In this study, twin wire arc spraying is used to bond wire mesh to the outside surfaces of stainless steel pipes in order to increase heat transfer surface area. At the optimum spray distance, the oxide content, porosity, and adhesion strength of the coatings are shown to be 6.6%, 2.1%, and 24 MPa, respectively. Pipes with different wire mesh configurations were placed in an oven and heated to temperatures from 300 °C to 900 °C. Water temperatures were measured at the inlet and outlet of the pipe for flow rates between 0.2 and 0.5 gpm. A maximum water temperature rise of 13 °C was achieved, corresponding to a total heat flux of 57 kW/m2. Heat transfer efficiency is shown to depend strongly on the quality of the bonds between the wire mesh and pipe and the spacing of wires in the mesh.

This work studies the manufacturability of pyramidal fin arrays produced using cold gas dynamic spraying. Near-net shape pyramidal fins of various sizes were formed and tested. The fin arrays were characterized and their heat transfer properties were assessed. Results obtained correlate well with data published for banks of tubes at a similar dimensionless pitch, and show that fins produced by cold spraying outperform traditional straight-cut fins at the same fin density.

A numerical investigation of fluid flow and heat transfer through thermal spray formed metal foam heat exchangers is presented. Experimentally obtained fluid flow and heat transfer parameters are used in the simulations. Analytically obtained values of effective thermal conductivity are used to model heat transfer. A 3D CFD model was created for a metal foam heat exchanger with a square cross-section. The external walls were deposited on the foam using a wire-arc process. The channel walls of the foam were exposed to a constant temperature of 400 K and an air flow with an inlet velocity of 2 m/s. The model was verified by comparing sample results to experiments. The effect of the foam on heat transfer was then studied by varying thermal conductivity values.

This paper demonstrates the use of commercial simulation software to evaluate a new heater design for cold gas spraying. The gas heater consists of a heating unit and a self-cooling housing. The heating unit is a coiled tube encased in an insulating enclosure. The housing is a double-walled shell through which gas continually circulates, carrying heat away from the outer surface of the insulating enclosure. Simulation results indicate that there is no heat loss in the design as verified by experimental testing.

Heat exchangers play a vital role in ongoing efforts to conserve energy. Plate-type heat exchangers typically consist of two flat separated flow paths in which heat transfer enhancing matrices are inserted. The combined effects of small irregular hydraulic diameters along with elevated heat transfer areas results in highly-efficient heat transfer to the external fluid. This allows for very versatile and compact heat exchanger designs. Typical plate-type heat exchanger fabrication methods such as brazing are labour intensive and limit post-processing operations like welding. In this paper, a novel micro-heat exchanger fabrication method using recently patented technologies is presented. The approach uses thermal spray processes such as Pulsed Gas Dynamic Spraying (PGDS) as an alternative to brazing for the production of a pressure barrier and integration of flow headers. Mesh wafer surfaces sealed using PGDS

Concentrating solar power (CSP) systems represent a zero emission method for conversion of sunlight to electricity. CSP systems use an array of mirrors to concentrate sunlight on the surface of a heat exchanger and heat a working fluid. These heat exchanger surfaces must have high absorptivity and low emissivity in the solar spectrum. In addition, they must be capable of extended operation at temperatures in excess of 600°C. Initial development of solar selective coatings using the air plasma spray process will be discussed. Eight different coating materials were deposited onto 304L stainless steel substrates. Solar absorptance and emittance were measured from each coating in three conditions: as-deposited, after heat treatment at 600°C for six hours, and after polishing to a 1 µm finish. A figure of merit based upon solar power tower (SPT) operation was calculated from these data and compared to the industry standard solar selective coating for SPT receivers, Pyromark Series 2500 high temperature paint. This comparison shows that Ni-5Al, 80WC-20Co, and CeO plasma-sprayed coatings all have potential as solar selective surfaces for SPT receivers.

Thermal spray coating processes have been employed in the current study to deposit well-adhered, dense skins on the surfaces of open-cell nickel foams. Using foam with 10 and 40 PPI (pores per inch) pore sizes, square channels were made with a height of 20mm and having a length of 250mm. In a unique process that prevents the deposited skin from penetrating the foam substrate via a paste comprised of a thermoset resin and powder particles, a dense stainless steel skin with an average thickness of 400 μm is applied to the exterior of the foam sample. The result is a channel that consists of a Ni foam core and a stainless steel skin wall that can be used as a compact heat-exchanger by directing the coolant flow through the foam. To study the feasibility of the metallic foam heat-exchangers, hydraulic and heat-transfer characteristics were investigated experimentally. The local wall and fluid temperature distribution and the pressure drop along the length of the heat exchanger were measured for heat-flux of 1540.35 – 9627.38 W/m 2 . Experiments were conducted using air as the coolant and varying flow velocity from 10 – 80 L/min. For non-Darcy flow with inertia effects in the porous media, the Dupuit and Forchheimer modification is employed with the experimental results to determine foam characteristics such as permeability (K), Ergun coefficient (CE) and the friction factor (f). To measure the heat-transfer performance of the metal foam filled channels, a length average Nusselt number is derived based on the local wall and fluid temperatures. Heat transfer was shown to have nearly doubled compared to that of a channel without a foam core.

