Upward-facing Surface Research Articles

Heat transfer characteristics of printed circuit heat exchangers under mechanical vibrations

PurposeThe purpose of this paper is to investigate the impact of mechanical vibration on the heat transfer and pressure drop characteristics of semicircular channel printed circuit heat exchangers (PCHEs), while also establishing correlations between vibration parameters and thermal performance.Design/methodology/approachBy combining experimental and numerical simulation methods, the heat transfer coefficient and pressure drop characteristics of supercritical carbon dioxide (S-CO2) in a semicircular channel with a diameter of 2 mm under vibration conditions were studied. Reinforce the research by conducting computational fluid dynamics studies using ANSYS Fluent 22.0, the experimental results were compared with the numerical simulation results to verify the accuracy of the numerical method.FindingsThe use of vibration has the potential to attenuate the degradation of wall heat transfer caused by buoyancy-induced PCHEs on the upward-facing surface. The heat transfer enhancement (HTE) was maximized by an increase of 18.2%, while the pressure drop enhancement (PDE) was elevated by over 25-fold. The capacity to enhance the heat exchange between S-CO2 and channel walls through increasing vibration intensity is limited, indicating maximum effectiveness in improving thermal performance.Originality/valueConducting heat transfer experiments on PCHEs with mechanical vibration enhancement and verifying the accuracy of the vibration numerical model. The relation based on the dimensionless factor is derived. To provide theoretical support for using vibration to enhance the heat transfer capability of PCHEs.

Toward a complete understanding of quasi-static bubble growth and departure

The distortion by gravity of a quasi-static bubble attached on an upward facing surface in a quiescent liquid is investigated. The contact angle evolution during the growth of such a bubble is studied, and the consequences on the motion of the contact line between the solid and the interface are discussed. From the initial case of a bubble attached to the rim of a nucleation site, the contact line can move inside the cavity for a highly wetting fluid, causing premature departure. For a higher wettability, the contact can either remain attached to the rim of the cavity or spread out of the cavity, depending on the cavity size and geometry. For the latter case, the bubble growth is investigated taking into account a contact angle hysteresis. Finally, a comprehensive map detailing various geometrical characteristics of bubbles is presented, ranging over several orders of magnitude of Bond numbers and normalized volumes. The map aims at being used as a tool for investigating bubble growth in a similar situation.

Complex-Shape Solid-State Photonic Droplets Prepared via Phase Separation and Microfluidics.

Complex-shape solid-state cholesteric liquid crystal (CLCsolid) droplets were prepared via solvent removal, phase separation, and photopolymerization of uniformly sized reactive CLC (rCLC)/fluorocarbon oil (FCO)/dichloromethane (solvent) droplets produced via a microfluidic method. The interfacial energies between rCLC and FCO, rCLC and water, and FCO and water of a rCLC/FCO droplet in an aqueous solution were precisely controlled through the specified surfactants. The shape of the rCLC/FCO droplet was strongly dependent on the balances among these interfacial energies, enabling the preparation of complex-shape droplets through the controlled concentration of the used surfactants. The complex-shape rCLC/FCO droplets showed photonic patterns consisting of a central reflection from a convex surface, cross-communication from a convex surface between adjacent particles, a photonic reflection band from the outer upward-facing concave surface, and total internal reflection from the inner upward-facing surface. Complex-shape CLCsolid particles obtained after photopolymerization and extraction of a nonreactive chiral dopant and FCO showed photonic patterns similar to those before photopolymerization without much deterioration of the photonic structure. These complex patterns make CLCsolid and rCLC/FCO droplets promising anticounterfeiting materials.

Mass finishing of additively manufactured Ti6Al4V parts: An investigation of surface finish dependency on build orientation and processing conditions

