Upward-facing Horizontal Surface Research Articles

Mixed convection boundary-layer flow on a horizontal flat surface with a convective boundary condition

The steady mixed convection boundary-layer flow on an upward facing horizontal surface heated convectively is considered. The problem is reduced to similarity form, a necessary requirement for which is that the outer flow and surface heat transfer coefficient are spatially dependent. The resulting similarity equations involve, apart from the Prandtl number, two dimensionless parameters, a measure of the relative strength of the outer flow M and a heat transfer coefficient γ. The free convection, M=0, case is considered with the asymptotic limits of large and small γ being derived. Results for the general, M>0, case are presented and the asymptotic limit of large M being treated.

A model to predict the effect of contact angle on the bubble departure diameter during heterogeneous boiling

Over the past eighty years, bubble release during heterogeneous boiling has been the subject of numerous investigations. However, current understanding of factors that influence this process is still incomplete. In this paper, a model is developed to describe the effect of contact angle on the bubble departure from an upward-facing horizontal surface. Based on the concept of macro- and micro-contact angles, this model gives an explicit theoretical relation between the bubble departure diameter and the contact angle. Agreement with previous experimental data confirms the predictive ability of the present model.

Numerical Study of Single Bubble Dynamics During Flow Boiling

Three-dimensional numerical simulation of single bubble dynamics during nucleate flow boiling is performed in this work. The range of bulk liquid velocities investigated is from 0.076to0.23m∕s. The surface orientations at earth normal gravity are varied from an upward facing horizontal surface to vertical through 30, 45, and 60deg. The gravity levels on an upward facing horizontal surface are varied from 1.0ge to 0.0001ge. Continuity, momentum, and energy equations are solved by finite difference method and the level set method is used to capture the liquid-vapor interface. Heat transfer within the liquid micro layer is included in this model. The numerical results have been compared with data from experiments. The results show that the bulk flow velocity, heater surface orientation, and gravity levels influence the bubble dynamics.

Free convective mass transfer at up-pointing truncated cones

Free convective mass transfer at the individual surfaces and total surface of up-pointing truncated cones of different height and base diameter were experimentally studied using the limiting diffusion current technique. It was found that the mass transfer rate is highest for the upward-facing horizontal surface. The experimental mass transfer coefficient for the combination of downward-facing horizontal and conical surface is lower than that obtained from the equivalent summed separate surfaces due to the fact that the conical surface is exposed to solution which has already been depleted in cupric ions. The total mass transfer was successfully correlated using a single characteristic dimension (defined as a surface area divided by perimeter projected onto horizontal plane) which takes into account all the relevant dimensions of a truncated cone, i.e. the height and two diameters.

Simultaneous heat and mass transfer by natural convection above upward facing horizontal surfaces

A comprehensive numerical study is presented for combined heat and mass transfer by natural convection over upward facing horizontal surfaces. A comparison of numerically calculated result for thermal buoyancy alone with experimental data agrees well. Both aiding and opposing buoyancies, and a wide range of parameters are considered. It is shown that most flows are unsteady, and the causes of instabilities are physically explained. The transient nature of the unsteady flows is investigated in detail. The influence of diffusion rates on the dispersion of thermal energy and species is delineated. Complete Nusselt and Sherwood number data are provided.

Filtration with Sedimentation: Application of Kynchs Theorems

Filtration at moderate rates on upward-facing horizontal surfaces is accompanied by sedimentation. Kynch theorems developed for sedimentation in the noncompression range can be adapted to explaine the evolution of the cake surface and the filtrate volume with time. The different relationships obtained have been tested with filtration + sedimentation runs and filtration without sedimentation runs carried out with calcium carbonate suspensions at moderate pressure differences from 17 to 97 mmHg. The differences between the average specific resistance observed at the different runs can be satisfactorily explained by the fundamentals of the theory.

