In this blog post, we’ll explore the principles of heat transfer in an ondol, how airflow—which varies depending on the shape of the ondol—affects heating performance, and its connection to modern heating technology.
The History and Importance of Ondol
A significant portion of the energy humans use is related to maintaining body temperature, and keeping body temperature stable is a crucial factor for survival. To achieve this, our ancestors developed various technologies, including clothing, housing, and heating methods. They primarily protected body temperature through clothing, shielded themselves from wind and cold through their dwellings, and, in addition, developed unique heating methods.
While Western cultures, which are based on a standing lifestyle, developed heating methods centered on radiant heat—such as fireplaces—East Asian cultures, which are based on a sitting lifestyle, developed heating methods that effectively utilize conduction and convection, such as the ondol. In particular, the ondol has evolved independently as Korea’s representative traditional heating technology, and its excellence has been confirmed through various documents and studies from ancient times to the present.
The Heat Transfer Process in the Ondol
In the ondol, heat transfer begins when hot smoke and heat generated by a fire in the hearth rise upward through the “bunemgi” (or “bulmok”). The bunemgi is a structure that slightly raises the boundary between the hearth and the ondol floor; heat transfer begins in earnest from this point.
The hot air flows along the flue (the passageway through which smoke and heat travel) beneath the gudeuljang (the stones covering the floor channels), transferring heat to the stones; this heat is then conducted to the floor of the room, creating a heating effect. The rooms are connected via connecting passages, and the heat that has traveled through them passes through the chimney junction—which connects the fire channel to the chimney—and is ultimately vented out through the chimney.
The “bang-gore” is the passageway through which flames and smoke travel beneath the ondol slab, while the “chimney opening” is a space located directly beneath the chimney that prevents smoke backflow and catches ash falling from the chimney. Thus, the various passages and structures within the ondol are crucial factors in determining heat transfer efficiency.
The Shape of the “Gorae” and Airflow
The “gorae” is a passageway structure formed inside the ondol to prevent hot smoke and heat from escaping easily. Representative forms of the “gorae” found in traditional houses include the straight gorae, fan-shaped gorae, scattered gorae, mixed gorae, single-hearth style, and onion-shaped gorae.
The straight gorae (jul-gorae) consists of straight passages formed by support stones; its simple structure made it easy to construct and widely used. The straight gorae has the advantage of efficiently transferring heat from the firebox to the gorae distribution area while preventing rapid heat loss to the outside, thereby maintaining warmth for a long time with relatively little fuel.
The fan-shaped stove arranges the firebricks in a fan-like pattern to help heat spread evenly throughout the room. The scattered-brick stove uses irregularly arranged stones to adjust the width of the passageway and the path to the chimney, thereby ensuring a smooth flow of smoke. The hybrid layout combines the advantages of the straight and scattered layouts, effectively transferring heat toward the outer walls to improve heating efficiency throughout the room.
The single-sided hearth design is constructed with the firebox facing the center of the room to ensure even heat distribution and achieves relatively high heating efficiency. The “onion-shaped” go-rae involves burying a large jar at the center of the go-rae structure and directing smoke through a flue to the chimney; the long distance between the hearth and the chimney allows heat to linger longer, resulting in minimal heat loss. As such, the shape of the go-rae determines the path and speed of airflow, which in turn affects heating performance.
Fluid Flow Based on Gudul Shape and Research Findings
The shape of the gudul also determines the path of fluid flow, which in turn affects heat transfer rates and heat retention characteristics. A representative example is the comparison between “Jul-gore” and “Deodong-gore” types. In related studies comparing the symmetrical “straight-whale” gu-deul with the asymmetrical “curved-whale” gu-deul, the straight-whale gu-deul showed a faster initial temperature rise during heating, whereas the curved-whale gu-deul exhibited a higher peak temperature and retained heat for a longer period.
Interpreting this in terms of the relationship between the gu-deul’s shape and gas flow, in the symmetrical, straight-line “Julgorae” gu-deul, gas moves relatively quickly, resulting in a fast initial heating rate; however, heat escapes quickly, leading to a relatively low heat retention effect. In contrast, in the asymmetrical spiral-shaped braziers, the gas flow path is longer and more complex; while the heating rate is somewhat slower, heat remains trapped inside for a longer time, resulting in excellent heat retention.
Explanation Based on the Principle of Continuity and Bernoulli’s Principle
To understand why gas flow and velocity vary depending on the shapes of the “whale” and “gudeul,” it is necessary to examine both the principle of continuity and Bernoulli’s principle. The principle of continuity states that when a fluid passes through a tube over a fixed period of time, the flow rate of the fluid passing through each cross-section remains constant. Therefore, when the cross-sectional area of a passage narrows, the fluid’s velocity increases, and when the cross-sectional area widens, the velocity decreases.
Bernoulli’s principle describes the relationship between a fluid’s velocity, pressure, and height; for an ideal fluid, it states that the sum of pressure energy, potential energy, and kinetic energy remains constant. It is generally expressed as follows:
P + ρgh + ½ρv² = constant
Taking an ondol as an example, while the internal passages of the “jul-gorae-gu” sections have a relatively constant width, those of the “deodong-gorae-gu” sections do not; in particular, there is a section where the width narrows sharply toward the end. According to the principle of continuity, the gas velocity increases in sections where the passage narrows and decreases in relatively wider sections.
In sections where the velocity increases, the kinetic energy term (½ρv²) becomes larger, resulting in relatively lower pressure. Conversely, in sections where the velocity is lower, the pressure becomes relatively higher, increasing the likelihood that hot gas will linger longer. These flow characteristics can be understood as a fluid dynamics-based explanation for why the “deodong-gorae-gu” exhibit relatively superior heat retention performance.
The Significance of Ondol and Its Modern Implications
The airflow within an ondol plays a crucial role in the overall heat transfer process. Although our ancestors did not mathematically calculate this heat transfer or fluid flow, they inductively understood the principles through long-standing experience and effectively applied them to daily life.
Ondol has been utilized not only in residential spaces but also in various fields such as agricultural greenhouses, and it stands as a prime example of the excellence of traditional heating technology. During the Joseon Dynasty, ondol technology was highly regarded as a superior heating method, and there are records indicating that the Korean ondol structure attracted interest even overseas.
Although ondol and modern hot-water heating differ in that their heat-transfer media are gas and liquid, respectively, they share the common feature of heating indoor spaces through conduction and convection. While modern hot-water heating typically uses pipes of a fixed diameter, research on various pipe designs that take into account fluid flow and heat transfer characteristics is also being conducted steadily.
Even today, research continues on various traditional ondol technologies, such as improved ondol systems and ventilated raised-floor ondol, and active research is underway to integrate the principles of ondol into modern heating systems. This research provides important insights for the development of new heating technologies that enhance energy efficiency, and ondol—our traditional technology—deserves to be continuously developed and preserved alongside the advancements of modern science and engineering.
