Choking Flow

7/31/2019 Choking Flow

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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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Choking Flow

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