Upward-facing Surface Research Articles

Experimental assessment of the effects of body force, surface tension force, and inertia on flow boiling CHF

The interfacial instabilities important to the modeling of critical heat flux (CHF) in reduced-gravity systems are sensitive to even minute body forces, especially for small coolant velocities. Understanding these effects is of paramount importance to both the reliability and safety of two-phase thermal management loops proposed for future space and planetary-based thermal systems. Unfortunately, reduced gravity systems cannot be accurately simulated in 1 g ground-based experiments. However, ground-based experiments can help isolate the effects of the various forces (body force, surface tension force and inertia) which influence flow boiling CHF. In this project, the effects of the component of body force perpendicular to a heated wall were examined by conducting 1 g flow boiling experiments at different orientations. Boiling experiments were performed using FC-72 in vertical and inclined upflow and downflow, as well as horizontal flow, and with the heated surface facing upward or downward relative to gravity. CHF was very sensitive to orientation for flow velocities below 0.2 m/s and near-saturated flow; CHF values for downflow and downward-facing heated surface were much smaller than for upflow and upward-facing surface orientations. Increasing velocity and subcooling dampened the effects of flow orientation on CHF. For saturated flow, the vapor layer characteristics fell into six different regimes: wavy vapor layer, pool-boiling, stratification, vapor stagnation, vapor counterflow, and vapor concurrent flow. The wavy vapor layer regime encompassed all subcooled and high-velocity saturated conditions at all orientations, as well as low-velocity upflow orientations. Prior CHF correlations and models were compared, and shown to predict the CHF data with varying degrees of success.

충돌제트/유출냉각기법에서 분사판의 홀배열이 열전달에 미치는 영향

Two perforated plates are used to investigate local heat/mass transfer characteristics in an impingement/effusion cooling system. A naphthalene sublimation method is conducted to determine the local heat/mass transfer coefficients on the upward facing surface of the effusion plate. Two plates are placed in parallel position with gap distances of 1, 2, 4 and 6 times of effusion hole diameter. The effects of hole arrangements of the plates are studied fur staggered, square, and hexagonal arrays. The experiments are conducted at Reynolds number of 10,000 based on the effusion hole diameter. The results show that the smaller hole size in the staggered array has the higher transfer coefficients on the stagnation region due to the formation of higher momentum flows through the impingement holes. In the square array, heat/mass transfer on the target plate is more uniform as the number of impingement holes increases. High and uniform heat/mass transfer coefficients are obtained for the hexagonal array.

The taphonomy of charcoal following a recent heathland fire and some implications for the interpretation of fossil charcoal deposits

In May 1995, fire burnt an area of heathland with stands of pine and birch trees in the Frensham Common Country Park near Tilford in Surrey, southeast England. Extensive areas were burnt by a rapidly spreading surface fire, and charcoal was produced from both the living plants and plant litter. Flames reached a height of no more than 2 m so that whilst much of the low growing vegetation was destroyed, the trees generally survived the burn. Samples were taken of unburnt living plants and unburnt litter as well as charcoal from heathland and adjacent woodland and small channels running through the locality. Much of the heathland charcoal was produced from living vegetation and included wood, leafy shoots, flowers and fruits of Calluna vulgaris (common heather) and rachises and pinnae of the fern Pteridium aquilinum (bracken). The surface peaty layer here was only charred to a depth of 1 or 2 mm. In contrast, the bulk of the charcoal from the Pinus sylvestris (scots pine) and Betula pendula (silver birch) trees was from litter, none of the lower branches of the trees having caught alight. Only a few fallen logs produced much wood charcoal. Fallen P. sylvestris female cones were typically only charred upon their upward-facing surface. Strong winds across the heathland following the fire gathered the charred C. vulgaris small leafy shoots, flowers and fruits into ripple concentrates. Movement of the charcoal by water following rain storms was also tracked from the heathland onto bare crossing paths and into depressions and channels as well as into a small nearby lake. Water transport resulted in selective bias in favour of wood charcoal. None of the other charred organs reached the lake. Scanning electron microscopy shows that a wide range of plant taxa and organs may be anatomically preserved by charcoalification, and plants covered with fungal hyphae (possibly indicating decomposition) were also found charred. Studies of samples in polished blocks show that all plant organs have increased reflectance under oil. Mean random reflectance under oil of all samples was 1.53 (range 0.13–6.22), which indicates that if preserved in the fossil record, they would be classified as the coal maceral semifusinite. Comparisons with fossil deposits are made, and this study may contribute to our understanding of charcoalified flower concentrates (e.g the late Cretaceous of Sweden and the USA) and deposits rich in fern charcoal (e.g. of the early Cretaceous of southern England).

