Air Distribution System Research Articles

Experimental parametric study on the temperature distribution of an underfloor air distribution (UFAD) system with grille diffusers

The underfloor air distribution (UFAD) system has attracted great attention from many researchers and designers due to its advantages of energy conservation and air quality enhancement. In a UFAD system, the conditioned air is directly delivered to the occupied zone, causing thermal stratification from the lower zone to the upper zone. This flow pattern provides better distribution of temperature and airflow than the overhead air conditioning system for its improved ventilation and indoor air quality at the breathing level. This paper focuses on a new and unconventional way of conditioning the interior zones in UFAD buildings by employing large-area grille diffusers in a full-scale experimental room. The influences of swirl diffusers and grille diffusers on the temperature distribution in the UFAD system are compared. The effects of supply air temperature, airflow rates, and number of diffusers on the performance of the UFAD system with grille diffusers are discussed in details. The results show that the large-area grille diffusers can create stratification comparable to the more traditional swirl diffusers and thus provide an alternative way of configuring a UFAD system.

Influence of air distribution system on energy efficiency of pneumatic percussion unit of circular impact machine

The article describes computer modeling of three options of pneumatic percussion unit in a circular impact machine. All options have similar structure diagrams and mechanical linkage parameters but different control of energy-carrier feed in a working chamber: valveless; with one or two circular elastic valves. The obtained data allow quantitative evaluation of operation of such systems given the accepted constraints for mechanical block.

Ventilation and Air Distribution Systems in Buildings

Ventilation and Air Distribution Systems in Buildings

An experimental investigation of the flow and comfort parameters for under-floor, confluent jets and mixing ventilation systems in an open-plan office

There is a new trend to convert the workplaces from individual office rooms to open offices for motivating money saving and better communication. With such a shift the ability of existing ventilation systems in meeting the new requirements is a challenging question for researchers. The available options could have an impact on workers' health in terms of providing acceptable levels of thermal comfort and indoor air quality. Thus, this experimental investigation focuses on the performances of three different air distribution systems in an open-plan office space. The investigated systems were: mixing ventilation with ceiling-mounted inlets, confluent jets ventilation and underfloor air distribution with straight and curved vanes. Although this represents a small part of our more extensive experimental investigation, the results show that all the purposed stratified ventilation systems (CJV and UFAD) were more or less behaving as mixing systems with some tendency for displacement effects. Nevertheless, it is known that the mixing systems have a stable flow pattern but has the disadvantage of mixing contaminated air with the fresh supplied air which may produce lower performance and in worst cases occupants' illness. For the open-plan office we studied here, it will be shown that the new systems are capable of performing better than the conventional mixing systems. As expected, the higher air exchange efficiency in combination with lower local mean age of air for corner-mounted CJV and floor-mounted UFAD grills systems indicates that these systems are suitable for open-plan offices and are to be favored over conventional mixing systems.

Heat Transfer Effect in an Air Cooled Ventilated Enclosure

Fluid flow and heat transfer in a mechanically ventilated enclosure using a mixed air-distribution system at various levels have been numerically simulated. Thermally activated fluid is supplied through various slots of the vertical channel wall, flush in with the isothermal ceiling and flush out through a port in the opposite wall. The horizontal walls are assumed to be impermeable and adiabatic. Steady, laminar, and incompressible flow under a Boussinesq approximation has been considered, and numerical results for the Navier–Stokes equation and energy equation are obtained using a finite volume method based on the semi-implicit method for pressure linked equations algorithm for the following parameters: locations of inlet and outlets, and different values of Grashof numbers and Reynolds numbers. The proposed numerical model has been validated by comparing its predictions with those of other models’ data in the literature. The validated model is then used to investigate the effects of the Reynolds number and room aspect ratio on the flow velocity and thermal characteristics within the room. The flow structures and thermal activities are studied for Richardson’s number in the range of . Variation of the convection coefficient along the wall for cold and hot fluids at different locations has been investigated, and cooling efficiencies are found to be maximum when the inlet and the outlet are placed at the middle and lower parts, respectively, of the hot and the cold walls.

