This paper investigates the thermal performance of prefabricated exterior walls using the Computational Fluid Dynamics method to reduce energy consumption. The thermal performance of the prefabricated exterior wall was numerically simulated using the software ANSYS Fluent. The composite wall containing the cavity is taken as the research object in this paper after analysis. The simulation suggests that when the cavity thickness is 20 mm and 30 mm, the heat transfer coefficient of the air-sandwich wall is 1.3 and 1.29, respectively. Therefore, the optimal width of the cavity is 20 mm, and the most suitable material is the aerated concrete block. In addition, a comparative analysis is conducted on the cavity temperature in the wall under different conditions. It is proven that an intelligent environment control system can significantly improve thermal efficiency and provide a solid theoretical basis for further research in the external insulation of prefabricated buildings.

The rapid development of modern society is inseparable from the reserve energy, a critical material basis for the continuation of human culture [

Building energy efficiency has become a rigid index for the modern construction industry [

This paper uses the average heat flux as the thermal performance evaluation index. The FLUENT software is used for experimental simulation to analyze the thermal performance of the exterior insulation board. Several significant factors affecting the heat transfer of the wall are discussed to determine the most crucial factors affecting the thermal performance of the insulation board [

There are diverse ways to classify walls in the building. According to their position in the building, they are divided into internal and external walls; according to the layout direction, they are divided into horizontal and longitudinal walls; according to whether they are stressed or not, they are divided into load-bearing and non-load-bearing walls; according to building materials and pouring methods, they are divided into solid, hollow, and composite walls. The research object reported here is the building’s exterior non-bearing thermal insulation wall.

The thermal performance of the air interlayer in the wall is closely related to the airflow in the wall. Computational Fluid Dynamics (CFD) is a branch of fluid dynamics. Its development primarily depends on the progress of the aerospace industry and is influenced by methods or theories, such as grid generation, numerical calculation, and differential equations [

The basic conservation laws of mass, momentum, and energy are derived based on the assumption of the energy continuum [

In

where

The diffusive flux in the energy equation can be expressed as:

where

The calculation domain is re-divided to solve the governing equation. The governing equation is discretized in time and space and separated into finite non-overlapping control bodies. The governing equation is discretized in integral form. The governing equation can be written as

In

In addition,

In

A high-quality computational grid needs to be characterized by reasonably sized cells. The automatic construction algorithm for adaptive-sized cells dramatically reduces the frequency and intensity of user interaction. The core data architecture of the grid generation module is extended. The liquid solution uses the standard CFD General Notation System format, and the solid solution adopts the Bitmap Distribution Format. The algorithm model is designed through the top-up process, and the boundary recovery algorithm is used to insert a specific number of Steiner points. The P-multi grid is different from the original grid and iterates with different precisions to speed up convergence. Its error frequency is high in high-order accuracy, and result correction can accelerate the convergence. The turbulence model used here is the Spalart-Allmaras model, which has low computational complexity and can achieve an excellent measurement effect on relatively complex boundary layer problems.

A prefabricated building is a new type of building. Its standard components or accessories (such as floors, wallboards, and stairs) are processed and produced in batches in the factory and then transferred to the construction site for direct assembly through specific connection methods [

At present, there are multiple mainstream assembly systems in the construction industry, including fully prefabricated shear wall systems, double-sided composite shear wall systems, and PCF + PC shear wall systems. The main difference among them lies in the design of exterior wall panels [

The indoor thermal environment of a building is a crucial index to measure the heat of the building cavity. There is an inevitable interplay between the indoor thermal environment and residents’ activities [

The heat of the indoor environment mainly comes from the following aspects:

(1) the heat emission of the human body in a building;

(2) sunlight transmitted to the room through walls and windows;

(3) the heat provided by the heating equipment inside the building;

(4) the heat emitted by household appliances in use.

The heat loss of the indoor thermal environment is manifested in the following aspects:

(1) heat loss through the floor;

(2) heat absorbed by indoor water evaporation;

(3) the heat transferred from the room with a higher temperature due to the temperature difference on both sides of the envelope structure, such as walls and windows.

The air interlayer placed in the multi-cavity composite wall has an excellent insulation effect due to the poor thermal conductivity of air. The built-in air interlayer is also called a cavity, which can balance the pressure inside and outside the wall and remove the moisture inside the wall [

The air interlayer in the multi-cavity composite wall can be divided into closed interlayers and circulating interlayers. The difference lies in whether the interlayer is sealed or not. Circulating interlayers involve mechanical circulation and natural circulation. In addition, the height and width of the air interlayer are relatively large, but the gap is very narrow. In addition, most of the air interlayers are tall and wide, but the gap is very narrow. Functionally, the air interlayer can improve indoor thermal comfort by reducing the cooling and heating load of the building by supplying cooling and heating.

It has been common to use the internal air interlayer in the building envelope for insulation in recent years. For example, the air interlayer is applied to the external windows and exterior walls. Exterior windows include multi-layer windows and multi-glazed windows [

Under the condition of stable heat transfer, when the air temperature difference between the two sides of the enclosure structure reaches 1°C, the heat transferred per unit area in unit time is the

In _{0} represents the heat transfer resistance of the wall.

