In the design of HVAC systems it is common practice to make important decisions regarding plant design based on construction practice or purely theoretical analytical bases. For example, we know that the possibility of creating full-air systems for winter The glass walls of environments are known to avoid the formation of condensation on them. We could go for a long time with examples in this regard. Although it is a theoretical and theoretical knowledge, it is an integral part of the experience of a designer.
In order to satisfy this growing demand for accuracy and precision, designers can resort to an extremely powerful tool: computational fluid dynamics , in short CFD. The use of CFD is to obtain a numerical solution, calculated through recursive methods, of the complex Navier-Stokes equations that, as is known, govern the dynamics of fluids. In practice, computational fluid dynamics makes it possible to give precise and punctual shape, in time and space to the often smoky analytical solutions which, even when obtainable, appear to be too complex for use in practical design.
Returning to the example, air exposure of rooms with high heights, we know that in order to limit the phenomenon of air stratification is necessary to provide considerable air flow (at least 5/6 volumes / hour). But exactly, what is the air flow needed to achieve acceptable air comfort? To this question, each designer will respond by providing his version, based on his previous experience. The CFD instead of working with certainty: by setting the model correctly and assuming adequate conditions it is possible to predict the temperature. In this way, thanks to the CFD,
An interesting consideration, therefore, can be made regarding the use of CFD for the design of HVAC systems: computational fluid dynamics Why this apparent contradiction? The CFD allows users to be pushed towards higher levels of accuracy.
To better understand the possibilities of CFD, but also the related criticalities, it is certainly useful to analyze the typical work-flow of a fluid dynamics simulation. The steps you need to get reliable and physically consistent results can be summarized as follows:
- Model validation
Pre-Processing includes all preparatory and necessary activities for the start of numerical simulations. First of all it will be the control volume, ie the control volume; in order to obtain consistent results, the geometry must have a high level of detail. Once the geometry has been established, the physical model is defined, setting the equations that define the object of the simulation Choosing the correct equations that govern the simulation of fundamental importance to the correctness This phase of the process is defined as the so-called boundary conditions, as well as the hypotheses Once the Pre-Processing is over, it is necessary to validate the model just defined to allow its use. However, the validation of the model is the most complex step in the field of computational fluid dynamics; during validation it is in fact necessary to carry out the following operations:
1 .. Mesh design : a “mesh” is defined as the discretization of the control volume. Mesh is made up of large cells of various shapes and sizes decided by the designer: for each cell the program will calculate the properties of the fluid (velocity, pressure, etc.). The realization of the mesh is, excluding the actual computational time, the longest operation in the field of CFD simulations and obviously has a fundamental importance. “Meshing”, as well as related issues, is available here [NOTES: Add link to the entry about “meshing”].
Figure 2: Mesh of a centrifugal pump – Source: truegrid.com/cfdgallery
2 .. Selection and setting of the solver : Once the mesh is built, it is necessary to choose the most appropriate solver for the application in question. Without going into the details of the complex algorithms that hide behind the abbreviations (PISO, SIMPLE, etc.) that are typically encountered when approaching CFD, it is necessary to know that each solver calculates the numerical solution through iterative algorithms. The starting equations are the same, but the order in which they are solved varies, as well as the use of numerical correctors necessary to improve the stability of the simulation.
3 .. Obtaining the preliminary solution : Once you have chosen the most suitable solution, you can finally launch a preliminary simulation. The simulator will return a solution. This must first of all be compared with the analytical solutions: is the solution If the answer is yes, you can proceed with the validation of the model.
4 .. Convergence controls : obtaining an acceptable solution is certainly not sufficient to consider the model realized as validated. Later it will be necessary to proceed with the simulation to refine the solution. If this step is also successfully passed, the model can finally be considered validated.
Figure 3: Air stream lines inside data center – Source: simscale.com
Once a correct and stable computational model has been developed, it will be possible to simulate the different conditions required in the design field. The results of the simulations, however, will be endless tables of numbers, in fact incomprehensible. To translate these results into something usable, post-processing tools must be used to get the most interesting results.
We want to give taste to the design that computational fluid dynamics can help solve, as well as give an idea of how to complex it can be correctly set up CFD simulations. However, it must be remembered that the use of CFD is not a substitute for traditional design tools: in order to implement correctly and physically sensible fluid dynamic model it is in fact necessary to have a good command of the analytical equations governing the dynamics of fluids, or the already mentioned Navier-Stokes equations. Furthermore, in order to obtain useful and coherent solutions, it is necessary that the designer correctly circumscribes the problem by sending it, avoiding waste of time and energy in useless, if not harmful, simulations.
Ing. Gaetano Trovato
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