UFAD System Research Articles

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.

Effect of blind angles on thermal decay in the UFAD system in summer

• Thermal decay depending on blind angle examined two heat transfer effects. • The effect of change in conduction at the bottom surface of the concrete slab on the rate of convective heat transfer. • The effect of change in solar radiation at the top surface of the raised floor on convective heat transfer at its bottom surface. • Optimization of the UFAD system based on analysis of thermal decay and chiller energy consumption, and their dependence on Phi. The objective of this study was to explain and quantify the effect of blinds on the heat transfer paths causing thermal decay and the resulting effects on diffuser discharge temperatures, temperature profiles, supply airflow rate, and energy consumption. To analyze the direct effects of convective heat transfer on thermal decay as blind angle changes, two processes must be examined. The first is the effect of the return temperature on convective heat transfer during operating hours, and the resulting change in conduction rate at the bottom surface of the concrete slab. The second is the effect of change in solar radiation at the top surface of the raised floor on convective heat transfer from its bottom surface, which affects thermal decay. This paper performed an analysis of thermal decay and cooling energy consumption according to the blind angle through a comparison of experimental data and simulation results. Phi means stratification in the room; lower values mean more stratification, which should be considered as a UFAD systems design. The most notable result of this study is the return temperature. It can be seen that during better temperature stratifications, the return temperature was higher due to the plume phenomenon. Although blinds decrease the total indoor cooling load with their shielding effect on the solar radiation load, the rise in return temperature increased the cooling coil load of the air conditioner and the heat storage in the concrete slab. This study will help to understand the relationship between the blind angle, the thermal stratification, and cooling energy usage in the UFAD systems.

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.

Improvement in airflow and temperature distribution with an in-rack UFAD system at a high-density data center

This paper introduced and analyzed a new concept where an under-floor air supply (UFAD) system with cold aisle containment (CAC) is replaced by a new in-rack UFAD system called an in-rack cold aisle (IR-CA). The IR-CA system is analyzed using CFD simulation, and on-site measurement was carried out to validate the feasibility and reliability of simulation models. The study is divided into eight cases with seven different dimensions for the rack air inlet (2.2 m × 0.6 m, 0.2 m × 0.6 m, 0.3 m × 0.6 m, 0.4 m × 0.6 m, 0.5 m × 0.6 m, 0.6 m × 0.6 m, and 0.7 m × 0.6 m), while an additional partition plane is placed in Case 8 with a 0.6 m × 0.6-m in-rack air inlet. The thermal distribution is compared and analyzed in the eight cases, while cooling efficiency and energy saving is compared between the original and optimal cases. The results showed that the optimal thermal distribution is achieved in Case 8 with a 0.6 m × 0.6 m IR-CA and partition plane, while the thermal distribution in Case 8 with SAT of 23 °C is still much better than that in the original DC. The application of a 0.6 m × 0.6 m IR-CA and partition plane can save approximately 98 kW h/day in electricity consumption in the studied DC. A new evaluation index named the MS index is proposed to evaluate the optimization effects of the optimization model based on the original model.

Computational optimization of a UFAD system using large eddy simulation

In the present study, the Large Eddy Simulation (LES) turbulence closure is implemented, for the first time, to the best of our knowledge, to investigate the air conditioning system in a large space. The results of LES simulations are validated against experimental measurements and the model is used to study the effect of different design variables, including the Air Changes per Hour (ACH), supply temperature, and return air vent height, on design objectives, such as local and global thermal comfort indexes and the energy saving parameter, via a systematic multi-objective optimization approach. The sensitivity analysis shows that the global and local thermal comfort indexes are most sensitive to the air supply temperature while the energy saving is sensitive to ACH and the supply temperature to the same extent. In addition, the return air vent height affects the energy saving more than the other objectives. Finally, with the best design proposed by the multi-objective optimization, an energy saving of 22.9% is achievable while keeping the thermal comfort indexes within the allowable range.

Numerical evaluation of the effect of air distribution system and location on performance of a portable air cleaner

Indoor air quality (IAQ) has been an important issue in air conditioning researches. Ventilation and air cleaning are two approaches to improve the air quality in closed spaces. Portable air cleaners (PACs) are devices, which are very efficient in removing air pollutants and recently have gained much attention in IAQ applications. In this study, a numerical study was developed to investigate the gas removal of a portable air cleaner under different air distribution systems. SF6 was used as a tracer gas to model the dispersion of gaseous pollutants in a simulated office involving a thermal manikin as an occupant. Clean air delivery rate (CADR) exposure reduction effectiveness and age of air are the concepts that were used to evaluate the air cleaner performance. Simulation results showed that the cleaner was more efficient under mixing and the wall-mounted system and did not perform well under under-floor air distribution system (UFAD) system. Location of the device is a key parameter affecting its air purification when there is a ventilation system, and it had great influence on CADR in UFAD and mixing system. The cleaner must be located in high concentration zones, which have high age of air.

