Underfloor Air Distribution System Research Articles

Numerical Investigation of the Indoor Environmental Factors With Stratum and Underfloor Air Distribution Systems in an Educational Institute Computer Laboratory

Abstract Ventilation with optimal airflow patterns is the most significant aspect in controlling airborne transmission as well as providing an improved indoor thermal environment and air quality in an energy-efficient way. Conducting the air-conditioned computer laboratory in educational institutes during the pandemic is challenging. This article presents a comparative study on various aspects related to indoor environmental quality factors, including thermal comfort indices, vertical air temperature differences, draught rate, air distribution effectiveness, CO2 concentration levels, energy saving, and dispersion of exhaled droplets of aerosols in different sizes (0.5 μm, 6 μm, 20 μm, and 45 μm). The study examines these factors by comparing 12 cases with different air distribution strategies (stratum, underfloor), air change rates (15 ACH and 20 ACH), and return outlet positions (1.3 m, 1.8 m, and 2.3 m). The numerical investigation is carried out in the educational institute computer laboratory model with the dimensions of 10 m (length), 5 m (width), and 3 m (height). The findings indicated that stratum ventilation and underfloor air distribution (UFAD) at a rate of 20 ACH, along with the placement of the return outlet in position 2, would ensure adequate ventilation, acceptable thermal comfort, and optimal energy savings. Compared to the overhead air distribution system, the maximum energy savings were obtained at 10.15% and 10.07% in both stratum and underfloor air distribution systems. Furthermore, the higher air distribution index and ventilation parameter were acquired in the UFAD system at 20 ACH. This study provides a better understanding of air distribution systems and aerosol particle dispersion in the indoor environment of a computer laboratory.

Walking-induced particle resuspension in a commercial airliner cabin under different ventilation systems

The potential health risk posed by particle resuspension in the air of an airliner cabin is a concern for passengers. This study employed computational fluid dynamics (CFD) to analyze particle dispersion patterns under different ventilation systems, such as mixing ventilation (MV), underfloor air distribution (UFAD), and aisle displacement ventilation (ADV), within a twin-aisle cabin environment. To ensure the accuracy of the CFD model, the study validated the simulated air velocity distribution against experimental airflow patterns from a small-scale water model reported in existing literature. Additionally, the research included direct measurements of particle resuspension within the breathing zone of a full-scale, twin-aisle cabin mockup, which featured five rows of seats. These measurements were taken in response to the disturbance caused by a volunteer walking over an aviation carpet in the aisle. While the CFD simulation results generally aligned with the experimental data, some discrepancies were noted. Pinpointing the exact causes of these discrepancies proved challenging due to the numerous uncertainties and approximations inherent in the study. Nevertheless, the investigation revealed that the air in the upper region near the moving body was pushed forward, creating a strong airflow that enveloped the body. This dynamic resulted in the formation of a wake that lifted particles from the floor level to the upper cabin area. The study observed that particle concentration within the breathing zone increased in the direction of movement. The airflow patterns produced by the three ventilation systems were different. Despite the cabin's symmetrical design, some particles can still transfer from one side to the other, especially under the MV system. The peak particle concentration in the breathing zone was recorded at 20 s, after which it decreased to negligible levels by 120 s. Among the ventilation systems analyzed, the MV system exhibited the highest peak particle concentration, while the UFAD system showed the lowest peak. However, the UFAD system also presented the possibility of very high particle concentrations at certain seats.

Impact of displacement ventilation and underfloor air distribution systems on concentrations of indoor particle in different seasons

