Piping Simulation Comparison – Chapter 5

Transient flows

The dynamic (transient) simulation features of Flowmaster and Flownex include a lot of capabilities:

  • The ability to start the simulation from steady-state or specified initial conditions
  • Variable time-step sizes
  • Controllers allowing to change value as function of time or any other results available in the network
  • Open loop events can be specified at different time steps (e.g. Varying or fixing of variables; Switching controllers on or off; etc.)
  • Plot values of multiple parameters are displayed on the screen.

The transient capabilities of hydrosystem are more simple, only waterhammer effect is available actually.

We will show below an example in the automotive industry for Flowmaster, an ECS application for Flownex and a waterhammer for Hydrosystem.

Flowmaster – Cooling system modeling

FM Auto Design

Thermal Management is a significant challenge in the design of modern vehicles and a well designed cooling system is vital in overcoming this challenge. Designing and optimising for packaging space restrictions, powertrain architecture variations and thermal loads is essential.

The Cooling Systems Modelling packages from Flowmaster enable you to model your cooling system and optimise its design before building the real thing, whether you vary the design parameters of the thermostat to ensure a constant temperature in the cooling system or resize key components to packaging constrains.

Modelling Cooling systems in Flowmaster can allow you to evaluate the following:

  • The impact of components for different suppliers on the overall cooling system
  • The effects of different operating conditions; warm-up cycles in winter
  • The sizing of the expansion tank or de-gas bottle and its effects on performance

Engine heat rejection and its control are critical to engine performance and vehicle emissions.

When you model an engine cooling system in Flowmaster, you can easily determine:

  • Heat rejection based on engine speed and load
  • The thermal flow paths between the engines metal structure with the coolant and underhood air flow
  • That the rate of heat transfer is realistic
FM Auto Result
Temperature of Coolant in the Radiator Versus Temperature
of Air Across the Radiator Over Time

To help you do all of this Flowmaster comes complete with a wide range of standard and cooling specific components, including:

  • Engine components including thermal inertia, conduction, convection & radiation
  • Fans, Pumps & Valves
  • Pipes & expandable hoses
  • Heat Exchangers
  • Standard & User Definable Coolants

Once constructed, these models can be run under various conditions to simulate steady or transient conditions. This enables you to accurately predict coolant pressure, flow rates, temperatures and other performance parameters as well as optimise component sizes and understand the effect of component changes on the entire system. By linking your coolant model and a Flowmaster lubrication model, the effects of the engine cooling system on the lubrication system, during scenarios such as warm-up or standard industry drive cycles, can be assessed.

Flownex – Environmental control & life support systems

Aircraft operate at varying altitudes, thus resulting in changes in ambient pressure and temperature surrounding the aircraft. Environmental Control Systems (ECS) control the temperature, pressure and air flow into the aircraft, which is essentially a pressure vessel and includes the cockpit, cabin and interior compartments. ECS also performs cabin altitude and cabin pressure differential monitoring. Typical transport aircraft ECS comprises various systems performing functions such as bleed air supply, bleed air leak detection, air conditioning and distribution, avionics cooling, cabin pressure control and oxygen supply. The system may also encompass wing anti-ice systems.

In a typical ECS systems, bleed air is bled from a compressor stage in the aircraft engine. The bleed air pressure is controlled with a pressure regulating valve which may include reverse flow protection. Bleed air temperature is controlled via a fan air valve which controls the fan (cold) air flow through the pre-cooler, which is an air-to-air heat exchanger used to cool the bleed air. The bleed air is then conditioned, which involves the regulation of temperature and humidity of the air, and then supplied to the cockpit and the cabin zones at the required mass flow rate. Provision is also made for recirculation of a portion of the cabin air whilst maintaining the required oxygen level and removal of particulate and odours in the re-circulated air by means of filtering systems. A cabin pressure control system regulates the pressure within the cabin by controlling the outflow of air by means of one or more outflow valves and a control system.

Flownex can be used for the design, optimisation and simulation of the ECS. The components making up the ECS can be simulated with Flownex, which allows simulation from basic design to detail design of individual components up to systems level. Flownex’s ability to simulate bleed air from the compressor, through the pressure regulating valve, pre-cooler, air conditioning system including filers and dehumidifier through to cabin pressure control makes it ideally suited for design and optimisation of the complete system. Transient simulation capability allows simulation and design of the system parameters during transient events such as altitude change (change in system inlet pressure and temperature as well as humidity levels) under normal operating conditions as well as for abnormal (accident scenarios) conditions. Flownex has the ability to mix gasses such as air and water vapour, thus allowing the prediction of condensation and icing on components and can also be used for humidity control design to achieve the required humidity levels. The design of control philosophies for complete systems or individual components can be done while control philosophies for existing systems can be improved by means of simulation before hardware is fabricated.

Schematic of a typical transport aircraft ECS

The Environmental Control and Life Support (ECLS) systems aboard a manned spacecraft or space station are responsible for maintaining a liveable environment within a pressurized crew compartment. These subsystems are required to sustain a liveable environment by providing oxygen, drinking water, waste processing, temperature control, ventilation and CO2 removal.

