Large-span air-conditioning plant rooms have a large roof area and suffer from serious solar radiation in summer. The outside roof surface temperature is very high, so cooling load of roof occupies a large proportion in the envelope structure cooling load of large-span air-conditioning plant rooms. Based on the Coanda airfoil air induction unit, the author combined with exhaust air in large-span air-conditioning plant rooms to design the roof air film cooling system of large-span air-conditioning plant rooms. The adherence air film formed on the outside surface takes away heat on the outside surface of the roof, so as to reduce outside roof surface temperature of the roof, decrease heat transfer temperature difference between inside and outside roof surfaces of, and reduce roof cooling cold. Furthermore, the mathematical model and numerical simulation method of considering fluid-structure interaction for heat transfer and influences of solar radiation on air film formation of outside surface and cooling were constructed. Moreover, the numerical simulation method was conducted the validation of effectiveness. Also, the author discussed the air film formation mechanism and air film cooling ability of outside surface in large-span air-conditioning plant rooms without natural wind, developed a new air film cooling technology for the roof of large-span air-conditioning plant rooms, and supplemented the existing roof cooling technology.
The roof of large-span air-conditioning plant rooms is often exposed to the strong solar radiation in summer. In addition, the outside surface of the surface has the high temperature and the roof area has the large proportion in the envelope structure of large-span air-conditioning plant rooms, so the cooling load of the roof has a large proportion in envelope structure cooling load of large-span air-conditioning plant rooms.
Domestic and overseas studies about roof energy-saving technology mainly include cooling roof technology [
The air film cooling technology started being used to cool high-temperature components in aircraft engines in the 1970s. Hunley [
Based on the Coanda airfoil air induction unit, the author combined with exhaust air in large-span air-conditioning plant rooms to design the roof air film cooling system of large-span air-conditioning plant rooms, constructed mathematical model and numerical simulation method of fluid-structure interaction between air film of outside surface and garret, as well as influences of solar radiation on outside surface air film on the roof, conducted the validation of effectiveness for numerical simulation method and discussed influences of Coanda airfoil air induction unit’s supply velocity on the outside surface air film formulation mechanism, cooling effect and inside roof temperature distribution.
The upper air movement above the roof of the large-span air-conditioning plant rooms studied in this paper referred to the low-speed incompressible turbulence flow. Due to high outside surface temperature in the large-span air-conditioning plant rooms, the hot buoyancy should be considered. As a result, Boussinesq assumption was used. In other words, viscous dissipation in flow was neglected. Except for the air density, other physical parameters were constants. The momentum equation showed that only air density in buoyancy was changed, while density of other items was unchanged. The control equation was shown in Formula (1), Formula (2) and Formula (3).
1) Mass conservation equation
∂ u j ∂ x j = 0 (1)
2) Momentum conservation equation
∂ ( ρ u i ) ∂ t + ∂ ( ρ u i u j ) ∂ x j = − ∂ p ∂ x i + ∂ ∂ x j [ η e f f ( ∂ u i ∂ x i + ∂ u i ∂ x i ) ] + ρ β ( T 0 − T ) g i (2)
3) Energy conservation equation
∂ ( ρ T ) ∂ T + div ( ρ u → T ) = div ( h C p grad T ) + S T (3)
In the above-mentioned equations, uj is the velocity, fi is the body force, ρ is density, P is pressure, μ is dynamic viscosity coefficient, v is kinematic coefficient of viscosity, ui is the average velocity, ui’ is the pulsation of velocity, T is temperature, h is the heat transfer coefficient, cp is the specific heat and ST is the source item.
The fluid-structure interaction heat transfer problem is formed between the air film area of outside roof surface in the large-span air-conditioning plant rooms and internal solid area of the garret. The boundary condition is dynamically determined by the heat exchange process, so it can’t be given in advance. In the numerical solution process, different equations were used to solve the fluid domain and solid domain and used continuity, thermal flux and temperature of equations as the exchange data of fluid-solid boundary.
