## 1 Introduction  
Buoyancy-induced convection in air from heated vertical parallel plates is of much interest for a number of thermal engineering applications, such as the electronic equipment cooling and the solar energy capture to name a few.  
The first documented work dealing with this subject was executed experimentally by [PERSON] [1], who, by using a pair of square plates suspended face to face in free air, obtained the optimal plate spacing for the dissipation of the maximum amount of heat. Investigations aimed at determining the optimal plate spacing for vertical parallel plate channels subjected to uniform wall temperature and uniform heat flux conditions were subsequently conducted theoretically by [PERSON] and [PERSON] [2], [PERSON] [3], [PERSON] et al. [4] and [PERSON] and [PERSON] [5], as well as both experimentally and theoretically by [PERSON] et al. [6, 7] and [PERSON] et al. [8] for channels consisting of a heated plate and an unheated plate insulated at the rear.  
Other studies dealing with natural convection in vertical parallel plate channels, which were mostly performed experimentally, were carried out by [PERSON] et al. [9], [PERSON] et al. [10], [PERSON] and [PERSON] [11], [PERSON] and [PERSON] [12], [PERSON] and [PERSON] [13], [PERSON] and [PERSON] [14], [PERSON] et al. [15], [PERSON] et al. [16], [PERSON] et al. [17] and [PERSON] et al. [18] for both conditions of symmetric and asymmetric heating, with the aim to obtain heat transfer correlations rather than to determine the optimal plate spacing.  
While in some engineering applications the plates could be staggered rather than positioned face to face, only two studies are readily available in the literature on buoyancy-driven convection from a pair of staggered vertical parallel plates. One was performed numerically by [PERSON] and [PERSON] [19] and the other was executed experimentally by [PERSON] [20]. Their common conclusion was that for any assigned plate spacing, an optimal plate staggering for maximum overall heat transfer rate does exist. In particular, they found that the staggering of the plates resulted in a considerable enhancement of the thermal performance of the lower plate and a relatively moderate degradation of the thermal performance of the upper plate. However, no correlation was developed and this was mainly due to the limited number of investigated configurations. Other works with a bearing on the subject are those executed for arrays of fully staggered plates, such as those authored by [PERSON] and [PERSON] [21; 22], [PERSON] et al. [23], [PERSON] [24] and [PERSON] and [PERSON] [25].  
Framed in this general background, which points out a meaningful lack of data, a comprehensive investigation of natural convection from pairs of staggered vertical plates heated at the same uniform temperature and suspended in free air is performed numerically. The study is conducted using the Rayleigh number based on the plate length, the horizontal plate spacing normalized by the plate length and the vertical plate staggering normalized by the plate length as controlling parameters. The main scope of the paper is to scrutinize the basic heat and momentum transfer features and to analyze how the independent variables affects the thermal performance of each plate and the pair of plates. Furthermore, the study also aims to develop dimensionless heat transfer correlating equations that will be useful for thermal engineering applications.