Built-In Environmental Construction Mechanism and Sustainable Renewal Strategies of Traditional Qiang Dwellings in Western China

Abstract

Indoor air quality (IAQ) has a significant impact on human health, as people spend 90% of their time in various indoor environments. Therefore, research on IAQ is extremely necessary. However, current research on traditional Qiang residences in western Sichuan mainly focuses on the indoor thermal environment and heritage protection, with relatively little attention paid to IAQ. This study investigates the IAQ of traditional Qiang residences in western Sichuan, which have open fire pits as the core of daily life, exploring the impact of passive renovation strategies on the indoor air quality. Using simulation methods, this study employs passive strategies, such as increasing the size of windward windows, changing ventilation methods, relocating the fire pit, and enlarging interior partition openings, to improve and optimize the IAQ through natural ventilation. The results show that when the windward window sizes are 0.8 m × 1.9 m and 0.7 m × 1.55 m, the reduction in the indoor CO₂ concentration is the greatest, with a maximum decrease of 0.024% at the 1.5 m plane. This paper proposes passive renovation strategies to improve the indoor air quality of Qiang residences in western Sichuan. These strategies effectively enhance the indoor air quality of Qiang residences and address the research gap on indoor air quality in regional Qiang residences in western Sichuan. The insights and methods presented contribute to the improvement of the indoor air quality in traditional buildings and support the sustainable development of traditional architecture.

1. Introduction

1.1. Background

Air quality has huge impacts on the comfort and healthiness of various environments [1]. Worldwide, people tend to spend approximately 90% of their time in different indoor environments [2]. Users of residential buildings can experience greater health impacts from living in poor-air-quality environments for prolonged periods. The existence of indoor air pollutants—such as carbon dioxide, sulfur dioxide, nitrogen dioxide, particulate matter, and total volatile organic compounds—is a critical issue for human health [3]. According to the EPA (Environmental Protection Agency), indoor levels of pollutants may be up to 100 times higher than outdoor pollutant levels and have been ranked among the top five environmental risks to the public [4]. The WHO (World Health Organization) reported that poor air quality caused 4.2 million deaths in 2016 [5]. In recent years, the engineering community has placed increasing emphasis on maintainability [6], alongside a growing focus on the sustainable development of buildings. More and more scholars have begun to pay attention to indoor air quality, hoping to provide a comfortable living environment for people’s health by effectively improving air quality. In recent years, with the widespread popularization of the concepts of energy savings and emission reduction, natural ventilation, a passive method, has become an important means to improve indoor air quality. Studies have shown that occupants of naturally ventilated buildings are comfortable over a wider range of temperatures than occupants of buildings with centrally controlled HVAC (heating, ventilation, and air conditioning) systems [7]. Therefore, this paper investigates and proposes improvement measures for indoor air quality by simulation and other means, using natural ventilation and other methods.

The Qiang, one of China’s ethnic minorities, is located in southwestern China. The Qiang people had already flourished on the Tibetan Plateau during the Stone Age [8]. Qiang traditional dwellings are renowned for their architectural features, described as “residing by the mountain, building with stacked stones, and reaching heights of over ten meters”. Based on materials and structures, Qiang dwellings can be classified into stone masonry, rammed earth, and board houses. In the early days, because of military defense requirements, Qiang dwellings had intricate structures with interconnected households, allowing for thorough connectivity. Inside these dwellings, the Qiang people constructed fire pits [9] as traditional heating devices. These fire pits not only provided warmth and facilitated food processing but also became cultural symbols of the Qiang community. Because of the high altitude and consistently low temperatures, the fire pits are rarely extinguished, earning them the name “eternal fire”. The Qiang people conduct daily activities around the fire pits, and the smoke from the burning flames is used for food preservation, showcasing the architectural ingenuity of the Qiang people. However, the combustion of fire pits produces a significant amount of smoke, resulting in poor indoor air quality.

Therefore, this study selects a typical dwelling in Taoping Qiang Village as the research subject. It aims to address the issue of reduced indoor air quality caused by the smoke from the fire pits by exploring natural ventilation strategies to improve the indoor air environment.

1.2. Research Status

1.2.1. Qiang Dwellings

Xiong et al. [10] investigated the damage levels of residential buildings in Taoping Qiang Village following the Wenchuan earthquake. Their study provided a detailed analysis of the structural aspects and seismic performance of these residences. The results indicated that the extent of the damage to the Qiang residential buildings varied significantly because of differences in building materials, construction techniques, and building scales. Zou [11] conducted a study on the seismic performance of residential structures in Taoping Qiang Village. Through basic mechanical performance experiments, shear wall performance analysis, and examination of factors affecting seismic performance, such as volume, tapering, and shared walls, the study proposed corresponding measures for seismic reinforcement, including small-volume low-rise structures, tapered walls, and shared wall reinforcements. Sun et al. [12], through field surveys and questionnaires, studied the site selection, layout, and architectural characteristics of residential buildings in Taoping Qiang Village, exploring the stability of the Qiang architecture. The results indicated that the stability is closely related to the unique local construction materials and the “four-sided pyramid” structure. Hu [13], through field investigations, identified the threats currently facing the Qiang residential heritage, including ongoing earthquake damage and unscientific reconstruction. This study highlights the importance for protecting the Qiang’s residential heritage and proposes corresponding countermeasures. Fu [14], by reviewing the construction materials and structural characteristics of Qiang residences, elaborated on the demands for the protection and inheritance of the Qiang residential heritage. The study proposed regional creation strategies that include recognizability, authenticity, systematicity, and adaptability.

