Field Synergy Number Research Articles

Experimental and numerical investigations of tube inserted with novel perforated rectangular V-shape vortex generators

The utilization of punched vortex generators has been established as an effective means to enhance heat transfer in heat exchangers. However, there are few studies on the use of perforated rectangular winglet pairs in heat exchange tubes and the study of perforation parameters based on this approach in the existing literature. In this paper, three parameters, including three height ratios, four perforation indices, and three pitch ratios, are considered experimentally and numerically. In the Reynolds number range of 7454–13,664, the effects of perforated rectangular vortex generator pairs on fluid flow and heat transfer are obtained by scaling parameters such as Nusselt number ratio, friction factor ratio, thermal enhancement factor, field synergy number, entropy generation, and exergy efficiency. The numerical results indicate that the perforation will lead to a reduction in turbulent kinetic energy in the local multi-longitudinal vortices on the rear side of the vortex generator, resulting in a decrease in local heat transfer as well as friction loss. Meanwhile, the pitch of perforated rectangular vortex generator pairs plays a crucial role in heat transfer and friction loss, enabling an increase in the Nusselt number of smooth tubes by 68–257%. Furthermore, the results are proved using the principle of entropy generation and exergy efficiency analysis. As Reynolds number increases, the thermal enhancement factor gradually decreases. Notably, the maximum thermal enhancement factor of 1.43 is achieved when the Reynolds number is equal to 7454, the height ratio is 0.08, the perforation index is 0.18, and the pitch ratio is 1.04.

Effect of structure parameters on thermal performance of novel punched V-shape VGs for hear exchangers: An experimental approach

The use of perforated technology on the surface of the vortex generator (VG) has been proven to further enhance heat transfer performance by reducing its pressure loss. Based on this technology, a new type of turbulence insert is proposed in this work, which is composed of a V-shaped fixed ring with perforated rectangular VGs. The influence of its number (N = 2, 4, 6), VG area (S = 10, 20, 30 mm2), and pitch (P = 25, 50, 75 mm) on heat transfer and pressure drop under turbulent conditions was investigated through experiments. The results show that the influence of N and S on heat transfer intensity and pressure drop is consistent, but the value of P is inversely proportional to pressure drop and heat transfer intensity. Regardless of the influence of parameters on heat transfer intensity, it is explained by the field synergy number. Within the parameter range studied in this article, the optimal TEF value of 1.42 is attained when S=10mm2, N = 6, and Re=7454.

Buoyancy and Velocity Field Synergy Principle in Convective Heat Transfer and Its Role in Thermo-Hydraulic Performance Improvement

Abstract This study proposes the buoyancy and velocity field synergy principle and aims to enhance thermo-hydraulic performance in convective heat transfer. A mechanical energy conservation equation concerning synergy between buoyancy and velocity was derived, which describes the mechanical energy transport and dissipation in convective heat transfer. Two new field synergy numbers, FsU,g and FsU,∇p, were proposed to characterize the degree of synergy between velocity and buoyancy, and the degree of synergy between velocity and pressure gradient over the fluid domain, respectively. The pressure drop of a channel subjected to convective heat transfer is related to not only Gr/Re2 but also FsU,g. Under a same Gr/Re2, a larger |FsU,g| leads to a smaller |FsU,∇p|, and thus the pressure drop is decreased. Furthermore, the multifield synergetic relationships among buoyancy, velocity, temperature gradient and pressure gradient were analyzed for convective heat transfer in channels. The correlation between FsU,∇p and (Gr/Re2)FsU,g, and the correlation between FsU,g and a traditional field synergy number characterizing convective heat transfer capability, Fc, were derived, which reveals the coupled mechanisms of mechanical energy dissipation and thermal energy transport in convective heat transfer. The proposed principle was applied in typical channel flows subjected to convective heat transfer, and its benefits were demonstrated. It is noted that both pressure drop reduction and convective heat transfer enhancement can be achieved in convective heat transfer using the proposed principle. This paper provides a new insight for improving thermo-hydraulic performance of heat exchangers.

