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.

Transient Temperature Profile of a Hot Wall Due to an Impinging Liquid Droplet

1978 ◽  
Vol 100 (1) ◽  
pp. 167-169 ◽  
Author(s):  
M. Seki ◽  
H. Kawamura ◽  
K. Sanokawa
Keyword(s):  
Surface Temperature ◽  
Turning Point ◽  
Liquid Drop ◽  
Water Drop ◽  
Liquid Droplet ◽  
Contact Temperature ◽  
Transient Temperature ◽  
Initial Surface ◽  
Heater Surface ◽  
Hot Surface

An experiment was made to investigate the heat transfer to a liquid drop impinging on a hot surface. The transient temperature of the heater surface was measured by a thin-film thermometer. The surface temperature fell to a contact temperature immediately after contact with the drop. The contact temperature increased with increasing initial surface temperature T0. In the case of the water drop, however, it was approximately constant for 200°C ≲ T0 ≲ 300°C; and it increased again for T0 ≳ 300°C. The surface temperature at the turning point, i.e., T0 ∼ 300°C, roughly coincided with the Leidenfrost point.

Effect of Subcooling and Nozzle Diameter on Heat Transfer Characteristics of Downward Facing Hot Surfaces Using Mist Jet

2018 ◽  
Author(s):  
Avadhesh Kumar Sharma ◽  
Monika Meena ◽  
Anirudh Soni ◽  
Santosh K. Sahu
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
Stagnation Point ◽  
Surface Heat Flux ◽  
Jet Impingement ◽  
Surface Heat ◽  
Nozzle Diameter ◽  
Initial Surface ◽  
Ir Camera ◽  
Impingement Cooling

The jet impingement cooling is always preferred over the other cooling methods due to its high heat removal capability. However, rapid quenching may lead to the formation of cracks and poor ductility to the quenched surface. Mist jet impingement cooling offers an alternative method to uncontrolled rapid cooling, particularly in steel and electronic industries. In mist cooling, the droplets are atomized by compressed air. Experiments are performed under transient conditions using two full-cone spray nozzles (Lechler Pneumatic atomizing nozzle 136.115.xx.A2 and 136.134.xx.A2) to study the effect of subcooling and nozzle diameter on surface heat flux. The hot surface used for the experiment is a stainless steel foil (AISI-304) of thickness 0.15mm. The initial surface temperature of the plate is maintained at 500±10°C with the help of an AC transformer. Infrared thermal image camera (A655sc, FLIR System) is used for data estimation. The IR camera and the nozzle are positioned on either side of the plate. The variation in surface temperature has been acquired at 8 different spatial points. It has been observed that that as we move away from the stagnation point then irrespective of air and water flow rates surface heat flux decreases. The maximum surface heat flux obtained at the stagnation point. With the increase in diameter surface heat flux increases irrespective of pressure values. The correlation between qm/qstag experimental and predicted values has been shown.

scholarly journals Effect of Surface Oxidation on the Onset of Nucleate Boiling in a Materials Test Reactor Coolant Channel

2016 ◽  
Vol 2 (2) ◽  
Author(s):  
Eric C. Forrest ◽  
Sarah M. Don ◽  
Lin-Wen Hu ◽  
Jacopo Buongiorno ◽  
Thomas J. McKrell
Keyword(s):  
Heat Flux ◽  
Surface Temperature ◽  
High Speed ◽  
Surface Oxidation ◽  
Nucleate Boiling ◽  
Temperature Measurements ◽  
Native Oxide ◽  
Oxide Surface ◽  
Surface Conditions ◽  
Test Reactor

The onset of nucleate boiling (ONB) serves as the thermal-hydraulic operating limit for many research and test reactors. However, boiling incipience under forced convection has not been well-characterized in narrow channel geometries or for oxidized surface conditions. This study presents experimental data for the ONB in vertical upflow of deionized (DI) water in a simulated materials test reactor (MTR) coolant channel. The channel gap thickness and aspect ratio were 1.96 mm and 29∶1, respectively. Boiling surface conditions were carefully controlled and characterized, with both heavily oxidized and native oxide surfaces tested. Measurements were performed for mass fluxes ranging from 750 to 3000  kg/m2 s and for subcoolings ranging from 10 to 45°C. ONB was identified using a combination of high-speed visual observation, surface temperature measurements, and channel pressure drop measurements. Surface temperature measurements were found to be most reliable in identifying the ONB. For the nominal (native oxide) surface, results indicate that the correlation of Bergles and Rohsenow, when paired with the appropriate single-phase heat transfer correlation, adequately predicts the ONB heat flux. Incipience on the oxidized surface occurred at a higher heat flux and superheat than on the plain surface.

HEAT TRANSFER WITH SURFACE BOILING

1948 ◽  
Vol 26a (4) ◽  
pp. 268-278 ◽  
Author(s):  
J. W. Knowles
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Surface Temperature ◽  
Boiling Point ◽  
Volume Flow Rate ◽  
Heat Fluxes ◽  
Bulk Liquid ◽  
Bulk Temperature ◽  
Liquid Temperature ◽  
Transfer Constant

These experiments were undertaken to investigate the transfer of heat from solid surfaces to flowing water. The bulk temperature of the cooling water was below the boiling point, and the surface temperature of the heat transfer tube went to about 100 °C. above the boiling point. Water velocities ranged up to 10 ft. sec.−1 and heat fluxes up to 720 w. cm.−2. The non-dimensionless heat transfer constant a in the Dittus and Boelter formula (Nu) = a(Re)0.8 (Pr)0.4, which under normal forced convection has a value of 2.30 × 10−2, increased to four times the normal value under the surface boiling conditions. It is shown graphically that the constant a bears a simple relation to the heat flux and the length–diameter ratio for surface temperatures below a certain value. For constant heat flux and volume flow rate, the surface temperature rises with rising bulk liquid temperature until it reaches the certain value of the surface temperature referred to above. The surface temperature then remains constant with further rise in the bulk liquid temperature until conditions become too unstable for measurements.

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