Regarding the thermal comfort of Qiang residential interiors, many scholars have conducted studies on the indoor thermal environment of Qiang residences and proposed corresponding renovation and improvement strategies, effectively enhancing the indoor thermal environment of traditional Qiang residences. Liao [15], using field measurements and questionnaires, investigated the indoor thermal comfort of traditional Qiang residential buildings in Taoping Qiang Village. Fu [16] conducted a study on the indoor thermal environment of typical stone-built residences of the Tibetan and Qiang peoples in the cold regions of Sichuan. He analyzed the existing indoor thermal environment problems and found that these residences urgently need improvements. He proposed passive renovation strategies suitable for the local area, considering factors such as building location, building form, and layout design and verified the feasibility of these strategies. Li et al. [17] conducted field surveys to study the ecological adaptation experiences and energy-saving technologies of traditional Qiang residences. They proposed modern green energy-saving renovation strategies, including improvements to fire pit spaces, energy systems, and construction techniques. Xiao [18], through field surveys, identified and summarized climate-adaptive design and construction techniques from traditional residential environments. By referring to climate-zoning methods, the study proposed “Northern Qiang” and “Southern Qiang” climate zones within the Qiang area and suggested corresponding overall layout and individual building design renovation strategies to effectively enhance the indoor thermal comfort of Qiang residences. Wang et al. [19] focused on the indoor lighting environment of traditional Qiang rammed-earth residences with poor physical indoor environments. Through field measurements and simulations, they proposed two improvement measures: increasing the reflectance using light-colored wooden walls and enhancing the utilization of natural light by employing light pipes.

Research on traditional Qiang ethnic dwellings by foreign scholars is relatively limited. However, many foreign scholars have conducted studies on the indoor thermal comfort of their own countries’ dwellings or traditional buildings, indicating a high level of international attention for improving the indoor thermal environment of traditional dwellings. Chkeir et al. [20] conducted a comparative study on the indoor thermal comfort of traditional and modern houses in Lebanon, using analysis of different environmental parameters and surveys. The study results showed that because of the use of natural and passive methods, traditional houses have better thermal comfort than modern houses. The study also pointed out that whether modern or traditional, there is still room for improvement in energy efficiency. Okafor [21] conducted field measurements in warm and humid southeastern Nigeria, recording indoor environmental changes in nine buildings, including traditional and modern buildings. The study results showed that traditional buildings have lower indoor temperatures compared to modern buildings; thus, passive practical methods should be adopted to improve the indoor thermal environment of traditional buildings. Aqilah et al. [22] provided an overview of the “comfort line” and energy-saving potential of residential buildings to determine the comfortable temperature range of residential buildings and elucidate the impacts of adaptive measures on the residential energy-saving potential. The study showed that the variation in the residential comfortable temperature mainly depends on the climate and building operation mode.

In summary, domestic research on Qiang residential buildings can be broadly divided into two aspects: the buildings themselves and the living environment. For the buildings, most scholars focus on their seismic performance and their protection and development, while for the living environment, most scholars focus on improving the indoor thermal comfort of the buildings. Research by foreign scholars on traditional dwellings also shows that the thermal comfort of traditional buildings is generally superior to that of modern houses. This indicates the necessity for researching and protecting traditional Qiang residences. However, there is currently a lack of research on the indoor air quality of Qiang residences, which significantly impacts residents’ health. Therefore, this paper argues that the research and optimization of the indoor air quality in Qiang residences are essential.

1.2.2. Fire Pits

Research on the primary heating device in traditional Qiang dwellings, the fire pit, is relatively limited and primarily concentrated in China. However, similar heating devices in other countries have also been studied, indicating a broad international interest in improving indoor thermal environments of traditional dwellings. Ren [9] used cluster analysis tools to study the layout characteristics, spatial forms, and material constructions of fire pits, generating corresponding clustering results and visual maps. The study confirmed that a residential building culture characterized by open-burning fire pits is shared across multiple ethnic groups and regions. Guan et al. [23] analyzed the structural form, functional settings, ergonomics, and spatial layout of fire pits, evaluating the design rationality of the fire pit in Yang’s Courtyard in Taoping Qiang Village. This research provides references for furniture and interior space design. Yan [24] compared traditional Chinese fire pits with Western fireplaces, deeply analyzing the differences in traditional residential cultures between China and the West. The study revealed that these cultural differences are influenced by geographical environments and cultural perceptions. Cheng et al. [25] studied fire pits in Western Hunan by analyzing spatial characteristics and using CFD simulations to quantify pollutant concentrations, providing an analysis of the indoor air quality. They proposed passive renovation strategies, such as increasing the window-to-floor ratio on the windward side and adding openings on the leeward side. Hua [26] addressed issues of poor thermal comfort and low heating efficiency in rural Western Hunan. Through questionnaires, on-site inspections, and simulation studies, an assembled-biomass fire-pit-heating system was proposed. The study results showed that this system effectively improved the indoor thermal environment of rural houses in Western Hunan.

