John D. Bernardin

Mapping of impact and heat transfer regimes of water drops impinging on a polished surface

Still and high-speed photographic techniques were used to record the impact behavior of water droplets on a hot aluminum surface. Drop velocity and surface temperature were two important parameters governing both the impact behavior and ensuing heat transfer. Droplet Weber numbers of 20, 60 and 220 identified three major classes of impact behavior, while surface temperatures ranging from 280 to 100°C were used to define heat transfer regimes corresponding to film boiling, transition boiling, nucleate boiling, and film evaporation. Temperatures corresponding to the critical heat flux and the Leidenfrost point showed little sensitivity to both droplet velocity and impact frequency. The photographic results and heat transfer measurements were used to construct droplet impact regime maps which identify the various boiling regimes for each of the three Weber numbers. These maps serve as a new useful foundation for understanding droplet impact behavior as well as future analytical or numerical modeling of droplet and spray heat transfer.

Contact angle temperature dependence for water droplets on practical aluminum surfaces

Abstract This paper presents an experimental investigation of the temperature dependence of the quasistatic advancing contact angle of water on an aluminum surface polished in accordance with surface preparation techniques commonly employed in boiling heat transfer studies. The surface, speculated to contain aluminum oxide and organic residue left behind from the polishing process, was characterized with scanning electron microscopy, surface contact profilometry, and ellipsometry. By utilizing a pressure vessel to raise the liquid saturation temperature, contact angles were measured with the sessile drop technique for surface temperatures ranging from 25 to 170°C and pressures from 101.3 to 827.4 kPa. Two distinct temperature-dependent regimes were observed. In the lower temperature regime, below 120°C, a relatively constant contact angle of 90° was observed. In the high temperature regime, above 120°C, the contact angle decreased in a fairly linear manner. Empirical correlations were developed to describe this behavior which emulated previous experimental data for nonmetallic surfaces as well as theoretical trends.

Effects of surface roughness on water droplet impact history and heat transfer regimes

Abstract Still and high speed photographic techniques and heat transfer measurements were used to study the impact of water droplets on heated surfaces with different roughness. The study encompassed droplet Weber numbers of 20, 60 and 220 and surface temperatures of 100–280°C, covering droplet stability and heat transfer regimes established previously by the authors for polished surfaces. Three different surface finishes, polished, particle blasted and rough sanded, with respective arithmetic average surface roughness values of 97, 970 and 2960 nm, were applied to the test surfaces. While the temperature corresponding to critical heat flux (CHF) was fairly independent of surface roughness, the Leidenfrost point (LFP) temperature was especially sensitive to surface finish. The parametric effects of Weber number and surface temperature were consolidated into droplet impact regime maps for each of the three surface finishes. Aside from depicting the commonly known boiling curve regimes of film, transition and nucleate boiling, and thin film evaporation, these maps illustrate the complex liquid-solid interactions which occur during the lifetime of the impacting droplet within each of the boiling curve regimes, thus serving as an effective reference for future modeling of droplet heat transfer. Copyright

Film boiling heat transfer of droplet streams and sprays

Abstract This paper presents an empirical approach to determining film boiling heat transfer of a spray from extrapolation of the heat transfer characteristics of an isolated droplet stream. First, an experimental investigation of film boiling heat transfer from a polished nickel surface to a continuous stream of monodispersed water droplets was performed for surface temperatures up to 400°C. Empirical correlations are presented for film boiling heat transfer rate and droplet heat transfer efficiency over a wide range of operating conditions. These single droplet stream correlations were then employed to predict film boiling heat transfer rates of both multiple droplet streams and sprays of uniform droplet size and velocity. By correctly accounting for differences in volumetric flux and droplet heat transfer efficiency, it is shown how the single droplet stream correlations facilitate the prediction of film boiling heat transfer of a dilute spray.

A Leidenfrost Point Model for Impinging Droplets and Sprays

This study presents, for impinging droplets and sprays, a model of the Leidenfrost point (LFP); the minimum liquid/solid interface temperature required to support film boiling on a smooth surface. The present model is an extension of a previously developed sessile drop model, based on bubble nucleation, growth, and merging criteria, as well as surface cavity size characterization. The basic concept of the model is that for liquid/solid interface temperatures at and above the LFP, a sufficient number of cavities are activated and the bubble growth rates are sufficiently fast that a continuous vapor layer is established nearly instantaneously between the liquid and the solid. For impinging droplets, the influence of the rise in interfacial pressure created by the impact of the droplet with the surface, must be accounted for in determining fluid properties at the liquid-solid interface. The effect of droplet impact velocity on the LFP predicted by the model is verified for single impinging droplets, streams of droplets, as well as sprays