Field synergy analysis on thermal-hydraulic behavior of phase change slurry in porous microchannel heat sink with graded porosity configuration

In this study, a novel design of porous microchannel heat sink (MCHS) with graded porosity configuration is proposed in the hope of attaining lower flow and thermal resistances than constant porosity (CP) configuration. The efficacy of new scheme is validated by utilizing a three-dimensional solid-fluid conjugate model based on the finite volume method, which considers the flow and heat transfer of microencapsulated phase change material slurry (MPCMS) inside the porous fins. It is reported that the thermal performance of porous MCHS with constant and graded porosity configurations is significantly better than that of non-porous MCHS. The feasibility of new design is verified by comparing the hydrothermal behavior of multiple graded porosity MCHSs and CP configuration for different fin designs (involving the number of graded equidistant fins N, graded length ratio R, and fin-to-pitch width ratio β) and operating conditions (including Reynolds number Re, mass fraction cm, and inlet velocity uin). The comprehensive performance improvement mechanism of MPCMS in graded porosity microchannels is discussed through the analysis of flow and thermal details, field synergy angle (θ) and field synergy number (Fc), and the overall performance of new design is evaluated by performance evaluation factor (PEF). Results indicate that the six types of porous MCHS with stepwise varying porosity exhibit lower thermal resistance than CP mode, and better thermal behavior and larger PEF can be obtained at N = 2 and β = 0.7. Smaller and larger R should be chosen for stepwise increasing porosity (SIP) and stepwise decreasing porosity (SDP) modes, respectively, to achieve higher heat transfer enhancement while avoiding greater friction losses. The Fc value of porous MCHS with various porosity configurations is about two orders of magnitude less than 1 and shows a monotonically reducing trend with Re, and the synergistic effect deteriorates at high Re. Compared to the CP mode, a low-θ “drift” toward the low porosity region of porous fin occurs within the graded porosity MCHS, which decreases and increases in the downstream and upstream directions of the “drift”, respectively. Among the novel MCHS designs, the z-directional SIP mode acquires the best overall performance due to the excellent synergy between the velocity and temperature fields, and its PEF consistently exceeds 1.13 and reaches up to 1.33. Generally speaking, the fin porosity of graded porosity MCHS near the channel cavity, channel top, and channel inlet should be greater for superior comprehensive performance.

Numerical study on flow and heat transfer characteristics of rectangular mini-channel of interpolated double S turbulators.

In this study, we propose a new type of small-channel plug-in, the double S turbulators, for passive heat transfer enhancement to improve the flow and heat transfer performance of the fluid in the channel. In the range of Reynolds number 254.51~2545.09, under constant wall temperature heating conditions, the effects of interpolated double S turbulators with different long axial radii (1mm, 1.5mm, 2mm) on the average Nusselt number, pressure drop, total thermal resistance and field synergy number in the rectangular mini-channel were studied. The simulation results show that compared with the smooth rectangular mini-channel, after interpolating double S turbulators with different long axial radii (1mm, 1.5mm, 2mm), the average Nusselt number increased by 81.74%~101.74%, 71.29%~94.06%, 67.16%~88.48%, the total thermal resistance decreased by 45.1%~50.72%, 41.72%~48.74%, 40.28%~47.2%, and the number of field synergies increased by 85.58%~111.65%, 74.1%~102.6%, 69.64%~96.12%. At present, there are few studies on the boundary condition of constant wall temperature, and this paper supplements the research on this aspect. At the same time, the heat transfer performance of the rectangular mini-channel of the interpolated double S turbulators is stronger than that of the ordinary smooth rectangular mini-channel, which not only provides a new idea for the manufacture of micro heat dissipation equipment, but also improves the heat transfer performance of micro heat dissipation equipment and improves its work efficiency. According to the simulation data, the prediction formula of average Nusselt number and pressure drop was established by nonlinear regression method, which can be used to predict the flow and heat transfer characteristics of the rectangular mini-channel of the interpolated double S turbulators.

