Fundamental Research On Unsteady Natural Convection Heat Transfer Under The Influence Of Concentration Boundary Conditions

The problem of natural convection heat transfer under concentration boundary conditions is a basic problem in the field of heat transfer.At the same time,it is also the basis of the research on heat transfer problem of building energy-saving wall.It not only provides reference for the application and development of building energy-saving wall,but also has important significance to reduce building energy consumption and realize building energy efficiency.Firstly,the effects of different concentration boundary conditions on the transient natural convection heat transfer are studied by means of numerical calculation.The results show: When two gradients are in the same direction,the horizontal and vertical temperatures are higher than the reference group 2.4K and 1.4K respectively;the stability time of flow field is shortened by nearly 750 s,and the boundary flow velocity is increased by about 0.0006m/s;the average Nusselt number is 3 larger than the reference value which improves the heat transfer intensity.When two gradients are reversed,the horizontal and vertical temperatures are higher than the reference group 2K and 1.2K respectively,and the temperature field and flow field are reversed;the stability time of flow field is shortened by nearly 250 s,the boundary flow velocity is decreased by 10 times,and the center flow velocity is increased by 15 times;the average Nusselt number is 63 larger than the reference value which improves the heat transfer intensity obviously.When two gradients are crossed,ranges of temperature changes in horizontal and vertical directions are 20 K and 5K respectively;the flow velocity is about 3 orders of magnitude higher than the other cases;the average Nusselt number is only 6,much lower than the reference value,and the whole heat transfer process is dominated by conduction.Secondly,the influence of different concentration boundary conditions on the transient natural convection heat transfer is studied by experiments.The experimental results show: whether two gradients are in the same direction or the opposite direction,the time to reach the new steady stage is about 5000 s.Compared to the case without concentration gradients,the informal stage time is shortened by 100 s and 300 s respectively.When two gradients are crossed,the informal stage time is 500 s,and the fluctuation of temperature is difficult to reach steady stage.The temperature variation trend obtained by experiment and numerical calculation are consistent basically.Finally,taking the exterior wall of civil buildings in Liaoning,Fuxin(severe cold regions)as an example,the problem of periodic natural convection heat transfer is studied.The study shows: In summer,flow stagnation occurs in the wall,which reverses the flow field,but the phenomenon is not obvious in winter.When the time is from 4 to 16,the air inside the wall flows anti-clockwise,and the flow velocity is the maximum at 12;When the time is from 16 to 20,the flow velocity in the wall decreases and stagnation occurs;When the time is from 20 to 24,the direction of the air has reversed,and the air inside the wall flows clockwise.The phenomenon of flow reversal reacts on the temperature field,and the second distribution of temperature field is affected.The change of the concentration only affects the indoor and outdoor plastering layer,and the relative humidity of the insulation layer and the base wall is 0.The heat transfer process of the wall reflects the thermal inertia of the exterior wall.In a word,the relationship between the concentration boundary conditions and the heat transfer intensity of natural convection is obtained.A natural convection heat transfer experimental platform with concentration boundary conditions is designed to reveal the variation of temperature with time in informal stage.The phenomenon of the flow field in the wall of Liaoning,Fuxin which moves from anti-clockwise direction to stagnation and then to clockwise direction is found.Flow reversal phenomenon caused by stagnation contributes to shortening the using time of indoor temperature control equipment in summer,and determining the starting and stopping of indoor temperature control equipment in the early stage of equipment operation.The purpose is to reduce building energy consumption and realize indoor temperature control in advance.