Test: Nucleate Boiling


10 Questions MCQ Test Heat Transfer | Test: Nucleate Boiling


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QUESTION: 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

Where, D is the average bubble diameter, G is the mass velocity of the bubble per unit area and δ f is the fluid viscosity

Solution:

This nucleate boiling regime is of great importance because of the very high heat fluxes possible with moderate temperature differences.

QUESTION: 2

The bubble diameter has been expressed by Fritz as

Where, C is a constant, σ is surface tension of the liquid and β is the bubble contact angle measured through liquid in degrees

Solution:

is a constant which has been evaluated as 0.0148 for hydrogen and water bubbles.

QUESTION: 3

What is the value of surface fluid constant for water-copper combination?

Solution:

The surface fluid regime is of great importance because of the very high heat fluxex possible with moderate temperature differences.

QUESTION: 4

What is the value of surface fluid constant for water-brass combination?

Solution:

For H2O and Brass combination, this value must lie between 0.0056 to 0.00062.

QUESTION: 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

Solution:

– T sat = (2 σ/r – p g) R v T V2/p v h f g.

QUESTION: 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

Solution:

Q= V I = 286 W and A = 9.81 * 10 -4 m2. Therefore heat flux = Q/A.

QUESTION: 7

Consider the above problem, find the boiling heat transfer coefficient

Solution:

Q = h A d t. So, h = 8330 W/mK.

QUESTION: 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

Solution:

Acc to Zuber relation, burn out (Q/A) = 0.18 p g h f g [p (p f – p g)/p 20.25 [p f/p + p f0.5.

QUESTION: 9

All the following statements are correct, except

Solution:

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.

QUESTION: 10

Milk spills over when it is boiled in an open vessel. The boiling of milk at this instant is referred to as

Solution:

This is an application of sub-cooled boiling.

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