In terms of residential buildings abroad, fireplaces are traditional heating devices, and numerous scholars have researched them. Alves et al. [27] analyzed emissions from the combustion of seven types of trees grown in Portuguese forests and coal blocks produced from forest biomass waste in fireplaces and wood stoves. By comparing emission levels with the literature data, significant differences were found between old household appliances and more efficient modern woodstoves and boilers. Because of the impacts of particulate emissions from fireplaces on the environment and human health, Karlsson et al. [28] studied the effects of user behaviors on particulate emissions and mitigation techniques. Through online surveys collecting responses from 146 participants, the study indicated that users’ emotions might influence their energy use and choices. Van der Walt et al. [29] measured the indoor air quality in seven South African households, using the PM2.5 concentration as the evaluation standard, comparing pollutant levels from open and closed fireplaces. The results showed that PM2.5 concentrations exceeded WHO standards within five months, with open fireplaces having slightly higher PM2.5 levels than closed fireplaces.

To date, many scholars have researched fire pit installations. However, most of these studies focus on the cultural aspects of fire pits. Even when the indoor air quality is considered, the regional differences limit their applicability to Qiang residences. In contrast, there have been numerous international studies the on indoor air quality related to open-burning devices. Therefore, it is essential to study the impact of fire pits on the indoor air quality in traditional Qiang residences, given their regional characteristics.

1.2.3. Indoor Air Quality

Indoor air quality (IAQ) refers to the suitability of certain elements in the air within a specific environment for people’s work and life, reflecting the specific needs of people for the indoor environment [30]. Poor indoor air quality not only reduces comfort but also leads to diseases when people are exposed for long periods. In recent years, an increasing number of scholars have conducted research on indoor air quality. Peng [31] used numerical simulation and other methods to study the diffusion characteristics of hazardous chemical pollutants in single-room and multi-room building models. The study identified the variation patterns of indoor pollutants, with the pollution source location under different window-opening conditions in single-room models, and the distribution of airflow fields and pollutant concentrations in different rooms under different window-to-wall ratios and ventilation paths in multi-room models. The study also quantified the relationship between the magnitude of the pollution source leakage and the probability of human fatalities under various ventilation conditions. Wang et al. [32] used CFD simulations to analyze the impacts of the window-opening size and windowsill height on natural ventilation, providing suggestions to enhance natural ventilation in buildings and, thus, offering references for buildings’ energy efficiency and window design optimization. Fonseca [33] and colleagues reviewed 171 articles, concluding that studying the impacts of the indoor air quality in healthcare facilities on patient safety and occupational health has promising prospects. Pamonpol et al. [34] researched the indoor air quality of a poorly ventilated office in a tropical region through data measurements and simulations, finding that door ventilation and plant cultivation effectively reduce the indoor CO₂ concentration, potentially saving up to 24.3% on electricity costs. Sánchez-Fernández [35] and colleagues conducted field measurements to study the indoor air quality of school classrooms in Spain, using the CO₂ concentration as the evaluation standard, providing references for school administrators to develop efficient natural ventilation strategies.

Research indicates that an increasing number of scholars are focusing on indoor air quality, with many using the indoor CO₂ concentration as a measure of assessment. The methods for studying the indoor air quality can be summarized into two categories: empirical measurements and CFD simulations.

1.2.4. Summary of Current Research Status

As previously mentioned, although many scholars have begun to focus on traditional Qiang residences, their research primarily treats these structures as cultural heritage, focusing on architectural culture and preservation. In the realm of indoor thermal comfort, most attention has been given to the thermal environment itself, with relatively few studies addressing the indoor air quality. Even the limited research on indoor air quality often does not apply to the western Sichuan region because of regional differences. By incorporating the evaluation standards and research methods used by international scholars in the study of the indoor air quality, this paper employs CFD simulation methods, using CO₂ as the standard for assessing the indoor air quality. This study aims to contribute to the research on the indoor air quality in traditional Qiang residences and fill the existing research gap in this area.

2. Geographical Information and Study Subjects

2.1. Geographical Information

Taoping Qiang Village (Figure 1) is located in Sichuan Aba Tibetan and Qiang Autonomous Prefecture, Li County, Taoping Township, on the banks of the Zagunao River. Belonging to the high mountains and very high mountainous areas of northwestern Sichuan Province, located in the upper reaches of the Minjiang River, in the basin of the Zagunao River, this terrain’s slopes are very steep, with deep valleys crisscrossed with high mountains [11]. Taoping Qiang Village is at an average altitude of about 1500 m, with a complex natural terrain, and its high-altitude location makes it have a large temperature difference between day and night and a cold climate. Because of its steep terrain, high mountains, and deep valleys, land resources are tight [36]; therefore, the Qiang people use local materials when building houses.

Figure 2a presents the summer climate information of Taoping Qiang Village. As shown in the figure, even in summer, the average temperature in Taoping Qiang Village is only 12 °C because of its high altitude, highlighting the importance of the fire pit. Figure 2b is the wind rose diagram of Taoping Qiang Village, indicating that the prevailing wind direction in summer is southeast, with an average wind speed of 0.97 m/s. Literature research reveals that many young Qiang people choose to move away from traditional residences mainly because the use of the fire pit, an open-heating device, leads to poor indoor air quality [18]. Therefore, it is necessary to improve the indoor air quality of traditional Qiang residences.

