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Effect of Localized Heating on Three-Dimensional Flat-Plate Oscillating Heat Pipe  [PDF]
S. M. Thompson,H. B. Ma
Advances in Mechanical Engineering , 2010, DOI: 10.1155/2010/465153
Abstract: An experimental investigation was conducted, both thermally and visually, on a three-dimensional flat-plate oscillating heat pipe (3D FP-OHP) to characterize its performance under localized heat fluxes while operating in the bottom heating mode and charged with acetone at a filling ratio of 0.73. The cooling area was held constant and three heating areas of 20.16? , 11.29? , and 1.00? were investigated, respectively. It was found that as the heating area was reduced and higher heat fluxes were imposed, the thermal resistance increased and the amplitude of thermal oscillations in the evaporator increased and became more chaotic. Using neutron radiography, it was observed that fluid oscillations did not occur in outer channels located away from the region of local heating. Although the thermal resistance increased during localized heating, a maximum heat flux of 180?W/ was achieved with the average evaporator temperature not exceeding . 1. Introduction Due to the ongoing miniaturization of electrical components and the general advancement of technology, the thermal management of high heat fluxes is becoming a pivotal requirement for product development and application. Although conventional, wicked heat pipes have provided adequate thermal solutions for many applications, their functionality at high heat fluxes is hindered by numerous limitations (i.e., form factor, operational limits, etc.). The oscillating heat pipe (OHP), as introduced by Akachi [1], provides a promising alternate solution for high heat flux thermal management. The OHP, a two-phase heat transfer device, typically has no wicking structure and exists either as a closed-loop, serpentine-arranged tube (T-OHP) or an engraved, serpentine-arranged channel on a flat-plate of metal that is sealed—also referred to as the flat-plate oscillating heat pipe (FP-OHP). The inner hydraulic diameter of the pipe (or channel) is made sufficiently small to allow a partially filled working fluid to disperse itself into liquid slugs and vapor plugs via capillary action during operation. The OHP operates when both ends of the tubing/plate thermally interact with a heat source and a heat sink. The heat exchange occurring at both ends continually disturbs the phase of the internal saturated working fluid, resulting in vapor condensation or liquid evaporation. The continual phase change results in a pressure imbalance along the channels of the OHP, which allows for the quasichaotic displacements of the internal working fluid slugs/plugs. A typical OHP allows for the working fluid to flow within a single plane.
Performance of a 2-Element Plane Reflector Augmented Galvanised Pipe Flat Plate Collector for Solar Water Pasteurisation
E.A. Onyango,T.F.N. Thoruwa,S.M. Maingi,E.M. Njagi
Journal of Food Technology , 2013,
Abstract: About 80% of all infectious diseases including diarrhoea, typhoid, cholera in developing countries are transmitted through consumption of contaminated water. This study reports on the concept of heating water using low cost solar thermal energy systems to kill disease-causing microorganisms and gives the design philosophy, construction and measured thermal performances of a 2-element plane reflector augmented flat plate solar collector. A solar water pasteurization system was designed and constructed using locally available materials for direct water heating using solar energy in a flow-through system made of copper pipes. Standard car radiator thermostat valve was used to regulate pasteurization temperature at 82°C. The results demonstrated that solar heating of contaminated water could be achieved through a 2 m2 flat plate collector with mounted with a 2-element plane mirror reflector with an aperture area of 4 m2. The experimental collector was constructed with 20 mm diameter galvanized pipe fixed onto mild steel absorber plate. The collector without reflectors was used as control experiment. Under fair weather conditions, it was observed that the use of mirrors tremendously improved the performance of the collector by up to 94% producing 49-95 L of pasteurized water using the galvanized pipe collector. The microbiology water quality tests using presumptive test for total coliform and confirmed by Eijkman test showed no presence of E. coli in the solar pasteurized water samples. On the basis of the results, it was concluded that the solar water pasteurization system developed provides a cheaper alternative for water disinfection in Kenya and other sunny countries.
