With the acceleration of urbanization in southern China in recent years, due to the high building density of the traditional urban blocks in the humid and hot areas, the urban heat island (UHI) effect is aggravated, which leads to the deterioration of the micro-climate in the urban blocks. On the other hand, high building density restricts the potential of natural ventilation in summer and increases the difficulty of thermal comfort and indoor air quality (IAQ) improvement in the blocks. Commercial buildings in these blocks provide consumption and recreational spaces that play a significant role in humans’ daily business activities. Their indoor thermal comfort directly affects their business and urban tourism [1
]. Both thermal comfort and IAQ are beneficial to human health [2
], since people spend almost 90% of their time indoors at present [3
]. A thermal discomfort or poor IAQ environment reduces the productivity of dwellers and even their wellbeing [4
One kind of colonnade shopping street (called qilou, see Figure 1
a) gradually emerged in southern China as an adaptive response to the local humid and hot climate. At the end of the 19th century, many qilou shopping streets were built in Guangzhou, Wuzhou, Haikou, Nanning, and other Lingnan regions, all of which boasted flourishing trade and rapid economic development. Some negative effects are that micro-climates in qilou streets have higher air temperature and lower air velocity than their surroundings [5
], which is referred to as the heat island effect [6
]. The qilou streets and their pavements trap a lot of solar energy daily and emit it at night. Hence, this increases the requirement for air conditioning in qilou street shops to guarantee customers’ thermal comfort [7
]. Though designed mainly for commercial use, qilou buildings integrate both residential and commercial functions, with the front part serving as a shop space and the rear part as a residential space. The colonnades of qilou also provide an environment protecting visitors from sunlight, wind, and rain. The shop space is connected with the colonnade walkway (see Figure 1
b) and is used for displaying goods and conducting business. The shop space is enclosed on three sides, and the doorway side is used for the introduction of natural lighting and ventilation (see Figure 1
c). Consequently, based on the requirements of indoor thermal comfort and air quality, the optimization to the ventilation scheme of air conditioning in qilou street stores is still a problem to be solved.
Summer is the peak season for traveling and shopping; the flow of people in a commercial street, such as a qilou, is denser; people would like to stay in the store for a quite long time; and there are higher requirements for the indoor thermal comfort and IAQ of commercial buildings [8
]. Therefore, in the study of urban blocks’ redesign, attention should be paid to their indoor thermal environment, and the research on the thermal environment of buildings is rising in recent years. At present, the investigation on the thermal environment of buildings focuses on urban planning, residential planning, campus planning, and monomer building design. However, most of them are related research results in residential areas, schools, urban squares, outdoor thermal environment, and energy-saving, which are less involved in dealing with the thermal comfort level and IAQ inside traditional street stores [10
The existing research on qilou streets has mainly focused on the perspective of historical preservation [12
]. Few studies have been reported on the thermal environment of qilou street shops. With increasing attention paid to the thermal environment of urban shopping streets, thermal adaptation and thermal comfort models considering local urban climate factors and street morphology have become a research hotspot [15
]. For example, Ruiz et al. [18
] evaluated a model for the thermal environment of arid-climate cities and predicted urban thermal comfort in these cities. The thermal comfort of pedestrian streets in severe cold areas of China has received the attention of researchers, who investigated the major parameters influencing thermal comfort in different seasons and an optimization strategy [19
]. They found that the mean radiant temperature (MRT) had the greatest influence on thermal comfort during the summer and winter, whereas the thermal resistance of clothing had the greatest influence on the thermal comfort of people during the transition seasons, i.e., spring and fall. Understanding the human thermal sensation in commercial buildings requires a field survey of the thermal environment in large commercial building complexes. Li et al. [20
] conducted a field test and questionnaire survey of a large shopping complex in Guangzhou and found inconsistencies between the predicted and actual thermal sensations. Based on this, they established an adaptive model for describing the variations in interior thermo-neutral temperature with the exterior air temperature in winter. Chen [21
] investigated the variation in thermal sensation with temperature in different areas of the same shopping complex using a similar method and obtained thermal adaptation models for four types of functional areas. Dang et al. [22
] conducted a test and subjective evaluation of the indoor thermal environment of a large commercial complex in Beijing during winter, established an equation for an adaptive predicted mean vote (PMV) correction, and proposed a mathematical model for assessing the winter thermal environment of large shopping malls.
