Application of Supercritical Fluids in Thermal- and Nuclear-Power Engineering

Supercritical Fluids (SCFs) have unique thermophyscial properties and heat-transfer characteristics, which make them very attractive for use in power industry. In this chapter, specifics of thermophysical properties and heat transfer of SCFs such as water, carbon dioxide, and helium are considered and discussed. Also, particularities of heat transfer at Supercritical Pressures (SCPs) are presented, and the most accurate heat-transfer correlations are listed. Supercritical Water (SCW) is widely used as the working fluid in the SCP Rankine “steam”-turbine cycle in fossil-fuel thermal power plants. This increase in thermal efficiency is possible by application of high-temperature reactors and power cycles. Currently, six concepts of Generation-IV reactors are being developed, with coolant outlet temperatures of 500°C~1000°C. SCFs will be used as coolants (helium in GFRs and VHTRs, and SCW in SCWRs) and/or working fluids in power cycles (helium, mixture of nitrogen (80%) and helium (20%), nitrogen and carbon dioxide in Brayton gas-turbine cycles, and SCW/“steam” in Rankine cycle).

Study on Cooling Heat Transfer of Supercritical Carbon Dioxide Applied to Transcritical Carbon Dioxide Heat Pump

Understanding the heat transfer characteristics of supercritical fluids is of fundamental importance in many industrial processes such as transcritical heat pump system, supercritical water-cooled reactor, supercritical separation, and supercritical extraction processes. This chapter addresses recent experimental, theoretical, and numerical studies on cooling heat transfer of supercritical CO2. A systematic study on heat transfer coefficient and pressure drop of supercritical CO2 was carried out at wide ranges of tube diameter, mass flux, heat flux, temperature, and pressure. Based on the understanding of temperature and velocity distributions at cross-sectional direction provided by the numerical simulation, a new prediction model was proposed, which agreed well with the experimental results. In addition, the effect of lubricating oil was also discussed with the focus on the change in flow pattern and heat transfer performance of oil and supercritical CO2.

Thermal-Mechanical Effects and Near-Critical Fluid Dynamic Behaviors in Micro-Scale

Supercritical CO2 fluid has been widely used in chemical extraction, chemical synthesis, micro-manufacturing, and heat transfer apparatus, and so forth. The current chapter deals with near-critical CO2 micro-scale thermal convective flow and the effects of thermal-mechanical process. When the scale becomes smaller, new, and detailed figures of near-critical thermal effects emerges. To explore this new area, theoretical developments and numerical investigations are discussed and explained in this chapter. From a theoretical point of view, the thermal-mechanical nature of near-critical fluid would play a leading role in small time and spatial scales. This effect is found dominant to the thermal dynamic responses and convective structures of micro-scale fluid behaviors. The scaling effects, boundary thermal-mechanical process, instability evolutions, mixing flows and characteristics, possible extensions, and applications are also discussed in this chapter.

Numerical Modelling of Hydrodynamic Instabilities in Supercritical Fluids

The case of a supercritical fluid heated from below (Rayleigh-Bénard) in a rectangular cavity is first presented. The stability of the two boundary layers (hot and cold) is analyzed by numerically solving the Navier-Stokes equations with a van der Waals gas and stability diagrams are derived. The very large compressibility and the very low heat diffusivity of near critical pure fluids induce very large density gradients which lead to a Rayleigh–Taylor-like gravitational instability of the heat diffusion layer and results in terms of growth rates and wave numbers are presented. Depending on the relative direction of the interface or the boundary layer with respect to vibration, vibrational forces can destabilize a thermal boundary layer, resulting in parametric/Rayleigh vibrational instabilities. This has recently been achieved by using a numerical model which does not require any equation of state and directly calculates properties from NIST data base, for instance.

Supercritical Fluids and Their Applications in Power Generation

Supercritical fluids have been studied and used as the working fluids in power generation system for both high- and low-grade heat conversions. Low-grade heat sources, typically defined as below 300 ºC, are abundantly available as industrial waste heat, solar thermal, and geothermal, to name a few. However, they are under-exploited for power conversion because of the low conversion efficiency. Technologies that allow the efficient conversion of low-grade heat into mechanical or electrical power are very important to develop. First part of this chapter investigates the potential of supercritical Rankine cycles in the conversion of low-grade heat to power, while the second part discusses supercritical fluids used in higher grade heat conversion system. The selection of supercritical working fluids for a supercritical Rankine cycle is of key importance. This chapter discusses supercritical fluids fundamentals, selection of supercritical working fluids for different heat sources, and the current research, development, and commercial status of supercritical power generation systems.

