Yoshihiro Iida

Boiling nucleation on a very small film heater subjected to extremely rapid heating

Abstract A very small film heater immersed in ethyl alcohol, toluene or water is heated at an extremely high rate to a maximum temperature rise of 93 × 106 K s−1. The nucleation process is photographed and the temperature at boiling incipience is measured. At a higher heating rate, a large number of tiny bubbles generate concurrently on the heater surface and the temperature at boiling incipience agrees with the homogeneous nucleation temperature for ethyl alcohol and toluene, whereas it is about 19°C lower for water. The number density of the nucleated bubble is compared with those calculated by the nucleation theory.

Transient boiling heat transfer characteristics of nitrogen (bubble behavior and heat transfer rate at stepwise heat generation)

Abstract Transient boiling heat transfer characteristics of liquid nitrogen at stepwise heat generation is investigated. A series of heat transfer experiments are carried out using a 0.10 mm diameter horizontal platinum wire as the test heater. Bubble behavior is observed by taking high-speed movie photographs. The history of heat transfer rate corresponds well with the bubble behavior during the transition stage to film boiling. In the case of a low heat generation rate, boiling transition occurs due to the coalescence of nucleate boiling bubbles. In the case of a high heat generation rate, a vapor sheath grows along the heater wire. Boiling transition occurs due to the settling of the vapor sheath around the wire and the heat transfer rate just after the boiling transition becomes much lower than that of stationary film boiling. In the case of an extremely high heat generation rate, a lot of fine bubbles grow rapidly and simultaneously when the heater temperature reaches the homogeneous nucleation temperature. Boiling transition occurs due to the filling of fine bubbles on the heater.

Effects of ultrasonic waves on natural convection, nucleate boiling, and film boiling heat transfer from a wire to a saturated liquid

A series of experiments were carried out to make clear systematically the effects of ultrasonic waves on natural convection, nucleate boiling, and film boiling heat transfer from a heated 0.2 mm diameter platinum wire to saturated water or ethyl alcohol. The effects on the maximum and minimum heat flux points were also examined. The test wire is set in liquid above a 20 mm × 40 mm vibrating surface of an ultrasonic transducer whose resonance frequency is 28 kHz and maximum power is 33.6 W. A distinctive augmentation effect on heat transfer is observed in both the natural convection and film boiling heat transfer regions. Though no effect is observed in nucleate boiling of water, a small effect is attained in low heat flux nucleate boiling of ethyl alcohol. An increase of about 20% in the maximum heat flux is obtained in both liquids by applying ultrasonic waves. The minimum heat flux point is raised at higher values of both the degree of superheat and the heat flux. The profiles of sound pressure along the centerline of the ultrasonic field and those of the heat transfer coefficients in natural convection and in film boiling were measured and compared. Heat transfer coefficients attained under ultrasonic waves are shown to be depend largely on the distance from the vibrating surface to the test position. In general, the sound pressure profiles resemble a deformed sine wave and are roughly identical with those of the heat transfer coefficient as regards their wavelength and their maximum and minimum. It is considered that the sound pressure of ultrasonic waves may, directly or indirectly, have some connection with the mechanism of heat transfer augmentation.

Pumping action by boiling propagation in a microchannel

The use of boiling propagation as the actuation mechanism of micropumps is proposed. The process of boiling propagation along a film heater surface, the propagation velocity, propagating bubble size, maximum repetition frequency, and the wall superheat conditions under which propagation occurs are investigated using ink for a thermal inkjet printer under pool boiling conditions. A prototype micropump that uses boiling propagation is developed. A film heater placed facing the microchannel is powered stepwise over a short duration and unidirectional boiling propagation over the entire heater length is triggered at a high wall superheat by generating a vapor bubble at the end of the heater. A continuous pumping action in a U-shaped microchannel via boiling propagation repeated at a prescribed frequency of up to 20 Hz is confirmed based on the head difference generated between liquid columns in the vertical sections.

Direct-contact heat transfer characteristics: Evaporation of a drop dropped onto a hot liquid surface

Abstract The evaporation time and the evaporation mode of liquid drops (n-pentane, dichloromethane, acetone and methyl alcohol) dropped onto the surface of a silicone oil at high temperature were measured, and the heat transfer characteristics between the two liquids were disclosed qualitatively over a wide range of temperature difference. The evaporation time curves generally showed the existence of a maximum and a minimum of apparent heat transfer rate at respective superheats of the hot liquid, similarly to what is observed for liquid-solid contact. The observed evaporation modes were grouped into twelve types.

