Author: Kuiyan XuPublisher:
ISBN:
Category : Ebullition
Languages : en
Pages : 188
Book Description
Pool Boiling Heat Transfer from Porous-coated Surfaces in FC-72, the Effects of Subcooling and Non-boiling Immersion Time
Author: Kuiyan XuPublisher:
ISBN:
Category : Ebullition
Languages : en
Pages : 188
Book Description
Proceedings of the ASME Heat Transfer Division
Author: Proceedings of the ASME Heat Transfer Division--2005
Author: Enhancement of Pool Boiling Heat Transfer Using Thermally-conductive Microporous Coating Techniques
Author: Joo Han KimPublisher:
ISBN: 9780542979903
Category : Mechanical engineering
Languages : en
Pages :
Book Description
The present research is an experimental study of the enhancement of boiling heat transfer using microporous coating techniques. The current research is divided into four major phases. During the first phase, the effects of different metal particle sizes in the coating compound for thermally non-conductive microporous coating on pool boiling performance of refrigerants and water are investigated. The test surfaces were solid copper blocks with 1-cm2 base at atmospheric pressure in saturated FC-72, R-123, and water. Results showed that the surface treatment by non-conductive microporous coating significantly enhanced both nucleate boiling and critical heat flux of FC-72 and R-123. However, the enhancement of boiling performance for water was merely shown. In the second phase, thermally conductive microporous coatings to enhance boiling performance of water were developed. The first phase motivated efforts to fabricate microporous coatings with conducting binder options. The second phase was stemmed from an effort to combine the advantages of both a mixture batch type (inexpensive & easy process) and sintering/machining method (low thermal resistance of conduction). Two categories of surface treatment processes were considered in the current research. The first can be achieved by a chemical process, Multi-Staged Electroplating (MSE), which uses electricity in a chemical bath to deposit a microporous structure on the surface. The second is a soldering process, Multi-Temperature Soldering Process (MTSP), which binds the metal particles to generate optimum microporous cavities. Scanning Electron Microscope (SEM) and optical microscope images were obtained for thermally conductive microporous coated surfaces. During the third phase, the pool boiling performance of developed MSE and MTSP from second phase was confirmed for water. Results showed that the MSE and MTSP augmented the boiling performance not only for refrigerants but also for water significantly compared to non-conductive microporous coatings. Further investigation for possible future industrial applications of microporous coatings, such as indirect cooling for electronic chips, nanofluids for high power generation industries, and freezing problem of water, were conducted in the final phase.
Pool Boiling Studies on Nanotextured Surfaces Under Highly Subcooled Conditions
Author: Vijaykumar SathyamurthiPublisher:
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.
Pool Boiling Heat Transfer with Highly-wetting Dielectric Fluids
Author: Seung Mun YouThe Effect of Chemical Nature of Heating Surfaces on the Heat Transfer Coefficients of Pool Boiling Liquids
Author: George William PreckshotPublisher:
ISBN:
Category : Heat
Languages : en
Pages : 62
Book Description
Pool and External Flow Boiling
Author: V. K. DhirPublisher:
ISBN:
Category : Ebullition
Languages : en
Pages : 504
Book Description
AN EXPERIMENTAL INVESTIGATION OF THE EFFECTS OF SURFACE CONDITIONS ON POOL-BOILING HEAT TRANSFER FOR VARIOUS MATERIALS.
Author: Shikha EbrahimPublisher:
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.
High-pressure Pool-boiling Heat Transfer Enhancement and Mechanism on Engineered Surfaces
Author: Smreeti DahariyaPublisher:
ISBN:
Category :
Languages : en
Pages :
Book Description
Boiling has received considerable attention in the technology advancement of electronics cooling for high-performance computing applications. Two-phase cooling has an advantage over a single-phase cooling in the high heat removal rate with a small thermal gradient due to the latent heat of vaporization. Many surface modifications have been done in the past including surface roughness, mixed wettability and, porous wick copper play a crucial role in the liquid-vapor phase change heat transfer. However, the mechanisms of high-pressure pool-boiling heat transfer enhancement due to surface modifications has not been well studied or understood. The properties of water, such as the latent heat of vaporization, surface tension, the difference in specific volume of liquid and vapor, decrease at high-pressure. High-pressure pool-boiling heat transfer enhancement is studied fundamentally on various engineered surfaces. The boiling tests are performed at a maximum pressure of 90 psig (620.5 kPa) and then compared to results at 0 psig (0 kPa). The results indicate that the pressure influences the boiling performance through changes in bubble dynamics. The bubble departure diameter, bubble departure frequency, and the active nucleation sites change with pressure. The pool-boiling heat transfer enhancement of a Teflon© coated surface is also experimentally tested, using water as the working fluid. The boiling results are compared with a plain surface at two different pressures, 30 and 45 psig. The maximum heat transfer enhancement is found at the low heat fluxes. At high heat fluxes, a negligible effect is observed in HTC. The primary reasons for the HTC enhancement at low heat fluxes are active nucleation sites at low wall superheat and bubble departure size. The Teflon© coated surface promotes nucleation because of the lower surface energy requirement. The boiling results are also obtained for wick surfaces. The wick surfaces are fabricated using a sintering process. The boiling results are compared with a plain surface. The reasons for enhancements in the pool-boiling performance are primarily due to increased bubble generation, higher bubble release frequency, reduced thermal-hydraulic length modulation, and enhanced thermal conductivity due to the sintered wick layer. The analysis suggests that the Rayleigh-critical wavelength decreases by 4.67 % of varying pressure, which may cause the bubble pinning between the gaps of sintered particles and avoids the bubble coalescence. Changes in the pitch distance indicate that a liquid-vapor phase separation happens at the solid/liquid interface, which impacts the heat-transfer performance significantly. Similarly, the role of the high-pressure over the wicking layer is further analyzed and studied. It is found that the critical flow length, [lambda]u reduces by three times with 200 [mu]m particles. The results suggest that the porous wick layer provides a capillary-assist to liquid flow effect, and delays the surface dry out. The surface modification and the pressure amplify the boiling heat transfer performance. All these reasons may contribute to the CHF, and HTC enhancement in the wicking layer at high-pressure.