**The continuity equation** is simply a mathematical expression of the principle of conservation of mass. For a control volume that has a **single inlet** and a **single outlet**, the principle of conservation of mass states that, for **steady-state flow**, the mass flow rate into the volume must equal the mass flow rate out.

ṁ_{in} = ṁ_{out}

Mass entering per unit time = Mass leaving per unit time

## Conservation of Mass

This principle is generally known as the **conservation of matter principle** and states that the mass of an object or collection of objects never changes over time, no matter how the constituent parts rearrange themselves. This principle can be use in the analysis of **flowing fluids**. Conservation of mass in **fluid dynamics** states that all **mass flow rates into** a control volume are equal to all **mass flow rates out** of the control volume plus the rate of change of mass within the control volume.

This principle is expressed mathematically by following equation:

ṁ_{in} = ṁ_{out} +^{∆m}⁄_{∆t}

Mass entering per unit time = Mass leaving per unit time + Increase of mass in the control volume per unit time

This equation describes **nonsteady-state flow**. Nonsteady-state flow refers to the condition where the fluid properties at any single point in the system may change over time. **Steady-state flow** refers to the condition where the fluid properties (**temperature, pressure, and velocity**) at any single point in the system **do not change over time**. But one of the most significant properties that is constant in a steady-state flow system is the system mass flow rate. This means that there is **no accumulation** of mass within any component in the system.

## Continuity Equation

**The continuity equation** is simply a mathematical expression of the principle of conservation of mass. For a control volume that has a **single inlet** and a **single outlet**, the principle of conservation of mass states that, for **steady-state flow**, the mass flow rate into the volume must equal the mass flow rate out.

ṁ_{in} = ṁ_{out}

Mass entering per unit time = Mass leaving per unit time

This equation is called **the continuity equation** for steady one-dimensional flow. For a steady flow through a control volume with many inlets and outlets, the net mass flow must be zero, where inflows are negative and outflows are positive.

This principle can be applied to a **streamtube** such as that shown above. No fluid flows across the boundary made by the **streamlines** so mass only enters and leaves through the two ends of this streamtube section.

When a fluid is in motion, it must move in such a way that mass is conserved. To see how mass conservation places restrictions on the velocity field, consider the steady flow of fluid through a duct (that is, the inlet and outlet flows do not vary with time).

## Differential Form of Continuity Equation

A general continuity equation can also be written in a **differential form**:

^{∂⍴}⁄_{∂t} + ∇ . (⍴ ͞v) = σ

**where**

- ∇ . is divergence,
- ρ is the density of quantity q,
- ⍴ ͞v is the flux of quantity q,
- σ is the generation of q per unit volume per unit time. Terms that generate (σ > 0) or remove (σ < 0) q are referred to as a “sources” and “sinks” respectively. If q is a conserved quantity (such as energy), σ is equal to 0.

## Continuity Equation – Multiple Inlets and Outlets

For a control volume with **multiple inlets and outlets**, the principle of conservation of mass requires that the **sum of the mass flow rates into** the control volume equal the **sum of the mass flow rates out** of the control volume. **The continuity equation** for this more general situation is expressed by following equation:

∑ṁ_{in} = ∑ṁ_{out}

Sum of mass flow rates entering per unit time = Sum of mass flow rates leaving per unit time

## Continuity Equation – Examples

### Flow rate through a reactor core

In this example, we will calculate the **flow rate** through a reactor core from continuity equation. It is an illustrative example, following data do not represent any reactor design.

ṁ_{in} = ṁ_{out}

(ρAv)_{in} = (ρAv)_{out}

____________________________

**Pressurized water reactors** are cooled and moderated by **high-pressure liquid water** (**e.g. 16MPa**). At this pressure water boils at approximately **350°C (662°F)**. Inlet temperature of the water is about **290°C** (**⍴ ~ 720 kg/m ^{3}**). The water (coolant) is heated in the reactor core to approximately

**325°C**(

**⍴ ~ 654 kg/m**) as the water flows through the core.

^{3}The primary circuit of typical

**PWR**is divided into 4 independent loops (piping diameter ~ 700mm), each loop comprises a

**steam generator**and one

**main coolant pump**. Inside the reactor pressure vessel (RPV), the coolant first flows down outside the reactor core (

**through the downcomer**). From the bottom of the pressure vessel, the flow is

**reversed up through the core**, where the coolant temperature increases as it passes through the fuel rods and the assemblies formed by them.

### Calculate:

- the primary piping volumetric flow rate (m
^{3}/s), - the primary piping flow velocity (m/s),
- the core inlet flow velocity (m/s),
- the core outlet flow velocity (m/s)

when

- the mass flow rate in the hot leg of primary piping is equal to
**4648 kg/s**, - Reactor core flow cross-section is equal to
**5m**,^{2} - Primary piping flow cross-section (single loop) is equal to
**0.38 m**^{2}

### Results:

Cold leg volumetric flow rate:

**Q _{cold} = ṁ / ⍴** = 4648 / 720 = 6.46 m

^{3}/s =

**23240 m**

^{3}/hodCold leg flow velocity:

A_{1} = π.d^{2} / 4

**v _{cold}** =

**Q**

_{cold}**/ A**= 6.46 / (3.14 x 0.7

_{1}^{2}/ 4) = 6.46 / 0.38 =

**17 m/s**

Hot leg volumetric flow rate:

**Q _{hot} = ṁ / ⍴** = 4648 / 654 = 7.11 m

^{3}/s =

**25585 m**

^{3}/hodHot leg flow velocity:

A = π.d^{2} / 4

**v _{hot}** =

**Q**= 7.11 / (3.14 x 0.7

_{hot}/ A_{1}^{2}/ 4) = 7.11 / 0.38 =

**18,7 m/s**

or according to **the continuity equation**:

**⍴ _{1} . A_{1} . v_{1} = ⍴_{2} . A_{2} . v_{2}**

**v _{hot}** = v

_{cold}. ⍴

_{cold}/ ⍴

_{hot}= 17 x 720 / 654 =

**18.7 m/s**

Core inlet flow velocity:

A_{core} = 5m^{2}

A_{piping} = 4 x A_{1} = 4 x 0.38 = 1.52 m^{2}

⍴_{inlet} = ⍴_{cold}

according to **the continuity equation**:

⍴_{inlet} . A_{core} . v_{inlet} = ⍴_{cold} . A_{piping} . v_{cold}

**v _{inlet}** = v

_{cold}. A

_{piping}/ A

_{core}= 17 x 1.52 / 5 =

**5.17 m/s**

Core outlet flow velocity:

⍴_{inlet} = ⍴_{cold}

⍴_{outlet} = ⍴_{hot}

according to **the continuity equation**:

⍴_{outlet} . A_{core} . v_{outlet} = ⍴_{inlet} . A_{core} . v_{inlet}

**v _{outlet}** = v

_{inlet}. ⍴

_{inlet}/ ⍴

_{outlet}= 5.17 x 720 / 654 =

**5.69 m/s**