Herman D. Haustein

Influence of micro-scale aspects and jet-to-jet interaction on free-surface liquid jet impingement for micro-jet array cooling

Arrays of impinging jets can cool large areas with good thermal uniformity and are often used in industrial processes, such as drying and electronics cooling. However, due to cross-flow of the spent liquid and interference between adjacent jets, a significant amount of the available cooling performance is lost. Under free-surface jet impingement the area beyond the hydraulic jump is associated with significantly reduced heat transfer, and locally increased temperatures, therefore the hydrodynamics in this area must be better understood. Specifically, the interaction of jets in the vicinity of this location is expected to shed light on improving multi-jet array cooling uniformity and performance. Beyond this, it has been shown that micro-scale (sub-millimeter) jets tend to behave somewhat differently from larger jets, due to the increased significance of surface tension, pumping noise and edge-effects (such as small recirculation zones, and jet widening due to contact-angle at the nozzle exit, noise and nozzle imperfections due to manufacturing). These become much more dominant at the micro-scale. These effects cannot usually be accounted for by traditional scaling laws or numerical simulations, and are preferably investigated experimentally. Moreover, at these scales a micro-machined fixed-geometry array of jets is typically used, leaving no possibility for geometric variation, optimization and limited observation.

A simple hydrodynamic model of a laminar free-surface jet in horizontal or vertical flight

A useable model for laminar free-surface jet evolution during flight, for both horizontal and vertical jets, is developed through joint analytical, experimental, and simulation methods. The jet’s impingement centerline velocity, recently shown to dictate stagnation zone heat transfer, encompasses the entire flow history: from pipe-flow velocity profile development to profile relaxation and jet contraction during flight. While pipe-flow is well-known, an alternative analytic solution is presented for the centerline velocity’s viscous-driven decay. Jet-contraction is subject to influences of surface tension (We), pipe-flow profile development, in-flight viscous dissipation (Re), and gravity (Nj = Re/Fr). The effects of surface tension and emergence momentum flux (jet thrust) are incorporated analytically through a global momentum balance. Though emergence momentum is related to pipe flow development, and empirically linked to nominal pipe flow-length, it can be modified to incorporate low-Re downstream diss...

Analytical re-examination of the submerged laminar jet’s velocity evolution

Understanding laminar submerged jet flight is important to many transport processes, although existing theory is insufficient within the most relevant near-nozzle region defined by the effective distance x′/(D·Re) < 0.05. A linearized convection diffusion momentum equation is employed to derive an approximate flow description within the jet core, for all archetypal issuing profiles. This is validated in the core region near the nozzle by numerical simulations and experimental measurements, and it provides novel insights and adaptation of the far-field (self-similar) Schlichting jet solution. It is employed here to reveal the detailed contour of each profile’s potential core and allows its differentiation from a new “boundary core” concept—the region unaffected by the change of the jet-edge shear transition from pipe flow to free-jet. This new concept reveals the minimal distance at which self-similarity can begin to exist, thereby analytically determining the virtual origin required to bring the existing far-field solution nearest to the nozzle. Thus, profile evolution and jet width become predictable within the near-nozzle region for all issuing profiles. As an alternative to the lengthy full prediction, current analysis also facilitates analytical rederivation of physically based parameters for two existing correlations describing the full uniform profile evolution and the centerline velocity decay for all other issuing profiles.Understanding laminar submerged jet flight is important to many transport processes, although existing theory is insufficient within the most relevant near-nozzle region defined by the effective distance x′/(D·Re) < 0.05. A linearized convection diffusion momentum equation is employed to derive an approximate flow description within the jet core, for all archetypal issuing profiles. This is validated in the core region near the nozzle by numerical simulations and experimental measurements, and it provides novel insights and adaptation of the far-field (self-similar) Schlichting jet solution. It is employed here to reveal the detailed contour of each profile’s potential core and allows its differentiation from a new “boundary core” concept—the region unaffected by the change of the jet-edge shear transition from pipe flow to free-jet. This new concept reveals the minimal distance at which self-similarity can begin to exist, thereby analytically determining the virtual origin required to bring the existing ...

