Leidenfrost Evaporation of Liquid Nitrogen Droplets

1994 ◽  
Vol 116 (4) ◽  
pp. 999-1006 ◽  
Author(s):  
S. Chandra ◽  
S. D. Aziz
Keyword(s):  
Surface Temperature ◽  
Liquid Nitrogen ◽  
Glass Surface ◽  
Evaporation Rate ◽  
Ambient Pressure ◽  
Droplet Diameter ◽  
Droplet Evaporation ◽  
Copper Surface ◽  
Droplet Impact ◽  
Bubble Nucleation

The evaporation of a single droplet of liquid nitrogen, levitated during film boiling above a solid, impervious surface, was studied experimentally. The droplet initial diameter (1.9 mm), surface temperature (~20°C), ambient temperature (~20°C), and ambient pressure (~0.1 MPa) were held constant. The principal parameters varied were the surface material (copper or glass), and roughness (0.35 to 50 μm). Measurements were made of the droplet diameter evolution and the surface temperature variation during droplet impact. Predictions from existing models of droplets in Leidenfrost evaporation agree well with measurements of the droplet evaporation rate. The droplet lifetime was found to be slightly longer on the glass surface than it was on the copper surface, corresponding to the greater cooling of the glass surface during droplet impact. The droplet evaporation rate was unchanged by small increases in surface roughness. However, ridges on the surface with a height of the same magnitude as the thickness of the vapor film under the drop caused vapor bubble nucleation in the droplet, and significantly reduced the droplet evaporation time.

Evaporation of Silver-Graphene Hybrid Nanofluid Droplet on its Nanostructured Residue and Plain Copper Surfaces at Elevated Temperatures

2020 ◽  
Author(s):  
Farooq Riaz Siddiqui ◽  
Chi Yan Tso ◽  
Sau Chung Fu ◽  
Huihe Qiu ◽  
Christopher Yu Hang Chao
Keyword(s):  
Substrate Temperature ◽  
Evaporation Rate ◽  
Elevated Temperatures ◽  
Droplet Evaporation ◽  
Copper Surface ◽  
Copper Plate ◽  
Hybrid Nanofluid ◽  
Copper Surfaces ◽  
Latent Energy ◽  
Droplet Sizes

Abstract Droplet evaporation is an efficient process as it removes a large amount of heat by using the latent energy, making it suitable for heat transfer applications. In this research, evaporation of the silver-graphene hybrid nanofluid (SGHF) droplet, because of its synergistic thermal conductivity, is investigated for substrate temperature in a range of 25–100 °C. The experiments for droplet evaporation were performed in an environmental facility for two droplet sizes, 3 μL and 30 μL volume, on a copper plate. A 100 W silicone heater mat was used to heat the copper plate from the underside, while two T-type thermocouples were used to monitor its surface temperature. As droplet evaporation ended, a porous residue was formed on the copper surface. Subsequently, a 3 μL volume of the SGHF droplet was dispensed on the porous residue surface. The results showed a tremendous rise in the evaporation rate (up to 160%) for the subsequent SGHF droplet sitting on the porous residue as compared to the non-wetted copper surface. Moreover, the evaporation rate of the SGHF droplet on the copper surface increased up to 56% as compared to the water droplet for a substrate temperature range of 25–100 °C.

Transient Cooling of Hot Porous and Nonporous Ceramic Solids by Droplet Evaporation

1994 ◽  
Vol 116 (3) ◽  
pp. 694-701 ◽  
Author(s):  
M. Abu-Zaid ◽  
A. Atreya
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
Solid Surface ◽  
Boiling Point ◽  
Droplet Diameter ◽  
Droplet Evaporation ◽  
Porous Solid ◽  
Temperature Measurements ◽  
Transient Temperature ◽  
Initial Surface

