###### Abstract  
As most existing office buildings in China lack fresh air systems for ventilation, natural ventilation with windows remains the main means of improving indoor air quality and adjusting indoor thermal comfort. However, knowledge of the ventilation characteristics of various window-opening forms in actual buildings is limited and current methods for evaluating ventilation performance lack a comprehensive consideration of ventilation rate and thermal comfort. In this study, the ventilation characteristics of different window-opening forms were systematically compared by conducting computational fluid dynamics (CFD) simulations. A full-scale experiment was conducted in a typical office room in a university in Tianjin to validate the CFD simulation. Two ventilation modes (window-driven cross-ventilation and temperature-driven single-sided ventilation), three window-opening angles, and seven window types were investigated. Additionally, the ratio of the ventilation rate to the absolute value of thermal sensation was used to quantify the indoor natural-ventilation performance. The results showed that a sliding window with a full opening has the highest discharge coefficients of 0.68 and 0.52 under wind-driven cross-ventilation and temperature-driven single-sided ventilation, respectively, and top-hung windows opening both inwards and outwards have better ventilation performance than other window types under the two ventilation modes. This study is applicable to the design and practice of natural ventilation. Keywords:natural ventilation; window-opening forms; ventilation performance; CFD +
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effects of the position of a large rectangular opening on the discharge coefficient under single-sided ventilation and concluded that the vertical position of the opening is the most important influencing factor. [PERSON] et al. [21] investigated the ventilation performance of side-hung windows and bottom-hung windows under single-sided ventilation and cross-ventilation and confirmed that the discharge coefficient varies with both the opening area and window type. [PERSON] et al. [22] conducted computational fluid dynamics (CFD) simulations to identify a positive correlation between the discharge coefficient and the opening angles. However, the aforementioned studies are limited in terms of certain window-opening forms or window configurations. In addition, there have been few studies on the window-opening angle [23]. To evaluate the performance of natural ventilation, indices such as air change per hour [24; 25] and age of air [26] are commonly used. The air change per hour is the ratio of ventilation rate per hour to room volume. The age of air refers to the time required for air from entering the room to reaching a certain point in the room, which reflects the freshness of indoor air. However, under unsteady conditions, the advantages and disadvantages of natural ventilation cannot be determined based on only the ventilation rate; the indoor thermal comfort should also be considered. For the evaluation of thermal comfort under natural ventilation, the commonly used indices are the predicted mean vote-predicted percentage dissatisfied (PMV-PPD) [27; 28] and the thermal comfort adaptability model [29]. PMV represents the average cold and heat felt by most people in the same environment. PPD represents the percentage of people dissatisfied with the thermal environment. [PERSON] et al. [30] discussed the advantages and disadvantages of the PMV and adaptive comfort models in evaluating the thermal comfort of natural ventilation and proposed that attention should be paid to their combination. In general, occupants open windows for ventilation for two main purposes: one is to meet the needs of indoor ventilation, and the other is to adjust the indoor thermal comfort. However, the current evaluation index considers only one aspect, that is, either the ventilation rate or indoor thermal comfort, and lacks a comprehensive consideration of both.