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From a practical engineering point of view, boiling can be categorized according to several criteria. Categorization by the flow regime: Pool BoilingFlow BoilingCategorization by the wall superheat temperature, ΔTsat: Natural Convection BoilingNucleate BoilingTransition BoilingFilm Boiling
Categorization by the flow regime: Pool Boiling. Perhaps the most common configuration, known as pool boiling, is when a pool of liquid is heated from below through a horizontal surface. In pool boiling, the liquid is quiescent, and its motion near the surface is primarily due to natural convection and mixing induced by bubble growth and detachment. The pioneering work on pool boiling was done in 1934 by S. Nukiyama, and he was the first to identify four well-known different regimes of pool boiling using his apparatus.Categorization by the wall superheat temperature, ΔTsat:
Four different boiling regimes of pool boiling (based on the excess temperature) are observed: Natural Convection Boiling ΔTsat < 5°CNucleate Boiling 5°C < ΔTsat < 30°CTransition Boiling 30°C < ΔTsat < 200°CFilm Boiling 200°C < ΔTsatDescription of Boiling Modes:
Boiling may also be classified according to whether it is subcooled or saturated: Subcooled boiling. I subcooled boiling, the temperature of most of the liquid is below the saturation temperature, and bubbles formed at the surface may condense in the liquid. T is condensation (collapsing) produces a sound of frequency ~ 100Hz – 1 KHz. This is why an electric kettle makes the most noise before the water becomes saturated boiling. T e term subcooling refers to a liquid existing at a temperature below its normal boiling point.Saturated Boiling. I saturated boiling (also known as bulk boiling), the temperature of the liquid slightly exceeds the saturation temperature. Bulk boiling may occur when system temperature increases or pressure drops to the boiling point. At this point, the bubbles entering the coolant channel will not collapse, and the bubbles will tend to join together and form bigger steam bubbles. Steam bubbles are then propelled through the liquid by buoyancy forces, eventually escaping from a free surface.The mass fraction of the vapor in a two-phase liquid-vapor region is called the vapor quality (or dryness fraction), x, and it is given by the following formula: The value of the quality ranges from zero to unity. A though defined as a ratio, the quality is frequently given as a percentage. F om this point of view, we distinguish between three basic types of steam. I must be added at x=0, and we are talking about the saturated liquid state (single-phase). Wet SteamDry SteamSuperheated SteamThis classification of steam has its limitation. Consider the system’s behavior, which is heated at a pressure that is higher than the critical pressure. In this case, there would be no change in phase from liquid to steam, and in all states, there would be only one phase. Vaporization and condensation can occur only when the pressure is less than the critical pressure. T e terms liquid and vapor tend to lose their significance. See also: Saturation See also: Throttling of Steam BoilingWe have discussed convective heat transfer in the preceding chapters with a very important assumption, and we have assumed a single-phase convective heat transfer without any phase change. This chapter focuses on convective heat transfer associated with the change in phase of a fluid. In particular, we consider processes that can occur at a solid-liquid or solid–vapor interface, namely, boiling (liquid-to-vapor phase change) and condensation (vapor-to-liquid phase change). Latent heat effects associated with the phase change are significant for these cases. Latent heat, also known as the enthalpy of vaporization, is the amount of heat added to or removed from a substance to produce a change in phase. This energy breaks down the intermolecular attractive forces and also must provide the energy necessary to expand the gas (the pΔV work). When latent heat is added, no temperature change occurs. The enthalpy of vaporization is a function of the pressure at which that transformation takes place. Latent heat of vaporization – water at 0.1 MPa (atmospheric pressure) hlg = 2257 kJ/kg Latent heat of vaporization – water at 3 MPa hlg = 1795 kJ/kg Latent heat of vaporization – water at 16 MPa (pressure inside a pressurizer) hlg = 931 kJ/kg The heat of vaporization diminishes with increasing pressure while the boiling point increases, and it vanishes completely at a certain point called the critical point. Above the critical point, the liquid and vapor phases are indistinguishable, and the substance is called a supercritical fluid.