Based on the direct influence of ventilation scheme on thermal comfort and air quality and the unique airflow distribution of hotel lobbies, Zhang [23
] numerically simulated the summer thermal environment of a hotel lobby using a computational fluid dynamics (CFD) method, comparatively analyzed the indoor thermal comfort levels under different air circulation modes, and explored the optimal air circulation solution. To develop approaches for improving the air quality of shopping malls, Huang et al. [24
] measured the air quality and thermal parameters of a shopping mall in Ma’anshan through a field survey as well as the subjective assessment of the indoor air quality by the shopping assistants and customers through a questionnaire survey. They found that users were quite unsatisfied with the air quality. Compared with commercial complexes, street shops in traditional blocks have a different thermal environment owing to the unique street layout. Their thermal comfort is also worthy of attention. For the purpose of evaluating human thermal sensation precisely, Henderson et al. [25
] suggested replacing the constant-temperature control strategy with a constant-comfort adjusting strategy in the control of air conditioning systems to maintain constant comfort instead of constant temperature. Under most circumstances, the constant-comfort based adjusting strategy also improves the indoor thermal comfort level at reduced energy consumption of the AC systems.
A salient characteristic of an urban climate is micro-climate differentiation. Different urban zones have diverse micro-climate environments owing to the influence of many factors, such as building layout, vegetation, water body, and spatial characteristics. Qilou street shops have markedly different building spaces than other street shops due to the unique colonnade walkway space. Namely, the urban features of qilou streets have enormous effects on their local thermal environment. Therefore, the research findings on the thermal environment mentioned above cannot be directly extended to qilou shopping streets. The reasons for this are as follows.
The existing research mostly focuses on ordinary shopping malls and large commercial complexes by measuring and qualitatively assessing their thermal environments. For traditional shopping streets in particular climates (such as humid and hot ones), the indoor thermal environment is greatly affected by the outdoor block layout and diverse human activities. Hence, the existing research findings are not sufficiently applicable to qilou shopping streets in humid and hot climates.
The two widely applied thermal comfort models, the PMV model proposed by Fanger [26
] and the adaptive model presented by de Dear [27
], do not take indoor air cleanliness into account. The IAQ, however, is also a research topic of significant and growing interest [28
]. In order to improve the IAQ of school classrooms in the subtropical region, Liu et al. [29
] analyzed the area ratio of AoA in the classroom plane to determine the optimal window orientation. For providing useful information for comprehending the situation of air quality in sensitive indoor and outdoor areas, Lucialli et al. [30
] performed a test to investigate the indoor and outdoor concentrations of benzene, toluene, ethylbenzene, and xylene in eight schools. The results are helpful to enhance environmental quality in school construction renovation programs. Since classrooms and shops have similarities in air quality requirements and crowd density, the study of IAQ in classrooms has enlightened the IAQ study in shops. For the purpose of investigating the main influencing factors of air quality in commercial settings, Zhang et al. [31
] continuously monitored IAQ in two large-scale comprehensive commercial places in Beijing and found that IAQ in functional areas of the shopping mall was related to human activities, ventilation, air freshness, and so on. The outbreak of COVID-19, furthermore, has increased the awareness of the importance of IAQ. Maintaining air freshness through healthy ventilation is critical for improving the IAQ of shops.
