A gas accumulator is a fixed volume divided by an elastic diaphragm where the pressure acting on the working fluid in one of the volumes are created from the compression of the gas in the adjacent volume.

The gas-charged accumulator can also represent a piston cylinder arrangement. Although the total volume of the gas-charged accumulator remains fixed, the volume occupied by the gas and consequently that occupied by the working fluid changes. Accumulators such as these are frequently used in hydraulic systems to ensure adequate mass flow rates in the system for certain time spans in the event of a loss of pumping power. The concept is also employed to dampen water hammer effects in certain flow systems (surge tanks).

The Boundary condition component is used to specify the boundary conditions of a network. Boundary condition components can only be connected to a node component. The boundary condition specified in the boundary component is applied to the node it is connected to. The boundary condition can be temperature, mass source, Pressure, gas mixture composition, heat source or sink and two phase fluid state.

The Junction component can be used to connect more than two elements and take the junction losses into account. Currently, Flownex includes junction characteristics for T-junction and jet pump junction types.

The Junction component is based on the Node and was introduced to allow the user to create simpler, more intuitive networks by restricting some of the node functionality and presenting the component with a unique icon on the diagramming canvas. Note that the functionality provided by the Junction component is therefore also available by using the Node component.

Simply put, nodes are the end points of elements. Nodes can also be used to model a tank or reservoir and elevation in a system. It is also possible to specify that a node can make a junction and that junction losses should be taken into account. In the case where a two-phase fluid is used in a reservoir, the node can be used to separate the fluid and gas to the bottom and top sections of the reservoir.

Level tracking can consequently be performed during dynamic simulations to determine the height of the liquid level and whether a liquid, gas or mixture is withdrawn from the outlets of the reservoir.

From the Basic components functionality group on the Flownex component library, it can be seen that there are five node type elements; Node, Junction, Open Container, Accumulator and Reservoir.

The Open Container component can be used to model an Open reservoir or tank. Open container can be modelled with compressible and incompressible fluid.

The Open Container component can be used as building block in the following systems/subsystems:

• Atmospheric Drain Tank
• Fire protection system reservoir
• Dams
• Lubrication and hydraulic oil tank

Reservoir

The Reservoir component can be used to model a reservoir or tank. The Reservoir component is based on the Node and was introduced to allow the user to create simpler, more intuitive networks by restricting some of the node functionality and presenting the component with a unique icon on the diagramming canvas. The Reservoir element assumes the fluid is completely mixed and therefore no level tracking.

The Two-phase tank component will take into account Level tracking performed during dynamic simulations to determine the height of the liquid level and whether a liquid, gas or mixture is withdrawn from the outlets of the reservoir. The two phase tank component can be used as building block in the following systems/subsystems:

• HP, IP and LP Steam drums
• Boiler headers
• Condenser

The General Empirical Relationship element can be used to model the pressure drop through heat exchangers, valves and experimental equipment and any equipment on the plant with empirical data available for pressure drop as function of Reynolds number and Prandtl number. The model applies to gas, liquid and two-phase flows.

This element is intended for use with user specified pressure loss characteristics obtained for flow through complicated flow paths such as leak flow paths. The actual leak flow path that is modelled with the Lumped Resistive Duct element can be of any shape or size and may also feature discontinuous changes in the geometrical properties.

A User-Specified Pressure Drop element is modelled in the same manner as a Fan/Pump element, except that the Δp₀ - value of the User-Specified Pressure Drop element curve is treated as a pressure decrease (Δp₀= p_0i – p_0e ) rather than a pressure increase. Compressible and incompressible flows can be modelled with the user-specified pressure drop element.

The User-Specified Pressure Ratio element can be used to model any pressure drop element for which flow characteristics are available (valves, restrictors, ducts, orifices etc.). Flownex determines the pressure drop by means of interpolation from the pressure ratio/corrected mass flow rate curve.

The Angle Valve is a Control valve. Control valves are devices for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications. The Angle valve is a type of the Control valve with loss coefficient.

Control valves are devices for closing or modifying the passage through a pipe line, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications.

The equations for incompressible flow are based on the standard hydrodynamic equations for Newtonian incompressible fluids. They are not intended for use when non-Newtonian fluids, fluid mixtures, slurries or liquid-solid conveyance systems are modelled. This model was derived from the ANSI/ISA standard 75.01–1985 (R1995).