Compared to winter, summer temperatures are more comfortable. Because many traditional residences have not introduced air conditioning as an active-regulation device, people generally consider opening windows and doors to utilize natural ventilation for improving the indoor air quality during summer [37,38]. Therefore, this study focuses on the indoor air quality of traditional Qiang residences during the summer months from June to August.

2.2. Building Features of Qiang Dwellings

Qiang traditional dwellings are renowned for their architectural features, described as “residing by the mountain, building with stacked stones, and reaching heights of over ten meters”, reflecting the Qiang people’s profound understanding of the natural environment and their architectural ingenuity [36]. The architectural forms of Qiang traditional dwellings can be categorized into three main types: stone masonry buildings, stilted board houses, and fortress-like watchtowers, primarily used for residence and defense [39]. This study focuses on the watchtower-style dwellings. As shown in Figure 3, the taller structures on the left are watchtowers, characterized by their tapering form—wider at the base and narrower at the top, with thicker walls at the bottom and thinner at the top. This design not only enhances military defense but also provides some seismic resistance. Regarding individual buildings, most current dwellings do not feature watchtowers and exist as standalone residential structures. Because of the constraints of the terrain, Qiang traditional dwellings often have irregular shapes. Typically, these dwellings are from three to four stories high [40]. The ground floor usually serves as a space for livestock, storage, and toilets. Over time, as the needs of the Qiang people have evolved, the ground floor has been increasingly used for commercial purposes and hosting guests. The middle floors serve as the core living areas, centered around the fire pit, with the kitchen, bedrooms, and other functional spaces arranged accordingly. The top floor is used for drying grains and storage, effectively utilizing the building’s space.

The walls of Qiang dwellings are typically over 40 cm thick, with the characteristic form of “tapering exterior walls and vertical interior walls”. Overall, the enclosure structure of traditional Qiang residences is shown in Figure 4. The wall structure of traditional Qiang dwellings is built from the inside out, consisting of a 10 cm layer of cement mortar, followed by a 10 cm layer of yellow clay, then a 400–600 cm stone wall, and, finally, another 10 cm layer of yellow clay [41]. The roof structure of traditional Qiang dwellings consists of layers from bottom to top: 200 cm diameter wooden beams, 80 cm diameter branches, 50 cm of sticky grass, 150 cm of compacted yellow earth, and, finally, 30 cm of cement mortar. The roof utilizes plant fibers and tiny gaps to form an air layer, effectively reducing heat loss and enhancing thermal insulation and heat storage performances [40].

The fire pit (Figure 5a) is a traditional Qiang heating device composed of a pit, a hearth frame, a hanging fire platform (Figure 5b), a platform, and an iron fence. It serves not only for heating and food processing but also as a decorative and sacred cultural symbol. For the smoke generated by the fire hearth, traditional Qiang dwellings have a unique smoke exhaust method: Directly above the hearth, there is an approximately 2 m² opening that connects to the upper space (as shown in Figure 5b), with a wooden frame suspended in the opening space. The smoke generated by the combustion rises through the opening above the hearth, utilizing the smoke to smoke and bake the food hanging on the wooden frame. Simultaneously, the smoke smokes the exterior walls of the dwelling, serving as a deterrent against termite damage to the walls. However, because the hearth operates as an open-combustion heat source, the smoke it generates remains indoors for extended periods, and human activities within traditional Qiang dwellings mainly revolve around the hearth. Therefore, the combustion in the hearth reduces the indoor air quality, and the lower indoor air quality poses a threat to human health.

2.3. Study Object

Through field investigations, this study selected a traditional dwelling owned by Yu Jizhong in Taoping Qiang Village, Li County, Aba Tibetan and Qiang Autonomous Prefecture, Sichuan Province, China, as the research object. Figure 6a shows the current state of the selected residence, reflecting the typical materials and interior characteristics of Qiang architecture.

The exterior wall of the dwelling is approximately 700 mm thick at the bottom and slightly tapers upward, with a thickness of 440 mm at the upper end. The main building materials used are slate, yellow clay, and wood, consistent with typical materials used in Qiang traditional dwellings [41]. The doors and windows of the dwelling are relatively small, with a minimum door width of 900 mm and a maximum of 1300 mm and a height of approximately 1900 mm. There are two sizes of window openings: 800 mm × 1100 mm and 700 mm × 750 mm. The internal layout of the dwelling is illustrated in Figure 6b, with the first floor comprising the living room, dining room, kitchen, and other public spaces, including the hearth. The second floor consists of bedrooms, living rooms, bathrooms, and other daily living areas, while a part of the third floor serves as a loft, and the remaining platform areas can be used for drying grain. The floor plan depicted in Figure 6b is based on measurements taken by the author on-site.

3. Methodology

3.1. CFD Model

Before solving the problem, it is essential to establish the governing equations. Typically, assuming no heat exchange occurs, the continuity equation and the momentum equation can be directly used as the governing equations. For steady-state problems, initial conditions are not required, whereas boundary conditions must be specified. These boundary conditions describe the variation in the solution variables or their derivatives with location and time and are necessary for all problems.

After establishing the governing equations, mesh generation is required to discretize these equations over the spatial domain. In this study, Airpack-3.0 software, which supports structured and unstructured meshes, is utilized. It can handle tetrahedral, hexahedral, and hybrid meshes and supports local mesh refinement to improve the mesh quality.

Following the mesh generation, the continuous governing equations need to be discretized. Common discretization methods include the finite difference method, finite element method, and finite volume method. For this study, Airpack employs the finite volume method for discretization.

