Underfloor Air Distribution Research Articles

Multi-objective optimization of supply air jet enhancing airflow uniformity in data center with Taguchi-based grey relational analysis

In most data centers (DCs), hot spots reduce the reliability, durability, and efficiency of server cooling. The hot spots are mainly attributed to the uneven airflow distribution and the resulting mixing of cold and hot air. To evenly distribute the airflow, the relay jet fan, an active local adjustment technology, is introduced into the under-floor air distribution in DCs. The jet fans are set in front of the closed cold aisle where the hot spots most often appear. The nozzle height, nozzle angle, horizontal position and attachment distance greatly influence the performance of the new system. To comprehensively evaluate the multiple levels of these affecting parameters, a large number of cases that need testing make it impossible to conduct research. An approach based on the Taguchi method with grey relational analysis optimizes the system and the L16 (44) orthogonal array is employed to design the experiments. The Taguchi method combined with computational fluid dynamics determines the optimal combinations of the Supply Heat Index (SHI), Return Temperature Index (RTI), β, and Index of Mixing (IOM). The significance of the affecting parameters is revealed and ranked, and through the main effect analysis and the analysis of variance, the nozzle angle is chosen as the most important among the four affecting parameters. After that, the Taguchi-based grey relational analysis transforms the multi-objective into a single-objective to determine the optimal combination of design variables. Finally, a confirmatory test was conducted, and the optimal combination reduces the server temperature by up to 7.2 °C.

Energy performance index of air distribution: Thermal utilization effectiveness

The energy performance index is crucial for the energy-efficient design and operation of air distribution. Heat Removal Efficiency (HRE) is a widely used energy performance index, and Energy Utilization Coefficient (EUC) and Effectiveness of Heat Removal (EHR) are alternatives to HRE. This study justifies that these existing indices are unreasonable and incompetent as energy performance indices. This study also proposes a new index called Thermal Utilization Effectiveness (TUE) and verifies its efficacy as an energy performance index via theoretical analyses and experiments on stratum ventilation, displacement ventilation, and underfloor air distribution. EUC is not a suitable energy performance index since it does not consider the relative contributions of the occupied and unoccupied zones to the energy performance of air distribution. EHR cannot qualitatively distinguish the energy performance of air distribution because of its floating benchmark point. HRE cannot quantitatively distinguish the energy performance of air distribution because it unequally weighs the thermal energy of the air temperature of the occupied zone. TUE accounts for the relative contributions of the occupied and unoccupied zones to the energy performance with the help of the exit air temperature, qualitatively distinguishes the energy performance because of its two fixed benchmark points, and quantitatively distinguishes the energy performance by equally weighing the thermal energy of the air temperature of the occupied zone. Therefore, TUE overcomes the drawbacks of EUC, EHR, and HRE, and is a reasonable and competent energy performance index of air distribution.

Effect of under-floor air distribution system to prevent the spread of airborne pathogens in classrooms

This study investigated the effect of the under-floor air distribution (UFAD) system compared to conventional mixed-mode ventilation system to prevent airborne pathogen spread in elementary school. To analysis airborne pathogen spread pattern, STAR-CCM+ is used. By applying the UFAD system airborne infection risk at region around the exhaust area of conventional ventilation system can be significantly reduced. However, increased infection risks around exhaust area of UFAD system still existed. Thus, it is recommended to install the exhaust vents (outlets) away from the breathing area. In addition, since both of conventional ventilation system and the UFAD systems have a high infection risk at the front of the classroom, novel method to reduce the infection risk from students to teachers should be considered.

An Alternative Air Distribution System in Economy-Class Passenger Airplane for Infection Reduction

Airplane transportation has played a major role in the spread of the new corona virus COVID-19 all over the world. The current air distribution system used in airplanes circulation assists motion of contaminants and hence spreads airborne diseases. In this work, a new air distribution system is proposed. A combined Under Floor Air Distribution (UFAD) and a new Personalized Ventilation (PV) systems with a front-back design has been adopted such that it satisfies thermal comfort for the passengers, as well as providing an air shield between the passengers in order to prevent the spread of airborne contaminants. Two configurations have been made for the economy class in a passenger airplane. Both had the same UFAD system, and PV system with air inlet located at the back of the ahead seat, but with different air inlet angles, normal to the seat back and inclined by 45°. It has been found out that the second configuration provides better control for contaminant spreading, as well as better thermal comfort, with predictive mean vote of 0.6, and predicted percentage dissatisfied of 17.6%.

