Underfloor Air Distribution System Research Articles

Determination of particle concentration in the breathing zone for four different types of office ventilation systems

Many factors affect the airflow patterns, thermal comfort, contaminant removal efficiency and indoor air quality at individual workstations in office buildings. In this study, four ventilation systems were used in a test chamber designed to represent an area of a typical office building floor and reproduce the real characteristics of a modern office space. Measurements of particle concentration and thermal parameters (temperature and velocity) were carried out for each of the following types of ventilation systems: (a) conventional air distribution system with ceiling supply and return; (b) conventional air distribution system with ceiling supply and return near the floor; (c) underfloor air distribution system; and (d) split system. The measurements aimed to analyse the particle removal efficiency in the breathing zone and the impact of particle concentration on an individual at the workstation. The efficiency of the ventilation system was analysed by measuring particle size and concentration, ventilation effectiveness and the indoor/outdoor ratio. Each ventilation system showed different airflow patterns and the efficiency of each ventilation system in the removal of the particles in the breathing zone showed no correlation with particle size and the various methods of analyses used.

Numerical study on temperature stratification in a room with underfloor air distribution system

In this study, numerical prediction using computational fluid dynamics (CFD) was utilized to investigate air temperature stratification in a room with an underfloor air distribution (UFAD) system. The numerical modeling using CFD computation was validated with physical test in a full size experimental room with an UFAD system. The different supply air conditions and heat loads were discussed. The results show that the effect of three parameters, heat load, supply volume flux and supply air velocity, on room air temperature would be expressed by the length scale of the floor supply jet. When the length scale increased from 0.8 to 1.56 m, the ratio of vertical temperature difference between 2.5 and 0.1 m at the occupied zone to the difference between return and supply air temperature decreased from 0.62 to 0.25. When there was only one local heat source in the room, there was a thermal stratified interface at the occupied zone. The interface height was about 1.42 times the length scale. The results may suggest ways to optimize UFAD design and operation.

Numerical Simulation for Under-Floor Air Distribution System With Swirl Diffusers

A finite volume renormalization group (RNG) k-ε turbulent model was employed to simulate an under-floor air distribution (UFAD) system consisting of eight swirl diffusers. Mesh generation was conducted using PRO/E and GAMBIT. Computational fluid dynamics (CFD) results using FLUENT show both flow and thermal patterns for an instrumented laboratory room (Building Technology Laboratory-BTL) located at the University of Nevada Las Vegas. Simulation results are presented using symmetrical boundary settings for the BTL. Stratification heights and clear zones are discussed. The application of CFD simulation provides insightful analyses in UFAD design and placement.

Performance of Underfloor Air Distribution in a Field Setting

Underfloor air distribution (UFAD) is a new method of supplying heated or cooled air throughout a building. Reported advantages of UFAD include energy savings and improved indoor air quality (IAQ). We measured several aspects of the performance of a UFAD system installed in a medium-size office building. The measured air change effectiveness was very close to unity, which is comparable to that measured in buildings with typical overhead air distribution. The pollutant removal efficiency for carbon dioxide was 13% higher than expected in a space with well-mixed air, suggesting a 13% reduction in exposures to occupant generated pollutants. The increase in indoor air temperatures with height above the floor was only 1 to 2 °C. This amount of thermal stratification could reduce the sensible energy requirements for cooling of outdoor air by approximately 10%. The occupants’ level of satisfaction with thermal conditions was well above average and this high satisfaction rating could possibly be due, in all or part, to the use of a UFAD system. The results of this study provide some evidence of moderate energy and IAQ-related benefits of UFAD. Before general conclusions are drawn, the benefits need to be confirmed in other studies.

