Static evaporator instabilities can occur in modern lignite-fired steam generators. This results in an uneven distribution of the medium temperatures in the several hundred parallel running evaporator tubes. The reason for this is the interaction of an uneven distribution of heat on the evaporator walls and an uneven distribution of the total mass flow on the individual evaporator tubes, especially in partial load operation. The aim of this work is to simulate this phenomenon by means of coupled simulation of the heat flux distribution due to the combustion and the mass flow distribution in the evaporator tubes, and to investigate possible remedial measures.
While heat flux distribution on the evaporator walls is directly dependent on the firing conditions, the mass flow distribution on the individual tubes is a function of the geometric, operational as well as thermo- and fluid dynamic boundary conditions. In this work, a selection of reliable models for the description of the heated two-phase flow as well as a descriptive explanation of the formation of the static evaporator instability is given.
In order to analyse the processes taking place in the real evaporator, two preliminary investigations are carried out first. With the help of a highly simplified model, geometric parameters such as pipe length or the occurrence of elbows are varied in addition to the heat supply, and operational boundary conditions are changed in order to determine the sensitivities of individual influencing variables on the temperature distribution at the evaporator outlet. In a next step, a detailed model of an actual evaporator geometry is homogeneously heated in order to be able to consider the effects of the geometric and water/steam-side influencing variables separately. It turns out that the mass flow distribution is influenced both by the geometry of the individual pipes and by the respective position at the distributor or collector. On account of this, an entirely uniform distribution of the total mass flow and thus an elimination of the evaporator instabilities are not possible.
In a further step the coupled simulation with a CFD furnace calculation takes place, i.e. a heterogeneous heat flux density distribution is applied onto the evaporator walls. With the uneven heat supply, a further influencing factor on the mass flow and temperature distribution at the evaporator outlet is added.
The simulation results show a very good qualitative agreement with measured values from power plant operation both for full load operation and for the two partial loads of 70 % and 50 %.
One possibility to attenuate static evaporator instabilities already installed in actual plants is offered by the so-called redistributing system, which is installed in the evaporator and which, by mixing and redistributing the total mass flow, significantly reduces the temperature differences at the evaporator outlet. Its mode of operation is analysed and presented. The maximum temperature difference at the evaporator outlet can thus be reduced to 50 - 75 % of its original value in the examined operating cases.
On this basis, an increased inner diameter of the evaporator outlet collector and controllable valves in the eight evaporator effluents are proposed and investigated as further remedial measures. By combining the measures, the maximum temperature difference occurring at the evaporator outlet can be more than halved compared to using the redistributing system alone.