Numerical study on settlement characteristics of inhalable particles in under-floor air distribution system

Particles deposited on the surface and floor will again enter the air in a suspended state due to airflow disturbance, which will threaten people's health. Thus, the purpose of this study is to explore the effects of different particle sizes on the settlement and trajectory of floor particles in the under-floor air distribution (UFAD) system. In this paper, the indoor temperature distribution and particle concentration of office room in Shanghai are simulated by numerical simulation, and verified by experiments. On the basis of ensuring that the numerical simulation results are consistent with the experimental results, the numerical model is used to further analyze the trajectory and settlement characteristics of particles with different sizes in the UFAD system. The results show that the particles less than 1 μm are easy to be suspended in the air under the influence of floor air supply, and the particles larger than 2.5 μm are less affected and mainly settle on the floor. Moreover, the particles close to the air inlet are easier to suspend, so the personnel stay area should keep a certain distance from the air inlet area. Finally, the mosquito coil smoke diffusion simulation is carried out to simulate the unsteady diffusion process of particles when there is particle pollution source on the floor. It is found that most mosquito coil particles could be discharged from the air outlet in 10 min.

Energy Saving Quantitative Analysis of Passive, Active, and Renewable Technologies in Different Climate Zones

The purpose of this study was to provide a guideline for the selection of technologies suitable for ASHRAE international climate zones when designing high-performance buildings. In this study, high-performance technologies were grouped as passive, active, and renewable energy systems. Energy saving technologies comprising 15 cases were categorized into passive, active, and renewable energy systems. EnergyPlus v9.5.0 was used to analyze the contribution of each technology in reducing the primary energy consumption. The energy consumption of each system was analyzed in different climates (Incheon, New Delhi, Minneapolis, Berlin), and the detailed contributions to saving energy were evaluated. Even when the same technology is applied, the energy saving rate differs according to the climatic characteristics. Shading systems are passive systems that are more effective in hot regions. In addition, the variable air volume (VAV) system, combined VAV–energy recovery ventilation (ERV), and combined VAV–underfloor air distribution (UFAD) are active systems that can convert hot and humid outdoor temperatures to create comfortable indoor environments. In cold and cool regions, passive systems that prevent heat loss, such as high-R insulation walls and windows, are effective. Active systems that utilize outdoor air or ventilation include the combined VAV-economizer, the active chilled beam with dedicated outdoor air system (DOAS), and the combined VAV-ERV. For renewable energy systems, the ground source heat pump (GSHP) is more effective. Selecting energy saving technologies that are suitable for the surrounding environment, and selecting design strategies that are appropriate for a given climate, are very important for the design of high-performance buildings globally.

The influence of air inlet angle in swirl diffusers of UFAD system on distribution and deposition of indoor particles

The objective of the present study was to investigate the effect of swirl angle on the distribution and deposition of contaminant particles in a chamber with swirl diffuser in an under-floor air distribution (UFAD) system. In the desired model, the analysis of flow field and particle distribution for swirling diffusers at four angles 30°, 45°, 60°, and 90° has been conducted by OpenFOAM® with Eulerian-Lagrangian approach. The results show that under uniform initial distribution condition for particles, the percentage of inhalable particulate matter in the breathing zone decreases faster for the rotation angles of 45° and 60°. In other words, at the inlet angle of 45° and 60° after 600 s, the percentage of 2.5 μm particles removed is 79.2 and 79.5, respectively. While the same percentage for the inlet angle of 30° and 90° is 67.5 and 75.8, respectively. For the considered particles, the inlet angle of 45° and 60° can cause to increase the number of exited particles in comparison with the inlet angle of 30° and 90. Also, when the particles enter the chamber from inlet air diffusers, the concentration of 1, 2.5, and 5 μm particles in the breathing zone decreases by increasing the inlet air angle. But, for the 10, and 25 μm particles, the concentration of particles in the breathing zone increases by increasing the rotation angle of inlet swirl diffusers. The percentages of 2.5 μm particles exiting the outlet are 27.8, 46.9, 53.7, and 58 for the inlet angle of 30°, 45°, 60°, and 90°, respectively.