Free convective mass transfer at down-pointing pyramidal electrodes

A limiting current technique was used for the measurement of the free convective mass transfer rate at entire down-pointing pyramids and also at the individual faces of the pyramids. The cathodic deposition of Cu 2+ ions at electrodes of varying geometries was used as the mass transfer process, the supporting electrolyte being sulphuric acid. The mass transfer rate at a single inclined pyramidal surface was correlated by the equation Sh L = 0.78(Ra L,θ) 0.25 for the Ra L,θ, range 9.3 × 10 6, to 2 × 10 11. The mass transfer data for the entire pyramid were correlated using a method which includes an interference factor taking account of the fact that the upward-facing horizontal surface is exposed to fluid which has already been depleted in copper ions. The dependence of the interference factor on the aspect ratio of the pyramid ( L/ b) was established.

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.

Natural Convection Heat Transfer from a Horizontal Surface. (Effect of the Size of a Rectangular Obstacle above the Heated Surface).

The effects of the width of a rectangular obstacle above an upward-facing horizontal surface on the natural convection were investigated both numerically and experimentally. The horizontal surface was heated at a uniform temperature with a strip heat source, while the downward-facing surface of the obstacle was kept in either adiabatic or uniform temperature condition. The width of the obstacle was varied in the range of one to four times the width of the strip heat source. From the result, the typical flow patterns were presented in detail, and they were discussed in connection with the heat transfer characteristics. In particular, the region where the heat transfer rate is small in the heated surface becomes widened with increasing of the width of the obstacle for the case of adiabatic condition when the distance between the heated surface and the downward-facing surface of the obstacle is small, while the heat transfer rate is insensitive to the width of the obstacle for the case of uniform temperature condition.

Measurements of Particle Deposition Rates Inside Southern California Museums

Rates of deposition of submicrometer particles were measured inside five Southern California museums using an automated particle-counting technique on a scanning electron microscope equipped with an energy-dispersive X-ray detector combined with bulk chemical analysis. Deposition velocities to vertical and upward-facing horizontal surfaces were determined as a function of particle size for each site by analysis of indoor suspended particles sampled onto filters and particles deposited on vertically and horizontally oriented deposition plates. Measured deposition velocities to vertical indoor surfaces were in the range of 10^(−6) to 10^(−5) m/s at all sites but varied from site to site in terms of their dependence upon particle size. Deposition velocities to horizontal surfaces were in the range of 10^(−6) to 10^(−3) m/s and showed the expected increase with particle size due to gravitational settling. The deposition velocities measured by single-particle analysis are compared to deposition velocities determined by bulk chemical analysis of deposition plates and indoor ambient filter samples for several chemical species. Advantages and drawbacks of the automated particle counting technique are discussed.

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.

Natural Convection Over Upward-Facing Horizontal Surface : 4th Report, Examinations of Instability of Periodical Flow

加熱された水平面上に生ずる自然対流の周期流れ場が,レーレー数の増加に伴って次第に乱れてゆく過程に関し,二次元計算と実験の結果との相違が主に実験装置の三次元性に起因するものかどうかを円形加熱面を用いて検討し,流れ場自身の持つ不安定性によることを確かめた.また,周期性が乱れてゆく過程を温度変化のスペクトル解析で検討した.さらに,ハワードの周期流れのモデルを改良してこの系にあてはめ,良い結果をえた.

ORIENTATION AND CURRENT-INDUCED FLOW IN THE STALKED ASCIDIANSTYELA MONTEREYENSIS

1. Ambient water currents enhance internal flow and feeding in Styela montereyensis by forcing water through the branchial basket. Induced flow depends on upstream orientation of the incurrent siphon.2. In open-coast and protected-outer-coast habitats, most animals occur in surge channels or on upward-facing horizontal surfaces where they sway freely with the surge. Orientation takes place as the flexible stalk bends with the passing of each wave, keeping the posteriorly-recurved incurrent siphon directed upcurrent.3. In calmer water, animals' growth form is characterized by relatively shorter stalks and ventrally-inclined incurrent siphons. Adults in these habitats are mostly oriented with the mid-saggital plane parallel to the surface of the water. This orientation allows the ascidians to utilize the dynamic force in water surges coming from a single direction.4. Observed patterns of microdistribution probably result from differential mortality rather than habitat selection. Larvae delay metamorphosis for a short time in complete darkness and show no strong preferences for shaded substrata. Otherwise, they seem almost non-discriminatory with regard to attachment sites.