Effects of return air vent height on energy consumption, thermal comfort conditions and indoor air quality in an under floor air distribution system

In this article, an under floor air distribution (UFAD) system with separate return and exhaust air vents is investigated. The proper angle of the swirl inlet diffuser is selected by comparing the general thermal comfort conditions obtained from three different inlet angles. At this proper inlet angle, the effects of return air vent height on energy consumption, thermal comfort conditions, and indoor air quality (IAQ) are investigated. To this end, thermal comfort indices (PMV–PPD), local thermal discomfort index (temperature gradient in vertical direction), and IAQ index (mean local age of air) are probed by CFD methods. Energy consumption reduction in an UFAD system equipped with a return air vent compared to MV system is 10.9, 15.3, 18.9, and 25.7% when the return air vent height is set at 2.0, 1.3, 0.65, and 0.3m, respectively. Reducing the height of return air vent from ceiling to floor leads to an increase in exhaust air temperature and temperature gradient at the vertical direction. It is found that selection of the 1.3m height from floor (upper boundary of occupied zone in seated mode) for return air vent height will cause a 15.3% reduction in energy consumption and would maintain the thermal comfort and IAQ in the specified range.

Prediction of thermal comfort, IAQ, and energy consumption in a dense occupancy environment with the under floor air distribution system

The objective of this study is to investigate an under floor air distribution (UFAD) system with combined and separate return and exhaust air vents regarding thermal comfort, indoor air quality (IAQ), and energy consumption. This ventilation system is used in a large indoor environment (dense occupancy), and two common types of inlet diffusers (direct and swirl) are used. Computational fluid dynamics (CFD) methods are used to predict thermal comfort conditions of occupants, IAQ, and energy consumption in this space. Based on the results, direct inlet diffusers cannot lead to an acceptable temperature gradient in vertical direction in none of the examined return air vent positions, but swirl inlet diffusers can lead to an acceptable one. By precisely considering thermal comfort conditions, and IAQ made by swirl inlet diffusers, opting the upper boundary of the occupied zone (1.7 m) for return air vent position (which is the optimum case study among all the examined case studies) can lead to the best results. Moreover, the optimum return air vent position in this UFAD system can result to 10.5 percent reduction in the amount of energy consumption compared to the mixing ventilation system.

Procedure for Balancing an Air Distribution System with Decentralised Fans

This paper presents results from an on-going project concerning new design procedures for mechanical ventilation systems with low energy use. Conventional constant air volume (CAV) systems are usually balanced using flat plate dampers. The purpose of using balancing dampers is to intentionally introduce pressure drops in the duct system thus nominal airflows are achieved throughout the ductwork within specified tolerances. However, introduction of pressure drops will increase energy requirement for the ventilation system and in addition, balancing a duct system is a time consuming task and often flawed. This paper presents a new procedure for balancing of C AV systems in combination with decentralised fans. The new system was based on replacing the balancing dampers with decentralised fans. By replacing the balancing dampers with decentralised fans, airflows can be balanced by adjusting the speed of the fans. In conventional air distribution systems the fan provides the necessary pressure to overcome the resistance in the branch with the highest pressure resistance. This gives an unnecessary overpressure in the remaining branches that does not provide any useful purpose. In order to decrease the fan pressure requirements the fan was dimensioned for the branch with the least pressure resistance. The decentralised fans then provided sufficient pressure to overcome the exact resistance in the corresponding branch. The results show that by using decentralised fans in duct systems instead of dampers there is a potential for energy saving. In this study the calculated saving in the power consumption for the main fan was found to be 40%. The results also show that when dampers are replaced with decentralised fans with the same efficiency the overall power saving potential was 16%. In the study a mock-up of the calculated ventilation system was tested. Results from the calculations and the measurements showed a good correlation and the balancing procedure was acceptable.

