ht quiz 8

boiling heat transfer

What is Boiling Heat Transfer?

occurs when temp of surface is sufficiently higher than T_SAT of liquid its in contact with

transforms some of it into vapour

most of energy which is transferred from surface to liquid is absorbed by change in phase from liquid to vapour

what is h_fg

latent heat of vaporisation

Why is boiling heat transfer such an effective method of removing thermal energy from a surface?

in single phase ht, thermal energy absorbed by fluid from hot surface increases temp of fluid

• more gentle temp gradient at surface

• reduces rate of wall ht

in boiling ht, due to high h_fg needed to convert a unit mass of liquid into vapour

• most of E absorbed by fluid from hot surface used to convert liquid to vapour

• maintains steep temp gradient at surface

• high wall heat flux rates

How does the pressure inside a vapour bubble relate to the surrounding pressure?

pressure inside > surroundings

difference between pressures inside and outside bubble is:

• proportional to surface tension of liquid

• inversely proportional to radius of bubble

  

What condition needs to be satisfied for a vapour bubble to grow?

Why is it necessary for the wall temperature to be several degrees higher than the saturation temperature in order to have boiling?

if wall temp = vapour T_SAT , P_SAT = surroundings

only bubbles with infinitely large radius would survive

• bc increase in P across bubble surface is

  

for boiling to start, surface temp needs to be high enough for critical radius to be same sixe as surface cavities (which act as nucleation sites)

Which are the dimensionless groups which characterise boiling heat transfer?

Explain the term Pool Boiling

initially stagnant pool of liquid is heated to its boiling point

List the four main regimes encountered in pool boiling, as the difference between the temperature of the heating surface and the liquid saturation temperature is gradually increased.

natural convection

nucleate boiling

transition boiling

film boiling

How can you explain the strong noise generated in kettles during the boiling process?

when temp of liquid above heated surface < boiling temp

vapour bubbles generated at heated surface collapse (re-condense)

• as they rise through sub-cooled liquid

the collapsing is the noise

In pool boiling, controlled through the gradual increase of the difference between the temperature of the heating surface and the liquid saturation temperature, identify the regime in which an increase in the difference between the difference in surface and liquid saturation temperature leads to a reduction in surface heat flux rate and explain why.

transition boiling regime

• increase in surface and liquid T_SAT = reduction in wall heat flux rate

because

• in this region, large proportion of heated surface covered by vapour film

• vapour film acts as thermal barrier

• as surface temp increases, proportion of surface covered by film increases

• reduces wall heat flux rate

at min. heat fux point, entire surface covered by vapour film

beyond this point, film boiling regime takes over

• wall heat flux rate increases with surface temp

In pool boiling, controlled through the gradual increase of the wall heat flux rate, explain what the Burnout Point is.

as heat flux rate increases

T_W - T_SAT also increases at first

• through natural convection and nucleate boiling regimes

at end of nucleate boiling regime, further increase in wall heat flux will need a large increase in wall temp to sustain it

• causes sudden change from nucleate boiling to film boiling regime

the sudden jump in surface temp raises temp above melting point of surface material

so, at end of nucleate boiling regime, the point where q_w reaches its local max = burn out point

For the Film Boiling regime, what are the modes of heat transfer from the heated wall to the liquid?

from heated surface to boiling liquid, across vapour film

• via. heat conduction

• as temp diff. increases, thermal radiation also contributes

In empirical heat transfer correlations for the Film Boiling regime, how is the augmented latent heat of vapourisation and why is there a need for it.

augmented latent heat of vaporisation = h’_fg

accounts for energy needed to change liquid → vapour

AND

for energy needed to raise its temp across film

In empirical heat transfer correlations for the Film Boiling regime, at what temperature are the vapour properties defined and why?

vapour properties defined at film temp = av of wall and saturation temps, because there is large diff between the two

[not necessary for nucleate boiling regime bc diff between wall and saturation temps is small]

Explain convection boiling

convection boiling occurs when liquid forced to flow past heated surface

which is at temp > liquid T_SAT

general effects of imposed fluid motion on boiling ht

imposed fluid motion increases coefficient of wall heat flux

increases q_max (the critical wall heat flux)

In external convection boiling over a flat surface:

how is the wall heat flux q calculated in the nucleate boiling regime

wall heat flux q in nucleate boiling regime

caculated through superposition method

• add q_b (nucleate boiling ht for pool boiling) to q_c (forced convection ht

• q = q_b + q_c

which two dimensionless groups determine augmentation of critical heat flux q_max in external convection boiling?

Webber Number

ratio between liquid and vapour densities

In internal forced convection boiling in an upward flow within a heated tube, with sub- cooled liquid at the inlet, list the flow regimes that develop, provided the tube is long enough.

single phase flow

bubbly flow

slug flow

annular flow

annular flow with entrainment

drop flow

single phase vapour

In internal forced convection boiling in an upward flow within a heated tube, with sub- cooled liquid at the inlet, list the modes of heat transfer that develop, provided the tube is long enough.

heat convection to liquid

sub-cooled boiling

saturated nucleate boiling

forced convective ht through annular liquid film

forced convective transfer to liquid deficient region

heat convection to vapour

In internal forced convection boiling in an upward flow within a heated tube, what is the Dry Out point and what is its practical significance under uniform heating conditions?

Dry out point = end of annular liquid film that covers heated surface of tube

practical significance = locartion of abrupt increase in wall temp

In internal forced convection boiling in an upward flow within a heated vertical tube which dimensionless groups are used in the calculation of the ratio of the actual local heat flux coefficient to that of fully-developed forced convection, based on the liquid properties?

In internal forced convection boiling in flow within a heated horizontal tube, how is the ratio of the actual local heat flux coefficient to that of fully-developed forced convection, based on the liquid properties calculated?

dependa on Froude number:

Fr > 0.04, high speeds

• gravitational effect negligible

• flow remains symmetric

• calculation procedure identical to vertical tube calc

Fr < 0.04, low speeds

• gravitational effects lead to non-symmetric flow

• Fr number included in correlation for heat flux ratio