This document discusses two-phase flow boiling in small channels. It begins with an introduction and classification of boiling as pool or flow boiling. Pool boiling is described along with the boiling curve and common correlations. Flow boiling is then discussed, noting it can be external or internal. Internal flow boiling, or two-phase flow, is the focus. Common flow boiling regimes and the challenges modeling microchannels are outlined. The document then presents the annular flow model for microchannels, including assumptions, parameters, equations, and results showing improved accuracy over empirical correlations. In conclusion, the annular flow model is shown to better capture heat transfer trends in microchannels with low error compared to other correlations.

1. TWO PHASE FLOW BOILING IN SMALL
CHANNELS
Presented by :- Ashutosh kushwaha

2. CONTENTS
 Introduction
 Classification
 Pool Boiling
 Boiling Curve
 Correlations and explanation
 Flow boiling
 Different correlations and parameters
 Annular Flow model
 Conclusion

3. INTRODUCTION
 Its applications
 Micro-channels heat sink
 How high htc can be obtained in micro-channels
 Drawbacks

4. CLASSIFICATION OF BOILING
 Boiling:- Sudden vaporization of liquid at solid-liquid interface at its
boiling point (the temperature at which vapor pressure of liquid is equal
to external pressure).
 Classification based on the bulk temperature
 Subcooled boiling
 Saturated boiling
 Further classification based on the motion
 Pool boiling
 Flow boiling

5. CONTD.
 Classification based on bulk temperature helps in better understanding
of transition film boiling and film boiling in pool boiling

6. POOL BOILING
 Boiling of stationary fluid or motionless fluid, any
motion in fluid is due to convection.
E.g. Domestically boiling of water on electrical heater

7. CONTD.
 Introduction to boiling curve
 Variation in boiling curve when heat flux or
temperature of heating surface is varied independently

8. CONTD.
 Different regimes
 Natural convection
 Nucleate boiling
 Transition film boiling
 Stable film boiling
 Parameters like
 Critical heat flux
 Burnout point
 leiden frost point
 Inflection point

11. CORRELATIONS
 An equation was proposed by Rohsenow for heat flux in nucleate pool
boiling.
 Here (l) denotes liquid property which has to be calculated at saturation
temperature and (v) denotes vapor properties which has to be mean vapor
temperature.
 The constant values like Csf and n can be seen from table.
 Another correlation for calculating heat transfer coefficient was proposed by
Mostinski
 high turbulence leads to high heat transfer.
 Applications of nucleate boiling:-
3
)(Pr
2/1
)(









 
 n
s
lvsf
eplvl
vl
LC
Tcg
Lq



CmWTPh ecb  /)(P)()(00341.0 566.0
r
33.23.2

12. CONTD.
 Critical heat flux or Burnout point
A simple correlation for calculating max. heat flux by Lienhard
 Heat transfer coefficient at stable film boiling is calculated by
At high surface temperatures (typically above 300°C), heat transfer across the vapor film by
radiation becomes significant and needs to be considered.
4/1
)(
149.0 2
max





 

v
vl
vv
g
Lq




13. CONTD.
 Zuber derived the following expression for the minimum heat flux (at
Leiden frost point) for a large horizontal plate.

14. FLOW BOILING
 Boiling in which fluid motion is
induced due to convection as
well as due to external sources
like pump.
E.g. boiling in small channels
 May be External flow boiling
or Internal flow boiling.
 In External flow boiling higher
the velocity, the higher the
nucleate boiling heat flux
and the critical heat flux.

15. CONDT.
 Internal flow boiling,
commonly referred to as
two-phase flow.
 no free surface for the vapor
to escape
 Both the liquid and the vapor
are forced to flow together.
 Boiling in two phase flow also
consist many regimes
 Here the heat transfer depends
on the quality.

16. CONTD.
 Nucleate boiling region
 Heat transfer coefficient is dependent on heat flux
 Less sensitive on mass velocity and quality
 Forced convection region
 Majorly dependent in mass velocity and quality
 Fairly independent on heat flux
 Annular flow region is of greater interest.
 Early transition to annular region from nucleate boiling is observed in
case of water taking as a test fluid nor refrigerants
 High surface tension leads to forms big bubbles

17. CORRELATIONS
 Many empirical correlations that were developed for macro channels were
applied in microchannels heat transfer correlations
 These correlations results showed large variation from
experimental results

18. CONTD.
 The other five correlations developed for mini/micro channels
showed satisfactory results but still not appreciable results
 Hence there was need to propose different models for precise
prediction of htc.
 Annular Flow model

19. ANNULAR FLOW MODEL
 Assumptions
 Primary parameters
 Mass flow rate
 Liquid film thickness
 Pressure gradient
 Interfacial stress
 Model construction
 Mass conservation
 Momentum conservation in liquid film
 Momentum conservation in vapor film
 Interfacial shear stress
 Solution procedure

20. RESULTS
 Comparison of experimental results and
model results

21. CONCLUSION
 The new model correctly captures the unique overall trend of heat
transfer coefficient with increasing vapor quality in regions of micro-
channels.
 Mean while the MAE(Maximum Absolute Error ) is very low it is near
to
13.3% which clearly agrees the better results obtained from this
annular flow model as compared to the 11 empirical correlations.

22. THANK YOU