Heat Transfer in Supercritical Fluids: Going to Microscale Times and Sizes

Heat Transfer in Supercritical Fluids: Going to Microscale Times and Sizes

Sergey B. Rutin (Institute of Thermal Physics, Ural Branch, Russian Academy of Sciences, Russia), Aleksandr D. Yampol'skiy (Institute of Thermal Physics, Ural Branch, Russian Academy of Sciences, Russia) and Pavel V. Skripov (Institute of Thermal Physics, Ural Branch, Russian Academy of Sciences, Russia)
Copyright: © 2017 |Pages: 21
DOI: 10.4018/978-1-5225-2047-4.ch009
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Results of experimental study of non-stationary heat transfer in supercritical fluids, which were obtained using the method of controlled pulse heating of low-inertia wire probe, are discussed. The aim of this study was to clarify the peculiarities of heat conduction mode at significant heat loads. A threshold decrease in the “instant” heat transfer coefficient, the more pronounced the closer the pressure value to critical pressure, has been found, as well as the absence of impact of the isobaric heat capacity peak known from stationary measurements on the experimental results. These results give new insights into selection of the operating pressure of supercritical heat transfer agent. Small time and spatial scale in the experiments (units of millisecond and units of micrometer) in combination with high-power heat release (up to 20 MW/m2) makes it possible to associate the results with the behavior of boundary layer region of heat transfer agent.
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The study of heat transfer in supercritical fluids (SCFs) has a long history (see, for example, Anisimov, 2011; Berche, Henkel, & Kenna 2009; Kurganov, Zeigarnik, & Maslakova, 2012; Pioro, & Duffey, 2007) 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. 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. It is curious that at the same time, supercritical fluids 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 high heat flux densities (Rutin & Skripov, 2012, 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, which are of interest for both fundamental and applied aspects. 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 (pc). Second, for all investigated substances (Rutin & Skripov, 2013b, 2013c, 2014; Rutin, Volosnikov, & Skripov, 2015), the revealed effect was observed in the range of reduced pressures from 1 to 3p/pc and completely disappeared in the vicinity of pressure p/pc = 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.

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