The particle motion in a room is affected by the thermal environment and the air supply conditions caused by seasonal changes. In the study, a Lagrangian discrete trajectory model combined with the Eulerian fluid method was adopted to simulate the characteristics of indoor airflow and the diffusion of two typical indoor particles with considering different air supply conditions in different seasons. Specifically, the thermal comfort perception, draught rating (DR) of the indoor personnel, and the contaminant removal effectiveness (CRE, ε) of indoor particles under the influences of displacement ventilation (DV) and underfloor air distribution (UFAD) systems were investigated. The results revealed that when considering the indoor thermal comfort under summer and winter air supply conditions, UFAD was found be more effective than the DV system in protecting the temperature difference of indoor personnel. Additionally, the removal effect of PM1 in the breathing area was superior compared to that of the DV system. Regarding the removal effectiveness of PM2.5, the removal performance of both ventilation systems decreased, but the removal effect of the DV system on PM2.5 in the breathing area (with the largest εl of 1.55) was significantly better than that of the UFAD system under winter operating conditions. It is essential for the selection of the ventilation system to consider the thermal comfort of indoor occupants, as different ventilation systems have varying capabilities to remove particles of different sizes. Therefore, choosing an appropriate ventilation system is crucial for enhancing indoor air quality, and the findings of this study provide a basis for improving indoor air quality.

Direct numerical simulation of contaminant removal in presence of underfloor air distribution system

Indoor contaminant removal over 0.5 ≤ FrT ≤ 5.0, 0.5 ≤ N ≤ 5.0, and 50 ≤ Re ≤ 500 was investigated numerically, wherein FrT refers to the Froude number, N refers to the buoyancy ratio, and Re refers to the Reynolds number. As demonstrated, the ventilation effectiveness increased with increasing contaminant source intensity and air supply intensity at a constant air temperature, indicating that increase the fresh air can effectively eliminate contaminants in this case. At high air supply temperatures, the heat retention time and contaminant transport was extremely short, and the fresh air induced by strong natural convection floating lift was rapidly discharged. Additioanlly, the air supply intensity had significant effects on contaminant removal. Quantification of the ventilation effectiveness under the combined effects of air supply intensity, air supply temperature and contaminant source intensity was determined based on the results of direct numerical simulations.

CFD-based multi-objective optimization of indoor air quality and thermal comfort in a classroom

Comfort conditions and air quality in educational buildings can affect students’ performance, achievement and productivity, but they can also threaten their health. In this study, a sample classroom in Trakya University, Edirne, which lacks a ventilation system, is discussed. Depending on the determined parameters, an optimization study was performed using computational fluid dynamics. Predicted mean vote (PMV) for general thermal comfort, draught rate (DR) for local thermal comfort and the carbon dioxide (CO2) level for indoor air quality are selected as target functions. Outdoor air temperature, heating/ventilation system, insulation thickness, number of people in the classroom and clothing factor were identified as parameters that could affect the target functions. According to the numerical results, the rank of importance of the parameters affecting the target functions was calculated using the Taguchi method. The effect percentages of the parameters were determined using the analysis of variance. Then, all target functions were optimized with the grey relational analysis. The best case obtained as a result of the optimization has PMV, CO2 and DR values of −0.12, 511 ppm and 5.37%, respectively. When the general results are evaluated, a comfortable and healthy environment is provided in the classroom with the underfloor air distribution system operating at 100% fresh air and a reduced number of occupants.

Effect of Local Floor Heating System on Occupants’ Thermal Comfort and Energy Consumption Using Computational Fluid Dynamics (CFD)

In this article, the influence of splitting a local underfloor air distribution system (UFAD) on indoor thermal comfort for three occupants was studied numerically. A validated computational fluid dynamics (CFD) model was employed in this investigation. The proposed heating system was evaluated and analyzed for different values of air temperature and supply velocity. Providing suitable thermal comfort and saving energy are considered the main evaluation indexes for this study. Three cases, cases 2, 3, and 4, of the proposed local UFAD system were compared with a traditional heating system case, case 1. The supplying air velocity and air temperature in the reference case were 0.5 m/s and 29 °C, while in cases 2, 3, and 4, they were 0.4 m/s and 29 °C, 28 °C, and 27 °C, respectively. The results show that acceptable indoor human thermal comfort and energy demand reduction were achieved by using the splitting UFAD concept.

