Luisa A. Cabrera

High velocity oxy-fuel thermal spray gun design

High-velocity oxy–fuel (HVOF) thermal spraying was developed in 1930 and has been commercially available for twenty-five years. HVOF thermal spraying has several benefits over the more conventional plasma spray technique including a faster deposition rate which leads to quicker turn-around, with more durable coatings and higher bond strength, hardness and wear resistance due to a homogeneous distribution of the sprayed particles. HVOF thermal spraying is frequently used in engineering to deposit cermets, metallic alloys, composites and polymers, to enhance product life and performance. HVOF thermal spraying system is a highly promising technique for applying durable coatings on structural materials for corrosive and high temperature environments in advanced ultra-supercritical coal-fired (AUSC) boilers, steam turbines and gas turbines. HVOF thermal spraying is the preferred method for producing coating with low porosity and high adhesion. HVOF thermal spray process has demonstrated to be one of the most efficient techniques to deposit high performance coatings at moderate cost. This publication is devoted to shed light on the design aspects and test equipment requirements for liquid fuelled HVOF thermal spray gun. The current research provides extensive understanding of several key process parameters to develop design tools for the next generation liquid fuelled HVOF thermal spray system for high temperature and harsh environment applications.

Flow Characterization of High Velocity Oxy-Fuel Thermal Sprays

High-velocity oxy–fuel (HVOF) thermal spraying system is a highly promising technique for applying durable coatings on structural materials for corrosive and high temperature environments in advanced ultrasupercritical coal-fired (AUSC) boilers, steam turbines, and gas turbines. HVOF thermal spraying is the preferred method for producing coatings with low porosity and high adhesion. The HVOF thermal spray process has demonstrated to be one of the most efficient techniques to deposit high performance coatings at moderate cost. A computational fluid dynamics (CFD) model has been developed to predict gas dynamic behavior in a HVOF thermal spray gun in which non-premixed oxygen and methane are burnt in a combustion chamber linked to a parallel-sided converging-diverging nozzle. The CFD analysis is applied to investigate axisymmetric, steady-state, turbulent, compressible, chemically reacting, subsonic and supersonic flow inside and outside the gun. The gas axial velocity, axial temperature, static pressure and Mach number distributions are presented for various locations inside and outside the gun. The calculated results show that the most sensitive parameters affecting the process are the equivalence ratio and total gas flow rate. Gas dynamic behavior along the centerline of the gun depends on both total gas flow rate and equivalence ratio. The numerical simulations show that the gas axial velocity, axial temperature, static pressure and Mach number distribution depend on both flow rate and equivalence ratio. The maximum velocity, temperature, and pressure are achieved at the highest flow rate and/or richest equivalence ratio. In addition, the results reported in this paper illustrate that the numerical simulation can be one of the most powerful and beneficial tools for the HVOF thermal spray system design, optimization and performance analysis.

Experimental Study of Thermoelectric Properties of Randomly distributed SWCNTs and SiC Nanoparticles

In this experimental study we have investigated the thermoelectric (TE) properties of Single wall carbon nanotubes (SWCNTs) and Silicon carbide (SiC) nanoparticles. The nanoparticles were randomly distributed on a nonconductive glass substrate and hot and cold junctions were created using silver epoxy and Alumel (Ni-Al) wire. The carbon nanotubes used were approximately 60% semiconducting and 40% metallic. Voltage (mV), current (µA) and resistance (Ω) were measured across the distributed nanoparticles within 200 o C temperature difference. The Seebeck coefficient for SWCNTs was 0.12 mV/ o C whereas SiC nanoparticles showed no TE effect. However, when SWCNTs (at 10, 25 and 50 wt%) were infused into SiC, substantial TE effect was present. Even though the Seebeck coefficient was in a similar range with different SWCNT concentrations, current, resistance and Power factor (P.F.) changed with wt% of nanotubes. Current and P.F. increased with the increased concentration of SWCNTs in the samples. However, resistance slightly decreased with the increase in temperature. Finally, the structures created were analyzed in a SEM (scanning electron microscope). It was revealed that fiber like SWCNTs created randomly distributed network with Nano junctions (NJ) inside the SiC matrix and infused the thermoelectric properties in the combined SiC+SWCNTs material system.