EFFECTS OF HORIZONTAL FINS ON NATURAL-CONVECTIVE HEAT TRANSFER IN A SQUARE ENCLOSURE

Holographic interferometry is utilized to qualitatively study the effects of the horizontal fins on natural-connective heat transfer performance over a heated vertical plate, which forms a square enclosure with a cooled vertical plate and nonconductive watts. A laser light sheet is employed to visualize the flaw field over the heated vertical plate with or without fins. It is disclosed that (1) connective heat transfer performance deteriorates with the installation of fins shorter than the boundary-layer thickness δ on the vertical plate, but the average Nusselt number enhances with an increase in the fin length and reaches as much as 1.3 to 1.7 times that without fins. (2) There is an optimum fin length to enhance the average heat transfer performance. (3) The upward-facing surface (downstream side) of the fins performs better convection heat transfer than the downward-facing surface (upstream side). Flow visualization reveals that fluid flow separates from the upward-facing surface of the fin tip and turbulent flow occurs inside the boundary layer behind the protruding tip. (4) Coanda-effect flows are observed at the restricted portion, that is, at the bottom corner of the heated surface, and moreover on the condition including the fin of a 20-mm height above the floor. Turbulent-effect flows are observed everywhere if there is no fin at the position of a 20-mm height above the floor. (5) The connective heat transfer resulting in the turbulent mechanism is superior to that resulting in the Coanda effect.

Absorption of solar radiation by clouds: Interpretations of satellite, surface, and aircraft measurements

To investigate the absorption of shortwave radiation by clouds, we have collocated satellite and surface measurements of shortwave radiation at several locations. Considerable effort has been directed toward understanding and minimizing sampling errors caused by the satellite measurements being instantaneous and over a grid that is much larger than the field of view of an upward facing surface pyranometer. The collocated data indicate that clouds absorb considerably more shortwave radiation than is predicted by theoretical models. This is consistent with the finding from both satellite and aircraft measurements that observed clouds are darker than model clouds. In the limit of thick clouds, observed top‐of‐the‐atmosphere albedos do not exceed a value of 0.7, whereas in models the maximum albedo can be 0.8.

Measurement of liquid helium heat transfer in a high-speed rotational field and consideration of effect of obstacles blocking channels on the heat transfer

The heat transfer of liquid helium is measured in a high-speed rotating field. The centrifugal acceleration is in the range of 900–4600 g. Vertical test surfaces with channel are used, as well as horizontal surfaces. Moreover, for the vertical surfaces with channels, heat transfer is investigated with a blocked inlet and outlet. We examine blocking at the orifice channel outlet and the cover channel inlet for the heat transfer. Results show that heat transfer on vertical surfaces and the upward facing horizontal surface can be approximately represented by correlation equations for constant property fluids. Channel gap size and coriolis force do not affect heat transfer. The heat transfer of a horizontal surface facing downward is 1 6 – 1 7 of the upward facing surface. The orifices and covers for simulating channel blocking have almost no effect on heat transfer. Thus, obstacles blocking the channels do not affect cooling with helium unless blocking is complete.

Mass transfer study of the electropolishing of vertical cylinders with active ends under natural convection conditions

Rates of electropolishing of vertical copper cylinders with active ends in H3PO4 were studied by measuring the limiting current under natural convection. Variables studied were H3PO4 concentration, cylinder diameter and aspect ratio. The rate of polishing of the whole cylinder was represented by the mass transfer equation $$Sh = 0.33(Sc Gr)^{0.32}$$ for the range 1.17 × 1010 < Sc Gr < 5.11 × 1011. Rates of mass transfer were measured also at vertical cylinder with insulated ends, and the upward facing surface (disc). Data for the vertical cylindrical surface were represented for the range 8.75 × 109 < Sc Gr < 1.1 × 1012 by the equation $$Sh = 1.206(Sc Gr)^{0.255}$$ while data at the upward facing disc were correlated for the range 0.11 × 1010 < Sc Gr < 46 × 1010 by the equation $$Sh = 0.17Sc^{0.396} (Sc Gr)^{0.146}$$ A comparison between the measured rate of mass transfer at the whole cylinder and the value calculated by adding the rates of mass transfer at the separate surfaces of the cylinder shows that the measured value deviates from the calculated value, the degree of deviation increases with increasing Sc × Gr. Deviation was attributed to flow interaction at the different cylinder surfaces.

Experimental Study on the Flow Behavior of a Liquid Film on an Inclined Downward-Facing Plate.

