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Analysis of thermal and flow phenomena in natural circulation boiler evaporator

Grądziel, Sławomir
Energy 2019 v.172 pp. 881-891
differential equation, energy conservation, enthalpy, mass flow, mathematical models, momentum, temperature
An original in-house mathematical model enabling the analysis of thermal and flow phenomena occurring in the evaporator of a natural circulation boiler is proposed herein. It is a distributed parameter model based on an original approach to the problem of solving the equations describing the principles of mass, momentum, and energy conservation. The equations are solved using the method of lines. Space derivatives are written on the left side of the equations and time derivatives are approximated using difference quotients. This system of ordinary differential equations is solved using the Runge–Kutta method. Such an approach is characterised by high stability and accurate calculations. The developed model is applied to obtain the fluid mass flow, pressure, and enthalpy (temperature) values in the boiler evaporator. It can be applied in single- and two-phase flows. This paper presents the original results of the testing of the flow phenomena occurring in the boiler evaporator in steady and transient states. Steady-state tests were performed for different boiler loads. The operating multiplicity of the fluid circulation in the boiler evaporator was determined. A flow meter was installed on a downcomer, which enabled a comparison between the measured values of the water mass flow and those calculated using the developed model. The presented model can also be applied for the analysis of the boiler evaporator operation at an abrupt increase or decrease in pressure in emergency situations. Such occurrences may cause water stagnation in the evaporator tubes; therefore, the tubes may crack.