a. Numerical simulation of the air-conditioning environment (1998)
This paper describes the development of software for simulating conditioned-air environments in a computer room. The finite volume method combined with a rectangular structured mesh system, and a k-ε, two-equation turbulent model were used (Nishihara, 1998).
The conclusions are that
(1) Flow and temperature distributions in computer rooms can be simulated with sufficient accuracy using this simulator; and
(2) When a mal-functioning occurs due in an air-conditioning problem, the simulator can help find the reason and develop measures to counter a reoccurrence of the problem.
b. The cooling characteristics on the air supply and return flow system (1998)
This paper describes a CFD study for the cooling characteristics of the supply and return air flow systems in a telecommunication cabinet room. Three systems were considered, namely, (A) horizontal air supply and return, (B) underfloor air supply and overhead return, and (C) overhead air supply and underfloor air return (Noh, 1998).
The conclusions are that
(1) The cooling performances of systems B and C supplying the cooling air directly in the telecommunication cabinet were improved much better than the COS; and
(2) The cooling performances of an underfloor air supply and overhead return flow system are the best among the three airflow systems.
c. Numerical investigation of transient buoyant flow (2001)
Experiments were carried out in an environmental test chamber equipped with displacement ventilation with supplementary cooling provided by a chilled ceiling for a range of operating conditions. CFD calculations (Rees, 2001) of the room airflow and heart transfer were carried out in parallel with the experiments using the test chamber geometry and experimental boundary conditions.
The conclusions are that
(1) The airflow is separated into two distinct regions of the room, one near the floor and up to the height of the top of the supply air diffuser, and the other region above the top of the diffuser and up to the ceiling;
(2) Complex quasi-periodic fluctuations in the room air flow have been observed in the predicated results;
(3) The air temperature and velocity profiles from the transient calculations show evidence the vertical extent of the recirculating flow in the upper region of the room when compared with experimental data; and
(4) The calculations of the fluctuations in the temperature field near the ceiling show similar results in the experimental data. This demonstrated that the CFD methods could be used to make predications of the presence of phenomena found in experiments.
d. A sensitivity study of floor and ceiling plenum energy model parameters (1997)
The heat transfer phenomena in plenums are complex and transient and include conduction, convection, and radiation modes. This project found the sensitivity of different geometries of plenums to convective heat transfer coefficients, surface conditions, and other model parameters. The public domain nodal program HLITE was used in this project. A companion program, VLITE, was used to solve for the radiation view factors. Three-dimensional models of ceiling and floor plenums with six different air distribution schemes were examined (Rock, 1997).
The conclusions are that
(1) The sensitivity of plenum models to various parameters was found and is presented; and (2) The required accuracy is given for each variable to which the computer models are
sensitive.
More work should be done on guidelines for HVAC designers to quantify the influence of plenums on the peak loads and system selection, and on load calculation models for design purposes including the effect of different types of ceiling and floor plenums.
e. Thermal climate assessment in office environment – CFD calculations and thermal manikin measurements (Awbi, 2000)
Thermal sensation is affected by many factors in the office environment especially thermal factors and effects from air movements caused by different ventilation systems. To investigate whether CFD calculations and measurements with a thermal manikin are able to predict the perceived climate in an office, CFD calculations as well as full-scale measurements were carried out. A mixing-ventilated office room was equipped with displacement-ventilation and evaluated by CFD simulations and supported by measurements.
The interactions between the air movements and thermal comfort conditions in the office
room were also studied.
The conclusions are that the CFD results show good agreement with the measurements made in the real environment.
More work should be done on development of the simulations for a human or manikin.
The manikin model should also include the same thermal behaviors as the real manikin.
Future research should aim at evaluating these numerical calculation results to find more accurate methods for evaluating ventilation and office room designs.
f. CFD simulation of concentration of gaseous impurities (Awbi, 2000)
A validated CFD model was employed to generate concentration distribution data for CO2, radon, and moisture in a typical Hong Kong industrial workshop with displacement ventilation. The indoor radon comes from building materials. The indoor CO2 and moisture are from the occupants. The analysis found that the concentration distribution is affected by factors such as the source type and location and its associated plume strength and human body convection. Because the upward free convection around a person brings the air from a form lower level to the breathing zone, the inhaled air is cleaner than the air at the same height. Prediction of contaminant distribution is more difficult than air temperature and flow distributions.
