ANSWERSPOOL QUESTION ANSWERS BOILING CLASS NOTES
POOL BOILING QUESTION
ANSWERS CLASS NOTES
Pool boiling phase conversion while
fluid is at rest. It is common in day
today life. There are empirical correlations
to find the heat transfer during pool
boiling. Boiling a heat transfer process in
which liquid changes to vapor at the
saturation temperature. In this, there is
latent heat transfer . The latent heat is very
high. Thus, rate of heat transfer is quiet
high. This heat transfer is 10 to 20 times
than the convective heat transfer without
phase change. When there is no phase
change, heat transfer is sensible heat
transfer.

List the various types of boiling.
There are four types of boiling namely
(i) Pool boiling or Nucleate boiling
(ii) Forced convection boiling
(iii) Subcooled boiling
(iv) Saturated boiling

Discuss pool boiling or nucleate boiling?
The liquid is at rest and is in contact with the heated surface. Motion of the liquid is due to density difference. This is due to Buoyancy effect on heating. Bubble formation and bubble detachment causes fluid motion. Phase change takes place at saturation temperature. The hot surface temperature is higher than the saturation temperature.

Q. Explain forced convection boiling?
Pump causes motion of the fluid. Phase chase takes place at saturation temperature. In this, rate of heat transfer is still higher. It takes place in boiler tubes.

Discuss subcooled boiling?
The liquid, in contact with the hot surface, changes into vapor. The remaining liquid subcools. It is at a temperature lower than the saturation temperature. Therefore, the bubble collapses as it tends to go up. Vapors do not reach the free surface. It is case of subcooled boiling.

Explain saturated boiling?
When the bulk of the liquid at saturation temperature, the bubbles forms at the hot surface. These bubbles rise and reach up to the free surface of the liquid. It is a case of saturated boiling.

What is excess temperature?
The difference between temperature of the hot surface and the saturation temperature at the prevailing pressure.
ΔT_{excess}=ΔT_{e=}T_{Surface} –T_{Saturation}

What is a critical heat flux? Explain is its importance.
It is the maximum heat flux during nucleate or pool boiling. It is burn out point.
Importance: If the temperature of the surface not controlled as per critical flux, the film boiling will start. Heat flux to boiling will decrease. The surface temperature will go on increasing and ultimately the metal will melt.
3. What are the two separate processes of nucleate boiling?

Subcooled boiling
The vapor forms at the hot surface. It starts rising in the liquid. But it condenses before reaching the free surface of the liquid.

Saturated boiling
With the increase in excess temperature, the vapor forms at the hot surface. The vapor rises through the liquid due to buoyancy effect and leaves the free surface.
4. List the parameters involved in boiling and condensation.

Latent heat

Surface tension

Surface characteristics

Properties of the two phase systems.

