boiling and condensation lecture presentation
- 2. Boiling Heat Transfer • Evaporation occurs at the liquid–vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a given temperat ure. • Boiling occurs at the solid–liquid interface when a liquid is brought into contact with a surface maintained at a temperature sufficiently above the saturation temperature of the liquid
- 3. Boiling • The boiling process is characterized by the rapid formation of vapor bubbles at the solid–liquid interface that detach from the surface when they reach a certain size and attempt to rise to the free surface of the liquid. • Bubbles owe their existence to the surface-tension at the liquid–vapor interface due to the attraction force on molecules at the interface toward the liquid phase. • The boiling processes in practice do not occur under equilibrium conditions, and normally the bubbles are not in thermodynamic equilibrium with the surrounding liquid.
- 4. Classification of boiling Pool Boiling • Boiling is called pool boiling in the absence of bulk fluid flow. • Any motion of the fluid is due to natural convection currents and the motion of the bubbles under the influence of buoyanc y. Flow Boiling • Boiling is called flow boiling in the presence of bulk fluid flow. • In flow boiling, the fluid is forced to move in a heated pipe or over a surface by external means such as a pump.
- 5. Subcooled Boiling • When the temperature of the main body of the liquid is below the saturation temperature. Saturated Boiling • When the temperature of the liquid is equal to the saturation temperature. Classification of boiling
- 6. Pool Boiling S. Nukiyama used electrically heated nichrome and platinum wires immersed in liquids in his experiments. Boiling takes different forms, depending on the DTexcess=Ts-Tsat
- 7. Natural Convection (to Point A on the Boiling Curve) • Bubbles do not form on the heating surface until the liquid is heated a few degrees above the saturation temperature (about 2 to 6°C for water) the liquid is slightly superheated in this case (metastable state). • The fluid motion in this mode of boiling is governed by natural convection currents. • Heat transfer from the heating surface to the fluid is by natural convection.
- 8. Nucleate Boiling • The bubbles form at an increasing rate at an increasing number of nucleation sites as we move along the boiling curve toward point C. • Region A–B ─isolated bubbles. • Region B–C ─ numerous continuous columns of vapor in the liquid .
- 9. Nucleate Boiling • In region A–B the stirring and agitation caused by the entrainment of the liquid to the heater surface is primarily responsible for the increased heat transfer coefficient. • In region A–B the large heat fluxes obtainable in this region are caused by the combined effect of liquid entrainment and evaporation. • After point B the heat flux increases at a lower rate with increasing DTexcess, and reaches a maximum at point C. • The heat flux at this point is called the critical (or maximum) heat flux, and is of prime engineering importance.
- 10. Transition Boiling • When DTexcess is increased past point C, the heat flux decreases. • This is because a large fraction of the heater surface is covered by a vapor film, which acts as an insulation. • In the transition boiling regime, both nucleate and film boiling partially occur.
- 11. Film Boiling • Beyond Point D the heater surface is completely covered by a continuous stable vapor film. • Point D, where the heat flux reaches a minimum is called the Leidenfrost point. • The presence of a vapor film between the heater surface and the liquid is responsible for the low heat transfer rates in the film boiling region. • The heat transfer rate increases with increasing excess temperature due to radiation to the liquid.
- 12. Burnout Phenomenon • A typical boiling process does not follow the boiling curve beyond point C. • When the power applied to the heated surface exceeded the value at point C even slightly, the surface temperature increased suddenly to point E. • When the power is reduced gradually starting from point E the cooling curve follows Fig. 10–8 with a sudden drop in excess temperature when point D is reached. C E D
- 13. Heat Transfer Correlations in Pool Boiling • Boiling regimes differ considerably in their character different heat transfer relations need to be used for different boiling regimes. • In the natural convection boiling regime heat transfer rates can be accurately determined using natural convection relations.
- 14. Heat Transfer Correlations in Pool Boiling ─ Nucleate Boiling • No general theoretical relations for heat transfer in the nucleate boiling regime is available. • Experimental based correlations are used. • The rate of heat transfer strongly depends on the nature of nucleation and the type and the condition of the heated surface. • A widely used correlation proposed in 1952 by Rohsenow:
- 15. Critical Heat Flux (CHF) Ccr is a constant whose value depends on the heater geometry, but generally is about 0.15. • The CHF is independent of the fluid–heating surface combination, as well as the viscosity, thermal conductivity, and the specific heat of the liquid. • The CHF increases with pressure up to about one-third of the critical pressure, and then starts to decrease and becomes zero at the critical pressure. • The CHF is proportional to hfg, and large maximum heat fluxes can be obtained using fluids with a large enthalpy of vaporization, such as water. • The maximum (or critical) heat flux in nucleate pool boiling was determined theoretically by S. S. Kutateladze in Russia in 1948 and N. Zuber in the United States in 1958 to be:
- 16. Minimum Heat Flux • the relation above can be in error by 50% or more. • Minimum heat flux, which occurs at the Leidenfrost point, is of practical interest since it represents the lower limit for the heat flux in the film boiling regime. • Zuber derived the following expression for the minimum heat flux for a large horizontal plate
- 17. Film Boiling • The heat flux for film boiling on a horizontal cylinder or sphere of diameter D is given by given by • At high surface temperatures (typically above 300°C), heat transfer across the vapor film by radiation becomes significant and needs to be considered. • The two mechanisms of heat transfer (radiation and convection) adversely affect each other, causing the total heat transfer to be less than their sum.