Micro-channel topology optimization based on enhanced heat transfer mechanism

Topology optimization modifies the material distribution in the design domain to produce micro-channel structure with improved thermal performance. In this work, five heat dissipation micro-channel structures with various design domain aspect ratios are optimally designed based on the bi-objective topology optimization method. The optimal design variable fields, temperature fields, and pressure fields are subsequently obtained for each operating condition, and the flow heat transfer effect and the enhanced heat transfer mechanism are investigated under various working conditions. On this basis, the flow heat transfer impact of micro-channels under various operating situations is optimized and studied by combining the field synergy concept and entransy dissipation theory. The findings show that when the Reynolds number rises in the laminar flow region, the complexity of the topological flow channels also rises. The average temperature, Tave,decreases, Nusselt number rises, the inlet and outlet pressure drop, ?P, gradually increases, the integrated enhanced heat transfer factor PEC gradually decreases, the field synergy number, Fc, increases, the pressure drop synergy angle, ?, gradually increases, the entransy dissipation, Evh, increases, and the flow heat transfer performance of each heat dissipation channel is also enhanced due to the complex channels and high Reynolds number in the domain. The investigation of micro-channels with various topologies revealed that the micro-channels with the same boundary conditions and a design domain aspect ratio of 25/64 had the best synergy effects of velocity-pressure gradient and velocity-temperature gradient, the best heat transfer effect, and the best flow characteristics.

Optimal design of liquid cooling structures for superfast charging cable cores under a high current load

Superchargers have become a focus of much research into new-energy vehicles, for which the cooling of high-current cable cores is a key problem that needs to be solved. To estimate influences of different core structures of liquid-cooled cables on the fluid flow and heat transfer characteristics in circular pipes, nine helical cable core structures with insertion of smooth pipes were designed taking dimethyl silicone oil as the coolant. The fluid flow and heat transfer in the pipes were studied through computational fluid dynamics (CFD) numerical simulation at different inlet velocities (0.2–2.0 m/s), and the models were verified by building an experimental platform. Numerical calculation results show that under conditions of the same Re and pitch of cable cores, the average surface temperature of three-core cables is lower than those of single-core and double-core cables. Moreover, the mean flow velocity v in pipes, heat transfer performance Nu, resistance factor f, evaluation factor for comprehensive heat transfer performance (PEC), and field synergy number Fc all increase with the decrease in the pitch of cable cores. This finding indicates that the heat transfer effect is gradually enhanced. The research implies that when the pitch p is 22.4 mm (cable C6), the velocity field is the most synergetic with the temperature field and the comprehensive heat transfer performance is optimal. This research provides an excellent cooling scheme for cable cores in superchargers under a high current load.

Power consumption and thermal performance of tube employing punched delta winglets

The method of enhancing heat transfer (HT) by combining jets and main longitudinal vortex (MLV) had been proposed and proven effective. Based on this method, this article further explored the interference effect of VG structure on jets, MLVs, and other forms of vortices, as well as the corresponding HT mechanism. The punched delta winglet vortex generators (PDWVGs) applied in this work had three inclined angles (α=30°,60°,90°), three numbers (N=2,4,6), and three pitch ratios (PR=4.89,3.27,1.63). The secondary vortices number (Se values were used to measure the strength and position of different vortices, and the field synergy number (Fc) was also applied to measure the underlying causes of HT rate. The results confirmed that MLVs (induced by PDWVGs in core-flow region) were created, which improved the mixing rate of the air. It was worth mentioning that the structure of MLVs and jets was different at different α values, specifically when α>30°, the jets would carry away heat from the wall surface and would also collide with the reflux MLVs, suppressing the further formation of the reflux region. The Se values of different cases clearly demonstrate the morphology and intensity of mixed vortices. Within the studied parameter range, the biggest TEFof 1.29 was observed at N = 2, PR = 1.63, α = 30° at Re=9090.

