Daily Question (January 27)

The phenomenon in which the inner wall surface of the evaporation tube comes into contact with steam and is no longer cooled by water, resulting in a significant decrease in the heat transfer coefficient from the tube wall to the working fluid and a sharp rise in the wall temperature, is known as degraded boiling heat transfer, or film boiling. There are two main reasons for the deterioration of boiling heat transfer: one is a high heat load ； First, the soda mixture has a high gas content by mass. When the heat load is too high, it increases the number of \"vaporization centers\" on the inner wall of the tube; numerous bubbles form a vapor film that adheres to the tube wall, preventing contact between the water and the tube wall. The heat load at this point is called the critical heat load qlj. When the vapor content in the mass of the soda mixture reaches a certain level, the continuous water film on the tube wall is torn apart, and a vapor film takes its place in contact with the tube wall. The mass vapor content at this point is referred to as the critical vapor content xlj. Under normal conditions, the evaporation tube maintains a continuous layer of water film in contact with the tube wall. Due to the excellent heat transfer properties of this flowing water film, the tube wall is well cooled, preventing the wall temperature from rising too high. Once boiling leads to a deterioration in heat transfer, the vapor film replaces the water film in contact with the tube wall, and heat is transferred to the water primarily through the thermal conduction of the vapor film. Steam has a much poorer thermal conductivity than water, which causes the heat transfer coefficient to drop rapidly, leading to a swift increase in wall temperature; in severe cases, this can damage the heated surfaces. Therefore, it is necessary to keep the heat load below the critical heat load and the vapor content below the critical vapor content in order to prevent the deterioration of boiling heat transfer.

The phenomenon in which the inner wall surface of the evaporation tube comes into contact with steam and is no longer cooled by water, resulting in a significant decrease in the heat transfer coefficient from the tube wall to the working fluid and a sharp rise in the wall temperature, is known as degraded boiling heat transfer, or film boiling. There are two main reasons for the deterioration of boiling heat transfer: one is a high heat load ； First, the soda mixture has a high gas content by mass. When the heat load is too high, it increases the number of \"vaporization centers\" on the inner wall of the tube; numerous bubbles form a vapor film that adheres to the tube wall, preventing contact between the water and the tube wall. The heat load at this point is called the critical heat load qlj. When the vapor content in the mass of the soda mixture reaches a certain level, the continuous water film on the tube wall is torn apart, and a vapor film takes its place in contact with the tube wall. The mass vapor content at this point is referred to as the critical vapor content xlj. Under normal conditions, the evaporation tube maintains a continuous layer of water film in contact with the tube wall. Due to the excellent heat transfer properties of this flowing water film, the tube wall is well cooled, preventing the wall temperature from rising too high. Once boiling leads to a deterioration in heat transfer, the vapor film replaces the water film in contact with the tube wall, and heat is transferred to the water primarily through the thermal conduction of the vapor film. Steam has a much poorer thermal conductivity than water, which causes the heat transfer coefficient to drop rapidly, leading to a swift increase in wall temperature; in severe cases, this can damage the heated surfaces. Therefore, it is necessary to keep the heat load below the critical heat load and the vapor content below the critical vapor content in order to prevent the deterioration of boiling heat transfer.

Film boiling: If the heat load outside the tube is high, the rate at which bubbles are formed exceeds the rate at which they detach from the tube wall. As a result, bubbles accumulate on the inner surface of the tube, forming a vapor film that separates the water in the center of the tube from the tube wall. This prevents the tube wall from being directly cooled by the water film, leading to overheating of the tube wall. This phenomenon is also known as type 1 heat transfer degradation. Film boiling generally occurs in the water-cooled wall tubes of subcritical-parameter boilers. This is because the latent heat of vaporization of water decreases significantly as pressure increases, making it easier for water in the fluid boundary layer near the inner surface of the water-cooled wall tubes to vaporize under subcritical conditions. As a result, more vaporization nuclei are formed, increasing the likelihood of film boiling. The occurrence of film boiling depends on various factors such as the heat load outside the water-cooled wall tubes, the quality vapor fraction of the fluid inside the tubes, the mass flow rate inside the tubes, the pressure of the fluid, and the tube diameter. However, it mainly depends on the heat load and the quality vapor fraction of the water-cooled wall. Operational experience and tests have shown that although the quality vapor fraction of the steam-water mixture at the outlet of the water-cooled wall tubes in subcritical-parameter boilers is usually only around 0.3–0.4, the possibility of heat transfer degradation remains high. The use of internally threaded tube water-cooled walls can help to suppress film boiling.