- 18. Water is to be boiled at atmospheric pressure in a mechanically polished stainless steel pan placed on top of a heating unit, as shown in Figure. The inner surface of the bottom of the pan is maintained at 108°C. If the diameter of the bottom of the pan is 30 cm, determine (a) the rate of heat transfer to the water and (b) the rate of evaporation of water. The excess temperature in this case is T = Ts – Tsat=108 - 100 = 8°C
- 19. The surface area of the bottom of the pan is Then the rate of heat transfer during nucleate boiling becomes
- 20. Water in a tank is to be boiled at sea level by a 1- cm-diameter nickel plated steel heating element equipped with electrical resistance wires inside, as shown. Determine the maximum heat flux that can be attained in the nucleate boiling regime and the surface temperature of the heater surface in that case.
- 21. Enhancement of Heat Transfer in Pool Boiling • The rate of heat transfer in the nucleate boiling regime strongly depends on the number of active nucleation sites on the surface, and the rate of bubble formation at each site. • Therefore, modification that enhances nucleation on the heating surface will also enhance heat transfer in nucleate boiling. • Irregularities on the heating surface, including roughness and dirt, serve as additional nucleation sites during boiling. • Heat transfer can be enhanced by a factor of up to 10 during nucleate boiling, and the critical heat flux by a factor of 3.
- 22. FLOW BOILING Internal forced convection boiling External flow forced convection boiling
- 23. Condensation • Condensation occurs when the temperature of a vapor is reduced below its saturation temperature. • Only condensation on solid surfaces is considered in this chapter. • Two forms of condensation: – Film condensation, – Dropwise condensation.
- 24. Film condensation • The condensate wets the surface and forms a liquid film. • The surface is blanketed by a liquid film which serves as a resistance to heat transfer. Dropwise condensation • The condensed vapor forms droplets on the surface. • The droplets slide down when they reach a certain size. • No liquid film to resist heat transfer. • As a result, heat transfer rates that are more than 10 times larger than with film condensation can be achieved. Condensation: Physical Mechanisms
- 25. Film Condensation on a Vertical Plate • liquid film starts forming at the top of the plate and flows downward under the influence of gravity. • δ increases in the flow direction x • Heat in the amount hfg is released during condensation and is transferred through the film to the plate surface. • Ts must be below the saturation temperature for condensation. • The temperature of the condensate is Tsat at the interface and decreases gradually to Ts at the wall.
- 26. Vertical Plate ─Flow Regimes • The dimensionless parameter controlling the transition between regimes is the Reynolds number defined as: • Three prime flow regimes: – Re<30 ─ Laminar (wave-free), – 30<Re<1800 ─ Wavy-laminar, – Re>1800 ─ Turbulent.
- 28. Heat Transfer Correlations for Film Condensation ─ Vertical wall Assumptions: 1. Both the plate and the vapor are maintained at constant temperatures of Ts and Tsat, respectively, and the temperature across the liquid film varies linearly. 2. Heat transfer across the liquid film is by pure conduction. 3. The velocity of the vapor is low (or zero) so that it exerts no drag on the condensate (no viscous shear on the liquid–vapor interface). 4. The flow of the condensate is laminar (Re<30) and the properties of the liquid are constant. 5. The acceleration of the condensate layer is negligible. Height L and width b
- 29. Hydrodynamics
- 30. • Since the heat transfer across the liquid film is assumed to be by pure conduction, the heat transfer coefficient can be expressed through Newton’s law of cooling and Fourier law as • The local heat transfer coefficient is determined to be • The average heat transfer coefficient over the entire plate is
- 31. Where the Jacob number is defined as • Thus • Rohsenow recommended using the modified latent heat ' h* h 0.68c (T T ) h (1 0.68Ja) fg fg p,l sat s fg Ja cp (Ts Tsat ) hfg
- 32. Effect of Vapor Velocity • When the vapor velocity is high, the vapor will “pull” the liquid at the interface along since the vapor velocity at the interface must drop to the value of the liquid velocity. If the vapor flows downward (i.e., in the same direction as the liquid), this additional force will increase the average velocity of the liquid and thus decrease the film thickness. This, in turn, will decrease the thermal resistance of the liquid film and thus increase heat transfer. • Upward vapor flow has the opposite effects: the vapor exerts a force on the liquid in the opposite direction to flow, thickens the liquid film, and thus decreases heat transfer.
- 33. The Presence of Noncondensable Gases in Condensers • Most condensers used in steam power plants operate at pressures well below the atmospheric pressure (usually under 0.1 atm) to maximize cycle thermal efficiency, and operation at such low pressures raises the possibility of air (a noncondensable gas) leaking into the condensers. • Experimental studies show even small amounts of a noncondensable gas in the vapor cause significant drops in heat transfer coefficient during condensation. • Therefore, it is common practice to periodically vent out the noncondensable gases that accumulate in the condensers to ensure proper operation.