| 著作名稱： | Accelerating flow simulations in the built environment by using the fast fluid dynamics initializer |
| 年度： | 2024 |
| 類別： |
期刊論文
Building and Environment
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| 摘要： | This study introduces the Fast fluid dynamics (FFD) based initializer as a novel approach to accelerate steady-state flow simulations. Three modes - Coarse-Mesh Mode (CMM), Large Timestep Mode (LTM), and Hybrid Mode (HM) - are developed on the OpenFOAM platform for efficiently simulating airflow in the built environment. All three modes exhibit faster computations than standard initialization methods by enabling quicker attainment of desired flow states and reducing the required matrix iterations. These initializers demonstrate better performance in flow problems characterized by higher Reynolds numbers. The CMM initializer achieves a nearly six-fold increase in speed by mapping initial solutions from a coarse mesh to a fine mesh. The LTM initializer allows for larger timesteps during initialization, resulting in a two-fold speed improvement in fluid simulation. The HM initializer combines the advantages of both CMM and LTM. However, it does not surpass the speed-up performance of the previous two modes. Apart from the benchmark test case, the proposed initializers can be applied to other indoor and outdoor flow scenarios. Furthermore, the simulation process is further streamlined by automating the execution of the initializer using scripts, eliminating the need for manual step-by-step inputs. |
| 關鍵字： | CFD; Simulation acceleration; Urban wind; Ventilation |
| 著作名稱： | Effects of urban tree planting on thermal comfort and air quality in the street canyon in a subtropical climate |
| 年度： | 2023 |
| 類別： |
期刊論文
Sustainable Cities and Society
|
| 摘要： | Planting trees is considered to relieve the thermal load. However, trees may "pollute" the air quality. Tree net effects should receive more attention, but only a few studies have simultaneously addressed thermal comfort and air quality. By computational fluid dynamics (CFD) simulations, we created an evaluation model of trees and investigated the overall effects of tree planting inside street canyons. We considered the following four parameters by comparing seven common tree species in Hong Kong (subtropical climate): three tree morphological indicators (leaf area index (LAI), tree height (Htree), and crown spread (Scrown)) and tree planting density (Ptree). The results demonstrated that under a high ambient wind speed, higher trees with a lower near-ground leaf area density are better options, which more greatly reduces the physiological equivalent temperature (PET) by up to 1.1 K but causes the least pollutant accumulation. Bigger-crown trees on the windward side are advocated, while smaller ones are suitable for the leeward side. In such a way, there was at least a 0.7 K decrease in PET on the tree-planted side while maintaining better air quality. Thermal comfort can be improved by increasing LAI or Ptree, but a higher LAI or Ptree causes a greater accumulation of pollutants. Increasing Ptree can cause a 1.1 K reduction in PET, while the effect of changing LAI is relatively limited. When the length of a street is reduced, varying tree factors have a comparable effect. As canyon depth increases, the effect of trees is limited. |
| 關鍵字： | Air quality, Outdoor thermal comfort, Computational fluid dynamics, Tree planting, Urban design |
| 著作名稱： | Numerical and experimental investigations on melting heat transfer performance of PCM in finned cold thermal energy storage |
| 年度： | 2023 |
| 類別： |
期刊論文
International Journal of Heat and Mass Transfer
|
| 摘要： | Cold thermal energy storage (CTES) is of great importance for the enduring decrease in fossil fuel energy consumption. Moreover, CTES with phase change materials (PCMs) can be an effective measure to accumulate the heat or cooling energy for overcoming the mismatch between the supply and demand of air conditioning loads, augmenting system dependability and flexibility in operations of power grids. This paper numerically and experimentally examines the melting characteristics of PCM inside a CTES tank with the installation of interior fins. The computational fluid dynamics (CFD) software ANSYS/Fluent® is applied to predict the time sequences of liquid fraction contours and temperatures in comparison with the photographed images of ice-liquid water interface and measurement data in the CTES tank to verify the accuracy of CFD predictions. The simulations by the validated CFD model are extended to evaluate the influences of fin height and total number of longitudinal fins on the heat transfer outcomes in the storage tank. The present study further proposes a novel design adopting stratified fins to enhance the melting performance of the CTES device. The estimated mean power with the new stratified fin design is notably greater than that without fins by 156.3%, achieving the effectively liquefaction enhancement of ice for guiding the development of finned cold thermal energy storage. |