This paper presents the results of an experimental investigation into transient cooling of low-thermal-conductivity porous and nonporous ceramic solids by individual water droplets. The initial surface temperature (Ts) of both solids ranged from 75 to 200°C. Both solids were instrumented with several surface and in-depth thermocouples and had the same thermal properties. This enabled investigation into the similarities and differences in the thermal behavior of porous and nonporous solids during droplet evaporation. The measured and theoretical contact temperatures, for both solids, were found to be in good agreement until they became equal to the boiling point of water (which occurs at an initial solid surface temperature of 164°C). Further increase in the initial solid surface temperature did not change the measured contact temperature. Instead, it became roughly constant at a value slightly greater than the boiling point of water. During the droplet evaporation process, surface and in-depth temperatures for the nonporous solid remain nearly constant, whereas for the porous solid there was a continuous decrease in these temperatures. A thermocouple in the porous matrix at the same location as that of the nonporous matrix cools faster under identical conditions, indicating an energy sink in the vicinity of the thermocouple. Also, evaporation time for the nonporous solid was found to be larger than that of the porous solid for the same droplet size and under the same conditions. These observations confirm that there is both in-depth and lateral penetration of water in the porous solid. The transient temperature measurements were used to determine the following quantities: (i) the recovery time (time required by the surface to recover to its initial temperature), and (ii) the size of surface and in-depth zones affected by the droplet. The instantaneous evaporation rate, and the instantaneous average evaporative heat flux for the nonporous solid, were also determined from video measurements of the droplet diameter on the solid surface and the transient temperature measurements. It was found that the average evaporative heat flux is higher for smaller droplets because of their smaller thickness on the hot surface.

Superheat Layer Thickness Measurements in Saturated and Subcooled Nucleate Boiling

1971 ◽  
Vol 93 (4) ◽  
pp. 455-461 ◽  
Author(s):  
J. R. Wiebe ◽  
R. L. Judd
Keyword(s):  
Mathematical Model ◽  
Experimental Study ◽  
Heat Flux ◽  
Layer Thickness ◽  
Glass Surface ◽  
Nucleate Boiling ◽  
Copper Surface ◽  
Bubble Nucleation ◽  
Temperature Profiles ◽  
Thickness Measurements

An experimental study of temperature profiles in water boiling on a horizontal copper surface is reported for incipient boiling conditions and for 20,000, 50,000, and 100,000 Btu/hr-ft2 heat flux at various levels of subcooling ranging from 0 to 105 deg F. The extrapolated superheat layer thickness results for the incipient boiling conditions lend support to Hsu’s mathematical model for bubble nucleation. Increasing heat flux and decreasing subcooling were observed to result in a decreasing extrapolated superheat layer thickness. Analysis of some additional results for Freon-113 boiling on a glass surface indicated that the thickness of the extrapolated superheat layer was governed by the bubble flux density which was influenced by both heat flux and subcooling.

Spray Cooling Enhancement by Addition of a Surfactant

1998 ◽  
Vol 120 (1) ◽  
pp. 92-98 ◽  
Author(s):  
Y. M. Qiao ◽  
S. Chandra
Keyword(s):  
Heat Transfer ◽  
Surface Temperature ◽  
Boiling Heat Transfer ◽  
Pure Water ◽  
Sodium Dodecyl ◽  
Nucleate Boiling ◽  
Copper Surface ◽  
Bubble Nucleation ◽  
Spray Droplets ◽  
Cooling Enhancement

An experimental study was done on the effect of dissolving a surfactant in water sprays used to cool a hot surface. A copper surface was heated to an initial temperature of 240°C and then rapidly cooled using a spray of either pure water or an aqueous solution containing 100 ppm by weight of sodium dodecyl sulfate. The variation of surface temperature was measured during cooling, and spray impact was photographed. Addition of the surfactant was found to enhance nucleate boiling heat flux by up to 300 percent. The surface temperature required to initiate vapor bubble nucleation was reduced from 118°C to 103°C. These effects were attributed to the surfactant promoting bubble nucleation and foaming in spray droplets. Nucleate boiling heat transfer enhancement was observed at all liquid mass fluxes and droplet velocities in the range of our experiments. The surfactant slightly reduced transition boiling heat transfer, and also reduced the temperature at which spray droplets started to wet the surface. Changing the orientation of the surface with respect to gravity had no effect on heat transfer.

