Boiling and Condensation

Contents

Define and classify Boiling heat transfer 1

Describe Boiling Regimes 2

Recall Nucleate Boiling 3

List the factors affecting Nucleate Boiling 4

Recall Condensation heat transfer 5

Describe Filmwise and Dropwise condensation 6

Recall laminar film condensation on vertical plate 7

Describe the influence of non-condensable gases on condensation heat transfer 8

Define and classify Boiling heat transfer

Boiling heat transfer refers to the process of heat transfer from a solid surface to a liquid or a vapour, resulting in the boiling or vaporisation of the liquid. It is an important type of heat transfer in many industrial and technological applications, such as power generation, refrigeration, and chemical processing.

Classification of Boiling Heat Transfer:

1. Natural Convection Boiling: This type of boiling occurs when the heat transfer from a solid surface to a liquid is driven solely by natural convection, i.e., the buoyant force caused by the density difference between the hot and cold fluid.
2. Forced Convection Boiling: This type of boiling occurs when the heat transfer from a solid surface to a liquid is driven by both natural and forced convection. Forced convection can be caused by a variety of factors, including the presence of a fluid flow or a heat transfer fluid, such as a coolant.
3. Pool Boiling: Pool boiling occurs when the heat transfer from a solid surface to a liquid takes place in a static pool of liquid, such as a pot of boiling water. In this type of boiling, the heat transfer rate is dependent on the temperature difference between the solid surface and the liquid, the surface area, and the heat transfer coefficient.
4. Flow Boiling: Flow boiling occurs when the heat transfer from a solid surface to a liquid takes place in a flowing stream of liquid, such as water flowing through a pipe. In this type of boiling, the heat transfer rate is dependent on the flow rate, the temperature difference between the solid surface and the liquid, and the heat transfer coefficient.
5. Film Boiling: Film boiling occurs when a thin layer of vapour separates the solid surface from the liquid, creating a barrier that slows down the heat transfer. This type of boiling is typically seen in high-heat-flux applications, such as the cooling of high-power electronics.

In conclusion, boiling heat transfer is an important type of heat transfer that occurs when a solid surface transfers heat to a liquid or vapour, resulting in boiling or evaporation. It can be classified based on the type of fluid flow, the presence or absence of a vapour layer, and the method of heat transfer. Understanding the different types of boiling heat transfer is important in the design and optimization of various industrial and technological systems.

Describe Boiling Regimes

Boiling regimes refer to the different stages or modes of boiling that occur during heat transfer from a solid surface to a liquid. There are several boiling regimes, each with its own characteristic features and heat transfer mechanisms.

1. Nucleate Boiling: Nucleate boiling is the initial stage of boiling, in which small vapour bubbles are formed at discrete points on the solid surface. In this stage, heat transfer is primarily driven by the formation and growth of these vapour bubbles, which carry heat away from the solid surface and into the liquid. The heat transfer rate in nucleate boiling is high, and it is dominated by the heat transfer coefficient.
2. Transition Boiling: Transition boiling is a transition stage between nucleate boiling and film boiling, in which the heat transfer rate decreases and the size of the vapour bubbles increases. In this stage, the vapour bubbles become more stable and start to form a continuous vapour layer, which begins to act as a barrier to heat transfer.
3. Film Boiling: Film boiling is the final stage of boiling, in which a continuous vapour layer separates the solid surface from the liquid. In this stage, the heat transfer rate is much lower than in the previous stages, and it is dominated by the thermal resistance of the vapour layer. Film boiling is typically seen in high-heat-flux applications, such as the cooling of high-power electronics.
4. Concurrent Boiling: Concurrent boiling refers to the simultaneous occurrence of nucleate and film boiling on a solid surface. In this regime, both nucleate and film boiling contribute to the overall heat transfer, and the relative importance of each depends on the heat transfer conditions.
5. Pool Boiling: Pool boiling refers to the boiling of a liquid in a static pool, such as a pot of boiling water. In this regime, heat transfer occurs from the solid surface to the liquid, resulting in the formation of vapour bubbles. The heat transfer rate in pool boiling is dependent on the temperature difference between the solid surface and the liquid, the surface area, and the heat transfer coefficient.

In conclusion, boiling regimes are the different stages or modes of boiling that occur during heat transfer from a solid surface to a liquid. Understanding the different boiling regimes is important in the design and optimization of various industrial and technological systems, such as power generation, refrigeration, and chemical processing.

Recall Nucleate Boiling

Nucleate boiling is a heat transfer process that occurs when heat is transferred from a hot surface to a liquid through the formation of vapour bubbles on the surface of the liquid. This process is important in many industrial and technological applications, including power generation, refrigeration, and heat exchangers.

