TWO PHASE FLOW BOILING IN SMALL CHANNELS

Presented by :- Ashutosh kushwaha

CONTENTS

Introduction Classification Pool Boiling

Boiling Curve Correlations and explanation

Flow boiling Different correlations and parameters

Annular Flow model Conclusion

INTRODUCTION

Its applications Micro-channels heat sink

How high htc can be obtained in micro-channels

Drawbacks

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

CONTD.

Classification based on bulk temperature helps in better understanding of transition film boiling and film boiling in pool boiling

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

CONTD. Introduction to boiling curve

Variation in boiling curve when heat flux or temperature of heating surface is varied independently

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

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:- Liquid boiling equipment in process industry. For cooling rocket motors nucleate boiling is done at lower velocities

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)(Pr

2/1)(

n

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eplvl

vl

LC

TcgLq

CmWTPh ecb /)(P)()(00341.0 566.0r

33.23.2

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)(

149.02

max

v

vl

vv

gLq

CONTD.

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

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.

CONDT. Internal flow boiling,

commonly referred to as two-phase flow.

no free surface for the vapor to escape

Both the liquid and the vaporare forced to flow together.

Boiling in two phase flow alsoconsist many regimes

Here the heat transfer depends on the quality.

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

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

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

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

RESULTS Comparison of experimental results and

model results

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.

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