查看更多>>摘要：Direct-contact total heat exchange with liquid desiccant produces enormous gains in both sensible and latent heat recovery of low-grade flue gas. Previous studies related to that has tended to focus on the system performance rather than the heat and mass transfer characteristics, which is equally essential to their design, simulation and operation. This paper establishes an experimental system to investigate the heat and mass transfer characteristics in direct-contact total heat exchanger for flue gas total heat recovery. Ceramic structured packing is adopted considering its corrosion and high-temperature resistances; calcium chloride aqueous solution is employed as the coolant and absorbent for its low cost and high latent heat recovery capability. Based on those, the effects of flue gas and solution inlet parameters on the performance indices, including sensible and latent heat recovery capacities, latent heat rate and total heat recovery rate, are tested and analyzed. Coupled heat and mass transfer models between flue gas and solution in this process are developed, which feature the enthalpy of dilution. By combining the models and experimental data, heat and mass transfer coefficients are calculated and fitted. In addition, the effect of packing height on total heat recovery capacity and ratio is investigated through simulations. The results show that the total heat recovery rate of 74.7% can be achieved at packing height to 1.5 m when liquid-gas ratio is 5.172.
查看更多>>摘要：Noise fluctuation is the most common issue of small heat flux measurement and it disturbs the measurement precision. In the previous studies, how to select an appropriate filter and determine its optimal operating parameter was not given systematically. In this paper, the aim is to explore an effective filter selection and optimal operating parameter determination method for the small heat flux signal. Firstly, six filters with the large range operating parameters were chosen to remove the noise of a set of 10,000 numbers time-series heat flux in the range of 5-10 W/m(2). Then, signal-to-noise ratio, standard deviation, mean absolute deviation and correlation coefficient were introduced to comprehensively assess the filtering results. Lastly, the intersection region method is firstly proposed to determine the optimal operating parameters based on the tradeoff between repeatability and fidelity. The numerical results indicated that the signal-to-noise ratio of the sampled heat flux was ranged from 13.310 db to 20.703 db. The standard deviation and correlation coefficient was decreased from 1.111 to 0.093-0.672 and from 1 to 0.640-0.883, while the mean absolute deviation was increased from 0 to 0.498-1.056. When the filter is fixed, the repeatability and fidelity levels of filtered data are contradictory. When comparing the filtering effects of filters, some filters simultaneously showed a high repeatability and fidelity levels. For the heat flux signal in this study, the moving average filter with 51 window width and Savitzky-Golay filters with 101 window width and three highest order are the best choice. The proposed intersection region method with four assessment metrics is effectively to find an appropriate filter with high repeatability and fidelity.
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.
查看更多>>摘要：Ultra-thin loop heat pipe (UT-LHP) is a high-performance heat transfer component that satisfies the requirement of thermal management problems in miniaturized electronic devices. In this study, a novel UT-LHP with capillary wick structures is developed for mobile electronics cooling. By using a novel print wick structuring process on copper substrates, the total thickness of the UT-LHP can be decreased to only 0.3 mm. The effects of the filling ratio and wick structure distribution have been investigated to optimize the vapor-liquid circulation in the UTLHP. The heat transfer performance of the UT-LHP has been evaluated under different heat loads and configurations. The minimal thermal resistance of the UT-LHP is 1.03 degrees C /W at a heat load of 3 W which corresponds to a heat flux of 3 W/cm(2). It is confirmed that the proposed UT-LHP can transfer a heat load of up to 3 W with low sensitivity to gravity. The proposed novel UT-LHP is a promising thermal solution for high-performance mobile electronic devices.
查看更多>>摘要：Improving of thermal efficiency is inevitable for further dissemination of adsorption heat pumps. Here, a highly heat-integrated multi-bed switching sequence for adsorption chillers is presented to maximize regeneration of both, sensible and latent heat. The thermal coupling of the beds is investigated together with properties of the adsorbent layer using a True-Moving-Bed approach. Results are compared to the limiting thermodynamic efficiency. To account for heat and mass transfer in a consolidated adsorbent together with the motion of the adsorbent beds, a spatially one + one dimensional model is used. Adsorbent layer thickness, process cycle time and heat regeneration are optimized for maximal thermal efficiency and a given specific power density. Due to the combined optimization of material properties and cycle design, a Coefficient of Performance of up to 1.57 could be reached which is 96% of the limiting thermodynamic efficiency.
查看更多>>摘要：It is significant to evaluate whether the classical correlations based on conventional tube size (>= 10 mm) is suitable for predicting the flow and heat transfers characteristics of the precooler composed of mini-tube bundles (around 1 mm). In this work, a full-scale heat transfer experiment of precooler is carried out, and the high temperature incoming air (686 K) in the experiment was produced by burning the mixture of air and alcohol. The numerical simulation of a unit precooler is also conducted with Computational Fluid Dynamics. The classical correlations for the design of heat exchanger are evaluated by both experiments and simulations. The results indicate that the prediction error of classical correlations of flow resistance is less than 10% for both the flow inside the mini-tube and across mini-tube bundles. The classical correlation of heat transfer inside tube can meet the engineering requirements. However, the maximum deviation of Nusselt number prediction for the outside flow across tube bundles reaches 32.1%. A new correlation for calculating heat transfer across a compact pre cooler composed of staggered mini-tube bundles with a prediction error below 5% is finally established based on the experimental and numerical results.