Nucleate boiling occurs when the temperature of the liquid reaches its boiling point and the vapour pressure of the liquid exceeds the atmospheric pressure. At this point, small vapour bubbles form at nucleation sites on the surface of the liquid, which can be found on any small imperfections or rough spots on the surface.

The heat transfer in nucleate boiling is enhanced by the presence of these vapour bubbles. As the vapour bubbles form and grow, they carry heat away from the surface of the liquid, reducing the temperature of the liquid and allowing more heat to be transferred.

In order to understand the process of nucleate boiling, it is important to understand the different types of boiling. There are two main types of boiling: natural boiling and forced boiling. In natural boiling, the bubbles form spontaneously due to the difference in pressure between the liquid and the vapour. In forced boiling, the bubbles are formed by external means, such as by the use of a stirrer or pump.

The efficiency of nucleate boiling depends on a number of factors, including the temperature and pressure of the liquid, the surface roughness of the liquid-vapour interface, and the presence of any impurities or contaminants in the liquid. To optimize the heat transfer in nucleate boiling, engineers may use techniques such as controlling the surface roughness of the liquid-vapour interface, using high-heat-transfer fluids, and controlling the flow rate of the liquid.

In conclusion, nucleate boiling is an important heat transfer process that occurs when heat is transferred from a hot surface to a liquid through the formation of vapour bubbles on the surface of the liquid. The efficiency of nucleate boiling depends on a number of factors, and engineers may use techniques to optimize the heat transfer in this process.

List the factors affecting Nucleate Boiling

Nucleate boiling is a heat transfer process that is influenced by several factors, including:

1. Liquid temperature and pressure: The temperature and pressure of the liquid play a major role in determining the boiling point and the vapour pressure of the liquid. If the temperature and pressure of the liquid are increased, the boiling point and the vapour pressure will also increase, resulting in an increase in the heat transfer rate.
2. Surface roughness: The roughness of the surface in contact with the liquid can affect the rate of nucleate boiling by providing nucleation sites for the formation of vapour bubbles. A rough surface can increase the number of nucleation sites, resulting in a higher heat transfer rate.
3. Flow rate: The flow rate of the liquid can affect the rate of nucleate boiling by affecting the convective heat transfer between the liquid and the hot surface. If the flow rate is increased, the heat transfer rate will also increase.
4. Impurities: Impurities in the liquid can affect the rate of nucleate boiling by altering the boiling point and the vapour pressure of the liquid. Impurities can also reduce the number of nucleation sites, leading to a lower heat transfer rate.
5. Liquid-vapour interface temperature: The temperature difference between the liquid-vapour interface and the hot surface can affect the rate of nucleate boiling. A larger temperature difference will result in a higher heat transfer rate.
6. Heat transfer coefficient: The heat transfer coefficient, which is a measure of the effectiveness of the heat transfer, can affect the rate of nucleate boiling. A higher heat transfer coefficient will result in a higher heat transfer rate.
7. Liquid properties: The properties of the liquid, such as the thermal conductivity, viscosity, and surface tension, can also affect the rate of nucleate boiling. For example, a liquid with a high thermal conductivity will transfer heat more efficiently than a liquid with a low thermal conductivity.

In conclusion, the rate of nucleate boiling is influenced by several factors, including the liquid temperature and pressure, surface roughness, flow rate, impurities, liquid-vapour interface temperature, heat transfer coefficient, and liquid properties. Understanding these factors is important for optimising the heat transfer in nucleate boiling applications.

Recall Condensation heat transfer

Condensation heat transfer refers to the transfer of heat from a vapour or a gas to a liquid or a solid during the phase change from vapour to liquid. The process occurs when the temperature of the vapour is lower than the saturation temperature of the liquid, causing the vapour to condense into its liquid form. This process releases a significant amount of heat, which is known as the latent heat of evaporation.

Condensation heat transfer is a crucial aspect of many industrial and technological processes, including power generation, refrigeration, air conditioning, and many others. In power plants, for example, condensation heat transfer is used to cool steam after it has been used to power a turbine. The cooled steam condenses back into water, releasing its latent heat and providing a source of warmth that can be harnessed for other purposes.

The rate of condensation heat transfer depends on several factors, including the surface area of the condenser, the temperature difference between the vapour and the liquid, the flow rate of the vapour, and the physical properties of the vapour and the liquid. The most commonly used methods for enhancing condensation heat transfer include increasing the surface area of the condenser, increasing the turbulence of the flow, and using materials with high thermal conductivity.

In conclusion, condensation heat transfer plays a crucial role in many industrial and technological processes, providing a source of heat that can be harnessed for various purposes. Understanding the principles of condensation heat transfer is important for optimising the efficiency of these processes and reducing energy costs.

