Session: 12-04-01: Heat and Mass Transfer in the Natural and Built Environments

Paper Number: 166853

Experimental Natural Convection in Vertical Cavities With Adiabatic Extensions. The Effects of the Distance Between Symmetrically or Asymmetrically Heated Staggered Walls 

Due to modern engineering applications such as solar collectors, passive ventilation of buildings, cooling of electronic equipment, and nuclear reactors, there is a renewed interest in the study of natural convection in vertical channels. The building sector in the European Union currently accounts for approximately 40% of total global energy consumption. To address this, researchers are actively developing innovative solutions to enhance heat storage in buildings while improving comfort and energy efficiency. Among passive techniques, various strategies are employed. A notable example is the Trombe-Michel Wall. Moreover, air cavities are widely utilized for cooling applications, including the thermal management of electronic components and solar panels. In the case of solar panels, the majority of which are made from crystalline silicon, their efficiency drops when temperatures exceed 25°C. To mitigate this issue, one effective technique is back ventilation. This can be optimized by adjusting specific cavity parameters, such as the height-to-length ratio of the cavity.

This experimental work aims to study the heat exchange inside open-ended cavities by varying the distance between the walls, considering the presence or absence of adiabatic extensions that follow or precede parts heated both symmetrically and asymmetrically.

The test section consists of two walls, called I and II, supported by a metal cage that allows the adjustment of the orientation in space and the possible inclination with respect to the vertical axis.

Each wall is composed of three modules that can be assembled in different ways. Two of the three modules are adiabatic panels measuring 20x46x2.5 cm and 5x46x2.5 cm, while the third is a plywood frame measuring 25x46x2.5 cm on which the heater, aluminum plate, thermocouples and thermal insulation are fixed to limit heat dispersion towards the outside of the duct. The aluminum plate measures 22x42x0.15 cm. Let L be the height of the aluminum plate (L = 22 cm) and let H be the overlap between the plates of the two walls, the configurations studied in this work are for H/L=1, H/L=0.1 and H/L=0.3 with and without adiabatic extensions in the case of heating of a single wall, symmetric heating and asymmetric heating with thermal powers set to 10 W, 22 W and 34 W. The walls were positioned at a distance s = 11-9- 7-5-3-2-1 cm and let B and D be the heights of the adiabatic sections upstream of L, while A and C are the heights of the adiabatic sections downstream of L. The Rayleigh number is 8,2∙10^4≤Ra_𝐿≤2,2∙10^6.

The main conclusions reached can be summarized as follows:

*Heating only Wall I (Q1) or only Wall II (Q2):

1. For Wall I, there is always a maximum as distance s varies. If adiabatic extensions are present, Wall II also always has a maximum;

2. The staggering benefits both walls;

3. In the absence of extensions and for high powers, 𝑁𝑢_𝐿2 improves thanks to the guiding effect of Wall I.

*Heating of both walls (Q1Q2):

1. H/L=1 the extensions do not change the trends, and 𝑁𝑢𝐿 decreases compared to the Q1 case;

2. H/L<1 Wall I always benefits, while Wall II is always disadvantaged. Adiabatic extensions improve the heat exchange.

Finally, the measured data were correlated through relations of the type:

Nu_L =f (Ra_L, s/L, H/L, A/L, B/L, C/L, D/L)

The resolution method is the Nonlinear Generalized Reduced Gradient. The objective function is represented by the sum of the least squares obtained as the difference between the measured and interpolated Nusselt number, while the decision variables are all the coefficients to be determined in the relation

Presenting Author: Ivano Petracci University of Rome &quot;Tor Vergata&quot;

Presenting Author Biography: Ivano Petracci, PhD, is an Associate Professor at the Department of Industrial Engineering of "Tor Vergata" University of Rome. He currently teaches "Thermodynamics and Heat Transfer 1" and "Energetics" for the Bachelor's and Master's degrees in Mechanical Engineering, the course "Thermodynamics and Heat Transfer" for the Degree in Energy Engineering and for the Master's Degree in Medical Engineering. His experimental activity focuses on methods for enhancing convective heat transfer with active and passive techniques (extended and treated surfaces, lattice ducts, acoustic fields applied to fluids and composite solutions). The research is supported by thermodynamic and fluid dynamic investigations (hot wire anemometry and flow visualization techniques such as Particle Image Velocimetry) and by CFD studies in thermo-fluid dynamic applications.

Authors:

Ivano Petracci University of Rome "Tor Vergata"
Alessandro Quintino Sapienza Università di Roma
Vincenzo Spena Sapienza Università di Roma
Massimo Corcione Sapienza Università di Roma
Angelo Spena University of Rome "Tor Vergata"