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QUESTION: 1

When evaporation takes place at the liquid-vapor interface, the heat transfer is solely due to free convection and the film coefficient follows the relation

Solution:

The functions f _{1 }and f_{ 2 }depend upon the geometry of the heating surface.

QUESTION: 2

Fritz criterion is given by

Solution:

Fritz formulated the following formula for water boiling at atmospheric pressure in free convection in a vertical tube headed from outside.

QUESTION: 3

A 0.10 cm diameter and 15 cm long wire has been laid horizontally and submerged in water at atmospheric pressure. The wire has a steady state voltage drop of 14.5 V and a current of 42.5 A. Determine the heat flux of the wire.

The following equation applies for water boiling on a horizontal submerged surface

H = 1.54 (Q/A) ^{0.75} = 5.58 (d t)^{ 3} W/m^{2 }K where Q/A is the heat flux rate in W/m^{2} and d t is the temperature difference between surface and saturation

Solution:

Q = E I = 616.25 W and A = 4.71 * 10 ^{-4} m^{2}.

QUESTION: 4

Consider the above problem, find the excess temperature of the wire

Solution:

1.54 (1.308 * 10 ^{6})^{ 2} = 5.58 (d t) ^{3}.

QUESTION: 5

Natural convection heat transfer coefficients over surface of a vertical pipe and a vertical flat plate for same height. What is/are the possible reasons for this?

(i) Same height

(ii) Both vertical

(iii) Same fluid

(iv) Same fluid flow pattern

Select the correct answer

Solution:

The fluids must be same so their flow pattern.

QUESTION: 6

The heat flux in nucleate boiling varies in accordance with

Solution:

Q/A = δ _{f }h _{f g} [(p _{f }– p _{g}) g/σ] ^{0.5 }[C _{f }d t/h _{f g }p C _{s f}] ^{3}.

QUESTION: 7

In nucleate pool boiling, the heat flux depends on

Solution:

The heat flux must depends on liquid properties material and condition of the surface.

QUESTION: 8

Identify the wrong statement with respect to boiling heat transfer?

Solution:

For boiling to occur, the heated surface must be exposed to a liquid and maintained at a temperature higher than the saturation temperature of the liquid.

QUESTION: 9

Estimate the peak heat flux for water boiling at normal atmospheric pressure. The relevant thermo-physical properties are

p _{f} (liquid) = 958.45 kg/m^{3}

p _{g} (vapor) = 0.61 kg/m^{3}

h _{f g }= 2.25 * 10 ^{6 }J/kg

σ = 0.0585 N/m

Solution:

(Q/A) = 0.18 p _{g }h _{f g }[σ (p _{f }– p _{g})/p _{g }^{2}] ^{0.25}.

QUESTION: 10

A 1.0 mm diameter and 300 mm long nickel wire is submerged horizontal in water at atmospheric pressure. At burnout, the wire has a current of 195 A. Calculate the voltage at burnout. The relevant thermos-physical properties are

p _{f }(fluid) = 959.52 kg/m^{3}

p _{g} (vapor) = 0.597 kg/m^{3}

h _{f g} = 2257000 J/kg

σ = 0.0533 N/m

Solution:

(Q/A) _{MAX }= 1480000 W/m^{2}. Let E _{b }be the voltage at burnout. Then electric energy input to wire is E _{b }I = 195 E _{b }W.

### Free Convection Theory

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