TRANSIENT FILM CONDENSATION ON UPWARD-FACING HORIZONTAL SURFACES

Both analytical and experimental results are presented for a wide range of conditions for transient film condensation on an upward-facing horizontal surface, including the effect of condensate run-off. The appropriate conservation equations are simplified by using several approximations valid for a variety of physical circumstances of interest, and the resulting ordinary nonlinear differential equations are integrated numerically. The theoretical results are shown to depend on a single new dimensionless group. The importance of run-off increases as this parameter increases, and the no-run-off case results when the parameter is zero. The theory correctly predicts zero film thickness for large times and nonzero values of the run-off parameter. A comparison of the predicted transient temperature response with those obtained in the associated experiments suggests the range of validity of the theroetical results. We find fair agreement for nonzero values of the run-off parameter and excellent agreement for zer...

Optical and electrochemical studies of transient free convection mass transfer at horizontal surfaces

The onset of free convection at upward facing horizontal surfaces adjacent to a free fluid environment has been studied using the electrochemical system involving the electrodeposition of Cu 2+ ions. A transient potential step technique was employed and photography of the ensuing developing convective structure was synchronised with the recording of current-time transients. This enabled the interdependence of instantaneous mass-transfer rate and convective flow structure to be visualised. A number of distinctive flow structures in the transient and steady states were observed and these have been related to the correlation regimes for the overall mass-transfer rate. Interesting phenomena involving periodic instabilities have been discovered at transitional Rayleigh numbers leading to pulsations in the convective plume and corresponding regular oscillations in the steady state mass-transfer rate. The effect of surface roughening on mass transfer and hydrodynamic performance during extended experiments has been observed and a method of overcoming surface roughening difficulties in the application of the electrochemical technique is suggested. Diffusion transient undershoot times are correlated in terms of plate Rayleigh number by the equations Dt d 2 = 32·5(Gr d,Sc) − 2 3 in the range 10 7 < Re d < 10 11 and Dt d 2 = 1·80(Gr d,Sc) −0·51 in the range 10 4 < Ra d < 10 7. The periodic fluctuation frequencies are similarly correlated by the equation Dt d 2 = 12·8(Gr dSc) − 2 3 .

Free convection mass transfer at horizontal electrodes

The free convection mass-transfer behaviour of upward-facing horizontal surfaces has been investigated experimentally using an electrochemical technique involving the measurement of limiting currents for the deposition of copper on copper electrodes from acidified cupric sulphate solutions. The data are well correlated by the equationsSh = 0·64(Sc . Gr)0·25 for 3 × 104 < ScGr < 2·5 × 107 andSh = 0·16(Sc . Gr)0·33 for 2·5 × 107 < ScGr < 1012. Electrode diameters varied between 0·1 and 12 cm and cupric sulphate concentrations between 0·01 and 0·3 M.The data compare favourably with previous experimental results for the corresponding heat-transfer case.

A Study on the Boiling Heat Transfer from Flat Surfaces under High Heat Flux : 2nd Report, Nucleate Boiling from Vertical Surfaces and Upward Facing Horizontal Surfaces

Pool boiling of water from a vertical surface and two upward-facing flat surfaces was experimentally studied under atmospheric pressure by making use of the apparatus described in the first report. The range of heat flux covered the entire region of nucleate boiling up to the burnour limit. The shape of the curve showing the relation between the heat flux and the temperature difference was similar to the curve for thin wires. When the heat flux exceeded 4×105 kcal/m2h, bubbles coalesced on the heating surface before leaving it. In the vicinity of the burnout limit, very large lumps of vapor rose intermittently from the heating surface. Experimental equations were set up by making use of Stanton number for the heat transfer coefficient, Reynolds number for the steam velocity leaving the surface, and the size of the heating surface.