Thus, as the boiling boundary moves up the tube, the total pressure drop falls, potentially increasing the flow in an unstable manner. Boiler tubes normally overcome this (which is effectively a 'negative resistance' regime) by incorporating a narrow orifice at the entry, to give a stabilising  pressure drop on entry.
As the boiling boundary moves up the tube, the total pressure loss drops, potentially triggering an unstable increase in flow. Boiler tubes usually overcome this (which is essentially a 'negative resistance' regime) by incorporating a constricted passage at the inlet, providing a stabilizing pressure drop on entry.