Droplet Evaporation of Cu–Al2O3 Hybrid Nanofluid Over Its Residue and Copper Surfaces: Toward Developing a New Analytical Model

2020 ◽  
Vol 143 (2) ◽  
Author(s):  
Farooq Riaz Siddiqui ◽  
Chi Yan Tso ◽  
Sau Chung Fu ◽  
Huihe Qiu ◽  
Christopher Y. H. Chao
Keyword(s):  
Analytical Model ◽  
Evaporation Rate ◽  
Heat Dissipation ◽  
High Heat ◽  
Droplet Evaporation ◽  
Copper Surface ◽  
Hybrid Nanofluid ◽  
High Heat Flux ◽  
High Heat Transfer ◽  
Critical Residue

Abstract Droplet evaporation-based cooling techniques, such as the spray cooling, give high heat transfer rates by utilizing latent energy and are usually preferred in thermal applications. However, with the significant rise in heat dissipation levels for high heat flux devices, these devices cannot be thermally managed due to the limited cooling capacity of existing thermal fluids. In this paper, we report the evaporation of the Cu–Al2O3 hybrid nanofluid (HNF) droplet on a copper surface as well as its own residue surface, developed from the evaporation of the first Cu–Al2O3 HNF droplet. As the main novelty, we identify the critical residue size and investigate the residue size effect, above and below the critical residue size, on evaporation rate of the succeeding Cu–Al2O3 HNF droplet resting over a residue surface. We also develop a new analytical model to estimate the Cu–Al2O3 HNF droplet evaporation rate and compare our results with other existing models. The results show that the Cu–Al2O3 HNF droplet gives 17% higher evaporation rate than a water droplet on a copper surface. Also, the evaporation rate of the Cu–Al2O3 HNF droplet on a residue surface sharply increases by 106% with increasing residue size up to the critical residue size. However, further increasing the residue size above its critical value has a negligible effect on the droplet evaporation rate. Moreover, the evaporation rate of the Cu–Al2O3 HNF droplet on its residue surface is enhanced up to 104% when compared to a copper surface.

Experimental Investigation on Silver-Graphene Hybrid Nanofluid Droplet Evaporation and Wetting Characteristics of its Nanostructured Droplet Residue

2019 ◽  
Author(s):  
Farooq R. Siddiqui ◽  
Edwin C. Y. Tso ◽  
Sau C. Fu ◽  
Christopher Y. H. Chao ◽  
Huihe Qiu
Keyword(s):  
Evaporation Rate ◽  
Volume Fraction ◽  
Spray Cooling ◽  
Droplet Evaporation ◽  
Copper Surface ◽  
Nanoporous Structure ◽  
Hybrid Nanofluid ◽  
Mixing Ratios ◽  
Static Contact ◽  
Wide Range

Abstract Droplet evaporation is a complex phase change process with a wide range of cooling applications, such as spray cooling and dropwise hotspot cooling in microelectronics, to name a few. The hybrid nanofluid droplet evaporation and its residue effects on evaporation of the subsequent hybrid nanofluid droplet is investigated in this research. Silver-graphene (Ag-GNP) hybrid nanofluid exhibiting synergistic thermal properties is investigated and prepared by dispersing silver nanoparticles along with graphene nanoplatelets in water at 0.1% volume fraction and with different mixing ratios, followed by ultrasonication. The evaporation rate and wetting characteristics of a 3 μl volume of Ag-GNP hybrid nanofluid droplet on a copper surface were studied using an optical tensiometer. Once dried, the nanoporous structure of the residue surface was examined using a scanning electron microscope, while the surface roughness was measured using an optical profiler. Experiments were continued to further investigate the evaporation rate and wetting effects of the subsequent Ag-GNP hybrid nanofluid droplet over the residue surface. The results showed improved wetting characteristics, with 88% reduction in initial static contact angle and 163–196% enhancement in evaporation rate of the subsequent Ag-GNP hybrid nanofluid droplets over the residue surfaces as compared to the copper surface.

