Modelling of displacement ventilation and chilled ceiling systems using nodal models
Cooling and ventilation of office spaces by displacement ventilation and chilled ceiling panel systems is potentially more energy efficient than conventional air conditioning systems. Heat transfer in this type of system is characterised by the presence of vertical temperature gradients and significant radiant asymmetry. The room heat transfer models used in current building simulation methods do not allow adequate representation of this type of system due to their reliance on a single node to define the internal air temperature. The overall aim of the work described in the thesis has been to develop a model of this type of system that is suitable for use in annual building energy simulation. The model presented, is intermediate in complexity between a CFD numerical model and the current single air node models, having ten air nodes. The operating characteristics of displacement ventilation systems, used both with and without chilled ceiling panels, have been studied by making experimental measurements in a test chamber and by reference to published experimental data. Numerical calculations of the flow and temperature fields have been made with a coupled flow and radiant exchange CFD code. Steady state calculations have been made of displacement ventilation using a conjugate heat transfer method. Interesting oscillations in the flow and temperature field of displacement ventilation operating with a chilled ceiling have been found through adopting a fully transient calculation procedure. The thesis describes how the experimental and numerical data has been used to develop the structure and define the parameters of a simplified nodal model. The logical development of the model structure, from the most elementary model to one which is able to capture the effects of the temperature gradients and incomp1ete mixing of the air in the room is described. A method is also presented whereby the parameters of the model are found directly from the experimental data by solving the heat and mass balance equations of the nodal model. The parameters of the model have been generalised by analysing these calculations and by making use of established convection coefficient correlations. The performance of the model is demonstrated by firstly making comparisons of the experimental and numerical data under steady state conditions, and also by demonstrating that the model is able to reproduce the characteristics of displacement ventilation and chilled ceiling systems under different operating conditions.