The fluid dynamics of an underfloor air distribution system

This paper discusses the fluid dynamics of an underfloor air distribution (UFAD) ventilation system in which, in contrast to conventional air-conditioning systems, cool air is delivered from diffusers in the floor rather than from overhead vents. In order to produce more realistic models of UFAD systems, we extend previous work on a simplified system consisting of a single heat source at floor level and a single cooling diffuser, to the case of multiple cooling diffusers and a single heat source located at different heights above the floor. We carry out experiments in which the heat source is represented as a buoyant plume, and the cooling diffusers are modelled using negatively buoyant vertical jets. The experiments show that the properties of the system are determined by the entrainment into the plumes and the negatively buoyant jets. We characterize these entrainment processes by entrainment coefficients, and develop a theoretical model based on layered models of ventilation flows. The model predictions are compared with the laboratory experiments, and used to determine the dependence of these parameters on the cooling load, the ventilation rate and the properties of the cooling diffusers.

Novel air distribution systems for commercial aircraft cabins

Air distribution systems in commercial aircraft cabins are important for providing a healthy and comfortable environment for passengers and crew. The mixing air distribution systems used in existing aircraft cabins create a uniform air temperature distribution and dilute contaminants in the cabins. The mixing air distribution systems could spread infectious airborne diseases. To improve the air distribution system design for aircraft cabins, this investigation proposed an under-floor displacement air distribution system and a personalized air distribution system. This study first validated a computational fluid dynamics (CFD) program with the experimental data of airflow, air temperature, and tracer-gas concentration from an environmental chamber. Then the validated CFD program was used to calculate the distributions of the air velocity, air temperature, and CO 2 concentration in a section of Boeing 767 aircraft cabin with the mixing, under-floor displacement, and personalized air distribution systems, respectively. By comparing the air and contaminant distributions in the cabin, this study concluded that the personalized air distribution system provided the best air quality without draft risk.

Experimental measurements and numerical simulations of particle transport and distribution in ventilated rooms

Prediction of particle dispersion and distribution in a room is very important for creating and maintaining a healthy indoor environment. The present study used a CFD program with a Lagrangian particle tracking method to predict particle dispersion and concentration distribution in ventilated rooms. Since the Lagrangian method could generate great uncertainty in particle concentration calculations, this study first investigated such uncertainty using a statistical approach. It was found that the stability of the concentration solution became well controlled as a sufficient number of particles were analyzed. Although the overall computational cost was considerable, the numerical results agreed well with associated experimental data. In all cases studied, particle size distribution was monodisperse, and particle diameter ranged from 0.31 to 4.5 μm. Particle deposition rate was neglected, and particles were hence removed only by the ventilation system. Thus the particle removal performance of different ventilation systems can be evaluated. Three ventilation systems have been studied, including ceiling and side wall supply systems and an underfloor air distribution (UFAD) system. It was found that the UFAD system had a better particle removal performance than the ceiling and side wall supply systems in the study. However, resuspended particles at the floor level can still cause problems in an UFAD system.

06/00768 Heat transfer and pressure drop characteristics of the underfloor air distribution system: Zukowski, M. Energy and Buildings, 2005, 37, (8), 890–896.

06/00768 Heat transfer and pressure drop characteristics of the underfloor air distribution system: Zukowski, M. Energy and Buildings, 2005, 37, (8), 890–896.

Numerical procedure for predicting annual energy consumption of the under-floor air distribution system

As compared with the mixing system, indoor air temperature stratification in the under-floor air distribution (UFAD) system offers an opportunity for cooling load reduction in the occupied zone. This stratification is a major feature that offers the energy saving potential, but it has not been thoroughly taken into account in most energy simulation programs. In this article, a numerical procedure, based on coupling two types of modeling, i.e., CFD (computational fluid dynamic) simulation and dynamic cooling load simulation, is proposed to predict annual energy consumption. The dimensionless temperature coefficient is first defined in the UFAD system and obtained from CFD simulation, based on the boundary conditions from a cooling load program ACCURACY. According to this coefficient, temperature stratification input to ACCURACY is then revised to calculate the updated supply and exhaust air temperatures for final annual energy prediction. To demonstrate the method, a small office room is investigated using Hong Kong weather data. With the constant air volume (CAV) supply in the UFAD system, it is found that the dimensionless temperature coefficient is almost a constant, when the locations of heat sources are fixed. As 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 coefficients of performance (COP) for chillers.

