Gomez-de la Cruz, Francisco J.Palomar-Torres, AmaliaPalomar-Carnicero, Jose M.Cruz-Peragon, Fernando...
16页
查看更多>>摘要：Nowadays, the design, control and development of rotary dryers are based on the study of the complete equipment. Rotary dryers are treated as a black box where only inlet and outlet parameters are known. This implies the need to know the main interactions of mass, energy and exergy throughout the trommel to help improve the drying process. In this paper, we carried out an analysis of energy and exergy during drying of olive stone from finites control volumes in an experimental rotary dryer. Mass, energy and exergy balances are applied to each control volume in the drying air. The design of experiments is based on three initial drying air temperatures (210 degrees C, 180 degrees C and 150 degrees C) and three drying air flows (576 kg/h, 425 kg/h and 280 kg/h), with a by-product mass flow of 40 kg/h and a rotational speed of 5.5 rpm. The results indicated that olive stone moisture content was reduced to less than half in the first third of the trommel where a big thermal shock is produced due to the interaction between the by-product and the drying air flow at high moisture contents and high temperatures, respectively. This fact led to the highest exergy destruction values, thereby diminishing the flow exergies for the following control volumes. Heat losses were considerable in all equipment, especially in the first two control volumes. Furthermore, drying process was analyzed from the enersgetic, exergetic and drying efficiencies and the unit energy consumption.
查看更多>>摘要：Currently there still exist a large base of subcritical coal-fired boilers remaining in operation worldwide. Improving their thermal efficiency is critical for these units to reduce the cost of electricity and meet the increasingly stringent efficiency regulations. This paper presented an economic high temperature upgrading retrofit solution for these subcritical units in which the final superheat and reheat steam temperatures are elevated from 540 degrees C to 600 degrees C level while the steam pressures remain at subcritical such that the retrofit can be made with minimum modifications to the boiler to reduce retrofit cost and technical risk. A systematic analysis was first conducted to deduce the best boiler heating surface modification option to achieve such significant increase of boiler steam temperatures. The analysis shows that replacing part of the furnace water wall below the furnace exit with wall superheater is the most viable and economic boiler modification design. A threedimensional computational fluid dynamics (CFD) simulation was then conducted for a typical 320 MW subcritical boiler design to verify the feasibility of this design and determine the area of furnace wall that needs to be modified to achieve the target steam temperatures. The results show that with only 3.6 m height of furnace wall replaced with wall superheater both the superheat and reheat steam temperatures can reach the target 600 degrees C level even without any modification made to the reheaters. With this heating surface modification design, the efficiency of the existing subcritical power plants can be improved considerably with minimum boiler retrofit work. More importantly, this technology is going to help the plant owners meet the increasingly stringent efficiency and emission regulations mandated by the government and place them in a better position to continue operating these units.
查看更多>>摘要：To improve the heat transfer performance in the anti-gravity direction, the multi-layer wick heat pipe with three pore sizes was fabricated. The wick consisted of a coarse powder layer and a nanoporous fine copper powder layer. The critical heat load of the multi-layer wick was as high as 79 W in the 90 degrees direction, while the homogeneous wick sintered from coarse powder and nanoporous fine powder is only 23 W and 30 W, respectively. Multi-layer wick heat pipes with high heat transfer capacity will broaden their applications in terrestrial environments.
查看更多>>摘要：The existing combined absorption refrigeration and liquid desiccant dehumidification systems have the disadvantages of small temperature difference in waste heat utilization and high cost of working pair. For effective and deep utilization of low-grade heat below 80 degrees C, a novel air conditioning system cascade driven by waste heat is established. In this system, the low-grade heat is used to drive an absorption chiller to produce chilled water for air cooling and then drive a liquid desiccant system for air dehumidification. The cheaper and non-corrosive KCOOH solution is selected as the comparative working pair of LiBr solution to conduct the experimental study in the system. Operation parameters such as inlet temperature, flow rates of both hot water and cooling water that directly affect the application of the system are studied. The results show that the system can be driven by 80 degrees C hot water, which is much lower than that of the conventional single absorption refrigeration system. Besides, both working pairs can be used for the system. As the inlet parameters of supply air are fixed, the cooling capacity of KCOOH solution is 10% lower than that of LiBr solution. The KCOOH solution has comparable COP compared to LiBr solution only when the dilute solution flow rate is low. The conclusion is summarized that considering the much lower cost, the KCOOH solution is a promising working pair for this novel air conditioning system, nevertheless, the structure of this system needs to be redesigned to match this working pair.
查看更多>>摘要：In order to improve the transverse thickness uniformity of amorphous ribbon by increasing the axial uniformity of cooling roller thermal characteristics during planar flow casting, a novel convex cooling roller was proposed. The flow characteristics, transient temperature, and thermal deformation of the proposed convex roller and existing straight roller were studied using a fluid-solid thermal coupling method based on a full-flow channel calculation model under axial variable boundary. In addition, the reliability of the simulation approach and results are verified by measuring the roller deformation on the spot. The results show that the heat transfer acting on the roller inner wall is unevenly and asymmetrically distributed in the axial direction. The full-flow channel model with coupling method provides high solution accuracy, the error between simulation and the experiment is about 7.5 %. Compare to the existing straight roller, the maximum heat transfer coefficient in the middle width direction increases by 108 %, the axial difference of the temperature and deformation decrease by 15.4 % and 26.9 %, respectively, when the convex roller with bending radius R = 300 mm. The maximum temperature, thermal deformation, and axial difference in the puddle contact zone decrease obviously with the increase of the roller bending radius. These results provide a promising strategy for improving the transverse thickness consistency of the amorphous ribbon.
Ji, JiadongGao, RunmiaoShi, BaojunZhang, Jingwei...
