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Historically, probably the most commonly-studied cases of two-
phase flow are in large-scale power systems. Coal and gas-fired
power stations used very large boilers to produce steam for use inturbines. In such cases, pressurised water is passed through
heated pipes and it changes to steam as it moves through the
pipe. The design of boilers requires a detailed understanding of
two-phase flow heat-transfer and pressure drop behaviour, whichis significantly different from the single-phase case.
Even more critically, nuclear reactors use water to remove heat
from the reactor core using two-phase flow. A great deal of study
has been performed on the nature of two-phase flow in such
cases, so that engineers can design against possible failures in
pipework, loss of pressure, and so on (a loss-of-coolant
accident (LOCA))
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Three types of Water reactors are there
1>Pressurized Water reactor
2>Boiling Water Rector
3>LMFBR
But Pressurized Water Rector is used in major nuclearpower plants. The coolant for nuclear power plant is
water and that circulates through a heat exchanger by
a pump.
As the water reaches the boiling temperature (form
two phase mixture) by heat of the nuclear rods(Fuel
Rods) the deep pressurizer pressurize the water tobecame subcooled (compressed liquid) at high
pressure.
At high pressure at temperature corrosion can
occur result into pipe break or crack or leakage.
Loss of coolant accident (LOCA) can takes placewhich result into very dangerous effect. We are
aware of fukushima accident in Japan. The reason
behind that accident was Loss of coolant.
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The rate of coolant blow down through a break
or a leak is most important to design the
Emergency Core Cooling System (ECCS).
Rate of coolant blow down determine
depressurization rate and the time of reactorfuel uncovery.
The mass flux discharge depends upon the leak or break configuration , Upstream condition,
thermodynamic property and it tend to be choke and hence maximum flow rate.
1> Homogeneous equilibrium (same velocity and same property)
2>Homogeneous non-equilibrium (same velocity and different property)
3>non-Homogeneous equilibrium (different velocity and same property)3>non-Homogeneous non-equilibrium (different velocity and different property)
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We shall first discuss the physical characteristic of critical flow under subcool inlet flow
conditions. Next based upon the observed characteristics, flashing process will be
defined and critical flow conditions will be predicted.
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General Model:Assuming two phase flow through a pipe with equal phase velocity (Homogeneous) and
thermodynamic equilibrium exist between vapor and liquid (Equilibrium).
To formulate the two phase flow phenomenon, the bernaulis equation, in its generalform of the enery balance for the flow of unit mass of fluid, can written as
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Now, this is basic design for pipe break.
Based on this design we simplify the mass
Flux equation.
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Critical Two Phase Flow : Critical Overview
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Critical Two Phase Flow : Critical Overview
In HEM, we assume the vapor and liquid are in thermodynamic equilibrium and having
same velocity. HEM gives good calculated value of mass flux for long pipes, so that
thermodynamic equilibrium can achieve. But break or leak can happen at hi-
pressure/temperature side mean near to the core. Hence HEM could not be the best
model but sufficient to fulfill the purpose of ICCS (Emergency core cooling system designed
on HEM calculations.As we done our previous calculation, we here assuming that our upstream conditions are
subcooled and all the processes are isentropic. We will use predictions of Levy, Starkman,
R.E. Henry and other paperwork.
Our assumptions are based upon Moody and
Henry-Fauske model.
Gmax is critical flow rate obtained at nozzle/orifice exit. C is sonic velocity and b is
dimensionless parameter. is P/Po. y is specific heat ration of vapour phase. K is slip ratio
u(l) / u(g).
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Comparison between experimental data and
HEM and HNEM with changing stagnationsteam quality at particular instant of time.
Time varying analysis of mass flow rate
With orifice diameter 300mm and L/Dratio L/D=1
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Comparison between observed mass flux with calculated
mass flux of HEM and HNEM
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