Takeshi Shimazaki

Second sound wave heat transfer, thermal boundary layer formation and boiling: highly transient heat transport phenomena in He II

Highly transient heat transport phenomena in He II are experimentally investigated. High sensitivity temperature measurements are carried out by means of a superconductive temperature sensor. It is found that the waveform of a thermal pulse in the second sound wave mode loses its dependence on heating time in the case of strong heating. Only a limited, small amount of thermal energy is transported in this mode. In such a situtation, dense quantized vortices are generated and accumulate in the vicinity of the heating surface to form a thermal boundary layer which has a large temperature gradient. The time variation of the temperature distribution in the thermal boundary layer is measured. Boiling in the layer is usually observed in the case of strong heating and the heat transport phenomena seem to be governed by boiling. The rest of the energy which is not transported in the second sound wave mode is slowly transported in a diffusive process or is consumed upon phase transition of the He II.

Visualization study of film boiling onset and transition to noisy film boiling in He II

The transient boiling phenomena occurring on a planar heater in He II are visually studied to investigate the onset condition and the bifurcation criterion. The whole flow field is visualized with the aid of Schlieren and shadowgraph optics. After step-wise heating, local boiling sites appear and develop, eventually covering the heater surface, followed by the onset of film boiling. The state may further proceed to noisy boiling if the hydrostatic pressure is sufficiently large. A distinctive feature between the noisy and silent boiling states is visually confirmed in the present experiment. The onset time of film boiling as a function of heat flux is obtained via a transparent heater. It is found that there is some distinction between cases with large and small q values in the functional relationship for the onset time on q, presumably originating from different states of the precursor nucleate boiling before film boiling.

Visualization study on the thermo-hydrodynamic phenomena induced by pulsative heating in He II by the use of a laser holographic interferometer

A visualization method for cryogenic thermo-hydrodynamic phenomena is established by using a laser holographic interferometer. The technique is applied to the study of some transient heat transfer, that is the thermal shock wave propagation and onset of boiling in He II. All these phenomena are induced by a pulsative heating from a thin-film planar heater. The evolution of an initial trapezoidal thermal pulse into a thermal shock wave and the subsequent propagation is visualized at various temperatures. The temperature rise due to diffusive process caused by the action of high density vortices, that is the formation of a thermal boundary layer adjacent to the heater, and subsequent boiling are observed. The finite-amplitude pressure wave is found to be generated in response to the onset of pulsative heating. Further, a visualization of evaporation from an He II free surface caused by the incidence of a thermal shock wave is made and the propagating speed of the vapor front is measured from visualization photos.

The initial thermo-fluiddynamic processes of boiling phenomena in He II

A systematic study on a series of thermo-fluiddynamic phenomena in the initial stage leading to boiling in He II is conducted with the aid of visualization methods and with a superconductive temperature sensor. Following the onset of a stepwise heating, a compression (first sound) wave and a second sound wave which develops into a thermal shock wave are emitted. Then high density quantized vortices are generated and accumulate near the heater which cause formation of a thermal boundary layer adjacent to the heater. Finally film boiling starts on the heater surface following precursory spotwise boiling. It is emphasized in the present study that such visualization methods as laser holographic interferometry and shadowgraph method, and a superconductive temperature sensor are complementarily utilized to investigate the thermo-fluiddynamic phenomena which result from a stepwise heating at a rather large heat flux from a planer heater.

Visualization study of noisy and silent film boiling phenomena on a plane heater in He II

Abstract The thermofluiddynamic phenomena resulted from boiling on a plane heater in He II is visually studied to investigate the characteristics of noisy and silent film boiling phenomena. The flow field is visualized with the aid of Schlieren optics and shadowgraph one. The frontal and side views of the resulted flow-field image are recorded with a still camera and a video recorder. A transparent heater consisted of evaporated indium oxide thin film on an optical glass substrate is utilized for the frontal view. These visualization images revealed a distinctive feature between noisy and silent boiling modes.

Laser holographic interferometer visualization of a thermal shock wave in He II

Thermal shock waves in superfluid helium (HeII) were studied using a laser holographic interferometer. We succeeded in the visualization of three typical types of thermal shock: frontal, back and double shock waves, each shock front shows up through the appearance of a sharp interference fringe. The evolution of a pseudo-trapezoidal finite-amplitude temperature disturbance to a shock was observed in a series of interferograms. A strong compression wave was also visually detected in the temperature range of back shocks, at higher temperatures, which was generated by the rapid formation of a vapor film on the heater. The critical curve for the onset of boiling in HeII was investigated from visualization interferograms in terms of the peak heat flux Q (W/cm2) the heating time tH (µs), and HeII temperature T (K). Further a visualization of evaporation from a HeII free surface caused by the incidence of a thermal shock was made and the propagating speed of the evaporating vapor front was evaluated from photos.

Visualization Study of Highly Transient Thermo-Fluid Dynamic Phenomena in He II

A laser holographic interferometer is applied to the study of the thermo-fluid dynamic phenomena in superfluid helium. A pressure wave accompanied by a thermal wave and a vapor wave by a thermal wave incidence to a liquid-vapor interface are investigated, all excited by pulse heating from a plane heater under the saturated vapor pressure condition. A new technique for interfcrometry — finite-fringe method — is introduced in order to observe the above phenomena not only visually but also quantitatively. It is understood from the visualized photos that a finite amplitude pressure wave of the first sound origin propagates with a speed of sound, corresponding to the local Mach number of unity in He II, in immediate response to the onset of heating, being ahead of a thermal shock wave. The density variation and propagating speed of the vapor wave with the variable heat flux from the heater are measured from the visualized photos. It is found that the two have their own limits with applied heat flux.

Experimental study of thermal shock wave deformation and decay due to tangled mass of quantized vortices in He II

Propagation of a thermal shock wave and of highly transient heat transport phenomena in He II are experimentally investigated with a superconducting temperature sensor. The variation of the wave height of a thermal shock wave during propagation is compared with the prediction of a energy conservation and the Burgers equation. The total amount of energy carried away by a thermal wave from a heating surface is evaluated from the area of the measured wave profile and its variation is investigated with varying heat flux and heating time. It is found that just a small amount of energy is transported through He II in the form of a second sound wave for the case of large heat flux and long heating time. The rest of the applied energy from the heater is diffusively transported since dense tangled vortices impede ideal heat transport in the second sound mode.

Measurement of characteristic time for quantized vortex tangle development in He II

The characteristic time of quantized vortex tangle development due to pulsed heating in He II is measured from the thermal pulse shape deformation with a superconductive temperature sensor. It is the time duration which is required for the effect of quantized vortex tangle on transient heat transfer phenomena to become appreciable. The relationship between the characteristic time and the applied heat flux is obtained. Its temperature dependence and the comparison between the time and the onset time of transient nucleate boiling are also discussed.

Measurement of Characteristic Time of Quantized Vortex Development Using a Thermal Shock Wave

Publisher Summary This chapter derives the characteristic time of quantized vortex development by analyzing the thermal shock wave profiles measured with a superconductive temperature sensor. In this study the characteristic time of quantized vortex development, defined from the point of view of a transient heat transfer, is derived by analyzing the thermal shock wave profiles measured with a superconductive temperature sensor. The result is also compared with the numerical result based on the vortex line density equation. A thermal shock wave generated by a pulsed heating from a planar heater is deformed through the interaction with quantized vortices. The deformation is measured by means of a superconductive temperature sensor. It is found that the characteristic time is inversely proportional to the square of applied heat flux. It is also found that appreciable effect appears on transient heat transfer if the vortex line density exceeds approximately 105cm/cm3.