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research-article  
Pradeep GV and K Rama Narasimha
J. Heat Transfer   doi: 10.1115/1.4041953
This paper describes the experimental investigations conducted on a closed loop pulsating heat pipe (CLPHP) for assessing the thermal performance. The pulsating heat pipe has a single closed loop made of copper. The working fluids used are water and titanium-di-oxide nano fluids with varying concentrations of TiO2 nano particles (1.5% and 1%) on weight basis. The TiO2 particles are mixed in water to form a stable suspension using a sonicator. The heat input is varied between 40W-100W in steps of 20W. All experiments are conducted in the bottom heating mode (evaporator at the top) in vertical and horizontal orientations. The parameters considered for evaluating the thermal performance are the temperature difference between evaporator and condenser, thermal resistance, heat transfer coefficient and thermal conductivity. The results of the investigation reveals that, the vertical orientation and increase in nano particle concentration favors better heat transfer performance of the PHP
TOPICS: Heat pipes, Nanofluids, Water, Nanoparticles, Thermal conductivity, Condensers (steam plant), Weight (Mass), Heat, Temperature, Heat transfer, Fluids, Copper, Particulate matter, Heating, Heat transfer coefficients, Thermal resistance, Titanium
research-article  
Oguzhan Der, Dr. Marco Marengo and Volfango Bertola
J. Heat Transfer   doi: 10.1115/1.4041952
A low-cost, flexible pulsating heat pipe (PHP) was built in a composite polypropylene sheet consisting of three layers joint together by selective laser welding, to address the demand of heat transfer devices characterized by low weight, small unit thickness, low cost, and high mechanical flexibility. A thin, flexible and lightweight heat pipe is advantageous for various aerospace, aircraft and portable electronic applications where the device weight and its mechanical flexibility are essential. The concept is to sandwich a serpentine channel, cut out in a polypropylene sheet and containing a self- propelled mixture of a working fluid with its vapour, between two transparent sheets of the same material; this results into a thin, flat enclosure with parallel channels hence the name "pulsating heat stripes" (PHS). The transient and steady- state thermal response of the device was characterised for different heat input levels and different configurations, either straight or bent at different angles. The equivalent thermal resistance was estimated by measuring the wall temperatures at both the evaporator and the condenser, showing a multi- fold increase of the equivalent thermal conductance with respect to solid polypropylene.
TOPICS: Heat, Plastics, Weight (Mass), Heat pipes, Aircraft, Condensers (steam plant), Thermal resistance, Transparency, Wall temperature, Heat transfer, Fluids, Composite materials, Transients (Dynamics), Laser welding, Thermal conductivity, Aerospace industry
research-article  
Sam Darr, Dr. Jason Hartwig, Jun Dong, Hao Wang, Alok Majumdar, Andre LeClair and Prof. Jacob Chung
J. Heat Transfer   doi: 10.1115/1.4041830
Recently, two-phase cryogenic flow boiling data in liquid nitrogen (LN2) and liquid hydrogen (LH¬2) were compared to the most popular two-phase correlations, as well as correlations used in two of the most widely used commercially available thermal/fluid design codes in Hartwig et al. (2016a). Results uncovered that the correlations performed poorly, with predictions significantly higher than the data. Disparity is primarily due to the fact that most two-phase correlations are based on room temperature fluids, and for the heating configuration, not the quenching configuration. The penalty for such poor predictive tools is higher margin, safety factor, and cost. Before control algorithms for cryogenic transfer systems can be implemented, it is first required to develop a set of low-error, fundamental two-phase heat transfer correlations that match available cryogenic data. This paper presents the background for developing a new set of quenching/chilldown correlations for cryogenic pipe flow on thin, shorter lines, including the results of an exhaustive literature review of 61 sources. New correlations are presented which are based on the consolidated database of 79,915 quenching points for a 1.27 cm diameter line, covering a wide range of inlet subcooling, mass flux, pressure, equilibrium quality, flow direction, and even gravity level. Functional forms are presented for LN2 and LH2 chilldown correlations, including film, transition, and nucleate boiling, critical heat flux, and the Leidenfrost point.
TOPICS: Quenching (Metalworking), Hydrogen, Nitrogen, Pipes, Flow (Dynamics), Temperature, Heat transfer, Fluids, Safety, Equilibrium (Physics), Boiling, Design, Pipe flow, Subcooling, Thermofluids, Nucleate boiling, Heating, Control algorithms, Critical heat flux, Databases, Errors, Pressure, Gravity (Force)
research-article  
Fabio Villa, Marco Marengo and Joël De Coninck
J. Heat Transfer   doi: 10.1115/1.4041708
Heat pipe characteristics are linked to the surface properties of the diabatic surfaces, and, in the evaporator, surface properties influence both the onset boiling temperature (TONB) and the critical heat flux (CHF). In this work the effect of surface wettability in pool boiling heat transfer is studied in order to understand if there could be a path to increment heat pipe thermal performance. This work analyses the effects of surface wettability on boiling (tested fluid is pure water) and proposes a new super-hydrophobic polymeric coating [1], which can have a very important effect in improving the heat pipe start-up power load and increasing the thermal performance of heat pipes when the flux is lower than the critical heat flux. The polymeric coating is able to reduce the TONB (-11% from 117°C to about 104°C) compared with the uncoated surfaces, as it inhibits the formation of a vapour film on the solid-liquid interface, avoiding CHT conditions up to maximum wall temperature (125°C). This is realized by the creation of a heterogeneous surface with SHS zones dispersed on top of a hydrophilic surface (stainless steel surface). The proposed coating has an outstanding thermal resistance: No degradation of SH properties of the coating has been observed after more than 500 thermal cycles.
TOPICS: Heat pipes, Coating processes, Coatings, Critical heat flux, Boiling, Surface properties, Cycles, Pool boiling, Stainless steel, Thermal resistance, Wall temperature, Water, Stress, Temperature, Heat transfer, Fluids

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