Poor surface finish of as-built metal Additive Manufacturing (AM) remains a key challenge in fabricated parts. Metal AM parts typically require postprocessing to improve surface finish before application. Furthermore, different surface orientations and scanning strategies in Powder-Bed-Fusion (PBF) metal AM are prone to result in non-uniform surface roughness across different surfaces of a part. In recent years, Mass Finishing (MF) technologies, such as Centrifugal Disc Finishing (CDF), are gaining popularity in improving AM surfaces due to its fixture-free, batch-processing, and relatively low costs. There is a critical need to study the surface finish dependency on AM build orientation and MF processing conditions. In this study, Ti6Al4V ASTM-E8b tensile bars were fabricated in 45⁰ and 90⁰ orientation via Laser Powder Bed Fusion (L-PBF) to investigate the effects of CDF processing conditions on upward facing surface (up-skin), downward facing surface (down-skin), and side facing surface (side-skin). The surface roughness evolution on each type of surface were collected via laser scanning profilometry. It was found that both processing speed and processing time of CDF were statistically significant factors in improving AM surface roughness, however, processing time had higher impact. Furthermore, different build orientation demonstrated different surface roughness reduction rate and material removal rate. It was found that side-skin had the highest average roughness reduction, and the down-skin exhibited the least roughness reduction. In addition, this study found that the surface with the highest initial surface roughness (down-skin) had the least material removal rate, and that the highest material removal rate was observed in surface with higher periodicity (side-skin). Findings from this study will help the AM community in decision-making of CDF conditions to prevent over-finishing and under-finishing. In addition, the investigation of surface evolution dependency on AM build orientation can further improve the understanding of key mechanisms of AM + MF hybrid manufacturing system.

An investigation into the effects of contouring process parameters on the up-skin surface characteristics in laser powder-bed fusion process

Laser powder-bed fusion (L-PBF) additive manufacturing is reshaping how parts are designed and built in the aerospace, automotive, biomedical, and other industries. However, in several applications, the quality of the components in terms of surface roughness, density, and mechanical properties is not comparable to that of parts made by conventional manufacturing processes. Multiple process parameters and associated uncertainties strongly influence the properties of L-PBF built parts. This study aims to understand the effects of contouring process parameters on the formation of up-skin surfaces and roughness variability in an inclined build scenario. Through Design of Experiment (DOE), the influence of the contour process parameters, such as the laser power, scanning speed, contouring offset, and incline angle (θ, between 30° and 60°), on upward-facing surfaces is investigated. Furthermore, a comprehensive multi-physics Computational Thermo-fluid model and Discrete Element Method (DEM) simulation framework is developed to shed light on how contouring scans affect the morphology of the upward-facing surface. The developed thermo-fluid model framework is used to perform multi-track multi-layer simulations of an L-PBF process. Numerical data suggests that a higher laser power enlarges the melt-pool size providing sufficient fusion at the border, hence, improving the density of up-skin regions. A higher density may primarily contribute to a better surface finish for up-skin features. The experimental result analysis shows that in the tested slope range (30° ≤ θ ≤ 60°), the up-skin surface roughness exacerbates with the increase of the incline angle and the scanning speed. On the other hand, a higher laser power will result in better surface finishes. The findings also indicate that a higher linear energy density (LED = Power/Speed) for contour scans is more favourable for the range of parameters tested in this study. Within the range of studied contouring process parameters, the maximum and minimum of average roughness (Ra) values for up-skin surfaces are 21.2 ± 2.9 µm and 8 ± 0.6 µm, respectively.

Demonstration of a Remotely Piloted Atmospheric Measurement and Charge Release Platform for Geoengineering

AbstractElectric charge is always present in the lower atmosphere. If droplets or aerosols become charged, their behavior changes, influencing collision, evaporation, and deposition. Artificial charge release is an unexplored potential geoengineering technique for modifying fogs, clouds, and rainfall. Central to evaluating these processes experimentally in the atmosphere is establishing an effective method for charge delivery. A small charge-delivering remotely piloted aircraft has been specially developed for this, which is electrically propelled. It carries controllable bipolar charge emitters (nominal emission current ±5 μA) beneath each wing, with optical cloud and meteorological sensors integrated into the airframe. Meteorological and droplet measurements are demonstrated to 2 km altitude by comparison with a radiosonde, including within cloud, and successful charge emission aloft verified by using programmed flight paths above an upward-facing surface electric field mill. This technological approach is readily scalable to provide nonpolluting fleets of charge-releasing aircraft, identifying and targeting droplet regions with their own sensors. Beyond geoengineering, agricultural, and biological aerosol applications, safe ionic propulsion of future electric aircraft also requires detailed investigation of charge effects on natural atmospheric droplet systems.

