The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas ...The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas the peak OH mass fraction increases significantly under normal gravity(g=9.8 m/s^2).For a very low jet velocity(e.g.,V=0.1 m/s),both the peak OH mass fraction and flame temperature under g=9.8 m/s^2 are lower than the counterparts under g=0 m/s^2.Analysis reveals that when V≥0.2 m/s,fuel/air mixing will be promoted and combustion will be intensified due to radial flow caused by the buoyancy effect.However,the flame temperature will be slightly decreased owing to the large amount of entrainment of cold air into the reaction zone.For V=0.1 m/s,since the heat release rate is very low,the entrainment of cold air and fuel leakage from the rim of tube exit lead to a significant drop of flame temperature.Meanwhile,the heat loss rate from fuel to inner tube wall is larger under g=9.8 m/s^2 compared to that under g=0 m/s^2.Therefore,the buoyancy effect is overall negative at very low jet velocities.展开更多
The buoyancy effect on heat transfer in a rotating,two-pass,square channel is experimentally investigated in curent work.The classical copper plate technique is performed to measure the regional averaged heat transfer...The buoyancy effect on heat transfer in a rotating,two-pass,square channel is experimentally investigated in curent work.The classical copper plate technique is performed to measure the regional averaged heat transfer cofficients.In order to perform a fundamental research,all turbulators are removed away.Two approaches of altering Buoyancy numbers are selected:varying rotation number from 0 to 2.08 at Reynolds number ranges of 10000 to 70000,and varying inlet density ratio from 0.07 to 0.16 at Reynolds number of 10000.And thus,Buoyancy numbers range from 0 to 12.9 for both cases.According to the experimental results,the relationships between heat transfer and Buoyancy numbers are in accord with those obtained under different rotation numbers.For both leading and trailing surface,a critical Buoyancy number exists for each X/D location.Before the critical point,the effect of Buoyancy number on heat transfer is limited;but after that,the Nusselt number ratios show different increase rate.Given the same rotation number,higher wall temperature ratios with its corresponding higher Buoyancy numbers substantially enhance heat transfer on both passages.And the critical exceed-point that heat transfer from trailing surface higher than leading surface happens at the same Buoyancy number for different wall temperature ratios in the second passage.Thus,the stronger buoyancy effect promotes heat transfer enhancement at high rotation number condition.展开更多
A variety of turbulence models were used to perform numerical simulations of heat transfer for hydrocarbon fuel flowing upward and downward through uniformly heated vertical pipes at supercritical pressure. Inlet temp...A variety of turbulence models were used to perform numerical simulations of heat transfer for hydrocarbon fuel flowing upward and downward through uniformly heated vertical pipes at supercritical pressure. Inlet temperatures varied from 373 K to 663 K, with heat flux rang- ing from 300 kW/m2 to 550 kW/m2. Comparative analyses between predicted and experimental results were used to evaluate the ability of turbulence models to respond to variable thermophysical properties of hydrocarbon fuel at supercritical pressure. It was found that the prediction performance of turbulence models is mainly determined by the damping function, which enables them to respond differently to local flow conditions. Although prediction accuracy for experimental results varied from condition to condition, the shear stress transport （SST） and launder and sharma models performed better than all other models used in the study. For very small buoyancy-influenced runs, the thermal-induced acceleration due to variations in density lead to the impairment of heat transfer occurring in the vicinity of pseudo-critical points, and heat transfer was enhanced at higher temperatures through the combined action of four thermophysical properties： density, viscosity, thermal conductivity and specific heat. For very large buoyancy- influenced runs, the thermal-induced acceleration effect was over predicted by the LS and AB models.展开更多
The buoyancy driven flow of a second-grade nanofluid in the presence of a binary chemical reaction is analyzed in the context of a model based on the balance equations for mass,species concentration,momentum and energ...The buoyancy driven flow of a second-grade nanofluid in the presence of a binary chemical reaction is analyzed in the context of a model based on the balance equations for mass,species concentration,momentum and energy.The elastic properties of the considered fluid are taken into account.The two-dimensional slip flow of such non-Newtonian fluid over a porous flat material which is stretched vertically upwards is considered.The role played by the activation energy is accounted for through an exponent form modified Arrhenius function added to the Buongiorno model for the nanofluid concentration.The effects of thermal radiation are also examined.A similarity transformations is used to turn the problem based on partial differential equations into a system of ordinary differential equations.The resulting system is solved using a fourth order RK and shooting methods.The velocity profile,temperature profile,concentration profile,local skin friction,local Nusselt number and local Sherwood number are reported for several circumstances.The influence of the chemical reaction on the properties of the concentration and momentum boundary layers is critically discussed.展开更多