Open pore foams can be used as gas filters, catalyst supports, and heat exchangers due to their high gas permeability and heat conductivity. In this study, Ni-Cr skins were deposited on each side of a foam sheet by HVOF spraying to form a sandwich structure for use as a heat exchanger. The microstructure of the skins and the interface with the nickel foam struts were examined and the hydraulic characteristics and heat transfer properties of the sandwich structure were experimentally determined. Pressure drops across the heat exchanger were measured and found to be proportional to the square of the velocity of the coolant and a least square fit was used to solve for the permeability, K, and form coefficient, C, of the foam.

Nickel-based superalloys can be used at temperatures up to 1050 °C in air. Superalloy open cell foam sheets with skin layers plasma sprayed on both sides can be used as high temperature heat exchangers provided that the two deposited skins are dense and well adhered to the open cell foam. In this study alloy 625 skins were deposited on each side of a sheet of metal foam by APS and HVOF to form a sandwich structure. Two densities of open cell foams, 20 and 10 pores per linear inch (ppi), were used in this study as the core. The initial Ni foam was converted to an alloy composition by plasma spraying aluminum and chromium on the foam’s struts with subsequent diffusion/solutionizing heat treatments before the alloy 625 skins were deposited. The microstructure of the coatings and the interface between the struts and skins was investigated. A layer of Ni-Al alloy was formed near the surface of the struts as a result of the heat treatment. The foam struts were imbedded more deeply into the coatings deposited by HVOF than the coatings deposited by APS.

The conventional manufacturing process of the automotive brazed heat exchanger includes complex preparation processes before brazing: aluminum brazing filler alloy is pre-claded on both sides of a fin by an extrusion method, and holed aluminum tubes are coated on both sides with Zn for corrosion protection by a wire arc spraying process. The intent of this study is to simplify the preparation process by kinetic spraying using all of the components, including Al-12%Si (for the brazing filler metal), Zn (for corrosion protection), and KAlF4 (flux powder). Four kinds of blended powder, with and without flux, were evaluated. The bond properties and composition distribution at the braze joint area were evaluated by SEM and an electron probe micro analyzer (EPMA). It was necessary to control the Zn content so that the corrosion resistance and brazeability of the aluminum heat exchanger would not be affected. An optimal kinetic spray condition was obtained, in order to fabricate the heat exchanger in this study. It was observed that the joints of the brazed specimens on each side of the brazing part were sounder than those achieved brazed by the conventional methods. Further, the kinetic sprayed heat exchanger showed acceptable corrosion protection.

Fireside corrosion and erosion of heat exchanger tubes is a serious problem. One of the methods to combat this is by applying corrosion and erosion resistant coatings. Nickel– chromium alloys have already been used as coatings to deal with oxidation environments at high temperature. The wear resistance of these coatings can be improved by adding different hard precipitates such as carbides of refractory metals and cemented carbides. In the present study, various compositions comprising of Ni-Cr with 15, 35, 60 and 100% wt% WC/Co were made using thermal spray grade powders. These were then coated on steel substrate by the HVOF method. After detailed characterization of the coatings, the performance of the coatings at high temperature was studied by exposing the coated samples in the temperature range of 600-700°C. Ni-alloy coatings with moderately (15%) added WC/Co showed better oxidation resistance than coatings with high percentage of WC/Co and also than the pure Ni-alloy coating in high temperature range of 600-800°C. From the characterization, it was found that Ni-Cr alloy with 15% WC/Co gave the optimum results.

In many industrial processes, metallic materials are subjected to high temperature corrosion and aggressive forms of erosion. Such conditions occur especially in thermal energy advanced systems using tube bundles of fluidised bed boilers. For these types of plants, availability and low costs determine the choice of materials and in most cases this choice is not optimised for middle or long-term behaviour. Moreover, few data exist to estimate material lifetime under such aggressive conditions. Such is the case for low carbon steel grades which constitute the basic materials for these plants. New FeAl intermetallic alloys have been developed in the framework of a European collaboration, and it has been seen that these materials have good corrosion and erosion resistance properties at high temperature. Heat exchanger tubes in low carbon steel have been coated by thermal-spray then tested in a new industrial plant burning a very poor fuel (coal residues). After 5000 hours of operation in a rich erosive high temperature environment (850-900°C), no significant wear was observed on coated tubes whereas the other tubes, without protection, showed an appreciable diameter reduction.

High temperature corrosion is a serious problem on tlie heat exchanger tubes of recuperators because they encounter an corrosive environment at maximum temperature around 900°C. These tubes were found to be corroded via oxidation, sulfidation and molten salt corrosion. Particularly molten salt corrosion could be the most severe corrosion mechanism. As a protective coating for recuperators, nickel and cobalt based self-fluxing alloys, iron based amorphous alloy and chromium carbide cermet coatings were considered. These coatings were prepared by an arc spray and or/not fusing or a HVOF spray. Their molten salt corrosion resistance was tested, and the high temperature corrosion resistance in a SO2 containing atmosphere was examined. Also microstructures of the coatings were studied after corrosion tests.