At very low ratios of pressure differential to absolute inlet pressure (Δp₀/p₁), compressible fluids are assumed to behave similarly to incompressible fluids. Under such conditions, the valve sizing equations for compressible flow can be traced to the standard hydrodynamic equations for incompressible fluids.

However, the increasing values of Δp₀/p₁ result in compressibility effects that require the basic equations to be modified by appropriate correction factors. The equations for compressible fluids are for use with gas and are not intended for use with multi-phase streams such as complex fluid, vapour-liquid or gas-solid mixtures. However, vapour-liquid two-phase flow can be handled and is modelled with separate equations.

Control valves are devices for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications.

Control valves are devices for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications.

The Gate Valve is a Control valve. Control valves are devices for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications.

The Globe Valve is a Control valve. Control valves are devices for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications.

Pressure regulating valves maintains a set downstream pressure independent of higher variable upstream pressures. As upstream pressure increases reaching the set pressure, the valve closes. It opens as soon as the downstream pressure decreases below the set pressure. One application of a pressure regulating valve is to protect filters from damaging pressure surges.

Pressure relief valves are designed to provide protection from over-pressure in gas, liquid or two-phase lines. A pressure relief valve is a self-operating valve that is installed in a process system to protect against over pressurization of the system. Relief valves are designed to continuously regulate fluid flow, and to keep pressure from exceeding a preset value. The pressure relief valve blows off fluid from the system when safe pressures are exceeded, then closes again when pressure drops to a preset level.

The Sharp edge diaphragm Valve is a Control valve. Control valves are devices for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications.

The Sluice Valve is a Control valve. Control valves are devices for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications.

The Smooth diaphragm Valve is a Control valve. Control valves are devices for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications.

The Y - Valve is a Control valve. Control valves are devices for closing or modifying the passage through a pipe, outlet, inlet, or the like, in order to stop, allow, or control the flow of media. These valves are used throughout industry in many applications.

A British standard orifice is a sharp edged orifice mounted in a pipe or duct. This type of element applies to both compressible and incompressible flows.

Cooling Orifice

This element is used to model the cooling orifices in the walls of aircraft engine combustion chambers.

The following two options are provided for the calculation of total pressure drop:

• Upstream total pressure minus downstream total pressure.
• Upstream total pressure minus downstream static pressure.

The total pressure downstream of the throat is calculated as the static pressure in the throat. The model only applies to gas flows and full compressible relationships are used. The model is able to handle choking in the throat of the element. The discharge coefficient, which is a function of the cross-stream Mach number, the pressure ratio and the length-to-diameter ratio, must be determined through linear interpolation from tables.

This type of element is used to model cooling orifices in the walls of aircraft engine combustion chambers. The Lucas correlation cooling orifice has the same modelling abilities as the Cooling Orifice element, except that the discharge coefficient is calculated by using the Lucas correlation.

The Restrictor with Discharge Coefficient element in Flownex can be used to represent a throttling process. An example of such a throttling process is flow through an orifice. The total pressure drop through a Restrictor with Discharge Coefficient element is equal to the dynamic pressure in the throat of the element. This implies that no static pressure recovery occurs downstream of the throat. The model applies to gas, liquid and two-phase flows. With regard to gas and two-phase flows, the model can handle choking in the throat of the element.

The Restrictor with discharge coefficient can be used to model :

• Leak flows
• Pipe breaks, etc.

Restrictor with Loss Coefficient

The total pressure drop through a Restrictor with Loss Coefficient element is equal to the loss coefficient times the dynamic pressure in the throat of the element. Pressure recovery may take place if the loss coefficient is smaller than 1. The model applies to gas, liquid and two-phase flows. With regard to gas and two-phase flows, the model is able to handle choking in the throat of the element.

The Pebble Bed Reactor element (Generation 2) is a simplified model of the pebble bed nuclear reactor. The purpose of the model is not to do detail reactor design, but rather to allow for the integrated simulation of the reactor together with the rest of the main power system within acceptable computer simulation times.

