talk on direct steam generation system a perspective on...

1

Talk

on

Direct steam generation system– A perspective on using

basic ideas of two-phase flow and heat transfer

By

Pradeep Kumar. P

Affiliation

Assistant Professor

Aerospace Engineering

Indian Institute of Space Science and Technology,

Valiamala

Trivandrum

Kerala -695547 ([email protected])

Design of Concentrated Solar Thermal Systems” during December 16 - 18, 2013 17 Dec 2013

Outline of the talk

Introduction to the topic

Some predictive methodologies

Recap on Single phase forced circulation system

Recap on single phase natural circulation

Natural circulation boiling loop

Understanding behaviour of boiling loop (natural circulation)

Discussion on the gross instabilities

Start-up

Limiting value of heat transfer

Summary

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3

Direct steam generation involves, the heated section sees the water (working fluid) and steam

is generated at the end of the heated section

A conceptual schematic of a

tower based direct steam

generation

P1

DMWT

ACC

N2

HXTS

C

DSG LOOP

N2

DA

RV RD

P4

FM2

FM4

FM3

Drain

HF

T

SH

H

P2

v

BU

CV

FM1

P3


4


P1

DMWT

ACC

N2

Drain1

HXTS

CV1

CDSG LOOP

N2

N2

SD

DA

P3

CV2

FM1

Superheating

Section

Main Heater

P2

PWT

RVRD

Aux

heater

H2

H1

P4

FM6

FM2

FM5

FM3

FM4

Drain2

IL

A conceptual schematic of a

parabolic trough based direct

steam generation


Deterministic prediction of flow behaviour of the system is important

- Are there any instability related issues?

- How reliably can we predictable them?

The system could be based on

- natural circulation ( relying on density gradients)

- forced circulation ( positively driven by pump)

Some considerations

5

Let us understand and get some elementary working knowledge of

these aspects.


6

Predictive estimates-Forced flow heated loop

A single phase situation (steady state):

In a single phase heated loop, consider that we

are given pump characteristics and heater power

One can easily estimate steady flow rate and

fluid temperature distribution

Conservation of momentum:

( integrated around the loop) 21 ( ) w

w

loop loop loop loop

Pp Au Hg

s A s A s

Expressing shear stress in term of Fanning friction factor

and after simplification of term , it can be brought to the

form 21 ( ) ww

loop loop loop loop

Pp Au Hg

s A s A s


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Predictive estimates-Forced flow heated loop

''

p surf heated

dTmc q P

ds

Note that heat flux is specified only in heated link and other portions they are zero. It is

easy to see (we talked about it in previous lecture) that the fluid temperature

distribution is linear in the heater

One can now easily simplify the energy equation also to

get

Now only mass flow rate is unknown which

can be easily estimated


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Predictive estimates-Natural circulation heated loop

Natural circulation:

The flow is driven by density variation and are

called gravity driven flows or thermosyphon

flow

Here density varies as temperature changes and

usually modelled as

One can use a Boussinesq appx.

This now makes momentum and energy

equations to be coupled.

Conservation of momentum:

( integrated around the loop)


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Predictive estimates-Natural circulation heated loop

Energy equation:

''

p surf heated

dTmc q P

ds

Heater

cooler

This can be simplified to

Assume mass flow rate

Compute temperature profile

Check if momentum eqn. is satisfied

Repeat till convergence


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Steady state flow rate in a boiling natural

circulation loop


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circulation loop

Objective:

The objective of the following analysis is to obtain the steady state mass

flow rate for a case when uniform heat flux applied to full length of the link 3 subject

to the stated assumptions. The power applied is taken to be 15kW. The loop is

assumed to be at a pressure of 0.5 Mpa-abs. The inlet subcooling to link3 is 15oC.

Assumptions:

• The loop is operating at steady state and all fluid properties are constant

at the loop operating pressure.

• One dimensional flow is valid and in the two phase region homogeneous

one dimensional theory is assumed to be valid.

