Heat Transfer and Fluid Flow of Supercritical Fluids in Advanced Energy Systems

Introduction

In the past decades, supercritical fluids, due to their highly efficient heat transfer effectiveness, have been worldwide extensively concerned to be used in many promising advanced energy conversion and power systems such as supercritical carbon dioxide power cycles configured to operate with a variety of heat sources, GEN IV supercritical water nuclear power technology and supercritical hydrocarbon fuels (e.g. kerosene and methane etc.) rocket and supersonic combustion scramjet engines. It is of great importance to figure out the supercritical fluids thermal hydraulics characteristics including heat transfer, pressure drops and flow instabilities for the purpose of accurate design and efficient safe operation of these advanced energy systems.

This chapter is started with an introduction on general background and characteristics of supercritical fluids. In section 1, the general backgrounds and associated thermal hydraulics key issues of several advanced energy systems using supercritical fluids as working fluid will be introduced. In addition, the nonlinear thermal physical properties such as specific heat, density, thermal conductivity and viscosity play an important role in determination of supercritical fluids heat transfer and fluid flow characteristics, thus the general thermal physical properties of supercritical fluids will be illustrated and analyzed.

The section 2 will focus on the heat transfer issues of supercritical fluids. The forced convection heat transfer models considering the effect of non-uniform and nonlinear properties in both spatial and temporal dimensions, the mixed convection heat transfer models considering the effect of buoyancy, the effect of acceleration on the heat transfer performance under high heat fluxes will be discussed.

The section 3 will focus on the hydraulic resistance issues of supercritical fluids. Accurate prediction of pressure loss must be well known for design purposes. Total pressure drop in supercritical systems consist of typically four components: frictional pressure loss, local flow obstructions, acceleration effects due to density changes over length, and gravitational pressure effects. For supercritical fluids, the frictional factor is significantly different from that of the conventional single phase fluids. In this section, several correlations will be selected and suggested for different conditions based on an extensive literature review of hydraulic resistances of supercritical fluids.

The section 4 will focus on the flow instabilities issues of supercritical fluids. Although there is no distinct liquid or gas phases exist, the physical properties of supercritical fluids also change sharply with temperature, in the neighboring region of the pseudo-critical temperature point. It means that the supercritical fluids with high density and low density may flow through the heated channel alternately. This phenomenon may finally lead to the flow instabilities of supercritical fluids, which is similar to the density wave oscillation of two-phase flows at subcritical pressures. Since the flow instabilities of supercritical fluids will lead to heat transfer deterioration and equipment damage, it has been the hot topic in the study of the safe use of supercritical fluids. In this section, the experimental and theoretical work carried out by various investigators over a period of several years will be summarized. Then, the published models for supercritical flow instabilities based on four different regional partitions are summarized and compared in this chapter, the differences and characteristics of these models will be clarified.

The section 5 will provide two case studies to further intuitively elaborate the thermal hydraulics of supercritical fluids within the advanced energy systems. The first one will focus on the heat transfer of supercritical CO₂ at prototypic printed circuit heat exchangers, which could be used for advanced supercritical CO₂ power cycles. The second one will discuss the stability boundaries obtained based on different partition methods and the advantages and disadvantages of each model are analyzed in detail, which will provide technical support for the design of efficient and safe ultra-supercritical steam boilers.