Abstract
In a steam turbine system, one of the main factors limiting the operational flexibility is the thermal stress associated with a high-temperature gradient within the control valves, which often leads to structural damage during frequent startup and shutdown cycles. One possible solution is to utilize an electric heating system with appropriate insulation to decrease the warm-up time. Here, an experiment and a numerical simulation were performed using a scaled turbine valve equipped with an electric heating system to understand the heat transfer process. The experiment, which was conducted at Shanghai Jiao Tong University, had a duration of 100 h and included three heating–cooling cycles and two heat preservation states. The results showed that the temperature distribution of the scaled valve was uniform, which is due to the high thermal conductivity of the valve body and the low conductivity of the insulation layer. The simulation, which used the commercial software ansysfluent 2019 R1, was performed to model the experimental heat transfer process and showed less than 10% deviation from the measured temperatures. A sensitivity study revealed that the valve temperature is very insensitive to the heat transfer coefficient of the outer surface of the insulation layer, which is attributed to the negative feedback regulation of radiation heat transfer. To further improve the computing efficiency, a simplified model based on the lumped parameter method and the quasi-steady-state assumption was proposed and validated. This model can predict the 100 h valve temperature history in less than 1 min and showed good agreement for all of the studied cases. The ability of the simplified model to simulate the valve heating–cooling cycles at a high efficiency could accelerate the thermal design process to improve the operational flexibility of steam turbines in the future.
