# Findings log

One entry per mismatch between the draft contract and an existing model, or between the two
reference codebases. Append, never silently resolve. Contract changes must appear here.

**Verdicts:** `contract flaw` (change the contract), `model flaw` (fix the model),
`family-specific` (legitimate difference; keep in the family namespace; test: does it
matter for cross-family comparison? If not, it stays family-specific).

**Format:** `F-NNN | area | what differs | verdict | action | status`

## Entries

Seeded by the Stage A side-by-side diff (2026-10-11). Sources compared:

- **HB** = the finned-tube code prototype (not public), at `d391da0`.
- **PB** = packed bed, `encapsulated_ltes` in [encapsulated-LTES](https://github.com/mmsg-warwick/encapsulated-LTES) at `023bf58` (clean).
  Models: `FullModel` (resolved capsule at every `x`) and `ReducedModel` (one x-averaged
  capsule).

| ID | Area | What differs | Verdict | Action | Status |
|---|---|---|---|---|---|
| F-001 | Coordinates | Flow coordinate is `z` in HB and `x` in PB (both on domain `"pipe"`). Storage: HB `y` on `"PCM vertical"` (cylindrical) and, in the mush model, `x` on `"PCM horizontal"`; PB `r` on `"capsule"` (spherical). PB's `x` clashes with HB's fin-gap `x`. | Flow coordinate: model flaw (PB). Storage domains: family-specific. | PB renames `x` to `z` (D12). Storage domain names are settled with the open naming item. | closed (F-039): the bed uses `z` on "flow path" (F-038) and `r` on "capsule" |
| F-002 | Parameter names | PCM properties: HB `"PCM solid phase conductivity [W.m-1.K-1]"`, `"PCM latent heat [J.kg-1]"`, `"PCM melting temperature [K]"`; PB has the same names without `"PCM "`. Flow: HB `"Inlet flow rate [m3.s-1]"` (volumetric, per pipe); PB `"Inlet velocity [m.s-1]"` (interstitial). `"Pipe length [m]"` is the bed length in PB. HB scales to the device with `"Number of pipes"`; PB has no cross-section, so everything is per unit bed area. | Contract flaw (no shared namespace yet) and model flaw in both (neither uses mass flow rate, D8). | Contract defines the material namespace and the device-level control (mass flow rate). PB needs a bed cross-section parameter so totals are in J and W. | closed (F-039): shared "PCM ..." names; mass flow rate control; "Bed cross-sectional area [m2]" |
| F-003 | Material classes | Not shared, despite DESIGN §1. HB: `PhaseChangeMaterial` with `PureMaterial` and `CompositeMaterial`, corners smoothed by `smooth_min`/`smooth_max` over `"PCM melting range [K]"` (default 0.5 K), `liquid_fraction(H)`, `k` linear in liquid fraction, `T = T_m` taken as solid, `H` scaled by `rho_L`. PB: `H2T`, `T2H`, `k` are methods on the parameter class, sharp `pybamm.minimum`/`maximum`, `k` piecewise with indicator functions, no liquid-fraction function. PB `T2H` gives `H = 0` at exactly `T = T_m` (both indicators false), and its guard `if self.T_m == T` compares symbols, so it never fires. Same underlying physics: `rho_L = rho_s L`, liquid sensible heat `rho_l c_p_l`. | Model flaw (PB); DESIGN §1 inaccurate. | Use HB's classes as the base (Stage B merge). Correct DESIGN §1 wording. | closed: one copy of the material classes (pytes.materials), used by both families |
| F-004 | Energy conservation | HB: exact, using the ledger. Sharp front uses the integral of `dH/dt`. Mush uses the integral of `dH/dt` between PCM and fin, and the same `h(T_c - T_f)` expression on both sides at the wall. Test tolerance 1e-9 of released energy. PB: the fluid source is `q = -k(H_surf) * BoundaryGradient(T_c)`, a separately discretised flux. The stored-energy ODE uses the extrapolated `T_out` (F-006) and the inflow is wrong (F-005). **Measured** with default `"Raul2018"`, 1 h, PyBaMM 26.7.1: max error in energy conservation / final stored energy = 3.7 % (`FullModel`), 4.3 % (`ReducedModel`). | Model flaw (PB). | Stage B: ledger coupling across the capsule surface (D6). | closed (F-039): the bed uses the ledger; conservation exact (4e-15) where PB was out by up to 2.9 % |
| F-005 | Inlet condition | HB works around the `pybamm.upwind` ghost node: `T_bc = (T_in + T_1)/2`, with constant extrapolation on `"pipe"`. PB uses a plain Dirichlet `T_in`, so the inflow is `2 T_in - T_1`. | Model flaw (PB). | Move the HB workaround into `compat` (tagged), and use it for both families via the shared fluid submodel. | closed: `pytes.compat.upwind` in the shared fluid, both families |
| F-006 | Outlet temperature | HB: value in the last cell (constant extrapolation), which is the value upwinding advects out. PB: default FiniteVolume extrapolation (linear), which is not the advected value. | Model flaw (PB). | Outlet from the fluid submodel only. Ties to the open question of deriving `T_out` from `h_out`. | closed: outlet is the advected last-cell enthalpy, `T(h_out)` (F-032), both families |
| F-007 | Power: definition, sign, units | HB `"Power [W]" = N rho_f c_p_f Q_f (T_out - T_in)`, positive on discharge, whole device. PB has no power variable. `"Stored energy per unit area [J.m-2]"` integrates `eps rho_f c_p_f u (T_in - T_out)`, which is positive on charge and per unit area. Both use `c_p dT`, not enthalpy. | Model flaw (both). | Stage A: power from `mdot (h_out - h_in)` via the fluid-property object (D8). PB gets this from the shared fluid in Stage B. | closed: power from enthalpy in the shared device energy, both families |
| F-008 | Energy variables | HB `"Thermal energy [J]"` = PCM + fins + **fluid**, relative to 0 K; `"Phase-change material thermal energy [J]"` includes fins; `"Released energy [J]"` is a solver state. PB `"Total enthalpy per unit area [J.m-2]"` = capsules + fluid, relative to 0 K; `"Variation in total enthalpy..."` relative to `T_0`. Neither uses the `T_low` reference (D14). Note: checklist item 7 says the finned pipe has no fluid inventory. That is outdated: the HB working tree includes it. | Model flaw (both). | Stage A: storage and total thermal energy with the zero at uniform `T_low`; HB "Released energy" becomes "Discharged energy". Update checklist item 7. | closed: storage and total thermal energy, zero at the reference temperature (F-030), both families |