A new formula for determining a minimum recommended value of inlet air velocity from UFAD system to prevent occupants from draught risk

During the winter period the natural convection flow along a cold window surface can be the cause of thermal discomfort. Warm air blown into the room near the window prevents occupants from draught risk. The paper presents a new formula for determining a minimum recommended value of inlet air velocity. A balance of a momentum flux of flow of the down draught currents and the warm air jet inflow from the UFAD (under floor air distribution) system was a base for the analytical solution. In order to estimate air movement in a typical room, numerical simulations are executed. A computational model of the room with the UFAD system is constructed within the CFD environment developed by Fluent Inc. Ten cases with varying inlet air parameters and a window U -value are considered. The analytical estimation of minimum inlet air velocity is confirmed by simulation results of temperature and velocity distributions.

Heat transfer and pressure drop characteristics of the underfloor air distribution system

This paper reports experimental data for forced convection heat transfer in a heat exchanger, which is a key part of the underfloor distribution system. The experiments were performed to study heat transfer and pressure drop characteristics of tested units with different geometry of flow ducts. The heat exchanger was made of Plexiglas and PVC. The top side of plastic exchanger was covered by a layer of a concrete screed with thickness of 0.035 m. The bottom side was insulated with 0.1 m of polystyrene. The length of the tested units was 1 m and the width 0.5 m. The fluid in the heat exchanger was air. The results of experimental study were presented in a dimensionless form. The average Nusselt number and Fanning friction factor were expressed in terms of the Reynolds number. The experimental results presented in this paper provide useful information for designing of an underfloor air distribution system in the residential housing sector.

A model for an under floor air distribution system

We present a simplified model of an underfloor air distribution (UFAD) system consisting of a single source of heat and a single cooling diffuser in a ventilated space. Laboratory experiments were carried out to simulate the flow and a model for the flow in this space is proposed. The model is based on plume theory for the heat source and a fountain model for the diffuser flow, and predicts a steady-state two-layer stratification in the room. The governing parameters are shown to be the buoyancy flux of the heat source, and the volume and momentum fluxes of the cooling diffuser. The results suggest ways to optimize UFAD design and operation.

床下チャンバーまわりの熱移動のモデル化 : 床吹出し空調システムに関する研究 その3

The convective heat transfer coefficient was measured with an experimental rig, and found to follow Jurges's equation when the airflow velocity exceeds 2 m/s. For lower airflow velocities, another equation was derived from the measurement results. Heat flows were measured around the air plenum of a full-scale experimental rig of underfloor air distribution system. Two newly proposed models, a "1-dimensional model" for a transition and a "Developed 1-dimensional model" for a horizontal distribution in a steady state, were shown to yield heat flows and temperatures almost equal to the measured values, using the above convective heat transfer coefficient equations.

B-28 床下チャンバー加圧型の床吹出し空調方式における床下チャンバーの特性 : その7 モデルによる定常及び非定常熱移動シミュレーション

B-28 床下チャンバー加圧型の床吹出し空調方式における床下チャンバーの特性 : その7 モデルによる定常及び非定常熱移動シミュレーション

床吹出し空調システムにおける室内温度プロフィールの予測 : 床吹出し空調システムに関する研究 その1

On the basis of the results of the temperature measurements made in a full-scale test room, a simplified prediction model for simulating room air flow patterns in the underfloor air distribution system was developed, consisting of two vertical areas and three horizontal zones. The model and some derived relationships regarding air flow and heat reproduced the measured temperature profiles with practical accuracy. It was shown by comparison with the uniform room air temperature case, that the underfloor system consumes less energy and allows a lower temperature rise with an increase in heat load.