**2. Specifics of thermophysical properties of SCFs**

Prior to a general discussion on specifics of forced-convective heat transfer at critical and supercritical pressures, it is important to define special terms and expressions used at these conditions [6, 9]. For a better understanding of these

terms and expressions their definitions are listed in Glossary (see below) (also, see **Figures 10**–**35**). Specifics of thermophysical properties at SCPs are described in Pioro et al. [23]; Handbook [6]; Mann and Pioro [24]; Gupta et al. [25]; Pioro and

*Supercritical-Fluids Thermophysical Properties and Heat Transfer in Power-Engineering…*

*Compressed fluid* is the fluid at a pressure above the critical pressure, but at a

*Critical point (also called a critical state)* is the point in which the distinction between the liquid and gas (or vapor) phases disappears (see **Figure 10**), that is, both phases have the same temperature, pressure, and specific volume or density. The critical point is characterized with the phase-state parameters: *T*cr, *P*cr and *v*cr

*Deteriorated heat transfer (DHT)* is characterized with lower values of the HTC compared to those for normal heat transfer (NHT); and hence, has higher values of wall temperature within some part of a heated channel (see **Figures 12, 13a, 24b, 25b, 27, 31**, and **35**) or within the entire heated length (see **Figure 14b**). *Improved heat transfer (IHT)* is characterized with higher values of the HTC compared to those for NHT; and hence, lower values of wall temperature within some part of a heated channel (see **Figures 12, 21, 25, 27b, 33**, and **34**) or within the entire heated length. In our opinion, the IHT regime or mode includes peaks or

*Normal heat transfer (NHT)* can be characterized in general with HTCs similar to those of subcritical convective heat transfer far from the critical or pseudocritical regions, when they are calculated according to the conventional single-phase Dittus-Boelter-type correlations: **Nu** = 0.0243 **Re**0.8**Pr**0.4 (see **Figures 12, 13a, 14a,**

*Overheated vapor* is the vapor at pressures below the critical pressure, and at temperatures above the saturation temperature, but below the critical temperature

*Pseudocritical line* is the line, which consists of pseudocritical points (see **Figure 10**). *Pseudo-boiling* is a physical phenomenon similar to subcritical-pressure nucleate

boiling, which may appear at SCPs. Due to heating of an SCF with a bulk-fluid temperature below the pseudocritical temperature (high-density fluid, i.e., "liquidlike") (see **Figures 10, 11, 13b** and **15**), some layers near the heated surface may attain temperatures above the pseudocritical temperature (low-density fluid, i.e., "gas-like"). This low-density "gas-like" fluid leaves the heated surface in a form of variable density volumes (bubbles). During the pseudo-boiling, the HTC usually

*Pseudocritical point* (characterized with *P* and *Tpc*) is the point at a pressure above the critical pressure and at a temperature (*Tpc* > *Tcr*) corresponding to the

*Pseudo-film boiling* is a physical phenomenon similar to subcritical-pressure film boiling, which may appear at SCPs. At pseudo-film boiling, a low-density fluid (a fluid at temperatures above the pseudocritical temperature, i.e., "gas-like") prevents a high-density fluid (a fluid at temperatures below the pseudocritical temperature, i.e., "liquid-like") from contacting ("rewetting") a heated surface.

*Supercritical fluid* is the fluid at pressures and temperatures that are higher than

the critical pressure and critical temperature (see **Figure 10**). However, in the

maximum value of specific heat at this particular pressure (see

Pseudo-film boiling leads to the DHT regime.

Mokry [26]; and Pioro and Duffey [9] (for more details, see **Table 4**).

temperature below the critical temperature (see **Figure 10**).

*DOI: http://dx.doi.org/10.5772/intechopen.91474*

(or *ρ*cr), which have unique values for each pure substance.

"humps" in the HTC profile near the critical or pseudocritical points.

**Glossary**

**21, 24, 25, 27**, and **30**–**34**).

increases (IHT regime).

**Figures 10, 11**, and **13b**).

**7**

(see **Figure 10**).


*Pressures for SC carbon dioxide, R-134a, and R-12 are equivalent for SCW pressure of 25 MPa, based on, so-called, reduced-pressure scaling: <sup>P</sup> Pcr Fluid* <sup>¼</sup> *<sup>P</sup> Pcr SCW.*

### **Table 4.**

*Selected list of literature sources on thermophysical properties of fluids, gases, and other materials.*

*Supercritical-Fluids Thermophysical Properties and Heat Transfer in Power-Engineering… DOI: http://dx.doi.org/10.5772/intechopen.91474*

terms and expressions their definitions are listed in Glossary (see below) (also, see **Figures 10**–**35**). Specifics of thermophysical properties at SCPs are described in Pioro et al. [23]; Handbook [6]; Mann and Pioro [24]; Gupta et al. [25]; Pioro and Mokry [26]; and Pioro and Duffey [9] (for more details, see **Table 4**).