WATER DROPLET EVAPORATION IN A CHAMBER ISOLATED FROM THE EXTERNAL ENVIRONMENT

2020 ◽  
Vol 6 (3) ◽  
pp. 8-22
Author(s):  
KSENIA A. Batishcheva ◽  
ATLANT E. Nurpeiis
Keyword(s):  
Water Vapor ◽  
Evaporation Rate ◽  
External Environment ◽  
Heat Removal ◽  
Droplet Evaporation ◽  
Droplet Volume ◽  
Evaporation Time ◽  
Aluminum Alloy Amg6 ◽  
Evaporation Of Droplets ◽  
Contact Diameter

With an increase in the productivity of power equipment and the miniaturization of its components, the use of traditional thermal management systems becomes insufficient. There is a need to develop drip heat removal systems, based on phase transition effects. Cooling with small volumes of liquids is a promising technology for microfluidic devices or evaporation chambers, which are self-regulating systems isolated from the external environment. However, the heat removal during evaporation of droplets into a limited volume is a difficult task due to the temperature difference in the cooling device and the concentration of water vapor that is unsteady in time depending on the mass of the evaporated liquid. This paper presents the results of an experimental study of the distilled water microdrops’ (5-25 μl) evaporation on an aluminum alloy AMg6 with the temperatures of 298-353 K in an isolated chamber (70 × 70 × 30 mm3) in the presence of heat supply to its lower part. Based on the analysis of shadow images, the changes in the geometric dimensions of evaporating drops were established. They included the increase in the contact diameter, engagement of the contact line due to nano roughening and chemical composition inhomogeneous on the surface (90-95% of the total evaporation time) of the alloy and a decrease in the contact diameter. The surface temperature and droplet volume did not affect the sequence of changes in the geometric dimensions of the droplets. It was found that the droplet volume has a significant effect on the evaporation time at relatively low substrate temperatures. The results of the analysis of droplet evaporation rates and hygrometer readings have shown that reservoirs with salt solutions can be used in isolated chambers to control the concentration of water vapor. The water droplets evaporation time was determined. The analysis of the time dependences of the evaporation rate has revealed that upon the evaporation of droplets in an isolated chamber under the conditions of the present experiment, the air was not saturated with water vapor. The latter did not affect the evaporation rate.

scholarly journals Propellant Combustion Wave Studies by Embedded Thermocouple and Imaging Method at Ambient Pressure

2020 ◽  
Author(s):  
Rakesh Kumar Kalal ◽  
Himanshu Shekhar ◽  
Prashant Sudhir Alegaonkar ◽  
Shrikant Pande
Keyword(s):  
Surface Temperature ◽  
High Speed ◽  
Ambient Pressure ◽  
Combustion Mechanism ◽  
Imaging Method ◽  
Flame Zone ◽  
Sem Images ◽  
Propellant Combustion ◽  
Double Base ◽  
Zone Temperature

This paper discusses the method for propellant combustion studies with embedded thermocouple and imaging method at ambient pressure. In this study, three different propellant compositions are experimentally evaluated for surface temperature, flame zone temperature with embedded thermocouple, and reaction zone thickness with high-speed imaging of propellant during combustion at ambient pressure. Preheat zone and flame zone temperature profiles are recorded with time and surface temperature is determined with available models. Observation of these experiments gives the difference between combustion mechanism of double-base propellant with diethylene glycol dinitrate (DEGDN) and 2,4-dinitrotoluene (DNT), composite propellant (CP) and CP with energetic binder. Scanning electron microscope (SEM) images analysis for pristine and quenched sample is also presented.

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