The phenomenon in which the inner wall surface of the evaporation tube comes into contact with steam and is no longer cooled by water, resulting in a significant decrease in the heat transfer coefficient from the tube wall to the working fluid and a sharp rise in the wall temperature, is known as degraded boiling heat transfer, or film boiling. There are two main reasons for the deterioration of boiling heat transfer: one is a high heat load ； First, the soda mixture has a high gas content by mass. When the heat load is too high, it increases the number of \"vaporization centers\" on the inner wall of the tube; numerous bubbles form a vapor film that adheres to the tube wall, preventing contact between the water and the tube wall. The heat load at this point is called the critical heat load qlj. When the vapor content in the mass of the soda mixture reaches a certain level, the continuous water film on the tube wall is torn apart, and a vapor film takes its place in contact with the tube wall. The mass vapor content at this point is referred to as the critical vapor content xlj. Under normal conditions, the evaporation tube maintains a continuous layer of water film in contact with the tube wall. Due to the excellent heat transfer properties of this flowing water film, the tube wall is well cooled, preventing the wall temperature from rising too high. Once boiling leads to a deterioration in heat transfer, the vapor film replaces the water film in contact with the tube wall, and heat is transferred to the water primarily through the thermal conduction of the vapor film. Steam has a much poorer thermal conductivity than water, which causes the heat transfer coefficient to drop rapidly, leading to a swift increase in wall temperature; in severe cases, this can damage the heated surfaces. Therefore, it is necessary to keep the heat load below the critical heat load and the vapor content below the critical vapor content in order to prevent the deterioration of boiling heat transfer.

The phenomenon in which the inner wall surface of the evaporation tube comes into contact with steam and is no longer cooled by water, resulting in a significant decrease in the heat transfer coefficient from the tube wall to the working fluid and a sharp rise in the wall temperature, is known as degraded boiling heat transfer, or film boiling. There are two main reasons for the deterioration of boiling heat transfer: one is a high heat load ； First, the soda mixture has a high gas content by mass. When the heat load is too high, it increases the number of \"vaporization centers\" on the inner wall of the tube; numerous bubbles form a vapor film that adheres to the tube wall, preventing contact between the water and the tube wall. The heat load at this point is called the critical heat load qlj. When the vapor content in the mass of the soda mixture reaches a certain level, the continuous water film on the tube wall is torn apart, and a vapor film takes its place in contact with the tube wall. The mass vapor content at this point is referred to as the critical vapor content xlj. Under normal conditions, the evaporation tube maintains a continuous layer of water film in contact with the tube wall. Due to the excellent heat transfer properties of this flowing water film, the tube wall is well cooled, preventing the wall temperature from rising too high. Once boiling leads to a deterioration in heat transfer, the vapor film replaces the water film in contact with the tube wall, and heat is transferred to the water primarily through the thermal conduction of the vapor film. Steam has a much poorer thermal conductivity than water, which causes the heat transfer coefficient to drop rapidly, leading to a swift increase in wall temperature; in severe cases, this can damage the heated surfaces. Therefore, it is necessary to keep the heat load below the critical heat load and the vapor content below the critical vapor content in order to prevent the deterioration of boiling heat transfer.

The phenomenon in which the inner wall surface of the evaporation tube comes into contact with steam and is no longer cooled by water, resulting in a significant decrease in the heat transfer coefficient from the tube wall to the working fluid and a sharp rise in the wall temperature, is known as degraded boiling heat transfer, or film boiling. There are two main reasons for the deterioration of boiling heat transfer: one is a high heat load ； First, the soda mixture has a high gas content by mass. When the heat load is too high, it increases the number of \"vaporization centers\" on the inner wall of the tube; numerous bubbles form a vapor film that adheres to the tube wall, preventing contact between the water and the tube wall. The heat load at this point is called the critical heat load qlj. When the vapor content in the mass of the soda mixture reaches a certain level, the continuous water film on the tube wall is torn apart, and a vapor film takes its place in contact with the tube wall. The mass vapor content at this point is referred to as the critical vapor content xlj. Under normal conditions, the evaporation tube maintains a continuous layer of water film in contact with the tube wall. Due to the excellent heat transfer properties of this flowing water film, the tube wall is well cooled, preventing the wall temperature from rising too high. Once boiling leads to a deterioration in heat transfer, the vapor film replaces the water film in contact with the tube wall, and heat is transferred to the water primarily through the thermal conduction of the vapor film. Steam has a much poorer thermal conductivity than water, which causes the heat transfer coefficient to drop rapidly, leading to a swift increase in wall temperature; in severe cases, this can damage the heated surfaces. Therefore, it is necessary to keep the heat load below the critical heat load and the vapor content below the critical vapor content in order to prevent the deterioration of boiling heat transfer.