Accuracy of CFD Simulations on Indoor Air Ventilation: Application of Grid Convergence Index on Underfloor Air Distribution (UFAD) System Design

Underfloor air distribution system (UFAD) mesh flow velocity was simulated using Computational Fluid Dynamics (CFD). Three mesh sizes were used to explore the domain's core x-y plane velocity contour and profiles. Compared to medium and fine, the coarse mesh underestimated the velocity significantly. A slight discrepancy occurred where the shear flow was dominant. The symmetrical flow velocity for both sides of the room length was shown in the xy-plane at the centre of the inlet. The mean error for coarse and medium mesh was larger than for the medium and fine mesh. It shows that the difference between the medium mesh and the fine was accepted. The computational time for medium mesh was acceptable for simulation, and it will not vary substantially even if the grid is refined further. The normalised mean square error (NMSE), the factor of two observations (FAC2), the factor of 1.3 observations (FAC1.3), and the fractional bias (FB) are used to measure the performance of the models and the value of the outcomes was exceptional. As a result, the accuracy of the finding can be improved by conducting additional research with manikins and in a fully occupied room under real-world conditions. In addition, this study could analyse and anticipate the optimal scenario regarding ventilation performance, etc.

Multi-objective optimization of supply air jet enhancing airflow uniformity in data center using Taguchi/CRITIC/TOPSIS triple method

Jet fans have been introduced into under-floor air distribution systems as an active local measure to reduce hot spot temperatures and improve airflow distribution uniformity in data centers. However, as the rack load increases, the two jet fans are insufficient to balance the airflow distribution in the hotspot area, more jet fans and the corresponding parameters need setting and optimizing. A couple of methods of criteria importance through intercriteria correlation/technique for order preference by similarity to ideal solution were introduced as a multi-criteria decision making method to solve this problem. The optimal design parameters of multiple jet fans were determined by combining the Taguchi method with Computational Fluid Dynamics. The contribution of each parameter to the evaluation index was calculated by signal-to-noise ratio analysis and analysis of variance using Taguchi method, and the nozzle angle was determined to be the most important parameter. Then, Taguchi method was combined with criteria importance through intercriteria correlation/technique for order preference by similarity to ideal solution to transform the multi-objective into a single-objective to determine the optimal combination of design parameters for multiple jet fans. Finally, the best combination case was verified by evaluation indexes including supply heat index, return temperature index, ratio of temperature differentials and index of mixing. In the optimal combination, the highest server temperature was reduced by 5.3 °C compared with two jet fans. The multi-criteria decision making method is suitable for choosing the optimal parameter combination of airflow organization when faced with complex decisions involving several dimensions.

Optimization of underfloor air distribution systems for data centers based on orthogonal test method: A case study

With the rapid development of cloud computing and artificial intelligence, it has been increasingly significant to reduce the energy consumption of data centers (DCs) by improving the thermal performance. Focusing on the layout of existing DCs, in this paper orthogonal test method was adopted to investigate the influencing factors of thermal performance of an underfloor air distribution system in a DC. The effects of geometric factors on the system thermal performance were experimentally and numerically investigated by means of the three-factor and four-level orthogonal design. A simulation was conducted using Computational Fluid Dynamics. First, eight experimental cases were conducted to validate the model. Then, a comparison among 17 cases with various plenum heights, perforation percentages and the angles of detachable baffles in plenums were conducted. Moreover, the effects of working conditions with various rack power, supply air temperatures, and air volumetric flowrates, were also investigated by means of three-factor and three-level orthogonal design. It was found that the plenum height was the dominant influencing factor on DC thermal performance, followed by the perforation percentage. The operation strategy of high-temperature and high-volume resulted in a better performance in the DC. The optimal configuration is suggested to have a plenum height of 0.7 m, a perforation percentage of 20% and the angle of plenum detachable baffle of 45°. This study provides a comprehensive reference for the optimization of DC thermal performance considering multiple factors.