Influences of different operational configurations on combined effects of room air stratification and thermal decay in UFAD system

The non-insulated underfloor supply plenum and the heat gain of the cool supply air into supply plenum results in significant magnitude of thermal decay defined as the supply air temperature rise. Eventually, these changes have influence on the room air stratification, causing negative effects throughout whole system. Therefore, for the optimization of UFAD system, it is important to understand these fundamentals and relevant effects on the overall system operation. In this study, comparative analysis was conducted during cooling period using validated EnergyPlus model, after employing different operational configurations related to room air stratification and thermal decay. The thermal behavior and cooling energy performance were analyzed by observing the convective heat transfer, thermal decay, airflow, stratification among different operational configurations. As a result, it was observed that the existence of non-insulated supply plenum have influences on the thermal behavior in occupied zone, and the corresponding heat transfer has load reduction effect of the space by about 40%. However, there were still significant amounts of cooling energy consumption in the form of thermal decay despite the reduced cooling load, indicating that unintended and additional energy consumption was occurred in supply plenum. In addition, return air temperature rise by the room air stratification increased the cooling coil load. Eventually, these effects canceled each other out. As a result, compared to CBAD system, standard UFAD system consumed more electric energy by approximately 30%, and for fully-insulated UFAD system, 6% energy saving could be achieved.

Detailed and simplified models of a terminal unit that combines an UFAD system with a floor cooling

This paper analyses the thermal behaviour of an underfloor air distribution system (UFAD) combined with a floor cooling, focusing on the thermal decay (TD). Two new models were implemented: a detailed model based on the convective heat transfer coefficient inside the plenum, and a simplified model based on the bypass factor. To validate the models we performed three experiments.Both, detailed and simplified models, reproduce well the measured TD. The maximum relative errors between the predicted and measured TD were 8.4% (transient state) and 1.3% (steady state). Although differences were higher at transient regimen, the terminal unit is designed to operate with smooth changes of the boundary conditions. As the error obtained by detailed and simplified models was near identical, the model based on the bypass factor is recommended because of its simplicity.A sensitivity analysis was performed. We evaluated: (1) the TD as a function of the supply air temperature and air flow rate, (2) the cooling capacity of the floor, and (3) the cooling capacity of the ventilation air. The paper concludes with a nomographic design chart that brings together all the features of the terminal unit, allowing a quick selection of the operating parameters.

The stratification study of public buildings microclimate parameters with supply and exhaust ventilation

The paper presents the results of the UFAD system numerical modeling where the air is supplied through diffusers installed in a floor of a room. Warm air is removed from the upper zone of the room through the exhaust duct. The purpose of this study is to determine the magnitude of the temperature stratification in various areas of an office premise in the conditions of the under floor air distribution. Enclosed volume with local heat sources has been chosen as the computational domain: people, computers, and lighting. The problem is solved in a stationary setting using the k-omega SST turbulence model. As a result of the study the temperature distribution throughout the room has been obtained and the influence dependence of wall temperature on an average air temperature in the room has been defined. The numerical data received are in good agreement with the results of the borrowed experimental study that demonstrate the adequacy of the developed numerical model as well as the possibility of its further use for the optimization of the considered ventilation systems.

A new terminal unit combining a radiant floor with an underfloor air system: Experimentation and numerical model

This paper investigates a novel terminal unit that integrates a radiant cooling floor (RCF) with an underfloor air distribution system (UFAD). The unit consists of rectangular modules that are installed above the standard structural floor of the building, creating a raised floor. The upper face of these modules incorporates a grid for easy installation of the pipes of the RCF, while the space between the raised floor and the structural slab is used as the air supply plenum of the UFAD system.The proposed system presents some advantages over the standard air systems: (1) it allows increasing production and zone set point temperatures, which improve the efficiency of the chiller plant, (2) improved comfort, and (3) improved air quality due to reduced mixing within the occupied zone. The proposed system can also solve or mitigate some of the drawbacks of cooling floors: (1) prevent water condensation over the surface of the floor, (2) increase the cooling capacity of the floor, and (3) reduce the response time to changes in the space thermal loads.The interactions between the RCF and the UFAD are not evident. In this paper, the thermal behavior of the combined unit is investigated by means of a numerical model implemented in COMSOL Multiphysics. The accuracy of this model is checked against three experiments undertaken in laboratory using a scale model of the system. In the first experiment, the air above the floor is kept quiescent and the cooling capacity is measured. The maximum relative error between the predicted and measured capacities is 19.5%. The numerical model is then used to study how the air movement within the plenum modifies the transient thermal response of the unit. The second and third experiments are designed to quantify the temperature change of the air circulating within the plenum (thermal decay or TD). In nominal operating conditions, TD is about 2.7°C. In short, the new terminal unit is like a radiant cooling floor and a UFAD system working separately, but integrated in the same terminal unit.