An Upward-Facing Surface Appears Darker: The Role Played by the Light-From-Above Assumption in Lightness Perception.

The human visual system can extract information on surface reflectance (lightness) from light intensity; this, however, confounds information on reflectance and illumination. We hypothesized that the visual system, to solve this lightness problem, utilizes the internally held prior assumption that illumination falls from above. Experiment 1 showed that an upward-facing surface is perceived to be darker than a downward-facing surface, proving our hypothesis. Experiment 2 showed the same results in the absence of explicit illumination cues. The effect of the light-from-left prior assumption was not observed in Experiment 3. The upward- and downward-facing surface stimuli in Experiments 1 and 2 showed no difference in a two-dimensional configuration or three-dimensional structure, and the participants' perceived lightness appeared to be affected by the observers' prior assumption that illumination is always from above. Other studies have not accounted for this illusory effect, and this study's finding provides additional insights into the study of lightness perception.

Surface Orientation Effect on Local Heat Transfer by Round Water Jet Impingement

This paper describes local heat transfer for a round water jet on upward-facing and downward-facing static plates. Impinging liquid jets have high heat transfer rates. One of the technological issues in the steel making process is uniformity in cooling between the top and bottom sides of a plate. The objective of this study is to investigate the surface orientation effect on transient heat transfer experimentally. Experiments were conducted for a range of water flow rate, surface orientation. Both surface orientations were conducted on the same apparatus. The heat transfer plate was stainless steel to deal with both supercritical flow and subcritical flow regions. The plate length was wide for laboratory scale. Initial plate temperature was 800 degrees Celsius. Pipe Reynolds numbers were laminar flow regime. Inverse solution was employed to estimate transient local surface temperature and heat flux. Heat transfers between upward-facing and downward-facing surfaces are the same in the order of magnitude at downward-facing surface flow rate from 1.2 times to twice that of upward-facing surface. Local downward-facing surface temperature is higher than that of upward-facing temperature at the same flow rate. The following points can be given as reasons. Downward-flowing impinging jet velocity is larger than that of upward-flowing jet at the same flow rate due to gravity direction. Heat transfer is affected by a geometric arrangement relation of radial spreading liquid film, vapor and hot plate.

Bubble Movement on Inclined Hydrophobic Surfaces.

The movement of a single air bubble on an inclined hydrophobic surface submerged in water, including both the upward- and downward-facing sides of the surface, was investigated. A planar Teflon sheet with an apparent contact angle of a sessile water droplet of 106° was used as a hydrophobic surface. The volume of a bubble and the inclination angle of a Teflon sheet varied in the ranges 5-40 μL and 0-45°, respectively. The effects of the bubble volume on the adhesion and dynamics of the bubble were studied experimentally on the facing-up and facing-down surfaces of the submerged hydrophobic Teflon sheet, respectively, and compared. The result shows that the sliding angle has an inverse relationship with the bubble volume for both the upward- and downward-facing surfaces. However, at the same given volume, the bubble on the downward-facing surface spreads over a larger area of the hydrophobic surface than the upward-facing surface due to the greater hydrostatic pressure acting on the bubble on the downward-facing surface. This makes the lateral adhesion force of the bubble greater and requires a larger inclination angle to result in sliding.

Hydrodynamics of nucleate boiling on downward surface with various orientation. Part I: Departure diameter, frequency, and escape speed of the slug

We investigated slug dynamics in nucleate pool boiling on downward-facing surfaces using high-speed camera visualization. The behavior of a single slug was observed according to orientation angle (120, 135, 150, 160, 170°) and heat flux conditions, to near critical heat flux. Using image processing, the translation speed, frequency, departing diameter of slug, and vapor generation rate were determined. In nucleate boiling on a downward-facing surface, the slug deformed in the normal direction to the surface and slowed down with the tangential direction of the surface, compared with that on an upward-facing surface (0°). Accordingly, a thinner and wider slug departed from the heating surface at a lower speed, as the orientation angle increased. Thus, the measured translation speed of the slug was strongly dependent on the orientation and radius of the slug. Moreover, there was a strong relationship between the departing frequency and diameter of the slug and the vapor generation rate from the heating surface. Based on the measured vapor generation rate from images, heat flux by evaporation was predicted, resulting in slight overestimation of the applied heat flux. These findings indicate that evaporation represented most of the heat transfer in downward nucleate boiling, rather than quenching.