However, most of these studies were performed on cold and humid or cold and dry types of climates, and there are very few studies on the hot and humid type of climatic conditions prevailing in southern China. There also has been a lack of research on the thermal environment and IAQ of traditional commercial buildings (like qilou street shops) in areas of severe summer. To fill this research gap, this study will investigate the indoor thermal environment of qilou street shops in Nanning (see Figure 2
a,b). Considering that the airflow mode is a major factor influencing the thermal comfort and IAQ in mechanically ventilated qilou street shops, the thermal environment of these shops will be evaluated using two indices, PMV and AoA, which are the time of air particles required to reach a certain location in the airflow field [32
]. The patterns influencing the ventilation scheme on the indoor thermal comfort and IAQ are analyzed, and approaches are proposed for effectively improving the thermal environment and IAQ of qilou street shops in hot and humid areas. This study aims to analyze the effects of mechanical ventilation scheme (air circulation modes for cooling) on thermal comfort and air quality in a qilou street shop with air conditioner. In Section 2
, we introduce the parameters of micro-climate in qilou streets, including monthly average temperature, hourly dry bulb temperature, monthly relative humidity, and the summer wind environment in qilou street areas. The indices (PMV and AoA) to evaluate the thermal performance and air quality in a qilou street shop are established in Section 3
, and the indoor thermal environment of a qilou street shop is measured and simulated in Section 4
. Then, the analysis of numerical simulations is conducted to optimize the ventilation scheme by evaluation in terms of PMV and AoA in Section 5
. Finally, the conclusions are provided in Section 6
2. Micro-Climate Environment in Qilou Street Block
Nanning is located south of the Tropic of Cancer and has a wet subtropical monsoon climate. The annual highs of monthly average temperature (MAT) occur from June to September (see Figure 3
]. The MAT values in these four months are 28.1, 27.9, 28.1, and 27.3 °C, respectively. The annual lows of MAT occur from December to February. The MAT values in these three months are 14.9, 13.9, and 14.4 °C, respectively. Thus, Nanning has a long period of high temperature annually, with four months having an MAT of above 27 °C. As shown in Figure 4
, the weather is fairly hot for most hours of the year. High air temperature in urban blocks can impair human health; as a consequence, reduction of indoor air temperature should be a key consideration inside a building and block micro-environment redesign.
The relative humidity (RH) in Nanning varies insignificantly throughout the year and is fairly high for most of the year. The monthly average RH is approximately 75–85% and is much higher than many other cities in China for most months of the year (see Figure 5
). In summer, a high humid environment increases the sensible temperature, causing sultriness and discomfort.
To further understand the climatic characteristics of qilou shopping streets in Nanning, the summer wind environment of the block was simulated using the PHOENICS software (see Figure 6
). The software PHOENICS that was first launched in 1981 by Cham UK is a well-known numerical simulation software, which is used to simulate wind fields and heat transfer in building environments [34
] and offers different kinds of turbulence models. The results showed that the airflow velocity was low in the building clusters, the wind-free areas are developed, and the ventilation is weak in the building clusters owing to the dense network of roads of different widths in the block. The poor ventilation and high density of the building clusters resulted in a relatively high temperature, and several high-temperature concentration zones and thermal discomfort in these areas are developed. Therefore, the indoor thermal discomfort level in qilou streets could not be solved through natural ventilation. Air conditioners and electric fans are commonly employed for cooling and ventilation.
5. Optimization and Discussion
Indoor space differs from the outdoor space that makes up the qilou street environment; the customers staying in the shop experience variations in thermal sensation due to ventilation schemes that directly affect humans’ thermal comfort and air freshness.
Generally, the ventilation schemes for the shop include the upper-inlet–bottom-outlet, upper-inlet–upper-outlet, and side-inlet–side-outlet [21
]. The thermal environments inside the shop under the three ventilation schemes will be simulated separately by using PHOENICS software. Figure 10
, Figure 11
and Figure 12
show the shop models, PMV, and AoA distribution in the occupied zone under three ventilation schemes, respectively.
