Solution → Mass and heat imbalance
Purpose · Steps · Inputs and units · Limitations · Related topics
Purpose
Check mass conservation and, for supported thermal cases, the energy balance and thermal stopping criteria.
Steps
- Run or resume the case and inspect the mass imbalance indicator.
- With heat transfer enabled, inspect the heat indicator. Hover over it for rates in watts and the sample time.
- Compare the balance with residuals and monitored quantities. If a steady thermal run reaches its iteration limit before converging, review the case and resume with a larger limit when appropriate.
Inputs and units
| Input or value | Meaning | Units |
|---|---|---|
| Mass imbalance | Difference between total incoming and outgoing mass flow, displayed as a relative imbalance. | % |
| Heat imbalance | 100 × (heat input − heat output) / heat input. If input is zero, nonzero output is used for normalization. | % |
| Heat rates | Boundary conductive heat, including turbulent thermal diffusion, and thermal energy transported by flow relative to TRef. Values update at each solver step or steady iteration. | W in the indicator tooltip |
| Transient storage | Positive stored energy is included in output; released stored energy is included in input. | Included in the heat-rate accounting |
| Heat indicator colors | Absolute imbalance: green below 1%; orange from 1% to below 3%; red at 3% or above. | % |
Limitations
- A small imbalance is one convergence check; it does not establish mesh independence or solution accuracy. Positive heat imbalance means more energy enters than leaves.
- The heat monitor supports single-region buoyantBoussinesqSimpleFoam and buoyantBoussinesqPimpleFoam cases with constant material properties and without radiation or fvOptions source terms. Transient storage supports Euler and backward time schemes.
- Other energy equations, CHT, radiation, source terms, missing data, and zero thermal throughput display N/A until complete accounting is available. N/A does not mean zero error.
- A saved complete sample is restored when a project is reopened. Older results need to be resumed with the monitor enabled to obtain a sample.
- For supported steady thermal cases, automatic stopping checks the final residuals: combined velocity ≤ 1e-4 and temperature ≤ 5e-4, unless residualControl targets are specified. Pressure and turbulence residuals are also checked. Absolute heat imbalance must remain below 1% for five consecutive complete iterations. If the solver stops or reaches its limit first, the log reports that thermal convergence was not reached.