| F-009 | Heat transfer coefficient | Both use the constant `"Heat transfer coefficient [W.m-2.K-1]"`. HB applies it at the pipe wall; the mush model puts a pitch's worth of wall heat through the fin root (`d_m/d_c` factor). PB applies it at the capsule surface, with specific area `3(1 - eps)/R`. Both define `"Heat transfer fluid conductivity"` and never use it. | Source: model flaw (both, D8). Where it applies: family-specific (coupling slot). | HTC comes from the fluid-property object (a constant for now). Keep `k_f` (maintainer, 2026-10-11): reserved for axial diffusion and the correlation hook. | closed: the coefficient is in the surface flux law (`NewtonCooling`, F-032), both families |
| F-010 | SOC and liquid fraction | HB `"State of charge"` = `"Average liquid fraction"`, a scalar. Sharp front: weighted by the composite annulus volume. Mush: weighted by the PCM gap volume, excluding fins. PB `"State of charge"` is a **field over `x`**: the capsule volume fraction with liquid fraction ≥ 1/2 (a phase indicator, not a liquid fraction). `"X-averaged state of charge"` thresholds the x-averaged `H`, which is not the average of the indicator. PB has no average liquid fraction. | Model flaw (both, D13). | Stage A: energy-based SOC on total energy (since removed, F-030). Contract fixes the definition of average liquid fraction: PCM-volume-weighted, fins and fluid excluded. | closed: SOC removed (F-030); average liquid fraction PCM-volume-weighted in both families |
| F-011 | Domain structure | Same pattern: `"pipe"` primary for the fluid; storage domain primary, with `"pipe"` secondary (tertiary for the mush model's PCM). | Family-specific (compatible). | None. Fits the D5 geometry switch. | closed |
| F-012 | Fluid equation | Both: state `T_f`, first-order upwind, no axial conduction (PB has it commented out). HB: `A_flow = pi R_f^2`, wall area per fluid volume `2/R_f`. PB: divides by `eps`, specific area `3(1 - eps)/R` per bed volume, `u` interstitial. Both match D5 with `A_flow` = pipe area or `eps` × bed area. | Family-specific. | Shared fluid submodel takes `A_flow` and the wetted perimeter from the family. | closed: one `AdvectedFluid` with `FlowPath` (flow area, number of paths) from each family |
| F-013 | Cut-off and operating mode | HB: `operating_mode` and `"Cut-off temperature [K]"`. PB: no events. | Model flaw (PB). | No longer applies: models have no cut-off events (F-031); PB needs none. | closed |
| F-014 | Storage temperature as a state | PB carries `T_c` as an **algebraic** variable (`T_c - H2T(H) = 0`), which makes the model a DAE with twice the storage unknowns. HB uses `T = H2T(H)` as an expression. | Model flaw (PB) under D7 (state list `[H]`). | Stage B: use the expression. | closed (F-039): temperature is the expression `H2T(H)`; the bed model is an ODE system |
| F-015 | Celsius variables | HB: shared helper `set_temperature_variables`. PB: written by hand for each variable. | Model flaw (PB). | Shared helper (D12). | closed: shared helper in both families |
| F-016 | Initial state | Both: uniform `"Initial temperature [K]"` mapped through `T2H`. | Model flaw (both, D14: initialise by SOC). | Stage A for HB. Initial temperature kept as an alternative (F-029). | closed: initial state by "Initial temperature [K]" (F-030), both families |
| F-017 | Defaults | Mesh: HB 40×40 (sharp front) and 20³ (mush); PB `r` 40, `x` 80. Solver: HB `IDAKLUSolver(rtol=1e-5, atol=1e-8)`, tuned and documented; PB default `IDAKLUSolver()`. Fluid extrapolation: see F-006. Quick-plot variables differ. Temperature scale 200 K, reference `T_m`, in both. Enthalpy scale: HB `rho_L` about mid-band; PB `T2H(T_0)` with no reference. | Mesh: family-specific. Fluid spatial method: contract (owned by the fluid submodel). Plot variables: D15. Solver and scaling: model flaw (PB). | Fluid spatial method set by the fluid submodel. Shared default plot variables. | closed (F-039): fluid method from `compat`; shared plot variables; bed mesh and solver chosen in F-039 |
| F-018 | Experiment runner | `step.py` control residuals recompute power as `C Q (T_out - T_in)` instead of reading `"Power [W]"`. The runner reads `"Inlet flow rate [m3.s-1]"` and `"Inlet temperature [K]"` directly, and removes the event by name. | Model flaw (HB runner). | Stage A: residuals use the model's power variable and the declared control set. Approved by the maintainer (2026-10-11). | closed (Stage A) |
| F-019 | Test coverage | HB: 72 tests (smoke, conservation, materials, experiments). PB: a version check and the template's entry-point tests, with no physics tests. PB has things HB lacks: experimental data (Nallusamy 2007), a mesh-refinement study, and a lower-fidelity `ReducedModel`. | Family-specific (coverage gap in PB). | Conformance suite covers PB in Stage B. `ReducedModel` is part of the PB hierarchy (maintainer, 2026-10-11); renamed `AverageCapsuleModel` on porting. | closed (F-041): the bed in the conformance suite (F-039); `ReducedModel` is the bed with `"storage": "averaged"` |
| F-020 | Naming and sign | PB `"Flux into phase-change material [W.m-2]"` is `-k dT/dr` at the surface, which is the flux **out of** the capsule. | Model flaw (PB). | Name by direction in the contract. | closed: the storage boundary flux is named by direction, into the storage ("<label> boundary heat flux [W.m-2]") |
| F-021 | Packaging | PB imports `scienceplots` at package import, so plotting is a hard dependency of the models. | Model flaw (PB), minor. | Plots go in the toolkit, as an optional extra. | closed: the bed models import no plotting package |
| F-022 | Control: flow rate | HB's control was `"Inlet flow rate [m3.s-1]"`, volumetric and per pipe. PyTES uses `"Mass flow rate [kg.s-1]"` through the whole device (D8: power is `mdot (h_out - h_in)`; D4: device-level interface). Default `1000 * 20 / 6e4` kg/s is the same flow. Step arguments `flow_rate` become `mass_flow_rate`; termination `flow_rate_above` becomes `mass_flow_rate_above`. | Contract flaw (HB contract). | Done in Stage A. The value no longer follows a change of fluid density, as a volumetric flow did. | closed |
