CFD (Computational Fluid Dynamics) is a subject that has been accompanied by the rapid rise of computer technology since the 1960s. After half a century of rapid development, this subject has become quite mature. Various CFD general software packages have appeared one after another, becoming commercial software, widely accepted by the industry, with increasingly perfect performance and expanding application scope. They are widely used in many fields, such as thermal power generation, aerospace, machinery, civil hydraulics, environmental chemistry and so on. HVAC industry is one of the important fields of CFD technology application.
The mathematical models of various CFD general software are mainly composed of Navier-Stokes equation and various turbulence models, plus multiphase flow model, combustion and chemical reaction flow model, free surface model and non-Newtonian fluid model. Most additional models add some additional source terms, additional transport equations and relationships on the basis of the main equation. With the continuous expansion of application scope and the emergence of new methods, new models are also increasing.
CFD numerical solution methods mainly include finite difference method, finite element method, boundary element method and finite analysis method. Among them, finite difference method and finite element method are the main methods. Starting from the differential equation, the finite difference method discretizes the calculation area, and uses the difference sum and difference quotient approximation to replace the differential and WeChat services, thus reducing the solution of differential equations and boundary conditions to a numerical solution of linear algebraic equations. For a long time, the numerical simulation of flow field, temperature field and concentration field in air-conditioned rooms is almost all carried out by finite difference method. The finite element method draws lessons from the core of discrete processing in finite difference method, and inherits the reasonable method of selecting trial function and integral solution region in variational calculation. In the finite element method, the definition and integral range of trial function is not the whole region, but the units divided from the region according to the actual needs. Through comparison, it is found that the simulation effect is the same for the research area with regular boundary shape, such as rectangular area. For areas with complex boundary shapes, the finite element method is more effective. At present, most commercial CFD software adopts finite element method.
In addition, CFD software is equipped with grid generation (preprocessing) and flow display (post-processing) modules. The quality of grid generation has a great influence on the calculation accuracy and stability, so the grid generation ability is an important factor to measure the performance of CFD software. Grids can be divided into two categories: structural grids and unstructured grids. At present, structural grid is widely used. For the complex solution domain, the structural grid should be divided into several subdomains according to its topological properties, and the subdomains are realized by partition docking or partition overlapping technology. Unstructured grids are not limited by the topological structure and boundary shape of the solution domain, which is convenient to construct and generate adaptive grids, and can automatically adjust the grid density according to the characteristics of the flow field, which is very beneficial to improve the calculation accuracy of local areas. However, the amount of memory and computational work required by unstructured grid is much larger than that of structured grid. Therefore, the combination of the two is the development direction of power generation technology. At present, FLUENT software (which will be introduced later) has this function.
The flow display modules of CFD software all have three-dimensional display function, showing various flow characteristics, and some can also demonstrate unsteady processes with animation function. Although there is no internal relationship between flow display and flow field calculation, for the convenience of users, the output of flow field calculation should be conveniently connected with graphics processing software. The post-processing module of FLUENT software is better.
The following introduces the application of FLUENT software according to the current market share.
The main features of 2.2. Smooth software
The appearance and commercialization of CFD general software package greatly promoted the application of CFD technology in engineering. During the period of 1998, Fluent, a CFD software with global market share, officially entered the China market, and it is the mainstream commercial software of CFD at present, with a market share of about 40%.
FLUENT software design is based on the concept of CFD computer software group. According to the characteristics of each flow physical problem, a numerical solution suitable for it is adopted to achieve the calculation speed, stability and accuracy.
The structure of FLUENT software consists of three modules: preprocessing, solver and post-processing. In FLUENT software, GAMBIT is used as a special pre-processing software to make the grid have various shapes. Triangular and rectangular grids can be generated for two-dimensional flow; For three-dimensional flow, tetrahedron, hexahedron, triangular prism and pyramid can be generated. Combined with concrete calculation, the hybrid grid can be generated, and its adaptive function can subdivide or coarsen the grid, and it can also generate discontinuous grid, variable grid and sliding grid. The second-order upwind scheme adopted by FLUENT software is a multidimensional gradient reconstruction method proposed by Barth and Yesbergen for unstructured grids, and it has been further developed. Using least square method to estimate gradient can better handle the calculation of distorted mesh. FLUENT took the lead in adopting unstructured grid to make it technically.