This is because even in turbulent flow, there is a stagnant fluid film layer (laminar sublayer) that isolates the surface of the heat exchanger. This stagnant fluid film layer plays a crucial role in the convective heat transfer coefficient. It is observed that the fluid comes to a complete stop at the surface and assumes a zero velocity relative to the surface. This phenomenon is known as the no-slip condition, and therefore, at the surface, energy flow occurs purely by conduction. But in the next layers, both conduction and diffusion-mass movement occur at the molecular or macroscopic levels. Due to the mass movement, the rate of energy transfer is higher. As was written, nucleate boiling at the surface effectively disrupts this stagnant layer. Therefore, nucleate boiling significantly increases the ability of a surface to transfer thermal energy to the bulk fluid. Two-phase Fluid FlowA multiphase flow can be a simultaneous flow of: Materials with different states or phases (e.g., water-steam mixture).Materials with different chemical properties but in the same state or phase (e.g., oil droplets in water).There are many combinations in industrial processes, but the most common being the simultaneous flow of steam and liquid water (as encountered in steam generators and condensers). In reactor engineering, a great deal of study has been performed on the nature of two-phase flow in case of a loss-of-coolant accident (LOCA), an accident of importance in reactor safety, and all thermal-hydraulic analyses (DNBR analyses). Characteristics of Two-phase Fluid FlowAll two-phase flow problems have features that are characteristically different from those found in single-phase problems. In the case of steam and liquid water, the density of the two phases differs by a factor of about 1000. Therefore the influence of gravitational body force on multiphase flows is of much greater importance than in the case of single-phase flows.The sound speed changes dramatically for materials undergoing a phase change and can be orders of magnitude different. This significantly influences a flow through an orifice.The relative concentration of different phases is usually a dependent parameter of great importance in multiphase flows, while it is a parameter of no consequence in single-phase flows.The phase change means flow-induced pressure drops can cause further phase change (e.g., water can evaporate through an orifice), increasing the relative volume of the gaseous, compressible medium and increasing efflux velocities, unlike single-phase incompressible flow where decreasing of an orifice would decrease efflux velocities.The spatial distribution of the various phases in the flow channel strongly affects the flow behavior.There are many types of instabilities in multiphase flow.Boiling Point - SaturationIn thermodynamics, the term saturation defines a condition in which a mixture of vapor and liquid can exist together at a given temperature and pressure. T e temperature at which vaporization (boiling) starts to occur for a given pressure is called the saturation temperature or boiling point. T e pressure at which vaporization (boiling) starts to occur for a given temperature is called the saturation pressure.When the vapor quality is equal to 0, it is referred to as the saturated liquid state (single-phase). On the other hand, when the vapor quality is equal to 1, it is referred to as the saturated vapor state or dry steam (single-phase). B tween these two states, we talk about vapor-liquid mixture or wet steam (two-phase mixture). A constant pressure addition of energy does not change the mixture’s temperature, but the vapor quality and specific volume change. References:Heat Transfer:Fundamentals of Heat and Mass Transfer, 7th Edition. Theodore L. Bergman, Adrienne S. Lavine, Frank P. Incropera. John Wiley & Sons, Incorporated, 2011. I BN: 9781118137253.Heat and Mass Transfer. Yunus A. Cengel. McGraw-Hill Education, 2011. I BN: 9780071077866.U.S. Department of Energy, Thermodynamics, Heat Transfer and Fluid Flow. D E Fundamentals Handbook, Volume 2 of 3. M y 2016.Nuclear and Reactor Physics: J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317W.S.C. Williams. N clear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.U.S. Department of Energy, Nuclear Physics and Reactor Theory. D E Fundamentals Handbook, Volume 1 and 2. January 1993.Paul Reuss, Neutron Physics. E P Sciences, 2008. I BN: 978-2759800414.Advanced Reactor Physics: K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2.E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.See above:Boiling and Condensation |
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