| F-023 | Variable renames | `"Released energy [J]"` → `"Discharged energy [J]"`. `"Thermal energy [J]"` (from 0 K) → `"Total thermal energy [J]"` (zero at uniform `T_low`); new `"Storage thermal energy [J]"`. `"Phase-change material thermal energy [J]"` is now from `T_low`, and in the mush model it is **PCM only** (it included the fins); the fins have `"Fin thermal energy [J]"`. Removed: `"Inlet flow rate [m3.s-1]"`, `"Heat transfer fluid heat capacity per unit flow rate [J.m-3.K-1]"`. New: inlet/outlet specific enthalpy, `"Heat loss power [W]"`, `"Thermal capacity [J]"`, `"Initial uniform temperature [K]"`, `"Initial uniform liquid fraction"`. | Contract flaw (HB names), per D12. | Done in Stage A; see `contract.md` §3. | closed |
| F-024 | Initial state and window | `"Initial temperature [K]"` is removed. New device parameters: `"Lower temperature limit [K]"` (default 10 °C), `"Upper temperature limit [K]"` (70 °C), `"Initial state of charge"` (1). With these defaults the initial state is the old one (uniform 70 °C, liquid). | Contract flaw (HB), per D14. | Done in Stage A, then superseded by F-030: the window is gone and the initial state is by temperature only. | closed |
| F-025 | SOC values | HB's `"State of charge"` was the average liquid fraction; it is now energy-based on total energy (D13). Values differ from HB's: the sensible heat counts, and SOC reaches 0 only at uniform `T_low`. `"Average liquid fraction"` is unchanged (reference match to 1e-7). | Contract flaw (HB), per D13. | Done in Stage A, then superseded by F-030 (no SOC). | closed |
| F-026 | PyBaMM 26.10 | From 26.10, `pybamm.Solution` has `__slots__`: HB's runner fails setting `step_description` (`.cycles` and `.steps` still work). Found running the reference on 26.10. | Contract-neutral; upstream change. | Workaround in `pytes.compat.solution`, read with `pytes.experiments.step_description`. | closed; superseded by F-057 |
| F-027 | Reference agreement | PyTES reproduces HB `d391da0` to the solver tolerance: at the default `rtol=1e-5`, outlet temperature within 1.1e-3 K and power within 2e-5 relative; at `rtol=1e-9`, within 3e-7 K (same discrete equations). The experiment matches to 1e-9 relative. HB itself moves by 1.7e-5 relative between PyBaMM 26.7 and 26.10. | None (agreement). | Pinned in `tests/data/reference_prototype_d391da0.npz`; its generator, which needs the non-public prototype, was removed on 2026-10-17. | closed |
| F-028 | Submodel interface | Submodels were plain objects with hand-written `requires`/`provides` and PyTES's own two-phase build (fundamental, then coupled variables). They are now small `pybamm.BaseModel`s that read other submodels' outputs as `pybamm.CoupledVariable`; `requires`/`provides` are read from the code, the model merges them with `pybamm.BaseModel.update`, and storage owns all its boundary conditions (couplings provide `"<label> boundary heat flux [W.m-2]"` or `"<label> boundary temperature [K]"`). PyBaMM resolves coupled variables only with `Discretisation(resolve_coupled_variables=True)`, which `pybamm.Simulation` does not set, so models resolve them when built (`compat.resolve_coupled_variables`). Reference agreement unchanged. | Contract flaw (submodel interface), at the maintainer's request (2026-10-12). | Done. The check "undeclared read" is replaced by a domain check and a cycle check. | closed |
| F-029 | Initial state by temperature | Models take `initial_state="state of charge"` (default) or `"temperature"`, alongside `operating_mode`. The temperature mode reads `"Initial temperature [K]"` (default 70 °C, the same state as SOC 1) and starts uniform at that temperature, solid at `T_m` (HB's convention). Both modes provide `"Initial uniform temperature [K]"` and `"Initial uniform liquid fraction"`, so the submodels and the experiment runner are unchanged. A model argument rather than a parameter-value switch because the SOC of a temperature depends on the model's capacity, so neither parameter can be given as an expression of the other in a shared parameter set. | Contract addition (D14), at the maintainer's request (2026-10-14). | Done. New conformance check `check_initial_temperature`. | superseded by F-030 |
| F-030 | State of charge and temperature window | Removed: `"State of charge"`, `"Thermal capacity [J]"`, the optional `"Storage state of charge"`, the parameters `"Lower temperature limit [K]"`, `"Upper temperature limit [K]"` and `"Initial state of charge"`, the `initial_state` model argument (F-029), and the terminations `state_of_charge_below/above`. Added: `"Reference temperature [K]"` (energy zero; default 10 °C, the old lower limit, so energies are unchanged) and `pytes.energy_between(model, T_a, T_b)`. The initial state is uniform at `"Initial temperature [K]"` (default 70 °C, the old SOC 1). The conformance check `check_soc_end_points` becomes `check_initial_state`. Reference agreement (F-027) unchanged; `energy_between(10 °C, 70 °C)` equals the old capacity. | Contract flaw (DESIGN D13, D14, maintainer, 2026-10-14): a SOC between 0 and 1 needs a "full" energy the device does not have, and setting an initial SOC is ill-posed. | Done. | closed |
| F-031 | Cut-off in the model | Removed: the `operating_mode` model argument, its event and the parameter `"Cut-off temperature [K]"`. The models have no events; the runner no longer filters one out. | Contract flaw (DESIGN D11, maintainer, 2026-10-14): the model is the same for charge and discharge, and two places to stop a simulation can disagree. | Done. Cut-offs are experiment terminations. | closed |
| F-032 | Fluid state, property interface and heat transfer coefficient | The fluid state is now `"Heat transfer fluid specific enthalpy [J.kg-1]"` (was the temperature), scaled like the temperature so that the scaled unknowns are unchanged for a constant-`c_p` liquid; the temperature is `T(h)` and the outlet temperature `T(h_out)`. Fluid-property functions take the state `(h, p)` (were `(T, p)`), plus `h(T, p)`. `heat_transfer_coefficient` is removed from the fluid-property object: couplings take a `SurfaceFluxLaw` (`pytes.submodels.flux_laws`), default `NewtonCooling` with the same `"Heat transfer coefficient [W.m-2.K-1]"`. New required variable: the fluid specific enthalpy. Results: sharp front unchanged to 1e-9; mush within solver tolerance (3e-4 K at `rtol=1e-5`, 2e-8 K at `rtol=1e-10`, same equations). Reference agreement (F-027) unchanged. The mush end-of-discharge power changed sign within solver error (outlet within 1e-5 K of the inlet), so the discharge test now solves at `rtol=1e-8, atol=1e-10` and keeps its strict check `power > 0`. | Contract change (DESIGN D5, D6, D8, maintainer, 2026-10-14). | Done. A test checks that a custom (cubic) flux law conserves energy exactly. | closed |
