The document discusses phase change heat transfer, particularly focusing on boiling and condensation processes. It outlines the mechanisms of boiling, such as pool boiling and forced convection boiling, as well as the two main types of condensation: drop-wise and film-wise. Key equations and comparisons between the heat transfer rates in different boiling and condensation modes are presented, highlighting the significance of these processes in engineering applications.

1. HEAT TRANSFER WITH PHASE
CHANGE
Guided By: Mr. Chintan Modi
Prepared By:
Gaurav Dave (130990136011)
1

2. Content
1. BOILING
 Introduction
 Modes of Boiling
 Regimes of Pool Boiling
2. CONDENSTATION
 Introduction
 Condensation of vapor
 Comparison between film and drop
condensations
2
 Introduction

3. Introduction
 Phase change heat transfer is a broad field that finds applications in almost
all of the engineering disciplines.
 Boiling and condensation are two of the most important phase change
processes.
3

4. Boiling
4

5. Introduction
 Boiling is convective heat transfer process that involves a change in phase from
liquid to vapour.
 The phenomenon of boiling ,opposite of condensation ,is commonly encounter in
the unit operation such as distillation, evaporation, and steam generation.
 Such a process occurs when the heat is transferred from the solid surface to liquid
in a contact and surface temperature is maintained at a temperature higher then
saturation temperature of liquid.
 Boling process is used in boilers for steam formation, heat absorption in
evaporators in refrigeration system, dehydration and drying of foods , distillation
of liquids etc.
5

6.  The heat transfer from solid surface at Tw to liquid at Tsat can be
written as:
q = h (Tw-Tsat) =h.Δte
 The temp. Difference ΔTe= (Tw-Tsat) is called excess temperature.
6

7. Modes of Boiling
1. Pool boiling
2. Forced convection Boiling
3. Sub cooled or local boiling
4. Saturated boiling or bulk boiling
7

8. Pool boiling:
 It refers to the condition in which the hot surface is submerged below the free surface of a
stagnant liquid and its motion the near the surface is due to free convection only.
Forced convection Boiling:
 In this case the fluid motion is provide by external means like a pump as in case of modern
boilers.
 The forced convection improves the rate of heat transfer due to increased heat transfer co-
efficient.
8

9. Sub cooled or local boiling
 This is the case of pool boiling in which the liquid temp. is below its saturation temp.
 In such a case the bubbles formed at the heated surface are condensed in the liquid as they
leave the surface.
Saturated boiling or bulk boiling
 This is also the case of pool boiling in which the liquid is at saturation temp.
9

10. Various Regimes of Pool Boiling
1. Free convection boiling
2. Nucleate boiling
3. Transition boiling
4. Stable film boiling
10

11. Pool boiling curve
11

12. Free convection boiling
 In this regime the excess temperature, Te is very small. in ,the heat is transferred from
wall to the liquid . Density of liquid at its surface decreases, the hot liquid moves up and
cold liquid descends and sets up free or natural convection currents. Thus, the heating in
this case is by natural convection.
 In this case vapour is produce at free surface of liquid by evaporation hence this regime
is also known as interface evaporation
Nucleate boiling.
 Heat flux increase rapidly with increase in excess temperature, ΔTe = (Tw – Ts). The
values of ΔTe The values of ΔTe is in the range of 5c to 30c.
 In this region the bubbles start forming at certain location on the heated surface. It
represents the start of boiling process. 12

13.  During period A to B called regime of pool boiling.
 The bubble formed are very few in numbers. These bubbles condense in the liquid and do
not reach up to the free surface of the liquid .This regimes is referred as isolated nucleate
boiling or unstable boiling.
 With the further increase in excess temperature ΔTe in regime (3) ,The large number of
bubble are form on almost all places of surface.
 This bubble growth in size and rise to the free surface of liquid and from slugs of vapor
which leaves the surface.
13

14. Transition boiling
 With the increase in temperature difference, the bubble formation is very high.
 These bubble start to combine and from a blanket of vapour film on heating surface.
 In this region vapour film is not stable and collapses. Due to this a part of the surface will have film boiling
and the remainder surface will have nucleate boiling. For this reason, this regime is called as the unstable
film boiling.
Stable film boiling
 Thus the region beyond point D up to E the film is stable on the surface and this region is called the regime
of stable film boiling.
14

15. Correlation in pool boiling heat transfer
Nucleate saturated pool boiling:
𝑪𝒑𝒍
(𝑻𝒘−𝑻𝒔)
𝝀
= Csf
𝑸
𝑨
𝝀𝝁𝒍
𝝈
𝒈(𝝆𝒍
−𝝆 𝒗
)
𝟏
𝟑
𝑵 𝒏
𝒑𝒓𝒍
 𝐶𝑝𝑙 = Specific heat of saturated liquid J/(kg.k)
 𝑇𝑤 − 𝑇𝑠 = Different between surface temperature and saturated temperature
 Nprl = Prandtl number of saturated liquid
 Q/A = Heat flux, W/m2
 λμl = Liquid viscosity Kg/ (m.s)
 σ = Surface tension of liquid-vapour interface, N/m
 ρl = Density of saturated liquid, Kg/m2
 ρv = Density of saturated vapour, Kg/m3
 Csf = Constant, For water-platinum Csf 0.013, for water-brass Csf 0.006, for copper Csf 0.013.
 n = 1 for water and 1.7 for other liquid
 𝜆 = latent heat-enthalpy of vaporisation, j/kg
 g = Gravitational acceleration, m/s2
 The subscripts l and v denote the liquid and vapour phases. 15