After completing the aforementioned steps, the discretized set of equations is solved using iterative methods until the solution converges to the desired accuracy.

For this study, the governing equations consist of the mass conservation equation, momentum equation, and energy equation, which are the fundamental equations describing fluid motion.

The mass conservation equation describes the conservation of the fluid mass. For incompressible fluids, the continuity equation is given by

$$\begin{array}{c}\nabla ·\mathbf{u}=0\end{array}$$

(1)

where u represents the fluid velocity vector.

For compressible fluids, the continuity equation is given by

$$\begin{array}{c}{\displaystyle \frac{\partial \rho }{\partial t}}+\nabla ·\left(\rho \mathbf{u}\right)=0\end{array}$$

(2)

where ρ denotes fluid density, u signifies the fluid velocity vector, and t represents time.

The momentum equation describes the conservation of the fluid momentum, taking into account the viscous effects of the fluid. For incompressible fluids, the momentum equation can be expressed as follows:

$$\begin{array}{c}\rho \left({\displaystyle \frac{\partial \mathbf{u}}{\partial t}}+\mathbf{u}·\nabla \mathbf{u}\right)=-\nabla p+\mu {\nabla }^2\mathbf{u}+\mathbf{f}\end{array}$$

(3)

For compressible fluids, the momentum equation can be expressed as follows:

$$\begin{array}{c}{\displaystyle \frac{\partial \left(\rho \mathbf{u}\right)}{\partial t}}+\nabla ·\left(\rho \mathbf{u}\mathbf{u}\right)=-\nabla p+\nabla ·\tau +\rho \mathbf{f}\end{array}$$

(4)

where ρ denotes the fluid pressure, u is the dynamic viscosity, τ represents the stress tensor, and f represents external body forces per unit of volume (such as gravity).

The energy equation describes the conservation of energy for compressible fluids and can be expressed as follows:

$$\begin{array}{c}{\displaystyle \frac{\partial \left(\rho e_t\right)}{\partial t}}+\nabla ·\left(\rho e_t\mathbf{u}\right)=-\nabla ·\mathbf{q}+\nabla ·(\tau ·\mathbf{u})+\rho \mathbf{f}·\mathbf{u}+Q\end{array}$$

(5)

For incompressible fluids, the energy equation is often reduced to a temperature equation as follows:

$$\begin{array}{c}\rho c_p\left({\displaystyle \frac{\partial T}{\partial t}}+\mathbf{u}·\nabla T\right)=k{\nabla }^{2}T+Q\end{array}$$

(6)

To investigate the impact of the natural ventilation on the indoor air quality of traditional Qiang dwellings using CFD simulations, an outdoor-wind-field computational domain must first be established. If the computational domain is too small, it will cause problems of flow field distortion and insufficient flow development, and if the computational domain is too large, it will cause unnecessary waste of computational resources [42]. Therefore, in this study, the distance between the inlet of the computational domain and the building is set at 10 h (where h = 9 m, the building height), and the distance between the outlet and the building is set at 5 h. The distances on both sides of the building are also set at 5 h, and the distance from the top of the computational domain to the highest point of the building is, likewise, 5 h. The wind direction is set to the southeast, consistent with the summer wind direction in Taoping Qiang Village, with a wind speed of 0.97 m/s. The established computational domain for the outdoor wind field is shown in Figure 7.

To ensure the accuracy and efficiency of the simulation, the building model was simplified for the indoor air quality (IAQ) simulation as follows:

• Because the fire pit is located on the first floor of the dwelling and primarily affects the air quality of this space, only the first floor of the dwelling was modeled for the IAQ simulation;
• Given that the combustion releases a significant amount of CO₂ and that high concentrations of CO₂ can cause discomfort, this study focuses on the distribution and concentration of the CO₂ within the indoor environment;
• The wind speed at the window openings was set based on the results of the outdoor-wind-field simulation, serving as the boundary wind speed condition for natural ventilation inside the dwelling. The dimensions of the building model were set according to the selected dwelling’s floor plan;
• This study only investigates the impact of the natural ventilation on the indoor air quality, without considering the indoor thermal environment. Therefore, the thermal properties of the exterior walls and floor materials were assumed to be the same;
• The indoor environment includes a fire pit, a standing human model, and a sitting human model, with each source simulating CO₂ emissions from combustion or respiration. During the simulation, the heat released from the fire pit and human respiration were considered, with the fire pit releasing 3500 W of heat and human respiration releasing 150 W [30].

Under normal conditions, the CO₂ content in the air is 0.03–0.04%, commonly expressed in parts per million (PPM). Although CO₂ itself is non-toxic, increased concentrations can cause discomfort [30]. In the simulation model, the CO₂ concentration in the exhaled air from humans is approximately 4–5% or around 40,000–50,000 ppm. Therefore, the CO₂ concentration for the exhaled air was set at 45,000 ppm. For the fire pit, which uses locally sourced wood and other materials as fuel, the CO₂ concentration released during complete combustion is typically 10–15% or 100,000–150,000 ppm. Thus, the CO₂ concentration for the fire pit combustion was set at 125,000 ppm.

After establishing the simulation model, an automatic mesh generation was performed using the Airpack-3.0 software. The quality of the mesh was evaluated to ensure it met the requirements of the simulation. In regions with higher wind speeds, such as door and window openings, local mesh refinement was applied manually to improve the mesh quality. Similarly, mesh refinement was also conducted above the fire pit to ensure more accurate calculations. A total of 485,592 mesh elements were generated.