| F-033 | Fins and the fin-root coupling | `SensibleConduction` (fin temperature as the state) is removed: the fins are `EnthalpyStorage` of the sensible `SensibleMaterial`, with `"Fin enthalpy [J.m-3]"` as the state, scaled like the old fin temperature. `EnthalpyStorage` gains an optional volumetric heat source and provides `"<label> conduction heating rate [W.m-3]"`; it provides the liquid-fraction variables only for phase-change materials. `FinRootCoupling` is now a ledger (a `LedgerWallCoupling` with flux factor `d_m / d_c`): the fluid's heat is minus the fins' integrated conduction heating rate, which stays exact for a non-constant conductivity. All ledger couplings use the conduction heating rate, which equals `dH/dt` without a source. `SensibleMaterial`'s conductivity parameter is now `k_param`, and `k(H, states)` a method, as for the other materials. Results: mush within 2.6e-8 K of the previous version at `rtol=1e-10` (same equations, as `k` is constant); reference agreement (F-027) unchanged. | Contract change (DESIGN D5, D6, maintainer, 2026-10-14). | Done. | closed |
| F-034 | Composition API | New: `pytes.ComposedModel(submodels, param, name=..., defaults_from=..., geometry=..., submesh_types=..., var_pts=..., spatial_methods=..., solver=..., parameter_values=..., quick_plot_variables=...)` builds a model from submodels with the built-in wiring checks, and raises if the geometry, submesh types or spatial methods miss a state domain; `BaseModel.print_submodel_info()`. Reusing submodels does not modify the model they came from (tested). The `options` dict is deferred to the first real variant. | Contract addition (DESIGN D3, maintainer, 2026-10-14). | Done; example notebook `examples/composing_models.ipynb`. | closed |
| F-035 | Melting range and smoothing | `"PCM melting range [K]"` was the width over which the corners of `H(T)` were smoothed; it is now the physical melting range (solidus `T_m - range / 2` to liquidus `T_m + range / 2`, `H` linear in `T` between them with the latent heat plus the mean solid-liquid heat capacity), and the smoothing has its own parameter, `"PCM smoothing width [K]"`. Defaults: range 0, smoothing 0.5 K, which reproduce the previous relations exactly (term by term), so results and the reference agreement (F-027) are unchanged. Both may be 0 (the 1e-9 K floor on the smoothing keeps derivatives finite; tested). Removed, as a melting range now fixes the liquid fraction of a uniform state: `equilibrium_enthalpy(T, phi)` (use `T2H`), `"Initial uniform liquid fraction"`, and the `phi` argument of `uniform_energy`. `validate_parameter_values` takes a list of non-negative names. Users who set `"PCM melting range [K]"` for smoothing must now set `"PCM smoothing width [K]"`. | Contract change (DESIGN D7, D14, maintainer, 2026-10-14). | Done. Tested: mushy-zone relations, finite derivatives with no smoothing, energy conservation and initial states with a 2 K range. | closed |
| F-036 | Power steps | `ConstantPowerInletTemperature(power, mass_flow_rate)` and `ConstantPowerFlowRate(power, inlet_temperature)` are replaced by one `ConstantPower(power, inlet_temperature=... \| mass_flow_rate=...)`, which adjusts the control left free (`step.adjusted`) and raises unless exactly one is given. The old names read as what is held but named what is adjusted. Results unchanged (reference experiment F-027 passes). No deprecation aliases (pre-v0.1). | Contract change (DESIGN D10, maintainer, 2026-10-14). | Done; example notebook `examples/experiments.ipynb` (the runner had none). | closed |
| F-037 | Temperature differences | Names containing "temperature difference" are K-only: `pytes.variables.is_temperature_difference`, `add_temperature_difference_variables` (no twin), and `add_temperature_variables` now refuses such names. `check_required_variables` exempts them from needing a `[degC]` twin and fails if one has a twin. No model provides one yet. | Contract change (DESIGN D12, maintainer, 2026-10-14). | Done. | closed; the helpers are replaced by `add_unit_twins` (F-043) |
| F-038 | Flow domain and "tube" | The shared flow domain is `"flow path"` (was `"pipe"`, which does not describe a packed bed), coordinate `z`, in every family. The finned family uses "tube", the usual heat-exchanger word: subpackage `pytes.models.finned_tube` (was `finned_pipe`), `FinnedTubeParameters`, `BaseFinnedTubeModel`, parameters `"Tube radius [m]"`, `"Tube length [m]"`, `"Number of tubes"` (were `"Pipe ..."`, `"Number of pipes"`), notebook `examples/finned_tube_models.ipynb`. Model classes unchanged. Earlier findings keep the old names. | Contract change (DESIGN D12, maintainer, 2026-10-15), before the packed-bed port. | Done. | closed |
| F-039 | Packed-bed port | New family `pytes.models.packed_bed`: `PackedBedModel` (fluid through the voids, flow area `eps A`; `EnthalpyStorage` of the PCM on a spherical "capsule" domain at each `z`, exchanging heat at `r = R`; `LedgerWallCoupling` with Newton's law; `DeviceEnergy`; `PerAreaTotals`), parameters `"Bed length [m]"`, `"Bed cross-sectional area [m2]"`, `"Bed porosity"`, `"Capsule radius [m]"` (outer radius) and the shared `"PCM ..."` names; sets `"Nallusamy2007"` (default; tank 360 mm x 460 mm from the article) and `"Raul2018"` (1 m2 placeholder area). The conformance suite passes unchanged. **Comparison with PB `FullModel` at `023bf58`** (Nallusamy, sharp melting, both on 40 x 80, `rtol=1e-8`): energy conserved to 4e-15 (PB: up to 2.9 %); outlet agrees to 3 mK and stored energy to 0.03 % at 4 h; up to 4.6 K apart while the front leaves the bed (about 22 min). PB's outlet extrapolation explains at most 0.5 K and its inlet condition about 0 (diagnostic runs); the rest is radial mesh error in both codes, which converge to the same answer from opposite sides (at `r` = 40 each is about 2.5 K from PyTES at `r` = 160) and slowly, because of the melting front inside the capsule. Default mesh 40 x 80 (3.5 s; about 1.8 K at most and 0.24 K on average in the outlet, 1 % in stored energy). **Heat transfer coefficient:** PB's parameter set has 100 W.m-2.K-1 but its validation notebook uses 1000; a sensitivity check against the Nallusamy data (not a fit: every other parameter is PB's set unchanged) gives an RMS error in the fluid temperature of 1.4 K (100), 1.1 K (300) and 1.1 K (1000), and about 4.6 K in the PCM at 0.8 R for all. The default stays at PB's parameter-set value, 100, which is not the best of the three; whether PB's set was itself tuned to these data is for the maintainer to confirm. Data copied to `examples/data/` with provenance; notebook `examples/packed_bed_models.ipynb`. | Port (Stage B). The `h` discrepancy is a model flaw in PB (set and validation disagree). | Done; the averaged variant (F-019) and the benchmarks follow. | closed |