Effectiveness of a personalized ventilation system in reducing personal exposure against directly released simulated cough droplets.

The inhalation intake fraction was used as an indicator to compare effects of desktop personalized ventilation and mixing ventilation on personal exposure to directly released simulated cough droplets. A cough machine was used to simulate cough release from the front, back, and side of a thermal manikin at distances between 1 and 4 m. Cough droplet concentration was measured with an aerosol spectrometer in the breathing zone of a thermal manikin. Particle image velocimetry was used to characterize the velocity field in the breathing zone. Desktop personalized ventilation substantially reduced the inhalation intake fraction compared to mixing ventilation for all investigated distances and orientations of the cough release. The results point out that the orientation between the cough source and the breathing zone of the exposed occupant is an important factor that substantially influences exposure. Exposure to cough droplets was reduced with increasing distance between cough source and exposed occupant. The results from this study show that an advanced air distribution system such as personalized ventilation reduces exposure to cough-released droplets better than commonly applied overhead mixing ventilation. This work can inform HVAC engineers about different aspects of air distribution systems’ performance and can serve as an aid in making critical design decisions.

Thermal uniformity in an open plan room with an active chilled beam system and conventional air distribution systems

An active chilled beam (ACB) system is known to be not only energy-efficient, but also advantageous to high indoor environment quality. For this reason, its application for indoor climate control is increasing in Europe, North America, and even Asia. The objective of this study is to evaluate the performance of the ACB system in terms of uniformity for the indoor thermal environment. To investigate the thermal uniformity of the ACB system, comparative experiments were conducted in a full-scale test bed, where conventional air distribution systems as well as the ACB system can be selectively operated. The thermal uniformity was evaluated with three performance indices: ADPI to evaluate the air diffusion performance, air velocity to evaluate the local discomfort due to cold draught, and vertical air temperature difference to evaluate temperature stratification. The result shows that the ACB system is successful in providing the acceptable thermal uniformity, even with less air flow rate than other conventional air distribution systems.

Statistical Modeling of Head Loss through Duct Fittings in Conditioned Air Distribution Systems

Fractions of the total head loss which constitute the loss through duct fittings, obtained in an earlier study, for varying numbers of supply outlets and lengths of index duct run were used in a regression analysis to obtain second order equations which gave a general increase of the fraction from 0.598 to 0.713 for an increase in number of outlets from 3 to 19 and for a corresponding increase of 11.2 m to 43.2 m in index duct length. The correlation coefficient between the fraction of loss through duct fittings and each of the variables of number of supply outlets and length of index duct run was 0.823, which was found to be acceptable for a 95% confidence level. The ratios are useful for quick estimate of total pressure losses in conditioned air distribution systems which are needed in determining the fan pressure requirements.

PH Control during the Struvite Precipitation Process of Wastewaters

The high concentration of phosphorus and nitrogen in wastewater and sludge could be lowered to a certain level by struvite (MgNH4PO4·6H2O) crystallization. One of the main factors for struvite formation is the solution pH. It can be adjusted by non-reagent carbon (CO2) dioxide stripping through the process of aeration. The intensity of the mass transfer between the air and the supernatant of dewatering sludge obtained from wastewater treatment plant is characterized by the volumetric liquid-side mass transfer coefficient, which can be estimated theoretically. It is found that the rate of pH increase depends strongly on the sparging area of the air distribution system while the air flow rate does not influence considerably the Dissolved Oxygen (DO) level which governs the CO2 stripping process. The theoretical calculated values of the volumetric mass transfer coefficient have been compared with those obtained experimentally. Based on the data obtained, relationships of pH/kLa (mass transfer coefficient) were developed. These correlations serve as a tool for prediction of pH during the struvite precipitation process.

