Waterhammer

Waterhammer, Acoustic Waves, and Other Fast Transients

Waterhammer is an impulse load created by sudden changes such as valve opening or closing. The resulting pressure loads can have catastrophic effects on pumps, pressure transducers, turbines, and valves. Waterhammer events typically occurs over a millisecond time frame, but may spand several seconds in large systems. The ability of SINDA/FLUINT and FloCAD® to capture fast transients allows the modeling of waterhammer and other fast transient events such as acoustic waves propagating through ducts that contain compressible and even two-phase fluids.

Historically, the Method of Characteristics (MOC) was often used to predict wave propagation phenomena, while FloCAD uses a finite difference/volume approach. What is the difference? FloCAD may be optionally used to model wave propagation events, while MOC-based software can't be used for much else. MOC is inapplicable to phenomena such as slow transients (much less steady states), heat transfer, two-phase flows with phase change, mixtures, etc.

Shown below is a comparison between FloCAD and a MOC solution, as documented in Wylie and Streeter's Fluid Transients (1982 edition, Example 3-1). The RMS error is about 1%: on the order of the interpolation error. To enable an "apples to apples" comparison, default FloCAD features had to be turned off such as the automatic inclusion of frictional losses, heating effects, heat transfer, fluid properties, etc.

Chart showing Comparison between FloCAD and a MOC solution

Other validations have included comparisons with column separation events … a flashing phenomenon not easily handled by MOC-based programs. In fact, SINDA/FLUINT is capable of full two-fluid nonhomogeneous phasic nonequilibrium analysis if needed. What does that mean? It means FloCAD is the only commercial off-the-shelf program that is capable of modeling waterhammer and similar oscillations and instabilities in two-phase multiple-component flows.

FloCAD's implicit solution techniques enable it to take large time steps, and the availability of simplifying assumptions means that FloCAD is just as applicable to parametric sweeps of steady-state heat exchanger performance as it is to simulating detailed waterhammer events. In other words, FloCAD doesn't have to concentrate on higher order effects like fast transients. But such capabilities are there if needed.

dispersed vs. coalesced front

Tuesday, June 26, 2018, 1-2pm PT, 4-5pm ET

This webinar describes flat-front modeling, including where it is useful and how it works. A flat-front assumption is a specialized two-phase flow method that is particularly useful in the priming (filling or re-filling with liquid) of gas-filled or evacuated lines. It also finds use in simulating the gas purging of liquid-filled lines, and in modeling vertical large-diameter piping.

Prerequisites: It is helpful to have a background in two-phase flow, and to have some previous experience with FloCAD Pipes.

Register here for this webinar

FloCAD model of a loop heat pipe

Since a significant portion of LHPs consists of simple tubing, they are more flexible and easier to integrate into thermal structures than their traditional linear cousins: constant conductance and variable conductance heat pipes (CCHPs, VCHPs). LHPs are also less constrained by orientation and able to transport more power. LHPs have been used successfully in many applications, and have become a proven tool for spacecraft thermal control systems.

However, LHPs are not simple, neither in the details of their evaporator and compensation chamber (CC) structures nor in their surprising range of behaviors. Furthermore, there are uncertainties in their performance that must be treated with safety factors and bracketing methods for design verification.

Fortunately, some of the authors of CRTech fluid analysis tools also happened to have been involved in the early days of LHP technology development, so it is no accident that Thermal Desktop ("TD") and FloCAD have the unique capabilities necessary to model LHPs. Some features are useful at a system level analysis (including preliminary design), and others are necessary to achieve a detailed level of simulation (transients, off-design, condenser gradients).

CRTech is offering a four-part webinar series on LHPs and approaches to modeling them. Each webinar is designed to be attended in the order they were presented. While the first webinar presumes little knowledge of LHPs or their analysis, for the last three webinars you are presumed to have a basic knowledge TD/FloCAD two-phase modeling.

Part 1 provides an overview of LHP operation and unique characteristics
Part 2 introduces system-level modeling of LHPs using TD/FloCAD.
Part 3 covers an important aspect of getting the right answers: back-conduction and core state variability.
Part 4 covers detailed modeling of LHPs in TD/FloCAD such that transient operations such as start-up, gravity assist, and thermostatic control can be simulated.