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Author: Vijaykumar Sathyamurthi Publisher: ISBN: Category : Languages : en Pages :
Book Description
Subcooled pool boiling on nanotextured surfaces is explored in this study. The experiments are performed in an enclosed viewing chamber. Two silicon wafers are coated with Multiwalled Carbon Nanotubes (MWCNT), 9 microns (Type-A) and 25 microns (Type-B) in height. A third bare silicon wafer is used for control experiments. The test fluid is PF-5060, a fluoroinert with a boiling point of 56°C (Manufacturer: 3M Co.). The apparatus is of the constant heat flux type. Pool boiling experiments in nucleate and film boiling regimes are reported in this study. Experiments are carried out under low subcooling (5 °C and 10 °C) and high subcooling conditions (20°C to ~ 38°C). At approximately 38°C, a non-departing bubble configuration is obtained on a bare silicon wafer. Increase in subcooling is found to enhance the critical heat flux (CHF) and the CHF is found to shift towards higher wall superheats. Presence of MWCNT on the test surface led to an enhancement in heat flux. Potential factors responsible for boiling heat transfer enhancement on heater surfaces coated with MWCNT are identified as follows: a. Enhanced surface area or nano - fin effect b. Higher thermal conductivity of MWCNT than the substrate c. Disruption of vapor-liquid vapor interface in film boiling, and of the "microlayer" region in nucleate boiling d. Enhanced transient heat transfer caused by local quasi-periodic transient liquid-solid contacts due to presence of the "hair like" protrusion of the MWCNT e. Enhancement in the size of cold spots f. Pinning of contact line, leading to enhanced surface area underneath the bubble leading to enhanced heat transfer Presence of MWCNT is found to enhance the phase change heat transfer by approximately 400% in nucleate boiling for conditions of low subcooling. The heat transfer enhancement is found to be independent of the height of MWCNT in nucleate boiling regime in the low subcooling cases. About 75%-120% enhancement in heat transfer is observed for surfaces coated with MWCNT under conditions of high subcooling in the nucleate boiling regime. Surfaces coated with Type-B MWCNT show a 75% enhancement in heat transfer in the film boiling regime under conditions of low subcooling.
Author: Vijaykumar Sathyamurthi Publisher: ISBN: Category : Languages : en Pages :
Book Description
Subcooled pool boiling on nanotextured surfaces is explored in this study. The experiments are performed in an enclosed viewing chamber. Two silicon wafers are coated with Multiwalled Carbon Nanotubes (MWCNT), 9 microns (Type-A) and 25 microns (Type-B) in height. A third bare silicon wafer is used for control experiments. The test fluid is PF-5060, a fluoroinert with a boiling point of 56°C (Manufacturer: 3M Co.). The apparatus is of the constant heat flux type. Pool boiling experiments in nucleate and film boiling regimes are reported in this study. Experiments are carried out under low subcooling (5 °C and 10 °C) and high subcooling conditions (20°C to ~ 38°C). At approximately 38°C, a non-departing bubble configuration is obtained on a bare silicon wafer. Increase in subcooling is found to enhance the critical heat flux (CHF) and the CHF is found to shift towards higher wall superheats. Presence of MWCNT on the test surface led to an enhancement in heat flux. Potential factors responsible for boiling heat transfer enhancement on heater surfaces coated with MWCNT are identified as follows: a. Enhanced surface area or nano - fin effect b. Higher thermal conductivity of MWCNT than the substrate c. Disruption of vapor-liquid vapor interface in film boiling, and of the "microlayer" region in nucleate boiling d. Enhanced transient heat transfer caused by local quasi-periodic transient liquid-solid contacts due to presence of the "hair like" protrusion of the MWCNT e. Enhancement in the size of cold spots f. Pinning of contact line, leading to enhanced surface area underneath the bubble leading to enhanced heat transfer Presence of MWCNT is found to enhance the phase change heat transfer by approximately 400% in nucleate boiling for conditions of low subcooling. The heat transfer enhancement is found to be independent of the height of MWCNT in nucleate boiling regime in the low subcooling cases. About 75%-120% enhancement in heat transfer is observed for surfaces coated with MWCNT under conditions of high subcooling in the nucleate boiling regime. Surfaces coated with Type-B MWCNT show a 75% enhancement in heat transfer in the film boiling regime under conditions of low subcooling.