Computational Fluid Dynamics Simulation of Indoor Air Quality and Thermal Stratification of an Underfloor Air Distribution System (UFAD) with Various Vent Layouts

The underfloor air distribution (UFAD) system has not been able to penetrate the residential and commercial air conditioning industry significantly until now. To date, the most notable applications are found in datacenters because of their more demanding thermal stratification and cooling requirements. In addition to highlighting the advantages of the UFAD system over the traditional overhead (OH) system, this study compares various ventilation layouts for a UFAD system. Four different UFAD ventilation layouts are compared and one OH layout. The results show that using multiple swirl-type diffusers creates a more uniform floor-to-knee temperature and less air recirculation than the rectangular grille-type diffusers. Placing the return vents on the sides of the room creates a cooler environment by isolating recirculating air to a smaller space. At the end of the study, a design guide matrix is presented to summarize how the supply and return vent layout affects air conditioning performance.

Influence of manikin movement on temperature stratification in a displacement ventilated room

Temperature stratification generally exists in an indoor environment with floor-level air-supply systems, such as displacement ventilation (DV) and underfloor air distribution (UFAD). The systems with vertical temperature differences possess many advantages over mixing ventilation, including lower energy consumption and better indoor air quality. Some dynamic factors, including the movement of the human body, the door opening and closing processes, can disrupt or weaken the temperature stratification. However, the mechanisms by which these factors affect temperature stratification remain unclear. In this study, a series of experiments was conducted to investigate the temperature stratification of the DV system in an environmental chamber by adopting a moving manikin. We employed temperature stratification stability (TSS) and temperature stratification recovery time (TSRT) to quantify the influence of dynamic factors on temperature stratification. The primary and secondary factors which influencing TSS and TSRT were identified using orthogonal experiments among four important indices, i.e. manikin moving velocity (v), manikin moving duration (t), manikin heat generation intensity (P) and air change rate of the chamber (ACH). Results show factors destruct the mean TSS, ranked from the most to least significant, are v, ACH, P and t, and those for mean TSRT are v, t, P and ACH. In the manikin activity area, TSS increases with the vertical height. Manikin moving velocity exerts a primary effect on temperature stratification. There is a negative correlation between Re number and TSS in ourexperiment. The area affected by the movement is observed mainly around the manikin, not the whole chamber. The temperature stratification still exists after the termination of the manikin movement, but is relatively weakened.

Spreading of SARS-CoV-2 via Heating, Ventilation, and Air Conditioning Systems—An Overview of Energy Perspective and Potential Solutions

Abstract The role of heating, ventilation, and air conditioning systems (HVAC) in spreading SARS-CoV-2 is a complex topic and has not been studied thoroughly. There are some existing strategies and technologies for health and high performance buildings; however, applications to other types of buildings come at large energy penalty: cost; design, regulations and standards changes, and varied public perception. In the present work, different factors and strategies are reviewed and discussed and suggested mitigations and solutions are provided including the required air flowrates with the presence of infectors with and without mask and disinfection techniques including ultraviolet (UV) light. Experimental and numerical research in open literature suggests that the airborne transmission of SARS-CoV-2 is sufficiently likely. However, in situ detailed experimental studies are still needed to understand the different scenarios of the virus spread. Displacement ventilation, underfloor air distribution, chilled beams, radiant ceiling panels, and laminar flow systems have varied effectiveness. High-efficiency particulate arrestance (HEPA) filters and UV light can clean viruses but at high energy cost. Suggested solutions to reduce the infection probability include recommended levels of ventilation and a combination of virus sampling technologies including cyclones, liquid impinger, filters, electrostatic precipitators, and water-based condensation.