The conclusions are that
(1) Low contaminant concentration may be obtained in the occupied zone when the contaminant source is associated with a heat source and the thermal plume generated by the heat source is sufficiently strong to reach the upper zone. Because the upward free convention around a person brings the air from a lower level to the breathing zone, the inhaled air is cleaner than the air at the same height. Because contaminant distribution depends very much on the air velocity field, and to less extent, on the air temperature field, prediction of contaminant distribution is more difficult than air temperature and flow distribution; and
(2) The displacement ventilation performed well for indoor concentration of CO2, radon,
and relative humidity for industrial workshops under Hong Kong conditions, providing that the system is designed, installed, maintained, and operated according to engineering practice.
g. Numerical study of a new ventilation tower system (Awbi, 2000)
This study used CFD to investigate the ventilation tower system. Outdoor air is supplied directly to the breathing zone without mixing it with the return air from the room. This is achieved by supplying outdoor air though a vertical duct rising from the floor and distributing outdoor air to breathing zone height. With this system, the indoor air quality at the breathing zone is improved and the requirement of the conditioned air is reduced in comparison with conventional air distribution system. The CFD simulation for isothermal conditions was carried out to compare the value of the air exchange efficiency between a COS and the ventilation tower system.
The conclusions are that
(1) The ventilation tower system proved that the IAQ could be upgraded by directly supplying outdoor air into the breathing zone; and
(2) Energy can possibly be saved while keeping the IAQ in the breathing zone almost at the conventional level.
h. Analysis of air velocity and temperature distribution in furnished rooms with underfloor air diffusers (Awbi, 2000)
A numerical model on the specific physical system for a RFS was carried out. A comparison between experimental and CFD results is presented for the case of different floor air diffusers in a typical office.
The conclusions are that
(1) The suitability of a CFD to correctly predict comfort indices was proved; and
(2) The CFD model can be used for design purposes, to determine the characteristics of the air flow field within internal spaces and to obtain useful information for different cases, having various values of air flow rate, room dimensions, positions of air supply and
exhaust grilles, and thermal gains.
i. Room air distribution and cooling load (Awbi, 2000)
The characteristics of the indoor environment are examined using a CFD simulation under various conditions of outdoor air. The control of the room air conditioning system (variable air volume system) is included in the calculation through changing the supply air flow rate to keep the task zone temperature at a target temperature. When the temperature of the outdoor air rises, the flow penetrates deeper into the room and mixes well with the indoor air.
The conclusions are that
(1) By keeping the air temperature at the task zone constant, when the temperature of the outdoor air increases, there is a tendency for the supplied air to flow deeper into the room and to mix well with the indoor air;
(2) When the temperature of the outdoor air increases, the air flow rate from the conditioned-air system increases in response, thus creating a larger temperature gradient at the task zone;
(3) The relative humidity distribution is under the influence of temperature fields because the absolute humidity distribution is almost constant through out the zone;
(4) The distribution of the age of the air is strongly influence by the flow fields; and
(5) The cooling load for the mechanical conditioned-air system increases with an increase of the temperature of the outdoor air and with a decrease in the outdoor air flow rate.
When varying the window opening area (vertical width of the opening), it is advantageous to make the opening larger.
j. Indoor air quality and climate control parameters (Awbi, 2000)
This paper develops a new ventilation assessment method with control parameters for settling particles. The age of air concept is complemented by flow parameters that describe air movements and particle settling in the room. A standard single person office unit is investigated using CFD calculations and measurements. The new method for particle settling
assessment in a room is tested. Indoor air quality, ventilation efficiency, and thermal comfort conditions are reported. A thermal manikin is used in the measurements.
7.3 Representative results
Figures 7.1 to 7.3 show several plots taken from the CFD studies discussed above (parts f and j in Section 7.2). The results are selected from the papers and are considered representative of the CFD results. The exact results of the studies vary because of the different simulation models, and the located articles should be consulted for details. The vertical temperature distribution is shown in Fig. 7.1. The temperature is stratified along the vertical direction in this plot. As shown in Fig. 7.2, CFD results are shown for CO2
concentration, relative humidity, and local age of air. Figure 7.3 displays values for the air velocity distribution and local age of air. The experimental and CFD results are shown side-by-side in Fig 7.4 to show the relation between air temperature differences and ventilation efficiency. The results in Figs. 7.1 to 7.4 illustrate the level of details for the air flow and temperature as well as contaminant distributions that can be provided by a CFD analysis.
Figure 7.1 Temperature distribution (Hsu, ROOMVENT 2000).
Figure 7.2 CFD results (Lin, ROOMVENT 2000).
a. Air flow. b. Local age of air.
Figure 7.3 Air distribution (Holmberg, ROOMVENT 2000).
Figure 7.4 Temperature difference and ventilation efficiency (Akabayashi, ROOMVENT 2000).