When the shop is air-conditioned adopting the upper-inlet–bottom-outlet scheme (see Figure 10
a), owing to the combined effect of the air curtain and the air-conditioning outlets, the cooling air first blows towards the floor, forming a zone of cool high-pressure air in the middle-to-front part of the shop, with an average PMV of 1.032. This zone is adjacent to the hot space near the doorway, resulting in a non-uniform PMV distribution inside the shop space, as shown in Figure 10
b. The rapid variation between cool and hot easily makes the occupants feel uncomfortable. Under this ventilation scheme, the airflow in the rear part of the shop space is restricted, resulting in a high AoA (with an average value of 253.12 s) and a low air freshness level in the rear space of the shop (see Figure 10
c), creating a feeling of sultriness.
Under the side-inlet–side-outlet scheme (see Figure 11
a), the inlet airflow zone in the upper side of the shop has a high level of air freshness. The inlet airflow descends owing to the low temperature. High air freshness level and low AoA occur in the place that is near the opposite wall from the inlet. The cool air descends along the wall and absorbs heat to move up to the ceiling level, creating an airflow circulation in the vertical direction of the shop space. The thermal sensation on the air inlet side is relatively cool, resulting in a certain level of temperature variance across the different parts of the human body and attenuated comfortable feeling. Figure 11
b indicates that the thermal sensation inside the shop is slightly hot, as the average PMV is 1.07 and PMV is an uneven distribution. The resulting air circulation makes the air linger for quite a long time inside the shop, with a mean AoA of 218.46 s. In particular, the rear space of the shop has a high AoA and low air freshness level, and the AoA varies considerably from point to point as shown in Figure 11
c, making the shop space unsuitable for a long occupancy. Therefore, the IAQ of the shop is relatively poor in the shopping area.
As illustrated in Figure 12
a,b, the upper-inlet–upper-outlet scheme produces a relatively uniform PMV field in the shop, which exhibits a slightly warm feeling in the front part and a slightly cool feeling in the rear part, and a warm–cool transition is reasonable, thus avoiding the discomfort caused by abrupt changes and minimizing the impact on the thermal sensation of the customer. Satisfying thermal comfort is provided for people standing beside the showcases to select and purchase goods, owing to this shopping area with a low value of PMV. Most of the space of the shop also has lower PMV values, with an average PMV of 0.995. As the indoor thermal comfort requirement in China is −1 ≤ PMV ≤ 1, the expected PPD of the indoor environment should not be greater than 25% [42
], so that the indoor thermal comfort under the upper-inlet–upper-outlet scheme meets the requirement of GB/T33658-2017. In addition, the indoor AoA has a uniform distribution and is young, with a mean AoA of 205.76 s in the breathing zone, indicating that there is a relatively high level of air freshness and it does not make the occupants feel stuffy in the store. Figure 12
c displays a uniform and low AoA distribution, which satisfies requirements for comfort and air hygiene developed in the shop.
Among the three ventilation schemes, the upper-inlet–upper-outlet scheme with the lowest PMV and highest air freshness level exhibits the most favorable indoor thermal environment. Figure 13
shows a comparison of the mean values of PMVs and mean AoAs under three schemes, and indicates that the upper-inlet–upper-outlet mode of air circulation is more conducive to creating the favorable PMV and AoA distribution. In addition, this kind of mechanical ventilation requires less AC energy consumption to provide the same level of thermal sensation. Hence, this ventilation scheme better facilitates energy saving. Therefore, the comparative analysis above shows that the upper-inlet–upper-outlet scheme can create a comfortable, healthy, and energy-saving thermal environment in the qilou street shop.
By analyzing the impact of the ventilation scheme on indoor thermal comfort and AoA, this study indicates that the ventilation performance of upper-inlet–upper-outlet mode is an optimal scheme. During simulation, some details of the shop have been simplified. In the shop, the effects of enclosing structures and different heights of inlets and outlets are not taken into account. It is recognized that the building model adopted in the present study was relatively simple in comparison to the real qilou street shops. However, the obtained results in the study do provide useful insight into the magnitude of effect that mechanic ventilation schemes are likely to have on qilou street shops. Since only indoor thermal environment under summer conditions is investigated in this paper, the future work will focus on investigating the indoor thermal environment of qilou street shops in different seasons, and describing the vertical distribution of PMV and AoA in these shops.