Critical heat flux enhancement on a downward face using porous honeycomb plate in saturated flow boiling

Enhancing the Critical Heat Flux (CHF) is extremely needed in some engineering applications. Many methods involving nanofluids and surface modifications have been investigated to enhance the CHF. Most of the previous works focus on pool boiling and upward-facing surface conditions. An experimental research on enhancing the CHF in downward-facing saturated flow-boiling conditions was performed by attaching a metal porous honeycomb plate. The experiments were conducted under four different flow rates: 160, 320, 640, and 1280kg/(m2s). It was found that the porous honeycomb plate can both enhance the CHF and reduce surface superheat at all four flow rates. However, the CHF increase ratio decreased as the flow rate increased, i.e., from 2.4 at 160kg/(m2s) to 1.1 at 1280kg/(m2s). The enhancement effect is attributed to the honeycomb structure and water supply through the porous media of the honeycomb. The role of the water supply through the metal porous media was investigated.

Buoyancy-induced turbulent mixing in a narrow tilted tank

We describe a series of experiments in which a constant buoyancy flux $B_{s}$ of salty dyed water of density ${\it\rho}_{s}$ is introduced at the top of a long narrow tank of square cross-section tilted at an angle ${\it\theta}$ to the vertical. The tank is initially filled with fresh clear water of density ${\it\rho}_{0}<{\it\rho}_{s}$, and we investigate the resulting buoyancy-driven turbulent mixing at various tilt angles ${\it\theta}$. Using a light-attenuation image analysis method, we determine the evolution of the reduced gravity $g^{\prime }=g({\it\rho}-{\it\rho}_{0})/{\it\rho}_{0}$ of the mixed fluid in time and space as it propagates towards the bottom of the tank. For all tilt angles tested (${\it\theta}=0^{\circ }$ to ${\it\theta}=45^{\circ }$), we focus exclusively on high-Reynolds-number experiments, where the flow remains turbulent both along the length and across the width of the tank. We find that when ${\it\theta}>0^{\circ }$, the cross-tank component of gravity acts to segregate the dense fluid from the relatively lighter fluid, and a statically stable gradient of $g^{\prime }$ across the width of the tank occurs more frequently than a statically unstable gradient, i.e. $(\partial g^{\prime }/\partial x)<0$ occurs more frequently than $(\partial g^{\prime }/\partial x)>0$. This is in contrast to the case when ${\it\theta}=0^{\circ }$, where instantaneous cross-tank gradients of reduced gravity may be positive or negative, but are equal to zero in an ensemble average. We observe that when ${\it\theta}>0^{\circ }$, the cross-tank gradient of reduced gravity induces a turbulent counterflow where dense fluid flows down the upward-facing surface of the tank and lighter fluid flows in the opposing direction above. We model the evolution of the cross-tank averaged, ensemble averaged reduced gravity $\langle \overline{g^{\prime }}\rangle _{e}$ as a diffusive process using Prandtl’s mixing length theory, building on the model of van Sommeren et al. (J. Fluid Mech., vol. 701, 2012, pp. 278–303) who considered purely vertical tanks. We model the fluctuations (from the cross-tank averaged quantity) of reduced gravity $\langle {\hat{g}}^{\prime }\rangle _{e}$ and counterflow velocity $\langle {\hat{w}}\rangle _{e}$ by characterising the mixing across the width of the tank with a cross-tank turbulent diffusivity ${\it\kappa}_{T,x}$, which we assume is constant in the cross-tank coordinate $x$. We show that the counterflow that exists when ${\it\theta}>0^{\circ }$ acts directly to enhance the effective along-tank turbulent diffusivity ${\it\kappa}_{T,z}$, and from experiments, we find that the mixing length increases approximately linearly with ${\it\theta}$, and that both ${\it\kappa}_{T,x}$ and ${\it\kappa}_{T,z}$ are proportional to $(\partial \langle \overline{g^{\prime }}\rangle _{e}/\partial z)^{1/2}$.

