Recent Advances in Condensation on Tube Banks

P M V SubbaraoProfessor

Mechanical Engineering DepartmentI I T Delhi

Reduce the Degree of Over Design!!!

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Condensation on Horizontal Bundles: Prediction of Heat Transfer Coefficient in Nth Tube Row

Falling Film Condensation on Horizontal Tubes

• Falling-film heat exchangers are attractive because they provide good heat transfer performance and low working-fluid inventories.

• The design of falling-film heat exchangers has been largely based on empirical data.

• A thorough understanding of the falling-film flow and heat transfer interactions is important.

• An ability to predict the falling film mode would allow better data correlation and improve the modeling and analysis of heat transfer and fluid flow.

• The ultimate aim of this advanced learning is to enhance economy of heat exchanging.

Modes of Condensation on Tube Bundle

The droplet mode The jet mode The sheet mode

Daily Load Variation on A Power Plant%

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Daily Variation of Flow Rate through Condenser

Flow Rate Vs Mode of Falling Film

Identification of flow Regimes

Identification of flow Regimes

Condensation on Horizontal Tube Bundles : Flow Map

• Hu and Jacobi (1996) proposed flow mode transition equations with ReΓ versus Ga+ (film Reynolds number vs the Galileo number)

• Following principal flow modes were defined : sheet flow, column flow and droplet flow.

• The mixed mode transition zones of column-sheet and droplet-column were also considered as regimes, bringing the total to five.

Flow Transition Map

Identification Condensation Mode

The true characteristic Dimension

The film thickness is the true characteristic dimension:

31

3

gfilm

Nusselt Number,

kg

h

kh

Nu film

31

3

Ripples & Waves

• Experimental results showed a positive difference when compared to Nusselt theory.

• This positive difference was attributed to interfacial instability and occurrence of ripples in film.

• The ripples enhances the surface area and lowers the film thickness.

• The effect of ripples are accounted in the Nusselt number through a new number called Archimedes number.

Archimedes Number,2

3

gdAr

Final Correlations for Condensation Heat Transfer

For the Sheet Mode

For the Jet Mode

For the Drop Mode

Onset of Turbulence & Turbulent Film Condensation

• The transition film Reynolds number for the tube bundle is adapted from a vertical plate turbulent transition criterion of 1600 (but also values of 1200, 1800 and 2000 have been proposed).

• Thus, the film will become turbulent on the tube bundle at ReΓ

equal to 1600 and thus when ReΓ > 1600 the following expression should be used.

Condensation on Horizontal Tube Bundles : Turbulent Flow

• Turbulent flow of the condensate film may be reached in a condenser, which significantly increases heat transfer.

• Comparatively little has been published on turbulent film condensation on tube bundles compared to the information available for laminar films.

• Butterworth (1983) recommends adapting the Labuntsov expression for turbulent film condensation on a horizontal tubes for predicting local turbulent film condensation on the Nth tube row in horizontal tube bundles

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Computation of Wall Temperatures

• It is often necessary to calculate the wall temperature by an iterative approach.

• The summarized procedure is: 1. Assume a film temperature, Tf 2. Evaluate the fluid properties (viscosity, density, etc.) at

this temperature 3. Use the properties to calculate a condensing heat transfer

coefficient.4. Calculate the wall temperature. The relationship will

typically be something like

coolantsat

oo

satwall TTAh

UATT

1

1

5. Use the wall temperature to calculate a film temperature 6. Compare the calculated film temperature to that from the

initial step. 7. If not equal, reevaluate the properties and repeat.

Extreme Limits Shell And Tube Heat Exchanger

Extreme Combinations• Large cold fluid to hot fluid specific heat ratios.• Large ratios of Volumetric flow rates.• Large ratios of Convection heat transfer coefficients.• Numerical value of these ratios tending to infinite.• Impossible to contain the large volume flow rates.• How to obtain a velocity of wetting without containing?

Power Plant Steam Generator : An extreme HX

Cylinder in Cross Flow

Generally the overall average Nusselt number for heat transfer with the entire object is important.

Correlations are developed from experimental data to compute Nu as a f(Rem,Prn)

Overall Average Nusselt number

6

4131

10Re1500Pr7.0

:for Valid

PrPr Pr Re

D

s

mDD C

kDhNu

•All properties are evaluated at the freestream temperature, except Prs

which is evaluated at the surface temperature.

Values for C and m

Expect an accuracy within 20% with these correlations

ReD C m

1 -40 0.75 0.4

40 - 1000 0.51 0.5

1000 – 2X 105 0.26 0.6

2X 105 - 106 0.076 0.7