Project: CFD simulation of HVAC system

Introduction: HVAC Analysis

This project carried out by ICEMM aims to perform a comparative analysis of five different HVAC systems for a large industrial building. CFD simulation will be used for this purpose. In the building there are thermal requirements both for human comfort and for the process that is carried out inside, which requires some materials whose temperature must be maintained above a minimum value. For confidentiality reasons, the result maps provided will simply be examples without quantitative data or relation with the real cases studied.

The HVAC simulations are carried out by modelling the walls with the enclosures designed for the building. Likewise, the air infiltrations are adjusted in the model to take into account this additional thermal load that will exist in the real building. These conditions will be the same in all the thermal analyses in order to have a real comparison between the systems. The alternatives that are compared in the HVAC simulations are:

  • Aerotherm system
  • Nozzle system
  • Underfloor radiant system
  • Radiant ceiling system
  • Radiant wall system

This study was conducted using MSC Cradle’s ScStream software.

Pre-dimensioning and creation of the CFD model

In the first phase, an analytical calculation is carried out on the basis of the heat load booklet. In this way, an initial size is assigned to the HVAC systems. This pre-design is based on the customer’s requirements and restrictions and on the data of the equipment of specialised suppliers.

This provides an estimate of the number of fan heaters and nozzles to be installed in each proposal, and the power that the radiant elements must be able to provide to achieve the target temperature.

In parallel to the pre-dimensioning, a CAD model is created containing the main geometry of the building, considering the external and internal elements that are of interest. A study of the infiltrations predicted by the CFD model is also carried out so that their effect on the model is adjusted to reality.

Starting from this base model, the five models corresponding to each HVAC system to be simulated are generated. The following images show the geometry of the building and an interior view with the preliminary nozzle system modelled.

Basic geometry of the building analysed by HVAC simulation

CFD simulation of HVAC systems

The first stage of the simulation is based on the pre-design. The five proposed HVAC systems are calculated. This type of analysis allows us to obtain maps of operating temperature, air speed, surface temperature and PMV (Predicted Mean Vote), among many other results.

The CFD simulations take into account the effects of heat transfer by convection, conduction (heat loss to the ground, for example) and radiation, which are particularly relevant in some of the proposed systems. They also resolve the fluid velocity fields, so that thermal comfort can be assessed comprehensively considering the effects of air flows.

To correctly simulate thermal effects, the meshing and physical models must be appropriate for the problem under consideration. In this study, grids of between 15 and 25 million elements (cells) are used to capture the phenomena of interest well.

In this first stage, it is found that some of the proposed systems are far from giving satisfactory results. This means that in the subsequent optimisation stage, only the systems that have given favourable results are compared. The following image shows the operating temperature results in a vertical cut of one of the systems analysed.

Temperature map of the building as a result of the CFD simulation of the HVAC system.

And in the next one you can see the PMV map calculated with the thermal insulation values by clothing and physical activity expected for the workers in the building. This is given in a horizontal cut at the usual working height.

PMV map in the building as a result of the CFD simulation of the air conditioning.

Optimisation of proposals

In the initial analyses, it is observed that convective systems may have an efficiency problem caused by air stratification in the enclosure, a common phenomenon in high-rise buildings. This means that hot air accumulates in the upper part of the building and leaves the occupied area at a lower temperature than expected.

To prevent this situation, the addition of destratifier fans in the aerotherms system and an increase of the discharge velocity in the case of nozzles is considered. Some examples of destratifiers are shown in the following picture.

CFD-simulated destratification fans in the building

The HVAC simulation results show that the use of destratifier fans has a significant impact on temperature only if the drive is at high speed. However, this generates high velocity zones that penalise thermal comfort. These effects are amplified by some corridors created by the fixed elements of the building, as can be seen in the following image.

Map of velocities on the building when placing the destratifiers at their maximum velocity

For radiant systems, it was decided to study other external conditions, simulating a less unfavourable case of temperature outside the building. In this way, the behaviour of the system can be better assessed with additional information.

The optimised solution that is finally chosen shows very satisfactory results in terms of effectiveness and efficiency. In the following images you can see the operational temperature maps in a vertical cut and PMV in a plan view obtained with this system.

Temperature map of the building as a result of the optimised HVAC simulation of the air conditioning.
PMV map in the hall as a result of the optimised HVAC simulation of the system.

Detailed study with model of human thermoregulation

As a final stage of the HVAC study, a series of simulations are carried out including a human thermoregulation model. These are chosen on the basis of the alternatives that have proved to be the most advantageous in terms of thermal comfort and energy efficiency.

The model used, known as JOS2, simulates the thermal behaviour of the human body and its interaction with the environment. It takes into account factors such as clothing, age and level of physical activity. This makes it possible to predict thermal comfort in detail in combination with CFD simulation. A schematic of how the model works is shown in the image, taken from the website of the software used in the analyses.

Schematic of the model of human thermoregulation included in Cradle

For the study, a number of dummies are placed at different positions on the building and the model is simulated as a whole. Three examples of skin temperature results in different scenarios are shown below. This data is a good indicator of the perceived thermal sensation and the actual thermal comfort of the occupants.

Skin temperature distribution of people simulated with the JOS2 model and CFD in the study of HVAC system.

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