specific tests showed that ammonia boiling could start at nearly saturation level in the case of a strongly overheated wall. In particular, for ammonia at saturation pressure above 12.5 bar, the required overheating of the wall of 19 K ensures the boiling of ammonia without superheating in the bulk of the liquid.