• Links 4 and 5 are assumed to be adiabatic and the flow quality remains

constant at the exit quality value of link3. Hence acceleration pressure

drops are neglected in these constant diameter two phase links.

• The single phase links 1 and 2 is assumed to have liquid density at 0.5

MPa abs The link 3 inlet enthalpy is taken at a subcooling Tsub of

150C.This enthalpy is assumed to be same for link 1 and 2 also.


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circulation loop

Loop balance

Drum balance

Energy balance:

One can now get the energy balance to the form

Momentum equation:

We will do it link by link before we assemble it to the final

form


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circulation loop

Frictional pressure drop

Heated link

We need to do it link by link before we assemble it to the final form

ln 1e fg

f

grav f NB Bfg

e

f

x v

vp g L L

vx

v


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circulation loop

For the about loop the momentum equation integrated around the loop can

be simplified to the form

A typical MATLAB code for secant algorithm can be written to solve the

above to estimate the flow rate


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Stable

Type II

DWO

region

Heating Power

Ma

ss F

low

ra

te

Unstable

Xe =0

Stable

Type I

DWO

region

A

Behaviour of natural circulation

boiling flow loop

Natural circulation:

Driving a steam generation flow loop

by natural may seem attractive.

The system has to pass through gross

instabilities especially at low pressure

level.

It is important to understand and

establish how the system will reach

the operating pressures.


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Type I oscillations are the ones occurring at low pressure

( -1% <x<1%) • Riser voids influence gravitational pressure drop • Riser pr.drop oscillate in phase & wall temp oscillate out of

phase • low frequency oscillation.

Type II oscilllation are the ones occurring at high quality

• Time period of oscillation is of the order of transit time of

Kinematic waves • High frequency oscillation(0.5 Hz to 1 Hz)& hence wall temp could

shoot • Time period of oscillation increases monotonously with subcooling


boiling flow loop


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boiling flow loop

0.2 0.4 10.6 0.8

Flow Quality

Vo

id F

rac

tio

n

0.2

0.4

10.6

0.8

Core RiserZ

1 2

Low pressure<20 bar

Flashing phenomenon

Vapour generation is not by outside heating but by temperature of fluid reaching its local saturation value and vapourising as it is flowing up.

Flashing typically initiates from the riser exit and the point of vapour

generation develops downwards

The incubation period of oscillation decrease with power rise.


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boiling flow loop

Geysering phenomenon

Occur at low heat fluxes , intermittent boiling (subcooled) in core. There occurs vapour generation, detachment, growth and condensation – Compound relaxation instability (Boure et al, 1973)

Condensation occurs at different heights above the location of vapour generation due to thermal non- equilibrium and hence period and amplitude not constant

Pressure rise damps the amplitude of oscillation and narrows its region of occurrence.


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boiling flow loop

Typical signatures of flashing and geysering seen in the flow oscillation


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'' , , ,CHF eq f G x p D

The correlations most widely used can be typically categorised as TYPE I and TYPE II correlation

'' , ( . ), , ,CHF in inq f G x or h p D L

TYPE I Correlation

TYPE II Correlation

These predictions are usually based on appropriate correlations

There could be local instabilites like key thermal hydraulic phenomenon that is concerning the

upper limit to the operating power - critical heat flux limit.

Maximum limit of pumping heat

Boiling crisis occurs when the heat flux is raised to such a high level that the heated surface can no longer

support continuous liquid contact. This heat flux is usually referred to as the critical heat flux (CHF).Failure of

the heated surface may occur once the CHF is exceeded

Broadly categorised as DNB and annular dry out type mechanisms


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Summary:

• The flow stratification in horizontal heaters would mean no uniform heat

removal around the circumference .

• The sudden heat flux fluctuations especially in cases where operating

pressures could be high needs careful study. Large cycling tmeperature

gradient are not good for the system.

• Parallel ( and connected ) heated boiling pipes , gross manometric

oscillation behaviour could occur due to non uniform distribution in each

tube.

• If pumps are not sized correctly, Ledinegg instability could be triggered.

Direct steam generation


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