Use of levitating liquid micro-droplets as tracers to study the evaporation in the vicinity of the contact line

Self-organization of a cloud of liquid micro-droplets into an ordered two-dimensional array, levitating over a heated layer of liquid due to upward vapor flow, has been observed in several recent experimental works. In the present paper, the levitating micro-droplet array is studied under the condition when the liquid layer ruptures and a dry spot form on the heater. It has been found, that when the levitating micro-droplets are moving from wetted heater area to dry heater area and passing over the contact line, the micro-droplets levitation height is drastically increased, which is indicative of the intensive evaporation occurring in the region of the contact line

Use of a thin liquid film moving under the action of gas flow in a mini-channel for removing high heat fluxes

Intensively evaporating liquid films shear-driven in a mini- or micro-channel under the action of cocurrent gas flow are promising for the use in modern cooling systems of semiconductor devices. In this work, we investigated the influence of liquid and gas flow rates on the critical heat flux in a locally heated film of water, moving under the action of air flow in a mini-channel. In experiments a record value of critical heat flux of 870 W/cm{2} was reached. Heat spreading into the substrate and heat losses to the atmosphere in total do not exceed 25 % at heat fluxes above 400 W/cm{2} . A comparison with the critical heat flux for water flow boiling in the channel shows that, for shear-driven liquid films the critical heat flux is almost an order of magnitude higher

An experimental study of high heat flux removal by shear-driven liquid films

Intensively evaporating liquid films, moving under the friction of a co-current gas flow in a mini-channel (shear-driven liquid films), are promising for the use in cooling systems of modern semiconductor devices with high local heat release. In this work, the effect of various parameters, such as the liquid and gas flow rates and channel height, on the critical heat flux in the locally heated shear-driven water film has been studied. A record value of the critical heat flux of 1200 W/cm2 has been achieved in experiments. Heat leaks to the substrate and heat losses to the atmosphere in total do not exceed 25% for the heat flux above 400 W/cm2. Comparison of the critical heat fluxes for the shear-driven liquid film and for flow boiling in a minichannel shows that the critical heat flux is an order of magnitude higher for the shear-driven liquid film. This confirms the prospect of using shear-driven liquid films in the modern high-efficient cooling systems

Use of a thin liquid film moving under the action of gas flow in a mini-channel for removing high heat fluxes

Intensively evaporating liquid films shear-driven in a mini- or micro-channel under the action of cocurrent gas flow are promising for the use in modern cooling systems of semiconductor devices. In this work, we investigated the influence of liquid and gas flow rates on the critical heat flux in a locally heated film of water, moving under the action of air flow in a mini-channel. In experiments a record value of critical heat flux of 870 W/cm{2} was reached. Heat spreading into the substrate and heat losses to the atmosphere in total do not exceed 25 % at heat fluxes above 400 W/cm{2} . A comparison with the critical heat flux for water flow boiling in the channel shows that, for shear-driven liquid films the critical heat flux is almost an order of magnitude higher

Levitation of Liquid Microdroplets Above A Solid Surface Subcooled to the Leidenfrost Temperature

Evaporation of liquid microdroplets that fall on a solid surface with the temperature of below the Leidenfrost temperature is studied. It has been found out that sufficiently small liquid droplets of about 10 microns can suspend at some distance from the surface (levitate) and do not reach the surface; at that, the rate of droplet evaporation is reduced by an order as compared to microdroplets, which touch the surface. It is determined that in contrast to microdroplets, which touch the surface, the specific evaporation rate of levitating droplets is constant in time

The Effect of Contact Angle on Dynamics of Dry Spots Spreading in a Horizontal Layer of Liquid at Local Heating

The effect of equilibrium contact angle on dynamics of dry spot spreading at disruption of a horizontal water layer heated locally from the substrate was studied using the high-speed Schlieren technique. Different methods of working surface processing were applied; this allowed variations of the equilibrium contact angle from 27±6° to 74±9° without a change in thermal properties of the system. It is found out that substrate wettability significantly affects the propagation velocity of dry spot and its final size. It is also found out that the velocity of contact line propagation is higher in the areas of substrate with a higher temperature

Evaporation Rate of Distilled Water Drop on the Surface of Non-Ferrous Metals

We studied experimentally the evaporation process of distilled water drops on the surfaces of non-ferrous metals. Investigations were conducted on the experimental setup using a shadow optical system. The main elements of this system are the source of plane-parallel light and photographic camera. According to the contact diameter change during the evaporation, three stages have been determined (spreading, pinning, depinning). It has been found, that the dependence of evaporation rate on drop volume at low temperatures appear to be well fit by a power function

Heat Flux at the Surface of Metal Foil Heater under Evaporating Sessile Droplets

Evaporating water drops on a horizontal heated substrate were investigated experimentally. The heater was made of a constantan foil with the thickness of 25 μm and size of 42 × 35 mm2. The temperature of the bottom foil surface was measured by the infrared (IR) camera. To determine the heat flux density during evaporation of liquid near the contact line, the Cauchy problem for the heat equation was solved using the temperature data. The maximum heat flux density is obtained in the contact line region and exceeds the average heat flux density from the entire foil surface by the factor of 5–7. The average heat flux density in the region wetted by the drop exceeds the average heat flux density from the entire foil surface by the factor of 3–5. This fact is explained by the heat influx from the foil periphery to the drop due to the relatively high heat conductivity coefficient of the foil material and high evaporation rate in the contact line region. Heat flux density profiles for pairs of sessile droplets are also investigated

Evaporation Rate of a Liquid Layer Streamlined by Gas Flow in Minichannel

Paper presents the experimental results of mass evaporation rate from ethanol layer surface into a moving in parallel way air flow. Dependences of the mass evaporation rate on the gas velocity, temperature difference between gas and liquid layer, and liquid layer thickness were obtained. The assumption about the formation of the concentration boundary layer at the interface was made. It is found that with increasing the gas flow velocity the gradient of vapor concentration in the gas phase increases; as a result the evaporation rate rises. When changing the liquid layer thickness, the local maximum of evaporation rate was observed, but for 2 mm thickness layer two maxima were found. Probably, it is due the appearance of unstable structures on the liquid surface