Premature transition to film boiling with stepwise heat generation 2nd report: Effect of wall material and surface condition

The effects of wall material and surface condition on the behavior of an initial boiling bubble of R113 subjected to transient heating were investigated using a heater with a large heat capacity. The behavior of the initial bubble is closely related to premature transition to film boiling of liquids with high wettability. An initial bubble, which is peculiarly shaped like a straw hat and leads to premature transition in saturated liquid nitrogen (as reported in a previous paper), also appears on a heated wall with large heat capacity and grows rapidly to cover the entire wall surface. From the observations using a high-speed video camera, the initial bubble is found to be a coalesced bubble into which small bubbles activated in succession along the heated surface are rolled. The growth rate of the initial bubble along the heated surface is not greatly affected by the thermal conductivity of the wall material but is affected markedly by the surface roughness.

Trial Manufacturing of MQL Oil Mist Density Sensing System for Environmentally Friendly Machine Tools

In connection with the worldwide increase of the environmental consciousness, eco-machining becomes one of the most important subjects in the mechanical production field. Consequently, MQL (Minimal Quantity Lubrication) machining process becomes the practical technology today. However, behavior of the mist particle at the tool-work interface in company with its lubrication and cooling effect in that interface has not been made clear. Indeed, although it is indicated that the mist particles floating in the surrounding are so small that the machining operator can be damaged by the particle, even the density of the mist at the arbitrary point can not be known. In this study, authors tried to develop a simple and small mist density sensing system by means of QCM (Quartz Crystal Microbalance) sensor conventionally used for the smell sensor. The developed system was evaluated by some experiments to check its sensitivity and reliability in the view point of the availability for the practical use. As a result, it was shown that the density of oil mist can be qualitatively obtained by measuring the resonance frequency of the circuit, and that this system is applicable to the actual production fields.

Vapor film collapse around hot liquid drops when an external pressure wave is supplied

An external pressure wave generated by a magnetic hammer is applied to a single or multiple hot LiNO 3 drops which have been in stable film boiling in subcooled ethanol. The process of vapor film collapse triggered by the pressure wave is studied by photographic observations and pressure history measurements. The vapor film begins to destabilize about 0.1 ms after pressure wave arrival at the drop. About 1.0 to 1.5 ms later, the vapor film begins to collapse from the lower part. Collapse of the vapor film develops from a portion around the drop preceded by a complex destabilization process. The ripple waves generated from a portion in the initial stage are concluded to be due to a Rayleigh-Taylor instability. Generation of interface disturbance and that of tiny bubbles as well as that of hot liquid fragments are observed, though no penetration of cold liquid jets can be observed in these experimental conditions

Process of Vapor Film Collapse around Hot Liquid Drops Induced by an External Pressure Wave.

An external pressure wave generated by a magnetic hammer is applied as an explosion trigger to a single or multiple hot LiNO3 drops which have been in stable film boiling in subcooled ethanol and the process of vapor film collapse is studied by photographic observation and pressure history measurement. The vapor film begins to destabilize within about 0.1ms after the pressure wave arrival to the drop position. At about 1.0-1.5ms later, the vapor film begins to collapse from the lower part. The vapor film initiates to collapse at a portion around the drop after a complex destabilization process. The ripple waves generated near the central part of the drop at the initial stage are supposed to be due to the Rayleigh-Taylor instability because of the wave length. Generation of interface disturbance and that of tiny bubbles or hot liquid fragments are observed, though no penatration of cold liquid jets can not be observed in this experimental condition.

Film Boiling Heat Transfer with Catalytic Decomposition Reaction.

Film boiling heat transfer of saturated methanol with decomposition reaction is investigated experimentally by using a cylindrical heater with catalytic surface. Platinum black powder which is stuck on the cylinder shows enough activity to catalyze the reaction. The heat transfer coefficient remarkably increases with the slight increase of the wall superheat and becomes about 50% higher than the Bromleys prediction of no reaction case. Such high heat transfer is considered to be due to the endothermic action and the evolution of hydrogen the thermal conductivity of which is about five times that of methanol vapor. However, once the wall superheat exceeds about 510K, the heat transfer coefficient decreases and approaches that of no reaction case. This may be due to the sintering of the catalyst used in the present study. The heat transfer coefficient which is predicted by taking account of the absorbed heat and the properties change of vapor phase due to the evolution of decomposed gases is in good agreement with the high coefficient measured by experiment.