A Revisit to Recent Developments in the Underfloor Air Distribution Systems

Underfloor air distribution, also known as UFAD, is a technique of providing the space conditioning in offices as well as other public spaces. Because of the substantial benefits which it can provide, it is progressively being regarded as a major alternative to the conventional ceiling-based air distribution systems. This is due to the fact that the UFAD is a technique of providing the space conditioning in the public spaces. This method delivers cooled air directly into the inhabited zone of the building by making advantage of the open space that is created (the underfloor plenum) that exists between the structural concrete slab as well as the underside of a raised access floor system. Air may be supplied through a multiplicity of the supply outlets positioned at the floor level (this is the most typical configuration), or it can be integrated into the structure of the furniture and walls. This paper provides a recent development in the field of HVACs that have employed UFAD systems for improving their effectiveness as well as thermal comfort of humans. UFAD has the potential to assist in the enhancement of a building’s energy efficiency, indoor air quality, occupant comfort, and sustainable practises. The future scope of UFAD is significant, and it has the potential to become a mainstream technology in the building industry.

Measurement of Breath Velocity and Volume on Speaking, and Performance of Local Exhaust System as Prevention Measure of Infection

This study proposes a novel approach combining a local exhaust ventilation system (LEV) and a whole underfloor air distribution system (WUFAD) in the consulting room. This study assumes that two persons (doctor and patient) are sitting face to face and talking without a mask in a simple room regarded as a consulting room. To model people talking, exhaled air velocity and volume from talking were required, so we estimated velocity speed using an ultrasonic anemometer and volume using a PET bag and mask. Then CFD steady analyses were carried out, setting various parameters (hood height, hood flow rate, horizontal hood position, and ACH) using the breath experiment results. Capture efficiency and contribution distribution for the hood (SVE5: scale for ventilation efficiency 5) for tracer gas have been calculated. Infection risk for the doctor also has been calculated using the Wells-Riley model. The estimation’s results showed that exhaled air velocity was 0.30m/s and volume was 5.2L/min from talking. The CFD results showed that hood flow rate significantly affected capture efficiency. SVE5 showed that ACH greatly influenced the hood’s effective area. The infection risk for the doctor revealed that the combination of the LEV and WUFAD could protect doctors from infection risk under low supply conditions: 60 m3/h/person (6 ACH).

An Aisle Displacement Ventilation System for Twin-Aisle Commercial Airliner Cabin

The environmental control system in most commercial airliner cabins supplies air from shoulder and ceiling level and exhausts air at floor level on both sides of the cabin walls. The ventilation system mixes air in the cabins to create a relative uniform air temperature distribution which is great for passengers’ thermal comfort. However, the mixing ventilation also enhances airborne contaminant transfer. Many displacement ventilation methods have been proposed to use in aircraft cabins, but the disadvantages of the ventilation approach are usually creating draft on the passengers’ ankles and high air temperature stratification between passengers’ heads and feet. This investigation developed an aisle displacement ventilation (ADV) system which can reduce air temperature stratification effectively without occupying the legroom space under the seats, so it is good for passengers and crew members’ comfort and friendly for luggage storage, in addition, it can also be easily installed in aircraft cabins. By installing the system in a five-row, twin-aisle cabin mockup, our study found that the ADV system can create a low air velocity distribution in the cabin and can maintain an acceptable air temperature stratification without draft. The system created an uprising airflow which can effectively remove airborne contaminant which was generated from index passengers’ respiratory activities. The experimental data were used to validate a computational-fluid-dynamics (CFD) program. The validated CFD program was used to compare the ADV with under-seat displacement ventilation (USDV) and underfloor air distribution (UFAD) system along the aisles. The comparison results show that the ADV had obvious better thermal comfort than the other two systems, and the cabin air quality of the three ventilation systems was similar, all far better than the “perfectly-mixed” ventilated condition.

A field study on indoor environmental quality and energy savings of an office building integrated with underfloor air conditioning system in India

This study aimed to examine the real-time indoor environmental quality (IEQ) factors in an office space environment with a dedicated underfloor air conditioning system. Nevertheless, underfloor air distribution (UFAD) system buildings in India do not have reliable studies to record the impact of occupant perception and IEQ factors. This study was conducted in the UFAD office building during the peak monsoon period of 16th – 25th October 2019 in a warm and humid climate region of Mumbai, India. IEQ factors of indoor thermal, air quality, lighting, and noise comfort were examined using an occupant questionnaire survey for sensation, preference and overall acceptability, as well as physical measurements. Furthermore, energy consumption and energy utilization coefficient were determined at different room set-point temperatures. Four hundred and thirty-five responses were collected during the field study. The comfort zone temperature was obtained between 22.24–25.3 °C with near-zero (±0.5) tolerance range. Moreover, 80-95% of the occupants felt comfortable and acceptable within the thermal environment of the room set-point at 22-25 °C. Results showed that the real-time IEQ score was perceived between 0.663-0.846. The real-time IEQ score can be used as a building management strategy that simultaneously improves thermal comfort and indoor air quality.