Numerical investigation of turbulent flow and heat transfer characteristics in the improved Kenics static mixer

AbstractThis study focuses on using computational fluid dynamics to explore how well a modified static mixing component called the Kenics static mixer (KSM) increases heat transfer. The purpose of the study is to assess this new static mixer's pressure drop and heat transfer properties. Convective heat transport of water within the improved Kenics static mixer (IKSM) was examined using three‐dimensional turbulent, steady, and incompressible flow simulations. The Nusselt number and friction factor simulation results in the KSM are in good agreement with previous literature findings. Between the simulated outcomes of the IKSM, empty pipe, and standard KSM, a comparison analysis was done. The study considered a range of Reynolds number (Re) values from 2000 to 20,000 and investigated the effects of the Re, slit width configuration (), as well as an aspect ratio () on heat transfer and mixing. Based on the performance evaluation criteria and field synergy number characteristics, the hydrothermal efficiency of the IKSM was evaluated, allowing the geometry to be modified for better heat transmission while lowering the friction factor. Notably, in the 2000–20,000 Reynolds number range, the IKSM with four slits showed a 38%–52% greater heat transfer coefficient than the traditional KSM.

Ionanofluid flow through a triangular grooved microchannel heat sink: Thermal heightening

With recent technological advances, thermal transport from different electronic and electrical devices is the most vital concern. The microchannel heat sink (MCHS) of liquid cooling is a useful device to remove over thermal load. Ionanofluid is a brand new and super potential cooling fluid for its ionic conductivity, non-flammability, negligible volatility, and high-level heat stability. In this research, the ionanofluid's velocity and thermal field characteristics through a triangular grooved MCHS are investigated using numerical tools. The combination of ionic liquid (IL) 1-Butyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide [C4mim]NTf2 and propylene glycol (PG) is used as base fluid whereas graphene (G) and single-walled carbon nanotube (SWCNT) are chosen as hybrid nanoparticles to make the working ionanofluid. The governing equations of nonlinear partial differential equations describing the physical phenomena along with proper border settings are resolved by applying the finite element method (FEM). Different ratios of hybrid nanoparticles (G: SWCNT) like (1: 0, 1/3: 2/3, 1/2: 1/2, 2/3: 1/3, 0: 1) are suspended in the base fluid mixture. In addition, the base fluid mixture is assumed in different combinations of (IL: PG) as (100: 0, 50: 50, 0: 100). The numerical results are displayed in the forms of streamlines, isothermal lines, and rate of thermal transfer for the pertinent parameters namely forced convection (Re = 100–900) and solid concentration (φ = 0.001–0.05). Also, pressure drop, field synergy number, relative fanning friction feature, relative Nusselt number, and temperature enhancement efficiency are calculated. The results indicate that a higher heat transport rate is found using the IL-based ionanofluid with the highest solid concentration. Moreover, the higher forced convection enhances the thermal efficiency of MCHS. Two linear regression equations along with very good correlation coefficients have been derived from the numerical results.

Microfluidic silicon interposer for thermal management of GaN device integration

With the popularity of the third generation semiconductor applications, silicon-based hetero-integration is driven to be applied massively by the miniaturization of microsystems and modules. The system-level power has a sharp increase in recent years, and consequently, hotspot thermal management is being increasingly concerned. A high-power GaN device with chip heat fluxes > 500 W/cm2 and hotspot heat fluxes > 30 kW/cm2 is implemented in this work to study extremely uneven hotspot cooling for silicon-based hetero-integration. A customized microjet silicon interposer (SI) is designed and fabricated based on the on-chip hotspot distribution, and another common microchannel SI in similar sizes is prepared for comparison. The GaN device is integrated on SI and micro-assembled in a test cube. The measurement results demonstrate that the maximum junction temperature of the GaN device on microjet SI is down to 150.3 ℃ at 70 ℃ ambient temperature with a pressure drop of 231.9 kPa. The overall thermal resistance (TR) of the test vehicle with microjet cooling has a 7.11 ∼ 7.72 % reduction and its heat transfer coefficient (HTC) increases by 12.3 ∼ 28.3 % compared to that with microchannel cooling with the pumping power of 0.5 ∼ 1.5 W. The material properties of the GaN chip and SI are extracted from the measurement results and then used in the analysis of the thermal and hydraulic properties of both microfluidic SIs numerically. Their cooling performance and efficiency are evaluated and compared based on the performance evaluation criteria (PEC), field synergy number and entropy generation rate. According to the evaluation results, while the PEC of microjet and microchannel SIs increases with the pumping power, the microjet SI can contribute to the enhancement of the synergetic relationship between the flow and temperature fields and reduction of total irreversibility in the heat transfer process compared to the microchannel SI in pursuit of low overall TR. The field synergy number improves by 90.40 % and the argument entropy generation rate is 0.376 when the overall TR is 1.35 K/W. The findings can provide valuable references for hotspot thermal management in silicon-based hetero-integration.

