The use of ducts to improve the control of supply air temperature rise in UFAD systems: CFD and lab study

doi:10.1016/j.apenergy.2014.08.002

Applied Energy

Volume 134, 1 December 2014, Pages 490-498

https://doi.org/10.1016/j.apenergy.2014.08.002 Get rights and content

Highlights

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Experimental data demonstrated the rise in air temperature in the underfloor.
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Using ducts “reverses” the typical temperature distribution.
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We made detailed CFD models of the underfloor plenum equipped with a fabric duct.
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The models well predicted the discharge air temperatures and airflows.

Abstract

Cool supply air flowing through the underfloor plenum is exposed to heat gain from both the concrete slab (conducted from the warm return air on the adjacent floor below the slab) and the raised floor panels (conducted from the warmer room above). The magnitude of this heat gain can be quite high, resulting in undesirable loss of control of the supply air temperature from the plenum into the occupied space. These warmer supply air temperatures can make it more difficult to maintain comfort in the occupied space (without increasing airflow rates), particularly in perimeter zones where cooling loads reach their highest levels. How to predict plenum thermal performance is one of the key design issues facing practicing engineers – evidence from completed projects indicates that excessive temperature rise in the plenum can be a problem.

One of the recommended strategies for addressing temperature rise in UFAD systems is the use of ductwork (flexible or rigid) within the underfloor plenum to deliver cool air preferentially to perimeter zones or other critical areas of high cooling demand.

Several experiments were carried out in a full-scale underfloor plenum test facility, in order to characterize all the phenomena that take place in an underfloor plenum equipped with a fabric or metal duct. Experimental data were collected for validation of a computational fluid dynamics (CFD) model of the plenum. This paper describes the first part of a more comprehensive work, whose aim is to use the validated CFD plenum model to conduct simulations of a broader range of plenum design and operational parameters. This work proves that using ductwork within the underfloor plenum reduce the temperature rise in the plenum.

Introduction

Underfloor air distribution (UFAD) systems use open spaces, called plenums, between the structural slab and raised floor panels to supply conditioned air into the occupied zone [1]. During the past two decades these systems have offered an alternative to traditional overhead (OH) systems, and are now commonly used all over the world. There are several potential advantages of using UFAD systems compared to OH systems:

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improved thermal comfort [1], [2], [3];
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improved indoor air quality [1], [3];
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reduced life cycle costs [1];
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reduce floor-to-floor height in new construction.

Among all features that characterize UFAD technology, room air stratification and temperature rise in the underfloor plenum play key roles in determining the operational success of these systems.

Under cooling operation, a properly controlled UFAD system produces air stratification in the conditioned space [4], resulting in higher temperatures at the ceiling level compared to the floor. Room air stratification has been widely investigated during the years with case studies, laboratory tests, and energy models [4], [5], [6], [7].

Temperature rise in the underfloor plenum is due to convective heat gain to the conditioned supply air as it travels through the underfloor supply plenum [8]. Bauman et al. [9] demonstrated using a steady-state model that the magnitude of heat transfer into the underfloor plenum through the raised floor and the floor slab is 35–45% of the total zone cooling load. Schiavon et al. [10], using whole-building energy simulations, found that the total heat transfer into the plenum is of similar magnitude, 22–37% of the total zone peak UFAD cooling load, depending on interior or perimeter zone location. The work done by Woods and Novosel [11] also provides clear evidence of the temperature rise in the underfloor plenum and its magnitude. In their study the researchers performed a comparative analysis of conventional overhead (OH) and UFAD system performance. They found that the room heat extraction rate for the UFAD system is 35–64% of the instantaneous heat gain to the room, compared to the OH system, where it is 66–93%. The authors found the cause of this difference to be the heat entering the supply plenum. The magnitude of the plenum heat gain can be quite high, and the warmer supply air temperature can make it difficult to maintain comfort in the occupied space (without increasing airflow rates), particularly in perimeter zones where cooling loads reach their highest levels [1]. How to predict and account for plenum thermal performance is one of the key design issues facing practicing engineers.

In a building conditioned with a UFAD system a desirable scenario would be to deliver the coolest supply air into perimeter plenum zones, allowing warmer plenum temperatures in interior zones. However, due to the common building HVAC configuration with primary supply air ducts in the building core, conditioned air is typically supplied into interior portions of the underfloor plenum and flows out to perimeter zone locations. As a results of the temperature rise in the plenum, buildings equipped with an open underfloor plenum system will tend to have warmer supply air entering the perimeter zone, and colder air into the interior zone. Three recommended approaches to improve the control of supply air temperature rise in UFAD systems are as follows [1]:

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Use ductwork to deliver cool air to/towards the perimeter. This is the subject of this research work.
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Direct plenum inlets with higher inlet velocities toward critical perimeter locations.
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Instead of the typical interior plenum inlet locations, consider designing the plenum with perimeter inlets (shafts), if possible.

In terms of the first recommended strategy above, the advantage of using ductwork is that it eliminates direct contact along the length of the duct with the slab and underside of the raised floor panels, thereby reducing temperature gain to the supply air. One disadvantage is the added flow resistance caused by the duct in comparison to airflow through an open plenum, which tends to increase the pressure drop through the system and required fan energy. In addition, once the supply air exits the duct and enters the plenum, it is still exposed to the same heat transfer from the top and bottom surfaces. The challenge and goal of this research is to develop a validated CFD model of underfloor air supply plenums equipped with flexible (or rigid) ducts.