12页
查看更多>>摘要：Based on conventional elastic tube bundle heat exchangers, three novel elastic tube bundle heat exchangers were proposed by improving the tube bundle structure and adding segmental baffles in the heat exchanger to obtain a better comprehensive heat transfer effect. The heat transfer performance and vibration-enhanced heat transfer performance of conventional elastic tube bundle heat exchanger without baffles (CETB-NB), improved elastic tube bundle heat exchanger without baffles (IETB-NB), conventional elastic tube bundle heat exchanger with baffles (CETB-HB) and improved elastic tube bundle heat exchanger with baffles (IETB-HB) were compared qualitatively and quantitatively by numerical simulation. Numerical results show that improving the tube bundle structure and adding baffles in the heat exchanger can remarkably enhance the vibration-enhanced heat transfer performance and heat transfer capability of the heat exchanger. At the same inlet velocity, the vibrationenhanced heat transfer capability of the CETB-NB, IETB-NB, CETB-HB and IETB-HB are increased by 2.55%, 6.53%, 5.09% and 7.96% respectively. In the inlet velocity range of 0.1-1.0 m/s, compared with the CETB-NB, the heat transfer capacity of the IETB-NB is improved by 8.44%, 6.91%, 5.50% and 2.41%, respectively. Compared with the CETB-HB, the heat transfer capacity of the IETB-HB is improved by 5.14%, 4.21%, 4.03% and 2.14%, respectively. At the same inlet velocity, the pressure drop of the heat exchanger is IETB-HB, CETB-HB, IETB-NB and CETB-NB from largest to smallest. Considering the heat transfer and pressure drop of the heat exchanger comprehensively, the IETB-HB has obvious advantages in vibration-enhanced heat transfer performance and thermohydraulic performance.
查看更多>>摘要：Nowadays, geothermal energy in shallow hot dry rock fields is not exploited enough due to the high economic and environmental impact as well as the lack of scalability of the existing technologies. Here, thermoelectricity has a great future potential due to its robustness, absence of moving parts and modularity. However, the efficiency of a thermoelectric generator depends highly on the heat exchangers. In this work, a novel geothermal thermoelectric generator is experimentally developed, characterizing different configurations of biphasic heat exchangers to obtain low thermal resistances that allow the maximum efficiency in the thermoelectric modules. As a result, robust and passive heat exchangers were obtained with thermal resistances of 0.07 K/W and 0.4 K/W in the hot and cold sides, respectively. The geothermal thermoelectric generator was built with the most effective heat exchangers and was experimented under different temperature and convection conditions, generating 36 W (17 W by a prototype with 10 modules and 19 W by a prototype with 6 modules) for a temperature difference of 160 degrees C between the heat source and the environment. Furthermore, the experimental development showed that it is possible to increase electricity generation with a more compact generator, since a decrease in the number of modules from 10 to 6 increases the efficiency from 3.72% to 4.06%. With this research, the feasibility of a novel and robust geothermal thermoelectric generator whose working principle is phase change has been experimentally demonstrated, as well as the importance of compactness to maximize its efficiency and thus, power generation.
查看更多>>摘要：In this work, we studied the wall heat flux partitioning during the pool boiling of water on thin metallic surfaces. We conducted boiling experiments on surfaces where we engineered nucleation sites by nanosecond-fiber-laser texturing. These nucleation sites form triangular lattice patterns with different pitches. We measured the time-dependent temperature and heat flux distributions on the boiling surface using an infrared camera. We devel-oped post-processing algorithms to measure, based on these distributions, all the fundamental boiling parameters used in heat flux partitioning models (e.g., nucleation site density, bubble wait and growth time, and bubble footprint radius) and the actual partitioning of the heat flux, i.e., how much heat is transferred by evaporation of the microlayer, rewetting of the surface, and convective effects. This work reveals that the mechanisms of heat transfer on substrates of small thermal capacity are very different compared to substrates of large thermal capacity. With water, the bubble microlayer typically does not dry out and the surface temperature at rewetting is practically the same as the rewetting fluid temperature. These effects limit the efficiency of microlayer evaporation and rewetting heat transfer. Instead, convective effects generated by the bubble growth process remove most of the energy from the heated surface. This behavior is captured by a heat flux partitioning model that we re-derived from first principles to describe the heat transfer mechanisms on substrate of small thermal capacity.
查看更多>>摘要：Heat recovery technologies are used to reduce the energy use and the operating costs for ventilation systems in buildings. Run-around heat recovery systems for ventilation are commonly used in buildings when cross-contamination between the air streams is not acceptable, in buildings with complex ducting and in retrofit projects with space limitations. The design and operation of run-around systems are rather complex, especially in ventilation systems with variable air flow rates since the coupling liquid flow rate must be adjusted with respect to the air flow rate.
查看更多>>摘要：This paper employs coupled multiphysics modelling of pile-soil heat exchange to quantify pile thermal interaction and its influence in diminishing the power output expected from a group of geothermal piles. Threedimensional finite element models, which account for the flow of heat carrier fluid through the circulation tubes and conductive heat transport in pile and soil, are developed for different group arrangements of geothermal piles. Finite element analyses (FEAs) of a pair of geothermal piles reveal the effects of spacing, diameter, orientation of embedded fluid circulation tubes, and thermal operation time of geothermal piles on thermal interaction between the piles. A simple analysis-based expression is proposed to calculate power reduction factor that quantifies thermal interaction between two simultaneously acting geothermal piles. The proposed factor is further employed, in conjunction with the principle of superposition, to estimate power output from a group of geothermal piles. Comparison of predictions using the proposed method with FEA results suggests that the proposed method can successfully predict total energy harvesting efficiency (i.e., power output) of a thermally interacting group of geothermal piles.
NSTL
Elsevier