Qualitative shape from shading, highlights, and mirror reflections

The human visual system has a remarkable ability to perceive three-dimensional (3-D) surface shape from shading and specular reflections. This paper presents two experiments that examined the perception of local qualitative shape under various conditions. Surfaces were rendered using standard computer graphics models of matte, glossy, and mirror reflectance and were viewed from a small oblique angle to avoid occluding contour shape cues. The subjects' task was to judge whether a marked point on each surface lay on a local hill or valley. In the first experiment, performance was lower for glossy surfaces than matte surfaces, which is contrary to findings in previous studies of quantitative shape. For mirror surfaces, performance was high despite the absence of occluding contours, and performance was increased when the environment map was brighter in the upper hemisphere as in a natural environment. The second experiment examined how subjects resolve a depth-reversal shape ambiguity where surfaces can be either upward or downward facing. An upward-facing surface prior that is known to exist for matte surfaces was also found to exist for glossy and mirror surfaces. Subjects relied entirely on this prior to resolve the depth-reversal ambiguity for matte and glossy surfaces, but relied on perspective cues as well for mirror surfaces.

Experimental study on critical heat flux on a downward-facing stainless steel disk in confined space

Critical heat flux (CHF) is experimentally studied on a relatively large downward-facing surface with a heated stainless steel disk diameter of D = 300 mm in confined space at atmospheric pressure using water as the working fluid. The bulk working fluid is subcooled. The gap size s can be adjusted to 0.9, 2.2, 2.6, 3.0, 3.2, 5.0, 7.0, 10.0, 13.0, 15.6, 19.5, 25.0, 36.0, 51.0 and 77 mm. We found that the average CHF under the present condition is approximately 0.25 MW/m 2 which is only about 23% of which occurs on an upward-facing surface without confined space in pool boiling. The CHF increases with the increase of the gap size when the gap size is smaller than 7 mm and it is a function of Bond and Jakob numbers when the gap size is larger than 7 mm.

Subcooled pool boiling of water on a downward-facing stainless steel disk in a gap

Pool boiling is experimentally studied on a relatively large downward-facing surface with heated stainless steel disk diameters of D = 100 and 300 mm in confined space at atmospheric pressure using water as the working fluid. The bulk working fluid is subcooled. The gap size s can be adjusted to 10, 15 and 20 mm for D = 100 mm and 0.9–77 mm for D = 300 mm. We found that pool boiling under the present condition is far weaker than that occurring on an upward-facing surface. Furthermore, we found that the larger the diameter of the stainless steel plate, the weaker the pool boiling heat transfer. The heat transfer rate may be predicted by the Kutateladze correlation for s > 20 mm and by its modified form for s < 20 mm for both D = 100 and D = 300 mm. Two different typical bubble circulation motions are found. Type I motion occurs with a probability of 90.9% and type II occurs with a probability of 9.1% according to the statistical calculations. Most coalesced bubble diameters are from 90 to 100 mm for D = 100 mm and from 100 to 200 mm for D = 300 mm.

IMAGING OF OXYGEN DYNAMICS WITHIN THE ENDOLITHIC ALGAL COMMUNITY OF THE MASSIVE CORAL PORITES LOBATA(1).

We used transparent planar oxygen optodes and a luminescence lifetime imaging system to map (at a pixel resolution of <200 μm) the two-dimensional distribution of O2 within the skeleton of a Porites lobata colony. The O2 distribution was closely correlated to the distribution of the predominant endolithic microalga, Ostreobium quekettii Bornet et Flahault that formed a distinct green band inside the skeleton. Oxygen production followed the outline of the Ostreobium band, and photosynthetic O2 production was detected at only 0.2 μmol photons m(-2) · s(-1) , while saturation occurred at ∼37 μmol photons m(-2) · s(-1) . Oxygen levels varied from ∼60% to 0% air saturation in the illuminated section of the coral skeleton in comparison to the darkened section. The O2 production within the Ostreobium band was lower in the region below the upward facing surface of the coral and elevated on the sides. Oxygen consumption in darkness was also greatest within the Ostreobium zone, as well as in the white skeleton zone immediately below the corallites. The rate of O2 depletion was not constant within zones and between zones, showing pronounced heterogeneity in endolithic respiration. When the coral was placed in darkness after a period of illumination, O2 levels declined by 50% within 20 min and approached steady-state after 40-50 min in darkness. Our study demonstrates the use of an important new tool in endolith photobiology and presents the first data of spatially resolved O2 concentration and its correlation to the physical structures and specific zones responsible for O2 production and consumption within the coral skeleton.

