L. C. Brodie

Premature transition to stable film boiling initiated by power transients in liquid nitrogen

Abstract We have investigated transient pool boiling in liquid nitrogen for a process in which the boiling is initiated by a step-function application of power to a thin wire immersed in the liquid. For such a process we find that a transition to stable film boiling can be observed for power values as low as about 40% of the steady-state peak heat flux. Such a ‘premature transition’ to stable film boiling can occur, even though the maximum observed values attained by the transient heat flux may lie substantially below the steady-state peak heat flux. Heating wires of platinum, copper, and constantan were employed in this investigation. Steady-state measurements are presented for comparison with the transient results, and a demonstration of the inhibition of the premature transition is described using a form of forced convection around the test wire.

Transient heat transfer into liquid helium I

We have investigated the transient transfer of heat from a crystal of bismuth into a bath of liquid helium I in response to a step‐function current applied to the crystal. The superheat temperature ΔT of the helium in contact with the crystal was monitored through the magnetoresistance of the bismuth which is strongly temperature dependent at liquid‐helium temperatures. It is found that as a function of time, the transient superheat temperature can rise above (i.e., ’’overshoot’’) its steady‐state value. Of particular interest is the fact that, within a certain range of bath temperatures and heat fluxes, these transient overshoots develop quite sharp peaks at a superheat temperature ΔTmc which is independent of the applied heat flux. This overshooting of the transient superheat temperature is discussed in terms of the delayed onset of nucleation. In particular, it is suggested that the sharp overshoots result from the rapid onset of homogeneous nucleation. The temperatures associated with the sharp oversh...

Photographic studies of light induced nucleation of boiling at the interface of a solid and superheated liquid helium I

Abstract We report the results of photographic studies which show that light induces the nucleation of vapour bubbles in superheated liquid helium I. We believe that these bubbles are associated with the cooling that we have previously observed to result from a flash of visible light applied to the surface of a bismuth crystal heater-thermometer immersed in the superheated liquid. We have used this photographic technique to study vapour bubble growth dynamics and compare our results with the theory.

Light induced enhancement of transient heat transfer from a solid into liquid helium I

Abstract We report preliminary studies of a new effect: enhancement of transient heat transfer from a bismuth crystal into liquid helium by a light pulse. We found that when a single crystal of bismuth is rapidly heated above a certain threshold temperature in a bath of liquid helium by a step-function electric current, the application of a light pulse of intensity 1 to 200 mW cm −2 and duration 2 μs to 1 ms causes a decrease of up to 75% in the total crystal superheat temperature within 3 ms. The threshold temperature is less than the homogeneous nucleation temperature of liquid helium. The marked increase in heat transfer across the solid-liquid interface, associated with the observed rapid cooling, is believed to be caused by increased bubble activity due to the light induced rapid nucleation of bubbles near the interface. Although the exact nature of the interaction mechanism involved is unclear at this time, several hypotheses are presented and discussed.

New Measurements of the Tensile Strength of Liquid 4He

A piezoelectric hemispherical transducer was used to focus high-intensity ultrasound into a small volume of superfluid helium. The transducer was gated at its resonant frequency of 566 kHz with gate widths less than 1 ms in order to minimize the effects of transducer heating and acoustic streaming. The onset of nucleation was detected with small-angle scattering of laser light from the cavitation zone by microscopic bubbles. The laser probe also was used to confirm calculations of the pressure amplitude at the focus based on the power radiated into the liquid, the geometry of the transducer, and the nonlinear attenuation of the sound. The cavitation pressure observed was significantly greater than previously reported in the literature and in agreement with homogeneous nucleation theory.

Quantum effects on the temperature dependence of surface tension of simple liquids

An equation of general applicability is proposed to describe the surface tension of simple liquids over an extended temperature range. This equation incorporates the quantum effects on the exponent μ, which characterizes the temperature dependence of surface tension. Far from the critical region, μ decreases with increasing de Boer parameter and appears to reach a constant value μ≃1 for 4He and 3He. It is pointed out that for quantum liquids μ crosses over to the nonquantum mechanical value of μ≃1.28 as the critical point is approached. The surface tension and its temperature dependence are predicted for hydrogen isotopes such as T2, DT, D2, HT, and HD.

Light induced nucleation of vapour bubbles at the interface of a solid and superheated liquid helium i: test of the photoelectron hypothesis

Abstract We report further studies of light induced enhancement of heat transfer from a solid into liquid helium 1. We have previously found that a flash of visible light applied to a solid surface immersed in the superheated liquid results in the production of vapour bubbles and a subsequent decrease in liquid layer superheat. Because there appeared to be a long wavelength cut-off we felt that this effect could be due to a mechanism involving photoemission of electrons into the liquid helium. In this hypothesis, electrons are first detached from the solid by the action of light and second, serve as bubble nuclei in the superheated liquid helium. The results reported in this paper fail to support this explanation.

Superheating limits of liquid helium I

Abstract It is shown that the maximum superheat temperature Δ T max in liquid helium I, determined under steady-state peak nucleate boiling conditions, cannot be realistically estimated from the thermodynamic limit of superheat (spinodal), as has been previously claimed. It is suggested that a more meaningful estimate of Δ T max can be obtained from the kinetic limit of super-heat (homogeneous nucleation temperature). Experimental data on the homogeneous nucleation superheat temperature, Δ T h , in liquid helium I over an extended temperature range are presented and compared with the Δ T max measurements of several investigators. It is pointed out that in any quantitative predictions of Δ T max , a knowledge of Δ T h alone is not sufficient. In fact, the measured value of Δ T max can lie either below or even above Δ T h depending on the bath temperature, conditions of the surface, and the thermophysical properties of both heater and liquid.

Transient Heat Transfer in Liquid Nitrogen

With the advent of the new high Tc superconductors, liquid nitrogen will be one of the preferred cryogens used to cool these materials. Consequently, a more thorough understanding of the heat transfer characteristics of liquid nitrogen is required. In our investigations we examine the transient heating characteristics of liquid nitrogen to states of nucleate and film boiling under different liquid flow conditions. Using a platinum hot wire technique, it is verified that there is a premature transition to film boiling in the transient case at power levels significantly lower than under steady state nucleate boiling conditions. It is also shown that the premature transition can be reduced or eliminated depending on the flow velocity. Similar results are obtained with no forced liquid flow when a train of constant power step pulses is used. For certain ranges of power levels and repetition rates, increasing the pulse rate is analogous to increasing the flow rate.

Sound Induced Enhancement of Heat Transfer from a Solid into Liquid Helium

We are reporting on a preliminary study of the enhancement of heat transfer from a germanium crystal heater-thermometer into liquid helium as a result of the application of an ultrasonic pulse. We found upon rapid heating of a germanium crystal in the helium that the total crystal superheat can be substantially lowered by the application of a sound pulse. Application of a short sound pulse, prior to reaching a certain threshold superheat, had little effect on the maximum crystal temperature. Application of the sound pulse after this threshold value was reached resulted in a rapid decrease in the crystal superheat. These results are quite similar to those experiments showing the enhancement of heat transfer due to a light pulse which have been published previously.