Avionics Bay Cooling Sizing

Avionics Bay Cooling Sizing (vehicle-design/sizing/avionics-bay-cooling-sizing)

Use when the task is sizing the cooling airflow for the avionics and equipment bay of an aircraft: the LRU heat dissipations must be carried away by a cooling air flow whose exhaust stays at or below the LRU inlet-air temperature limit, and each LRU case must stay within its case temperature limit. This leaf implements the standard bay cooling model in pure Python, stdlib only: heat load rollup, cooling mass flow from the allowable temperature rise, volumetric flow and CFM conversion, and the per-LRU case temperature check. It pairs with vehicle-design/sizing/environmental-control-sizing (the cabin conditioning leaf, which stops at the conditioning pack flow and never sizes bay airflow or LRU case temperatures) and with avionics/do160/environmental-qualification (the LRU temperature test specification side, which does not size airflow).

Domain quick reference

• Bay heat load rollup: Q_total = sum of the LRU heat dissipations P_i, each LRU power in W. The bay is a collection of line-replaceable units (LRUs) inside the avionics/equipment bay.
• Cooling mass flow: m_dot = Q / (cp * (T_exhaust_limit - T_supply)), with cp = 1005 J/(kg K) for dry air. The exhaust temperature is sized to equal the LRU inlet-air temperature limit, giving the maximum cooling benefit without exceeding the LRU inlet limit.
• Volumetric flow: V_dot = m_dot / rho with rho = 1.2 kg/m3 near sea level; CFM = V_dot * 2118.88 (ft3/min per m3/s).
• LRU case temperature: T_case = T_inlet_air + P / (h A), where h A is the case-to-air conductance in W/K (default 12 W/K shared across the bay). A zero-power LRU sits exactly at the inlet air temperature.
• Case verdict: PASS when T_case <= T_limit, else FAIL; margin is T_limit - T_case (positive for PASS, negative for FAIL).
• Bay verdict: FAIL when any LRU case exceeds its limit or the mass flow is zero with positive heat, else PASS.
• Units are W, deg C, kg/s, m3/s, ft3/min (CFM), K.
• FAR 25.1309 frames the equipment installation context; the relations above are standard engineering methodology, summary-only.

Workflow

1. Collect the LRU heat dissipations in W and roll up the bay heat load with bay_heat_load.
2. Fix the bay cooling supply temperature and the LRU inlet-air temperature limit, then size the cooling mass flow with cooling_mass_flow; the returned exhaust temperature equals the limit by construction.
3. Convert the mass flow to the volumetric flow and CFM with volumetric_flow.
4. For each LRU, compute the case temperature from the dissipated power and the case-to-air conductance with lru_case_temperature, then grade it against the LRU case limit with case_verdict.
5. Run the whole bay in one call with bay_cooling_summary to get the heat load, flows, exhaust temperature, per-LRU case temperatures, case verdicts and the bay PASS/FAIL verdict that gates the bay ventilation layout.
6. Confirm the deterministic checks with the contract test scripts/test_avionics_bay_cooling_sizing.py.

Worked example

Reference bay with six LRUs dissipating 400, 350, 300, 550, 450 and 450 W (2500 W total), supply air at 25 C, LRU inlet-air limit 55 C, air density 1.2 kg/m3, case-to-air conductance 12 W/K for every LRU, and case limits [60, 60, 60, 65, 60, 60] C.

• Bay heat load: bay_heat_load returns total_w 2500.0 W.
• Cooling mass flow: cooling_mass_flow(2500.0, 25.0, 55.0) returns 0.08292 kg/s (2500 / (1005 * 30)); exhaust_temp_c 55.0 C.
• Volumetric flow: volumetric_flow returns 0.06910 m3/s and 146.4 CFM.
• 300 W LRU: lru_case_temperature gives rise 300 / 12 = 25.0 K and case_temp_c 25 + 25 = 50.0 C, versus the 60 C limit a PASS with margin 10.0 K.
• 550 W LRU: rise 550 / 12 = 45.83 K, case_temp_c 70.83 C, versus the 65 C limit a FAIL with margin -5.83 K; this is the undersized heat path the check exists to catch.
• Bay summary: bay_cooling_summary over the six LRUs reports the 2500.0 W load, 0.08292 kg/s, 146.4 CFM, exhaust 55.0 C, case temps {0: 58.33, 1: 54.17, 2: 50.0, 3: 70.83, 4: 62.5, 5: 62.5} C with verdicts PASS, PASS, PASS, FAIL, FAIL, FAIL and a bay verdict FAIL, so the bay layout must be revised (heat path or limits) before release. Re-running the summary with the 550 W LRU reduced to 350 W gives an all-PASS set and a bay verdict PASS.