Author: Sharan Ram Sriraman Publisher: ISBN: Category : Languages : en Pages :
Book Description
The effect of nano-structured surfaces on pool boiling heat transfer is explored in this study. Experiments are conducted in a cubical test chamber containing fluoroinert coolant (PF5060, Manufacturer: 3M Co.) as the working fluid. Pool boiling experiments are conducted for saturation and subcooled conditions. Three different types of ordered nano-structured surfaces are fabricated using Step and flash imprint lithography on silicon substrates followed by Reactive Ion Etching (RIE) or Deep Reactive Ion Etching (DRIE). These nano-structures consist of a square array of cylindrical nanofins with a longitudinal pitch of 1 mm, transverse pitch of 0.9 mm and fixed (uniform) heights ranging from 15 nm - 650 nm for each substrate. The contact angle of de-ionized water on the substrates is measured before and after the boiling experiments. The contact-angle is observed to increase with the height of the nano-fins. Contact angle variation is also observed before and after the pool boiling experiments. The pool boiling curves for the nano-structured silicon surfaces are compared with that of atomically smooth single-crystal silicon (bare) surfaces. Data processing is performed to estimate the heat flux through the projected area (plan area) for the nano-patterned zone as well as the heat flux through the total nano-patterned area, which includes the surface area of the fins. Maximum heat flux (MHF) is enhanced by ~120 % for the nanofin surfaces compared to bare (smooth) surfaces, under saturation condition. The pool boiling heat flux data for the three nano-structured surfaces progressively overlap with each other in the vicinity of the MHF condition. Based on the experimental data several micro/nano-scale transport mechanisms responsible for heat flux enhancements are identified, which include: "microlayer" disruption or enhancement, enhancement of active nucleation site density, enlargement of cold spots and enhancement of contact angle which affects the vapor bubble departure frequency.
Author: Shikha Ebrahim Publisher: ISBN: Category : Languages : en Pages :
Book Description
In this research, minimum film boiling temperature (Tmin) is quantitatively determined as a function of the initial substrate temperature, liquid subcooling, surface thermophysical properties and surface conditions. Since Tmin defines the boundary between the film and transition boiling regimes, its value is significant for the design of an emergency core cooling system following a hypothetical loss-of-coolant accident (LOCA) in a nuclear power plant. When a sufficiently heated surface is plunged in a water pool, a vapor blanket is generated around the test section acting as a heat transfer insulator due to the poor thermal conductivity of the vapor. At temperatures lower than Tmin, the heat transfer is dramatically enhanced owing the collapse of the vapor film allowing direct physical contact between the water and the heated surface. Therefore, it is very important to explore methods and techniques that increase this temperature in order to improve the safety of nuclear reactors. A test facility was designed and constructed to conduct quenching experiments using vertical rods. Seven cylindrical test samples were fabricated with embedded thermocouples inside the cladding material. The thermocouples were connected to a data acquisition system in order to measure the temperature history during the experiments. The temperature and heat flux at the surface were calculated using an inverse heat conduction code along with an advance image processing technique to quantitatively characterize the liquid-vapor interfacial waves, vapor layer thickness, Tmin, quenching temperature, quenching time, and quench front velocity in the film boiling heat transfer regime. Visualization of the boiling behavior was captured by a high-speed camera at a frame rate of 750 frames per second (fps). The thermocouple data and the captured videos were synchronized to couple the behavior of the vapor layer with the thermal behavior of the heated sample. Various characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM) associated with Energy-dispersive X-ray spectroscopy (EDS), and field emission scanning electron were employed to identify the phases, chemical composition, and surface microstructure of the Inconel-600 before and after being used in a 7 x 7 rod bundle facility. Micro- and nanoparticles composed of nickel, chromium, and iron oxides were observed at the surface of the oxidized Inconel samples. It was found that the porous microstructure coupled with the increase in liquid spreading played a significant role in the enhancement of the film boiling heat transfer. Finally, the heat transfer behavior in the film boiling regime was investigated by calculating the heat transfer coefficient and Nusselt number for various cases. The novelty of this research is the coupling between the results of the quenching experiments and the surface characterization analyses that prompted the development of a new correlation for Tmin. This correlation adequately captures the effects of liquid subcooling, porosity of the oxide layer, and system pressure.
Author: Vijaykumar Sathyamurthi
Author: Vijaykumar Sathyamurthi
Author: Sharan Ram Sriraman
Author: Shikha Ebrahim![Ìtan Ìnú Ìwé Mimó. [With illustrations.]. PDF](https://books.google.com/books/content/images/frontcover/vsTVjwEACAAJ?fife=w80-h100)
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