查看更多>>摘要：It is well known that non-adsorbable gas, e.g. residual air, can dramatically slow down the adsorption stage of adsorption heat transformation cycles. So far, this effect has been studied for the cycles initiated by temperature change (temperature-initiated cycles). This work addresses the dynamic effect of residual air for another way of adsorption initiation, namely, by changing vapour pressure over adsorbent (pressure-initiated cycle). Comparison of the pressure- and temperature-initiated cycles is also made. The effects are studied for the working pairs "AQSOA FAM-Z02 - water" and "LiCl/(silica gel) - methanol" promising for adsorption heat transformation. The residual partial air pressure Delta Pair was varied from 0 to 5 mbar. The main finding of this study is that the pressure-initiated adsorption is less sensitive to the presence of residual air than the temperature-initiated adsorption. This is especially true at a low partial air pressure Delta Pair. For instance, at Delta Pair <= 0.5 mbar, residual air has little or no effect on the pressure-initiated adsorption dynamics compared to the temperature-initiated one. A qualitative explanation of this finding is proposed. Thereby, closed adsorption heat transformation cycles based on the pressure-initiated process are more robust and resistant against the presence of residual air that could be a significant practical advantage.
查看更多>>摘要：To solve the instability of conventional single source heat pump and further improve energy efficiency, this paper presented a hybrid solar/air dual source assisted heat pump (S-A-AHP) water heater. Based on the system, a quasi-steady-state mathematical model was constructed to study the mode switching criterion and operation performance. The simulation results showed that it is more suitable to use the mass flow ratio of refrigerating medium (i.e., the ratio of the photovoltaic/thermal collector/evaporator to the whole system) as the criterion of system mode switching than directly using environmental factors, and the critical ratio for switching between the solar-air mode and the solar mode is about 0.75. In addition, compared with the conventional solar assisted heat pump (SAHP) and air source heat pump (ASHP), under the low environmental benefit conditions of ambient temperature of 0 degrees C and solar radiation intensity of 100 W/m(2), the average COP of S-A-AHP is 29.7% and 19.9% higher than that of SAHP and ASHP, respectively. And the S-A-AHP system's heating time is only 36.7% of the SAHP and 83.7% of the ASHP. And the impact of collector area on system performance was also discussed.
查看更多>>摘要：This study proposed a novel micro-scale falling film absorber with its experimental data on a complete absorption refrigeration system. Micro-scale liquid-distributor has been carefully designed and manufactured by special techniques. An experimental system for absorption refrigeration has been established to test the novel absorber. This unique absorber applies ammonia/lithium nitrate as working pair which is considered to own application potential for small-scale absorption refrigeration units. Experimental analysis has been done under various of working conditions for making contrasts. Relationships between significant working parameters and absorption parameters have been discussed. Under series of working conditions, absorption rates ranging from 1.2 to 2.6 x 10(-3) kg/(m(2).s) have been obtained. Approach to equilibrium factor which characterize the perfection of absorption process ranges from 0.5 to 0.98 while absorption ratio ranges from 0.02 to 0.12. With the increase of Reynolds number from 3 to 52, the mass transfer coefficient rises from 12 to 98 x 10(-6)m/s and Sherwood number ranges from about 30 to 60 respectively. According to the analysis, absorption pressure potential and cooling water temperature are two major factors which enhance the absorption intensity. Absorption pressure potential and inlet solution subcooling degree owns nearly linear relationship with parameters such as absorption rate, absorption ratio and approach to equilibrium factor. However, normalized absorption rate and approach to equilibrium factor gain negative correlations with absorption pressure potential and Reynolds number respectively. Effects of inlet solution flowing features on absorption performance are distinguished under different working condition series. In general, solution flowing intensity inside absorber is a positive factor for absorption process. All the experimental results reveal that, this micro-scale falling film absorber has its priority on absorption perfection and absorption rate for per unit of mass transfer area. Compact structure makes it more valuable for further research and application on small-scale absorption refrigeration unit with the working pair of ammonia/lithium nitrate.