Field-synergy and nanoparticle’s diameter analysis on circular jet impingement using three oxide–water-based nanofluids

The field synergy study is carried out using three oxide nanofluids impinging circular jet on the horizontal circular disc to analyse the synergetic interaction of cooling processes between temperature and flows fields. The h eat transfer effect o f the nanofluid is examined by rising the Reynolds number and the nanoparticle concentration depending on field synergy number. For jet impinged cooling process, the scale of synergy between the nanofluid flow speed and temperature is decayed with the increase of Reynolds number. Hence, it is contributed to a lower heat transfer efficiency of the nanofluid. Whe reas, the scale of synergy between the nanofluid flow speed and temperature can be enhanced by rising the particle concentration. Thus, the heat transfer efficiency of the nanofluid is increased. Analysis showed that Al2O3 nanofluid has the maximum relative field synergy among selected three oxide nanofluids. It is evident that the nanoparticle concentration, nanoparticle material and Reynolds number have significant effect on the heat transfer augmentation. In addition, the study is explored by varying jet-disk spacing. Moreover, the investigation has shown that the reducing heat transfer effect for the materials is Al2O3, CuO and TiO2 subsequently. It is revealed that the heat enhancement is higher for smaller nanoparticle’s diameter (i.e., 20 nm) than bigger nanoparticle’s diameter (i.e., 80 nm) of the same material.

Experimental and numerical study on the heat transfer and flow characteristics of convex plate heat exchanger based on multi-objective optimization

The convex plate heat exchanger is one of the new types of plate heat exchangers that have specific functional properties compared to conventional plate heat exchangers, the structural parameters of convex plate heat exchangers are important in affecting the heat transfer performance. In this paper, based on two-layer multi-objective optimization which is aimed at maximum performance evaluation criterion (PEC) and maximum field synergy number (Fc), the heat transfer and flow characteristics of convex plate heat exchanger are investigated experimentally and numerically. The optimization findings are broken down into single factor, response surface, and sensitivity analysis. The optimal results under different objective functions are analyzed and the optimal structural parameters obtained which are the welding hole radius (R) is 5.81 mm, the ellipse radius (L) is 5.02 mm, and the welding hole spacing (P) is 64.00 mm; Compared to the structure before optimization, the size-optimized structure may enhance the temperature differential between the inlet and outflow of the cooling water by less than 8.27%, the heat transfer performance is increased by 20.43%, and the comprehensive performance is improved by 2.3∼19.59%.

Investigation on the effects of non-uniform porosity catalyst on SCR characteristic based on the field synergy analysis

Selective catalytic reduction catalyst was commonly used in the denitrification of exhaust from the internal combustion engine. In the paper, a three-dimensional mathematical model was developed and employed to study the flow and heat transfer in the non-uniform selective catalytic reduction catalyst system. The denitrification reaction of selective catalytic reduction catalyst was employed to predict the NOx conversion efficiency. In addition, the field synergy principle is employed to analyze the effects of velocity and temperature fields on the heat transfer process. The results show that the non-uniform porosity catalyst can effectively reduce the catalyst back pressure. At 100 g/s, the pressure drop in the non-uniform porosity catalyst was 0.91 kPa lower than the high porosity catalyst. The total heat transfer rate of the non-uniform porosity catalyst was on average 1.1% lower than the uniform porosity catalyst. The low heat transfer rate ensures that the temperature drop is reduced. Thus, the denitrification reaction is improved by the high temperature. In addition, the field synergy number of non-uniform porosity catalysts is on average 0.53% lower than uniform porosity catalysts, which indicates energy transfer from the diameter change region to the outside is reduced. This proves the non-uniform porosity structure is more favorable for the denitrification reaction.

