Experimental and Theoretical Investigation of Instability within a Heated Meniscus.

摘要

The behaviour of heated, evaporating menisci in capillary tubes was experimentally observed and theoretically predicted. Testing was performed with n-pentane as a working fluid evaporating in both ambient air (multicomponent, multiphase environment) and a chamber filled with its saturated vapour (single-component, multiphase environment). Capillary tubes with both thin and thick walls with comparable internal diameters were tested. Thermocouples were embedded inside the thick tubes and epoxied onto the thin tubes. A carbon composite resistive element was deposited onto each tube for electrical heating. A meniscus was positioned between tube thermocouples and electrical heating was provided until the meniscus was observed to oscillate in the axial direction, at which time it was considered destabilised. The experimental temperature gradient at the onset of instability was found to be significantly different for thin and thick tubes of comparable inner diameters. Thin tube results were determined to be more representative of the axial temperature gradient at the tube inner wall. Oscillations were observed and recorded using a microscope-camera assembly. Meniscal instability occurred at a higher temperature gradient for testing while exposed to ambient air than for testing in a pure n-pentane environment. A meniscus inside a smaller inner diameter tube was found to be capable of sustaining a larger temperature gradient than that for a meniscus inside a larger inner diameter tube.;A leading-order scaled equation describing the evolution of the thin film was derived and a perturbation analysis was applied to obtain a stability criterion. A characteristic height was chosen as the height at which the evaporative mass flux was a maximum. The meniscus half channel width was chosen as the characteristic length normal to the characteristic height. A numerical thin film model was created to obtain the characteristic height from the thin film solution profile together with the calculated value of the stability criterion. Comparison with experimental results showed that the model has some predictive ability in determining the onset of evaporating meniscal instability in a heated capillary tube. Results of this study can be used to design and characterise the operational limits of phase change heat transfer devices using evaporation from a meniscus as would occur in heat pipes.