Nobuhiro Seki

Visual observation of natural convective flow in a narrow vertical cavity

Experimental visualization of natural convective flow was carried out by using several kinds of fluid contained in a narrow vertical rectangular cavity with one vertical wall heated, the opposing vertical wall cooled and the upper and lower walls insulated. The effects of the Prandtl number Pr of the working fluid and the width of the cavity W on the flow pattern are discussed qualitatively in the present paper. The occurrence of flow patterns consisting of unicellular flow, steady secondary flow, tertiary flow and transition (from laminar to turbulent) flow is categorically demonstrated by the photographs taken. Moreover, experimental measurements of the net heat transfer through the vertical fluid layer are given for aspect ratios of 6-30 and Prandtl numbers of 4-12 500.

Heat transfer in a confined rectangular cavity packed with porous media

Abstract This paper presents an experimental investigation of convective heat transfer in a confined rectangular cavity packed with porous media, on the opposing vertical walls of which different temperatures are imposed. Measurements are made for each of two kinds of solid particles using three kinds of fluids, i.e. water, transformer oil and ethyl alcohol. The present experiments cover a wide range of Rayleigh number Ra∗ between 1 and 105, Prandtl number Pr∗ between 1 and 200 and geometrical aspect-ratio H W between 5 and 26. The experimental results indicate that Nusselt number Nu∗ is correlated by the following relationship: Nu∗ = 0.627 Pr∗ 0.130 ( H W ) −0.527 Ra∗ 0.463

Forced Convection Heat Transfer on Heated Bottom Surface of a Cavity

Experiments to measure the heat transfer characteristics for various cavities situated at a duct-wall were performed. Flow visualization, measurements of pressure and temperature distributions on the heated bottom surface of cavity were carried out. It was observed that the effects of main flow stream, reattachment of separated flow, and vortex flow in the cavity on heat transfer unexpectedly large. It was found that heat transfer did not always decrease monotonously with an increase of aspect (depth-width) ratio D/W, in the flow range of laminar to turbulent. Correlations between Num and Rew were made in laminar and turbulent heat transfer ranges.

Free convective heat transfer with density inversion in a confined rectangular vessel

An experimental and analytical investigation pertaining to the effect of density inversion on steady free convective heat transfer of water in a confined rectangular vessel in which different temperatures are imposed on the opposing vertical walls is carried out. Water, as a testing fluid, has its maximum density at 4‡C. Temperature of the cold wall is maintained at 0‡C, while that of the hot wall is varied from 1‡C to 12‡ C. Photographs and analytical descriptions of the flow patterns, temperature distributions and average Nusselt number are presented. Moreover, the effect of dimensions of a rectangular vessel on the average Nusselt number is investigated. From the present investigation, it can be demonstrated both experimentally and analytically that the density inversion of water have an influential effect on the free convection heat transfer in the prescribed water layer, moreover, the average Nusselt number is a peculiar function of temperature difference between the cold and the hot walls, unlike the previous results for common fluids without density inversion.ZusammenfassungDer Einflu\ der Dichteumkehr von Wasser auf die stationÄre freie Konvektion in einem geschlossenen rechtwinkligen BehÄlter, dessen gegenüberliegende senkrechte WÄnde auf verschiedenen Temperaturen gehalten werden, ist experimentell und analytisch untersucht. Wasser hat sein Dichtemaximum bei 4‡C. Die kalte Wand ist auf 0‡C gehalten, die Temperatur der warmen Wand liegt zwischen 1‡C und 12‡C. Die Strömungsform, die Temperaturverteilung und die mittlere Nu\elt-Zahl werden durch photographische Aufnahmen und analytische Rechnungen beschrieben. Auch der Einflu\ der BehÄlterdimensionen auf die mittlere Nu\eltZahl ist angegeben. Es zeigt sich, da\ die Dichteumkehr den WÄremübergang in der Wasserschicht grundsÄtzlich verÄndern kann und da\ die Nu\elt-Zahl in eigentümlicher Weise vom angelegten TemperaturgefÄlle abhÄngt, im Gegensatz zu Älteren Ergebnissen für übliche Flüssigkeiten ohne Dichteumkehr.