The Effect of Surface Orientation on Radially Spreading Flow

An experimental study is performed to investigate the position of falling liquid film apart from downward-facing surface for impinging jet. Besides, we investigate the circular hydraulic jump radius on upward-facing surface to compare with the radius of falling liquid film apart from downward-facing surface. The results show that both of the two radii have the same dependency with variation of tube Reynolds number. Water columns in dropping region are formed concyclic at regular intervals. It is substantiated that the interval is approximately the critical wavelength of Rayleigh-Taylor instability.

Free convective mass transfer at circular thin disk electrodes with varying inclination

The electrochemical limiting diffusion current technique was used to evaluate free convective mass transfer rates at free-standing circular disk electrodes of varying inclination and diameter. The electrochemical system used was copper deposition from acidified cupric sulphate solution. Experiments were carried with the single faces active and also with both faces simultaneously active. A Toepler–Schlieren system was used to photovisualise the ascending convection streams in order to obtain flow pattern information. For the upward-facing surface, the flow separates at a certain distance from the leading edge of the inclined disk, whereas the flow on the downward-facing surface always stays attached. The data correlations for the doubled disks show an exponent in the Sh– Ra correlation ranging from 0.25 (vertical case), indicating laminar flow, to 0.294 (horizontal case), indicating a mixed effect of turbulent separated flow on the upward-facing side and attached flow on the downward-facing surface. A universal correlation Sh d = f A ( Ra d ) f B was obtained for disk diameters d from 5 to 80 mm and inclination angle from 0° to 90° (from the vertical) where f A and f B are functions of inclination.

SIMULTANEOUS NATURAL-CONVECTION HEAT TRANSFER ABOVE AND BELOW AN ISOTHERMAL HORIZONTAL THIN PLATE

Numerical studies were conducted to investigate the simultaneous natural-convection heat transfer above and below an isothermal thin plate in an infinite space. The full two-dimensional conservation equations for laminar flows were solved. The numerical approach was based on the finite-volume technique with a nonstaggered grid arrangement. The SIMPLE algorithm was used for handling the pressure-velocity coupling. Plate width and temperature were used to vary the Rayleigh number over the range 1.0 2 10 5 to 1.7 2 10 7 . The expressions for numerical local and average Nusselt number were correlated, and horizontal velocity and temperature profiles were provided. It was found that the upper-surface Nusselt numbers are smaller than the corresponding lower-surface Nusselt numbers. Numerical results for upward- and downward-facing isothermal horizontal plates (single-sided heat transfer) were also shown for comparison. For these cases, Nusselt numbers on the upward-facing surface are larger than those on the downward-facing surface.

Flow and Heat (Mass) Transfer Characteristics in an Impingement/Effusion Cooling System With Crossflow

The present study is conducted to investigate flow and heat/mass transfer characteristics in an impingement/effusion cooling system with crossflow. To simulate the impingement/effusion cooling system, two perforated plates are placed in parallel and staggered arrangements with a gap distance of two times of the hole diameter, and initial crossflow passes between the plates. Both the injection and effusion hole diameters are 10 mm, and the Reynolds number based on the hole diameter and hole-to-hole pitch are fixed to 10,000 and six times of the hole diameter, respectively. To investigate the effect of crossflow, the flow rate of crossflow is changed from 0.5 to 2 times of that of the impinging jet, and the results of impingement/effusion cooling with crossflow are compared with those of the crossflow in the channel and of an array of impingement jets and the effusion cooling system. A naphthalene sublimation method is used to determine the local heat/mass transfer coefficients on the upward facing surface of the effusion plate. The flow patterns are calculated numerically using a commercial package. With the initial crossflow, the flow and heat/mass transfer characteristics are changed significantly from the results without the crossflow. Jet flows ejected from the injection plate are deflected by the crossflow, so that the stagnation points of the impinging jets move downstream. The heat/mass transfer rates on the effusion (target) plate decrease as the velocity of crossflow increases, since the crossflow induces the locally low transfer regions formed at the mid-way between the effusion holes. However, the impingement/effusion cooling with crossflow presents higher heat/mass transfer rates than the array jet impingement cooling with the same initial crossflow.