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The SW also allows the inclusion in the calculations of the information related to thermal efficiency of

Figure 2 The SW also allows the inclusion in the calculations of the information related to thermal efficiency of

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Fig. 1. Geometry of HVOF torch used for calculations Calculations illustrating Some Capabdiities of tne developed model and SW were done using kerosene as a fuel and a geometry of a HVOF torch depicted on Fig. 1. This type of torch has been used for experimental validation of the calculations. The radial powder injection into the low pressure zone was chosen to avoid the disadvantages related to high pressure powder feeders which are used for an axial powder feeding [5].The torch geometrical parameters are the following: throat diameter - D=6 mm; diameter of the barrel exit - Dpe=10 mm; barrel length - Lp=100 mm. The throat surface area is almost 2 times smaller in comparison with JP5000 throat area that results in significantly less oxygen flow rates and related expenses. The throat diameter was chosen after preliminary calculations which showed that for a 6 mm throat, the influence of the barrel walls on emperature and velocity of combustion products is still very insignificant. Results of the calculations below were obtained using kerosene as the fuel, with he oxygen/kerosene mass flow ratio m./m, = 3.5. Kerosene was chosen because it is a relatively inexpensive fuel. NiCr was mainly used to illustrate he particle behavior during HVOF process. Tungsten carbide particles were considered in some cases.
Fig. 1. Geometry of HVOF torch used for calculations Calculations illustrating Some Capabdiities of tne developed model and SW were done using kerosene as a fuel and a geometry of a HVOF torch depicted on Fig. 1. This type of torch has been used for experimental validation of the calculations. The radial powder injection into the low pressure zone was chosen to avoid the disadvantages related to high pressure powder feeders which are used for an axial powder feeding [5].The torch geometrical parameters are the following: throat diameter - D=6 mm; diameter of the barrel exit - Dpe=10 mm; barrel length - Lp=100 mm. The throat surface area is almost 2 times smaller in comparison with JP5000 throat area that results in significantly less oxygen flow rates and related expenses. The throat diameter was chosen after preliminary calculations which showed that for a 6 mm throat, the influence of the barrel walls on emperature and velocity of combustion products is still very insignificant. Results of the calculations below were obtained using kerosene as the fuel, with he oxygen/kerosene mass flow ratio m./m, = 3.5. Kerosene was chosen because it is a relatively inexpensive fuel. NiCr was mainly used to illustrate he particle behavior during HVOF process. Tungsten carbide particles were considered in some cases.
Fig. 3. Influence of combustion chamber pressure on temperature of the combustion products: AnT—Z7 ~asr.
Fig. 3. Influence of combustion chamber pressure on temperature of the combustion products: AnT—Z7 ~asr.
Nonunitormity of Spatial alstrioution of particles temperature and velocity, even if the distribution is relatively narrow, may significantly impact various conditions of the particle interaction with substrate. Therefore, the local properties of the coating could differ substantially from point to point on the sprayed surface. However, one of the main requirements of the thermal spraying process is to ensure that the integral coating properties remain constant over the entire sprayed surface. To satisfy this requirement, spraying can be carried out in such a manner, so that he coating formation on any part of the surface akes place through the deposition of many layers, whose axis are displaced relative to each other. Ona macroscopic scale, this procedure will homogenize he properties of the coating. However, on a microscopic scale, the coating consists of a complex mixture of particles that experience vastly different hermal and dynamic histories and interactions with he substrate. The conditions of formation and the properties of such a complex system should be described by a statistically based model [1]. At the same time, the fact that any microscopic part of the coating comprises a “particle mixture” indicates that