查看更多>>摘要：This article describes the design, fabrication, and heat transfer characteristics of a novel type of loop thermosyphon with a horizontal porous evaporator (LTHPE) of the kW-class heat transfer performance. The main goal of this investigation is to improve the heat transfer intensity by increasing the fluid circulation in the evaporator annular channel. The Bond number Bo <= 1-2 is typical of this new device. Such thermosyphon with the evaporator annular thickness equal to, or larger than, the capillary limit exhibits some new properties. An additional (to the gravity field) mechanism of liquid flow enhancement with self-exciting oscillation due to the bubble generation in the annular gap is used for intense and efficient fluid circulation through the thermosyphon evaporator in a preferential direction. Experimental data on heat transfer coefficients in boiling and evaporation inside the porous wick were obtained in the evaporator annular horizontal gap flooded and partially saturated with liquid. Due to the two-phase fluid flow along the annular tube, stabilization of heat transfer has been achieved including the subcooled and saturated liquid boiling with evaporation inside the wick. A thin porous coating on the evaporator wall ensures a 2-2.5-fold increase of heat transfer in comparison with the flow boiling heat transfer in a smooth tube. The thermal performance of the LTHPE was investigated at different filling ratios and LTHPE inclination angles to the horizon. The evaporator envelope was made from a copper tube of length 200 mm. A copper sintered powder wick was installed inside the evaporator, and pure water was used as the working fluid. The wick thickness was less than 1 mm. The thermal resistance of the condenser was equal to 0.01.C/W. The LTHPE evaporator and condenser are connected with each other by flexible minipipes for transferring vapor and liquid. The thermal resistance of LTHPE is relatively insensitive to any changes in inclination, when the angle of the latter to the horizon exceeds 18.. The total thermal resistance of LTHPE does not exceed 0.03 K/W (thermal resistance of evaporator is near 0.02 K/W) with the heat load of 100-1750 W. This device guaranties a shortened start-up time, has a decreased evaporator wall temperature, has a small temperature hysteresis on increasing/decreasing the heat load, and suppresses the temperature pulsations inside the evaporator. The determination of the thermal resistance of evaporator and condenser and the analysis of the temperature field along the LTHPE were the main goals of this research.
查看更多>>摘要：The major challenge of multi-energy complementary systems consists of improving the thermal efficiency for latent heat thermal storage pools (LHTSPs). To address this, a new multi-tube LHTSP with tree-shaped fins is designed herein. The enthalpy-porosity approach is employed to model the charging/discharging process in LHTSPs, focusing on the role of inclinations in melting/solidification heat transfer. Moreover, experimental validation is conducted to ensure numerical reliability. The results show that tree-shaped fins effectively improve thermal efficiency and temperature uniformity. Compared with traditional LHTSPs, the innovative LHTSP shortens the total melting/solidification time by up to 29.4% and 22.8%, respectively, and improves the temperature uniformity by 12.3%similar to 19.2%. The difference in the influence regime of natural convection during melting and solidification lies in its onset and duration. Moreover, the thermal performance of LHTSPs is mainly related to the later charging/discharging stage. The inclination significantly affects the thermal charging performance, while it has less influence on the discharging processes. Compared to the horizontal arrangements, the innovative vertical LHTSP has a 46.3% reduction in the melting duration. Interestingly, there is a transition point in the heat storage efficiency of horizontal and vertical LHTSPs, and the LHTSP arrangement with a higher efficiency differs before and after this point.
查看更多>>摘要：This paper concerns the modeling of vertical Borehole Heat Exchangers (BHEs) for Ground Source Heat Pump (GSHP) applications. Focus is devoted to the analyses of Thermal Response Test (TRT) simulations aimed at understanding the main factors that influence the ground thermal conductivity and the effective borehole thermal resistance estimations. The conventional infinite line-source (ILS) model does not include any influence of the external heat transfer rate on the BHE/ground property evaluation. Analyses of numerically simulated TRTs show this omission can sometimes produce an error in the estimate of the ground thermal conductivity. The error may be between +/- 10% and +/- 22% for coaxial boreholes (800 m depth), if the ground has a significant geothermal gradient. On the other hand, for single and double U-pipe BHEs the error is less than +/- 5% under similar conditions. The parameter q(ratio) is identified as an indicator of when the error is significant. This parameter is equal to the external heat rate (injection or extraction) divided by a natural heat rate that is related to the geothermal gradient. Errors greater than +/- 10% tend to occur for coaxial boreholes with a center-pipe fluid inlet when vertical bar q(ratio)vertical bar < 1. Under the same conditions but with the annulus as the fluid inlet, the error is less than +/- 6%.