| F-040 | Analytical benchmarks (packed bed) | New `pytes.validation` with Schumann's solution (`schumann(xi, eta)`, checked against its own equations) and the quasi-steady melting of a sphere (`sphere_melting_time`). Benchmarks in `tests/validation/`, both composed from the packed bed's submodels with `ComposedModel`: (1) **Schumann**: capsules of a sensible material (Biot 0.008), NTU 5.5; the outlet temperature is within 2.4 % (200 cells) and 1.3 % (400 cells) of the temperature step, halving with the cell size (first-order upwinding; tested: < 2 % at 400 cells and a ratio of 2 ± 20 %). (2) **Spherical Stefan**: one capsule at its melting temperature, surface 1 K above (Stefan number 0.011); with 160 cells, the time to melt 50 %, 75 %, 90 % and 99 % is within 0.9 % of the quasi-steady solution (tested at 2 %). The first 10-25 % of melting is not compared: with a sharp melting point the front crosses the large outer cells one at a time and the melted fraction rises in steps (up to 50 % off on coarse meshes). | Validation (DESIGN D1, Stage B criteria). | Done; notebook `examples/validation.ipynb`. | closed |
| F-041 | Options and the averaged storage | First option (DESIGN D3): `"storage": "resolved" \| "averaged"` in all three models. `BaseModel` gains `options`, `allowed_options` (first value the default), `process_options`, `check_options` and `set_spatial_variables` (spatial variables are made after the options, as an averaged storage's exclude the flow path), and `pytes.OptionError`. `StorageGeometry(flow_path=...)` marks a storage averaged along a flow path; `LedgerWallCoupling` (and so `FinRootCoupling`) then uses the fluid state averaged along the flow path and spreads the heat uniformly along it, still exact. In the mush model the fin-PCM unit is averaged, fins and PCM separate (maintainer). Conformance runs over every option combination (`tests/conftest.py::MODELS`, 6 variants); all pass unchanged. Averaged agrees with resolved when the fluid is uniform (100 x flow: < 0.01 % in stored energy) and differs as a front forms (default flow: 8 %, 8 % and 2 % for sharp front, mush and bed), 3-4 times faster. **Known artefact, inherited from PB:** with the uniform spreading of the averaged coupling, the fluid near the outlet loses heat it does not have early in a charge, and the default bed's outlet dips to 27 °C (initial 32 °C, inlet 70 °C); energy stays exact, so conformance does not see it. Kept, as the maintainer's published reduced model (decision 2026-10-15, DESIGN D4); the local-driving-force alternative is recorded there. | Contract addition (DESIGN D3, D4, maintainer, 2026-10-15); closes F-019 (PB `ReducedModel` = bed with `"storage": "averaged"`). | Done. | closed |
| F-042 | Storage domain names | `"PCM vertical"` → `"storage depth"` (`y`, into the storage from the heat-exchange wall) and `"PCM horizontal"` → `"fin gap"` (`x`, between two resolved fins). The old names described a figure, not the geometry; the new ones also fit a future Cartesian channel ("annulus" was rejected for that reason). Coordinates unchanged. Earlier findings keep the old names. | Contract change (DESIGN D12, maintainer, 2026-10-15). | Done. | closed |
| F-043 | SI units and unit twins; variable tiers | Submodels define every variable in SI units; `pytes.variables.add_unit_twins`, run in `BaseModel.build_model`, generates `[degC]`, `[kW]` and `[kWh]` twins of every `[K]`, `[W]` and `[J]` variable (not temperature differences) and rejects twins written by hand. Replaces `add_temperature_variables` and `add_temperature_difference_variables` (F-037) and the hand-written `Power [kW]` and `Discharged energy [kWh]`; new twins in every model, e.g. `Total thermal energy [kWh]`, `Heat loss power [kW]`. `check_celsius_twins` becomes `check_unit_twins`. Extra variables cost nothing measurable (build and solve times unchanged; PyBaMM evaluates variables on request). Tiers: `Heat transfer fluid thermal energy [J]` is required; `Mean storage temperature [K]` dropped (undefined, unused); `Pressure drop [Pa]` reserved (`RESERVED_VARIABLES`). | Contract change (DESIGN D12, maintainer, 2026-10-16). | Done. | closed |
| F-044 | Parameter names; time twins | Rule (DESIGN D1): material and fluid properties start with their layer prefix, geometry with the geometric object, never a material prefix; no family prefix. Renamed: `"PCM outer radius [m]"` → `"Storage depth [m]"` (geometry, not a PCM property; now the extent of the storage from the tube wall, so the outer radius is `R_f + depth`, which also fits a future planar geometry; default 6.75 mm, the same geometry) and `"Fin spacing [m]"` → `"Fin pitch [m]"` (it is the centre-to-centre distance). `Time [min]` and `Time [h]` are now generated twins of `Time [s]` (`UNIT_TWINS` allows several twins per unit; `twin_name` becomes `unit_twins`). | Contract change (DESIGN D1, D12, maintainer, 2026-10-16). | Done. | closed |
| F-045 | Per-submodel parameters | `BaseSubModel.parameters`: the parameter names a submodel uses, read from its equations, conditions, variables, events and state scales (`parameter_names` helper). `BaseModel.geometry_parameters`: those in the default geometry (domain lengths, which enter through the mesh, not the equations). Both shown by `print_submodel_info`. A test checks that every parameter in each model's default set is attributed, except the fluid conductivity and viscosity (hooks). Closes the open metadata item: requires, provides and parameters read from the code; assumptions declared. | Contract addition (DESIGN D3, maintainer, 2026-10-16). | Done. | closed |