Advanced Pebble Bed Reactor (Generation 3)

The Advanced Pebble Bed Reactor model (third generation reactor model) is based on the fundamental equations for the conservation of mass, momentum and energy for the compressible fluid flowing through a fixed bed, as well as the equations for the conservation of energy for the pebbles and core structures. The equations are in a form that is suitable for incorporation in an integrated systems CFD code. This formulation of the equations results in a collection of one-dimensional elements (models) that can be used to construct a comprehensive multi-dimensional model of the reactor (two-dimensional axi-symmetrical model).

The elements account for the pressure drop through the reactor; the convective heat transport by the gas; the convection heat transfer between the gas and the solids; the radiative, contact and convection heat transfer between the pebbles and the heat conduction in the pebbles. Despite the increased complexity it retains the simplicity of the network approach.

The phenomena that cannot be simulated in the previous pebble bed reactor model (second generation) include the following:

• The presence of a central reflector column that implies that the core itself does not extend outward from the centre but has an inner and outer diameter.
• The addition and extraction of gas via purpose provided channels and/or leak flow paths along the inner or outer perimeters of the core.
• The simulation of heat transfer and fluid flow through porous and solid core structures surrounding the core.
• The simulation of fluid flow and heat transfer, including radiation and natural convection, in purpose provided cavities between core structures with a two-dimensional rather than one-dimensional nature.
• The ability to specify normalised radial power distribution profiles within the different axial layers in the core.
• The ability to take into account heat generation that may occur in any of the core structures.

Gearbox

Thermo-hydraulic components, such as turbo machines, generators, electric motors, pumps, etc., transfer mechanical power by means of a rotating shaft. The Gearbox element serves the purpose of facilitating speed ratios between these components. The Gearbox element may have an arbitrary number of load and power shafts, each with its own, unique and fixed gear ratio relative to the reference shaft.

The Flownex Shaft element serves as a connecting element to facilitate the interaction, i.e. the transfer of mechanical power, between rotating equipment. Furthermore, the Shaft element is used for performing steady-state and transient power matching between these components by allowing the user to specify certain power matching options in the input property pages. It must be remembered, however, that this is only applicable when a Gearbox element is not connected to the shaft, in which case the power matching is performed according to the Gearbox element specifications. The user can also specify a shaft inertia which is used when power-matching routines are done for transient events.

The Heat Transfer element in Flownex is used to model heat transfer in solid structures. The HT element is able to model conduction, convection and radiation heat transfer to and from solid structures. The HT element can consist of a number of material layers, each of which can be divided into a number of increments. Heat Transfer elements can have both thermal resistance and thermal inertia and can be connected to flow elements or flow nodes with either single- or two-phase flows.

The HT elements also include a variable heat transfer area in the component direction, Cross component heat transfer and radiation heat transfer between the end surfaces of two HT elements. An option was also included to allow radiation and convection heat transfer from the end surface of a HT element to ambient conditions.

The heat transfer component can be used as building blocks for:

• Boilers
• Superheater
• Heat Lost to atmosphere.
• Heat Steam RG
• Pipe wall and insulation layers
• Air heaters
• HRSG
• Condensers

Flownex has two purely Conductive Heat Transfer elements. The one is used only for linear, one-dimensional heat transfer that is only in one direction. The other element is called the cross conductive heat transfer element.

Convection elements serve as the heat exchange link between a fluid and a solid surface. It is possible to connect the convection element to either an un-incremented flow element or a flow node. The other side of the convection element would always be a solid node. A convection element cannot be connected between either two flow nodes or two flow elements or a combination of the two. For a convection element, the user should specify the heat transfer area, and either the Dittus-Boelter relationship or a local heat transfer coefficient.

Cross conductive heat transfer elements are used in conjunction with ordinary Conductive Heat Transfer elements to model heat transfer in a two-dimensional domain. Cross Conductive Heat Transfer elements are used to connect adjacent Conductive Heat Transfer elements to create such a two dimensional domain The Cross Conductive Heat Transfer element can be connected directly to a flow node, or an un-incremented flow element. The heat transfer they represent is only one-dimensional.

The Radiation heat transfer element is used in Flownex to model radiation heat transfer. The radiation can be between two solid nodes or between a solid node and a flow node or between two flow nodes.