Mahmoudi, FarhadNajafabadi, Tooraj AbbasianMoarefianpour, Ali
17页
查看更多>>摘要：Stress control of boilers is a major concern in the aged power plants. The studied forced circulation boiler suffers from a malfunction of the combustion control system. In order to reduce cyclic stress on boiler equipment and prevent undesirable shutdowns of power unit during fuel shortage events, an operating instruction is proposed. In order to predict dynamic behavior of the boiler, a nonlinear state space model is developed using mathe-matical modelling approach. A gray box identification procedure is introduced using only steady state mea-surements of plant. The dynamic model is verified compared to actual plant data and accuracy of model parameters is evaluated by sigma-normalized sensitivity analysis. Considering aim of steam pressure regulation, a three-step optimization approach is proposed based on golden section search, parabolic interpolation and interior-point methods to optimize arbitrary parameters of the proposed manual control scheme. Simulation results for different loads and various fuel limitation scenarios show that proposed instruction improves the cyclic stress in superheater tubes at least 84.5%. In addition, Monte Carlo experiments illustrate despite a 10% operators' error in implement of the instruction, the stress is still reduced by an average of 78.1% for a given case.
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.
查看更多>>摘要：Cryogenic air separation plants produce oxygen at 99.5% purity at 150, 230 and 300 bar to fill cylinders. From the production location, liquid oxygen is often transported to a cylinder-filling plant located at a far-off location where it is pumped to high pressure and gasified in ambient vaporizers. Among the methods of cylinder filling, external vaporization is the safest, operationally most flexible, and allows the main heat exchanger to be designed at low pressure. Despite these advantages, external vaporization is not preferred for filling cylinders "in situ" because of high power consumption attributed to the loss of the entire cold exergy of pumped liquid to the environment. An appropriate modification of the plant helps to utilize the thermal energy of pumped liquid oxygen that is externally vaporized to fill cylinders at the plant site. A reduction of specific power consumption by 23% to 29% is possible while the main heat exchanger still operates at low pressure. Cylinders can also be filled at any of these pressures from the same plant with a penalty of 2% to 3% in SPC. The paper shows ways to adjust the operation of the air compressor to obtain oxygen at varying pressures with utilization of the thermal energy of liquid oxygen.
查看更多>>摘要：Vapor quality and mass flux are crucial parameters for heat transfer coefficient and pressure drop of flow boiling. In this paper, they are actively adjusted by means of draining and refilling liquid refrigerant in the paths for evaporator performance enhancement. Principles of the vapor-liquid adjustment evaporator are introduced and its corresponding configuration is proposed. The mathematical model of this novel evaporator is established and validated by experiment data. Under different separation efficiencies, inlet vapor qualities and inlet mass flowrates, the vapor-liquid adjustment evaporator (AE) is investigated and compared to the conventional evaporator without vapor-liquid adjustment (CE) from the perspectives of overall performance and local behaviors. At the studied conditions, the improved heat transfer coefficient and reduced pressure drop are simultaneously obtained at a separation efficiency of 40%. Its superiority over the conventional evaporator is generally confirmed in terms of total temperature penalization. However, the excessive separation efficiency leads to the deteriorated performance due to the appearance of superheating. The major differences of local heat transfer coefficient and pressure drop between the vapor-liquid adjustment evaporator and conventional evaporator take place in the first and second paths, which are the consequences of the competition of the enhanced vapor quality and decreased mass flux. The stratified wave flow is mostly encountered for both evaporators. The vapor-liquid adjustment evaporator can be further enhanced by optimization. This study offers an innovative approach for flow boiling heat transfer enhancements.
NSTL
Elsevier