Advances and Challenges of Boiling Heat Transfer

*Igor L. Pioro*

### **Abstract**

Boiling is a heat-transfer process during which vapor bubbles are created on a heated surface (nucleate boiling) or inside overheated liquid (bulk boiling). Boiling has been used by humans for tens of thousands of years for cooking, however, its application in industry started somewhere in the seventeenth century. Moreover, actual research into boiling-heat-transfer phenomena started only around 1920s. In general, several major types of boiling process can be identified: natural-convection pool boiling vs. forced-convection flow boiling and nucleate boiling vs. bulk boiling. Major nucleate-pool-boiling characteristics are as the following: Onset of Nucleate Boiling (ONB); Heat Transfer Coefficient (HTC); Critical Heat Flux (CHF); HTC at film pool boiling; minimum heat flux at film pool boiling; and HTC at transition boiling. Quite similar characteristics correspond to flow-boiling: Onset of subcooled Nucleate Boiling (ONB); Onset of Significant Void (OSV); HTC; CHF; and Post-DryOut (PDO) heat transfer. In spite of more than 100 years of active research and many years of applications, boiling phenomena/heat transfer are still not fully investigated and understood. There are some attempts to develop boiling-phenomena theories, but, unfortunately, they are not so practical yet. Therefore, more or less all practical calculations of various boiling characteristics/parameters rely heavily on empirical correlations, which were obtained experimentally. Due to this sophisticated studies are performed into boiling phenomena in the world.

**Keywords:** pool boiling, flow boiling, nucleate boiling, heat transfer coefficient, critical heat flux

#### **1. Introduction: History notes**

Based on various sources (Wikipedia, 2023), there is some evidence that ancient humans have started to boil water as early as 30,000 years ago during the Upper Paleolithic period. Later on, i.e., about 26,000 years ago, cracked "boiling stones" were discovered in caves, which have been used by early modern humans. Around 20,000 years ago, pottery has appeared for more conventional boiling. Therefore, for tens of thousands of years, the boiling process has been used for cooking.

The earliest steam engine was the scientific novelties of Hero of Alexandria in the first century CE, called as the aeolipile (https://www.britannica.com/technology/stea m-engine [Accessed: December 10, 2023]).This device is the first known one to

transform steam energy into a rotary motion. However, like many other early machines, they have demonstrated basic mechanical principles and were simply regarded as a curiosity or a toy and have not been used for any practical purposes.

Only in the seventeenth century, there were attempts to made steam engines for practical purposes (https://www.britannica.com/technology/steam-engine [Accessed: December 10, 2023]). As such, in 1698 Th. Savery patented a pump with hand-operated valves to raise water from mines by suction produced with condensing steam. Somewhere around 1712, Th. Newcomen, has developed a more efficient steam engine with a piston separating the condensing steam from the water. In 1765, J. Watt quite significantly improved the Newcomen engine by adding a separate condenser to avoid heating and cooling the cylinder with each stroke. And finally, he has developed a new engine that rotated a shaft instead of providing the simple up-and-down motion of a pump and added many other improvements to produce a practical power plant.

In 1769, N.J. Cugnot has built the first steam carriage for roads in France (https:// www.britannica.com/technology/steam-engine [Accessed: December 10, 2023]). After that R. Trevithick in England was the first to use a steam carriage on a railway; and, in 1803, he built a steam locomotive. In 1829, English engineer G. Stephenson has adapted a steam engine to railways, which became a commercial success. In 1802, W. Symington has built the first practical steamboat. And in 1807, R. Fulton has proposed to use a steam engine for a passenger boat in the United States.

In 1892, L.P. Perkins and W.E. Buck have patented a heat-transmitting device, which was the first two-phase thermosyphon (or, also, it can be named as a wickless heat pipe) operating with boiling-condensation cycle (for more details on these devices, see Bezrodny et al. [1]; Pioro and Pioro [2]).