Uniformity of Supply Air in the Plenum for Under-Floor Air Distribution Ventilation in a Circular Conference Room: A CFD Study

Underfloor air distribution (UFAD) systems are increasingly used for their advantages in improving energy savings, indoor air quality, and thermal comfort. In UFAD systems, an underfloor plenum delivers conditioned air to the air supply diffusers. The distribution of internal air velocity and static pressure in plenums determines the uniformity of the airflow to the occupied zones. As a result, the plenum has a detrimental effect on the characteristics of the supply air and, thus, the resulting indoor air quality and thermal comfort. Nevertheless, most existing studies on underfloor plenums focused on small-scale plenums with a single internal air duct. Large plenums and multiple air ducts in UFAD equipped in large premises are underexplored. In this study, a circular underfloor plenum with a large scale (radius of 15 m, height difference of 0.9−2.9 m) and 503 under-seat diffusers in a conference room was studied using computational fluid dynamics (CFD) simulation (ANSYS Fluent (16.0)). The distributions of airflow velocity and static pressure inside the plenum were analyzed and compared to one concentrated air supply mode and three uniform air supply modes. Based on the air velocity at the center of under-seat diffusers, the outgoing airflow uniformity from the diffusers under four cases was evaluated by the index of air velocity uniformity. The results showed that the multiple supply ducts with bottom air outlets yielded the best uniformity of supply air. The findings of this paper are expected to provide a technical basis for realizing the optimal design of the UFAD system in terms of uniformity of supply air.

Ceiling displacement ventilation system in an experimental theatre: A method to optimize thermal comfort

Compared to traditional theatres, experimental theatres have a lower floor height and the lights are closer to the audience. The large vertical temperature gradient caused by the underfloor air distribution (UFAD) system extremely impairs thermal comfort. Thus, an improved method by introducing the ceiling displacement ventilation (CDV) system was evaluated. CFD models of the experimental theatre were established and verified by field tests. The influence of designing and operation parameters of the CDV systems on thermal comfort was investigated. Effects on energy consumption and air quality were also analyzed to improve application feasibility. The results indicated that the increment of air temperature at the audience head (AHA temperature) was reduced by 51.0% with the help of the CDV system. Thermal comfort was improved as the installation height was extended higher. Furthermore, under constant total air supply rate, the AHA temperature of the last row was decreased with the increase in the air supply ratio of the UFAD to CDV systems (RUtoC), and the total power consumption was declined by 3.1% through adjusting RUtoC compared to the traditional UFAD system. Additionally, the air quality could be significantly improved. The results are helpful to the design and operation of HVAC systems in experimental theatres.

Multi-objective integrated BES-CFD co-simulation approach towards pandemic proof buildings

COVID-19 has posed an extraordinary burden to those professionals responsible for properly operating and safely maintaining facilities throughout this disaster. Considering this global pandemic, the common spaces in buildings must be reconsidered to accommodate a future in-presence existence. Governments address human health and safety as the most vital considerations worldwide; thus, Heating, Ventilation, and Air Conditioning (HVAC) designs, airflow patterns, and temperature distribution must all be reconsidered to achieve such healthy circumstances. Based on this, a Building Energy Simulation-Computational Fluid Dynamics (BES-CFD) validated model has been analysed in terms of various HVAC designs. The simulations assessed the proposed solutions in terms of energy-saving, operational CO2 emissions, thermal comfort enhancement, and infection control. The results were closely examined and showed that the Underfloor Air Distribution (UFAD) system generates approximately laminar vertical airflow, reducing the likelihood of indoor infections and viral transmission. Supply air is delivered to the inhabitants’ zone without sacrificing mixing efficiency, ensuring long-term indoor environmental quality. Moreover, the UFAD model proved to be more cost-efficient compared to the Conventional Overhead Distribution (COHD) and has a lower carbon footprint and energy consumption. In terms of thermal comfort, the dynamic simulations showed a noticeable enhancement in PMV. Additionally, the UFAD provides a vertical temperature gradient profile that is sufficiently uniform. Moreover, the integrated DOAS-UFAD systems’ effectiveness was proved through a techno-economic analysis with a Return on Investment of 8.25% and a Payback period of 7.3 years.

