CFD simulation of floor level air supply method and evaluation of system energy savings

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CFD simulation of floor level air supply method and evaluation of system energy savings

 

Author: Xu, Hongtao
Title: CFD simulation of floor level air supply method and evaluation of system energy savings
Degree: Ph.D.
Year: 2005
Subject: Hong Kong Polytechnic University -- Dissertations.
Air flow -- Mathematical models.
Fluid dynamics -- Mathematical models.
Heating -- Control.
Air conditioning -- Control.
Energy conservation.
Department: Dept. of Building Services Engineering
Pages: xxi, 212 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1896800
URI: http://theses.lib.polyu.edu.hk/handle/200/432
Abstract: The supply diffuser influences air flow patterns in the space with the heating, ventilating and air-conditioning (HVAC) system. How to accurately quantify its impact is essential for the proper design of air distribution systems. Some simplified CFD (Computational Fluid Dynamics) modeling methods are available. Among them, the box and momentum methods are the most popularly used. Different from the mixing system, the underfloor air distribution (UFAD) system employs the supply diffuser in the occupied zone and takes advantage of room air stratification for thermal comfort and energy saving. The design of this system is more sophisticated but requires practical tools, and the objective of this thesis is to develop such tools based on the supply diffuser simulation and air temperature stratification models. The box method cannot be employed where the buoyancy force dominates indoor air flow, as is the case with the UFAD system. In this thesis, the fully represented geometry method (FRGM) was proposed to model the supply diffuser. The aim is to reveal the influence of diffuser configurations on the air flow pattern near the supply diffuser. This method was validated with the experimental data from a precombustion chamber. Employing the standard K-E turbulence model, the FRGM presented more accurate results near the supply diffuser than the momentum method. With this method, the need to measure the input conditions near the supply diffuser can potentially be eliminated, which is difficult and inaccurate to do due to the large gradients. The RNG K-E turbulence model was also investigated with this method and it was found more suitable for predicting flow patterns in the downstream recirculation zone. With the FRGM, two supply diffusers, named the swirling and square diffusers, were employed in UFAD systems for different thermal environment investigations. It was found that indoor air temperature stratification can be maintained with both supply diffusers, but the thermal environment in the occupied zone was greatly influenced by the configuration of the supply diffuser. The swirling diffuser generated a more uniform thermal environment than the square diffuser due to the 'swirling' characteristic. Furthermore, the effects of the vane declining angles of the swirling diffuser were investigated, and it was found that two different flow patterns can be created, the horizontal and vertical flows. The horizontal flow is preferable for better thermal comfort. Indoor air temperature stratification in the UFAD system offers an opportunity for cooling load reduction and energy saving potential, as compared with the mixing system. Most energy simulation programs should be revised to take into account the stratification in the UFAD system. In this thesis, a numerical method by which the energy analysis program ACCURACY is coupled with the CFD technique was presented. The dimensionless temperature coefficient was defined and obtained through the CFD simulation with the boundary conditions calculated by ACCURACY. In conditions where the indoor heat sources dominated the whole cooling load in an office room, it was concluded that, for a constant air volume (CAV) supply, the dimensionless temperature coefficient was affected by the elevation of the electronic equipment when other heat sources were fixed. For a variable air volume (VAV) supply, the supply air flow rate dominated the dimensionless temperature coefficient when all heat sources were fixed. When compared with the mixing system, the UFAD system derives its energy saving potential from the following three factors: an extended free cooling time, a reduced ventilation load, and increased Coefficient of Performance (COP) for chillers. The cooled ceiling (CC) is a representative of HVAC systems. When it was combined with a DV (displacement ventilation) or UFAD system, the typical indoor air temperature stratification was suppressed and the indoor air temperature at head level (1.1 m) was almost equal to the exhaust air temperature in a small office room, signifying that the uniform indoor air temperature model can be employed for the energy analysis of the combined systems, CC/DV and CC/UFAD. Based on the same thermal sensation index (PMV=O) at head level, the combined systems presented a more uniform thermal environment in the occupied zone as a result of the employment of CC. Compared with the CC/DV and CC/UFAD system, the mixing system offers a longer free cooling time due to the larger air flow rate, when the economizer operation is assisted by a constant speed supply fan. The higher temperature required by the ceiling panel in the combined system makes it possible to have a separate chiller system running at much higher evaporator temperature conditions for the chiller energy saving.

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