This study focuses on the impact of the natural ventilation on the indoor air quality in traditional Qiang dwellings. Therefore, the simulation is categorized into the following five scenarios:

• Extreme Condition without Door and Window Openings: The distribution of the indoor CO₂ concentration in a completely sealed environment, i.e., without any natural ventilation;
• Normal Operating Condition: The distribution of the indoor CO₂ concentration under typical living conditions;
• Variation in the Window-to-Wall Ratio on the Windward Side: Exploring the effects of the window openings on the distribution of the indoor CO₂ by altering the window-to-wall ratio on the windward side;
• Enhanced Ventilation Methods: Adding window openings on the leeward side of the building to investigate the impacts of additional ventilation methods on the distribution of the indoor CO₂;
• Impact of Fire Pit Placement: Investigating how the placement of the fire pit affects the distribution of the indoor CO₂.

3.2. Evaluation Criteria

In this paper, the CO₂ content is used as the standard for evaluating the indoor air quality, and the specific evaluation standards are shown in

Table 1. The relationship between the carbon dioxide content and the indoor air index and its influence on human physiology [30].

Mode of Action	Carbon Dioxide Content (%)	Indoor Quality Index and Its Influence on Human Physiology
CO2 content as an index parameter of the indoor air pollution	0.03–0.04	Outdoor air concentration range
	0.07	Room tolerance for a large number of people
	0.1	The allowable indoor value under normal conditions
	0.15	Ventilation calculation reference value
	0.2–0.5	The indoor air quality is considered to be very poor.
Effects of the CO2 content on human physiology	>0.5	The indoor air quality is considered to be poor.
	0.07	A few sensitive people feel ill.
	0.1	More people feel ill.
	3	Breathing deepens and accelerates.
	4	The human body feels dizzy. Headaches and tinnitus are experienced, vision is affected, and blood pressure rises.
	8–10	The body feels difficulty in breathing, rapid heartbeat, general weakness, etc.
	>18	Deadly

.

In the simulation results, the CO₂ concentration is shown as a molar concentration, and it is extremely important to convert the molar concentration to a percentage content. First, calculate the PPM of the CO₂ as follows:

$$\mathrm{P}\mathrm{P}\mathrm{M}={\displaystyle \frac{ n\times M}{\rho \times 1000}}\times {10}^6$$

(7)

where n is the number of moles, M is the molar mass of the CO₂ (44.01 g/mol), and ρ is the air density (1.225 g/L).

Convert the PPM to a percentage:

$$percent={\displaystyle \frac{\mathrm{P}\mathrm{P}\mathrm{M}}{{10}^4}}$$

4. Results

To comprehensively analyze the impact of the natural ventilation on the indoor air quality in Qiang traditional dwellings, this study conducted simulations for the five types mentioned earlier, resulting in five sets of outcomes. These simulations help to identify the key factors affecting the indoor air quality and propose corresponding passive retrofit strategies to effectively enhance the indoor air quality. Considering that Qiang residents perform activities around the fire pit in both standing and sitting postures, the simulation results are presented for 0.825 m and 1.5 m above the ground. The 0.825 m level corresponds to the nose and mouth heights of a seated person, while the 1.5 m level corresponds to the height of a standing person. In this section, red-colored blocks in the model diagrams indicate the research variables. In this section, the simulation results of the typical living conditions are used as a baseline for comparison across various simulation groups, illustrating the changes in the CO₂ concentration under different conditions. Typical living conditions refer to a normal summer ventilation scenario, where all the windows and doors of the residential building are open.

The simulation results for the initial model show that the indoor CO₂ concentration significantly decreases after the addition of the door and window openings, indicating that natural ventilation plays a crucial role in improving the indoor air quality. The CO₂ concentration in the dining area, where the fire pit is located, is significantly higher than those in the adjacent kitchen and living room spaces. The CO₂ concentration in the dining area decreases radially from the fire pit, with lower concentrations further from the fire pit and closer to the windows. Additionally, comparing the simulation results for sitting and standing postures reveals that the CO₂ concentration at the standing level is lower than that at the sitting level.

Figure 8 shows the simulation results for the scenarios without any door or window openings and the initial model. The simulation results for the model without any openings indicate that the average indoor CO₂ concentration is 0.122 mol, equivalent to 0.44%. According to the indoor air quality assessment standards mentioned earlier, when the indoor CO₂ concentration is between 0.2% and 0.5%, the indoor air quality is considered as being very poor. Therefore, the indoor air pollution caused by the fire pit is significant, and improving the indoor air quality of Qiang dwellings is essential.

Because of the presence of the interior walls, the distribution of the CO₂ concentration indoors is uneven, making the minimum and average concentrations less valuable for references. To better optimize the indoor air quality, the highest indoor CO₂ concentration is selected as the evaluation criterion for this study. In the initial model simulation results, the highest CO₂ concentration at the 1.5 m level is 0.014 mol, equivalent to 0.05%, which is 0.39% lower than the CO₂ concentration in the scenario without any openings. At the 0.825 m level, the highest CO₂ concentration is 0.015 mol, equivalent to 0.05%, which is also 0.39% lower than the CO₂ concentration in the scenario without any openings. According to the evaluation standards mentioned earlier, the indoor CO₂ concentration at this time falls within the permissible range for scenarios with a high number of occupants.