Several experiments were carried out in a full-scale underfloor plenum test facility, in order to characterize all the phenomena that take place in an underfloor plenum equipped with a fabric duct. Fabric ducts were chosen due to their ease of installation and flexibility once installed. Experimental data were collected for validation of a computational fluid dynamics (CFD) model of the plenum. This paper describes the first part of a more comprehensive work, whose aim is to use the validated CFD plenum model to conduct simulations of a broader range of plenum design and operational parameters. In particular the validated model will be used to compare the use of ductwork, to deliver cool air to/towards the perimeter vs. the use of plenum inlets with higher inlet velocities (the CFD model for this previous open plenum configuration was developed by Jin et al. [12] using the same plenum test facility), in order to highlight the limits and the capabilities of these two technical solutions to reduce the temperature rise in the underfloor plenum.

Section snippets

Methods

The technical approach includes the following tasks:

(1)
Full-scale experiments in underfloor plenum test facility.
(2)
Development of computational fluid dynamics (CFD) model of underfloor plenum.
(3)
Validation of CFD plenum model by comparison with full-scale experimental database.

Test measurement results and discussion

The most important information is related to the diffusers, because they can be considered as the interface between the supply plenum and the room. The airflow through each diffuser was measured with the airflow-meter. As shown in Fig. 4, the airflows through the four perimeter diffusers are on average higher than for the interior diffusers. This is due to the VAV diffuser’s damper setting.

It is also interesting to see that in configuration #2, even if the four perimeter VAV diffusers are all

Conclusions

The tests conducted using the underfloor plenum facility have demonstrated the rise in air temperature as it flows through the underfloor plenum. The tests proved also that using fabric ducts (as well as rigid ducts) to deliver cool air directly into the plenum perimeter “reverses” the typical temperature distribution, with colder air in the perimeter and warmer air in the interior zone.

Using the test results it was possible to make two detailed CFD models of the underfloor plenum equipped with

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An “Underfloor Air Distribution” (UFAD) system employs the open area underneath a raised floor system to drive conditioned air to the supply outlets situated in the floor. To understand a UFAD, think of it as an “upside-down version” of an overhead (OH) system. The idea behind it is to increase both energy efficiency as well as the quality of air in a home. In this paper, an auditorium model has been simulated for analysing the temperature distribution as well as thermal comfort of human occupants provided with 4 different cases of ventilation. The auditorium has two AC inlets ducts, each 2.50 m wide further branched into a number of small diffusers over the auditorium ceiling in order to distribute the inlet air evenly throughout the theatre space. The first case involves an upward ceiling outlet at the front of the theatre near the screen area while the second case has a downward outlet on the floor in the screen area. The third case is similar to the second one with human occupants designed as cuboids on every staircase while for the fourth case, the outlets have been provided under every seat on the staircase. It has been observed in the analysis that the downward outlets in case 2 provide better temperature distribution than the upward outlets of case 1. But the position of outlets below every seat, as in case 4, in the auditorium provides the best thermal comfort for the occupants than all the other cases. Hence, for the “maximum thermal comfort” of the human occupants, the outlets are advised to be installed downward on the floor under every seat.
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Citation Excerpt :
Ignore the influence of lines, pipes and supports in the plenum, as well as the influence of some facilities such as corridors, pillars and pig troughs. In view of the constrains and assumptions in the simulation, the airflow environment is in a continuous and stable state, and the steady-state simulation is selected as the model of the fluid flow state [35,36]. The direction of gravitational acceleration is vertically downward, and the magnitude is 9.8 m/s2.
Modern agricultural production puts forward higher requirement for environmental control. Uniform air distribution can form the stable temperature, humidity and velocity of indoor air, which is particularly important for agricultural production. In this paper, theoretical research is conducted on a nursery piggery and the velocity uniformity is defined to evaluate the performance of perforated ceiling ventilation. A new optimization method for airflow organization is developed, which improves the airflow organization based on the suitable air supply angle. It shows that the velocity uniformity is increased and then decreased with the increase of air supply angle, and the best air supply angle is around 35°. The optimal air supply angle changes with the plenum height, and there is a threshold for the plenum height. The best velocity uniformity can be achieved when the height of plenum is 0.5 m. Besides, the air supply velocity has almost no effect on the velocity uniformity. The optimization model for airflow organization based on the air supply angle is established and its accuracy is verified. Finally, a specific case is selected to compare and analyze the air distribution under different air supply methods, which verifies the feasibility of the optimization method proposed in this paper.
Investigation on pre-cooling potential of UFAD via model-based predictive control
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The increased energy consumption of the UFAD could be avoided by increasing the insulation between the room and the plenum. On the other hand, ductwork added under the plenum could resolve the supply air temperature increase, which was investigated using a CFD simulation [19]. The higher energy consumption of the UFAD reported by Yu et al. [24] is somewhat controversial compared to Xue and Chen [21].
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.
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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.
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The use of ducts to improve the control of supply air temperature rise in UFAD systems: CFD and lab study

Highlights

Abstract

Introduction

Section snippets

Methods

Test measurement results and discussion

Conclusions

Appl Energy

Energy Build

Comput Meth Appl Mech Energy

Underfloor air distribution (UFAD) design guide

ASHRAE

Integrated design and UFAD

ASHRAE J

Performance of underfloor air distribution in a field setting

Int J Vent

Underfloor air distribution: thermal stratification

ASHRAE J