Nonunitormity of Spatial alstrioution of particles temperature and velocity, even if the distribution is relatively narrow, may significantly impact various conditions of the particle interaction with substrate. Therefore, the local properties of the coating could differ substantially from point to point on the sprayed surface. However, one of the main requirements of the thermal spraying process is to ensure that the integral coating properties remain constant over the entire sprayed surface. To satisfy this requirement, spraying can be carried out in such a manner, so that he coating formation on any part of the surface akes place through the deposition of many layers, whose axis are displaced relative to each other. Ona macroscopic scale, this procedure will homogenize he properties of the coating. However, on a microscopic scale, the coating consists of a complex mixture of particles that experience vastly different hermal and dynamic histories and interactions with he substrate. The conditions of formation and the properties of such a complex system should be described by a statistically based model [1]. At the same time, the fact that any microscopic part of the coating comprises a “particle mixture” indicates that
Uy stops growing when the spray distance is around the combustion chamber pressure and_ spray distance. Calculations show that increasing of the combustion pressure from 0.7 MPa up to 1.3 MPa allows increasing of mean particles velocity by 40- 50%. However, mean particle temperature experiences only a slight increase, depending on p,, within the investigated range of combustion chamber pressures. Results of the calculations depicted on Fig. 6 allow the making of the supposition that the optimal spray distance depends significantly on the combustion pressure. For example, for p,, =0,7 MPa,
Uy stops growing when the spray distance is around the combustion chamber pressure and_ spray distance. Calculations show that increasing of the combustion pressure from 0.7 MPa up to 1.3 MPa allows increasing of mean particles velocity by 40- 50%. However, mean particle temperature experiences only a slight increase, depending on p,, within the investigated range of combustion chamber pressures. Results of the calculations depicted on Fig. 6 allow the making of the supposition that the optimal spray distance depends significantly on the combustion pressure. For example, for p,, =0,7 MPa,
Conditions of powder injection into the barrel is the another very important parameter influencing the particle temperature and velocity. The conditions depend primarily on the position and the geometry of powder injector(s), the number of injectors, the carrier gas flow rate, and the powder feed rate. To estimate the influence of the powder injection on mean particle velocity, it was assumed that particles are evenly distributed around the axis of the barrel. Radius of the area occupied by the particles is Rp and the non-dimensional radius characterizing the R a p
Conditions of powder injection into the barrel is the another very important parameter influencing the particle temperature and velocity. The conditions depend primarily on the position and the geometry of powder injector(s), the number of injectors, the carrier gas flow rate, and the powder feed rate. To estimate the influence of the powder injection on mean particle velocity, it was assumed that particles are evenly distributed around the axis of the barrel. Radius of the area occupied by the particles is Rp and the non-dimensional radius characterizing the R a p
Fig. 8. Influence of powder injection velocity on mean values of particles velocity and temperature at the barrel exit (p.-=0.9 MPa, d,=30 um, Lp=100 mm)
Fig. 8. Influence of powder injection velocity on mean values of particles velocity and temperature at the barrel exit (p.-=0.9 MPa, d,=30 um, Lp=100 mm)
Optimization of the carrier gas flow rate and velocity of particles Vpo in a powder injector is, probably, the most important part of the powder injection optimization for the case of radial injection and for a particular design of an HVOF torch. Fig. 8 provides an illustration of that: adjustments of Vo and consequent changes in particle concentration in a barrel result in significant changes of particle velocity and temperature. Optimized powder feeding conditions result in a higher concentration of particles around the barrel axis that causes a higher mean velocity, shorter dwell time and, consequently, a lower mean temperature of the particles.
Optimization of the carrier gas flow rate and velocity of particles Vpo in a powder injector is, probably, the most important part of the powder injection optimization for the case of radial injection and for a particular design of an HVOF torch. Fig. 8 provides an illustration of that: adjustments of Vo and consequent changes in particle concentration in a barrel result in significant changes of particle velocity and temperature. Optimized powder feeding conditions result in a higher concentration of particles around the barrel axis that causes a higher mean velocity, shorter dwell time and, consequently, a lower mean temperature of the particles.
Related topics:
Numerical SimulationsComputational Fluid Dynamics (CFD) modelling and simulationCFD AnalysisHVOF coatingsDispersed Phase
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