Effect of Wavy Wall and Plate Bifurcations on Heat Transfer Enhancement in Microchannel

Abstract Miniaturization of electronic components requires compact and effective cooling techniques to dissipate large heat flux without a significant increase in pumping power. Microchannel heat sink with liquid as working fluid is a suitable technique for the purpose. In this study, heat transfer characteristics in presence of vertical bifurcation placed downstream of the microchannel passage are studied numerically. Six types of bifurcating plates are considered under two categories: (i) thick-plate and (ii) wavy thin-wall. Water is taken as the working fluid and the flow rate has been varied in the Reynolds number range, 100 ≤ Re ≤ 1000. The effect of bifurcations on pressure drop, heat transfer, and the overall thermal resistance are analyzed and compared with those of plane microchannel without bifurcation. The numerical results show that the usage of bifurcation in the microchannel reduces the overall thermal resistance. Field synergy number, entropy generation number, and hydrothermal performance index are calculated to quantify the overall performance improvement in the microchannel with bifurcations. Constant wavy thin-wall bifurcation has been found to improve the overall performance of the microchannel. The detailed geometry of the bifurcation, the resulting convective heat transfer characteristics, and percentage improvement in the performance are reported.

Optimal shape design and performance investigation of helically coiled tube heat exchanger applying MO-SHERPA

In this paper, the corrected thermodynamic model of the heat exchanger is proposed, the application of the Entransy Loss Theory in the helically coiled tube heat exchanger is modified, three modeling methods are proposed and selected, Entransy Loss Number (NgHE) combining with Performance Evaluation Criteria (PEC) and Field Synergy Number (Fc) as the objective function, the numerical method verified by the experiment combined with MO-SHERPA is used to carry out the geometry optimization with 120,000 sets of different cross-sections under the same heat exchange area. The results show that the proposed modeling method combined with MO-SHERPA can effectively avoid sharp phenomena in the structure, and which has obvious advantages in terms of optimized structure search and optimized structure search speed; Multi-objective optimization results in optimal solutions for different objectives, making the selection of optimization results more flexible under different working conditions; The comparative analysis of different heat exchangers under the same heat exchange area shows that the PEC of the tube side is increased by 22%, and the NgHE can be reduced by up to 66%; the PEC of the shell side is increased by 4% ∼ 20%.

Hydrothermal Investigation of the Performance of Microchannel Heat Sink with Ribs Employed on Side Walls

Abstract In the present study, conjugate heat transfer and fluid flow performance of microchannel heat sink has been investigated using dimensionless parameters. Novel ribs of four different types are introduced on the side walls of channel, which include trapezoidal ribs, rectangular ribs, hydrofoil ribs, and elliptical ribs. The performance evaluation has been conducted by comparing friction factor (f), Nusselt number ( N u Nu ), fluid bulk temperature ( T f {T_{f}} ), wall shear stress (τ), field synergy number ( F c Fc ), irreversible heat loss ( Q d {Q_{d}} ), and Bejan number ( B e Be ) in a Reynolds number, ranging from Re = 100 \mathit{Re}=100 to Re = 1000 \mathit{Re}=1000 . The results revealed that the addition of these novel ribs are helpful in improving the overall thermal and hydraulic performance of microchannel heat sink. From the results of Bejan number, it has been revealed that more than 96 % of losses are because of heat transfer. However, at low Reynolds number, the frictional losses can be neglected, because of very low fluid velocity. Moreover, it has been revealed that synergetic relation between velocity and temperature gradient becomes weaker at higher Reynolds number. Furthermore, it is clear from this study that elliptical ribs performed better in thermal aspects, whereas hydrofoil ribs performed better at hydrodynamic aspects.

