Teruhito Iida

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.

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.

Applications of a Laser Doppler Velocimeter and Some Visualization Methods to the Measurement of He II Thermo-Fluid Dynamic Phenomena

We have been engaged in the development of the application techniques of a laser Doppler velocimeter (LDV) and some flow visualization methods, a tracer method and a laser holography interferometer method, for the measurement of He II thermo-fluid dynamic phenomena. An LDV was applied to directly measure the normal fluid component velocity in a thermal counterflow jet by using hydrogen-deuterium micro solid particles as tracer particles. He II thermal counterflow jet was also visually studied by tracer method by adopting hydrogen-deuterium solid particles and hollow glass spheres both of which are neutrally buoyant in He II. The translational motion of a large-scale vortex ring was also investigated by this method. Some wave phenomena in He II were also visually investigated by applying a laser holography interferometer method. In this report, the application techniques of these methods are described and some typical results are presented to demonstrate their validity and to point out some problem areas.

Temperature Measurement in Transient Heat Transport Phenomena through a Thermal Boundary Layer with High Vortex Density in He II

Highly transient heat transport phenomena are investigated with a superconductive temperature sensor. The heat transport process in He II is categorized into three major processes, the second sound wave mode, the dissipative process through a high vortex density thermal boundary layer and the consumption for the phase transition of He II. The contribution of each process is evaluated by analyzing the measured temperature variation. The result is plotted in a map characterized by a parameter q p t H 1/2 . Experimental data are also compared with the analytical solution obtained by Dresner. It is found that if the contribution of the diffusive process can be properly evaluated, the thermal behavior in the diffusive region fairly agree with the Dresner’s approach in the highly transient case.

Control of Secondary Flow in a Double‐inlet Pulse Tube Refrigerator

Secondary flow in a double‐inlet pulse tube refrigerator with a double bypass valve configuration was evaluated to determine the advantage of this configuration in controlling the secondary flow over a conventional single valve configuration. In the double‐valve configuration two needle valves was connected in parallel and their respective valve directions were in opposite directions, namely, one valve was directed toward the hot end and the other toward the inlet of the regenerator. The effects of valve configuration on the flow behavior of the secondary flow, on the cooling performance, and on the pressure‐volume (work) diagram for gas were investigated. The secondary flow was visualized by using a smoke‐wire flow visualization technique, and cooling performance was estimated based on the temperature difference between the hot and cold ends. Results showed that the double‐valve configuration allowed the operating conditions of the refrigerator to be optimized to maximize the work and minimize the heat l...