Fluctuation characteristics of heating surface temperature near an obstacle in transient boiling two-phase flow in a vertical annular channel

When a flow obstruction such as a spacer is set in a boiling two-phase flow within an annular channel, the inner tube of which is used as a heater, the temperature on the surface of the heater tube is severely affected by the existence of the spacer. In some cases the spacer has a cooling effect, and in other cases it causes the dryout of the cooling water film on the heating surface resulting in the burnout of the tube. In the present paper we will discuss temperature fluctuations in relation to the change in the flow configuration near the cylindrical spacer in transient boiling two-phase flow caused by the stepwise change of the operation parameters such as the heat flux, the mass flow rate, and the inlet quality of the test section. As a result it is concluded that: Even if the flow pattern changes rapidly by a stepwise change of an operation parameter, the transition of flow proceeds safely provided that the change causes the increase of vapor velocity, i.e. the increase of the shear force which is acting on liquid film flow. On the other hand if the change causes the decrease of the vapor velocity, it must be noticed that the transient burnout possibly occurs even when the operating condition after the change is less than the steady burnout condition. (C) 2002 Elsevier Science B.V. All rights reserved.