Hakaru Saito

Study on Properties and Growth Rate of Frost Layers on Cold Surfaces

Etude experimentale des proprietes et de la vitesse de croissance de couches de givre sur une plaque verticale refroidie dans un ecoulement de convection naturelle. Deduction, par analyse dimensionnelle, de parametres sans dimension permettant la prevision des densites et des conductivites thermiques du givre et de ses vitesses de croissance

Effect of aging, thickness and color on both the radiative properties of polyethylene films and performance of the nocturnal cooling unit

This study presents the effects of the changing in color due to aging, thickness and the new color of polyethylene films on the radiative properties. Also, the influences of the radiative properties on the performance of the nocturnal cooling unit used to cool flowing water were studied. The measured data of the transmittance of the polyethylene films having a thickness of 50 μm and having changes in its color by 5, 30 and 100 days aging and also new film data were used to study its effects on the performance of the cooling unit theoretically. Also, two films of thickness 25 and 50 μm and two films having a thickness of 25 μm, with colors of light blue and colorless, were used to examine their effects on the performance of the cooling unit experimentally. The results showed that aging of the polyethylene films leads to substantial degradation in its transmissivity from average values of 0.72, 0.69, 0.57 and 0.42 corresponding to a new one, 5, 30 and 100 days aging in the wavelength range of 8–13 μm. Also, neither thickness of the films nor their new colors have significant effects on the transmittance. It has been found that the effects of aging of the polyethylene film by 100 days led to the reduction in performance of night cooling by 33.3%. The decrease in thickness of the polyethylene films from 50 to 25 μm leads to an increase in its radiative properties (transmittance) and the performance of the night sky radiation unit by 8.6%, approximately.

Combined forced and free laminar convective heat transfer from a vertical plate with coupling of discontinuous surface heating

Combined free and forced laminar air convective heat transfer from a vertical composite plate with isolated discontinuous surface heating elements has been studied numerically and experimentally. The problem has been simplified by neglecting heat conduction in unheated elements of the plate to accomplish a better understanding of the complicated combined/complicated convection problem. In this study, it is most important in explaining the heat transfer behaviour to clarify the interactions between buoyancy and inertia forces in the convective field and also the coupling effects of unheated elements upon the combined flow fields. Therefore, the temperature distributions of the wall surface and local Nusselt number, obtained by numerical calculations and experiments, have been discussed based on the various parameters associated with the present convection problem, i.e., Grashof number GrL, Reynolds number ReL, geometry factor D/L and stage number N. Heat transfer characteristics Nut/Re1/2L of this combined...

Transient behavior of heat removal from a cylindrical heat storage vessel packed with spherical porous particles

AbstractThe transient heat transfer behavior in the case of heat removal from a cylindrical heat storage vessel packed with spherical particles was investigated experimentally for various factors (flow rate, diameter of spherical particles packed, temperature difference between flowing cold air and spherical particles accumulating heat, and physical properties of spherical particles). The experiments were covered in ranges of Reynolds number based on the mean diameter of spherical particles packed Red= 10.3–2200, porosityɛ=0.310 to 0.475, ratio of spherical particle diameter to cylinder diameterd/D = 0.0075–0.177 and ratio of length of the cylinder to cylinder diameterL/D=2.5–10. It was found that especially the flow rate and the dimension of spherical particles played an important role in estimating the transient local heat transfer characteristics near the wall of the cylindrical vessel in the present heat storage system. As flow rate and diameter of spherical particles were increased under a given diameter of the cylinder heat storage vessel, the mean heat transfer coefficient between the flow cold air and the hot spherical particles increased and the time period to finish removing heat from the vessel reduced. In addition, the useful experimental correlation equations of mean heat transfer coefficient between both phases and the time period to finish removing heat from the vessel were derived with the functional relationship of Nusselt numberNud=f [modified Prandtl numberPr* (d/D), Red) and Fourier numberFo = f(d/D, L/D, Pr*, Red).ZusammenfassungDas Übertragungsverhalten beim Wärmeentzug aus einem zylindrischen Wärmespeicher mit Kugelschüttung wurde experimentell untersucht. Dabei wurden verschiedene Einflußgrößen berücksichtigt, wie beispielsweise der Volumenstrom, der Kugeldurchmessser der Schüttungsteilchen im Speicherbehälter, die Temperaturdifferenz zwischen der strömenden kalten Luft und der Schüttung sowie die physikalischen Eigenschaften des Speichermaterials.Die Experimente wurden in folgenden Bereichen durchgeführt:Reynoldszahl, bezogen auf den Kugeldurchmesser des Schüttguts im SpeicherRed = 10.3–2200.Porosität der Kugelschüttungɛ=0.310–0.475.Verhältnis von Kugeldurchmesser der Schüttung und Zylinderdurchmesser des Speichersd/D=0.0075–0.177 undVerhältnis von Zylinderlänge und Zylinderdurchmesser des SpeichersL/D=2.5–10.0. Es wurde festgestellt, daß insbesondere der Volumenstrom und die Abmessungen der kugelförmigen Teilchen der Schüttung von großer Bedeutung bei der Bewertung des Übertragungsverhaltens beim Wärmeentzug, speziell im Bereich der Speicherwand, sind. Wenn der Volumenstrom und der Kugeldurchmesser des Schüttguts erhöht wird, bei einem vorgegebenen Durchmesser des Speicherbehälters, steigt der mittlere Wärmetransportkoeffizient zwischen zuströmender kalter Luft und der heißen Schüttung; der Zeitraum der Entladung verkürzt sich.Die experimentell ermittelte Abhängigkeit des mittleren Wärmetransportkoeffizienten zwischen der strömenden Luft und der Kugelschüttung von der Entladungszeit des Wärmespeichers ermöglicht die Bestimmung der Abhängigkeit dieses Entladevorgangs von der NusseltzahlNudf (modifizierte PrandtlzahlPr*, d/D, Red) und der FourierzahlFo =f(d/D, L/D, Pr*, Red).