| F-046 | Surface flux law interface | `SurfaceFluxLaw.heat_flux(fluid, h_f, T_f, T_s)` becomes `heat_flux(fluid, fluid_state, T_s)`, with `pytes.submodels.FluidState` (`h`, `T`, `mass_flow_rate` of the whole device, `p` reserved, default None): new fields never break existing laws. Geometry a law needs goes to its constructor, from the family. Couplings now also read `"Mass flow rate [kg.s-1]"`. Results unchanged (Newton's law ignores the flow). Closes the last open item of §4 for Stage B. | Contract change (DESIGN D6, maintainer, 2026-10-16). | Done. | closed |
| F-047 | Stage C: plots, design study, packaging, CI | `pytes.plotting` (`plot` extra; not imported by `import pytes`): `plot_summary`, `plot_power_against_energy`, `plot_fluid_profiles`; power and energy axes include zero without an offset (a held power shows as a line at its value). Notebook `examples/design_study.ipynb`: a finned-tube unit and a packed bed with the same PCM and storable energy (sized with `energy_between`) on the same 5 kW test. Packaging: `requires-python = ">=3.10,<3.16"` (PyBaMM's range), SciPy declared (used by `pytes.validation`), classifiers. CI (`.github/workflows/tests.yml`, not yet run: the repository has no remote): pre-commit; tests on Python 3.10 to 3.15 against the latest PyBaMM release (3.15 non-blocking: PyYAML has no 3.15 wheel and its source build crashes); slow tests; PyBaMM `main` non-blocking. Locally: 263 tests pass on 3.10 and 3.14. Pre-commit's ruff pinned to the dev ruff (0.17.0; 0.6.9 formatted differently); its first run with the new pin could not be done here (GitHub blocked). PyBOP example dropped (maintainer). | Stage C (DESIGN §5, D15, D16). | Done. CI runs on every push since the repository was hosted (2026-10-16); 0.1.0a1 and 0.1.0a2 are released. | closed |
| F-048 | Default parameter sets | The families' default parameter values become the named compositions of `pytes.parameter_layers`, with values checked against the literature by the maintainer (2026-10-21). Against the earlier flat sets: water at 20 °C (CRC) has density 998.21 kg.m-3 and specific heat capacity 4184.1 J.kg-1.K-1, for 1000 and 4186; aluminium 897 J.kg-1.K-1, for 900; the PCM smoothing width is 0.01 K, for 0.5 K; "Nallusamy2007" flows at 1/30 kg.s-1 (2 l/min), for 0.0331 kg.s-1 from an interstitial velocity; "Raul2018" has its bed cross-section, pi 0.085² m², for a 1 m² placeholder, and 8.2 l/min of Hytherm 600 (0.0985 kg.s-1), for an interstitial velocity of 1 cm.s-1 over that placeholder; the finned-tube default becomes an illustrative one-tube battery (0.02 kg.s-1 of water, PCM 60). With all these changes, the Nallusamy (2007) fluid temperatures fit the measurements as before: 0.9 K to 2.0 K RMS, for 0.9 K to 1.9 K. | None (data, not contract: names, units and signs are unchanged). | The layer values are used. The reference results are regenerated from PyTES by `tests/data/make_reference.py` into `tests/data/reference_regression.npz`, a regression test; the prototype comparison of F-027, which needed the earlier finned-tube set, is retired with `reference_prototype_d391da0.npz`. The experiment tests' powers are scaled to the new battery. | closed |
| F-049 | Surface flux law | Flow-dependent heat transfer coefficients (issue #6). New option `"heat transfer coefficient"`: `"constant"` (default, unchanged results) or a Nusselt correlation, `"Gnielinski"` in the finned tube and `"Wakao-Kaguei"` in the packed bed (`pytes.submodels.NusseltCorrelation`, `Gnielinski`, `WakaoKaguei`). The correlation constants are part of each law, so there are no new parameter names; the geometry (tube radius and number of tubes; capsule radius and bed area) comes from the family. `SurfaceFluxLaw.variables` is a new hook, and the coupling now provides `"Heat transfer coefficient [W.m-2.K-1]"` in every model, plus `"Surface Reynolds number"`, `"Surface Prandtl number"` and `"Surface Nusselt number"` with a correlation. `"Heat transfer fluid viscosity [Pa.s]"` becomes a required name of a heat transfer fluid layer (it was optional, unused). `NewtonCooling` gains `heat_transfer_coefficient(fluid, fluid_state)`. In the default sets the correlations give: finned tube Re 4244 (transitional), h 2278 W.m-2.K-1 against the constant 1000; Nallusamy2007 Re 18, h 152 against 100, with the fluid temperatures fitting the measurements slightly better over 4 h (0.7 K to 1.8 K RMS, for 0.9 K to 2.0 K); Raul2018 Re 6.9, h 113 against 100, below the fitted range of Wakao and Kaguei (Re > 15). | Contract flaw (additions only: an option value per family, variables, a required fluid name). | Implemented; conformance runs over every option combination (12 variants). The constant coefficients of the default sets are kept, as the default option. Correlations are registered by flow geometry (maintainer, 2026-10-22): `pytes.submodels.register_correlation(name, nusselt, flow)` adds one, from PyTES or a user, as an option value of the models of that flow (`TubeCorrelation`, `PackedBedCorrelation` supply the geometry), and `pytes.conformance.check_model` checks a model using it (DESIGN D6). | closed |
| F-050 | Options | The option `"storage": "resolved" \| "averaged"` (F-041) is renamed `"storage along flow path"`, with the same values (maintainer, 2026-10-22). `"storage"` named a slot of `model.submodels`, not what the option varies; the lumped capsule (issue #5) adds a second option for the variation inside a storage element, `"storage element"`, and the two names now say which direction each one is about. A clean break, as PyTES is in alpha: the old name raises `pytes.OptionError` naming the new one (`BaseModel.renamed_options`). Results are unchanged. | Contract flaw (an option name). | Renamed in the models, tests, notebooks, README, getting-started page, contract and DESIGN D3, D4. Breaking changelog fragment. | closed |