However, for Qiang dwellings, because production activities are conducted on a household basis with fewer occupants, this study will further investigate methods to reduce the indoor CO₂ concentration and improve the indoor air quality to provide a more comfortable living environment for Qiang residents. Additionally, this study will explore the impacts of passive strategies for natural ventilation on the indoor air quality.

4.1. Increased Windward Window Size

According to the previous simulation classification, the first simulation conducted examines the impact for varying the size of the windward window openings on the indoor air quality. The results are shown in Figure 9, where, as previously, the research variables are marked with red blocks, and the results for two planes at heights of 0.825 m and 1.5 m are analyzed. In Figure 9, from Window 1 to Window 4 represent the sequential increase in the size of the windward window openings. The initial window sizes in the model are 0.8 m × 1.1 m and 0.75 m × 0.7 m. In this section, the size of the windward window openings is altered by increasing the window height in increments of 0.2 m from Window 1 to Window 4.

When extracting CO₂ concentration results, the same legend scale is used, so it is observed in Figure 9 that as the size of the windward window openings increases, the indoor CO₂ concentration decreases accordingly, and the indoor CO₂ distribution becomes more uniform. For the simulation results, we first analyze the CO₂ concentration at the 0.825 m plane, representing a sitting posture. The CO₂ contour plots show that with the increase in the windward window size, the indoor CO₂ concentration decreases from an initial 0.054% to 0.036%, a total reduction of 0.018%. These results indicate that increasing the window size can effectively reduce the high indoor CO₂ concentration caused by the fire pit to the same level as that of the outdoor environment. For the standing posture, represented by the 1.5 m plane, the indoor CO₂ concentration decreases from an initial 0.054% to 0.03%, a reduction of 0.024%. Because the CO₂ released by the fire pit diffuses from low to high, the CO₂ concentration at the nose and mouth levels in the standing posture is lower than that in the sitting posture. Increasing the size of the windward windows to enhance the natural ventilation has a more significant effect on improving the CO₂ concentration at the 1.5 m plane compared to the 0.825 m plane. This is because of the limitations on the size range of the window openings.

These analysis results indicate that increasing the size of the windward windows to enhance the natural ventilation significantly improves the indoor air quality, with a maximum reduction in the CO₂ concentration of 0.024%. In this scenario, the dimensions of the windward windows are 0.8 m × 1.9 m and 0.7 m × 1.55 m, respectively.

4.2. Optimized Ventilation Mode

As shown in Figure 10, this section studies the impacts of different ventilation methods on the indoor air quality for the windward side, leeward side, and sides of the building.

First, a window opening measuring 0.9 m wide and 1.1 m high was added to the windward side of the fire pit space. The results show that the maximum CO₂ concentration at heights of 0.825 m and 1.5 m decreased from the initial 0.054% to 0.047% and 0.043%, respectively, with reductions of 0.007% and 0.011%. However, this not only changed the ventilation method of the room but also increased the window size on the windward side, enhancing the ventilation volume on the windward side.

Next, a window opening measuring 2 m in width and 1.1 m in height was added to the north wall, parallel to the hearth. The simulation results showed that the maximum CO₂ concentrations at 0.825 m and 1.5 m heights were 0.043% and 0.04%, respectively, representing decreases of 0.012% and 0.014% compared to the initial model. Then, an additional window of the same size was added 2 m from the previous window on the north wall. The simulation results demonstrated that the maximum CO₂ concentrations at 0.825 m and 1.5 m heights were 0.04% and 0.036%, respectively, representing decreases of 0.014% and 0.018% compared to the initial model. Adding windows to the north wall is more effective in reducing the indoor CO₂ concentration. However, this approach also involves increasing the window-to-wall ratio of the north wall. Therefore, these results indicate that combining improved ventilation methods with increased window sizes is more effective in reducing the indoor CO₂ concentration.

Finally, a window opening measuring 2 m in width and 1.1 m in height was added to the leeward side of the hearth space. The simulation results indicated that the maximum CO₂ concentrations at 0.825 m and 1.5 m heights were 0.043% and 0.04%, respectively, representing reductions of 0.011% and 0.014% compared to the initial model.

The simulation results of the different ventilation methods indicate that changing the ventilation mode of the residence, such as by increasing the convective ventilation, can effectively reduce the indoor CO₂ concentration to levels comparable to that of the outdoor air’s CO₂ concentration (Table 1). Comparing the improvement in the indoor CO₂ concentration across the different ventilation methods, adding two window openings on the north side of the fire pit room yields the best results. However, considering that adding two window openings on the north side significantly increases the indoor ventilation, the optimal approach from the ventilation method perspective is to add one window opening on the north wall of the building. In this scenario, the CO₂ concentration can be reduced by up to 0.014% at standing height and by up to 0.012% at sitting height.

4.3. Replacement of the Fire Pit Location

Figure 11 shows the simulation results for changing the fire pit location, which can be categorized into two types: changing the location within the fire pit room and placing the fire pit in other rooms. First, the fire pit was moved 2 m to the east side of the building each time. As shown in the figure, changing the location of the fire pit within the fire pit room not only fails to effectively reduce the indoor CO₂ concentration but also increases the CO₂ concentration because of the local accumulation caused by the reduced space.