The turbulent flow of vertical plane wall plume with concentration variation was studied with the finite analytical method. The k-epsilon model with the effect of buoyancy on turbulent kinetic energy and its dissipati...The turbulent flow of vertical plane wall plume with concentration variation was studied with the finite analytical method. The k-epsilon model with the effect of buoyancy on turbulent kinetic energy and its dissipation rate was adopted. There were similarity solutions in the uniform environment for the system of equations including the equation of continuity, the equation of momentum along the flow direction and concentration, and equations of k, epsilon. The finite analytic method was applied to obtain the similarity solution. The calculated data of velocity, relative density difference, the kinetic energy of turbulence and its dissipation rate distribution for vertical plane plumes are in good agreement with the experimental data at the turbulent Schmidt number equal to 1.0. The variations of their maximum value along the direction of main flow were also given. It shows that the present model is good, i.e., the effect of buoyancy on turbulent kinetic energy and its dissipation rate should be taken into account, and the finite analytic method is effective.展开更多
Three-dimensional simulations of diesel particulate matter （DPM） distribution inside a single straight entry for the Load-Haul-Dump loader （LHD）-truck loading and truck hauling operations were conducted by using A...Three-dimensional simulations of diesel particulate matter （DPM） distribution inside a single straight entry for the Load-Haul-Dump loader （LHD）-truck loading and truck hauling operations were conducted by using ANSYS FLUENT computational fluid dynamics software. The loading operation was performed for a fixed period of 3 min. The dynamic mesh technique in FLUENT was used to study the impact of truck motion on DPM distribution. The resultant DPM distributions are presented for the cases when the truck were driving upstream and downstream of the loading face. Interesting phenomena were revealed in the study including the piston effect, layering of DPM in the roof region, and backflow of diesel exhaust against ventilation. The results from the simulation can be used to determine if the areas inside the face area and straight entry exceed the current U.S. regulatory requirement for DPM concentration （〉160 pg/m3）. This research can guide the selection of DPM reduction strategies and improve the working practices for the underground miners.展开更多
The exhaust smoke dispersion for a generic frigate is investigated numerically through the numerical solution of the governing fluid flow, energy, species and turbulence equations. The main objective of this work is t...The exhaust smoke dispersion for a generic frigate is investigated numerically through the numerical solution of the governing fluid flow, energy, species and turbulence equations. The main objective of this work is to obtain the effects of the yaw angle velocity ratio and buoyancy on the dispersion of the exhaust smoke The numerical method is based on the fully conserved control-volume representation of the fully elliptic Navier-Stokes equations. Turbulence is modeled using a two-equation （k-ε） model The flow visualization tests using a 1/100 scale model of the frigate in the wind tunnel were also carried out to determine the exhaust plume path and to validatc the computational results. The results show that down wash phenomena occurs for the yaw angles between ψ=10° and 20°. The results with different exhaust gas temperatures show that the buoyancy effect increases with the increasing of the exhaust gas temperature. However, its effect on the plume rise is less significant in comparison with its momentum. A good agreement between the predictions and experiment results is obtained.展开更多
Previous research was limited to flat-façade buildings when evaluating the indoor and outdoor ventilation performance in a multi-story building.However,envelope features can provide the shading effect to induce t...Previous research was limited to flat-façade buildings when evaluating the indoor and outdoor ventilation performance in a multi-story building.However,envelope features can provide the shading effect to induce the temperature difference between surfaces exposed to direct solar radiation and those without solar radiation.This temperature difference between surfaces can enhance the thermal buoyancy and change indoor and outdoor ventilation performance.We conducted scaled outdoor experiments to examine the impact of various envelope features on indoor and outdoor ventilation performance in multi-story buildings.Compared to the flat-façade multi-building,the average normalized horizontal airflow velocity of overhang,small wing wall,and large wing wall multi-buildings increased by 12.41%,10.56%,and 5.56%,respectively.Cross-ventilation is more susceptible to envelope features than single-sided ventilation in air change per hour(ACH).Specifically,the ACH values of cross-ventilation for large wing wall,small wing wall,and balcony multi-buildings decreased by 69.98%,25.79%,and 12.12%relative to the flat-façade building.For the same envelope feature building,the ACH values of single-sided ventilation on the windward side are better than those on the leeward side,particularly the building with small wing walls,with an improvement of 12.77%compared to flat-façade.This study contributes to advancing the understanding of urban ventilation,and provides a valid basis for designing envelope features in urban buildings.展开更多