The phenomenon in which the inner wall surface of the evaporation tube comes into contact with steam and is no longer cooled by water, resulting in a significant decrease in the heat transfer coefficient from the tube wall to the working fluid and a sharp rise in the wall temperature, is known as degraded boiling heat transfer, or film boiling. There are two main reasons for the deterioration of boiling heat transfer: one is a high heat load ； First, the soda mixture has a high gas content by mass. When the heat load is too high, it increases the number of \"vaporization centers\" on the inner wall of the tube; numerous bubbles form a vapor film that adheres to the tube wall, preventing contact between the water and the tube wall. The heat load at this point is called the critical heat load qlj. When the vapor content in the mass of the soda mixture reaches a certain level, the continuous water film on the tube wall is torn apart, and a vapor film takes its place in contact with the tube wall. The mass vapor content at this point is referred to as the critical vapor content xlj. Under normal conditions, the evaporation tube maintains a continuous layer of water film in contact with the tube wall. Due to the excellent heat transfer properties of this flowing water film, the tube wall is well cooled, preventing the wall temperature from rising too high. Once boiling leads to a deterioration in heat transfer, the vapor film replaces the water film in contact with the tube wall, and heat is transferred to the water primarily through the thermal conduction of the vapor film. Steam has a much poorer thermal conductivity than water, which causes the heat transfer coefficient to drop rapidly, leading to a swift increase in wall temperature; in severe cases, this can damage the heated surfaces. Therefore, it is necessary to keep the heat load below the critical heat load and the vapor content below the critical vapor content in order to prevent the deterioration of boiling heat transfer. 1# Stand in the east

The phenomenon in which the inner wall surface of the evaporation tube comes into contact with steam and is no longer cooled by water, resulting in a significant decrease in the heat transfer coefficient from the tube wall to the working fluid and a sharp rise in the wall temperature, is known as degraded boiling heat transfer, or film boiling. There are two main reasons for the deterioration of boiling heat transfer: one is a high heat load ； First, the soda mixture has a high gas content by mass. When the heat load is too high, it increases the number of \"vaporization centers\" on the inner wall of the tube; numerous bubbles form a vapor film that adheres to the tube wall, preventing contact between the water and the tube wall. The heat load at this point is called the critical heat load qlj. When the vapor content in the mass of the soda mixture reaches a certain level, the continuous water film on the tube wall is torn apart, and a vapor film takes its place in contact with the tube wall. The mass vapor content at this point is referred to as the critical vapor content xlj. Under normal conditions, the evaporation tube maintains a continuous layer of water film in contact with the tube wall. Due to the excellent heat transfer properties of this flowing water film, the tube wall is well cooled, preventing the wall temperature from rising too high. Once boiling leads to a deterioration in heat transfer, the vapor film replaces the water film in contact with the tube wall, and heat is transferred to the water primarily through the thermal conduction of the vapor film. Steam has a much poorer thermal conductivity than water, which causes the heat transfer coefficient to drop rapidly, leading to a swift increase in wall temperature; in severe cases, this can damage the heated surfaces. Therefore, it is necessary to keep the heat load below the critical heat load and the vapor content below the critical vapor content in order to prevent the deterioration of boiling heat transfer.

Film boiling is a phenomenon in which the inner surface of the evaporation tube comes into contact with steam and is no longer cooled by water; as a result, the heat transfer coefficient from the tube wall to the working fluid drops significantly, causing the wall temperature to rise sharply. This includes film boiling and dry out. In a externally heated main pipe, as water flows from bottom to top under the pressure of a pump, it is first heated due to forced convection; the water in contact with the pipe wall forms bubbles at certain heights, leading to local boiling. Further aggregation of these bubbles results in a continuous stream of steam, thereby giving rise to film boiling. This phenomenon can be observed in the evaporation tubes of conventional boilers, the heat transfer tubes of nuclear power plant steam generators, and the coolant channels in reactor cores. When a vapor film forms between the pipe wall surface and the fluid, the insulating effect of this vapor film reduces heat exchange significantly, causing the wall surface to reach very high temperatures and posing a risk of burnout.

The phenomenon in which the inner wall surface of the evaporation tube comes into contact with steam and is no longer cooled by water, resulting in a significant decrease in the heat transfer coefficient from the tube wall to the working fluid and a sharp rise in the wall temperature, is known as degraded boiling heat transfer, or film boiling. There are two main reasons for the deterioration of boiling heat transfer: one is a high heat load ； First, the soda mixture has a high gas content by mass. When the heat load is too high, it increases the number of \"vaporization centers\" on the inner wall of the tube; numerous bubbles form a vapor film that adheres to the tube wall, preventing contact between the water and the tube wall. The heat load at this point is called the critical heat load qlj. When the vapor content in the mass of the soda mixture reaches a certain level, the continuous water film on the tube wall is torn apart, and a vapor film takes its place in contact with the tube wall. The mass vapor content at this point is referred to as the critical vapor content xlj. Under normal conditions, the evaporation tube maintains a continuous layer of water film in contact with the tube wall. Due to the excellent heat transfer properties of this flowing water film, the tube wall is well cooled, preventing the wall temperature from rising too high. Once boiling leads to a deterioration in heat transfer, the vapor film replaces the water film in contact with the tube wall, and heat is transferred to the water primarily through the thermal conduction of the vapor film. Steam has a much poorer thermal conductivity than water, which causes the heat transfer coefficient to drop rapidly, leading to a swift increase in wall temperature; in severe cases, this can damage the heated surfaces. Therefore, it is necessary to keep the heat load below the critical heat load and the vapor content below the critical vapor content in order to prevent the deterioration of boiling heat transfer.