Multi-component Convection In The Solar Chimney And Ventilated Enclosure Driven By Thermal Buoyancy

Low energy buildings,particularly for buildings using solar energy,has been attracted more attentions due to it could contribute to the building energy cost reduction and indoor air environment improvement simultaneously.Regarding of low energy building and its air environment,buoyancy driven fluid flow,heat and species transports inside building enclosures have been numerically,theoretically and experimentally investigated in the present work.This research could provide a novel way for solar energy utilization,building energy conservation and indoor air environment enhancement.Representatively,thermal driven natural convection in the vertical solar chimney,thermal driven natural convection in the inclined solar chimney,natural convective thermal and species removals from the partial enclosure inserted with a baffle,and combined buoyancy driven and mechanical driven flows in the slot-ventilated enclosure are discussed respectively.This study could be significant for guiding future solar energy buildings and its relevant extensions.Firstly,fluid flow and thermal transports structures in a vertical window solar chimney are numerically and theoretically investigated.Numerical results are particularly presented to illustrate the effects of thermal Rayleigh(Ra)number on flow patterns.Results demonstrate that the reverse flow not only slows down the volumetric flow rate but also accelerates backflow presented in the solar chimney when Ra increases beyond a certain value,which makes the solar chimney lost its ventilation function.The thermal boundary layer development in different flow regimes are investigated by the scaling analysis,which directly give the mathematical relations between the reverse flows and other governing parameters.The corresponding steady-state time,thermal boundary layer temperature and velocity,volume flow rate scales are given.The results clearly illustrate linear correlation between the calculated volume flow rate and the scaling prediction before the volume flow rate decrease due to reverse flow effects.The reverse flow is characterized by the penetration depth.The scale of the penetration depth of the reverse flow demonstrates the correlation between the penetration depth,Prandtl(Pr),Ra and aspect ratio.Approximately linear correlation between the numerical data and the scaling prediction confirms the dependence of the penetration depth on Ra,Pr and aspect ratio when Ra is less than a certain value.In order to suppress the reverse flow and enhance the volume flow rate,discrete heat sources are attached to the glazing wall.The results shown that the heat source height D0=1,the distance between the bottom wall and the heat source S0=1 can abruptly increase the volume flow rate induced by the solar chimney.The effects of solar chimney inclination angle on ventilation rate have been investigated numerically.The inclination angle a is defined as the angle between the heat wall and the horizontal surface.Numerical results show that the reverse flow are suppressed at the heat wall side,meanwhile they become serious at the glazing side as increasing Ra number,whereas the inclination angle a is identical.The reverse flow presented the glazing side becomes more serious with the increase of a at the same Ra number.The smaller Ra number will lead to the reverse flow presented adjacent to the glazing side as increasing inclination angle a.Dimensionless volume flow rate V*increases with increasing Ra in the range of inclination 00?300.For 450???900,dimensionless volume flow rate V*increases with the increase of Ra but decreases beyond a certain Ra value.The V*is firstly an increasing function of the inclination angle ? at the same Ra,later it becomes a decreasing one of the inclination angle.The V*increases with the increase of the inclination a when Ra changes from 103 towards 104.For Ra = 105,V*reaches its maximum at the inclined angle of 750.For Ra = 105,3×106 and Ra = 4×106,V*reaches their maximum at the inclined angle of 150 respectively.According to different Ra value,the optimum inclined angle is varied as V*reached its maximum.The effects of inclined angle on the ventilation rate indicate that solar chimney could enhance the ventilation rate when the same discrete heat source is applied to the inclined solar chimney.Secondly,experiments were carried out for a window solar chimney model with a fixed height-to-gap ratio A=5.Temperature distributions across the chimney gap at Y=3.5 and Y=5.5 were obtained from Z=1,Z=2.5 and Z=4 respectively.It can be seen that those air temperature inside the solar chimney were coinciding with each other for Z=1,Z=2.5 and Z=4 respectively,which indicated the assumption of two dimensional flow could be valid in the experiment and numerical simulation.Temperature profiles across the chimney gap at different ambient temperatures and uniform heat inputs shown the temperature gradients adjacent to the heated wall were higher than those away from the heated wall,the temperatures rose slightly when approaching the opposite surface of the heated wall.This temperature rise is caused by radiation from the heated wall.Its distribution is not uniform,with higher air temperatures near the heated surface.The experimental data are little less than numerical results adjacent to the heated wall,while the temperature distributions away from the heated wall are slightly higher than those of numerical results due to radiation from the heated wall.It clear from the figures that numerical results showed a good agreement with the experimental results.The dimensionless temperature and the temperature difference between the experimental measurements and the ambient air are used to shown the temperature distributions inside the solar chimney model.Two expressions are identical in essence.The difference between the location of thermocouples and ambient air along the heated wall at given heat input indicated that the temperature did not increase linearly and even drop near the outlet.Thirdly,natural convection on a heated and polluted strip within a partial enclosure is numerically investigated,where the baffle is attached to the right wall.Numerical solutions demonstrate that the baffle in the partial enclosure could enhance heat and mass transfer rates only when Ra is higher than the critical value and its dimensionless deviation E between the baffle and horizontal x-axis lies between-0.8 and-0.5.When Ra?105 and E =-0,5 are imposed,heat and mass transfer rates for the dimensionless length of the baffle L=0.8 are higher than those of L=0.2 and L=0.5;whereas,for Ra?8.00×105 and E =-0.8,convective transfer rates of L=0.5 and L=0.8 are higher than those of L=0.2.These trends could be reversed when the regimes of low Ra is established.Steady transition flows are observed as Ra varies from 1.25×105 towards 1.30×105,where L=0.2 and E=-0.6 are maintained;As Ra turns up to 8.00×105 from 7.90×105,Nu and Sh could be enhanced sharply for L=0.8 and E--0.7.Moreover,huge divergences are observed for temperature and concentration distributions along the central axis when the steady transition flow appears in the enclosure.Especially,the temperature and concentration gradients adjacent to the heating and pollutant source in the lower regions of the enclosure.Finally,mixed convection on a heated and polluted strip within a slot ventilated enclosure in which the displacement and mixing flow schemes are considered.Contours of streamfunction,heatfunction,and massfunction are presented to clearly scrutinize the mechanism of heat and airborne pollutant transports.For the displacement flow scheme,thermal Nusselt(Nu)and pollutant Sherwood(Sh)numbers under different Reynolds numbers remain almost constant as the value of Gr/Re2 decreases down to the regime of forced convection dominated.However,as Archimedes(Ar)number increases up to the regime of natural convection dominated,both Nu and Sh increase sharply with Ar(Gr/Re2).Similar trends could be observed for the situation of mixing ventilated flow scheme.In the mixing scheme,non unique steady flow solutions could be observed for the range of transitional flow regime when different initial conditions were imposed.The expected airflow and heat and mass transfer structures could be achieved upon imposing different initial conditions,which are employed to improve the local heat and pollutant removals from a slot vented enclosure,regarding of cooling,increasing air change rates,and reducing building energy consumption.