Investigation on Heat Transfer Characteristics in Fully Open Cylindrical Cavity Based on Variable Properties
|Course||Power Engineering and Engineering Thermophysics|
|Keywords||Cylindrical cavity Natural convection Thermal radiation Conduction Constant wall heat flux Combined heat loss Numerical investigation|
The investigation on fully open cavity by different wall heating conditions can bewidely used in the cooling of electronic devices, regenerative engineering, buildingenergy conservation and other power engineering field, especially in solar cavityreceiver. The cavity receiver is the key component for converting solar radiation tothermal energy of the solar parabolic dish system. The heat losses from the cavityreceiver include three contributions: conductive heat loss from the insulation layer ofthe receiver, radiative heat loss through the receiver aperture and convective heat lossthrough the receiver aperture. So, it is of great importance to improve its performanceand eventually thermal efficiency.This dissertation is aimed to study the heat losses (heat transfer) characteristics incylinderical cavity, considering variable properties of air and radiation from the innerwall by numerical simulation. To solve the control equations in the cavity and the cavitywall, thus the temperature and velocity field can be obtained. The effects of the tilt angelof the cavity, heat flux, surface radiation and aspect ratio on the heat transfercharacteristics in the cavity have been analyzed. At the same time, the heat transfercharacteristics under condition of pure nature convection in cyliderical cavity werecompared with conjugated heat transfer by different wall conditions. Three cases withdifferent heating boundary conditions were examined:(i) a case with only bottom wallsurface heated and side wall surface insulated,(ii) a case with only side wall surfaceheated and bottom wall surface insulated, and (iii) a case with bottom and side wallsurfaces heated. The heated walls were maintained at constant heat flux.The results showed that: without considering the radiation heat loss when the heatflux is small, the average temperature at the inner wall of the cavity increases withincreasing wall heat flux and the average Nusselt number increases, the increased tiltangle of receiver leads to the reduced average Nusselt number. Not only the wall heatflux and tilt angle of cavity are factors that strongly affect the temperature and velocitycontours inside the cavity and in the opening surface, the mean temperature differencebetween bottom and side wall surfaces, the mean Nusselt number inside the cavity, etc.,but also the wall heating boundary condition does. Furthermore, the natural convectionfor case i with only bottom wall heated has a great distinctive, in comparison with thecase ii and iii both considered with side wall surfaces heated. Consiering radiation heat loss, tilt angle φ, aspect ratio Ar and emissivity of theinner wall surface ε all can affect the convection heat loss Nusselt number Nucandradiation heat loss Nusselt number Nurunder a certain wall heat flux q, but show adifferent characteristics of increment or decrease. When the total heat loss value isconstant, the conduction and radiation heat losses increase while the nature convectionheat loss decreases with the increasing tilt angle of cavity. Meanwhile, the proportion ofthe conduction heat loss to radiation heat loss depends on the value of total heat loss.When the heating quantity Q, aspect ratio Ar, surface emissivity ε are all the same,the temperature distribution in the cavity for case ii is the most evenly among the threewall boundary conditions. The average temperature of the outer surface of the cavity Tmand the average temperature difference between the bottom and the side of the cavityΔTmof case i are both the maximum with case iii the second and case i the least. Nucand Nurof case ii are both a little larger than case i and case iii. When Q, φ and ε are allthe same, under different Ar, Tmof case i are both the maximum with case iii the secondand case ii the least, and ΔTmof case i is much larger than that of case ii and case iii.The differences of Nucamong the three cases are very small, while Nurof case i is alittle larger than the other two when Ar is small. Under different φ, ΔTmof all the threecases increases with the increasing Ar, reaching its maximum at moderate tilt angles.In addition, it can be found that the consideration of radiation can make thetemperature distribution in cavity more evenly for all the three cases. And with theincrease of ε, ΔTmreduces. At the same q, the Nucof the case without considering theradiative heat loss and conductive heat loss from the insulation is smaller than thecorresponding values of considering those.