In _{i} denotes the heat exchange resistance of the internal surface of the wall; _{e} signifies the heat transfer resistance of the external surface of the wall;

In areas with indoor heating, heat loss from buildings is roughly a unidirectional but dynamic process in summer or winter. In this case, it is not enough to evaluate the heat transfer only by using the value of heat transfer coefficient

The thermal storage coefficient refers to a material’s ability to exchange heat with the surrounding environment. When the ambient heat on both sides of the wall changes and the wall surface temperature fluctuates to 1°C, the maximum heat flux introduced into the object only depends on the thermophysical properties of the material itself. The thermal storage coefficient is solved by

In

The thermal inertia index

In

Heat flux is a vector to measure the amount of heat transferred per unit interface area per unit time. Heat can be transferred via different mechanisms, like conduction and convection. The magnitude and direction of the heat flux reflect the extent and route of heat transfer. Heat flux is calculated according to

In

The τ is calculated according to

In

The composite air-sandwich wall has two different thermal principles: heat preservation and insulation. Heat preservation means that the wall prevents the transfer of indoor heat to the outside, reducing the heat loss in the building in cold winter; heat insulation means that the wall prevents the outdoor heat from entering the building to maintain the low-temperature state of the wall in summer [

The analysis of the heat transfer coefficient

The heat transfer process of wallboard is quite a complex problem in real life. The wall panel structure, the material of the insulation layer, and the temperature difference between the indoor and outdoor all significantly impact the heat transfer of the wall panel. To simplify the problem and facilitate the simulation experiment, it is assumed that the convection of the air interlayer inside the wall is natural convection in the heat transfer process of the wall panel; the air density inside the interlayer is only affected by the ambient temperature; the influence of indoor and outdoor temperature difference on the physical properties of wall materials is not considered.

The Time in the FLUENT Solver is set to steady state, and the Space is set to a three-dimensional structure. The governing equations contain the flow, energy, and turbulence equations. The air density parameter is 1.225, the constant pressure specific heat is 1006.43, and the thermal conductivity is 0.0242. There are 2,716,985 meshes to enhance the simulation effect. The air inlet is set as the speed inlet, and the air outlet is set as the pressure outlet. The fluid type of other indoor objects is set as the wall, the air value on the wall is set as 0, and the power value of lamps in the building is 40 W. The uncoupled implicit algorithm in FLUENT is adopted to solve the fluid equation. This fairly mature algorithm has been extensively validated in applications to simulate CFD problems of low-velocity flows. The finite element method is used as the spatial numerical scheme of this paper to divide the solution region into connected and non-overlapping finite sub-regions. The approximate solution is the product of the node function value and the basis function. The numerical solution can reach second-order or even higher accuracy [

No. | Number of grid points | Specific heat at constant pressure | Mean heat capacity | Thermal conductivity |
---|---|---|---|---|

1 | 234 | 946.75 | 33.12 | 0.0134 |

2 | 246 | 927.35 | 65.23 | 0.0337 |

3 | 279 | 1023.13 | 34.67 | 0.0234 |

4 | 258 | 892.56 | 79.13 | 0.0267 |

5 | 284 | 1011.45 | 67.14 | 0.0246 |

6 | 357 | 1005.34 | 68.13 | 0.0278 |

7 | 345 | 1239.12 | 76.35 | 0.0168 |

8 | 376 | 1123.78 | 68.94 | 0.0137 |

9 | 385 | 1234.01 | 78.47 | 0.0196 |

10 | 378 | 1213.12 | 96.38 | 0.0258 |

The influencing factors of the average heat flux of wallboards are analyzed by software to study the thermal properties of insulation boards. In this experiment, FLUENT software is used to compare the temperature differences between the inside and outside of the multi-cavity wall under different cavity thicknesses, materials, and wall thicknesses.

Furthermore, this experiment simulates the effect of wall height on the temperature change and average heat flux at the current measurement location of the multi-cavity composite wall. The FLUENT software sets the wall heights to 600 mm, 900 mm, 1200 mm, and 1500 mm in sequence.

FLUENT sets two experimental groups with a temperature difference of 16°C and 8°C to compare and analyze the temperature changes in the cavity. The experimental results in

The above results prove that the cavity thickness, the material and thickness of the wall panels on both sides of the cavity, and the temperature difference between the inner and outer walls all change the thermal insulation performance of the wall. The temperature difference inside and outside the cavity and the wall material significantly affects the heat flux. Nonetheless, wall height has no significant effect on heat flux. In short, the 20 mm thick thermal insulation wallboard with aerated concrete blocks can maintain the best thermal insulation effect when the temperature difference between the inner and outer walls is 8°C.

Manoram et al. [

The total energy resources in the world are limited. With the increasing demand for traditional energy in modern society, energy conservation and emission reduction has become an inevitable social development trend. Based on the relevant fluid mechanics principles, the thermal performance of prefabricated buildings’ enclosure structures is studied through software simulation analysis. The experimental results show that the average heat flux, the cavity thickness, the wall materials’ thickness, and the temperature difference between the two sides of the cavity all affect the thermal insulation performance of the composite wall. The clear inference is that using 20 mm air interlayers can significantly improve the thermal insulation performance of prefabricated buildings. Due to the air interlayer in the multi-cavity composite wall, particular components need to be added between the inner and outer panels to increase stability. Still, these components may form a building thermal bridge. Although this paper has achieved the expected outcomes, there are still some deficiencies. On the one hand, the model is simplified for convenience. On the other hand, the influence of the thermal bridge effect formed by stability maintaining components on building energy consumption is not considered, which may lead to some errors in the experimental results, which may cause some errors in the experimental results. Therefore, future research will establish a more comprehensive model and comprehensively analyze building energy consumption factors. Besides, the multi-cavity composite wall of prefabricated buildings will be further optimized to reduce the building energy consumption and promote energy conservation and emission reduction in the construction industry.