The roof heat transfer process of large-scan air-conditioning plant rooms is the steady-state heat transfer. The material is the color steel sheet roofing, showing the high thermal diffusivity. When the outside roof surface is conducted air film cooling, internal temperature change of roof is fast, so it has no need to consider thermal storage of garret. The heat transfer equation of the roof gets involved in constant property, no internal heat source and steady-state heat conduction equation. The specific form under the rectangular coordinate system is shown in Formula (4):
∂ 2 T ∂ x 2 + ∂ 2 T ∂ y 2 + ∂ 2 T ∂ z 2 = 0 (4)
In this paper, the author extracted the calculation model from the panel convective heat transfer experiment of R.S. AbduNour [
The geometric model simulation calculated by numerical modeling is present in
The section speed distribution comparison is illustrated in
standard k-ε model to do mesh encryption in the near wall region. Meanwhile, the wall-function method was used to deal with the near wall region. The numerical modeling result could be reliable, including dimensionless speed and dimensionless flow distance.
The roof air film cooling physical model of large-span air-conditioning plant rooms and computational domain are shown in
In this paper, hexahedral mesh was used to divide the computational domain. Moreover, the local cipher should be conducted in the air distributor of cooling induction unit and outside roof surface.
The air distributor of air induction outlet is the speed entrance. The supply air temperature is T = 305 k. The even air supply velocity of the cabin is u0. The air supply velocity is vertical to the split type air distributor. The outside roof surface can be the coupling surface, while constant temperature of inside roof temperature is T = 303 K. The calculation of extraterritorial outside boundary is the export boundary. Both sides can be called as the symmetric boundary condition. The absorbance of roof for solar radiation is 0.8, while heat transfer coefficient is 3.87 W/m2∙k.
The motion pattern of air induction unit is illustrated in
The normal section velocity profile along the roof method is present
stable. With the increase of movement distance and gradual increase of air film areas, the adherence velocity of air film is gradually reduced. In X = 1 m, air film has the minimal area, while air film has the maximum adherence velocity. At X = 32 m, the air film area is the biggest one, while air film has adherence speed. The aril film area is minimal. Air film area is more obvious with the increase of air supply speed.
The air temperature profile at 1 cm above roof of the large-span air-conditioning plant rooms is shown in
The air temperature cloud figure above the outside roof surface of large-span air-conditioning plant rooms is illustrated in
The surface temperature comparison of large-span air-conditioning plant rooms under the different air supply velocity is shown in
With the research object of the roof air film formation mechanism and cooling law of large-span air-conditioning plant rooms, the author designed the roof air film cooling system based on Coanda airfoil air induction unit, constructed mathematical model and numerical simulation method of fluid-structure interaction between air film of outside surface and garret, as well as influences of solar radiation on outside surface air film on the roof, conducted the validation of effectiveness for numerical simulation method and discussed the air film formation mechanism and air film cooling ability on outside roof surface in large-span air-conditioning plant rooms without natural wind. The main conclusions were shown as follows:
1) Air film has the good formation on the roof. With the increase of air flow distance, air film area is gradually increasing and air film adherence velocity is reduced. Moreover, with the increase of air supply velocity, the air film area is more obvious.
2) Air temperature is increasing with the increase of air flow distance. There is the maximum temperature rise in the end of the roof. Moreover, with the increase of air supply velocity, the temperature rising is reducing. As closing to the outside roof surface, the air temperature is higher. Air film constantly takes away outside roof surface heat with the increase of air flow distance.
3) The outside roof surface is increased with the increase of air flow distance. with the increase of air supply velocity, the outside roof surface has the lower temperature, cooling temperature difference is larger and cooling effect will be better.
The authors declare no conflicts of interest regarding the publication of this paper.
Peng, X.Y., Zhang, Y., Gu, W.L., Xiong, H. and Li, Y. (2018) Study on Air Film Formation Mechanism and Cooling in Large-Span Air-Conditioning Plant Rooms without Natural Wind. Journal of Applied Mathematics and Physics, 6, 1596-1605. https://doi.org/10.4236/jamp.2018.68135