Constant temperature during phase change

High heat transfer coefficients due to latent heat

Small temperature differences as excess temperature.
Q. Explain the mechanism of bubble formation and collapse in pool boiling.
When excess temperature (Tsur –T_{sat}) is > 5 ^{0}C and <10^{0}C, vapor formation starts from the heated surface due to absorbing of latent heat. But the liquid above the bubble is not so hot and hence bubble collapse as it tries to rise. It is in pool boiling since liquid is at rest.
Q. Write note on the critical diameter of a bubble.
Critical diameter of a bubble is the maximum diameter of a bubble. It is given by an equation given below:
d_{c} = Cβ( σ_{lv} / σ_{ls}) ( σ_{lv }/(g(ρ_{L} —ρ_{v}))^{0.5}
Where C = constant = 0.0148 for a water bubble
β=angle formed by a bubble with the horizontal surface = 30^{0} = for a water bubble
σ_{lv} = surface tension between liquid and vapor N/m
σ_{ls } = Surface tension between liquid and solid surface N/m
d_{c} = maximum or critical diameter of a bubble
ρ = density, kg/m^{3}
Nucleate boiling correlations
There are five Pool boiling correlations.
(i) General correlation for no phase change q=hA ΔT_{e}
Where h is heat transfer coefficient for nucleate boiling
(ii) ROSENHOW correlation for Nucleate boiling
Q= µ_{l}h_{fg }(g(ρ_{l}–ρ_{v})/σ)^{0.5} (C_{pl}ΔT_{e}/(C_{sl}h_{fg}Pr_{l}^{n}))^{3}
Where n=1 for water
n=1.7 for other fluids
C_{sl} = 0.013 for water copper
C_{sl} = 0.060 for water brass
(iii) Jacob relation for nucleate boiling over a horizontal plate
Nu = 0.16 (Gr. Pr) ^{0.33}
(iv) Jacob relation for nucleate boiling over a vertical plate
Nu= 0.61 (Gr. Pr) ^{0.25}
(v) ZUBER EQUATION TO FIND CRITICAL HEAT FLUX IN NUCLEATE BOILING
q^{.”}_{chf} = 0.18 h_{fg} ρ_{V}^{0.5} [σg (ρ_{L}–ρ_{V})]^{0.25}
Q WHAT IS CONVECTIVE BOILING OF SATURATED LIQUID?
Ans. During boiling amount of vapor is increasing and it is a two phase phenomenon. Relations are available for single phase flow heat transfer.
Chen correlation gives the combination of single phase liquid convection and nucleate boiling heat flux
Therefore q’’ = h_{c}(T_{sur}—T_{l})+ h_{nb}(T_{sur}—T_{sat})=h’(T_{sur}—T_{sat}) =h_{eq} (T_{sur}—T_{sat})
Nucleate boiling heat flux
q= h_{nb}(T_{sur}—T_{sat}) )
Single phase liquid convection
q= h(T_{sur}T_{l}).
where h’ is single heat transfer coefficient for forced convection
h_{eq } can be directly calculated by modified Dittus Boelter equation
i.e. h’ =h_{eq}=0.023 (k_{s}/D)F Rel^{0.8} Pr_{l}^{0.4}
where F is a multiplier or correction factor for two phase flow
Remember DittusBoelter equation is for liquid flow only
F=2.35(1/x+ 0.213)^{0.736} if 1/x >0.1
F=1 if 1/x ≤ 0.1
Q. What are film boiling correlations?
(v) Correlation for film boiling
q=h^{f}A ΔT_{e}
Where h^{f} is heat transfer coefficient for film boiling
h^{f} = h_{conv}^{4/3} +( h_{rad})^{1/3}
where h_{conv} from Nusselt Eq for free or forced convection
h_{rad} = 5.67 x 10^{8} Є(T^{4}_{super} — T^{4}_{sat})/(T_{super} –T_{sat})
Nu= h_{av}D/k_{v }
= c((g(ρ_{l}–ρ_{v})h’_{fg} D^{3})/(ν_{v}k_{v}(T_{sur}—T_{sat}))^{1/4}
Where c=0.62 for cylinders
c=0.67 for spheres
Q. Factors does the shape and size of bubble depend on the following three factors:

Nature of the heating surface: thermal conductivity of the surface

Condition of the heating surface: rough or smooth

Surface tension of the liquid at the solid liquid interface. Surface tension is the wetting capability.
(a) When surface tension is less or low wettability. Small but more number of bubbles are readily form and rise up.
(b) If the surface tension is medium type, less wettability. Large size and less number of bubbles will form. These require more buoyant force for detachment.
( c) If the surface tension is high, very very less wettability. Bubbles become still larger size. These are unable to detach from the surface.
Q. What are various parameters for bubbles formation?
Ans. Critical diameter of bubble = d^{c}
d_{c }= C β(σ_{lv}/σls)( σ_{lv}/(g(ρ_{l}ρ_{v})))^{0.5}
where σ_{lv }is surface tension between liquid and vapor
σ_{ls} is surface tension between liquid and solid surface
C is a constant and for water its value is 0.0148
β is the angle formed as show in figure.
d_{c} is the critical diameter of the bubble.
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