Acoustic, video and thermoacoustic registration of boiling up phenomena in He II (original) (raw)
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Study of film boiling in He II by pressure and temperature oscillation measurements
Cryogenics, 1999
In the present study, the discussion on ®lm boiling states in He II is presented on the basis of some new experimental results. Experiments were conducted under various thermal conditions, e.g., dierent bath temperature, immersion depth and heat¯ux. In subcooled ®lm boiling state, a¯ame-shaped vapor bubble is seen to be quivering on a planar heater accompanied with a little audible noise, and the pressure oscillation is detected, which results from both vapor bubble oscillation and liquid column oscillation of bulk He II in a cryostat. In noisy boiling state in saturated He II, a big vapor bubble grows and erupts repeatedly on a planar heater accompanied with a large acoustic noise, and the pressure oscillation is mainly caused in accordance with vapor bubble crush and formation. A three-dimensional boundary map indicating dierent boiling states is constructed by taking account of eects of the bath temperature and the heat¯ux as well as the hydrostatic pressure. Temperature oscillation during noisy ®lm boiling is also measured by the aid of superconductor temperature sensors. It is found that the large temperature rise is caused by both the passage of thermal boundary layer and repeated expansion and crush of a vapor bubble on the planar heater.
The boiling-up process in He II: optical measurements and visualization
Cryogenics, 1992
The boiling-up process in superfluid helium was measured by an optical method and compared with previous experiments of other authors. The influence of the different pulse triggering rates was removed by introducing a dimensionless parameter. It appears that the reported new method is much more sensitive than those used in former investigations and gives the possibility of a visualization of the whole process. Further, vapour bubbles with finite dimensions (diameter of the order of 1 cm) were observed in the bulk liquid. Depending on the remaining liquid column (or hydrostatic pressure head) above the heater, these bubbles have a lifetime of up to 50 ms. The lower the pressure head, the longer is the lifetime of the bubbles.
Study of pressure oscillation during noisy film boiling in He II
Cryogenics, 1997
In this paper we study the cause of loud audible noise generation during noisy film boiling in He II. Measurement of the pressure oscillation as well as a visualization study is conducted for this purpose. As a result, it is found that loud audible noise is repeatedly generated at each instance of explosive evaporation of He II which comes into direct contact with an overheated heater surface after a vapour bubble crush. The pressure oscillation measurement over a wide range of hydrostatic pressure also revealed that three film boiling modes appear in He II: boiling in subcooled He II (referred to as subcool boiling); noisy boiling; and silent boiling, as the hydrostatic pressure decreases. However, very close to the h-temperature, subcool boiling makes a direct transition to silent boiling without noisy boiling. Noisy boiling does not appear in subcooled He II, where subcool boiling appears with very weak audible noise of very high frequency.
Experimental investigation of the film boiling heat transfer in He II: Heat transfer coefficient
Cryogenics, 2005
He II film boiling is of both academic and applied interests. However, information about He II film boiling is still inadequate and further study is needed from both the technical application and the scientific aspects. In the present study, a thin stainless steel foil heater (10 lm thick) is employed to induce boiling in He II. The average heater surface temperature is measured to evaluate the heat transfer performance of He II film boiling under different thermal conditions. And meanwhile, the pressure and the temperature oscillations induced by the film boiling are also measured. It is found that the pressure oscillation and the temperature oscillation highly correlate with each other, which indicates that the vapor bubble is vibrating on the heater surface during film boiling. The heat transfer coefficient of the film boiling depends on both the pressure over the heater surface and the He II bath temperature. The heat transfer coefficients of three kinds of boiling states: noisy film boiling, transition boiling and silent film boiling, are measured in the present study. The visualization of the boiling process is also carried out.
PIV Measurement of Flow Field around Film Boiling in He II
Physics Procedia, 2015
Film boiling phenomena in He II were studied using the PIV (Particle Image Velocimetry). Noisy and silent film boiling states were generated on/around a planar or a cylindrical heater. Tracer particles were neutrally buoyant solid H 2-D 2 particles. The motions of vapour bubble and of tracer particles were PIV-analyzed. The velocity field was found to be composed of AC and DC velocity components. The AC component is caused by the dynamic behaviour of vapour, and the DC results primarily from the thermal counter flow. The time-averaged PIV velocity field provides with the flow field characteristics of each boiling mode.