Development of Heat Transfer in a Two-Dimensional Wavy Falling Film of Water and its Influence on Wave Stability

Heat exchangers employing falling films are relevant to a multitude of industrial applications using water-based liquids. In the present study, periodic, two-dimensional waves are imposed by excitation on a vertically falling film of water, which is then heated by a uniform heat flux, within the laminar and transitional flow range (39<Re<200). Liquid-film thickness is measured by confocal chromatic imaging and surface temperature is measured by high-speed IR thermography. As the 2D waves travel downstream they destabilize in the spanwise direction and evolve 3D structures (bumps). Further wave destabilization, under relatively low heating, was observed to coincide with the appearance of local thermal flows (“hot streaks”), though no deformation of the liquid surface could be measured. These flows are understood to be induced by thermo-capillary forces, which in extreme cases are known to lead to the formation of rivulets, film rupture and heater burnout. Understanding these initial stages of thermo-capillary flow is crucial to its suppression.Analysis of the thermal images reveals several significant streamwise length scales: a thermal inlet length based on the emergence of the thermal boundary layer (Lt), a thermal inlet length based on reaching thermally developed conditions (Lh), and the length at which “hot-streaks” first appear (Ls). In addition the dominant (most unstable) spanwise wavelength of the hot streaks, Lz, was identified through FFT analysis of the thermal profile beyond Ls. First the independence of the thermal inlet lengths from the heat-flux was established. Next, the influence of the nominal flow conditions (Reynolds number and excitation frequency) on Lt, Lh and Lz was examined — thereby extending the range of previous studies to higher Reynolds numbers. The thermal inlet lengths Lt and Lh were found to increase with flow rate, whereas they had opposing trends with regard to frequency. Lz consistently decreased with an increase of the flow rate, as smaller (turbulent) scales became more dominant, and it was found to be indifferent to excitation frequency over a wide range. Some future directions and methods of hot streak suppression are discussed, as well.© 2013 ASME

Evaluation of the sensitivity and response of IR thermography from a transparent heater under liquid jet impingement

The feasibility of a visible/IR transparent heater and its suitability for IR thermography is experimentally examined. The most common transparent conductive coating, Indium Tin Oxide (ITO), is quite reflective and its optical properties depend on thickness and manufacturing process. Therefore, the optical properties of several thicknesses and types of ITO, coated on an IR window (BaF2), are examined. A highly transparent Cadmium Oxide (CdO) coating on a ZnS window, also examined, is found to be unusable. Transmissivity is found to increase with a decrease in coating thickness, and total emittance is relatively low. A thick ITO coating was examined for IR thermography in the challenging test case of submerged water jet impingement, where temperature differences were characteristically small and distributed. The measurements under steady state conditions were found to agree well with the literature, and the method was validated. Comparison of two IR cameras did not show the LWIR low-temperature advantage, up to the maximal acquisition rate examined, 1.3KHz. Rather the MWIR camera had a stronger signal to noise ratio, due to the higher emissivity of the heater in this range. The transient response of the transparent heater showed no time-delay, though the substrate dampens the thermal response significantly. Therefore, only qualitative transient measurements are shown for the case of pulsating free-surface jet impingement, showing that the motion of the hydraulic jump coincides with thermal measurements. From these results, recommendations are made for coating/window combination in IR thermography.

A New Empirical Model for Bubble Growth: Boiling in an Infinite Medium and on a Wall at High Superheat

At high superheat bubble growth is rapid and the heat transfer is dominated by radial convection. This has been found, in the case of a droplet boiling within another liquid and in the case of a bubble growing on a heated wall, leading to similar bubble growth curves. Based on experiments conducted for the first case, an empirical model is developed for the prediction of bubble growth within the radial convection dominated regime (the RCD model), occurring only at high superheat (0.26<Ste<0.41). This model shows a dependence of R∼t1/3 equivalent to Nusselt number decreasing over time (Nu∼t1/3 ) as opposed to R∼t1/2 appearing in most other models, leading to a highly unlikely constant Nusselt number. The new model is shown to give accurate predictions for the first case and for the second case at medium-high superheat (0.19<Ste<0.30, experimental data taken from literature). A comparison of the RCD model to other models, shows a more consistent and accurate prediction. However, in the second case (nucleate boiling) the RCD model requires the foreknowledge of the departure diameter, for which a reliable model still is lacking.Copyright