16. Peak heat flux in nucleate pool boiling
 Zuber has developed the following expression for calculating the critical/peak heat flux in
nucleate pool boiling from an infinite horizontal plate facing up
[
𝑄
𝐴
]max =
𝜋
24
𝜆𝜌𝑣
𝜎𝑔(𝜌𝑙−𝜌𝑣)
𝜌𝑣2
1
4 𝜌𝑙+𝜌𝑣
𝜌𝑙
1
2
 The properties λ, ρl ,σv , and σ are evaluate at the saturation temperature of liquid.
16

17. Stable film pool boiling
hc =0.62
𝑘𝜐3 𝜌𝜐 𝜌𝜄−𝜌𝜐 𝑔∗ 𝜆+0.4𝐶𝑝𝑟 𝑇𝑤−𝑇𝑠
𝐷𝜇𝜈(𝑇𝑤−𝑇𝑠)
 Where hc is the convection heat transfer coefficient and D is the outside diameter of the
tube.
 The vapor properties are evaluated at the arithmetic mean of the surface and saturation
temperature i.e= at ,TW+TS/2 and the liquid density is evaluated temperature of
liquid (Ts).
17

18. Solved numerical
 Calculate the stable film boiling heat transfer coefficient assuming the film
boiling of saturated water at atmospheric pressure on an electrically heated
horizontal platinum wire of 1.6 mm diameter with an excess temperature of
255 K. also calculate the heat loss per unit length of the heater.
 Conditions:
1) hr < hc , h = hc + ¾hr
2) hr > hc , h = hc (hc/hr)1/3 + hr
18

19. 19

20. CONDENSATION
20

21. Heat transfer in the condensation of
vapor
 The change from liquid to vapor is known as vaporization and
that from vapor to liquid is known as condensation. In the
condensation of a pure vapour, it is necessary to remove the latent
heat of vaporization.
 When a saturated vapour comes into contact of a cold surface, for
example in surface condensers,
 Heat transfer from a vapour to the surface takes place and the
vapour gets condensed on the surface.
 Condensation occurs by two distinct mechanisms at very different
rates of heat transfer. 21

22. The two distinct mechanisms are
1. DROP-WISE CONDENSATION
2. FILM-WISE CONDENSATION
22

23. DROP-WISE CONDENSATION
 Dropwise condensation occurs when the vapor is in contact with a
non-wet-able surface, such as Teflon.
 In this the droplet formed are merged and falls down under action
of gravity from the surface.
 Condensation is difficult to sustain reliably.
23

24. FILM-WISE CONDENSATION
 Film-wise condensation is when a liquid film is formed on the
cooled surface by the vapors.
 This occurs when a clean, wet-able surface, such as most metals,
is in contact with the saturated vapor.
 The flow of the liquid film may be laminar if the flow velocity is
relatively slow. If the flow velocity is relatively fast, the flow of
the liquid film may be turbulent.
24

25. Comparison between film and drop
condensations
Film condensation
 The film formed on the surface offers
a thermal resistance to heat transfer.
 Due to low thermal conductivity of
the film the rate of heat transfer from
vapor to surface are reduced.
Drop condensation
 In case of vapor of drop wise
condensation the vapor condenses in the
form of droplets which grow in size and
finally they roll of the surface under the
influence of gravity.
 Thus there is no such thermal resistance
due to film in case of drop condensation
and the vapor directly comes in contact
with the surface.
25
“The heat transfer rates in drop wise condensation may be 10 times more
than film wise condensation.”

26. References
 http://en.citizendium.org/wiki/Condensation_(phase_transition)
 Eduardo Cao (2009). Heat Transfer in Process Engineering, 1st Edition. McGraw-Hill. ISBN 0-
07-162408-2.
 Satish G. Kandlikar. Masahiro Shoji and V.K. Dhir (Editors) (1999). Handbook of Phase Change:
Boiling and Condensation, 1st Edition. CRC Press. ISBN 1-56032-634-4.
 http://www.learnengineering.org/2012/12/introduction-to-heat-transfer.html
 http://wins.engr.wisc.edu/teaching/mpfBook/node9.html#SECTION00450000000000000000
 http://www.thermopedia.com
 http://www.nzifst.org.nz/unitoperations/httrtheory8.htm
 http://web.iitd.ac.in/~prabal/MEL242/(25)-boiling-1.pdf
 http://web.iitd.ac.in/~prabal/MEL242/(27)-condensation.pdf
 http://nptel.ac.in
26

27. 27