An investigation of thermal comfort, IAQ, and energy saving in UFAD systems using a combination of Taguchi optimization algorithm and CFD

ABSTRACT The present study investigates underfloor air distribution (UFAD) systems by using the combination of computational fluid dynamics (CFD) and the Taguchi optimization algorithm. A multi-objective optimization approach is used to analyze the efficiency of UFAD systems from the viewpoints of thermal comfort, indoor air quality (IAQ), and energy consumption. The supply air temperature, the air change rate per hour (ACH), and the height of the return air vent factors are considered in order to achieve the optimum operating condition. First, numerical validation has been done for an office room model, and then optimization has been performed for this space. The optimization results state that setting the supply air temperature and ACH at 19°C and 4.0, respectively, and placing the return air vent at the height of 1.6 m would lead to the optimum state. The CFD simulation results for this condition show that the thermal comfort in the occupied zone and the desirable IAQ in the breathing zone are established, corresponding to the predicted mean vote (PMV) and mean age of air (MAA) values equal to 0.13 and 640 s, respectively. Furthermore, energy consumption has a considerable reduction of 21.5%, compared to the conventional mixing ventilation (MV) systems.

Multi-objective optimization of underfloor air distribution (UFAD) systems performance in a densely occupied environment: A combination of numerical simulation and Taguchi algorithm

Underfloor air distribution (UFAD) systems are on the way of development owing to their substantial energy saving potential as well as their capability for establishing thermally comfortable environments. Comprehensive evaluating these systems necessitates considering multiple effective factors ranging in several levels, which significantly augments the number of test cases and can even make the conducting of research impossible. Proposing a methodology for combining CFD results and the Taguchi optimization algorithm using the multi-response signal to noise (MRSN) analysis, the present study addresses this issue. In order to investigate the performance of UFAD systems applied in a densely occupied classroom from three fundamental viewpoints of thermal comfort, indoor air quality (IAQ), and energy saving, this research employs a multi-objective optimization approach. The utilized algorithm concludes that setting the following conditions provides the optimal state: (i) The underfloor diffusers beside the seats of occupants, (ii) The return vent at the height of 1.5 m from the floor, (iii) ACH equal to 14, (iv) Supply air temperature equal to 18 °C, and (v) Vertical air distribution from the underfloor diffusers. The ultimate simulation outcomes demonstrate that, according to the PMV and mean age of air values equal to 0.24 and 147 s in occupied and breathing zones, respectively, the thermal comfort conditions and desirable IAQ are established. Furthermore, energy saving compared to conventional mixing ventilation systems is a significant amount of 13.2%. In addition, local thermal comfort in terms of the vertical temperature difference and local convective cooling (draft) is achieved.

Variations of Stratification and Cooling Energy Performance According to Diffuser Condition in UFAD System

AbstractThe underfloor air distribution (UFAD) system, which is a building energy-saving technology, is an air-conditioning system that enables maintenance of the indoor temperature with a relative...

Influences of three types of air-conditioning systems on human thermal comfort

The influence of three ventilation systems, namely, the displacement ventilation system, the stratum ventilation system, and the underfloor air distribution system, on human thermal comfort was investigated at different supply air temperatures. We compared the air velocity, CO2 concentration, temperature, human thermal comfort, and energy utilization coefficient of the three systems at a supply air temperature range of 18–24 °C. The results showed that the underfloor air distribution system provided the best performances for CO2 dispersion and cooling at a supply air temperature of 18 °C. For optimal human thermal comfort, the underfloor air distribution should be chosen at a supply air temperature of 18 °C, the displacement ventilation system is suggested at a supply air temperature of 19 °C, 20 °C, and 24 °C, and the stratum ventilation system would be most suitable at a supply air temperature of 21 °C, 22 °C, and 23 °C.