Pitfalls

• Checking the airflow but not the individual LRU cases: the bay verdict FAILs when any LRU case exceeds its limit even if the mass flow is adequate - in the worked bay three LRUs fail their case limits (550 W gives 70.83 C against 65 C) while the flow itself closes the 30 K rise.
• Sizing the flow on a temperature difference that is not positive: cooling_mass_flow requires the LRU inlet-air limit above the supply temperature; a limit at or below supply raises ValueError because there is no cooling potential.
• Reading the exhaust temperature as a margin: the sized exhaust EQUALS the LRU inlet limit by construction (55.0 C in the worked example); it is the per-LRU case temperatures that carry margin.
• Sharing one conductance blindly: the case temperature uses the case-to-air conductance h A (12 W/K default) per LRU, and doubling the conductance halves the case rise; a high-dissipation LRU with poor local heat path fails while the average looks fine.
• Treating this leaf as the cabin conditioning pack: the bay airflow and LRU case checks are this leaf's claim; environmental-control- sizing stops at the conditioning pack flow, and DO-160 environmental-qualification covers the LRU test side.
• Feeding non-physical bay inputs: an empty LRU list, negative power, density or conductance at or below zero, and a case limit below absolute zero all raise ValueError.

Verification

• Confirm bay_heat_load([400, 350, 300, 550, 450, 450]) returns total 2500.0 W.
• Confirm cooling_mass_flow(2500.0, 25.0, 55.0) returns 0.08292 kg/s (inside the independently verified 0.08 to 0.09 kg/s bound) and that zero heat returns a 0.0 kg/s flow.
• Confirm volumetric_flow converts the reference flow to 0.06910 m3/s and 146.4 CFM, and that 1 kg/s at 1.2 kg/m3 gives 0.8333 m3/s and 1765.7 CFM.
• Confirm the identities: doubling the allowable temperature rise at fixed heat halves the mass flow, doubling the heat doubles the flow, doubling the case-to-air conductance halves the case rise, and a zero-power LRU sits at the inlet air temperature.
• Confirm the 300 W LRU case is 50.0 C (PASS, margin 10.0 K) and the 550 W LRU case is 70.83 C (FAIL, margin -5.83 K), and that the bay verdict is FAIL when any LRU fails and PASS only when every LRU passes.
• Confirm every non-physical input raises ValueError: empty LRU list, negative power, temperature difference not positive (limit at or below supply), density at or below zero, conductance at or below zero, and a case limit below absolute zero.
• Run the contract test offline: python3 scripts/test_avionics_bay_cooling_sizing.py (32 tests, deterministic).

Related leaves

• vehicle-design/sizing/environmental-control-sizing: the cabin conditioning leaf; its equipment heat line sits inside the cabin load and it stops at the air conditioning pack, it does not size bay airflow or LRU case temperatures.
• vehicle-design/sizing/ice-protection-sizing: surface thermal ice protection for the aerodynamic surfaces, a different thermal sizing problem.
• avionics/do160/environmental-qualification: the test specification side of LRU temperature qualification, not airflow sizing.

Behavior contract (gate 3)

Run the deterministic contract test (stdlib unittest, offline):

python3 scripts/test_avionics_bay_cooling_sizing.py

The test covers the reference bay sizing contract (2500 W heat load, 0.08292 kg/s mass flow, 146.4 CFM, 55.0 C exhaust, 300 W LRU case 50.0 C PASS, 550 W LRU case 70.83 C FAIL and the FAIL bay verdict), the flow and case scaling identities, the volumetric conversion references, the convenience dict keys, determinism and the ValueError rejection of every non-physical input.

Compliance

• Standards referenced, not reproduced: FAR 25.1309 is the equipment installation context (case of an avionics bay in an aircraft); the bay cooling relations above are standard engineering methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.