Thermal comfort assessment of an office building served by under-floor air distribution (UFAD) system – A case study

The occupants' thermal comfort in an office building served by an under-floor air distribution (UFAD) system has been assessed using a number of different methods: field survey, physical and subjective measurements, and computational fluid dynamics (CFD) analysis. Results show that for rooms conditioned by UFAD systems, the Predicted Mean Vote (PMV) index was misleading, whereas the Air Distribution Performance Index (ADPI) was more indicative of the relative level of occupants' thermal comfort in the room. The CFD model search for the best operating conditions that maximize the occupant's thermal comfort resulted in a 90% improvement in the ADPI compared to the existing conditions. This was accomplished by setting the supply air temperature to 21 °C and inlet velocity to 1 m/s.

Channel depth, air mass flow rate and air distribution duct diameter optimization of photovoltaic thermal (PV/T) air collectors linked to residential buildings

Photovoltaic thermal (PV/T) air collector design requires an accurate determination of key parameters such as the channel depth and the air mass flow rate. This paper focuses on PV/T air collectors linked to an air distribution system with the aim of optimizing the channel depth, the air mass flow rate per unit collector area and the air distribution duct diameter considering the whole system performance. A weighted effective thermal energy output which includes electrical and thermal energy generated and fan power usage was utilized to examine the energy performance of the whole system. This study models PV/T air collectors with various collector areas (Ac = 10, 15, 25 and 30 m2) and different length to width ratios (L/W = 0.5, 1, 1.5 and 2) linked to an air distribution system of a typical residential building in a climate with a mild winter (e.g. Sydney). For a constant temperature rise (ΔT = 10 °C), and maximizing the rate of effective thermal energy output per unit collector area (Q˙eff) delivered to the building, the optimum channel depth (Dopt), the air mass flow per unit collector area (m˙/Ac), and the air distribution duct diameter were optimized. The optimum value of m˙/Ac is almost constant and approximately equal to 0.021 kg/s m2. The optimum depth (Dopt) value varies between 0.09 and 0.026 m and the optimum air distribution duct varies between 0.3 and 0.5 m. The optimum depth increases as the collector L/W ratio and the collector area (Ac) increase.

설비형 칠드빔 시스템 및 일반 공조 시스템 적용 공간의 온열환경 균일도 비교 평가

It is one of major objectives to achieve a uniform thermal environment and to minimize the local discomfort, when designing or operating air distribution systems. An active chilled beam (ACB) system, which is gaining much popularity owing to reduced fan energy consumption, needs to be evaluated in terms of uniform thermal environment, because it is operated with much smaller air flow rate than conventional all-air systems. The uniform thermal environment, or thermal uniformity, can be evaluated by analyzing air diffusion performance index (ADPI), local discomfort parameters within conditioned space. This study presents an evaluation method on the thermal uniformity in a room with different air distribution systems. To do this, a test bed facility was constructed so that four different air distribution systems, including ACB system, can be separatively operated. The ADPI of the ACB system was 80.7%, which is lower than that of all-air system, however, it could satisfy the recommended level (80%) of ADPI by ASHRAE. Although the ACB system resulted in the relatively low draught risk, averagely 6.7%, it was found that some regions near the chilled beam can be exposed to the draught risk due to the descending cold air. Vertical air temperature difference was not higher than , showing that the ACB system resulted in efficient air mixture in the vertical direction. It can be concluded that the ACB system can attain an acceptable thermal uniformity with much less air flow rate than other conventional air distribution systems.

Experimental investigation of personal air supply nozzle use in aircraft cabins

To study air passengers' use of individual air supply nozzles in aircraft cabins, we constructed an experimental chamber which replicated the interior of a modern passenger aircraft. A series of experiments were conducted at different levels of cabin occupancy. Survey data were collected focused on the reasons for opening the nozzle, adjusting the level of air flow, and changing the direction of the air flow. The results showed that human thermal and draft sensations change over time in an aircraft cabin. The thermal sensation response was highest when the volunteers first entered the cabin and decreased over time until it stablized. Fifty-one percent of volunteers opened the nozzle to alleviate a feeling of stuffiness, and more than 50% adjusted the nozzle to improve upper body comfort. Over the period of the experiment the majority of volunteers chose to adjust their the air flow of their personal system. This confirms airline companies' decisions to install the individual aircraft ventilation systems in their aircraft indicates that personal air systems based on nozzle adjustment are essential for cabin comfort. These results will assist in the design of more efficient air distribution systems within passenger aircraft cabins where there is a need to optimize the air flow in order to efficiently improve aircraft passengers' thermal comfort and reduce energy use.