Based on the first and second laws of thermodynamics,the heat transfer and flow(thermohydraulic)characteristics of horizontal supercritical pressure CO_(2)(S-CO_(2))in a circular pipe under heating conditions were inv...Based on the first and second laws of thermodynamics,the heat transfer and flow(thermohydraulic)characteristics of horizontal supercritical pressure CO_(2)(S-CO_(2))in a circular pipe under heating conditions were investigated numerically.Heating flows in two different diameters(d)of 4 and 6 mm were simulated in pipes with pressures of 8 MPa,mass fluxes(G)of 300 and 400 kg/(m^(2)·s),and heat fluxes(q)of 50,75 and 100 kW/m^(2).In the d=4 mm pipe,the peak heat transfer coefficient(hb)was about 3 times higher than in the d=6 mm pipe,while the entropy production due to fluid friction in the 4 mm pipe was on average 1.1 times higher,and the entropy production due to heat transfer was on average about 67%lower.A 4 mm tube was employed to further evaluate the influence of the applied wall heat flux,the results demonstrated that the irreversibility due to heat transfer was on average more than 4 times higher when heat flux density was 100 kW/m^(2)than when the heat flux density was 50 kW/m^(2),while the peak of heat transfer coefficient increased by 1.4 times as q was decreased from 100 to 50 kW/m^(2).The effect of thermal acceleration was ignored,while the buoyancy effect resulted in secondary flow and significantly affected the flow and heat transfer features.The jet flows were found in the vicinity of the lower wall of the pipe,which made the two fields of velocity and temperature gradient more synergistic,leading to an enhancement in heat transfer in the vicinity of the upper wall.The aggravation of heat transfer resulted in high irreversibility of heat transfer in the cross-sectional area near the wall,while the local friction irreversibility was less affected by the buoyancy effect,and the distribution was uniform.The uneven distribution of thermophysical properties also confirmed that the enhanced heat transfer occurred near the wall area at the bottom of the pipe.展开更多
基金Project(51576084)supported by the National Natural Science Foundation of China。
文摘The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas the peak OH mass fraction increases significantly under normal gravity(g=9.8 m/s^2).For a very low jet velocity(e.g.,V=0.1 m/s),both the peak OH mass fraction and flame temperature under g=9.8 m/s^2 are lower than the counterparts under g=0 m/s^2.Analysis reveals that when V≥0.2 m/s,fuel/air mixing will be promoted and combustion will be intensified due to radial flow caused by the buoyancy effect.However,the flame temperature will be slightly decreased owing to the large amount of entrainment of cold air into the reaction zone.For V=0.1 m/s,since the heat release rate is very low,the entrainment of cold air and fuel leakage from the rim of tube exit lead to a significant drop of flame temperature.Meanwhile,the heat loss rate from fuel to inner tube wall is larger under g=9.8 m/s^2 compared to that under g=0 m/s^2.Therefore,the buoyancy effect is overall negative at very low jet velocities.
文摘The buoyancy effect on heat transfer in a rotating,two-pass,square channel is experimentally investigated in curent work.The classical copper plate technique is performed to measure the regional averaged heat transfer cofficients.In order to perform a fundamental research,all turbulators are removed away.Two approaches of altering Buoyancy numbers are selected:varying rotation number from 0 to 2.08 at Reynolds number ranges of 10000 to 70000,and varying inlet density ratio from 0.07 to 0.16 at Reynolds number of 10000.And thus,Buoyancy numbers range from 0 to 12.9 for both cases.According to the experimental results,the relationships between heat transfer and Buoyancy numbers are in accord with those obtained under different rotation numbers.For both leading and trailing surface,a critical Buoyancy number exists for each X/D location.Before the critical point,the effect of Buoyancy number on heat transfer is limited;but after that,the Nusselt number ratios show different increase rate.Given the same rotation number,higher wall temperature ratios with its corresponding higher Buoyancy numbers substantially enhance heat transfer on both passages.And the critical exceed-point that heat transfer from trailing surface higher than leading surface happens at the same Buoyancy number for different wall temperature ratios in the second passage.Thus,the stronger buoyancy effect promotes heat transfer enhancement at high rotation number condition.
