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BOILING AND CONDENSATION CHAPTER 5 - Coggle Diagram
BOILING AND CONDENSATION
CHAPTER 5
Boiling Heat Transfer
Evaporation occurs at the liquid–vapor interface
Vapor pressure is less than the saturation pressure of the liquid at a given temperature
Boiling occurs at the solid–liquid interface
Liquid is brought into contact with a surface maintained at a temperature sufficiently above the saturation temperature of the liquid
Classification of boiling
Flow Boiling
Presence of bulk fluid flow
Fluid is forced to move in a heated pipe or over a surface by external means such as a pump
Saturated Boiling
When the temperature of the liquid is equal to the saturation temperature
Pool Boiling
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 buoyancy
Subcooled Boiling
When the temperature of the main body of the liquid is below the saturation temperature
Pool Boiling
Natural Convection (to Point A on the Boiling Curve)
Heat transfer from the heating surface to the fluid is by natural convection
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
Nucleate Boiling
Region A–B ─isolated bubbles
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
Region B–C ─ numerous continuous columns of vapor in the liquid
The bubbles form at an increasing rate at an increasing number of nucleation sites as we move along the boiling curve toward point C
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 ΔTexcess, 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
Transition Boiling
When ΔTexcess is increased past point C, the heat flux decreases
A large fraction of the heater surface is covered by a vapor film, which acts as an insulation
Both nucleate and film boiling partially occur.
Film Boiling
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
Beyond Point D the heater surface is completely covered by a continuous stable vapor film
The heat transfer rate increases with increasing excess temperature due to radiation to the liquid
Point D, where the heat flux reaches a minimum is called the Leidenfrost point
Burnout Phenomenon
When the power applied to the heated surface exceeded the value at point C even slightly, the surface temperature increased suddenly to point E
A typical boiling process does not follow the boiling curve beyond point C
When the power is reduced gradually starting from point E the cooling curve with a sudden drop in excess temperature when point D is reached
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
Heat Transfer Correlations in Pool Boiling ─ Nucleate Boiling
The rate of heat transfer strongly depends on the nature of nucleation and the type and the condition of the heated surface
No general theoretical relations for heat transfer in the nucleate boiling regime is available
A widely used correlation proposed in 1952 by Rohsenow
Experimental based correlations are used
The values in Rohsenow equation can be used for any geometry since it is found that the rate of heat transfer during nucleate boiling is essentially independent of the geometry and orientation of the heated surface
The correlation is applicable to clean and relatively smooth surfaces
Error for the heat transfer rate for a given excess temperature: 100%
Error for the excess temperature for a given heat transfer rate: 30%
Critical Heat Flux (CHF)
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:
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 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
Ccr is a constant whose value depends on the heater geometry, but generally is about 0.15
Minimum Heat Flux
Zuber derived the following expression for the minimum heat flux for a large horizontal plate
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
The relation above can be in error by
50% or more
Film Boiling
Experimental studies confirm that the critical heat flux and heat flux in film boiling are proportional to g1/4
The heat flux for film boiling on a horizontal
cylinder or sphere of diameter D is 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
Two mechanisms of heat transfer (radiation and convection) adversely affect each other, causing the total heat transfer to be less than their sum
Enhancement of Heat Transfer in Pool Boiling
Surfaces that provide enhanced heat transfer in nucleate boiling permanently are being manufactured and are available in the market
The effect of surface roughness is
observed to decay with time
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
Irregularities on the heating surface, including roughness and dirt, serve as additional nucleation
sites during boiling
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
Modification that enhances nucleation on the heating surface will also enhance heat transfer in nucleate boiling
Condensation
Occurs when the temperature of a vapor is reduced below its saturation temperature
Only condensation on solid surfaces is considered
Dropwise condensation
The droplets slide down when they reach a certain size
As a result, heat transfer rates that are more than 10 times larger than with film condensation can be achieved
The condensed vapor forms droplets on the surface
No liquid film to resist heat transfer
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
Film Condensation on a Vertical Plate
Heat in the amount hfg is released during condensation and is transferred through the film to the plate surface
Liquid film starts forming at the top of the plate and flows downward under the influence of gravity
Ts must be below the saturation temperature for condensation
δ increases in the flow direction x
The temperature of the condensate is Tsat at the interface and decreases gradually to Ts at the wall
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.
The Reynolds number increases in the flow direction
Heat Transfer Correlations for Film Condensation ─ Vertical wall
Assumptions:
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.
Heat transfer across the liquid film is by pure conduction.
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).
The flow of the condensate is laminar (Re<30) and the properties of the liquid are constant.
The acceleration of the condensate layer is negligible.
It is observed to underpredict heat transfer because it does not take into account the effects of the nonlinear temperature profile in the liquid film and the cooling of the liquid below the saturation temperature
When ρv«ρl (and thus ρl-ρv≈ρl). Using this approximation and substituting equation at x =L into the Reynolds number definition by noting that δx=L=kl/hx=L and havg=4/3hx=L give
Then the average heat transfer coefficient in terms of Re becomes
The results obtained from the theoretical relations above are in excellent agreement with the experimental results
Wavy Laminar Flow on Vertical Plates
The increase in heat transfer due to the wave effect is, on average, about 20 percent, but it can exceed 50 percent
Kutateladze (1963) recommended the following relation
The waves at the liquid–vapor interface tend to increase heat transfer
Turbulent Flow on Vertical Plates
Labuntsov (1957) proposed the following relation for the turbulent flow of condensate on vertical plates:
The physical properties of the condensate are to be evaluated at the film temperature
Nondimensionalized Heat Transfer Coefficients