查看更多>>摘要：Experimental and numerical investigations of a brazed plate heat exchanger are performed in this study. Three approaches are considered in numerical modeling of brazed plate heat exchanger in order to realize the difference amid simulations and experimental data. The departures between simulations and experiments are strongly associated with the complicated configurations of the plate geometry. Hence, modeling regarding the modified geometry, modeling of original geometry, and modeling of original geometry by considering brazing joints are conducted in numerical simulations for elaborations. Results show that the third geometry yields the best agreement with the experimental data, indicating the importance of considering brazing joints in the modeling of brazed plate heat exchangers. However, the modified geometry has the worst agreement with the experimental data. This approach has been adopted by many existing literatures. Flow field for all approaches illustrates that a zig-zag pattern from the inlet to outlet nozzles exists. This is because of large chevron angle of plates (75 degrees). Comparison of Nusselt number and friction factor of this brazed plate heat exchanger with the existing correlations demonstrates the necessity of developing new correlations for brazed plate heat exchangers. Correlations for Nusselt number and friction factor of the modeled heat exchangers are developed in a wide range of Reynolds number (50-10,000) with mean deviations of 5.1% and 3.4%, respectively.
查看更多>>摘要：A novel multistage air-gap diffusion distillation (MS-AGDD) system powered by low-grade sensible heat (LGSH) was proposed for fully recovering heat from LGSH to produce freshwater. The effect of operating parameter variations on the system performances was investigated theoretically based on the epsilon-NTU model for AGDD. The operating parameters include the hot water inlet temperature (T-hs(in)), the feed seawater flowrate (V-c,n(in)), salinity (S-c,n(in)) and temperature (T-c,n(in)), the temperature differences at the hot end of heat exchanger (Delta t(h)) and at the cold end of coolers (Delta t(c)). Results showed that under the given structure and size parameters of the system and the operating conditions, the variation of T-hs(in), V-c,n(in) or Delta t(h) affected the system performances significantly, but the variation of S-c,n(in) n or Delta t(c) affected those relatively less. The total freshwater yield and the recovered heat from the hot water enlarged with the increase of T-hs(in) or V-c,n(in), but they decreased with the increase of Delta t(h). The gained out ratio (GOR) of the system increased with the increase of T-hs(in), but it decreased with the increase of V-c,n(in) or Delta t(h). The irreversibility of the system increased with the increase of T-hs(in) or Delta t(h), but it decreased with the increase of T-c,n(in) or V-c,n(in).
查看更多>>摘要：Soil thermal imbalance has become a common problem for ground source heat pump (GSHP) system, leading to the fluctuation of ground temperature and decline of heat transfer efficiency. However, compared with vertical ground heat exchanger (GHE), there is a lack of research and evaluation on the soil thermal imbalance for horizontal GHE. Therefore, in this paper, to investigate the extent of soil thermal imbalance and its effects on performance of GSHP system, a sandbox experiment embedded with horizontal stainless-steel GHE was implemented. Various operation conditions were designed to explore effects of intermittent time, water content (rainfall) and buried depth of pipes on soil temperature distribution and restoration performance. The experimental data was validated by a numerical simulation. The results indicated that 5-day intermittent operation of GSHP leads to an increase of 6.41 celcius in soil temperature. After 2-day natural recovery, soil temperature recovery rate can only reach 0.72. Increasing soil moisture content and intermittent control are effective solutions to mitigate thermal accumulation and improve thermal performance. The heat transfer performance of GHE with an intermittent ratio of 6:18 which GSHP system runs for 6 h and stops for 18 h during a day is 33% higher than that with the intermittent ratio of 14:10 and 10:14. The higher the water content, the lower the soil temperature fluctuation and the stronger the soil temperature recovery capacity. This can be attributed to the increase of soil thermal conductivity and capacity, while the decrease of thermal diffusivity.
查看更多>>摘要：The multi-section absorption heat pump (AHP) is applied in the novel district heating (DH) system of China. This new type of AHP exhibits improved performance in DH systems, but involves a more complicated structural design and the risk of internal flow instability problems such as reversed pressure difference and liquid accu-mulation. The flow circulations of the solution and refrigerant consist of several gravity-driven flow processes that are rarely observed in conventional AHPs, and these processes have not been addressed in previous studies. The present work studies the dynamic response of gravity-driven flow inside a multi-section AHP in a DH system. A dynamic model capable of calculating the liquid level, flow rate, and pressure change of various gravity-driven flow processes is developed and validated. The steady-state conditions calculated by the dynamic model are compared with a validated model in the literature. The gap between the temperature and pressure calculations is less than 0.3 K and 0.05 kPa, respectively. The dynamic simulation results are compared with the start-up process test results of a multi-section AHP. The pressure values and trend change fit the test results well. The standard deviation (SD) and root-mean-square error (RMSE) of the pressure calculation errors are less than 5% and 5.5%, respectively. The model is used to study typical transient processes of multi-section AHPs in a DH system. In the start-up process, the pressure of the generator is 1-2 kPa lower than that of the absorber after the solution circulation begins. This reversed pressure difference leads to solution accumulation at the bottom of the generator. The solution flow rate into the absorber is 20% lower than the designed value. After the heat source is input into the generator, the reversed pressure difference disappears. Solution accumulation expedites the flow of the solution into the absorber. The flow rate is 75% higher than that of the designed value, leading to solution accumulation in the absorber. In the heat source sudden drop process, reversed pressure difference and solution accumulation problems also occur. The reasons for flow instability in each process and possible methods for preventing them are discussed.
NSTL
Elsevier