Numerical Simulation of Temperature Decrease in Greenhouses with Summer Water-Sprinkling Roof
Abstract
:1. Introduction
2. Materials and Methods
2.1. Description of the Greenhouse
2.2. Mathematical Model
- (1)
- Due to the short duration of the sprinkling time, the impact of plant respiration on temperature in the greenhouse was ignored.
- (2)
- The influence of maintenance of structures on the thermal environment of the greenhouse was ignored.
- (3)
- The gas in the greenhouse was incompressible, which was in agreement with the Boussinesq hypothesis.
- (4)
- The gas in the greenhouse was modeled as a Newtonian fluid.
- (5)
- All surfaces included in the thermal radiation modeling were gray and diffuse.
- (6)
- The flow rate on both sides of the dropper was consistent and stable.
2.2.1. Porous Medium Model
2.2.2. Radiation Model
2.2.3. Multiphase Flow Model
2.3. Model Parameters and Boundary Conditions
- (1)
- The sprinkle outlet was set as a mass-flow inlet, with the proportion of the liquid water component set to 1. The pattern of water flow pressure on the top of the roof was calculated by measuring the mass-flow rate in the pipe and the outlet area. Input parameters for model were the turbulence intensity IR and hydraulic diameter DH. Turbulence intensity IR was obtained according to Equation (10) [31]. Due to the impact of the solar radiation, the outlet temperature of the water pipe varied with time, ranging from 36 °C to 30 °C. The relationship between the ambient air and outlet water temperature, and a user-defined function (UDF) between the two was added into the model.
- (2)
- The outlet of water flow area on the roof was set as the out-flow boundary condition.
- (3)
- The plant was modeled as a porous media with porosity rate of 0.5.
- (4)
- Solar radiation was calculated using the DO model in FLUENT and setting the time zone and coordinates.
- (5)
- The walls were set as the coupling boundaries.
2.4. Model Applications
3. Results and Discussion
3.1. Roof Film Temperature Distribution and Indoor Air Flow Vector
3.2. Effect of Sprinkler Flow Rate on Temperature Distribution
3.3. Model Validation
3.4. Discussion
4. Conclusions
- (1)
- With an increase in the sprinkle flow rate, the temperature of the roof film would be lower and the temperature distribution on the film would be more uniform.
- (2)
- For a greenhouse with plastic film as the enclosure material—as is the case in Southern China—for an increase in the sprinkle flow rate, the air temperature would decrease faster, but the increased flow rate would have little effect on the average air temperature in the greenhouse, at the end of the sprinkle process. When the air temperature in the greenhouse was 40 °C, the average air temperature in the greenhouse would decrease by about 1.65 °C, after a five-minute sprinkle.
- (3)
- Furthermore, for a greenhouse with plastic film as the enclosure material, when the air temperature in the greenhouse is 40 °C, an increased sprinkle flow rate would give a more uniform temperature distribution within the greenhouse. However, the effect on temperature of the plants is relatively small.
- (4)
- The simulation results agreed well with the experimental results that verified the accuracy of the model.
Author Contributions
Funding
Conflicts of Interest
References
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Equation | Generalized Source Terms (S) | ||
---|---|---|---|
Momentum equation of X direction | u | η+ηt | |
Momentum equation of Y direction | v | η+ηt | |
Momentum equation of Z direction | ω | η+ηt | |
Turbulent energy equation | k | ||
Turbulent energy dissipation equation | ε | ||
Energy equation | T | 0 |
cμ | c1 | c2 | σk | σε | σT |
---|---|---|---|---|---|
0.09 | 1.44 | 1.92 | 1.0 | 1.3 | 0.9~1.0 |
Name | Air | Water | Plastic Film | Plant |
---|---|---|---|---|
Density (kg/m³) | 1.23 | 998.2 | 1000 | 1000 |
Thermal conductivity (W/m·K) | 0.024 | 0.6 | 1.50 | 0.2 |
Absorption constant (1/m) | — | — | 0.10 | 0.3 |
Scattering constant (1/m) | — | — | 0.25 | 0.2 |
Specific heat capacity (J/kg·K) | 1006.43 | 4182 | 1380 | 2300 |
Refractive index | 1.0 | 1.0 | 1.52 | 1.52 |
Name | Set Value |
---|---|
Temperature of the film on roof (K) | 318.15 |
Air temperature (K) | 313.15 |
Water temperature (K) | UDF |
Plant temperature (K) | 312.15 |
Test Points | T1 | T2 | T3 | T4 | T5 | T6 | T7 | T8 |
---|---|---|---|---|---|---|---|---|
Test value/°C | 36.90 | 38.60 | 37.90 | 37.80 | 39.90 | 40.50 | 42.60 | 40.00 |
Simulation value/°C | 38.71 | 38.89 | 38.1 | 38.89 | 38.43 | 38.44 | 38.43 | 38.44 |
Prediction error/% | 4.91 | 0.75 | 0.53 | 2.88 | 3.68 | 5.09 | 9.79 | 3.90 |
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Guo, J.; Liu, Y.; Lü, E. Numerical Simulation of Temperature Decrease in Greenhouses with Summer Water-Sprinkling Roof. Energies 2019, 12, 2435. https://doi.org/10.3390/en12122435
Guo J, Liu Y, Lü E. Numerical Simulation of Temperature Decrease in Greenhouses with Summer Water-Sprinkling Roof. Energies. 2019; 12(12):2435. https://doi.org/10.3390/en12122435
Chicago/Turabian StyleGuo, Jiaming, Yanhua Liu, and Enli Lü. 2019. "Numerical Simulation of Temperature Decrease in Greenhouses with Summer Water-Sprinkling Roof" Energies 12, no. 12: 2435. https://doi.org/10.3390/en12122435