Liquid films are encountered in many industrial applications such as chemical process equipment and exchangers. Most of the previous experimental studies on the hydrodynamics of liquid films have been performed with liquid film flow on a vertical surface or an inclined upward-facing surface. This study concerns flow behavior of thin water with or without surfactant films on an inclined downward-facing plate. Here, the dependence of behavior such as flow (10<Re<500) on experimental parameters (surface roughness, inclination angle of surface, surface tension of liquid, initial film thickness) was investigated through flow visualization by a tracer injection method. Several wave patterns which appeared at the free surface of flowing film were observed under certain flow conditions. Adding surfactant has the effect of damping of waves. As a result of demensional analysis for lF, the flow-distance until flowing film detouches a smooth plate, lF is in proportion to the product of Re1/2 and Fr1/3. Under the experimental conditions here, surface roughness has more effect on the film flow behavior than surface tension.

Experimental investigation of natural convection from a horizontal ice surface melting in pure water

A study on natural convection from a horizontal ice surface melting in pure water was conducted experimentally for the ambient water temperature from 2▿C to 10°C. Natural convection flow around upward-or downward-facing horizontal ice plate was divided into three regions according to the temperature variation of ambient water. The flow patterns of three regions were no flow, two-dimensional steady laminar flow and unsteady flow. Mean Nusselt number for the upward-facing surface had its maximum value at about 3°C of ambient water temperature. However, in the case of the downward-facing surface it increased as the ambient water temperature increased.

Transient Heat Transfer Characteristics of Mist Cooling from Horizontal Upward-Facing Surface.

The effects of cooling rate and thermal conductance of a heat transfer plate on the heat transfer coefficient of mist cooling are experimentally investigated in the high-temperature region. Experiments are conducted for horizontal upward facing surfaces made of silver, stainless steel and fused quartz. The experimental conditions of mist flow are as follows: the air velocity is Va=20m/s, the temperature of the water droplet is Tl=21°C, and the volumetric-droplet-flow-rate is D=0.00043-0.00472m3/m2s. It is found that, in the case where the horizontal surfaces face upward, thermal conductance of the surface material does not significantly affect the heat transfer coefficient, but the cooling rate of the surface affects the heat transfer coefficient if the heat capacity of the surface decreases to less smaller than a critical value.

Condensation by heterogeneous nucleation in a thermal boundary layer

Condensation can occur by the nucleation and growth of water droplets on particles in the air. This mode of mass transport can readily occur in a thermal boundary layer. The primary condition required for droplet nucleation is the existence of supersaturation in the boundary layer. Equations are developed to described droplet nucleation an d transport for the boundary layer on a horizontal, upward facing surface. These equations allow diffusion and nucleation mass fluxes and droplet concentration at a point in the boundary layer to be calculated. Experimental data were collected and compared to the theoretical calculations. Accurately predicting droplet concentration is difficult, but the presence of nucleation condensation can be readily predicted.

Measurements of Laminar Mixed Convection Flow Adjacent to an Inclined Surface

Measurements of laminar mixed forced and free convection air flow adjacent to an upward and a downward facing, isothermal, heated inclined surface (at 45 deg) are reported. Local Nusselt number and the velocity and temperature distributions are presented for both the buoyancy assisting and the buoyancy opposing flow cases for a range of buoyancy parameter 0 ≤ ξ ≤ 5 (ξ = Grx/Rex2). The measurements are in good agreement with predictions which define a laminar mixed convection regime for buoyancy assisting flow as 0.1 ≤ ξ ≤ 7, and for buoyancy opposing flows as 0.06 ≤ ξ ≤ 0.25 for this inclination angle of 45 deg. Simple mixed convection correlations for the local and average Nusselt numbers for inclined surfaces are also presented and they agree very well with predicted results. As expected, the local Nusselt number increases with increasing buoyancy parameter for assisting flows and decreases for opposing flows. For a given buoyancy parameter and Reynolds number, a downward facing surface provides essentially the same Nusselt number as the upward facing surface for the conditions examined in the experiment.

Natural convection from a horizontal surface (Effect of the existence of a rectangular obstacle above an upward-facing horizontal surface with a strip heat source)

The effect of the existence of a rectangular obstacle above an upward-facing horizontal surface on natural convection were investigated theoretically. The horizontal surface was heated at a uniform temperature with strip heat source, while the downward-facing of the obstacle was kept in both insulated and uniform temperature conditions. The analysis was carried out for the Prandtl number Pr=0.7 (air) in the range of the Grashof number Gr=105107. The typical flow and the temperature fields were presented, and they were discussed in connection with the heat transfer. The results obtained were compared with the experiment and reasonably good agreement was obtained.

Boiling heat transfer in porous bed with water injection and critical water level.

An Experimental study has been conduced to determine the influence of the saturated-water injection into packed bed on boiling heat transfer or a upward facing surface covered with the bed. Three kinds of the particles of alumina, glass, and steel balls were utilized as the testing porous beds. Injection of the saturated water was found to increase the heat-transfer performances over the range of injection rate investigated. It was also observed that there might be a critical water level for which a maximum boiling heat-transfer coefficient takes place at a given ΔTs.