Using CFD to Efficiently Reducing Thermal … CFD to Efficiently Reducing Thermal Stratification at Small and Medium-scale Industrial Facilities 2 conditions. A convective heat transfer

Using CFD to Efficiently Reducing Thermal Stratification at Small and Medium-scale Industrial Facilities

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Problem Statement

In order to satisfy production and storage requirements, small and medium-scale industrial

facilities commonly occupy spaces with ceilings ranging between twenty and thirty feet in height.

While these large spaces are ideal for manufacturing, air handling units are challenged with

ensuring the climate remains near a desired set point. Natural convection within these spaces

causes warm air to rise towards the ceiling, sometimes creating a significant temperature gradient

known as thermal stratification. To overcome this temperature difference, air handling units

increase the air circulation by forcing more conditioned air into the space, resulting in higher

energy use. One alternative to this practice reduces thermal stratification through the use of large,

high efficiency, ceiling fans.

In New York State, the climate requires facilities to be heated for roughly 7 months of the

year. Often times many facilities will have heating ducts at the top of the building, near the ceiling.

Because of this the heat stays at the ceiling, thus, not using the energy produced by HVAC units

as effectively as possible. Since the heating season is so long in this area of the country, this

concept affects many industrial facilities in the community of Central New York.

Project Summary/Background

High ceilings make it difficult to effectively distribute conditioned air to the area. The

effects of natural convection cause hot air to rise to the ceiling. In order to maintain work space

temperatures close to specified set point, air handling units must force more air into the space,

increasing air circulation, thus resulting in higher energy use. One solution to this problem is to

install ceiling fans to reduce stratification.

Industry currently recommends ceiling fan use based on the existence of a significant

(>5°F) temperature gradient between the floor and ceiling. Average surface temperatures are found

using a thermal camera and are assumed to closely match air temperatures based on steady state


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conditions. A convective heat transfer coefficient is calculated using the difference in temperatures

between the floor and ceiling. That coefficient is later combined with room specifications to

determine the current amount of energy being used by the facility. The cost savings associated

with the installation of ceiling fans is based on the reduced heating load resulting from better air

circulation.

This project is aimed towards developing a better understanding of the effects ceiling fans

have on temperature profile, in order to optimize the configuration of destratification fan systems.

Traditional methods of determining optimal fan placement are dependent upon fan specifications

and ceiling areas. In this method, Computational Fluid Dynamics (CFD) models will be generated

for various room sizes, air conditions, and fan specs in order to determine the optimal placement

for the fans, whereas the previous method was more arbitrary. By improving the methodology of

fan placement, the optimal placement of fans will be determined, which will in turn optimize the

best use of heating and air conditioning equipment. It is believed that by enhancing the

methodology used analyze and reduce thermal stratification, we can increase work zone comfort

while simultaneously lowering the energy and emissions associated with climate control.

Other ways of optimizing fan placement that were considered were using a small drone-

like approach, which would determine a temperature profile before and after the installation of

ceiling fans; using a fish-tank to model flow of dye in water when fans are used; and creating a

zonal analysis using engineering equations. Using CFD modeling was chosen because it is

believed to give the optimal and most accurate depiction of ceiling fans and flow. It also is

beneficial because model characteristics can be changed more easily and quickly than with other

methods that were considered.

Relationship to Sustainability


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Optimizing the approach taken to decrease thermal stratification in industrial facilities has

social, economic and environmental benefits. In the winter, the fans can be used to pull cool air

up from the floor, which gently pushes warm air near the ceiling towards the floor. This practice

mixes the air while minimizing the velocity of the air near the floor, limiting body heat that would

be lost to convective heat transfer. During warmer months, the rotational direction of the fan can

be switched to force air down, maximizing air velocity near the floor, and thereby increase the

body heat lost to convective heat transfer. Ceiling fans provide a level of comfort that cannot be

acquired by air handling units alone.

The social benefit comes from increase in employee comfort. When the workers of an

industrial facility are more comfortable, productivity will be increased. By placing the ceiling fans

in areas that will be of the best use of ceiling space, the productivity of employees can also be

increased.

Economically, the installation of ceiling fans provides year round cost savings due to their

seasonal versatility. Based on information previously explained, ceiling fans can be used for both

the heating and cooling season, based on their rotational direction. There is an additional capital

cost when purchasing and installing ceiling fans, and an electrical cost to run the fans, but the

reduced cost of energy usage by the heating and cooling equipment outweighs the other cost

factors. The HVAC equipment will run more efficiently, thus reducing associated costs.

