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：Flexible thermoelectric generators have attracted a tremendous amount of attention as they enable waste heat recovery in a wide range of applications ranging from flexible displays to biomedical devices. However, design limitations still exist in flexible micro thermoelectric generators, in particular for applications with a heat source array such as in flexible micro-light-emitting diodes. Hence, this paper proposes a thin film flexible annular thermoelectric generator design that specifically aims for heat recycling in flexible micro-light-emitting diodes for the first time. In order to evaluate the generator performance, a three-dimensional steady-state model is constructed and a systematic study on design parameters is carried out based on the finite element method. The model first takes into account the heat transfer from a heat source array to the thermocouples. Seebeck, Peltier, as well as Joule heating effects, are then considered to evaluate the specific power density of the generator. Additionally, critical design parameters including thermocouple structural dimensions, thermocouple numbers, and substrate thickness have been thoroughly investigated. After model validation using experimental data from the literature, the results show that the relationship between the thermocouple design parameters and the output power is nonmonotonic and for each parameter, a certain optimum range can be found, in which the obtained output power does not vary significantly (within 95% of the maximum output power Pmax). The thickness of the flexible substrate is inversely correlated to the output power. Moreover, the metallic thermocouples serve as a heat sink that helps to lower the temperature of the active region by - 3 degrees C. Overall, the model predicts an impressive specific power density of - 40 mu W/cm2.K2, thus demonstrating the potential applicability of using flexible thermoelectric generators to recycle heat from micro-light-emitting diodes while providing a path to further improve the performance of flexible thermoelectric generators.
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：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：The main purpose of this work is to conduct detailed comparisons of spray behavior among four important components of gasoline and to investigate the effects of fuel properties and aerodynamic breakup on spray behavior under flash boiling conditions. Isooctane, hexane, pentane, and ethanol were used as test fuels. Pressure ratio (Rp) was used as an indicator for the superheated degree and varied from 0.14 to 1.1 by adjusting the ambient pressure. Both Diffused Backlight Imaging (DBI) and Phase Doppler Anemometry (PDA) measurements were used to obtain macroscopic and microscopic characteristics of sprays. Four dimensionless numbers were used to evaluate the aerodynamic breakup processes. The results showed that as the reduction of Rp, the flash boiling intensity increased, leading to dramatic spray morphology changes and smaller droplet size, regardless of fuel type. Spray plumes merged into the spray center when lowering Rp from 1.0. Spray collapsed at an Rp of 0.28 for isooctane, hexane, and ethanol, and at an Rp of 0.18 for pentane. Fuel properties also had significant effects on spray behaviors. Spray with pentane had the smallest penetration length, and a dramatic far-field angle reduction was observed at Rp between 0.2 and 0.4, due to its higher vapor pressure and lower latent heat of vaporization. Under the same Rp, ethanol sprays had relatively larger near-field angles, far-field angles, and Sauter mean diameter (SMD), due to its high latent heat of vaporization. All the dimensionless numbers showed that the spray with pentane had the largest aerodynamic breakup intensity, followed by hexane and isooctane, while ethanol had the lowest aerodynamic breakup intensity under the tested conditions. Aerodynamic breakup still played an important role under low flash boiling intensity conditions (Rp over 0.8). Microscopic results showed that SMD of isooctane, hexane, and pentane under an Rp of 0.28 had a similar value of 10 mu m.
查看更多>>摘要：The current paper presents the broad-range sensitivity analysis and uncertainty quantification for the performance of a thermoacoustically-driven pulse tube cryocooler to make it robustly perform using a low-grade heat source. Accordingly, the onset and steady-periodic operations of the cryocooler are numerically simulated using Rott's one-dimensional equations. The effects of increasing hot/cold core's number, engine's phase-shifter length, and mean pressure on the cooling features and hot-source temperature are investigated. In this regard, a looped-branched, octa-engine, four-stage cryocooler is proposed that operates with a steady temperature difference of 132 K at a no-load cold temperature of 77 K. In contrast to previous findings, the results show that increasing the number of stages or the charge pressure does not necessarily improve thermodynamic performance. Moreover, the uncertainty in the operation of a quad and octa-engine, four-stage cryocooler caused by the geometric and material characteristics is estimated using the Morris method. It is demonstrated that the overall operation is more sensitive to the hot-core dimensions than to the cold-core dimensions. Besides the hot core dimensions, gas properties and solid thermal conductivity strongly influence the transient and steady periodic features of the cryocooler, respectively. Furthermore, doubling the hot cores in the four-stage system increases the uncertainty of the acoustic power up to five times, leading to more uncertainty in the cooling features. Drawing on the results of this paper, a multi-stage cryocooler can be designed to optimize the overall performance and reliability.
