Yuhichi Mitsutake

Estimation of Surface Temperature and Heat Flux Using Inverse Solution for One-Dimensional Heat Conduction

An analytical method has been developed for the inverse heat conduction problem using the Laplace transform technique when the temperatures are known at two positions within a finite body. On the basis of these known temperatures, a closed form to the inverse solution can be obtained to predict surface conditions. The method first approximates the measured temperatures with a half polynomial power series of time as well as a time lag, which takes for a monitor to sense the temperature change at the point. The expressions for the surface temperature and the surface heat flux are explicitly obtained in the form of the power series of time

Enhancement of heat transfer due to bubbles passing through a narrow vertical rectangular channel (Change in heat transfer along flow)

The objective of this paper is to make clear how heat transfer coefficient changes along the flow with the passing bubbles through a narrow vertical rectangular channel (20 mm wide, 2 mm deep and 450 mm long). The experiments were done using subcooled water of 80, 60, and 40 K at atmospheric pressure in which the air bubbles were injected into the channel at a designated period from 0.125 to 1.0 s and their length was controlled to be equal to 0.03, 0.02, and 0.01 m. The experiment shows that the heat transfer coefficients decrease along the flow and then reach a constant value beyond a certain distance from the leading edge of the heated surface where the flow becomes fully developed in both the velocity and the thermal conditions. Under the fully developed conditions, the heat transfer coefficients are predicted well by the existing theoretical analysis in which both the convective term and evaporation on the interface are ignored.ZusammenfassungDie Untersuchung hat das Ziel zu zeigen, wie sich der Wärmeübergangskoeffizient in Strömungsrichtung ändert, wenn Blasen das Fluid in einem engen, senkrechten Rechteckkanal (20×2 mm2, Länge 450 mm) durchsetzen. Die Experimente wurden mit Wasser bei 80, 60 und 40 K Unterkühlung unter Atmosphärendruck ausgeführt, in das Luftblasen im zeitlichen Abstand von 0,125 bis 1,0 Sekunden mit einer kontrollierten Länge von 0,03, 0,02 und 0,01 m eingebracht wurden. Die Experimente zeigen, daß die Wärmeübergangskoeffizienten in Strömungsrichtung abnehmen und in einer bestimmten Entfernung vom Eintritt in die beheizte Kanalstrecke (wo hydrodynamisch und thermisch abgeschlossener Einlauf erreicht ist) konstante Werte annehmen. Diese lassen sich gut mit bestehenden theoretischen Modellen vorausberechnen, in denen sowohl der konvektive Term, wie die Verdampfung an der Grenzfläche vernachlässigt werden.

Enhancement of heat transfer due to bubbles passing through a narrow vertical rectangular channel

Abstract A theoretical analysis has been made of heat transfer enhancement due to bubbles passing through a narrow vertical channel. The mechanism of enhancement for a constant heat flux is: heat is first transported by a latent heat of evaporation on the bubble interface covering the heated surface and is followed by sensible heating of the succeeding liquid. The latent heat transport is calculated using an integral method and the sensible heating is determined exactly. For low heat fluxes, the calculated heat transfer coefficients are in fairly good agreement with the experiments; at high heat fluxes, however, the calculated values are approximately half the experimental values, since flow along the heated surface is not taken into account.

Critical heat flux of natural circulation boiling in a vertical tube: Effect of oscillation and circulation on CHF

Abstract An experimental study has been made to elucidate an effect of oscillated flow induced near critical heat flux (CHF) and natural circulated flow of vapor and liquid in a vertical tube on the CHF. The experiment has been carried out for the condition of heated tube length of L=100–1000 mm, tube diameter of D=4–9 mm and the CHF is measured under the condition that the exit of the unheated tube connecting the heated tube is extruded into a vapor chamber to prevent a liquid flowing into the heated tube from the top. The experiment reveals that the CHF for D=7 and 9 mm and any tube length from 100 to 1000 mm becomes identical to that for the case of the tube top in the liquid, while the CHF for D=4 and 5 mm is smaller than that for the case of the tube top in the liquid due to no liquid supply from the top. It is found that the oscillation induced near the CHF increases the CHF.