Review of underfloor air distribution technology

Underfloor air distribution (UFAD) system, which is a relative new method of providing conditioned air and ventilation to the commercial buildings and data centers, is focused by researchers and designers for its potential advantages of better indoor thermal comfort (ITC) and indoor air quality (IAQ), layout flexibility, reduced life cycle costs, and energy saving. However, the application of UFAD system are still obstructed by the gap in the fundamental understanding of some new elements, such as the cooling load calculation, control of the thermal stratification, and evaluation of energy consumption. This paper reviews the current situation of UFAD technology, and the existing problems in recent researches are summarized. To improve the performance of UFAD system, several possible directions for the further studies are suggested.

Experimental comparison of zone cooling load between radiant and air systems

Radiant cooling systems work fundamentally differently from air systems by taking advantage of both radiant and convective heat transfer to remove space heat. This paper presents an experiment investigating how the dynamic heat transfer in rooms conditioned by a radiant system is different from an air system, and how such differences affect the sensible cooling load and cooling load calculation methods for radiant systems. Four tests with two heat gain profiles were carried out in a standard climatic chamber. For each profile, two separate tests were carried out to maintain a constant operative temperature: one with radiant chilled ceiling panels; and a second with an overhead mixing air distribution system. The experiments show that, during the periods the heat gain was on, the radiant system has on average 18–21% higher instantaneous cooling rates compared to the air system, and 75–82% of total heat gains were removed, while for the air system only 61–63% were removed. Based on the study, we conclude that a new definition must be used for radiant system cooling load. Calibrated dynamic energy simulation based on a fundamental heat balance approach showed good accuracy. Simplified cooling load calculation methods may lead to incorrect results for radiant systems.

The use of phase change materials in domestic heat pump and air-conditioning systems for short term storage: A review

Abstract Heat pumps for space heating and cooling are a mature and highly efficient technology that can take advantage of renewable energies. They can also provide energy savings by load shifting when they operate together with thermal energy storage (TES). This paper presents a literature review of TES systems using phase change materials (PCM) potentially applicable to domestic heat pumps used in residential and administrative buildings. The paper describes the systems proposed by the different authors and presents the main conclusions of the studies. The TES systems presented are not only used as energy storage to shift the load demand but also for other applications such as heat recovery or defrosting in air-conditioners. The PCM have the suitable melting temperature to work together with standard heat pumps in each application. Moreover, some systems where the heat pump is coupled to latent heat thermal energy storage (LHTES) units and other energy sources or where the TES system is incorporated in a radiant floor or air distribution system have also been included.

An experimental study on a full-scale indoor thermal environment using an Under-Floor Air Distribution system

Abstract Characteristics of thermal environment in a full-scale indoor space utilizing an Under-Floor Air Distribution (UFAD) system are investigated by conducting and analyzing experimental measurements. The indoor air stratification of different locations in the steady state for three supply flow rate options is presented. This research focuses on the influence of supply air flow rate, and its corresponding momentum and buoyancy fluxes, on the vertical temperature profile in the indoor environment. Temperature measurements show that the indoor vertical temperature profile is influenced highly by the distance away from the supply diffuser position. Experimental results also show that the supply air flow rate has a strong effect on the vertical temperature profile. When the supply air flow rate of a given diffuser increases, and then the supply air momentum flux increases as well, the gradient of the vertical temperature profile becomes gentler. The stratification height in the indoor environment ascends with a higher total flow rate. The almost same throw height at the supply diffuser is observed for the identical flow rate option in two sets of experiments with different total flow rates.