ECLS system process flow diagram of the International Space Station

Typically most of these ECLS systems rely on thermo fluid cycles which include pressurized gas containers, filters, heat exchangers, humidifying/dehumidifying equipment, fans, control valves, pumps, storage tanks, piping and ducting as well as control equipment to manage and maintain the required conditions to sustain life.

Flownex can be used for the design, simulation and optimization of ECLS systems in spacecraft. Flownex has the ability to track the gas concentration of multi gas mixtures in a system, thus enabling the user to determine gas concentrations during transient events. Flownex also has the ability to mix gasses such as air and water vapour, thus allowing the prediction of condensation and icing on components and can also be used for humidity control design to achieve the required humidity levels. Flownex allows the simulation of multiple subsystems with each other consisting of multiple fluids (gas, liquid and two phase fluids) in a single network, thus assisting in the integration design of complete ECLS. Flownex can assist with the design and simulation of insulation systems in order to regulate, control and minimise temperature variations within spacecraft. The Flownex control library can be used to simulate and design control philosophies of the ECLS. The ability to simulate complete systems allows the user to simulate off design and accident conditions to determine subsystem interaction under abnormal conditions.

Hydrosystem – Waterhammer only

hydro WH
Surge Analysis

Transient process analysis (waterhammer calculation) is performed basing on classic waterhammer theory equations developed by Joukowsky. These equations present the simplified form of general equations of transient fluid flow in pipes for isothermal conditions at low Mach numbers and weak dependency of fluid density and sound velocity on pressure. Corresponding quasi-linear first order partial differential equations set (with pressure and velocity derivatives) is transformed using Riemann invariants and solved by means of finite difference method on a rectangular grid. An explicit running-count scheme is used with program-defined time step and corresponding pipe length step.

If outer diameter and modulus of pipe elasticity are entered, then at shock wave speed calculation  both isothermal liquid speed of sound and pipe material elasticity correction for thin-wall pipes are considered. Isothermal liquid speed of sound can be calculated by using WaterSteamPro, GERG-2008, STARS or SimulisThermodynamics libraries. Note that “Properties” library as well as default thermodynamic model applied in Simulis Thermodynamics is not able to calculate this parameter (in Simulis one can use the LKP model to calculate it). Thus in these cases (and also in case of manual fluid properties input) speed of sound is assumed to be equal to 1000 m/s for waterhammer calculation.

It is recommended to input the outer diameter and modulus of elasticity of pipe for thin-wall large dimeter pipes during the calculation because pipe elasticity correction could be significant in this case.

We also have to mention that both column separation and distributed cavitation are taken into account.

hydro WH2
Distributed vaporous cavitation during surge analysis

Pressure losses calculation is based on the hypothesis of quasi-stationary – i.e. transient process calculation uses the same friction factors and local losses coefficients as the steady state flow hydraulic calculation. Flow regulator model used in waterhammer calculation assumes that flow regulator is capable to maintain the same constant flow rate in the pipeline as in the steady state flow “no matter what” (that could really not be the case). If flow regulator is not capable to maintain the same flow rate it is recommended to calculate Kv of regulator in steady state process and then use this value in waterhammer calculation.

Flow parameters change in dynamic process can be driven by one or multiple (simultaneous or not) events. Current version of program considers the following events:

  • valve opening and closing
  • pump starting and shutting down.

Program allows to specify  the sequence, duration and type (instant, linear etc.) of  each of these events. Valve opening and closing is simulated as valve Kv change according to selected law – instantaneous or continuous (linear or piecewise smooth). Pump starting and shutting down model also assumes forced flow rate change in pump that obviously appear to be very simplified case and suitable mostly for the displacement pumps.

As mentioned above, waterhammer calculation is basied on steady state flow analysis – both isothermal (including diameter selection calculation) or thermohydraulic. In a latter case initial temperature change (and corresponding fluid properties change) in pipeline could also be taken into account. However the temperature change during transient process is not considered.

Terminal nodes are assumed to maintain  constant pressure or constant inflow/outflow. Boundary conditions for intermediate nodes are flow rate balance in node and equality of pressure values.

It is now possible to simulate a dumper (arrestor) as a gas-liquid cap that smooths the waves due to the gas fraction. The dumper is simulated as a special type of apparatus specified on the scheme with its geometric characteristics (total volume, gas volume and input orifice area).

The program also allows you to determine the unbalanced forces that occur in the elements of the pipeline during waterhammer. These additional forces are made up of forces arising due to additional pressure on those elements where pressure is not balanced (elements with a change in the flow direction, merging or splitting the flow, closed valves etc.), plus forces fluctuations arising from for internal friction in pipes and local resistances. The latter for the waterhammer case usually are an order of magnitude smaller than the first, and therefore, they are not currently taken into account in calculation in program.

Each situation is different

The needs of your company are not necessarily the same as another society, they may also have changed with years.

Do not hesitate to contact Fluids & Co to have a personalized study of your project.