Dynamics of a Vapor Bubble in Film Boiling and the Superheat Effect
WSEAS TRANSACTIONS ON HEAT AND MASS TRANSFER
This study aims at developing an improved numerical simulation of the film boiling regime phenomenon to understand and visualize the growth of vapor bubble at a heated surface during low and high superheats. The simulation of the bubble dynamics including the bubble growth, departure, coalescence, rising, and frequency of detachment under different wall superheats is numerically investigated. The continuity, momentum, and energy equations are solved for the two immiscible fluids phases using the finite volume method. The phase change model and the results exhibited a good agreement with the theoretical models. The obtained results show that the velocity of bubble growth and its frequency of emission promotes heat exchange. It is found that the shape of a bubble has been influenced by the wall superheat. It is also found that the high superheat generates a large amount of steam in which the steam bubble takes the shape of a fungus. So, a clear correlation exists between heat transfer...
Study of He II boiling flow field around a heater
IOP Conference Series: Materials Science and Engineering, 2015
We studied boiling phenomena in He II based on the flow velocity measurement data by using a PIV (Particle Image Velocimeter). Noisy and silent film boiling modes together with non-boiling state were generated on/around a horizontal planar or a cylindrical heater. For PIV tracer particles, we used H 2-D 2 solid particles that were neutrally buoyant in He II. Video images showing the development and collapse of vapour bubble or film and the motions of tracer particles were PIV-analysed. We found the PIV velocity field was composed of AC and DC velocity components of the normal fluid. The AC component follows the dynamic behaviour of vapour, and the DC results primarily from the thermal counter flow and secondarily is induced by the asymmetric vapour bubble motion. We also investigated unsteady velocity component. The objective of this series of study is to compare the characteristic features of the flow field of He II film boiling states and peculiar He I boiling state in He II and to make clear the difference in the heat transfer performance of each boiling mode. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
The Dependence of the Heat Transfer Coefficient on Film Boiling Modes in He II
Journal of Cryogenics and Superconductivity Society of Japan, 2006
This study was carried out to clarify the difference in the boiling heat transfer modes in He II. Experiments were performed using a cryostat at a wide range of hydrostatic pressures in He II, from atmospheric to saturated vapor pressure. A thin stainless steel foil heater was used to initiate film boiling and simultaneously served as a temperature sensor as well. According to experimental results it was confirmed that the two film boiling modes appear above the lambda pressure: the strongly subcooled film boiling mode and the weakly subcooled film boiling mode. Measurements of the heater temperature variations indicated the difference between the thermo-fluid dynamic boiling features of the weakly subcooled film boiling and that of the noisy film boiling. In case of the weakly subcooled film boiling, the heat transfer coefficients become smaller as pressure increases. However, during the strongly subcooled film boiling, pressure has nearly no affect on heat transfer coefficient. Note that the peak in the heat transfer coefficient appears at a pressure of about 8 kPa, which is in the transition region between the weakly subcooled and the noisy film boiling modes.
Cryogenics, 1996
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.
Computations of film boiling. Part II: multi-mode film boiling
International Journal of Heat and Mass Transfer, 2004
Film boiling on horizontal periodic surfaces is investigated by direct numerical simulations. A front tracking/finite difference technique is used to solve the momentum and the energy equations in both phases and to account for inertia, viscosity, and surface deformation. Effect of the unit cell size W on the interface dynamics, heat transfer, and fluid flow is studied for different wall superheats. The simulations are carried out over sufficiently long times to capture several bubble release cycles and to evaluate the quasi steady-state Nusselt number hNui. While instantaneous Nusselt number will change as result of a change in the system size, statistically steady-state Nusselt number remains almost the same. Simulations of two-dimensional systems in large unit cells, 5k d2 < W < 10k d2 , show a distribution of bubble spacing in the range of 0.61k d2 -1.46k d2 . At relatively low superheats (Ja 6 0.064) the bubbles are released periodically from the vapor film, but at intermediate superheats (0.064 < Ja < 2.13) permanent vapor jets are formed with no bubble break off. At sufficiently high superheats, the vapor jets start to interact. It is shown that the average bubble spacing does not change with changes in the wall superheat.