| F-051 | Storage, surface flux law | Lumped capsule (issue #5). New packed-bed option `"storage element": "resolved" \| "lumped" \| "lumped with Jeffreson"` (the correction was first a separate option, `"internal resistance"`, which needed a lumped capsule; folded into one value so that no combination is invalid, maintainer, 2026-10-23). New `LumpedStorage` and `LumpedGeometry` (one enthalpy per capsule, `dH/dt = (3 / R) q`), providing the same variables as `EnthalpyStorage`; the ledger coupling is unchanged and energy is conserved exactly. Every storage now provides `"<label> thermal conductivity [W.m-1.K-1]"`. Surface flux laws gain an optional `coupled_variables` (the variables they read), which the coupling requires; the new `Jeffreson` law (`h_eff = h / (1 + Bi / 5)`, `Bi = h R / k`; Jeffreson, AIChE Journal 18, 1972, doi 10.1002/aic.690180225, checked by the maintainer) wraps any Newton law and adds `"Effective heat transfer coefficient [W.m-2.K-1]"` and `"Surface Biot number"`. Results: on Schumann's benchmark the lumped sensible bed is as close as the resolved one (1.3 % of the step on 400 cells, all of it upwinding error). Against the resolved capsule in the Nallusamy (2007) charge, the largest error in stored energy, over its change, is 48 % (Jeffreson 20 %) with the real PCM (`Bi` 7 to 18), 1 % (0.3 %) with `k` = 10 W.m-1.K-1 and 0.1 % (0.04 %) with `k` = 100: lumping the default capsules is not accurate. | Contract flaw (additions only: an option, a storage variable, a flux law hook). | Implemented; tests against Schumann, against the resolved capsule as `Bi` falls, and of Jeffreson's coefficient; notebook `examples/lumped_capsule.ipynb`. | closed |
| F-052 | Fluid numerics | A second-order upwind scheme for the fluid (issue #7), the default (maintainer, 2026-10-23): `pytes.compat.LimitedFiniteVolume`, MUSCL with the van Albada limiter applied to gradients, so it works on non-uniform meshes; the inflow and outflow faces stay first order, and for the gradients the inflow value sits on the inflow face and the last cell's value on the outflow face, so a steady linear profile is exact. `compat.fluid_spatial_method()` returns it, and `fluid_spatial_method("first order")` PyBaMM's upwinding. Energy is conserved exactly (the flux still telescopes), on uniform and stretched meshes, and no new extremum appears. Measured: on Schumann's benchmark the largest error in the scaled outlet temperature is 1.9 %, 0.8 %, 0.46 % on 25, 50, 100 cells against 12 %, 7.5 %, 4.3 % with first order (1.3 % on 400), at 1 to 4 times the solve time per cell; on the models the cost per cell is about the same. In the default finned-tube discharge the outlet error falls about 4 times per halving of the cells (0.15 K on 20 cells against 0.82 K). In the default packed-bed charge, on the earlier default mesh (40 by 80), 3.7 K at most and 0.22 K on average in the outlet temperature against 8.0 K and 0.34 K; the packed bed's `default_var_pts` docstring had claimed 1.8 K at most, against a first-order reference that was itself far from converged along the bed. Default meshes rebalanced so that each direction contributes about the same error: sharp front 20 by 20 (from 40 by 40; `y` 0.054 K mean and 0.11 % energy, `z` 0.066 K and 0.12 %, about 1 s); packed bed 40 by 40 (from 40 by 80; `r` 0.27 K and 1.8 %, `z` 0.063 K and 0.33 %, a 5.4 K spike as the front leaves the bed, about 1.5 s); mush unchanged (`z` 0.061 K and 0.11 %). The Nallusamy (2007) fluid temperatures fit the measurements to 0.9 K to 2.1 K RMS, for 0.9 K to 2.0 K. | Model flaw (numerics: first-order diffusion), and a documentation error. | Implemented as the default; regression reference regenerated; tests (steady linear profile, a bounded and sharper step, energy on a stretched mesh in every model, Schumann with either scheme); notebook `examples/advection_scheme.ipynb`; DESIGN D16. Breaking changelog fragment (results change). | closed |
| F-053 | Renames | PyTES had no way to deprecate a name: renames were clean breaks (F-036, F-050), and the only record of an old name was `BaseModel.renamed_options` (F-050), for options. Issue #17 adds `pytes.deprecation`: one registry of renames (`RENAMES`), `PyTESDeprecationWarning`, a module `__getattr__` for renamed classes and functions, and the option hook in `process_options`. `BaseModel.renamed_options` (unreleased) is replaced by the registry entry for F-050, whose error message is unchanged apart from the model name prefix. Option values are deferred (none renamed yet; the issue asked for them), and so are variable and parameter names: PyBaMM looks them up itself. No result changes. | Contract change (DESIGN D3, D17, maintainer, 2026-10-23). | Done; tests in `tests/toolkit/test_deprecation.py`; pytest turns the warning into an error; `CONTRIBUTING.md` "Renaming and deprecating". | closed |
| F-054 | Names: steps, couplings | Names that said less than, or something other than, what the code does (issue #18, maintainer, 2026-10-23). `step.ConstantInputs` becomes `step.Inlet` and `step.ConstantPower` becomes `step.Power`: their values need not be constant, and the names now follow `pybamm.step` (named after what is held). New `step.Rest`: no flow, with the inlet temperature given or, by default, held where the previous step left it (the initial temperature for a first step), through a new solver input the runner sets from the previous step; until now a rest was `ConstantInputs(T_in, 0)`. `LedgerWallCoupling` becomes `WallCoupling` ("ledger" names how energy is accounted, not the physics) and `FinPCMCoupling` becomes `FinStorageCoupling` (the material between the fins need not change phase); its default submodel name becomes "Fin-storage coupling", and its keyword arguments `pcm_label` and `pcm_geometry` are kept, as the registry cannot deprecate keyword arguments. The old names are aliases through the registry (F-053): they warn until 0.2.0. Not changed: no `mass_flow_rate_below` termination (backwards flow is not supported), `Target`, `ConstantPropertyLiquid`. No result changes (the reference experiment is unchanged). The rename test for user-facing files no longer reads the changelog's copy in `docs/`, which keeps the names of its time. | Contract flaw (names). | Renamed everywhere, aliases in `pytes.deprecation.RENAMES`; DESIGN D10 and the contract updated; tests for `Rest`. | closed |
