We note in the figure that

1. A free unconfined get spreads in the surrounding
2. Spreading of the jet is due to entrainment of the surrounding
3. Entrainment of the surrounding is due to the difference in momentum flow rate within the jet and that within the surrounding. Entrainment of surrounding continues till the difference becomes zero.
4. Entrainment of surrounding  depends on mass flow rate and jet velocity

In furnaces jet is confined by furnace walls. Due to confinement, mass of the surrounding is limited to the amount equal to secondary air. The primary air mixes rapidly with the fuel at the nozzle but the remaining air must be entrained and for this purpose excess momentum flux in the jet is required. If the confined jet has momentum flux in excess of that required for the complete entrainment of the secondary air, products of combustion will entrain and a circulatory jet is produced. Figure 27.3 shows recirculation of products of combustion in the jet

Figure 27.5:

Entrainment of products of combustion in the jet. At point  secondary air is entrained. Beyond point A the products of combustion entrain due to the excess momentum in the jet.