Design and Fabrication of a Novel Hybrid-Structure Heat Pipe for a Concentrator Photovoltaic  [PDF]
Hsin-Jung Huang,Sheng-Chih Shen,Heiu-Jou Shaw
Energies , 2012, DOI: 10.3390/en5114340
Abstract: This study presents a design method to fabricate a novel hybrid-structure flat plate heat pipe (NHSP heat pipe) for a concentrator photovoltaic. The NHSP heat pipe is composed of a flattened copper pipe and a sintered wick structure, and a coronary-stent-like rhombic copper mesh supports the structure. The coronary-stent-like supporting structure enhances the mechanical strength and shortens the reflux path of the working fluid. Experiments demonstrate that the sintered capillary heat pipe reduces the thermal resistance by approximately 72%, compared to a traditional copper mesh-screen heat pipe. Furthermore, it can reduce thermal resistance by 65% after a supporting structure is added to the heat pipe. The results show that the NHSP heat pipe provided the best performance for the concentrator photovoltaic, which can increase photoelectric conversion efficiency by approximately 3.1%, compared to an aluminum substrate.
Development and Investigation of a Miniature Copper-Acetone Loop Heat Pipe with a Flat Evaporator  [PDF]
Yury F. Maydanik, Vladimir G. Pastukhov, Mariya A. Chernysheva
Journal of Electronics Cooling and Thermal Control (JECTC) , 2015, DOI: 10.4236/jectc.2015.54006
Abstract: The paper presents the results of development and investigation of a copper miniature loop heat pipe (LHP) with acetone as a working fluid. The device was equipped with a flat evaporator measuring 80 × 42 × 7 mm and vapor and liquid lines with an outside diameter of 3 mm, whose lengths were 145 mm and 175 mm, respectively. The LHP was tested at heat loads from 5 W to 60 W, different orientations in the gravity field and heat-sink temperatures from -40°C to +50°C. It is shown that the LHP retains its efficiency at all testing conditions. It is also mentioned that at a heat-sink temperature of +50°C the device operates in the mode of constant conductivity in the whole range of heat loads, and in this case a minimum thermal resistance of the “heat source-heat sink” system equal to 0.16°C/W is achieved, which is independent of the LHP orientation in the gravity field.
Evaluation of heat loss coefficients in solar flat plate collectors  [PDF]
Y. Rajasekhar,K. V. Sharma,M. Basaveswara Rao
Journal of Engineering and Applied Sciences , 2009,
Abstract: Flat Plate Collectors (FPC) is widely used for domestic hot-water, space heating/drying, for applications requiring temperatures less than 100oC. The absorber plate of the FPC transfers solar energy to liquid flowing inside the tubes. The flow takes place by thermosyphon effect or by forcing water through the tubes. However, some of the energy absorbed by the plate is lost to the atmosphere as temperature of the plate is higher than the ambient temperature. The efficiency of FPC is dependent on the temperature of the plate which in turn is dependent on the nature of flow of fluid inside the tube, the emissivity of the plate and glass cover, wind loss coefficient, inclination of the FPC with respect to horizontal. The objective of the present work was to evaluate theoretically and experimentally the heat loss coefficient from flat plate collector. An experimental system was designed and fabricated to conduct experiments at different heat flux conditions. The effect of other significant parameters was evaluated by conducting the experiment.
Evaluation of the thermal and hydraulic performances of a very thin sintered copper flat heat pipe for 3D microsystem packages  [PDF]
S. Tzanova,L. Kamenova,Y. Avenas,Ch. Schaeffer
Computer Science , 2008,
Abstract: The reported research work presents numerical studies validated by experimental results of a flat micro heat pipe with sintered copper wick structure. The objectives of this project are to produce and demonstrate the efficiency of the passive cooling technology (heat pipe) integrated in a very thin electronic substrate that is a part of a multifunctional 3-D electronic package. The enhanced technology is dedicated to the thermal management of high dissipative microsystems having heat densities of more than 10W/cm2. Future applications are envisaged in the avionics sector. In this research 2D numerical hydraulic model has been developed to investigate the performance of a very thin flat micro heat pipe with sintered copper wick structure, using water as a refrigerant. Finite difference method has been used to develop the model. The model has been used to determine the mass transfer and fluid flow in order to evaluate the limits of heat transport capacity as functions of the dimensions of the wick and the vapour space and for various copper spheres radii. The results are presented in terms of liquid and vapour pressures within the heat pipe. The simulated results are validated by experiments and proved that the method can be further used to predict thermal performance of the heat pipe and to optimise its design.