An Experimental Study of Free Corrective Heat Transfer in a Parallelogrammic Enclosure

Experimental measurements are presented for free convective heat transfer across a parallelogrammic enclosure with the various tilt angles of parallel upper and lower walls insulated. The experiments covered a range of Rayleigh numbers between 3.4 × 104 and 8.6 × 107 , and Prandtl numbers between 0.70 and 480. Those also covered the tilt angles of the parallel insulated walls with respect to the horizontal, φ, of 0, ±25, ±45, ±60, and ±70 deg under an aspect ratio of H/W = 1.44. The fluids used were air, transformer oil, and water. It was found that the heat transfer coefficients for φ = −70 deg were decreased to be about 1/18 times those for φ = 0 deg. Experimental results are given as plots of the Nusselt number versus the Rayleigh number. A correlation equation is given for the Nusselt number, Nu, as a function of φ, Pr, and Ra.

A Criterion of Onset of Free Convection in a Horizontal Melted Water Layer With Free Surface

The problem considered in this paper is the determination of the Rayleigh number marking the onset of free convection in a horizontal melted layer of ice heated from above under uniform radiant heat flux. In this analysis, linear perturbation techniques are used to derive a sixth-order differential equation subject to hydrodynamic and thermal boundary conditions. The series-solution method is utilized to obtain an eigenvalue equation for the case where the lower surface (ice surface) is kept at 0°C and the upper free surface is subjected to the general thermal conditions. An experimental determination of the onset of free convection, when the heat transfer mode changes from conduction to convection, is obtained from the fact that the temperature distribution in a melted water layer starts to deviate from its linear profile. At the same time, a peculiar variational inflection of the water-surface temperature occurs. From the present investigation, it can be demonstrated both analytically and experimentally that the critical Rayleigh number Ra c in a horizontal melted water layer with a density inversion is dependent on the free water-surface temperature T2 for T2 < 8°C, while Ra c for T2 ≧ 8°C is independent of T2 .

Effect of Maximum Density of Water on Freezing of a Water-Saturated Horizontal Porous Layer

This experiment simulates the freezing of water in a layer of earth by the use of a porous bead layer. The layer is cooled from above. The beads are 1, 5, and 11-mm-dia glass and 11-mm-dia steel. When the predominant heat transfer mode in an unfrozen layer is conduction, the freezing rate is not affected by the bead diameter. However, in natural convection the freezing rate is greatly affected by the maximum density of water at 4C. An approximate numerical analysis which does not treat the maximum density effects predicts well the freezing rate in the range of 0C < T{sub 2} < 4C and 26C < T{sub 2} (T{sub 2} = the lower plate temperature).

An analysis of incipient frost formation

An analytical and experimental study of frost growth on a cooled plate being exposed to forced convective humid air stream was carried out. Attention is mainly focused on the incipient phenomena of the frost formation including the growth of supercooled water droplets which are based on condensation of water vapor leaving the air. Two kinds of the plates having different contact angles to water droplet (110 deg and 43 deg) are used as the testing ones. The effects of the velocity, humidity, and temperature of the air stream, the contact angle to water droplet, and the temperature of the cooled plate on the frost formation are extensively determined. An analytical model which is based on the experimental observations is found to closely predict the general trends in the growths of the supercooled water droplets and the porous frost layer.ZusammenfassungEs wurde eine analytische und experimentelle Untersuchung der Reifbildung an einer gekühlten Platte, die einem feuchten Luftstrom ausgesetzt war, durchgeführt. Dem Wachstum von unterkühlten Wassertropfen aus kondensiertem Wasserdampf der Luft während des Anfangsstadiums der Reifbildung wurde dabei besondere Aufmerksamkeit geschenkt. Für die Versuche wurden zwei Platten mit verschiedenen Kontaktwinkeln (110° und 43°) zu den Wassertropfen verwendet. Die Einflüsse der Geschwindigkeit, der Feuchte und der Temperatur des Luftstroms sowie des Kontaktwinkels zu den Wassertropfen und der Temperatur der gekühlten Platte auf die Reifbildung werden umfassend geklärt. Ein mit Hilfe der experimentellen Ergebnisse erstelltes analytisches Modell ermöglicht eine allgemeine Vorhersage des Wachstums der unterkühlten Wassertropfen und der porösen Eisschicht.

