### 1.13.2. Concept Generalized to 3D

Equation 1–292 becomes a diffusion-only transport equation with a uniform source term of unity:

| (1–299)

Dirichlet conditions of are applied on boundaries of interest (such as walls), and Neumann (or zero flux) conditions are applied on other boundaries. Equation 1–298 is then evaluated, replacing with ( is always positive) and is interpreted as the distance to the nearest boundary where was set. This gives the following expression for desired distance:

| (1–300)

Because is always positive, the wall distance is also always positive.

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