High-Power Heat Transfer in Supercritical Fluids: Microscale Times and Sizes

Introduction

The study of heat transfer in supercritical fluids (SCFs) has a long history (see, for example, Anisimov, 2011; Ivanov, 2008; Kurganov, Zeigarnik, & Maslakova, 2012; Levelt Sengers, 1976; Sengers, 1966) and is closely associated with the task of increasing the efficiency of thermoengineering devices. Application of heat transfer agents at supercritical conditions can significantly improve the thermal efficiency of thermoengineering units. Indeed, their prospects are justified due to the impossibility of boiling crisis phenomenon at supercritical pressures and the anomalous increase in thermophysical properties (namely, heat capacity and thermal conductivity values) in the near-supercritical region. It is generally accepted that the latter is attributed to the development of large-scale fluctuations (Levelt Sengers, Greer, & Sengers, 1976). However, insufficient understanding of the physical processes occurring in the flow presents an obstacle for the use of, for example, supercritical water as a heat transfer agent in the nuclear reactors (Kurganov et al., 2014; Pioro, & Duffey, 2007). The fundamental problems of heat and mass transfer in SCFs appear to be unresolved. At the same time, SCFs have long been successfully used as working fluids and hundreds of power plants in the world are working on supercritical water for decades. It is obvious that the requirements for manageability and predictability of the supercritical water behavior under conditions of extreme thermal loads are significantly higher in nuclear power industry than in the conventional thermal power industry. Therefore, the transfer of experience accumulated in the conventional thermal power industry to the nuclear power industry proved to be a non-trivial task.

In the opinion of the authors, of note are two important issues. First, there is no theoretical model that would be able to describe all heat transfer modes that were experimentally observed. Secondly, the majority of experimental studies of heat transfer in SCFs and measurements of the thermophysical properties of substances at supercritical conditions have been performed by stationary or quasi-stationary methods. Taking into account the extremely high convective instability of SCFs and gravitational sensitivity of parameters in the near-critical region, the known pattern of transfer phenomena in SCFs is not complete. In other words, there is a shortage of research methods for such complex objects as SCFs.

This motivated development of the specialized techniques for studying heat transfer, operating at small characteristic sizes and times. Moreover, these techniques allow one to set up experiments under conditions of relatively high heat flux densities not achievable in stationary techniques (Rutin & Skripov, 2013a, 2013b), as shown below. This approach made it possible to virtually avoid the influence of convection and gravity on the experimental results and obtain the data on conductive heat transfer mode in the course of high-power heat release. The most important results obtained at small characteristic sizes and times can be formulated as follows. First, the effect of threshold decrease in the heat transfer intensity was revealed in course of a fast transition between compressed liquid and supercritical fluid states along the isobar. The effect was more pronounced at pressures (p) close to the critical pressure (p_c). Second, for all investigated substances (Rutin & Skripov, 2013b, 2013c; Rutin, Yampol’skii, & Skripov 2014; Rutin, Volosnikov, & Skripov, 2015), the revealed effect was observed in the range of reduced pressures from 1 to 3p/p_c and completely disappeared in the vicinity of pressure p/p_c = 3. Third, it was found that the peaks of isobaric heat capacity and excess thermal conductivity, which are known from stationary measurements, do not affect the experimental results. Obviously, these unusual results are of interest from both fundamental and applied perspectives. The latter includes not only thermal power, but also a wide range of supercritical engineering technologies (Gumerov & Le Neindre, 2016; Polikhronidi, Batyrova, Aliev, & Abdulagatov, 2019).