How the flame burns in flame spraying

       Taking gas fuel flame combustion as an example, there are the following two combustion methods.



1. Diffusion combustion



This combustion method refers to the combustion method in which the fuel gas and the combustion-supporting gas are injected into the combustion chamber from their respective nozzles, and they are diffused and mixed while burning. The diffusion combustion flame is very long, the combustion flame stability is good, and the tempering phenomenon will not occur, but the flame temperature is not too high.



2. Premixed combustion



This combustion method refers to the combustion method in which the gas fuel is evenly mixed with part or all of the auxiliary gas before entering the combustion chamber for combustion. Premixed combustion belongs to dynamic combustion. In the partial premixed combustion, the flame has two cones, namely the inner flame and the outer flame. In the outer flame region, it is actually the diffusion combustion of the gaseous fuel. The flame shortens with the increase of the auxiliary gas. When complete premixed combustion is achieved, the fuel burns very fast, the combustion can be completed almost instantaneously, and the outer flame no longer exists. Using the premixed combustion method can achieve high-load combustion, but there is a risk of flashback.

Thermal spraying often uses a premixed combustion method. In premixed combustion, the mixing methods of fuel gas and auxiliary gas mainly include injection-suction mixing and isobaric mixing. Jet-suction mixing means that the combustion gas and the auxiliary gas are mixed according to the principle of the injector, and the auxiliary gas is generally used as the injection gas, as shown in Figure 3-2. Isobaric mixing is that both fuel gas and auxiliary gas are injected into the mixing chamber at a critical speed for mixing (Figure 3-3). Gas flow can be divided into parallel flow (mixed by turbulent flow), and a certain angle or vertical phase flow, and both are mixing between rotating airflows.

The premixed combustion stability of isobaric mixing is higher than that of jet-suction mixing, and the gas flow (flame power) and adjustment range of premixed combustion are also larger than those of jet-suction premixed combustion.