查看更多>>摘要：Based on the existing waste heat recovery technology and seawater desalination technology, a water-heat combined supply system based on waste heat from a costal nuclear power plant is proposed. The proposed system can improve the heating capacity of district heating systems and reduce the energy consumption of watergenerating seawater reverse osmosis systems by recovering the waste heat from exhaust steam in a nuclear power plant. Through the simultaneous delivery of water and heat, the long-distance conveyance cost is reduced. The thermodynamic characteristics, energy-saving properties, environmental benefits, and economic efficiency of the proposed system are discussed and analysed. The results show that the heating capacity of the proposed system is increased by 55.5% compared to the original extraction steam heating load. The exergy efficiency is 16.07% higher and the heating cost is 12.32 yen /GJ lower than those of traditional cogeneration district heating systems. The water supply cost of the proposed system is 2.95 yen /m3 lower than that of the conventional desalination water supply system. The current results of this study will be helpful in solving problems related to heat and water resource shortages and the high costs of long-distance heat and water conveyance in coastal areas.
Sarmadian, A.Dunne, J. F.Jose, J. ThalackottorePirault, J-P...
21页
查看更多>>摘要：A temperature control approach using evaporative spray cooling of vibrating surfaces in the nucleate boiling region is proposed and verified experimentally. This is relevant to temperature control of heat-generating automotive vehicle components. By exploiting an experimentally calibrated dynamic correlation model to represent evaporative spray cooling of a flat test-piece, a PID controller has been adopted with emphasis focused on the choice of gain parameters to ensure both stability of temperature control, and favourable responses in terms of relevant performance measures. Optimum linearisation of the correlation model has been achieved by solving an appropriate Wiener-Hopf equation, mainly to undertake a practical stability assessment of the closedloop temperature control system. To verify the predicted control system performance, experimental measurements have been obtained from an instrumented, and spray-evaporatively-cooled, flat test-piece exposed to displacement vibration from a shaker. Experimental testing, appropriate to automotive vehicle component applications, includes large-amplitude, low frequency vibration at 12 mm and 1.9 Hz, and low amplitude, highfrequency vibration at 0.02 mm and 400 Hz. To assess the effects of different PID controller gains on the thermal performance of the thermal management system, a coefficient of performance (COP) is used, defined as the ratio of heat power removal to the required pumping power. To achieve a reduction in the settling time, and an increase in the rise time of stable control, a PID controller with a negative proportional gain showed most promising results. A 10.5% increase in COP was achieved in comparison to a PID controller with positive gains. This information is useful for the design and optimization of thermal management systems using evaporative spray cooling.
查看更多>>摘要：Engine downsizing with a higher compression ratio and a higher boosting pressure improves the thermal efficiency of engines. However, with further downsizing of engines, knock intensity is aggravated until super knock occurs which is accompanied by a detonation wave that potentially can destroy engines rapidly. Therefore, it is essential to reveal the parameters that influence the knock intensity and the formation of a super knock. Selfdesigned detonation bomb experiments and corresponding numerical simulations were conducted to explore the end-gas combustion mode as well as the knock intensity problem. The experiment was conducted at three typical in-cylinder pressures for representing different mixture energy densities, which classify the knock phenomenon into subcritical (lower pressure), critical (medium pressure) and supercritical (higher pressure) conditions. Spark ignitions with low and high intensity realized by adjusting the spark ignition energy were given in each condition, respectively. The in-cylinder combustion process and pressure oscillation process were monitored by the synchronous acquisition of three pressure sensors that were installed in different positions of the chamber. It is found that irrespective of the low or high ignition intensity, the super knock as well as the detonation would not occur at the lower pressure, and only mild knock occurs. At the medium pressure, the high ignition intensity would result in a detonation wave, while the low ignition intensity would not. At the higher pressure, despite the intensity of ignition, a detonation wave as well as the super knock would always occur. Therefore, the ignition intensity and the in-cylinder pressure are two essential factors that affect the knock intensity of engines.
Sudhan, A. L. SriramSolomon, A. BruslySunder, Shyam
18页
查看更多>>摘要：Grooved Heat Pipes (GHPs) are the essential heat transfer devices for space applications, as they can effectively work with and without the aid of gravity. In the present study, the heat transport limitations of anodized grooved heat pipes were modelled by incorporating an anodised surface coating parameters under gravity and anti-gravity conditions. Variations in the heat transport limitations such as capillary limit, boiling limit, and entrainment limits due to the anodization over non-anodized grooved heat pipes are presented. The heat transfer performance of an internally anodized and non-anodized grooved heat pipe charged with ammonia as a coolant is experimentally investigated and compared. Two sets of grooved heat pipes are fabricated with 40 numbers of rectangular grooves. Semi-spherical pores are formed by carefully adjusting the voltage and current supply between the electrodes during the anodization process. Experiments in grooved heat pipes are conducted at different inclinations (0-90 degrees) under gravity and anti-gravity for various heat inputs of both heat pipes. Due to the anodization, thermal resistance reduction of 51.1% and enhancement in heat transfer coefficient of 36.6% was obtained at an optimum inclination of 45 degrees for the grooved heat pipes with 0.4 mm groove width. Also, a maximum of 35 W heat input was transferred due to the anodization under anti-gravity with an inclination of 30 degrees. Results suggest that the grooved heat pipes with semi-spherical pores charged with ammonia as a coolant was the best combination for enhanced heat transfer, preferably for space applications.
NSTL
Elsevier