COMIS and TRNSYS Application Example

11.5.6.1 Building

This example considers the thermal summer condition evaluation of a large, naturally ventilated test laboratory hall at EMPA (Fig. 11.50).

11.5.6.2 Purpose of the Study

The large, civil engineering test laboratory hall at EMPA is going to be re­furbished. Large parts of the roof will be glazed for maximum daylight use. Solid walls will be better insulated.

The simulation study should give answers to the following questions:

1. Overheating risk under summer conditions

2. Temperature reduction potential using passive cooling by natural nighttime ventilation

Different ventilation-opening control strategies in terms of compliance with thermal comfort requirements

4. Risk of draft due to cold airscreams falling from the cold roof windows in the winter season

I 1.5.6.3 Approach

Due to the thermally driven air exchange and the large building masses in­volved, the problem must be studied using a dynamic thermal building model with an integrated ventilation model.

The study is performed for a representative section of the hall. A network model was established for COMIS, considering doors, openings at floor level, and the large openable ventilation hoods on the roof. Relationships are estab­lished for the air-exchange rate as a function of the temperature difference be­tween inside and outside for different opening configurations. The effect of a temperature gradient in the hall is evaluated additionally. As a conservative approach, wind effects are neglected.

The relationships between air exchange rate and temperature difference were determined using COMIS (Fig. 11.51) and then integrated as the ventila­tion model in the thermal model. The thermal behavior is modeled with the TRNSYS multizone type, considering the hall and the room below the thick concrete test floor slab. For the hall, a room model with two air temperature nodes (one for the occupied zone and one for the rest of the hall) and geomet­rically detailed radiation exchange is used.

O , R Rt’-

Openings fully open * ° — Openings 75% open Openings50% open

5 10 15 20 Temperature difference: indoor — outdoor, A 7 (K)

I FIGURE I 1.51 Characteristics of the natural air change rate in the hall as a function of the differ­ence between indoor and outdoor air temperature, as calculated using COMIS. These characteristics were then integrated into the thermal model (TRNSYS).

For the determination of downdraft risk in the winter case, three-dimensional and transient CFD computations were performed using the TASC flow code. Boundary conditions were defined from the results of the thermal modeling.

Ventilation Model

The ventilation model is a simple flow network with one zone and the different openings modeled as airflow links from the hall to outside (Fig.

6. 52). For the flow through the roof hood, two additional nodes were considered between the different cross-sections through which the air flows (Fig. 11.53).

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Root hoods

©	Shed window	©
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Vents

^Ven

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Concrete test floor slab

FIGURE 11.52 The ventilation model consists of one zone (the hall), which is connected to the external nodes by the different opening links.

I 1.5.6.4 Simulation Results for the Summer Case

Thermal Comfort Evaluation

The thermal comfort was evaluated with hourly mean values of the air temperature in the occupied zone, plotted against the maximum I h mean out­door temperature value of the day. Only the period from April 1 to October 30 and only working hours (7 a. m. to 6 p. m.) are considered. This evaluation method is based on the Swiss standard SIA V382/2.15 The minimum and max­imum allowable comfort temperatures are adapted to the usual activity and clothing levels of the workers in the hall (see Figs. 11.55 and 11.56).

Winter Case

The draft risk due to cold air pillows under the roof glazing dropping into the occupied zone was determined by transient CFD calculations. As can be seen from Fig. 11.57, velocities do not exceed 0.2 m/s. Therefore, the draft risk was assumed to be marginal.

I 1.5.6.5 Conclusions from the Example Case

Nighttime ventilation is necessary under summer conditions to keep tem­peratures in the comfort range. The ventilation openings should be closed if

• Outdoor air is warmer than inside temperature. This is, of course, possible only for such a hall where the air volume is very large in comparison to the required airflow rate.

• Outdoor temperature at night falls below a certain threshold, in order to prevent too low temperatures in the hall in the morning.

Time (date) FIGURE 11.55 Air change rate, outdoor air temperature (TJ and room air temperature in the occupied zone (T,). for a four-day summer period. Ventilation openings are opened 0-24 hours if T, > T0. The moment when T„ becomes greater than T, is highlighted on the first day, with the air exchange dropping to zero.

I 1.5.7.2 From Simple to Complex Models

Simple, single-zone models are suitable for quick bur rough estimates, e. g., of the cooling potential of nighttime ventilation. However, with such sim­ulation tools, a detailed analysis of the thermal comfort and air quality situa­tion in the working area of a room normally is not possible. If a specific configuration must be checked, e. g., against required design values, then only the combined modeling with both a detailed thermal and a detailed ventilation model may produce satisfying results. This is especially true in cases with sev­eral zones, more complex ventilation openings, and sophisticated glazing and solar protection systems.

I t.5.7.3 CFD vs. Combined Thermal and Ventilation Modeling

CFD is appropriate in cases where the detailed flow field is of interest in a configuration with mostly known or at least steady-state boundary conditions (surface temperatures). Combined thermal and ventilation modeling is more suited to cases where the dynamic behavior of the building masses and the changing driving forces for the natural ventilation are of importance.

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