Effect on the Performance of Underfloor Air Distribution(UFAD) system under different supply air parameters

Underfloor Air Distribution (UFAD) systems have received increasing attention during the past decades due to thermal stratification and energy saving potential over conventional mixed air conditioning system. Different supply air parameters have significant impacts on the UFAD system performance. In this study, experimental study and energy simulation software EnergyPlus were used to investigate the UFAD performance in a full size experimental room. The distance between diffusers and human, supply air temperature and velocity were discussed. The orthogonal experiments were employed to discuss the effect of supply air parameters on the temperature distribution, thermal comfort and indoor air quality (IAQ). By the analysis, the most important factor is the distance between diffusers and human. Furthermore, the preferable supply air parameters are suggested by means of controlling parameters. The results show that better thermal stratification and thermal comfort can be achieved when the distance between the swirl diffusers and human is 0.7m, the Supply Air Temperature (SAT) is 18-20°C and the Supply Air Velocity (SAV) is 1.2-1.5m/s. Another contribution of this paper is that the energy consumption of UFAD system is calculated by EnergyPlus. The calculation shows that the satisfied thermal stratification and prominent energy saving can be achieved simultaneously by using reasonable supply air parameters, such as the SAT is 18°C and the SAV is 1.2m/s.

Numerical Study on Effects of Air Return Height on Performance of an Underfloor Air Distribution System for Heating and Cooling

The exhaust/return-split configuration is regarded as an important upgrade of traditional under-floor-air-distribution (UFAD) systems due to its higher energy efficiency. Moreover, existing studies are mostly focused on the effect of the return vent height on the performance of an UFAD system under cooling conditions. Knowledge of the performance under heating conditions is sorely lacking. This paper presents a numerical evaluation of the performance characteristics of an UFAD system with six different heights of the return vents in heating operation by comprehensively considering thermal comfort, air quality, and energy consumption. The results show that, in the heating mode, the general thermal comfort (predicted mean vote-predicted percentage dissatisfied (PMV-PPD) values) and indoor air quality indices (mean age of air and volatile organic compounds (VOCs) concentration) were greatly improved and energy consumption was slightly reduced with a lower return vent height. Although these were opposite to the findings of our previous study regarding the performance in cooling mode, an optimal return vent height in terms of the comprehensive all-year performance can be recommended. This method provides insight into the design and optimization of the return vent height of UFAD for space heating and cooling.

Numerical simulation of thermal stratification and air quality in an underfloor air distribution system (UFAD)

The penetration of underfloor air distribution systems (UFAD) in residential and commercial air conditioning has been rather slow. The most notable applications would be on data centers, where thermal stratification requirements are more demanding. The present study supports and strengthens recent work in the design and development of UFAD systems, by augmenting literature on proper vent positioning and design. In UFAD systems where thermal stratification is more pronounced, significant energy savings may be achieved through proper positioning of supply and return vents. Using a validated numerical simulation model in ANSYS CFX, four UFAD vent layouts are investigated with regards to their implications on thermal stratification and indoor air quality. Results show that not only ventilation layout, but also vent type selection can significantly affect the performance of a UFAD system. Spreading multiple, smaller supply diffusers is preferable than having large supply diffusers on the perimeter of the rooms, both from a temperature distribution and indoor air quality perspective. Notably, air flow is significantly poor in the perimeter layout, causing warmer temperature at the center of the room.

CFD simulation of airflows and ammonia emissions in a pig compartment with underfloor air distribution system: Model validation at different ventilation rates