Enhancement of CHF in open thermosyphon with heated bottom chamber

An experiment has been carried out to elucidate the critical heat flux (CHF) of an open thermosyphon with a bottom heated chamber and to enhance the CHF using a concentric-tube connecting between the heated chamber and an upper cooling chamber. The CHF data are measured for the saturated liquid of R113 at a different pressure and different dimension of concentric tubes. The CHF data without the inner tube are in good agreement with the existing correlation and analytical result. Heat supplied to the bottom chamber is absorbed by evaporation of liquid and vapor generated thereby makes a counter-current flow of vapor and liquid in an ordinary thermosyphon without the concentric-tube. On the other hand, in the thermosyphon with the concentric-tube by which the counter-current flow can be controlled well, the CHF can be improved as much as several times of the CHF without the inner tube at an optimum condition. The CHF is increased with an increase in the diameter of inner tube up to its optimum diameter and then decreases continuously as the inner tube diameter approaches the outer tube diameter.

Experimental study of critical heat flux in open two-phase thermosyphon

Critical heat flux in an open two-phase thermosyphon has been measured employing water, R113 and R22 for a wide range ofL/D=4.8 to 960 and density ratio ofρL/ρG=6.17 to 1602. The CHF data measured, are found to be in fairly good agreement with those calculated from a criterion that the CHF takes place when reaches a maximum liquid falling rate in counter-current annular flow of liquid and vapor. Normal operation of the thermosyphon cannot be obtained at any liquid of water, R113 and R22 for a small tube diameter of less than 2 mm.ZusammenfassungDer kritische Wärmefluß in einem offenen, Zweiphasen-Thermosiphon wurde für Wasser, R113 und R22 in den weiten Variationsbereichen 4.8≤L/D≤960 und 6.17≤g9L/ϱG≤1602 gemessen. Diese Daten stimmen gut mit jenen überein, die sich rechnerisch aus einem Kriterium ergeben, das unterstellt, der kritische Wärmefluß würde bei Gegenstrom-Ringströmung von Flüssigkeit und Dampf dann auftreten, wenn der Massenstrom des fallenden Flüssigkeits-films ein Maximum erreicht. Für Rohrdurchmesser kleiner als 2 mm läßt sich für keine der drei Versuchsflüssigkeiten ein normaler Thermosiphonbetrieb aufrecht erhalten.

Critical heat flux in a two-phase thermosyphon : Relationship between maximum falling liquid rate and instability on interface of falling liquid

Critical heat flux in a two-phase thermosyphon is usually dealt with in one of two different ways: one uses the maximum falling liquid rate obtained from the envelope method, the other uses the instability. We first clarify the difference between the maximum and the instability criteria. The CHF in the thermosyphon is shown to be well predicted by the maximum liquid rate obtained using the maximum criterion and this prediction is better than that using the instability criterion. This comparison implies that CHF phenomenon in a thermosyphon is caused by the falling liquid reaching a maximum value rather than by instability of the falling liquid on the interface.

CRITICAL HEAT FLUX IN TWO PHASE THERMOSYPHON ( RELATIONSHIP BETWEEN MAXIMUM FALLING LIQUID RATE AND INSTABILITY ON INTERFACE OF FALLING LIQUID )

Critical heat flux (CHF) in a two-phase thermosyphon is usually dealt with in two different ways : one is a maximum falling liquid rate due to envelope method (Maximum criterion), and the other one is that due to instability (Instability criterion). The difference between both criteria is first clarified. CHF in the thermosyphon is shown to be predicted better by the maximum falling liquid rate due to the maximum criterion than by that due to the instability criterion. In addition, comparison of the two different methods implies that CHF phenomenon in the thermosyphon occurs when the falling liquid reaches the maximum value rather than when the instability of the falling liquid on the interface is brought about.

Characteristics of Transient Boiling Heat Transfer

In this paper, one dimensional inverse heat conduction solution is used for a measurement of pool boiling curve. The experiments are performed under atmospheric pressure for copper, brass, carbon steel and gold. Boiling curves, including unsteady transition boiling region, are found can be traced fairly well from a simple experiment system by solving inverse heat conduction solution. Boiling curves for steady heating and transient heating, for heating process and cooling process are compared. Surface behavior around CHF point, transition boiling and film-boiling regions are observed by using a high-speed camera. The results show the practicability of the inverse heat conduction solution in tracing boiling curve and thereby supply us a new way in boiling heat transfer research.Copyright

Critical Heat Flux in Saturated Forced Boiling with a Plane Jet.

Critical heat flux (CHF) has been measured on rectangular heated surfaces 40 and 80 mm in length and 20 mm in width during saturated forced boiling with a plane jet. The experiments were carried out at jet velocities of 3 to 15 m/s and system pressures of 0.1 to 0.4 MPa. The experimental result shows that there is no difference in occurrence of the CHF between upward-and downwardfacing heated surfaces. It is found that no existing correlation can predict the CHF over a wide range of pl/pg from pl/pg=6.6 to 1 603. A revised correlation is proposed to give the CHF over the range of pl/pg.