Heat Fluxes and Airflow Patterns Through Roof Windows in a Naturally Ventilated Enclosure

Abstract

Experiments were carried out to study the airflow and heat transfer through continuous vertical roof openings of a naturally ventilated greenhouse. The major goals of this work were to investigate the effect of ambient wind speed and direction with respect to the greenhouse on the mean and turbulent characteristics of the air velocity and heat transfer through openings with and without insect-proof screens. Air velocity and temperature were measured simultaneously at two edges of the opening using one-dimensional sonic anemometers and miniature thermocouples. It is shown that when the wind is not perpendicular to the plane of the openings there are outflow and inflow, at the windward and leeward edges of the openings respectively. A wind blowing from the back of the openings and nearly perpendicular to them reduced the mean air velocity at the two edges but did not change the turbulent velocity much. Over a considerable part of the day the mean and turbulent air velocities scale with the ambient wind; the total heat flux (mean plus turbulent) scales with the product of the ambient wind and temperature while the turbulent heat flux does not. The integral length scale of the inflow was approximately equal to the height of the opening and was larger than that of the outflow. Installing screens on the openings generated smaller scales and increased the spectral decay rate in comparison with openings without screens. A quadrant analysis enabled the determination of the events that significantly contribute to the turbulent heat flux. It is shown that at the outflow the turbulent heat flux is mainly due to eddies of cool air entering from the surrounding to the greenhouse while at the inflow it is due to both warm and cool eddies leaving and entering the greenhouse respectively. Nomenclature C p = specific heat of air, J kg −1 K −1 Co = co-spectra of velocity and temperature fluctuations, m s −1 K E = spectral density function, m 3 s −2 f = frequency, Hz F u = velocity flatness F θ = temperature flatness H = threshold H c = total heat flux, W m −2 L = integral length scale, m r uθ = correlation coefficient between u and θ 22 M. TEITEL AND J. TANNY R (t) = autocorrelation function of velocity S i,H = normalized conditional heat flux S u = velocity skewness S θ = temperature skewness T = temperature, K T 0 = ambient temperature, K u = fluctuating air velocity through the opening, m s −1 u = RMS of velocity, m s −1 U = mean air velocity through the opening, m s −1 U 0 = ambient wind speed, m s −1 U n = normalized mean air velocity through the opening u n = normalized RMS of velocityGreek Symbols θ = temperature fluctuation, K θ = RMS of temperature, K ρ = air density, Kg m −3 T = temperature difference between air at the opening and the ambient, K T a = temperature difference between average greenhouse air and the ambient, K

Analysis of Turbulent Air Flow Characteristics Due to the Presence of a 13 × 30 Threads·cm−2 Insect Proof Screen on the Side Windows of a Mediterranean Greenhouse

“…That is to say, uy and uz components may take a minor contribution to the mass-flow rate and may even be the consequence of rejected flow or formation of small ); outer side of the IPS at the window ( gronomy 2022, 12, x FOR PEER REVIEW 15 of 18 through the porous mesh. For the total airflow velocity u, the results obtained agree with the observations made by Teitel and Tanny [27], which confirms that the turbulence energy decreases and the spectral decay rate is increased. It is obvious that this loss of energy at high frequency is what produces the reduction in the velocity fluctuations of ux (seen in Figure 5).…”

“…However, in the longitudinal component u x , the energy transported in the larger scales (at low frequency) is maintained after passing through the mesh, whereas the energy transported by the smaller scales (high frequency) is dissipated or lost when passing through the porous mesh. For the total airflow velocity u, the results obtained agree with the observations made by Teitel and Tanny [27], which confirms that the turbulence energy decreases and the spectral decay rate is increased. It is obvious that this loss of energy at high frequency is what produces the reduction in the velocity fluctuations of u x (seen in Figure 5).…”

“…The reduction in energy suffered after passing through the IPS is clearly observed in the plot of the energy density spectra on the inner and outer faces of the window. Teitel and Tanny [27] used a one-dimensional sonic anemometer to certify that the use of a high-mesh IPS leads to a reduction in turbulence levels and scales and an increase in the spectral decay rate. However, since they used a one-dimensional anemometer, they could not observe the effect on each airflow vector field component.…”

“…Turbulence is a field in fluid dynamics with many unresolved questions about the generation of turbulence; thus, its control is not easy, and efforts are not oriented toward its characterization. Teitel and Tanny [27] analyzed turbulence flow data from a onedimensional sonic anemometer positioned at the roof openings of a four-light greenhouse.…”

Computational fluid dynamics in greenhouses: A review

“…Different turbulence models cause differences in speed, temperature and humidity patterns, confirming the importance of choosing the model that most closely matches the actual conditions of turbulence (Roy and Boulard, 2005). Teitel and Tanny (2005) showed that the output of the turbulent heat flux is mainly due to cold air entering the greenhouse, which produces hot and cold eddies that enter and leave the greenhouse. Roy and Boulard (2005) showed the effects of wind direction on climate parameters inside the greenhouse are usually simulated with the use of different turbulence models available, to determine the energy balance between the flow of perspiration and the flow of radiation.…”