DYNAMIC ENERGY MODELING OF AN EXPERIMENTAL BUILDING EQUIPPED WITH AN UNDERFLOOR AIR DISTRIBUTION (UFAD) SYSTEM

In this study, experimental and simulation studies were performed in a building equipped with an UFAD system in order to investigate the applicability of a new approach designed for prediction of the energy consumption of a residential building with an UFAD system. In this approach, a zonal model, Pressurized zOnal Model with the Air diffuser (POMA), was coupled with a thermal jet model and integrated into a traditional multizone thermal model. The coupled model was verified experimentally. This integrated model has the ability to take into account the characteristics of an UFAD system and thus accurately simulate its energy consumption. A case study was carried out using both approaches: Multi-zone approach and the new developed integrated zonal/jet/multizone model. A quantitative comparison, in terms of energy demand of a building with an UFAD system, shows that the difference can reach 14% and thus indicates that the traditional multizone modeling approach is not appropriate to use for UFAD system in the building energy prediction.

The numerical simulation and experimental study on rack cooling effect in date center with UFAD system

In order to analyze the way of improving the cooling effect of the data center with the UFAD system, the HVAC simulation software—Airpak—was applied. Numerical simulations for the interior temperature of the rack at different porosity ratios were carried out, and the simulated results indicate that the way of partly enhancing the porosity ratio of the front door and back door can improve the cooling effect. To solve the issue of the thermal current recycle in the exhaust outlet and airflow distribution disorder, the measure of the cold aisle regional closure was taken, and the comparison of data before and after the closure of the cold aisle proved its positive influence on the cooling effect: to lowering the inlet air temperature of racks, to improve airflow distribution inside and outside of the rack and to solve the thermal current recycle in the exhaust outlet.

Study of Ventilation Character for an UFAD Room with Cooling Wall

This study mainly investigated the wall temperature affects on the ventilation performance of the UFAD system with multi-zone mode simulation and experiment methods. A room which has dimensions 4.4×2.7×2.5m and consists of a single source of heat and a cooling wall was developed. Laboratory experiments were carried out to measure the air temperature distribution of the main space and the cooling boundary layer under three cases. From the model simulated and the experimental results it is obvious that the stratification in the room disappears as the temperature of the cooling wall decreases, the downdraught generated by the cooling wall reverses with the main zone airflow. The conclusions can provide the guidance on designing the UFAD systems.

Effect of supply air temperature on conservation of energy in air conditioning of space

In HVAC applications, huge amount of energy is utilized in fans and blowers to maintain the flow. In this paper energy savings associated with air distribution is discussed. In a most commonly used air distribution system, uniform thermal environment in the occupied space is established. An alternative to this method is the under floor air distribution system (UFAD) which is in its fantasy state. Thermal stratification can be established in this method due to the buoyancy flow of the air. In this paper assessment of the impact of temperature sensors in energy savings is done in UFAD system. It is observed that by the placement of temperature sensors in the occupied space, supply air temperature can be controlled while maintaining the comfort conditions. By optimal conditions of the temperature and volume flow, energy savings can be achieved due to reduction in energy requirements in refrigeration and ventilation. The comfort criteria of ASHRAE standard 55-92 is taken.

Performance analysis of an integrated UFAD and radiant hydronic slab system

In this paper, an EnergyPlus model was used to simulate the operation of a novel integrated HVAC system. This system combines an underfloor air distribution system with a cooled radiant ceiling slab. A cooling tower supplies water to pre-cool the structural slabs during the night and early morning period. The paper compares the performance of this system to both a typical overhead system and a typical UFAD system in the cooling season for the Sacramento, California climate. When compared to the overhead system, the integrated UFAD/Radiant system shows a 22–23% reduction in total energy consumption during the peak cooling months (June to August) and a 31% reduction in peak hourly electricity demand. When compared to the UFAD system, these reductions are 21–22% and 24% respectively. An investigation of the simulation results showed that the integrated UFAD/Radiant system also improves occupant thermal comfort and reduces thermal decay issues in the underfloor plenum.