Phase change driving mechanism and modeling for heat pipe with porous wick
Wei Liu,ZhiChun Liu,Kun Yang,ZhengKai Tu
Chinese Science Bulletin , 2009, DOI: 10.1007/s11434-009-0220-5
Abstract: According to heat pipe theory, capillary force is the only driving force for the circle of working fluid in heat pipe with porous wick. By developing a simulating circuit of liquid and vapor flow in heat pipe with porous wick, this paper presents a new driving mechanism which is from phase change of fluid. Furthermore, by analyzing transport process of working fluid between evaporation and condensation interfaces, a mathematical model is developed to describe this driving mechanism. Besides, calculating examples are given for heat pipe with water as working fluid to predict its driving force and flow resistance. By applying the model presented in the paper, thermal design and calculation for heat pipe with porous wick, especially for miniature heat pipe, can be made correctly, and phase change driving mechanism of working fluid can be explained, which thereby leads to a better understanding of heat transfer limitation of heat pipe with porous wick.
Heat Transfer Optimization in Air Flat Plate Solar Collectors Integrated with Baffles  [PDF]
Ramadhani Bakari
Journal of Power and Energy Engineering (JPEE) , 2018, DOI: 10.4236/jpee.2018.61006
Abstract: This paper presents an experimental analysis for comparisons of conventional flat plate solar collectors and collectors integrated with different numbers of baffles. Heat transfer between absorber plate and drying fluid (air) has been one of the major challenges in the design and operations of the indirect solar dryer systems. In this experiment, efficiency of air flat plate solar collector integrated with 2, 3, 4 and 8 baffles was studied and compared with the ordinary collector. The results showed that integrating solar collector with baffles significantly increased the efficiency of the system. It was noted that collector with 2, 3, 4 and 8 baffles had a mean efficiency of 29.2%, 31.3%, 33.1% and 33.7% respectively while with no baffles was 28.9%. The analysis showed that when there were less than four baffles in the collector, heat transfer was dominant over pressure drop and hence high efficiency. However, when the number of baffles exceeded four, the effect associated with an increase in pressure drop highly observed compared to heat transfer coefficient, thus resulted to insignificant increase in efficiency. Therefore, the optimum number of four baffles was commended for the designed model for optimum efficiency.
An investigation on the performance characteristics of solar flat plate collector with different selective surface coatings  [PDF]
Madhukeshwara. N, E. S. Prakash
International Journal of Energy and Environment , 2012,
Abstract: In the present work, investigations are made to study performance characteristics of solar flat plate collector with different selective surface coatings. Flat plate collector is one of the important solar energy trapping device which uses air or water as working fluid. Of the many solar collector concepts presently being developed, the relative simple flat plate solar collector has found the widest application so far. Its characteristics are known, and compared with other collector types, it is the easiest and least expensive to fabricate, install, and maintain. Moreover, it is capable of using both the diffuse and the direct beam solar radiation. For residential and commercial use, flat plate collectors can produce heat at sufficiently high temperatures to heat swimming pools, domestic hot water, and buildings; they also can operate a cooling unit, particularly if the incident sunlight is increased by the use of reflector. Temperatures up to 70 oC are easily attained by flat plate collectors. With very careful engineering using special surfaces, reflectors to increase the incident radiation and heat resistant materials, higher operating temperatures are feasible.
Transient conjugate free convection from a vertical flat plate in a porous medium subjected to a sudden change in surface heat flux  [PDF]
Jian-Jun Shu,I. Pop
Physics , 2015, DOI: 10.1016/S0020-7225(97)00040-2
Abstract: The paper presents a theoretical study using the Karman-Pohlhausen method for describing the transient heat exchange between the boundary-layer free convection and a vertical flat plate embedded in a porous medium. The unsteady behavior is developed after the generation of an impulsive heat flux step at the right-hand side of the plate. Two cases are considered according to whether the plate has a finite thickness or no thickness. The time and space evolution of the interface temperature is evidenced.
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