Experimental and numerical analysis of the temperature transition of a suspended freezing water droplet

Abstract. The heterogeneous freezing temperatures of supercooled drops were measured using an acoustic levitator. This technique allows one to freely suspend single drops in the air without any wall contact. Heterogeneous nucleation by two types of illite (illite IMt1 and illite NX) and a montmorillonite sample was investigated in the immersion mode. Drops of 1 mm in radius were monitored by a video camera while cooled down to −28 • C to simulate freezing within the tropospheric temperature range. The surface temperature of the drops was contact-free, determined with an infrared thermometer; the onset of freezing was indicated by a sudden increase of the drop surface temperature. For comparison, measurements with one particle type (illite NX) were additionally performed in the Mainz vertical wind tunnel with drops of 340 µm radius freely suspended. Immersion freezing was observed in a temperature range between −13 and −26 • C as a function of particle type and particle surface area immersed in the drops. Isothermal experiments in the wind tunnel indicated that after the cooling stage freezing still proceeds, at least during the investigated time period of 30 s. The results were evaluated by applying two descriptions of heterogeneous freezing, the stochastic and the singular model. Although the wind tunnel results do not support the time-independence of the freezing process both models are applicable for comparing the results from the two experimental techniques.

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“…Figure 2a shows an example of temperature development during freezing. According to Hindmarsh et al (2003), it can be divided into four stages:…”

Section: Methodsmentioning

confidence: 99%

Particle surface area dependence of mineral dust in immersion freezing mode: investigations with freely suspended drops in an acoustic levitator and a vertical wind tunnel

Diehl¹,

Debertshäuser²,

Eppers³

et al. 2014

Atmos. Chem. Phys.

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“…Actually, the shape change of the water droplet during freezing is significant as the protrusion may appear due to the density discontinuity of liquid water and solid water. Hindmarsh et al [9] verified that a single droplet adhered to a thermocouples and put in cold air changed its original spherical shape and formed a protrusion on the top side of itself. The researchers attributed the protrusion to volume dilatation.…”

mentioning

confidence: 99%

Deformation of freezing water droplets on a cold copper surface

et al. 2006

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Freezing processes of water and peanut oil droplets on a cold surface are investigated in this paper. We observed during our experiments that the base surface of a water droplet that is in direct contact with the cold surface keeps its original shape, but the other part of the droplet shows an obvious growth along the direction normal to the base surface. One small protrusion appears on the top of the water droplet at the end of the freezing process. The experimental observations also show that no obvious shape change happens during the freezing of peanut oil droplets. It is postulated that the effects of surface tension and volume dilatation resulted from liquid-to-solid phase change cause the shape change and protrusions formation. Based on this postulation, a physical and mathematical model is developed. The results of the model of a water droplet's freezing process correspond with our experimental observations. The observed phenomenon that frost-growth speed on the protrusion is higher than that on the other part of the water droplet is also analyzed.Frost formation is a very common phenomenon in cryogenic, refrigeration, and air conditioning engineering. The temperature of heat exchange units will undergo a gradually decreasing process during their thermal defrosting process. When humid air comes in contact with a cold wall whose temperature is below the dew point of air, vapor contained in the humid air will condensate and deposit on this cold surface in the form of small water droplets. Continuous condensation of vapor and glomeration of the small droplets will produce larger water droplets. Meanwhile, when the temperature of the cold surface is lower than the freezing point of water, the water droplets formed on the cold surface will freeze. Subsequently, frost will deposit on these frozen droplets. Frost deposition on heat transfer surfaces will produce a series of negative effects on heat transfer equipment. For example, heat transfer resistance and pressure loss will increase, resulting in poor heat transfer performance. In addition, the air passage may be blocked and mal-

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“…It has been postulated that with the occurrence of nonequilibrium solidification in rapidly freezing droplets the uniform temperature assumption was not valid, 23 even when the Biot number was Ͻ0.1. In previous work 8 it was found that the assumption of a uniform temperature profile in the droplet was justified and accurate for the droplet sizes and freezing conditions used in this study. As a consequence, all models formulated to predict the freezing of water and sucrose solution droplets have used the uniform temperature assumption.…”

Section: Numerical Modelsmentioning

confidence: 80%

“…Re ϭ 2R a a (8) where v, a , and a are the air velocity, density, and viscosity, respectively. The Schmidt number (Sc) is…”

Section: Droplet Surface Heat Fluxesmentioning

confidence: 99%

NMR verification of single droplet freezing models

et al. 2005

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Experimental and numerical analysis of the temperature transition of a suspended freezing water droplet

Cited by 200 publications

References 31 publications

Particle surface area dependence of mineral dust in immersion freezing mode: investigations with freely suspended drops in an acoustic levitator and a vertical wind tunnel

Particle surface area dependence of mineral dust in immersion freezing mode: investigations with freely suspended drops in an acoustic levitator and a vertical wind tunnel

Deformation of freezing water droplets on a cold copper surface

NMR verification of single droplet freezing models

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