Boiling and condensation are crucial heat transfer processes in many engineering applications. These phenomena involve phase changes between liquid and vapor states, dramatically affecting heat transfer rates and system efficiency.
Understanding boiling regimes and condensation types is essential for designing effective heat exchange systems. From 's high heat transfer rates to 's superior performance, these concepts are key to optimizing thermal management in various industries.
Pool Boiling Regimes
Mechanisms and Characteristics
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Pool boiling occurs when a heated surface is submerged in a large volume of stagnant liquid, and the liquid near the surface undergoes to vapor
The boiling process can be divided into different regimes based on the excess temperature (ΔT), which is the difference between the surface temperature and the of the liquid
The boiling regimes, in order of increasing ΔT, are: natural convection, nucleate boiling, transition boiling, and
Regimes and Their Descriptions
In the natural convection regime, heat transfer occurs primarily through convection currents in the liquid, without significant bubble formation
Example: Heating in a pot before bubbles start to form
Nucleate boiling is characterized by the formation and growth of vapor bubbles at nucleation sites on the heated surface, leading to high heat transfer rates
Example: Water boiling in a pot with bubbles forming at the bottom and rising to the surface
Transition boiling occurs when the vapor bubbles begin to coalesce and form larger vapor patches, resulting in a decrease in heat transfer coefficient
Example: Large vapor patches forming on a heated surface, causing the liquid to intermittently contact the surface
In the film boiling regime, a stable vapor film covers the heated surface, acting as an insulating layer and reducing the heat transfer rate
Example: Leidenfrost effect, where a droplet of liquid levitates on a cushion of its own vapor when placed on a sufficiently hot surface
Critical Heat Flux in Boiling
Factors Influencing Critical Heat Flux
The (CHF) is the maximum heat flux that can be achieved during nucleate boiling before the transition to film boiling occurs
Factors that influence the critical heat flux include:
Surface roughness: Rougher surfaces provide more nucleation sites, increasing the CHF
Surface wettability: Highly wettable surfaces promote liquid contact and delay the onset of film boiling, increasing the CHF
Liquid properties: Liquids with higher of vaporization, higher thermal conductivity, and lower tend to have higher CHF values
System pressure: Increasing the system pressure generally increases the CHF
Estimating Critical Heat Flux
Correlations, such as the Zuber correlation, can be used to estimate the critical heat flux based on fluid properties and system parameters
where C is a constant, ρv and ρl are vapor and liquid densities, hfg is the latent heat of vaporization, σ is the surface tension, and g is the gravitational acceleration
Example: Estimating the CHF for water at atmospheric pressure using the Zuber correlation with C=0.131, ρl=958kg/m3, ρv=0.6kg/m3, hfg=2257kJ/kg, and σ=0.0589N/m
Film vs Dropwise Condensation
Mechanisms and Characteristics
Condensation occurs when a vapor comes into contact with a surface at a temperature below the saturation temperature of the vapor
is characterized by the formation of a continuous liquid film on the cooling surface, with the condensate flowing down the surface due to gravity
Example: Steam condensing on a cold vertical plate, forming a continuous liquid film
Dropwise condensation occurs when the condensate forms discrete droplets on the cooling surface, which grow, coalesce, and are eventually shed from the surface
Example: Water vapor condensing on a non-wettable surface, such as a treated metal or a hydrophobic coating
Heat Transfer Coefficients
The heat transfer coefficient for film condensation can be estimated using the Nusselt analysis, which accounts for the thermal resistance of the liquid film
where ρl and ρv are liquid and vapor densities, g is the gravitational acceleration, hfg is the latent heat of vaporization, kl is the liquid thermal conductivity, μl is the liquid dynamic viscosity, Tsat and Ts are the saturation and surface temperatures, and L is the characteristic length
Dropwise condensation typically results in higher heat transfer coefficients compared to film condensation due to the reduced thermal resistance and the continuous exposure of the surface to the vapor
Promoting dropwise condensation requires the use of non-wettable surfaces or the application of surface treatments to prevent the formation of a continuous liquid film
Boiling and Condensation Applications
Problem-Solving Approach
Boiling and condensation heat transfer find applications in various fields, such as power generation, refrigeration, and process industries
In boiling heat transfer problems, the objective is often to determine the heat transfer rate, surface temperature, or critical heat flux for a given set of conditions
In condensation heat transfer problems, the goal may be to calculate the heat transfer rate, surface temperature, or condensation rate for a given set of conditions
When solving problems, it is essential to:
Identify the boiling or condensation regime
Select the appropriate correlations or models
Account for the relevant fluid properties and system parameters
Calculating Heat Transfer Rates
The heat transfer rate in boiling can be calculated using Newton's law of cooling:
q=h×A×ΔT
where h is the boiling heat transfer coefficient, A is the surface area, and ΔT is the excess temperature
The heat transfer rate in condensation can be determined using the appropriate heat transfer coefficient correlation:
Nusselt analysis for film condensation
Empirical correlations for dropwise condensation
Example: Calculating the heat transfer rate for film condensation of steam on a vertical tube with a surface temperature of 80°C, given the saturation temperature of 100°C, tube length of 1 m, and tube diameter of 0.02 m
Key Terms to Review (17)
Clausius-Clapeyron Equation: The Clausius-Clapeyron equation is a fundamental relationship in thermodynamics that describes the phase change between two phases of a substance, particularly between liquid and vapor. This equation connects the change in vapor pressure of a substance to its temperature and the enthalpy of vaporization, providing insights into boiling and condensation processes. It highlights how changes in temperature can influence the equilibrium between phases, which is crucial for understanding mass transfer during phase changes.
