Experimental investigation of the flow and heat transfer dynamic response of supercritical water in the thermal system

With the swift advancement of renewable energy sources, notably solar and wind power, there is an escalating demand for conventional thermal power units to perform deep peaking-shaving and valley-filling operations. Gaining a comprehensive understanding of the dynamic response during the deep peaking process is essential for ensuring the operational safety of thermal power units. This research offers an extensive experimental investigation into the dynamic response of supercritical fluids to step input across a broad temperature range, with a particular emphasis on their behavior under supercritical pressures exceeding 22.074 MPa. By examining the dynamic behaviors across both the time and frequency domains, this study unveils pivotal insights into the system's response characteristics, which are crucial for optimizing the performance and reliability of thermal system. The investigation uncovers that the dynamic behavior of the heating flow system is sensitive to the bulk temperature. In liquid-like conditions ( T/T _pc <0.95), the system rapidly achieves a new equilibrium state, indicative of a first-order response. Conversely, as the bulk fluid temperature escalates, the response duration extends, demanding more time to reach stability and potentially manifesting as zero-damping periodic oscillations in the vapor-like phase ( T/T _pc >1.05), necessitating the application of higher-order functions for accurate modeling. The temperature response to step input is observed to be markedly different across various temperature regions. The transfer function analysis indicates that in the liquid-like region, the system behaves akin to a first-order function with a gradually increasing time constant to a new steady state, characterized by a damping ratio greater than one ( ζ > 1). However, in the vicinity of the pseudocritical region, the time constant exhibits a nonlinear and rapid increase, with a corresponding rise in the damping ratio, complicating the system's approach to a new steady state (1> ζ > 0). This is accompanied by an amplified amplitude of fluctuations. In the vapor-like region, the emergence of periodic oscillations with zero damping ( ζ = 0) is noted, which can be effectively captured by a fourth-order transfer function. With the proposal of transfer functions for different operational regions, providing a valuable tool for predicting dynamic responses in complex flow systems. These findings contribute to a deeper understanding of supercritical fluid dynamics and have implications for the design and operation of systems subjected to supercritical conditions.