| 關鍵字： | Melting process;Phase change materials;Fin structure;Cold thermal energy storage;Demand side management;CFD simulation |
| 著作名稱： | Effects of control zone, exhaust rate and station extent on smoke dispersal during emergency concourse fires in underground stations |
| 年度： | 2023 |
| 類別： |
期刊論文
International Journal of Thermal Sciences
|
| 摘要： | Effective mass rapid transit plays an essential role in public transportation for sustainable urban development. This study investigates the temporary spreading behavior of smoke in the concourses of underground stations. The computational approach implements the software package FDS® 6.7.4 along with a post-processing visualization module (Smokeview) to simulate the smoke dispersal processes for determining the unsteady distributions of temperature and visibility during emergency concourse fires. The interfacial temperature between the lower air layer and upper smoke layer is employed to estimate the smoke layer height, as guided by NFPA-92B. The FDS predictions agree reasonably well with the measured fire plume temperatures at the heights of 7.0 m and 13.0 m above the fire source from the available literature in the time duration of 600 s for model verification. FDS calculations are then extended utilizing the validated computational model to explore the effects of the control area, exhaust rate and station size on the smoke spreading progressions. The simulated results reveal the effectiveness of installing static barriers to restrict smoke dispersal within a specific control space; however, the implementation of suitable mechanical exhaust system is required to extract the dense smoke out of the concourse. For the scenario of a high volume MRT underground station, the design adopting the smoke control area of 20 m × 15 m in conjunction with the exhaust rate of 15 m3/s can attain clear visibilities for emergency evacuation. However, the simulations show the inappropriateness of arranging the same design layout of control area and exhaust rate as safety measures of underground fires for a medium volume station. |
| 關鍵字： | Computational methods; Fires; Rapid transit; Smoke abatement; Subway stations; Urban growth; Urban transportation; Visibility |
| 著作名稱： | Effect of void space arrangement on wind power potential and pressure coefficient distributions for high-rise void buildings |
| 年度： | 2023 |
| 類別： |
期刊論文
Journal of Building Engineering
|
| 摘要： | High-rise void building complexes can augment the permeation of airflows over buildings, and thereby strengthen urban wind energy harvesting for realization of the sustainable development goals (SDGs). This paper considers the wind over a typical 2 × 2 compact high-rise building array with the openings to analyze the wind power potential at varying void sizes, locations of voids, and direction of incoming wind. The predicted mean wind speeds and turbulence intensities are compared against the measured data with 4 generic high-rise building models having voids in an atmospheric boundary layer wind tunnel for validation of the computational model. Based on the comparative results from four common Reynolds-averaged Navier-Stokes (RANS) turbulence models including the standard, realizable, renormalization group (RNG) k-ε, and shear-stress transport (SST) k-ω models as well as the Reynolds stress model (RSM), the RSM approach can provide the most accurate predictions of streamwise mean velocity and turbulence intensity. The present study then conducts CFD simulations to explore the effects of opening configuration and wind direction on the wind field around high-rise void buildings. Considering the variations of void sizes of 10 × 10 m2, 12.5 × 12.5 m2, 15 × 15 m2, void heights of z/H 0.25, 0.5, 0.75 and wind directions of 0°, 45°, respectively, the simulated results indicate elevated wind power densities with acceptable turbulence intensities over the upstream gap passage and void channels to achieve prospective urban wind power harvest and mitigate wind load on structures of high-rise void buildings. |
| 關鍵字： | CFD simulation; Urban wind energy; Void buildings; Wind engineering; Wind-tunnel experiment |
| 著作名稱： | Effect of wind-based climate-responsive design on city breathability of a compact high-rise city |
| 年度： | 2023 |
| 類別： |
期刊論文
Journal of Building Engineering
|
