# Heat Transfer Questions and Answers – Free Convection Boiling

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This set of Heat Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Free Convection Boiling”.

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
a) Nu = f 1 (G r) f 2 (P r)
b) Nu = 2 f 1 (G r) f 2 (P r)
c) Nu = 3 f 1 (G r) f 2 (P r)
d) Nu = 4 f 1 (G r) f 2 (P r)

Explanation: The functions f 1 and f 2 depend upon the geometry of the heating surface.

2. Fritz criterion is given by
a) h = 1.973 (Q/A) 0.45
b) h = 1.973 (Q/A) 0.55
c) h = 1.973 (Q/A) 0.65
d) h = 1.973 (Q/A) 0.75

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

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/m2 K where Q/A is the heat flux rate in W/m2 and d t is the temperature difference between surface and saturation
a) 1.308 * 10 8 W/m2
b) 1.308 * 10 7 W/m2
c) 1.308 * 10 6 W/m2
d) 1.308 * 10 5 W/m2

Explanation: Q = E I = 616.25 W and A = 4.71 * 10 -4 m2.
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4. Consider the above problem, find the excess temperature of the wire
a) 18.01 degree Celsius
b) 19.01 degree Celsius
c) 20.01 degree Celsius
d) 21.01 degree Celsius

Explanation: 1.54 (1.308 * 10 6) 2 = 5.58 (d t) 3.

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
a) iv
b) i and ii
c) i
d) iii and iv

Explanation: The fluids must be same so their flow pattern.

6. The heat flux in nucleate boiling varies in accordance with
a) h f g
b) (h f g) 0.5
c) 1/(h f g) 2
d) (h f g) 3

Explanation: 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.

7. In nucleate pool boiling, the heat flux depends on
a) Liquid properties, material and condition of the surface
b) Material of the surface only
c) Material and roughness of the surface
d) Liquid properties and material of the surface

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

8. Identify the wrong statement with respect to boiling heat transfer?
a) The steam boilers employing natural convection have steam raised through pool boiling
b) Boiling occurs when a heated surface is exposed to a liquid and maintained at a temperature lower than the saturation temperature of the liquid
c) Leiden-frost effect refers to the phenomenon of stable film boiling
d) The nucleation boiling is characterized by the formation of bubbles at the nucleation sites and the resulting liquid agitation

Explanation: 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.

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/m3
p g (vapor) = 0.61 kg/m3
h f g = 2.25 * 10 6 J/kg
σ = 0.0585 N/m
a) 1.53 * 10 8 W/m2
b) 1.53 * 10 7 W/m2
c) 1.53 * 10 6 W/m2
d) 1.53 * 10 5 W/m2

Explanation: (Q/A) = 0.18 p g h f g [σ (p f – p g)/p g 2] 0.25.

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/m3
p g (vapor) = 0.597 kg/m3
h f g = 2257000 J/kg
σ = 0.0533 N/m
a) 6.15 V
b) 7.15 V
c) 8.15 V
d) 9.15 V

Explanation: (Q/A) MAX = 1480000 W/m2. Let E b be the voltage at burnout. Then electric energy input to wire is E b I = 195 E b W.

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