The core part of FLUENT software is the solution module of Naville-Stokes equation. The pressure correction method is used as the calculation method of low-speed incompressible flow, including SIMPLE, SIMPLER, SIMPLEC, PISO and so on. The finite volume method is used to discretize the equation, and its calculation accuracy and stability are better than the finite difference method used in traditional programming. The discrete scheme is a second-order upwind interpolation scheme for convection terms-Quick scheme, which has low numerical dissipation, high accuracy and simple structure. For compressible flow, the coupling method is adopted, that is, the continuity equation, momentum equation and energy equation are solved simultaneously.
Turbulence model is the main part of CFD software including FLUENT software. FLUENT software is equipped with various levels of turbulence models, including algebraic model, one-equation model, two-equation model, turbulence stress model, large eddy simulation and so on. The most widely used two-equation model is k-ε model, and the software includes standard k-ε model and several modified models.
The post-processing module of FLUENT software has three-dimensional display function, showing various flow characteristics, and can demonstrate the unsteady process with animation function, showing the simulation effect in an intuitive form, which is convenient for further analysis.
Application of 3.3. Jingtong HVAC
The ultimate goal of air conditioning design is to achieve the required indoor climate environment (temperature distribution, airflow, pollutant concentration, etc.). Through economically and technically reasonable system design and equipment selection. In order to control these environmental parameters reasonably, it is necessary to master their distribution characteristics. CFD is a method to analyze the distribution characteristics of indoor three-dimensional air flow in detail besides model experiments.
Taking the indoor air distribution of traditional wall-mounted air conditioner as the object, the temperature field is numerically simulated by FLUENT software.
3.3. 1 model building
Set the original parameters of the air conditioner as follows:
Geometric dimension: 770mm×240mm× 180mm.
Air outlet angle: 45; Wind speed: 6m/s for strong wind.
Refrigeration (heat) capacity: 2500 watts
Air outlet temperature during cooling: 18? C, 40 when heating? C
According to the known parameters, a suitable finite element model of the room is established. In order to make the results as close as possible to the real situation, a three-dimensional model of the room is established, with the length, width and height of 5m, 4m and 3m respectively. air conditioning equipment
Installed on the narrow side of the room. The simplified mannequin is placed in the center of the room, Figure 2: Three-dimensional air-conditioned room model.
Sitting posture, itself does not give off heat, and there is no heat source indoors. Figure 2 is a three-dimensional air-conditioned room model made by GAMBIT.
The boundary conditions are set as follows: 6 walls of the room are insulated, and the initial indoor temperature is 30? C, 5 when heating? C.
The whole continuous grid structure is adopted, and GAMBIT is used to generate grids, and the number of computational grids is 35,000. K-ε turbulence model is adopted, and the first-order unsteady separation calculation is adopted in the calculation. Considering the influence of gravity, the SIMPLE algorithm is used to solve the equations.
Calculation result
The post-processing display function can clearly and intuitively reproduce the cooling process, and product designers can evaluate the changes of human body area on this basis. It is found that there is a temperature gradient in the vertical direction of human body, and the temperature of head is higher than that of foot. This is because the density of cold air is higher than that of hot air, and the downward trend is obvious, resulting in the lower part of the room cooling faster than the upper part. The temperature difference will cause local thermal discomfort of human body, and the temperature gradient will gradually decrease with time. When the temperature field reaches a steady state, the temperature of the head is 2? C, the temperature difference between PMV and PPD meeting the international comfort index is 3? C.
4. Conclusion
This paper introduces the general structure and development of CFD technology, analyzes the main characteristics of ——FLUENT, the mainstream CFD software, and finally illustrates its application in HVAC field with examples. Using commercial software to calculate is an important means of scientific research and engineering design. Using FLUENT for fluid analysis does not need programming, which can save a lot of time from programming and other operations, make people have more time and energy to consider the physical nature of the problem, optimize algorithm selection and parameter determination, and greatly improve work efficiency.