An Experimental Study of Air Flow and Heat Transfer in an inclined Rectangular Channel with Wood Strips on the Bottom Plate

D. C. Diarra1*, L. Candanedo2, S. J.Harrison1, and A. Athienitis2

department of Mechanical and Materials Engineering, Queen’s University, K7L 3N6, Kingston, Canada Telephone: 1 6135332591, Fax: 1 613 5336489, email: diarra@me. queensu. ca 2 Department of Building, Civil and Environmental Engineering, 1455 de Maisonneuve Blvd. West, Montreal, QC, H3G 1M8, Concordia University, Montreal, Quebec *Corresponding Author: diarra@queensu. ca


Designs and configurations of building-integrated photovoltaic thermal (BIPV/T) air systems are based on the type of PV modules, the location, and the geometry of the framing on which the modules are to be mounted. Moreover, the unpredictable behavior of airflow in Building Integrated Photovoltaic ducts under natural convection requires high accuracy velocity measurement techniques to successfully predict the airflow rate and its pattern. Using an asymmetrically heated channel from the top to represent a BIPV configuration under field conditions, the air flow pattern and the temperature distribution of the system components were investigated. A Particle Image Velocity (PIV) system was used to study the air flow pattern in the system.

The results obtained gave a better understanding of the air flow pattern and the heat transfer mechanisms in practical roof-integrated BIPV and BIPVT system configurations and the formulation of some design guidelines for building integrated Photovoltaic systems in natural convection.

Keywords: heat transfer, wood strips.

1. Introduction

The introduction of BIPV market, the diversification of the designs and the various efforts to improve the systems, led to practical application of air-cooled PV/T and BIPVT systems across the world [1]. Consequently, building integrated photovoltaic systems are increasingly popular. There are many previous studies of natural and forced convection over vertical and horizontal plates, and through inclined channel formed by smooth parallel plates [2, 3] outlined some design procedures for smooth PVT ducts under natural convection, while [4], proposed a correlation for the calculation of local and average Nusselt number for asymmetrically heated channels with inclination angles ranging between 18° and 30°. Liao et al. [5] also presented a computational fluid dynamics (CFD) study of heat transfer for a BIPV/T facade. Brinkworth and Sandberg [6], reported the effects of ribs on the buoyant flow induced in a duct. It is found that the additional hydraulic resistance due to the presence of the ribs does not affect the flow-rate greatly, since the flow varies roughly as the cube root of the total resistance and secondly, an additional resistance will have a noticeable effect only if it is greater than the fixed values already present, arising from the flow losses at the inlet and along the duct walls.

Usually natural ventilation by air has low flow rates especially in residential areas. Heat and energy transfer processes in a practical BIPVT system (under field conditions) is a complex scenario. Currently limited information and data are available on BIPV/T energy systems analysis taking into account the real configurations and framing of the air duct. The current experimental test apparatus was designed to represent a practical model, configuration, and setup of BIPVT systems operating under field conditions. The thermal components of the PV module were simulated by an aluminium plate heated by uniform heat fluxes, and a particle image velocity system was used to study the air flow pattern in the channel.

2. Experimental Model

2.1 Components of the Channel


Figure 2 shows a schematic of the experimental PVT air duct system.

The plate was 2.4 m long and 0.34 m wide, and made of 0.002m thick aluminium sheet with an emissivity of 0.27. The bottom plate was also 2.4 m long, 0.34 m wide and made of Plexiglas with 0.006 m of thickness. Both side plates were also in plexiglass with 0.003 m thickness, and 0.14m height. The surface emissivity of the plexiglass was 0.97. The space (H) between the two plates could be varied by moving the bottom plate up or down. To ensure a two-dimensional flow, the channel height used in the experiment could be adjusted in the range of 0.015 m to 0.050 m. The channel was mounted on a steel frame with an adjustable tilt angle of 20° to 44°.

2.2 Measurements

The temperature distribution along the top surface was measured with copper/constantan thermocouples starting at 0.15 m and spaced at 0.3 m intervals. Three rows of eight thermocouples each were placed along the centre left-hand and right-hand sides of the top plate, giving an average temperature (Tt ) representing the top plate temperature. On the bottom plate of the channel, eight thermocouples were placed along the centre line opposite those as the top wall to give the average temperature (Tb ) of the

bottom plate. Temperatures of the insulation TftM(top plate insulation), Tbins (bottom plate insulation),

T (side insulation) were also measured at different points along the channel. Two Vaisala humidicap

sensors (Vaisala HMT 333) were used to measure the relative humidity and temperature of the air at both the inlet and outlet of the duct. Figure 3 illustrates the temperature sensors’ location for the air between and under the wood strips along the channel.


Figure 3: Wood strips and thermocouple location
inside the duct

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