Heat Transfer Problems on “Nucleate Boiling”.
1. Reynolds number is replaced by a modulus significant of the agitation of the fluid particles in nucleate boiling. Such a dimensionless modulus is defined by the relation
a) Re b = 2 D b G b/δ f
b) Re b = D b G b/δ f
c) Re b = ½ D b G b/δ f
d) Re b = 3 D b G b/δ f
Answer: b
Clarification: This nucleate boiling regime is of great importance because of the very high heat fluxes possible with moderate temperature differences. Where, D b is the average bubble diameter, G b is the mass velocity of the bubble per unit area and δ f is the fluid viscosity.
2. The bubble diameter has been expressed by Fritz as
a) D b = C d β [2 σ/g (p f – p g)]1/2
b) D b = C d β [2 σ/g (p f – p g)]3/2
c) D b = C d β [2 σ/g (p f – p g)]5/2
d) D b = C d β [2 σ/g (p f – p g)]7/2
Answer: a
Clarification: C d is a constant which has been evaluated as 0.0148 for hydrogen and water bubbles. Where, C d is a constant, σ is surface tension of the liquid and β is the bubble contact angle measured through liquid in degrees.
3. What is the value of surface fluid constant for water-copper combination?
a) 0.010
b) 0.011
c) 0.012
d) 0.013
Answer: d
Clarification: The surface fluid regime is of great importance because of the very high heat fluxex possible with moderate temperature differences.
4. What is the value of surface fluid constant for water-brass combination?
a) 0.004
b) 0.005
c) 0.006
d) 0.007
Answer: c
Clarification: For H 2 O and Brass combination, this value must lie between 0.0056 to 0.00062.
5. Spherical bubbles of 3 mm diameter are observed in the bulk fluid boiling of water at standard atmospheric pressure. Assuming pure water vapor in the bubble and vapor pressure equal to 101.325 k N/m2, calculate the temperature of the vapor
a) 100.217 degree Celsius
b) 200.217 degree Celsius
c) 300.217 degree Celsius
d) 400.217 degree Celsius
Answer: a
Clarification: T v – T sat = (2 σ/r – p g) R v T V2/pv h f g.
6. An electric wire of 1.25 mm diameter and 250 mm long is laid horizontally and submerged in water at 7 bar. The wire has an applied voltage of 2.2 V and carries a current of 130 amperes. If the surface of the wire is maintained at 200 degree Celsius, make calculations for the heat flux
a) 0.0915 * 10 6 W/m2
b) 0.1915 * 10 6 W/m2
c) 0.2915 * 10 6 W/m2
d) 0.3915 * 10 6 W/m2
Answer: c
Clarification: Q= V I = 286 W and A = 9.81 * 10 -4 m2. Therefore heat flux = Q/A.
7. Consider the above problem, find the boiling heat transfer coefficient
a) 5330 W/m2 K
b) 6330 W/m2 K
c) 7330 W/m2 K
d) 8330 W/m2 K
Answer: d
Clarification: Q = h A d t. So, h = 8330 W/m2 K.
8. Which of the following parameters affect burnout heat flux in the nucleate boiling region
(i) Heat of evaporation
(ii) Temperature difference
(iii) Density of vapor
(iv) Density of liquid
(v) Surface tension at the vapor-liquid interface
Mark the correct answer from the codes indicated below
a) i, ii, iii and v
b) i, iii, iv and v
c) i, ii, iii and iv
d) i, iii and v
Answer: b
Clarification: Acc to Zuber relation, burn out (Q/A) = 0.18 p g h f g [p (p f – p g)/p g 2] 0.25 [p f/p g + p f] 0.5.
9. All the following statements are correct, except
a) Nucleate boiling gets promoted on a smooth surface
b) In subcooled heating, the temperature of the heating surface is more than the boiling point of the liquid
c) Film boiling region is usually avoided in commercial equipment
d) There occurs transition from nucleate to film boiling burn-out point on the boiling curve
Answer: a
Clarification: A rough surface gives a better heat transmission than when the surface is either smooth or has been coated to weak its tendency to get wetted.
10. Milk spills over when it is boiled in an open vessel. The boiling of milk at this instant is referred to as
a) Interface evaporation
b) Sub-cooled boiling
c) Film boiling
d) Saturated nucleate boiling
Answer: b
Clarification: This is an application of sub-cooled boiling.