Evaporator elements are mostly used in Cold Air Units to pre-cool the air before it enters the primary heat exchanger after leaving the compressor. The evaporator effectiveness is either specified as a fixed value or interpolated from a table that gives the effectiveness as a function of the air mass flow rate.

The Finned-Tube Heat Exchanger model can be used to simulate conventional and cylindrical type finned-tube complex heat exchangers.

Flownex uses the Effectiveness-NTU method to calculate the heat transfer in the Heat Exchanger element. This method offers many advantages for analysis of problems in which comparison between various types of heat exchangers must be made for the purposes of selecting the type best suited to accomplish a particular heat transfer objective. The major advantages of this method is the ability to calculate the heat transfer without knowing any of the outlet temperatures and no detail or complex geometry is needed in the calculations.

A recuperator is a heat exchanger used to recover thermal energy from a system by transferring heat that would otherwise be lost to a stream where the heat is required or where the energy would improve the system effectiveness. Similar to the Heat Exchanger element a recuperator is also modelled as a primary and secondary element representing a hot and cold fluid stream. The assignment order of either a hot or cold stream to the primary or secondary Recuperator elements is irrelevant. The recuperator is modelled in a discretised fashion rather than a lumped system. In the case of transient flow, the thermal inertia of the metal is taken into account when calculating the heat transfer to the two fluid streams. The inertia of the fluid is also taken into account when calculating the pressure drops through the hot and cold fluid passages in the case of transient flows. The Recuperator element can handle two-phase flow.

Shell-and-Tube Heat Exchanger

The Shell-and-tube heat exchanger consists of a shell containing a number of baffle plates to direct the flow in a serpentine path. The purpose of the baffles is to induce turbulence in the flow and thereby increase the shell side heat transfer coefficient. The shell contains a number of tubes where the second fluid flows through. The Shell-and-Tube Heat Exchanger element is only capable of handling two-phase flow for the tube side.

A compressor is a device used to add energy in the form of a pressure increase in a gas stream. Flownex has the capability to model axial, centrifugal and positive displacement compressors. The performance characteristics, i.e. the pressure ratio and the efficiency/corrected work, are determined by means of interpolation from values specified in a separate compressor data reference in the data reference library.

The compressor component can be combined with other components to model following:

• Compressor with Intercooler heat exchanger.
• Compressor with silencer (Orifice used to model silencer).
• HP and LP compressors
• Gas line compressor stations
• Air compressors

The Compressor Stage element in Flownex can be used to model axial compressor stages for which the characteristic performance charts are known. In terms of the performance of a compressor, a stage consists of a rotating blade row followed by a stationary blade row. These stages can be added in series (“stage stacking”) to form multistage axial compressors in which the interstage variables are known enabling subsequent modelling of bleed flows between stages. Each stage can be assigned its own performance characteristics (if known) or the same characteristics can be used for several stages. With the compressor stage model, the flow conditions before and after each stage are solved. The compressor stage element is capable of modelling both steady-state flow conditions as well as transient events, but only for compressible fluids with subsonic through-flow. During a transient the characteristics are assumed to be quasi-steady.

The compressor stage component combined with other components can be used to model the following:

• Bleed flows between stages
• Surge prediction on a stage by stage level

The pressure increase over a Fan or Pump element is determined by means of interpolation from the fan or pump curve (i.e. the Δp₀- Q curve where Δp₀ = pressure increase [kPa] and Q = volume flow rate [m³/s]). The model takes into account the effect of energy transfer to the fluid and the temperature increase through the fan or pump. Compressible and incompressible flows can be modelled.

Labyrinth Seal

The labyrinth seal is used in both hydraulic and compressible flow turbo machines for sealing purposes. In Flownex, the Labyrinth Seal element can be used to model two different types of labyrinth seals, namely the axial and the radial type of seals. The model can be used for straight and staggered seals. The model can also be discretised in detail or modelled as a lumped model.

Positive Displacement Compressor

This model applies to reciprocating compressors and is valid for compressible and incompressible flows. For single-phase flow, the user can specify a polytrophic coefficient, polytrophic efficiency, isentropic efficiency or isothermal efficiency. When a two-phase fluid is used, the user can only specify an isentropic efficiency.

