The existing energy system consists of a variety of energy forms, of which CSP is an emerging form of energy generation. Concentrated solar power (CSP) is a novel technology for sustainable clean energy generation [1]. According to predictions, the proportion of traditional energy will be greatly reduced in the future, and clean energy such as CSP will become an important part of the energy system [2]. At the same time, CSP can form a large-scale renewable energy cluster with wind power and photovoltaic power to achieve joint regulation of energy systems [3]. Molten salt pump (MSP) is a key equipment in the CSP system, which is used to circulate molten salt cycle [4]. At present, CSP is designed for large capacity systems to deliver uninterrupted energy supply, which demands higher technical competence of MSP [5].
In order to improve the performance and stability of the MSP, a large number of studies have been carried out on the internal flow field and structure. Shao et al. [6,7] experimentally investigated the unsteady flow field of the MSP in which the velocity distribution was analyzed and a conversion method of MSP pumps was proposed for different mediums. The physical properties of molten salt vary significantly with the change in temperature of the medium, which affects the performance of the MSP. The influence of viscosity on the flow and energy loss characteristics of a MSP was illustrated by Cheng and Shao [8,9]. The objective of the study was to suggest design ideas for the transmission of different viscous molten salts. It was observed that the solid-liquid multiphase flow also exists in the molten salt flow. A greater impact of solid particle diameter on the pump performance was observed than that of volume fraction [10]. Wang et al. [11] suggested the modified design of auxiliary blades on the rear cover of the impeller and the sharp corners of the outlet pipe to improve hydraulic efficiency and reduce excitation of the MSP.
The common method to characterize the hydraulic performance and losses of fluid machinery is to express them in the form of pressure drop, however, it does not specify the magnitude and the cause of these losses [[12], [13], [14], [15]]. The ways to improve performance and reduce losses are based on finding the influence of geometrical parameters of the model on the performance and flow field distribution [[16], [17], [18], [19]]. Therefore, a lot of research works have been directed towards identifying the method that can analyze the flow losses in detail.
In recent years, there have been a series of studies on the exploration of energy loss characteristics of fluid machinery. The main focus of these studies was on the entropy production theory of the flow field. The distribution characteristics of energy loss in hydraulic components of PAT were summarized by Ghorani et al. [20] and found that the turbulent dissipation accounts for the main part of energy loss and more than 50% of the energy loss occurs in the runner. Wang et al. [21] established a diagnostic method based on the entropy production theory to analyze the cavitation flow in a pump-turbine model and showed that the entropy generation at the interface of pump-turbine is closely related to the evolution of cavitation. Li et al. [22] explored the hysteresis characteristics of a pump as a turbine using entropy generation theory and discovered that the backflow and separation vortices were causing the hysteresis.
Energy conversion processes affect the performance of fluid machinery and therefore, obtaining the energy transport and transformation characteristics of the hydraulic components is imperative to improve the efficiency of energy transformation in the flow field. Isberg et al. [23] established the energy flow equations of fluid surface waves to improve the power absorption in wave energy converters. A flow visualization method was discussed by Meyers et al. [24] for finding the kinetic energy flux path based on the concepts of momentum and energy transport. Bohr et al. [25] analyzed the energy transport in a moving fluid and determined the rates of energy transfer across the boundaries and energy dissipation from the arbitrary control volume. Whereas, the energy transport in multiscale/fractal-generated turbulence flow and special geometry structure was investigated by experimental methods [26,27].
The energy loss characteristics and energy conversion processes of fluid machinery can be used to guide the performance optimization of fluid machinery. Gu et al. [28] identified the energy loss distribution based on the entropy production theory and found an improvement in the hydraulic efficiency of the high-power pump by changing the clocking position to reduce the flow loss. An optimization method of the ultra-low specific speed centrifugal pump was proposed by Hou et al. [29], which combined the local entropy production theory and orthogonal design, aiming to reduce the total entropy production in impeller and volute. Finally, the performance of ultra-low specific speed centrifugal pumps was improved. Ji et al. [30,31] analyzed the energy loss characteristics in the flow field of the mixed-flow pump with different tip clearance and blade thickness based on entropy production theory, which is used to guide the engineering design of mixed flow pumps. An analysis method of energy production in centrifugal pump impeller was established by Zhang [32], and put forward a new design method under the guidance of energy production characteristics. Therefore, it is beneficial to improve the internal flow and performance by analyzing the energy loss and energy production of the fluid machinery flow field.
From the literature review, it came to know that greater attention was given to the investigation of performance and flow field distribution of hydraulic machinery, while the relationship between energy transport and hydraulic performance was not fully studied. Since there is no study available in the literature to find the relation between the energy transport and the flow characteristics of the molten salt pump of CSP, the present study is carried out to unfold the characteristics of the flow energy transport in the molten salt pump and its relationship with the energy conversion of different medium.
In this study, the energy transport characteristics of MSP with different mediums were carried out by using the computational fluid dynamics (CFD) method and energy transport theory. After that, the energy transport equation was separated according to cause, then the position and magnitude of energy production and energy loss in the flow field were obtained. Finally, the influence law of medium physical properties on the energy transport process was summarized. The research work provides a new perspective for analyzing energy transport in the centrifugal pump flow field, which can be used to guide the design and optimization of centrifugal pumps.