Heat Transfer and Fluid Flow Modeling for Supercritical Fluids in Advanced Energy Systems

This chapter aims to model the supercritical fluids thermal hydraulics behaviors including heat transfer, pressure drops, and flow instabilities for the purpose of accurate design and efficient safe operation of advanced energy systems. At first, the convection heat transfer models considering the effect of nonlinear properties and the effect of buoyancy and acceleration have been provided and discussed. Secondly, the hydraulic resistance models for supercritical fluids have been selected and suggested for different conditions. Thirdly, the published models for supercritical flow instabilities based on four different regional partitions are summarized and clarified. At last, two typical case studies have been provided to further intuitively elaborate the thermal hydraulics of supercritical fluids within the advanced energy systems.

Supercritical Natural Circulation Loop

Supercritical natural circulation loop is a compelling technology for cooling of modern nuclear reactors, which promises enhanced thermal-hydraulic performance in a simple design. Being a new concept, related knowledge base is relatively thin and involves several conflicting theories and controversies. The chapter summarizes the observation till date, starting from the very fundamentals. The phenomenon of natural circulation under steady state condition and suitability of supercritical medium as working fluid are discussed in detail. Different methods of analyses, including analytical, simple 1-d numerical, and multidimensional computational codes, as well as experimental, are elucidated. A comprehensive discussion is presented about the effect of various geometric and operating parameters on the system behavior, from both thermal-hydraulic and stability point of view. Finally, a few recommendations are included about the operation of such loops and future direction of research.

Principles, Experiments, and Numerical Studies of Supercritical Fluid Natural Circulation System

Due to the unique thermal and transport properties, supercritical natural circulation loop (NCL, or thermosyphon) has been proposed in many energy systems, such as solar heater, nuclear cooling, waste heat recovery, geothermal, etc. This chapter presents the principals of supercritical natural circulation loop and its application challenges. A specially designed experimental prototype system is introduced and compared with numerical findings. The system is operated in wide range of pressures from around 6.0 MPa to 15.0 MPa in the near-critical region. It is found that in a supercritical natural circulation system, very high Reynolds number natural convection flow can be achieved only by simple heating and cooling. Thermal performance analysis and parameter effects are carried out along with the experimental development. The heat transfer dependency on operation and its mechanisms are also explained and summarized in this chapter. The comparison of experimental and numerical results contributes to better understanding of NCL stability phenomena and applications in energy systems.

Effect of Flow Acceleration and Buoyancy on Thermalhydraulics of sCO2 in Mini/Micro-Channel

Supercritical fluids have found enhanced applications in several sectors. High efficiency and high compactness associated with supercritical carbon dioxide power cycle are of major interest to the thermal engineers. Additionally, due to environment friendly properties, such as zero ODP, considerably lower GWP, non-toxic and nonflammable supercritical carbon dioxide has emerged as a potential substitute of conventional refrigerants. The peculiar properties of supercritical fluids ensured distinct flow and thermal characteristics of supercritical systems. Therefore, the chapter is aimed to discuss the thermalhydraulic characteristics of supercritical carbon dioxide in minichannel and microchannel. Both experimental and numerical studies on flow and thermal behavior of supercritical carbon dioxide will be discussed. The focus of this chapter is to examine the effect of buoyancy and flow acceleration on heat transfer performance. Considering the widespread applicability, the comprehensive discussion introduced in the chapter will affirmatively help the researchers.

High-Power Heat Transfer in Supercritical Fluids

Non-stationary heat transfer in supercritical fluids at relatively small temporal and spatial scales was studied experimentally. The aim of the study was to clarify the peculiarities of conductive heat transfer mode at significant heat loads. An unexpected stepwise decrease in the instant heat transfer coefficient has been revealed in the course of crossing the vicinity of the critical temperature along the supercritical isobar. This means that the peaks of isobaric heat capacity and excess thermal conductivity, which are known from stationary measurements, do not affect the experimental results. It is assumed that the action of considerable gradient in temperature and the presence of heat-transfer surface in pulse heated system can serve as factors that suppress large-scale fluctuations, leading to a “smoothing” the critical enhancement of the thermophysical properties. As an important consequence, this study gives new insight into selection of the operating pressure of supercritical heat transfer agent.