| 摘要： | As urban development advances, there is an increasing interest in compact cities as the most sustainable form. To suggest a pronounced connection between dense urban morphology and wind environments, this study adopts the new approach of wind-based climate-responsive design to investigate the influences of urban density, building corner modification, and wind direction on urban ventilation outcomes for the enhancement of city breathability. The airflow rates are estimated using a high-rise building array layout to optimize urban penetrability for enhancing breathability. The results of the CFD parametric study indicate that compact urban layouts with larger λp values experience good infiltration rates, ensuring wind comfort at the pedestrian level. Conversely, sparse urban forms with reduced λp values can achieve better permeation outcomes, resulting in elevated wind speeds at the middle level and near the roof of buildings, as well as higher airflow rates through the urban canyon layer. Additionally, the implementation of round corners reduces average wind speeds at the pedestrian level by 39% and increases wind speeds above the middle level of high-rise buildings by 20%, as compared to those of sharp corners. These findings offer valuable insights for high-rise building planning strategies, serving as both immediate and long-term solutions to realize the sustainable objectives of wind-based climate-responsive design in compact cities. Present research has offered the multifaceted trends of wind energy assessments on the realization of urban wind power. This paper also provides a theoretical framework that can assist urban designers in implementing the wind-based climate-responsive designs and enhancing living environments in contemporary compact cities. |
| 關鍵字： | Compact city; Urban configuration; Urban morphology; Urban outdoor ventilation; Urban sustainability |
| 著作名稱： | Heat transfer of bubbly flows in microchannels at varied aspect ratios and hydraulic diameters |
| 年度： | 2023 |
| 類別： |
期刊論文
International Journal of Heat and Mass Transfer
|
| 摘要： | With the fast progress of new technologies applied to cloud computing, artificial intelligence, and fifth generation communication, the researches on the heat dissipation of high-power electronic devices have concentrated on the two-phase flows in the microchannel, with the phase change used to achieve excellent cooling efficiency. The objective of this paper is to conduct the computational and experimental studies for investigating the thermal-fluid behaviors of subcooled flow boiling in microchannels. In the computational fluid dynamics (CFD) model, the numerical solver is built in ANSYS/Fluent® implementing the multiphase formulation with the full consideration of the effects of turbulence, surface tension, and phase change to appropriately imitate the interfacial movements between liquid water and vapor for probing the subcooled boiling and bubble nucleation processes over the microchannel. In the experimental investigation, a syringe pump and a programmable DC power supply is employed to regulate the heat flux and subcooled temperature for determining the heat transfer and pressure drop outcomes across the microchannel at varied aspect ratios and hydraulic diameters. In addition, this study sets up a high-speed camera with a LED fiber optical light source to acquire the close-up observation images over the complex flow boiling phenomena. The accuracy of CFD predictions is assessed by comparison against both the measured heat transfer coefficients and pressure drops as well as the photo-captured interfacial behaviors. The numerical simulations are then conducted to resolve the temperature gradients on the heating wall surface due to intense disturbances incited by the confined bubbly flow and sweeping flow at high aspect ratios, enhancing the cooling performance over the microchannels. The experimental measurements show that a reduction in inlet subcooling from 65 to 50 °C tends to enlarge the average heat transfer coefficient and pressure drop by 21.5 and 17.1%, respectively. The design impact study also reveals the average heat transfer coefficient and pressure drop at an aspect ratio of 5 greater than those at an aspect ratio of 2 by up to 41.1 and 27.2%, respectively. In contrast, increasing hydraulic diameter from 0.92 to 1.38 mm can strengthen the thermal and frictional outcomes by 17.2 and 20.3%. The microchannel design having an aspect ratio of 5.0 can realize the estimated coefficient of performance (COP) up to 116,277.8, achieving the satisfactory overall thermal and frictional outcomes. |
| 關鍵字： | Computational fluid dynamics; Microchannel; Subcooled boiling; Vertical upflow |
| 著作名稱： | Numerical and experimental investigations on thermal management for data center with cold aisle containment configuration |