The Relation Between Airflow Pattern and Indoor Air Quality in a Hybrid Personalized Ventilation with Under-Floor Air Distribution System

The Relation Between Airflow Pattern and Indoor Air Quality in a Hybrid Personalized Ventilation with Under-Floor Air Distribution System

Establishing multi-criteria optimization of return vent height for underfloor air distribution system

Return vent height significantly influences energy consumption, indoor air quality (IAQ), and thermal comfort of rooms ventilated by underfloor air distribution (UFAD) systems. Previous research took the optimal return vent height as the one requiring the least cooling load among alternatives meeting requirements on IAQ and thermal comfort presented by standards (e.g., ASHRAE Standard 55, ISO 7730). However, the rankings among cases satisfying standards were determined solely by the cooling load, ignoring differences in other indices. Moreover, the supply rate and supply temperature were fixed among various alternatives in previous research, while they might be adjustable for demand-controlled systems. This study aims to optimize the return vent height of a UFAD system by a multi-criteria ranking preference method, technique for order preference by similarity to ideal solution (TOPSIS), based on evaluation indices covering cooling load, IAQ, and thermal comfort. The results are listed as follows: a) predictive mean vote (PMV) in the occupied zone (OZ) under all return vent heights are lower than −0.5 when the supply temperature is 18 °C, exceeding the suggested range for general thermal comfort; b) the optimal return vent height is 1.5 m ranked by the TOPSIS method; c) the optimal return vent height changes to 2.3 m when the supply temperature is adjusted to keep the occupied zone thermoneutral (i.e., |PMV|<0.5).

Investigation on pre-cooling potential of UFAD via model-based predictive control

This study investigated the operation cost-saving potential of an underfloor air distribution (UFAD) system by applying model-based predictive control (MPC). Based on the measurements from an actual building, the grey-box building model was constructed using a decentralized approach. The simulation case study was then carried out to quantify the saving potential of the MPC compared to feedback control.For the cooling season, the saving percentage of the MPC exceeded 30%, while those of the intermediate season were reduced by approximately 20%. More savings were achieved when the outdoor air temperature and solar radiation were high. The size of the plenum was not sensitive to the saving rate.The large savings were attributed to precooling that utilizes the cheaper utility cost of the off-peak and higher efficiency of the HVAC plant. This suggests that the larger thermal mass of the under plenum, including the air and still structure (bracket), is suitable for maximizing pre-cooling of the system.

Effect of Different Ventilation Systems on Concentrations of Indoor Particle Emitted from Floor of the Office

A three-dimensional numerical simulation of airflow, temperature, and pollutant concentration distributions with underfloor air distribution (UFAD) system and displacement ventilation (DV) systems are presented for different operating conditions. Since the particles deposited on the floors can be re-introduced into the air through re- suspension and then constitute a threat to human’s health, an optimal ventilation system is required to reduce the indoor air particle concentrations. In the present study, the effects of different systems on the concentrations of indoor particles are numerically investigated by an Eulerian-Lagrangian method, and the RNG k-ε model is adopted for the simulation. Three different supply air velocities (0.2-0.4 m/s) and different supply air temperatures (20 ℃, 22 ℃ and 24 ℃) are considered in the simulation. Meanwhile, the thermal effect of the human body on the micro-environment and its interaction with the surrounding environment are comprehensively evaluated. The simulation results shows that for the UFAD and DV systems, good performance of particle removal is obtained with the high air supply speed for the DV system, while under larger temperature gradient of the indoor environment, the UFAD system is capable of reducing the concentration of the particles emitted from the floors with lower air supply speed.