Plume Width by Height

This chart shows how the buoyant cooking plume expands laterally as it rises from the cooking surface to the hood. The plume diameter at hood height — not the cooking surface width — determines the minimum hood size required for capture.

Reference readings

Representative values below are computed directly from the same plume-geometry module (physics/plume.mjs) driving the instrument above, for the papers’ reference source — a medium gas grill (Q_c = 8.2 kW, D_eff = 0.51 m, z_0 = −0.37 m, 24-inch cooking surface; RB-001 Tables 3.1–3.2); live values update as you move the height control. The table is regenerated by scripts/generate-reference-tables.mjs, never typed by hand.

The capture diameter is already about 27 inches at the cooking surface — 3 inches wider than the 24-inch grate, because the virtual origin sits 0.37 m below it — and grows to about 41.4 inches by a 30-inch mounting height (RB-002 Table 3.3a prints 1.05 m, 41 inches) and 50.1 inches by 48 inches (1.27 m) [RB-002 §3.4]. Growth is linear at 0.48 inch of diameter per inch of rise, four times the Heskestad half-width slope of 0.12 [RB-002 App. A.3]. The recommended hood width is 1.38 × the capture diameter — 57 inches at 30 inches over this grill [RB-002 §3.6, Table 3.7] — so the overhang beyond the cooking surface is about 17 inches per side at that height and grows with every inch the hood is raised [RB-005 §3.1].

Key Findings

The plume is always wider than the cooking surface at hood height. Even at the lowest standard mounting height (18 inches), the capture diameter exceeds the cooking surface width for every source type. At 30 inches — the most common mounting height — the capture diameters of the eight modeled sources run from about 37 inches (small gas grill) to 48 inches (high-output gas grill), the W_min column of RB-002’s sizing tables [RB-002 §3.6].

The reference line tells the story. The “typical 36-inch grill width” line shows that every source type produces a plume wider than the grill at every mounting height. A hood sized to match the grill will always miss plume material at the edges [RB-002 §4.1].

Charcoal produces the widest plume for its cooking surface. The 22-inch charcoal kettle’s capture diameter at 30 inches is about 45 inches — wider than the 41 inches over the 24-inch medium gas grill — because its strongly negative virtual origin (z_0 = −0.47 m) starts the expansion further below the cooking surface [RB-001 Table 3.2; RB-002 §4.3].

Governing Equations

The live instrument draws the papers’ reference plume, not a separate curve per source type:

b_T = 0.12 · (z − z_0) — Gaussian temperature half-width [RB-001 §2.2] d_capture = 0.48 · (z − z_0) + D_eff — Heskestad capture diameter, the 98% flux contour plus the source width [RB-002 §2.7, App. A.3] W_rec = 1.38 · d_capture — recommended hood width, base margin K = 1.25 × 1.10 [RB-002 §3.5–3.6]

with z the height above the cooking surface and z_0 the tabulated virtual origin from RB-001 Table 3.2. The three slopes that appear in the papers describe different quantities and are not in conflict: 0.12 is the Heskestad temperature half-width b_T, 0.132 is the MTT point-source radius b = (6/5)αz, and the velocity half-width is b_u ≈ 1.2 b_T [RB-002 §2.7; RB-003 §2.2]. The instrument uses b_T.

Source Papers

• RB-002: Entrainment and Lateral Plume Spread — Plume width metrics (§3.4), sizing tables (§3.6), formulas (App. A.3)
• RB-001: Buoyant Plume Behavior — Source parameters D_eff and z_0 (Tables 3.1–3.2)