| 年度： | 2022 |
| 類別： |
期刊論文
Applied Energy
|
| 摘要： | This study proposes the container data center with the featured cold aisle containment (CAC) as effective thermal control strategy. In design, the overhead downward flow system is implemented with a heat exchanger arranged right above the data center on the air side and an evaporative water chiller on the water side to form the cooling approach. The cold airflows and hot exhausts of racks are separately transported by the contained cold and hot aisles to alleviate the problem of cold and hot air mixing. The measurements of air temperature and velocity of racks are used to validate the prediction accuracy of the computational fluid dynamics (CFD) model. The performance metrics in terms of the rack cooling index (RCI), return temperature index (RTI), supply heat index (SHI) are used to examine the design effectiveness of the proposed test data center. The simulations are then extended to assess the air distribution and thermal management at varied supply air temperatures and velocities for a large-scale data center to be built in the green energy technology demonstration site of the Shalun smart green energy science city. Overall, the calculated average PUE of 1.38 for the large-scale data center is notably less than the average PUE of 1.59 from the results of 2020 data center industry survey, indicating the potential savings of cooling energy and cost. This paper demonstrates a generalized approach as an easily adaptable, cost-effective solution for data centers to be deployed in tropical and subtropical areas. |
| 關鍵字： | Data centerAirflow managementRack cooling indexReturn temperature indexSupply heat indexCFD simulation |
| 著作名稱： | Air side performance characterization of wavy Fin-and-tube heat exchangers having elliptic tubes with large waffle heights |
| 年度： | 2022 |
| 類別： |
期刊論文
Applied Thermal Engineering
|
| 摘要： | Demand of less energy consumption and higher energy efficiency suggests increasing heat transfer performance of a fin-and-tube heat exchanger. This paper experimentally and numerically examines the air side performance of wavy fin-and-tube heat exchangers (FTHXs) with elliptic tubes. In a properly regulated and evenly distributed airflow supply loop, the inlet/outlet temperatures and pressure drops across the test section are obtained using two measuring meshes and a differential pressure transducer to resolve thermal and frictional outcomes in terms of the heat transfer coefficient and pressure drop. The theoretical formulation considers the thermo-fluid analysis of crossing airflows over the wavy finned tube heat exchanger. In this research, the test heat exchanger contains elliptic tubes incorporated with 4-row wavy fins, while the proposed design adopts a large setting of 3.24-mm waffle height and 3.02-mm fin pitch, respectively. The predicted heat transfer coefficients and pressure drops are in good agreement with the measured data at varied inlet air velocities to validate the CFD model. Numerical simulations are then extended to investigate the influences of waffle height, fin pitch, wave length and inlet air velocity on thermofluid characteristics of FTHXs. Six common correlations of Colburn factors (j) and friction factor (f) factors are compared with predictions to evaluate their applicabilities for reasonable estimates of thermal and frictional performance of FTHXs. It is noticed that the correlation by Wang et al. can achieve the most accurate results in calculating the j and f factors. |
| 關鍵字： | Fin-and-tube heat exchangerAir side performanceHeat transferElliptic tubeCFD Simulation |
| 著作名稱： | Characterization of Melting Process of PCM in Cold Thermal Energy Storage Tanks with Varied Fin Configuration |
| 年度： | 2022 |
| 類別： |
會議論文
|
| 摘要： | Cold thermal energy storage (CTES) is of enormous significance for the continuing reduction in fossil energy utilization. Moreover, CTES with phase change materials (PCMs) can be a crucial solution to solve the disparity between the supply and demand of energy. This paper numerically investigates the melt enhancement of PCM in a CTES unit through the installation of internal fins. To validate the computational model, the predictions of time sequences of liquid fraction contours of PCM agree reasonably well with the visualized images inside the CTES tank. The effect of fins on the melt progression is then examined. The CFD results indicate that the installation of fins can enlarge heat transfer areas to enhance conductive heat transfer for positively accelerating the PCM melt event. The melting time for the scenario with fins is 38.4% faster than that without fins. |
| 關鍵字： | Cold Thermal energy storage (CTES), Phase change material (PCM), CFD simulation |