基金funding support from National Natural Science Foundation of China (No.51406005)Defense Industrial Technology Development Program of China (No.B2120132006)
文摘A variety of turbulence models were used to perform numerical simulations of heat transfer for hydrocarbon fuel flowing upward and downward through uniformly heated vertical pipes at supercritical pressure. Inlet temperatures varied from 373 K to 663 K, with heat flux rang- ing from 300 kW/m2 to 550 kW/m2. Comparative analyses between predicted and experimental results were used to evaluate the ability of turbulence models to respond to variable thermophysical properties of hydrocarbon fuel at supercritical pressure. It was found that the prediction performance of turbulence models is mainly determined by the damping function, which enables them to respond differently to local flow conditions. Although prediction accuracy for experimental results varied from condition to condition, the shear stress transport （SST） and launder and sharma models performed better than all other models used in the study. For very small buoyancy-influenced runs, the thermal-induced acceleration due to variations in density lead to the impairment of heat transfer occurring in the vicinity of pseudo-critical points, and heat transfer was enhanced at higher temperatures through the combined action of four thermophysical properties： density, viscosity, thermal conductivity and specific heat. For very large buoyancy- influenced runs, the thermal-induced acceleration effect was over predicted by the LS and AB models.
基金United Arab Emirates University,Al Ain,UAE with Grant No.31S363-UPAR(4)2018.
文摘The buoyancy driven flow of a second-grade nanofluid in the presence of a binary chemical reaction is analyzed in the context of a model based on the balance equations for mass,species concentration,momentum and energy.The elastic properties of the considered fluid are taken into account.The two-dimensional slip flow of such non-Newtonian fluid over a porous flat material which is stretched vertically upwards is considered.The role played by the activation energy is accounted for through an exponent form modified Arrhenius function added to the Buongiorno model for the nanofluid concentration.The effects of thermal radiation are also examined.A similarity transformations is used to turn the problem based on partial differential equations into a system of ordinary differential equations.The resulting system is solved using a fourth order RK and shooting methods.The velocity profile,temperature profile,concentration profile,local skin friction,local Nusselt number and local Sherwood number are reported for several circumstances.The influence of the chemical reaction on the properties of the concentration and momentum boundary layers is critically discussed.
基金Project supported by the National Natural Science Foundation of China (Nos.50479038 and 50679061)
文摘The turbulent flow of vertical plane wall plume with concentration variation was studied with the finite analytical method. The k-epsilon model with the effect of buoyancy on turbulent kinetic energy and its dissipation rate was adopted. There were similarity solutions in the uniform environment for the system of equations including the equation of continuity, the equation of momentum along the flow direction and concentration, and equations of k, epsilon. The finite analytic method was applied to obtain the similarity solution. The calculated data of velocity, relative density difference, the kinetic energy of turbulence and its dissipation rate distribution for vertical plane plumes are in good agreement with the experimental data at the turbulent Schmidt number equal to 1.0. The variations of their maximum value along the direction of main flow were also given. It shows that the present model is good, i.e., the effect of buoyancy on turbulent kinetic energy and its dissipation rate should be taken into account, and the finite analytic method is effective.
文摘Three-dimensional simulations of diesel particulate matter （DPM） distribution inside a single straight entry for the Load-Haul-Dump loader （LHD）-truck loading and truck hauling operations were conducted by using ANSYS FLUENT computational fluid dynamics software. The loading operation was performed for a fixed period of 3 min. The dynamic mesh technique in FLUENT was used to study the impact of truck motion on DPM distribution. The resultant DPM distributions are presented for the cases when the truck were driving upstream and downstream of the loading face. Interesting phenomena were revealed in the study including the piston effect, layering of DPM in the roof region, and backflow of diesel exhaust against ventilation. The results from the simulation can be used to determine if the areas inside the face area and straight entry exceed the current U.S. regulatory requirement for DPM concentration （〉160 pg/m3）. This research can guide the selection of DPM reduction strategies and improve the working practices for the underground miners.
