Blender Fire Simulation Heat Emission Rate Explained

In Blender's Mantaflow fluid simulation system, the heat emission rate dictates the temperature difference between the emitted fluid and the surrounding domain environment. This setting directly controls the thermal buoyancy of fire and smoke, determining how fast flames rise, disperse, and interact with ambient forces. Understanding how heat settings interact with domain physics is essential for creating physically accurate fire dynamics, whether simulating a slow-burning campfire or an intense explosion.

What is Heat in Blender's Gas Simulation?

In Blender, heat is a property assigned to the Flow Object within a gas or fire simulation. It does not simply represent visual glow or color; instead, it acts as a physical force multiplier.

When a Flow object emits gas, the Heat value specifies the temperature delta relative to the domain's ambient temperature. Higher temperatures generate lower density relative to the surroundings, causing the fluid to accelerate upward due to thermal buoyancy.

Key Effects of the Heat Setting

  • Upward Acceleration: A positive heat value makes fire and smoke rise quickly. The higher the value, the stronger the upward lift.
  • Neutral Buoyancy: A heat value of 0.0 means the emitted fluid has the exact same temperature as the ambient air. It will not rise or fall on its own and will only move if pushed by initial velocity, wind, or turbulence.
  • Negative Buoyancy (Sinking Fluid): Setting a negative heat value makes the fluid colder and denser than the ambient air, causing it to fall toward the ground—ideal for cold smoke, dry ice effects, or heavy chemical vapors.

How Flow Heat Interacts with Domain Buoyancy

The behavior of emitted heat is governed by the Domain Object physics settings:

  1. Flow Object Heat: Sets the local temperature multiplier at the emission source.
  2. Domain Buoyancy (Heat): Found under the Domain’s Gas Settings, this parameter controls how strongly the entire domain reacts to temperature differences.

If the Domain's Buoyancy Heat is set to 0, the flow object's heat value will have no effect on upward movement, regardless of how high it is set. The final lifting force is the product of the flow object’s heat and the domain’s heat buoyancy factor.

To fine-tune fire generation alongside heat, adjust these interconnected settings:

  • Fuel: Dictates the amount of combustible material emitted per frame. Higher fuel values prolong combustion and sustain heat generation longer.
  • Reaction Speed: Controls how fast the fuel burns. A higher reaction speed creates a short, hot flame, while a lower speed allows flames to stretch higher before consuming the fuel.
  • Flame Smoke: Determines the volume of smoke generated per unit of burned fuel.
  • Subframes / Sampling Steps: High heat values can cause fast-moving flames to escape domain boundaries or create stepped artifacts. Increasing simulation substeps ensures smooth, continuous heat diffusion.