And only around 1920s, actual research into boiling-heat-transfer phenomena has been started. One of the most significant results, which is important even today, was obtained experimentally by Professor Sh. Nukiyama (http://www.htsj.or.jp/en/nukiyama [Accessed: December 10, 2023]). In 1934, Professor Nukiyama published his pioneering paper entitled "The Maximum and Minimum Values of the Heat Q Transmitted from Metal to Boiling Water under Atmospheric Pressure." In this paper he has clarified and provided an overview of the boiling phenomena in the form of the Nukiyama Curve (nowadays, quite often used just "Boiling Curve") (updated version of this curve is shown in **Figure 1**). Therefore, for more than 100 years, studies into the boiling heattransfer phenomena at various conditions within a wide range of working fluids, pressures and temperatures, surfaces, etc. are performed non-stop in many research organizations, companies, universities, laboratories, etc. around the world.

### **2. Classification of boiling cases**

In general, boiling is a heat-transfer process during which vapor bubbles are created on a heated surface (nucleate boiling) or inside overheated liquid (bulk boiling) (for properties of various fluids on saturation line, see [3, 4]). Boiling is only possible at subcritical pressures, because at critical and supercritical pressures fluid is a singlephase substance, therefore, there are no such terms as liquid and vapor (see **Figure 2**). However, due to significant variations of all thermophysical properties within critical and pseudocritical regions fluid undergoes a transition from high-density fluid (liquidlike) to low density fluid (vapor-like). Therefore, we have quite similar heat-transfer processes to those at subcritical pressures, which are called pseudo-boiling, pseudo-film-boiling, and deteriorated heat flux (for details, see [5–7]).

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

An important condition for boiling is that the temperature of a heated surface should be higher, at least, by several degrees than that of saturated liquid (see **Figure 1**). For subcooled boiling this difference in temperatures can be significantly higher.

Boiling process (heat transfer) can be classified in general as the following (for details, see **Figures 1**–**16**):

	- Internal boiling (inside tubes, pipes, channels, etc.) (**Figures 6**–**8**);
	- External boiling (over heated surfaces, annular channels, rod bundles, cross flow, etc.); and

#### **Figure 1.**

*Boiling curve for saturated water at atmospheric pressure (first time was obtained by Professor Sh. Nukiyama (Tohoku University, Japan) at the beginning of 1930s and for long time called as "Nukiyama's boiling curve"). In the current view the boiling curve is updated with* q*max and* q*min values and melting temperatures of selected common metals/alloys.*

**Figure 2.** *Thermodynamics diagrams for water: (a) pressure—temperature and (b) temperature—specific entropy.*

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

#### **Figure 3.**

*Nucleate pool boiling of water on horizontal copper plate (see Figure 5 for test section and electron-microscope images of boiling surfaces use in this experimental setup) at sub-atmospheric pressures: Parameters of working fluid —volume 120 ml and level 5.8 mm; height of boiling-condensation chamber 38 mm; scale—pitch between two thread-rods (black vertical posts on photos) equals to 40 mm.*


#### **Figure 4.**

*Nucleate pool boiling of refrigerants on horizontal high-density polyethylene plate (see Figure 5 for text section): Parameters of working fluid—volume 120 ml and level 5.8 mm; height of boiling-condensation chamber 38 mm; scale—width of photos equals to 120 mm in actual chamber. Photos 1 and 2—boiling in slots and photo 3 boiling on surface. Polyethylene has very low surface roughness due to that only several vapor-bubble-generating centers are seen.*



In addition, the following special cases of boiling can be identified:


#### **Figure 5.**

*Dimensions (in mm) of boiling-condensation chamber: Heating surfaces used—Aluminum, brass, copper, st. st., and high-density polyethylene (see Figure 15a-e for electron-microscope images of boiling surfaces and Tables 3 and 4 for their thermophysical properties and surface-roughness parameters):* H*b-c—height of boiling-condensation chamber.*