Pig buildings with underfloor air distribution (UFAD) system provide the animal area with efficient cooling and better air quality compared ceiling air inlets. This is because the air inlets near the floor and the exhaust opening at the ceiling allow the excess heat and the old air to be displaced from the animal area to the upper part of the building. Typical inlet types of UFAD systems in Flanders (Belgium) are the slatted floor units placed in service alley. The main drawback in pig buildings with UFAD systems is that during winter, cold draughts often occur in the animal area, and due to the air displacement ventilation process, there is a risk of NH3 transport from the slurry pit. The effect of NH3 emissions is important as it leads to acidification and eutrophication. Modelling airflows and NH3 emissions using Computational Fluid Dynamics (CFD) can promote the development of techniques for mitigating NH3 emissions, improving indoor air quality and animal thermal comfort. A steady-state CFD model was therefore developed to predict the indoor airflow and NH3 distribution in a pig compartment with a UFAD system. An advanced NH3 emission modelling approach was implemented in the CFD model to simulate NH3 generation in the slurry pit and pen floor. In order to validate the CFD model, two experiments were performed. The first experiment was conducted in a compartment with mock-up pigs at different ventilation rates to validate the modelled air velocity and temperature fields. The second experiment was conducted in a compartment occupied by real pigs to validate the modelled airflow patterns, temperature, CO2 and NH3 concentrations. Overall, there was a good agreement between the simulated and measured results. The field experiment and CFD model results confirmed that NH3 was transported from the slurry pit to the compartment. The air exchange rate of the slurry pit in the CFD model increased from 10.4 to 26.1 h−1 as the ventilation rate was increased from 11 to 92 m3 h−1 pig−1 due to the air displacement in the slurry pit.

THE SIMULATION ANALYSIS OF A PROPOSED ‘HYDRONIC RADIATOR’ SYSTEM TOWARDS LOW COST HOUSING OPERATION IN TEMPERATE AND HOT TROPICAL CLIMATES

ABSTRACT Fuel poverty is one of the global concerns affecting not only users' financial capacity or affordability for maintaining housing operation but also the occupants' health and wellbeing. Space heating and cooling require a relatively large amount of domestic energy use in housing. Therefore, this study was formed with the aim to propose an innovative approach to utilising free, clean renewable sources of energy applicable to the space heating and cooling of housing in both cold and hot regions. Accordingly, housing test facilities based in Melbourne, Australia, and Kuching, Malaysia, were selected and used for this study that examined the thermal performance of a proposed ‘hydronic radiator’ (HR) system through simulation and onsite measurements. The geothermal heat capacity of a ‘vertical ground heat exchanger’ (VGHE) installed in the house in Melbourne was examined previously by the authors and the VGHE measured data was also applied to this HR performance simulation. The water that circulates through the HRs is heated by sunlight and VGHE or cooled by night sky radiation. This study drew conclusions that the sole utilisation of renewable sources through these proposed HR space heating and cooling systems can provide thermally accessible or comfortable indoor living environments in both heating or cooling dominant regions. Thus, fuel poverty issues may be alleviated through HR system application. The HRs can remove a ‘sensible’ portion of metabolic heat, but they cannot effectively contribute to the ‘latent’ heat removal. Thus, the future potential use or effect of ‘flow-through’ HRs, which are integrated into a underfloor air distribution (UFAD) plenum, was also dsicussed in this study. In the test house located in Melbourne, the flow-through HR UFAD system is currently under development. Therefore, the performance will be measured once the system has come into operation for further testing.

Detailed Comparison of the Operational Characteristics of Energy-Conserving HVAC Systems during the Cooling Season

To provide useful information concerning energy-conserving heating, ventilation, and air-conditioning (HVAC) systems, this study used EnergyPlus to analyze in detail their operational characteristics and energy performance. This study also aimed to understand the features of the systems under consideration by investigating the dry-bulb temperature, relative humidity, and airflow rate at major nodes in each system’s schematic. Furthermore, we analyzed the indoor environment created by each HVAC system, as well as examining the cooling energy consumptions and CO2 emissions. The HVAC systems selected for this study are the variable air volume (VAV) commonly used in office buildings (base-case model), constant air volume (CAV), under-floor air distribution (UFAD), and active chilled beam (ACB) with dedicated outdoor air system (DOAS). For the same indoor set-point temperature, the CAV’s supply airflow was the highest, and VAV and UFAD were operated by varying the airflow rate according to the change of the space thermal load. ACB with DOAS was analyzed as being able to perform air conditioning only with the supply airflow constantly fixed at a minimum outdoor air volume. The primary cooling energy was increased by about 23.3% by applying CAV, compared to VAV. When using the UFAD and ACB with DOAS, cooling energy was reduced by 11.3% and 23.1% compared with VAV, respectively.