Modelling Approaches for Displacement Ventilation in Offices

Hensen, J.L.M. M.J.H. Hamelinck and M.G.L.C. Loomans: in Proc. 5th Int. Conf. on Air Distribution in Rooms; ROOMVENT '96, Yokohama

(Note: the following is a short presentation only ; ie this is not the complete paper)

INTRODUCTION

A building comprises various dynamic sub-systems with many time- variant interactions. For many practical problems, an integral approach is essential.

The building as an integrated dynamic system

Performance prediction of displacement ventilation in offices is an example and is used here to demonstrate the merits and drawbacks of various computer modelling approaches for HVAC design and performance prediction.

Main environmental engineering evaluation criteria:

energy consumption: cooling or heating demand; electricity consumption by fans
thermal comfort: whole body thermal comfort; local thermal comfort
indoor air quality: ventilation efficiency; contaminant concentration distribution

MODELLING APPROACHES

Modelling and simulation of HVAC systems is gaining interest. Focussing on the air flow modelling part of HVAC system simulation a range of modelling approaches can be distinguished.

Level A
+	problem definition and calculation procedure
+	incorporation in mass flow network for bulk flow through the whole building
-	representation of real temperature and flow conditions within a room

Level B
+	problem definition and calculation procedure
+	incorporation in mass flow network
+	global indication of temperature profiles within a room
-	definition of inter-zonal flows
-	radiation exchange between subzones can result in calculation problems

Level C
+	detailed information on the flow and temperature field within the room
-	problem definition and computational effort
-	predictions for specific points in time only
-	integration in general building simulation program difficult

CASE STUDY

Application of a displacement ventilation system in a "standard office module" located in the Netherlands [temperate sea climate].

Although displacement ventilation is a proven technology for factories and workshops, advantages over a mixing type of ventilation system are not so clear cut for office type applications.

LEVEL A: FULLY MIXED

The office is modelled as a fully-mixed zone

No distinction between fully mixed and displacement ventilation, since the vertical temperature gradient is not taken into account.
Whole body thermal comfort predicition is possible, but local not.
Global whole building information on indoor air quality.

LEVEL B: INTERMEDIATE APPROACH

Thermal stratification is approximated by sub-dividing the office in

stacked (fully-mixed) sub-zones, seperated by "fictitious" floors.

Energy consumption for cooling is lowest in case of displacement system when casual gains < 35 W/m² and when cooled ceiling is not necessary.
If cooled ceiling required then energy consumption for cooling is lowest for mixing system.
In terms of cooling energy plus fan electricity, both systems are very similar.
Whole body thermal comfort and local discomfort due to vertical gradients can be predicted.
global whole building information on indoor air quality.

LEVEL C: FLOW FIELD APPROACH

Computational Fluid Dynamics is used to predict flow and temperature field within the office.

Boundary conditions             Temperature contours for 
for reference case              reference case

No information on e.g. annual energy consumption.
Cooling effectivity higher for displacement ventilation than for mixing ventilation.
Extended whole body thermal comfort and local discomfort predicitions.
Requires considerable experience regarding problem definition, solution strategy and results analysis.

CONCLUSIONS

Demonstration of merits and drawbacks for three levels of air flow modelling approaches for HVAC design and performance prediction by evaluating displacement vs mixing ventilation for offices.

Each approach has its own (dis)advantage and different approaches should be used, depending on the question to be answered.

FUTURE

Enabling the incorporation of a vertical temperature gradient in approach A
Modelling of "ficticious surfaces" in approach B
Integration of CFD in general building energy simulation for approach C
Prediction of thermal comfort as affected by the flow and temperature field within a room