Rotating Annulus

The Flownex Rotating Annulus element can be used to model the flow of fluid through an annulus with the inner cylinder rotating and outer cylinder stationary. The model applies to annuli with gap width to mean radius ratio smaller than 0.1. Typically the rotating annulus element is used to model flow through rotor-stator systems such as the bearings of turbo machinery.

Rotating Pipe

The Flownex Rotating Pipe element can be used to model flow through a pipe/conduit rotating about a centreline. The prime area of application of the Rotating Pipe element is the modelling of flow through the cooling flow channels found in the rotating disk and blades of gas turbines. The model is, however, not restricted to this particular area of application alone.

Simple Turbine

The Flownex Simple Turbine element can be used to model a pressure and temperature decrease using user specified values for a discharge coefficient or a loss and contraction coefficient and isentropic efficiency. It is basically a turbine with only one working point, i.e. constant isentropic efficiency and total pressure decrease. The model applies only to two-phase flows and can handle choking in the throat of the element.

Turbine

The performance characteristics of the Turbine element, i.e. the pressure ratio and the efficiency, are determined by means of interpolation from data specified in separate turbine charts in the Charts and Lookup Tables Library. The user is able to select a turbine as a master unit and then add other turbines, compressors and a generator to the shaft associated with the master turbine. This enables the user to construct composite turbo machines. The model is able to calculate the rotational speed transients during transient simulations. With regard to steady-state simulations, the model is able to power-match all the components on the shaft (associated with the master turbine) by either keeping the speed constant while varying the master turbine blade angle or keeping the blade angle constant while varying the master turbine shaftspeed.

The Turbine component combined with the correct performance characteristics can be used to model the following:

• Boiler Feed pump Turbine
• Steam Turbines
• Gas Turbines
• Combustion Turbines
• High Pressure (HP) Turbines
• Intermediate Pressure (IP) Turbines
• Low Pressure (LP) Turbines

Variable Speed Pump

 

The Variable Speed Pump/Fan element can be used to model a hydraulic pump and a fan (with negligible increase in gas density). The pressure increase over a Variable Speed Pump or Fan element is determined by means of interpolation from the fan/pump curves for different speeds of rotation (i.e. the Δp₀- Q curve where Δp₀ = pressure increase [kPa] and Q = volume flow rate [m³/s]). The variable speed pump or fan is a fan/pump that can have its speed adjusted during transient simulations. The variable speed pump can also be connected to a shaft.

 

 

The variable speed pump component combined with the correct performance characteristics can be used to model the following:

• Boiler Feed pump
• Condensate extraction pump
• Vacuum pump
• Reactor coolant pumps

The Basic Pipe element in Flownex can be used to model flow in round pipes. It takes friction, inlet and outlet losses into account. For more advanced options, like ducting with non-circular variable area, user specified secondary losses, pipe wall elasticity, dividing the pipe into increments etc, the Pipe element should be used.

The Bend element in Flownex can be used to model flow through short circular arc bends. It takes the friction within the bend, pressure losses associated with the curvature of the bend and the downstream, reattachment into account using an empirical gross secondary loss factor.

Pipe

The Pipe element in Flownex can be used to model flow in pipes and ducting with non-constant, arbitrary cross sectional area. It takes friction and secondary losses into account. In dynamic simulations, pressure waves due to sudden changes in boundary conditions can be modelled. In the case of incompressible liquid flow, this would represent the water hammer effect.

The Pipe component can be used to model any flow conduit such as ducts and pipes. This element calculates a Reynolds number dependent flow resistance and can also have heat added or removed from the working fluid.

Pipes are used in conjunction with other components as building blocks for most thermal subsystems:

Resistive Pipe

The Resistive Pipe element in Flownex can be considered a simple flow component that only takes the flow resistance into account to simulate pressure drop over a pipe.

The Resistive Pipe component can be used to model all of the resistance losses normally modelled as pipes or flow channels to reduce the computational time and complexity, especially in terms of the amount of geometrical inputs. The resistive pipe can therefore also be used to model any of the systems or sub-systems mentioned under the Pipe section.

Restrictive Duct

The Resistive Duct element was initially developed to model the flow resistance of coal mine tunnels. This element can, however, also be used for non-circular ducts such as air-conditioning ducts. The element is compatible with single-phase compressible and incompressible flows and can be subdivided into a number of increments.