A fundamental study of combined free and forced convective heat transfer from a vertical plate followed by a backward step

Combined forced and free laminar convective heat transfer on a vertical plate with a backward‐facing step has been studied numerically and experimentally, considering the effects of the interaction between the buoyancy and inertia forces which play a significant role in this phenomenon with the step‐geometry factor of d/L. The convective heat transfer behavior in connection with the reattachment and recirculation flows appearing in the step region has been investigated based on the numerical calculations and Mach‐Zehnder interferometer measurement under the wide range of the thermal condition. The behaviors of local Nusselt number NuL, velocity and temperature boundary layers and streamline fields in the recirculating region have been discussed for the various parameters of Grashof number GrL, Reynolds number ReL and the geometry factor d/L. The characteristic behavior of this convection heat transfer, including the vortex flow mode in the recirculating region and the unstable fluctuating mode near the re...

Relationship Between Some Physico-Mechanical Properties and Numerical Indexes of Graphite Shape in Compacted-Vermicular Graphite Cast Irons

The main purpose of this paper is to present some data on the mechanical and physical properties of compacted-vermicular graphite cast iron and to give a reasonable interpretation to the characteristics by using numerical indexes indicating the shape of graphite flakes in the structure. After describing the preparation of the material and a new method for measuring thermal diffusivity of the iron, the influence of kind and amount of alloy added for treatment and of cooling rate upon the graphite shape is discussed by using the indexes of the structure. The thermal diffusivities and mechanical characteristics such as tensile strength and hardness are shown as functions of the indexes to clarify the relationship between them. Furthermore, the present indexes are compared with those proposed earlier to find which is most suitable for the cast iron treated in this work. Authors propose a definition of compacted-vermicular graphite cast iron, which is reasonable from the physico-mechanical point of view, and also show some typical mechanical properties and measures required to produce such cast iron with desirable features.

An experimental study on natural convective heat transfer from a vertical wavy surface heated at convex/concave elements

Natural convective heat transfer from a vertical wavy surface has been studied experimentally. The wavy surface has isothermally heated convex or concave semicircular cylinders. Such a problem involves the effect of complex geometries and heat conduction in unheated elements on natural convection flow. In this problem, the clarification of the interaction between the fields of fluid convection and heat conduction in the unheated elements is the most important part of explaining the heat transfer behavior. Therefore, a new proposed parameter S (= BR/GrD14) was introduced through a vectorial dimensional analysis, taking into account the thermal properties of the material of the unheated elements. Heat transfer characteristics for cases of both convex and concave heating surface are discussed on the basis of the parameter S and visual observations of the velocity and temperature fields. Empirical formulas for the cases of convex and concave heating have been derived to correlate the heat transfer characteristic Nu/GrD14 and the parameter S.

Visualization of Natural Convection Heat Transfer from a Vertical Wavy Surface Composed of Isothermally Heated Convex

Visualization of Natural Convection Heat Transfer from a Vertical Wavy Surface Composed of Isothermally Heated Convex/Concave Semicircular Cylinders.by Koki Kishinami *, Hakaru Saito *, Ikuo Tokura t, Norihiro Bando ** Natural convective heat transfer from a vertical wavy surface having heated convex/concave semicircular cylinders has been studied experimen-tally, considering the effect of heat conduction in unheated elements.Heat transfer characteristics are discussed based on visual observations of the velocity and temperature fields by using micro-particle trajectories and a Mach-Zehnder interferometer.The parameter S (= B R / Gro1/4 ) was introduced through a vectorial di-mensional analysis and clarified that it indicates to play an important role in the interaction between the fields of fluid convection and the heat con-duction regions.

Natural convective heat transfer on a vertical plate with discontinuous surface-heating (Effect of the heat conduction of a plate)

Natural convective heat transfer along vertical plates with discontinuous heating elements has been studied by numerical computation and experiments. It has been found that the heat transfer on the surface of a plate and the temperature distribution of unheated elements change depending upon the ratio of the thermal conductivity of convective fluid and that of the unheated element material, the ratio of thickness and length of unheated elements, and the length and surface temperature of the heating elements. The heat transfer data are also found to be correlated by using a parameter S=B··Rλ/G1/4rL, presented in this paper through vectorial dimensional analysis, and the observation of convective behavior visualized by a Mach-Zehnder interferometer and computational results. An empirical formula has been presented to predict heat transfer coefficients of discontinuously heated plates based on the new parameter S.

An experimental study on free convection heat transfer from vertically lined horizontal cylinders set between parallel walls.

Experiments have been carried out on the free convection heat transfer from horizontal cylinders in array of staggered arrangements. Average heat transfer coefficients of individual cylinders and whole cylinder arrays were measured in order to clarify dependabilities of those coefficients upon the geometric figures including spacing and off-set distance between two cylinder lines, . Visualization of flow fields around the array has also been tried to explain the heat transfer behavior of cylinders in array. The experiments showed that there exist some optimum values of cylinder spacings and the off-set distances to increase the heat transfer coefficients and that the promoting effect in heat transfer can be expected especially when the array is set between parallel walls.