Moving the fire pit from the dining room to the living room, with the fire pit near the doorway and air convection in the dining room, results in the maximum indoor CO₂ concentration decreasing to 0.03% and 0.025% at heights of 0.825 m and 1.5 m, respectively. However, relocating the fire pit to this position hinders the daily activity flow, causing inconvenience in entering and exiting the residence. In summary, the initial location of the fire pit is relatively appropriate, reflecting the traditional construction wisdom of the Qiang people.

4.4. Increase in the Size of the Doorway in the Partition Wall

Figure 12 illustrates the simulation results for enlarging the interior partition wall openings in residential buildings. The residence contains two interior partition walls, each with door openings measuring 1 m by 1.8 m.

First, increasing the size of the partition wall openings to 1.5 m by 1.8 m resulted in the maximum indoor CO₂ concentrations of 0.05% and 0.047% at heights of 0.825 m and 1.5 m, respectively, representing reductions of 0.004% and 0.007% compared to the initial model. Second, by further enlarging the partition wall openings to 2 m by 1.8 m, the simulation showed that the maximum CO₂ concentrations at heights of 0.825 m and 1.5 m were 0.047% and 0.043%, respectively, indicating decreases of 0.007% and 0.011% compared to the initial CO₂ concentrations.

These simulation results show that increasing the size of the door and window openings in interior partitions helps to reduce the indoor CO₂ concentration and improve the indoor air quality. However, compared to enhancing the ventilation methods and increasing the ventilation volume through the building envelope, enlarging the openings in interior partitions does not significantly reduce the CO₂ concentration. Although the CO₂ diffusion area increases, leading to a slight reduction in the indoor CO₂ concentration, the overall indoor environment does not improve substantially. Additionally, considering that further increasing the size of the door openings may not be practical, the door and window openings were not further enlarged.

5. Conclusions and Prospects

Indoor air quality is an important factor affecting human health, and as people’s requirements for living quality continue to improve, it is essential to optimize and improve the indoor air quality. For the indoor open-fire heating device, which is the core of the living culture in the western Sichuan Qiang ethnic group, the CO₂ emitted by the fireplace will greatly degrade the indoor air quality. However, because of geographical factors and economic conditions, passive strategies are more suitable for optimizing and improving the indoor air quality in Qiang ethnic group dwellings. Currently, there are few studies on the indoor air quality in traditional Qiang ethnic group dwellings in western Sichuan, so this paper selects a typical Qiang ethnic group dwelling in Taoping Village, Sichuan, and uses the CFD simulation method to explore the impact of the natural ventilation on the indoor air quality in Qiang ethnic group dwellings and propose suitable renovation strategies for Qiang ethnic group dwellings.

This study focuses on the typical dwellings of Qiang people located in Taoping Qiang Village in western Sichuan, China, selects the summer condition, and uses the CO₂ concentration as the indoor air quality evaluation standard. It conducts passive optimization strategies for the indoor air quality in Qiang ethnic group dwellings by exploring 1. the size of window openings in the windward side of the building, 2. the indoor ventilation method, 3. the placement of the fireplace in the indoor space, and 4. the size of door openings in interior partitions. The research results show that:

• The best passive optimization strategy for improving the indoor air quality in Qiang ethnic dwellings is to increase the height of the window openings facing the wind;
• Increasing the height of the window openings facing the wind by 0.8 m is the best, at which time, the CO₂ concentration in the indoor space can be reduced by 0.024% at the 1.5 m plane;
• Adding two window holes 2 m wide and 1.1 m high 2 m apart on the north wall parallel to the fire pit on the north side of the building, changing the ventilation mode, and changing the ventilation volume can effectively improve the indoor air quality;
• Although the effect for transferring the stove from the dining room to the living room is good, its location has impacts on production and life, so this method is not suitable.

For the traditional Qiang ethnic dwellings in western China, the importance of their traditional building techniques and culture is self-evident. This study aims to improve the indoor living quality while protecting traditional building techniques through passive transformation measures. At the same time, it proposes sustainable development strategies that are applicable to traditional Chinese dwellings facing similar situations. The current research results are a preliminary exploration of this goal, and we will continue to expand and improve our work in the future as follows:

• Explore the impacts of multi-factor composite transformation strategies on the indoor air quality;
• Expand the research sample and select Qiang ethnic dwellings with geographical factors, climate factors, etc. to explore the general passive transformation strategies for natural ventilation;
• Pay more attention to other factors that affect the indoor air quality, use a diversified evaluation standard to evaluate the indoor air quality, and explore the most important factors affecting the indoor air quality;
• By supplementing and improving this work, a suitable framework for the sustainable development of Qiang ethnic traditional dwellings will be established. This framework can be applied and promoted in traditional dwellings with the same indoor air quality problems as those in western China.

This article uses field survey methods and CFD simulation means to explore the impact of the natural ventilation on the indoor air quality in traditional Qiang ethnic dwellings located in western China and propose the most suitable passive transformation strategy for Qiang ethnic dwellings. This study improves the indoor air quality of local Qiang ethnic traditional dwellings and fills the gap in the indoor air quality research of Qiang ethnic traditional dwellings in the Sichuan Basin. It also provides a reference for improving the indoor air quality in residential buildings using open-heating methods. Meanwhile, it offers new insights into the sustainable development of traditional buildings.