A Cavity Activation and Bubble Growth Model of the Leidenfrost Point

This study presents a new mechanistic model of the Leidenfrost point (LFP); the minimumliquid/solid interface temperature required to support film boiling on a smooth surface.The model is structured around bubble nucleation, growth, and merging criteria, as wellas surface cavity size characterization. It is postulated that for liquid/solid interfacetemperatures at and above the LFP, a sufficient number of cavities (about 20 percent) areactivated and the bubble growth rates are sufficiently fast that a continuous vapor layer isestablished nearly instantaneously between the liquid and the solid. The model is appli-cable to both pools of liquid and sessile droplets. The effect of surface cavity distributionon the LFP predicted by the model is verified for boiling on aluminum, nickel and silversurfaces, as well as on a liquid gallium surface. The model exhibits good agreement withexperimental sessile droplet data for water, FC-72, and acetone. While the model wasdeveloped for smooth surfaces on which the roughness asperities are of the same magni-tude as the cavity radii (0.1–1.0 mm), it is capable of predicting the boundary or limitingLeidenfrost temperature for rougher surfaces with good accuracy.@DOI: 10.1115/1.1470487#Keywords: Boiling, Bubble Growth, Cavities, Heat Transfer

Experimental and statistical investigation of changes in surface roughness associated with spray quenching

Abstract Surface contact profilometry data measured on aluminum surfaces were statistically analyzed to investigate the changes in surface roughness which occur during heat treatment. The tested samples included commercially pure aluminum (Al-1100) with polished, particle blasted, and milled finishes; and aluminum alloy Al-2024 with polished, particle blasted and extruded finishes. Statistical results along with scanning electron microscope photographs indicate each heat-quench cycle was accompanied by measurable changes in surface roughness, whose magnitude was dependent upon initial surface finish and alloy composition. The changes included both small scale roughness features, which influence cooling rate by increasing the number of bubble nucleation sites during transition and nucleate boiling, and, more importantly, large features which influence the impact and spreading of spray drops as well as the Leidenfrost temperature for spray quenching. The primary cause for roughness was determined to be a hydrogen diffusion phenomenon resulting from breakdown of water vapor at high temperatures inside moisture laden furnaces.

Embedding Sensors in FDM Plastic Parts During Additive Manufacturing

In additive manufacturing, there is a necessity to qualify both the geometrical and material characteristics of the fabricated part, because both are being created simultaneously as the part is built up layer by layer. Increased availability of open source fused deposition modeling machines has expanded the parameter space for which the user has control during the build process. This work quantifies the effects of operator choices, such as print speed, printer head and build plate temperatures, layering thickness, or building in a thermally controlled or fully open environment, on the quality and reproducibility of the build. Modal analyses were performed on completed builds using an electrodynamic shaker and integrated circuit piezoelectric accelerometers embedded in the parts during the build process. Experimental measurements of the fused deposition modeled parts were benchmarked against eigenvalue analysis results for an idealized part with homogenous material properties to gauge the suitability of such analysis to fused deposition modeling additive manufacturing. Follow on work will use this embedded technique for state-of-health monitoring in deployed systems and real-time diagnostics and control of the build process.

The mechanical design and dynamic testing of the IBEX-H1 electrostatic analyzer spacecraft instrument

This paper presents the mechanical design, fabrication and dynamic testing of an electrostatic analyzer spacecraft instrument. The functional and environmental requirements combined with limited spacecraft accommodations, resulted in complex component geometries, unique material selections, and difficult fabrication processes. The challenging aspects of the mechanical design and several of the more difficult production processes are discussed. In addition, the successes, failures, and lessons learned from acoustic and random vibration testing of a full-scale prototype instrument are presented.

A Framework for Additive Manufacturing Process Monitoring & Control

Fused deposition modelling, like other additive manufacturing methods, has largely remained an open loop process in the absence of rigorous process monitoring and diagnostic functionality. By creating a framework that integrates quantitative diagnostic tools whose measurements are coordinated with the printing process and the system which commands the printer hardware, this paper demonstrates the feasibility of closing the loop in additive manufacturing systems. Specifically, this paper introduces the use of ultrasonic excitation as a means of detecting filament bonding failures introduced by manipulating the print bed temperature during the fused deposition modelling build process. Furthermore, this work demonstrates the capability of correcting these filament bonding failures using a correction mechanism introduced through tunable control of another process parameter of the printer. By demonstrating the detection and correction of filament bonding failures in situ, this work has demonstrated the progress toward fully closed loop control for fused deposition modeling processes.