基金the financial support received from the Scientific Research Projects Office of Istanbul Technical University,stanbul, Turkey for the project titled as ‘Numerical and Experimental Investigation of Exhaust Smoke Dispersion for Naval Ships’
文摘The exhaust smoke dispersion for a generic frigate is investigated numerically through the numerical solution of the governing fluid flow, energy, species and turbulence equations. The main objective of this work is to obtain the effects of the yaw angle velocity ratio and buoyancy on the dispersion of the exhaust smoke The numerical method is based on the fully conserved control-volume representation of the fully elliptic Navier-Stokes equations. Turbulence is modeled using a two-equation （k-ε） model The flow visualization tests using a 1/100 scale model of the frigate in the wind tunnel were also carried out to determine the exhaust plume path and to validatc the computational results. The results show that down wash phenomena occurs for the yaw angles between ψ=10° and 20°. The results with different exhaust gas temperatures show that the buoyancy effect increases with the increasing of the exhaust gas temperature. However, its effect on the plume rise is less significant in comparison with its momentum. A good agreement between the predictions and experiment results is obtained.
基金supported by the Key Laboratory of Ecology and Energy Saving Study of Dense Habitat,Ministry of Education,Tongji University(Grant No.20220104)the National Natural Science Foundation of China(Grant Nos.52378026 and 42175095)the Shenzhen Science and Technology Program(No.20220809120650001).
文摘Previous research was limited to flat-façade buildings when evaluating the indoor and outdoor ventilation performance in a multi-story building.However,envelope features can provide the shading effect to induce the temperature difference between surfaces exposed to direct solar radiation and those without solar radiation.This temperature difference between surfaces can enhance the thermal buoyancy and change indoor and outdoor ventilation performance.We conducted scaled outdoor experiments to examine the impact of various envelope features on indoor and outdoor ventilation performance in multi-story buildings.Compared to the flat-façade multi-building,the average normalized horizontal airflow velocity of overhang,small wing wall,and large wing wall multi-buildings increased by 12.41%,10.56%,and 5.56%,respectively.Cross-ventilation is more susceptible to envelope features than single-sided ventilation in air change per hour(ACH).Specifically,the ACH values of cross-ventilation for large wing wall,small wing wall,and balcony multi-buildings decreased by 69.98%,25.79%,and 12.12%relative to the flat-façade building.For the same envelope feature building,the ACH values of single-sided ventilation on the windward side are better than those on the leeward side,particularly the building with small wing walls,with an improvement of 12.77%compared to flat-façade.This study contributes to advancing the understanding of urban ventilation,and provides a valid basis for designing envelope features in urban buildings.
基金supported by the European Union’s Horizon 2020 Research and Innovation Programme Project(No.882628)(Guo,https://cinea.ec.europa.eu/programmes/horizon-europe_en)(acceseed on 08 October 2024),and the Fundamental Research Funds for the Central Universities(buctrc202406)(Guo,https://english.buct.edu.cn/)(accessed on 08 October 2024).
文摘Based on the first and second laws of thermodynamics,the heat transfer and flow(thermohydraulic)characteristics of horizontal supercritical pressure CO_(2)(S-CO_(2))in a circular pipe under heating conditions were investigated numerically.Heating flows in two different diameters(d)of 4 and 6 mm were simulated in pipes with pressures of 8 MPa,mass fluxes(G)of 300 and 400 kg/(m^(2)·s),and heat fluxes(q)of 50,75 and 100 kW/m^(2).In the d=4 mm pipe,the peak heat transfer coefficient(hb)was about 3 times higher than in the d=6 mm pipe,while the entropy production due to fluid friction in the 4 mm pipe was on average 1.1 times higher,and the entropy production due to heat transfer was on average about 67%lower.A 4 mm tube was employed to further evaluate the influence of the applied wall heat flux,the results demonstrated that the irreversibility due to heat transfer was on average more than 4 times higher when heat flux density was 100 kW/m^(2)than when the heat flux density was 50 kW/m^(2),while the peak of heat transfer coefficient increased by 1.4 times as q was decreased from 100 to 50 kW/m^(2).The effect of thermal acceleration was ignored,while the buoyancy effect resulted in secondary flow and significantly affected the flow and heat transfer features.The jet flows were found in the vicinity of the lower wall of the pipe,which made the two fields of velocity and temperature gradient more synergistic,leading to an enhancement in heat transfer in the vicinity of the upper wall.The aggravation of heat transfer resulted in high irreversibility of heat transfer in the cross-sectional area near the wall,while the local friction irreversibility was less affected by the buoyancy effect,and the distribution was uniform.The uneven distribution of thermophysical properties also confirmed that the enhanced heat transfer occurred near the wall area at the bottom of the pipe.