Describe Filmwise and Dropwise condensation

Filmwise condensation and dropwise condensation are two distinct modes of condensation heat transfer, which refer to the way in which a vapour condenses onto a surface.

Filmwise condensation occurs when a thin film of liquid forms on a surface, and the condensation takes place uniformly across the entire surface. The liquid film acts as an insulator, reducing the rate of heat transfer. Filmwise condensation is typically observed on smooth, flat surfaces, such as glass or metal, and is common in condensers in power plants and refrigeration systems.

Dropwise condensation, on the other hand, occurs when the condensate forms into discrete droplets, rather than a uniform film, and the droplets fall off the surface. Dropwise condensation is typically observed on rough or highly textured surfaces, such as fins or corrugated surfaces. The formation of droplets increases the surface area available for heat transfer, leading to a higher rate of condensation heat transfer compared to filmwise condensation. Dropwise condensation is less common than filmwise condensation but is more desirable in many applications due to its higher heat transfer rate.

In conclusion, filmwise and dropwise condensation are two distinct modes of condensation heat transfer, which differ in the way in which a vapour condenses onto a surface. Filmwise condensation is characterized by a uniform film of liquid and is typically observed on smooth surfaces, while dropwise condensation is characterized by discrete droplets and is typically observed on rough or textured surfaces. Understanding the differences between these two modes of condensation heat transfer is important for optimising the efficiency of condensation processes in various industrial and technological applications.

Recall laminar film condensation on vertical plate

Laminar film condensation on a vertical plate refers to the process of condensation heat transfer in which a vapour condenses onto a vertically oriented surface, forming a thin, laminar film of liquid. This mode of condensation heat transfer occurs under laminar flow conditions, where the fluid flow is smooth and uniform, with low turbulence.

In laminar film condensation on a vertical plate, the condensate forms a uniform film that covers the entire surface of the plate. The film acts as an insulator, reducing the rate of heat transfer between the vapour and the plate. The thickness of the film is determined by the rate of condensation, the surface temperature, and the physical properties of the vapour and the liquid.

The rate of heat transfer in laminar film condensation on a vertical plate is affected by several factors, including the temperature difference between the vapour and the surface, the surface area of the plate, and the properties of the vapour and the liquid. Increasing the temperature difference between the vapour and the surface or increasing the surface area of the plate will increase the rate of heat transfer.

In conclusion, laminar film condensation on a vertical plate is a mode of condensation heat transfer that occurs when a vapour condenses onto a vertically oriented surface, forming a thin, uniform film of liquid. The rate of heat transfer in this mode of condensation is affected by several factors, including the temperature difference between the vapour and the surface, the surface area of the plate, and the properties of the vapour and the liquid. Understanding the principles of laminar film condensation on a vertical plate is important for optimising the efficiency of condensation processes in various industrial and technological applications.

Describe the influence of non-condensable gases on condensation heat transfer

Describing the influence of non-condensable gases on condensation heat transfer, refers to the ability to understand how the presence of non-condensable gases in a condensation system can affect the heat transfer process.

Condensation heat transfer is the transfer of heat from a hot, vapour-filled medium to a cooler surface, resulting in the condensation of the vapour into a liquid. This heat transfer process is widely used in many industrial applications, such as refrigeration, power generation, and chemical processing.

However, in some cases, non-condensable gases, such as air or nitrogen, may be present in the condensation system. These gases can have a significant impact on the condensation heat transfer process. Some of the key ways in which non-condensable gases influence condensation heat transfer are as follows:

1. Reduced heat transfer coefficient: The presence of non-condensable gases in a condensation system can reduce the heat transfer coefficient, which is a measure of the heat transfer rate between the vapour and the cooler surface. This reduction in the heat transfer coefficient can lead to a decrease in the overall heat transfer rate, making it more difficult to condense the vapour.
2. Interference with flow: Non-condensable gases can interfere with the flow of the condensing vapour, reducing the turbulence and preventing the efficient transfer of heat. This can also lead to a decrease in the heat transfer coefficient.
3. Increased pressure drop: The presence of non-condensable gases can increase the pressure drop in the condensation system, making it more difficult to condense the vapour and increasing the required pumping power.
4. Formation of voids: Non-condensable gases can form pockets or voids in the condensation system, reducing the overall heat transfer area and limiting the amount of heat that can be transferred.

In conclusion, the presence of non-condensable gases in a condensation system can have a significant impact on the heat transfer process, reducing the heat transfer coefficient, interfering with flow, increasing the pressure drop, and forming voids that limit heat transfer. Understanding the influence of non-condensable gases on condensation heat transfer is essential for optimizing the performance of condensation systems and improving the efficiency of the heat transfer process.