Radiative melting of a horizontal clear ice layer

In this paper the horizontal layer of clear ice sticking to the substrate is melted by comparatively short wave radiation similar to solar radiation for the purpose of removing ice from the surface of the material subject to atmospheric icing. The radiating source used for melting is 300 wattages halogen lamps whose color temperature is 3200‡K at 100 voltages. From the present investigation, a typical phenomenon of backmelting is observed clearly and it can be found that the predicted results including the melting rate of upper and lower layers which are melted by radiant energy impinged on or penetrated the ice layer are in good agreement with the experimental results.ZusammenfassungEine waagerechte Klareisschicht, die auf einer Unterlage aufgefroren war, wurde durch kurzwellige Strahlung, Ähnlich der Sonnenstrahlung, zum Schmelzen gebracht, um die Entfernung von Eis nach atmosphÄrischer Vereisung zu untersuchen. Die Strahlungsquelle war eine 300 Watt-Halogenlampe mit einer Farbtemperatur von 3200 Kelvin bei 100 Volt. Als typische Erscheinung wurde ein “Rückseiten-Schmelzen” gefunden, im übrigen sind die vorausberechneten Schmelzraten an der Ober- und der Unterseite durch aufgenommene oder durchgelassene Strahlungsenergie in guter übereinstimmung mit den Messungen.

Radiative melting of ice layer adhering to a vertical surface

This paper is concerned with melting of a vertical ice layer adhering to the substrate by using radiating heat source of halogen lamps having a large fraction of short wave beam or nichrome heater having a comparatively large fraction of long wave one. From the present experimental results, it can be seen that the heating of short wave radiation produces a peculiar melting behavior of strongly rough melting-surface due to the internal melting at the grain boundary of ice-surface. On the other hand, for the case of long wave radiation the melting-surface becomes very smooth. The melting rate of clear ice layer by short wave radiation obtained from halogen lamps is smaller than that of cloudy ice layer due to the good penetration of short wave fraction through the clear ice layer. Moreover, the raising of temperature of ice-substrate interface could offer a feasibility of removing ice layer from the structure subject to atmospheric icing. Concludingly, it is clarified that the melting rate of ice layer could be predicted numerically by using the band model of extinction coefficient or absorption coefficient presented in this study.ZusammenfassungDiese Arbeit behandelt das Schmelzen einer senkrechten Eisschicht auf einer Unterlage mit Hilfe von Halogen-Lampen mit einem hohen Anteil an kurzen Wellen und Nichromheizern mit einem hohen Anteil an langen Wellen. Aus diesen Versuchen läßt sich ableiten, daß die Heizung durch kurzwellige Strahlung ein eigentümliches Schmelzverhalten mit sehr rauher Oberfläche hervorruft, verursacht durch Schmelzen an den Korngrenzen der Eisoberfläche. Bei langwelliger Heizung wird die Oberfläche sehr glatt. Die Abschmelzrate einer Klareisschicht bei kurzwelliger Heizung durch Halogen-Lampen ist geringer als die einer Opaleisschicht wegen des besseren Eindringens der kurzen Wellen in das klare Eis. Der Temperaturanstieg an der Grenze Eis — Unterlage bietet die Möglichkeit der Enteisung von Bauteilen, die der atmosphärischen Vereisung ausgesetzt sind. Es folgt, daß die Abschmelzrate einer Eisschicht, numerisch vorausberechnet werden kann, indem man das Bandmodell des Extinktions- und des Absorptionskoeffizienten dieser Arbeit verwendet.