Heat Release Rate Comparison

This chart compares the total and convective heat release rates for the representative source types analyzed in the research program. The convective fraction (Q_c) — not the total heat output — is the parameter that drives plume buoyancy, velocity, and entrainment.

Reference readings

Representative values below are the RB-001 Table 3.1 source parameters carried by physics/heat.mjs and the 30-inch centerline velocity computed from them by physics/plume.mjs — the same values driving the instrument above; live values update as you select a source. The table is regenerated by scripts/generate-reference-tables.mjs, never typed by hand.

Going from the 25,000 BTU small gas grill to the 60,000 BTU large one — 2.4× the rated heat and 2.4× the convective output — raises the plume’s centerline velocity at 30 inches only from about 342 to 444 fpm, a 30% gain: velocity scales with the cube root of Q_c [RB-001 §2.2]. This sublinear scaling is why a “twice the BTU” grill does not need twice the hood or twice the CFM; RB-008 puts the CFM penalty for doubling Q_c at 26% [RB-008 §2.4].

Key Findings

Convective fraction varies sharply by fuel type. Gas grills convert 70% of their heat to convective plume energy; charcoal in glowing-ember mode converts only 40% — the rest is thermal radiation to the food and surroundings [RB-001 Table 3.1]. The 15,000 BTU charcoal kettle’s 1.8 kW convective output is about a third of the 25,000 BTU small gas grill’s 5.1 kW, and its plume is correspondingly slower: about 230 fpm at 30 inches against 342 fpm.

Gas grills dominate the high end. The 80,000 BTU high-output gas grill produces 16.4 kW convective — about 485 fpm at 30 inches — the strongest and tallest plume in the program and the most demanding source for hood sizing [RB-001 §3.5].

Pellet smokers span a wide range. From 1.5 kW in low-smoke mode (about 222 fpm at 30 inches) to 5.7 kW in high-temperature grilling mode (about 355 fpm), pellet smokers operate at very different heat release rates depending on the temperature setting and pellet feed rate [RB-001 Table 3.1, note 4].

The charcoal paradox. Despite comparable BTU ratings, charcoal grills produce the weakest plumes per BTU because of their low convective fraction. This is counterintuitive — charcoal produces copious visible smoke, but the plume has less buoyant force than a gas grill of equal rating [RB-001 §4.3].

Why Convective Fraction Matters

The Heskestad plume correlations use Q_c (not Q_total) as the input parameter. All plume properties — velocity, temperature, diameter, mass flow — scale with Q_c. Two sources with the same total BTU but different convective fractions produce fundamentally different plumes [RB-001 §2.4].

Source Paper

• RB-001: Buoyant Plume Behavior — Heat release rate characterization (Table 3.1), virtual origins (Table 3.2), centerline velocity (§3.5)