| F-055 | Variables | Outputs for comparison with measurements (issue #14, maintainer's choices 2026-10-24). Required in every model: `Volume flow rate [m3.s-1]` (`mdot / rho(h_in)`, twin `[l.min-1]`), `Pumped volume [m3]` (its time integral, a solver state, twin `[l]`), `Inlet-outlet temperature difference [K]` (`T_in - T_out`, the example name of D12, which no model provided), `Heat transfer fluid velocity [m.s-1]` (mean velocity in the flow area; interstitial in a packed bed), `Charged energy [J]` (`-Discharged energy`), and `Discharged throughput [J]` and `Charged throughput [J]` (time integrals of `max(P, 0)` and `max(-P, 0)`, solver states). Every storage provides its volume-weighted averages, `<label> average temperature [K]` and, along the flow path, `<label> cross-section average temperature [K]`; and `Storage average temperature [K]` and `Storage cross-section average temperature [K]` (required) combine all the storage, volume-weighted, so that the mush model (fins and PCM apart) compares with the sharp front (homogenised, local equilibrium). One name in every family (the maintainer weighed "capsule average" for the bed and kept the shared name; in a packed bed the cross-section average is the capsule's volume average, as the docs say). This reverses F-043's dropping of `Mean storage temperature [K]` as undefined: it is now defined. `Surface Reynolds number` and `Surface Prandtl number` are provided with the constant coefficient too: `NewtonCooling` takes an optional flow geometry for reporting only. `Pressure drop [Pa]` stays reserved. Side effects: the three new solver states move the reference experiment within the solver tolerance (at most 1.5e-5 relative), so `tests/data/reference_regression.npz` is regenerated; and `check_unit_twins` compares to relative 1e-10 instead of 1e-12, as the `[l.min-1]` twin differs from the converted value by a few round-offs (2.6e-12), depending on the order in which the solver evaluates it. No physical result changes. | Contract flaw (additions; a tolerance). | Implemented; `tests/models/test_output_variables.py`; contract §3 and DESIGN D12 updated. | closed |
| F-056 | Storage, coupling | Packed-bed fluid temperatures were not monotonic during a charge: at every height the fluid rose, fell by 1.2 K to 2.1 K, then rose again (default Nallusamy charge, default mesh), seen in the tutorials' plots. The storage's wall condition turned the flux law's heat flux into a gradient with the conductivity at the extrapolated wall enthalpy, `k(H_surf)`, while the heat that enters the storage is `k grad(T)` on the wall face, where PyBaMM extrapolates the node values of `k`. The two agree when `k` varies smoothly; the default paraffin melts at a sharp point (melting range 0 K) and its conductivity drops from 0.4 to 0.15 W.m-1.K-1, so while the cells by the wall melted the storage took 19 % to 29 % of the flux law's heat (0.15 of the bed height, 12 to 16 min). Energy stayed exactly conserved (the ledger, D6, telescopes to the actual wall flux), so the conservation tests could not see it, and the coupling's docstring wrongly said the two "differ slightly". Evidence that it was numerics, not physics: the dips grew as the capsule mesh was refined (0.5 K on 10 cells to 3 K on 160), vanished with equal solid and liquid conductivities, and vanished with a melting range of 2 K. | Model flaw (numerics). | The wall condition uses `boundary_value(k)`, the conductivity `k grad(T)` uses on the wall face, so exactly the flux law's flux enters (`EnthalpyStorage`). The charge is now monotonic, converging as the capsule mesh is refined (largest dip 0.17 K on 20 cells, 0.04 K on 40, none from 80); energy is conserved as before (3e-8 J of 8.3 MJ). Results move: the Nallusamy (2007) fluid temperatures fit the measurements better at every height (RMS 0.98, 0.68, 1.41 and 1.85 K at a quarter, half, three quarters and the full height, for 1.09, 0.92, 1.63 and 2.05 K); the finned tubes, whose PCM melts over 2 K with conductivities 0.6 and 0.5, move by at most 0.006 K. Regression reference regenerated. Tests: the fluid's heat equals the flux law's flux over the capsule surface, and the default charge heats the fluid monotonically at four heights. | closed |
| F-057 | Experiments | The runner dropped skipped steps (termination condition already met at the start) from `solution.cycles[i].steps`, and a cycle whose steps were all skipped from `solution.cycles`, so positions stopped matching the experiment's steps. To say which step a solution belonged to, it labelled each one with its description, which needed a workaround once `pybamm.Solution` took `__slots__` (F-026) and a draft upstream request for a metadata field. PyBaMM needs neither: it holds a skipped step's place with `pybamm.EmptySolution` and matches step solutions to steps by position. | Contract-neutral; runner flaw (its return value). | As in PyBaMM, a skipped step's place is held by `pybamm.EmptySolution("Event exceeded in initial conditions", t=start)`; so is a skipped cycle's, which PyBaMM leaves out (maintainer, 2026-11-01). So `solution.cycles[i].steps[j]` always belongs to `experiment.cycles[i][j]`. `pytes.experiments.step_description` is removed without an alias (maintainer, 2026-11-01): read `experiment.cycles[i][j].description`. `compat/solution.py` and the upstream request are dropped. DESIGN D10 updated. Results are unchanged. Tests: placeholders for a skipped step and a skipped cycle; the reference experiment's steps match by position. | closed |

**Pinning note.** HB was pinned at `d391da0` on 2026-10-11 (the working tree diffed above),
and PB at `023bf58`.

## Checklist for the side-by-side diff

Compare, and log every difference:

1. Variable names, units, and sign conventions (power, energy).
2. Parameter naming and structure.
3. Material classes (are they truly identical?) and melting-range smoothing (form and default
   width).
4. Conservation approach: is the heat across the fluid-solid coupling computed from the
   storage side's discrete energy balance, or as `h*A*dT` on both sides?
5. Fluid equation: state variable, discretisation, upwinding workaround, inlet condition,
   outlet temperature.
6. Power definition: from enthalpy or `mdot*cp*dT`.
7. Fluid inventory: is it in the energy variables? (Finned pipe: yes, at `d391da0`; see F-008.
   Both energies are now required.)
8. Heat transfer coefficient: constant, correlation, or hard-coded.
9. Liquid fraction: averaging weights over the capsule volume and bed length, versus the
   finned-pipe definition.
10. Domain structure: macro/micro layout, secondary domains, naming.
11. Cut-off events and operating mode.
12. Default geometry, mesh, spatial methods, solver, and plot variables.
13. Test coverage: which existing tests have no counterpart in the other repo.