Numerical study on uniformity of temperature difference field in a spiral tube heat exchanger

• Two equations are developed to construct the temperature difference field. • Two methods is proposed to measure the uniformity of continuous temperature field. • The proposed methods can be applied for other real-world applications. • New methods can characterize the heat transfer efficiency. The characteristic of heat transfer of an organic working fluid driven by a built-in spiral low-temperature heat source is investigated numerically. Field synergy number is a function of the heat transfer factor j with the coefficient of correlation (r) all above 0.9. The temperature difference field is built based on the time space and position information about the objective. New metrics for quantifying the uniformity parameters of continuous temperature fields are developed by temperature difference field modeling. Those metrics are illustrated by using numerical modeling and present significant correlation between field synergy number or heat transfer factor. Furthermore, both the presented temperature field data and the other numerical temperature field is used to verify the effectiveness of this method. This method not only can significantly quantify variation on the uniformity of a temperature difference field under the same working conditions, but can also characterize the heat transfer efficiency at different times under the same working condition. This shows that the uniformity factor established in this study can reflect the heat transfer efficiency of heat exchanger as a whole, can be used as a new evaluation index of continuous field heat transfer efficiency, and can provide a new idea of the synthesis and optimization of heat exchanger.

Assessment of pollutant dispersion in urban street canyons based on field synergy theory

Vehicle emissions are an important factor in the deterioration of air quality in urban street canyons. To improve the air quality in urban street canyons, it is necessary to have a deep understanding of the mechanism of pollutant dispersion within the urban boundary layer. The transport process of pollutants in street canyons is essentially a convective mass transfer process. In this paper, a computational fluid dynamics (CFD) simulation is conducted to investigate the effects of wind speed, the setting of the viaduct, and the roof shapes of the street canyons on pollutant dispersion. According to the field synergy theory, Sherwood number and field synergy number are used to evaluate the three factors on the diffusion of pollutants in street canyons quantitatively. The results show that the Sherwood number decreases with increased viaduct construction height and decreased wind speed, which is compatible with the observed growth of pollutant fraction in the street canyon. When the pollution source on the viaduct is considered, a less substantial influence of construction height on pollutant diffusion is observed. Among the four types of roofs, upward pitched roofs resulted in the highest pollution, with a pollution concentration degree of 1.6–2.3 times that of flat roofs, followed by double pitched roofs, with a pollution concentration degree of 1.2–2 times that of a flat roof. By the method of analogy analysis and numerical simulation, the field synergy theory has been extended from applications in heat transfer in small-scale confined spaces to those in urban street canyon pollution simulation in large-scale open spaces.

Field synergy analysis of pollutant dispersion in street canyons and its optimization by adding wind catchers

The microenvironment, which involves pollutant dispersion of the urban street canyon, is critical to the health of pedestrians and residents. The objectives of this work are twofold: (i) to effectively assess the pollutant dispersion process based on a theory and (ii) to adopt an appropriate stratigy, i.e., wind catcher, to alleviate the pollution in the street canyons. Pollutant dispersion in street canyons is essentially a convective mass transfer process. Because the convective heat transfer process and the mass transfer process are physically similar and the applicability of field synergy theory to turbulence has been verified in the literature, we apply the field synergy theory to the study of pollutant dispersion in street canyons. In this paper, a computational fluid dynamics (CFD) simulation is conducted to investigate the effects of wind catcher, wind speed and the geometry of the street canyons on pollutant dispersion. According to the field synergy theory, Sherwood number and field synergy number are used to quantitatively evaluate the wind catcher and wind speed on the diffusion of pollutants in asymmetric street canyons. The results show that adding wind catchers can significantly improve the air quality of the step-down street canyon and reduce the average pollutant concentrations in the street canyon by 75%. Higher wind speed enhances diffusion of pollutants differently in different geometric street canyons.