Convective Heat Transfer Coefficient: The convective heat transfer coefficient is a measure of the heat transfer between a solid surface and a fluid flowing over it. This coefficient depends on the nature of the flow, the properties of the fluid, and the characteristics of the surface, making it crucial for understanding how heat is transferred in various situations involving convection.
Critical Heat Flux: Critical heat flux (CHF) refers to the maximum heat flux that a surface can handle before a transition from nucleate boiling to film boiling occurs, leading to a significant drop in heat transfer efficiency. This phenomenon is crucial in understanding the limits of heat removal in systems such as nuclear reactors and heat exchangers, where maintaining efficient heat transfer is vital for safety and performance.
Distillation Columns: Distillation columns are vertical vessels used in the process of separating components in a mixture based on differences in their boiling points. They play a crucial role in the distillation process, which involves both boiling and condensation, allowing for the efficient separation of liquids into their individual components through repeated vaporization and condensation cycles.
Dropwise Condensation: Dropwise condensation is a process where vapor condenses into small droplets on a surface, allowing for more efficient heat transfer compared to filmwise condensation. In this mode, the droplets form, grow, and then detach from the surface, which significantly reduces the thermal resistance to heat transfer. This process is particularly relevant in enhancing the performance of heat exchangers and improving energy efficiency in various applications.
Film Boiling: Film boiling occurs when a liquid comes in contact with a surface that is significantly hotter than its boiling point, resulting in the formation of a vapor film that insulates the liquid from the hot surface. This phenomenon typically happens at high temperatures and low pressures, leading to a distinct mode of heat transfer that can influence cooling processes and equipment efficiency. The presence of the vapor film can inhibit effective heat transfer, making it a critical aspect in thermal management systems.
Film Coefficient: The film coefficient is a measure of the heat transfer rate between a solid surface and a fluid flowing over it, indicating how effectively heat is transferred through the layer of fluid adjacent to the surface. It reflects the convective heat transfer characteristics of the fluid and is influenced by factors such as fluid velocity, viscosity, and temperature. The film coefficient plays a crucial role in determining the overall heat transfer in processes involving both forced convection and phase changes, like boiling and condensation.
Film Condensation: Film condensation is the process where vapor condenses into a liquid film on a surface, typically occurring on cooler surfaces where heat is removed from the vapor. This phenomenon is crucial in heat transfer applications, as it influences the efficiency of heat exchangers and condensers. The formation of a liquid film can enhance heat transfer due to the large surface area available for thermal exchange and plays a significant role in various industrial processes, such as refrigeration and power generation.
Heat Exchangers: Heat exchangers are devices designed to efficiently transfer heat from one medium to another, often between liquids or gases, without mixing them. They play a crucial role in various applications, such as in heating, cooling, and energy recovery systems, facilitating the transfer of thermal energy through conduction and convection.
Latent Heat: Latent heat is the amount of heat absorbed or released by a substance during a phase change without a change in temperature. This concept is critical in processes like boiling, condensation, drying, and biological systems, where energy transfer occurs while materials transition between solid, liquid, and gas states. Understanding latent heat helps explain how substances interact with heat during these transformations and is essential for efficient thermal management.
Nucleate Boiling: Nucleate boiling is a phase change process where vapor bubbles form on a heated surface and then grow as heat is transferred from the surface to the liquid. This process occurs when the temperature of the surface exceeds the saturation temperature of the liquid, leading to the formation of discrete vapor bubbles that detach and rise through the liquid. Nucleate boiling is significant because it represents an efficient mode of heat transfer and is commonly observed in many industrial applications, including cooling systems and heat exchangers.
Nusselt Number: The Nusselt number is a dimensionless quantity used in heat transfer that represents the ratio of convective to conductive heat transfer across a boundary. It helps to characterize the efficiency of convective heat transfer in fluid flows, making it essential for understanding processes involving both heat and mass transfer.
Phase Change: Phase change refers to the transition of a substance from one state of matter to another, such as from solid to liquid, liquid to gas, or vice versa. This process involves the absorption or release of heat, which is crucial in various applications like heating, cooling, and energy transfer. Understanding phase change is vital in contexts where temperature and pressure influence a material's state, impacting efficiency and performance in thermal systems.
Refrigerants: Refrigerants are substances used in refrigeration and air conditioning systems to absorb and transfer heat from one area to another, typically transitioning between liquid and gas phases. Their ability to change states is crucial during boiling and condensation processes, enabling these systems to maintain desired temperature levels efficiently. Understanding the properties of refrigerants helps in optimizing heat transfer and ensuring effective cooling solutions.
Saturation Temperature: Saturation temperature is the temperature at which a liquid and its vapor are in equilibrium at a given pressure. This means that at saturation temperature, any heat added to the liquid will cause it to start boiling and change into vapor, while any heat removed from the vapor will cause it to condense back into liquid. Understanding saturation temperature is crucial for analyzing boiling and condensation processes, as it helps determine the conditions under which phase changes occur.
Surface Tension: Surface tension is the cohesive force at the surface of a liquid that causes it to behave like a stretched elastic membrane. This phenomenon occurs due to the imbalance of molecular forces at the liquid's surface, where molecules are attracted more strongly to each other than to the air above. In the context of boiling and condensation, surface tension plays a significant role in the formation of bubbles during boiling and affects how droplets form and behave during condensation.
Water: Water is a colorless, odorless, and tasteless liquid that is essential for life and has unique properties, particularly in the context of phase changes such as boiling and condensation. Its ability to exist in three states—liquid, solid, and gas—makes it a crucial medium for heat transfer, influencing various thermal systems and processes, including heat pipes and thermosyphons.