How does gas-shroud HVOF (warm spray) system work?

[Lalit]

I was reading some scientific articles on warm spraying in which in-flight powder particles travel with high velocity and reduced surface temperature. There is an extra shroud with continuous supply of nitrogen is attached to the barrel of JP-5000 torch. Can someone explain me how the velocity of in-flight particles is increased by using a shroud. I can understand the effect of using nitrogen in the shroud which is to quench the flame and consequently reduction in particle temperature. Can we achieve the same condition by increasing the barrel length of jp5000 torch and supplying nitrogen from auxiliary port placed just after the de-laval nozzle or powder feeding port?

[djewell]

I don't see how a shroud gas could possibly impart any additional velocity to the particles. In order to accelerate the JP5000 gas stream, you must expand it after the choke. Introducing additional gas in the nozzle is going to compress the gas stream. Would you post the article to which you are referring?

Warm spray is a strange technological development, in my opinion. By quenching the flame with nitrogen, the operator is basically synthesizing air. In other words, they are turning a JP5000 into an HVAF gun. They get some nice looking coatings, of course, because they are synthesizing the HVAF spray conditions. However, in the process, they still have the same drawbacks of the JP, namely radial powder injection, water cooling, short nozzle life, etc. If the researchers would start with an HVAF gun, they could also take advantage of HVAF's lower operating cost, higher deposit efficiency, and longer component life.

[Lalit]

Dear djewell, thanks for reply and I am convinced with your idea that they are presenting hvaf in different style. One thing I would like to know that if you can tell me why the barrel length is more in d-gun as compared to Jp5000. Because I think the shroud also work in the same way. You can read the papers of seiji kuroda and Watanabe, NIMS, JAPAN. In every paper they have written that shroud increases the particle velocity.
Few days back I attended 6th Asian thermal spray conference in India where professor Seiji kuroda delivered a lecture on warm spraying. He compared hvof and warm sprayed wc-co coating. Initially hvof coayings were superior than warm sprayed coating but I was surprised to know that he was claiming to get a high fracture toughness value (17 MPa.√m) of wc-co coatings which was very near to sintered wc-co material. He got this high value of warm sprayed coayings after conventional heat treatment (800 °c) whereas no improvement was shown by HVOF coating.

[djewell]

In normal operation, a liquid fueled HVOF gun typically consumes approximately 1,000 l/min. I found a presentation from Mr. Kuroda on the Swiss Federal Laboratories web site. He achieved the best looking coatings from his warm spray gun with 1,000 l/min nitrogen flow. His actual oxygen concentration in the flame is less than 50%. This is not much different than air (~21% oxygen content).

Regarding your original question, I did find that Mr. Kuroda injected the nitrogen shroud into the combustion chamber before the nozzle choke, which would increase the pressure and lower the flame temperature, which could increase the gas density. As the gas expanded, it could achieve a higher velocity. You would have to look at some combustion and gas flow equations to confirm this.