Environmentally, there will be a reduction in emissions. Because the heating and cooling

loads will be lowered, there will be a corresponding decrease in emissions. If the air handling unit

uses a natural gas heating system, the cost savings associated with a reduced heating load are

directly related to a decrease in the amount of emissions released into the atmosphere.


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Some of the tradeoffs associated with this solution involve the increase in electrical energy

consumption. When installing ceiling fans, the heating load, which is most often produced using

natural gas or fuel oil, will be decreased. Simultaneously, will be an increase in electrical

consumption, which will still create emissions. However, there are more environmentally friendly

ways of producing electricity that will be purchased from the grid, such as hydro-power, wind,

solar, and other renewable energy sources.

Materials and Methods

Throughout the project there were several tasks to be performed. The biggest task was

modeling the various scenarios using CFD modeling software. The following describes the

methodology of the CFD process.

Production areas in a small and medium-scale industrial facilities typically have ceiling

heights in the range of 20 to 30 feet. For the basic scenario the room has fixed height of 30 feet,

width of 50 feet and length in the range of 70 to 200 feet. There are two ceiling fans that has placed

at height of 25 feet from room’s floor. The CFD model for the basic scenario is sketched in Figure

1.

Figure 1: CFD model for basic scenario

The parameters of ceiling fans are chosen from the commercial fans “AirVolution 10”

(MacroAir Company). Each fan has volume flow rate of 48,882 CFM and diameter of 10 ft. The

mesh of the three dimensional CFD model contains 1.1 million cells and was generated in the


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software Pointwise. The boundary conditions are presented in Figure 1 (right). The ceiling is set

as a no-slip wall with temperature of 80° F and the floor is set as a no-slip wall with temperature

of 60° F. The room’s walls are set as no-slip wall with isolated temperature. The gravity of -9.8

m/s² is turned on due to the buoyancy effect. The symmetry boundary condition is applied to

decrease the number of cells and minimize simulation time. The structure mesh is applied in this

study and 3-D RANS simulation is demonstrated in the commercial CFD solver Ansys Fluent. The

viscous model is k-e (2 equations) model and operating conditions pressure is 101325 Pa. The

coupling between pressure and velocity is solved using the SIMPLE scheme. In control of the

convergence of the solution all the continuity, x-velocity, y-velocity, and turbulent viscosity

residuals are down to at least 10-4. The “Fan Model” is chosen to model the ceiling fans. In this

model the total pressure rise is specified in order to satisfy volume flow rate.

First two members of the team worked to research potential solutions to the problem of

thermal stratification. They also researched benefits of employee comfort on productivity, and cost

savings associated with heating and cooling energy savings. Once it was established that the team

would use CFD to model various fan simulations, two members of the team worked to find various

fan sizes of a vendor. The various fan sizes and specifications were tabularized and given to the

third team member, who completed the CFD analyses. Once the various models were obtained,

the team began to put the report together.

Results, Evaluation and Demonstration

As mentioned, in this study small and medium-scale industrial facilities typically have

ceiling heights in the range 30 feet. Hot air rises to the ceiling, and excessive heat loss through

ceiling requires additional energy to heat the floor area to the set point temperature in Figure 2

(left). One potential solution to this problem is to install ceiling fans to reduce stratification. The


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effects of thermal stratification are investigated using three-dimensional CFD simulations. The

pathline of air from two ceiling fans is described in Figure 2 (right). In this configuration,

freestream air is ingested into the inlet of the ceiling fans located at height of 25 ft, energized using

the fan, and expelled at the outlet of the fan in the direction from ceiling to floor. Figure 2 provides

a visual presentation of flow colored velocity magnitude. The ceiling fans force more air into the

space and increase air circulation. The strong air circulation reduces the heat loss through ceiling

and provides additional heat energy to the floor area to maintain work space temperatures close to

the specified set point.

Figure 2: Thermal stratification in heating season (left) and ceiling fans reduce stratification.

The velocity contour of airflow in room is shown in Error! Reference source not found.

(left), shown below. The maximum velocity at the outlet of fans is 5 m/s, and then reduces along

vertical direction from ceiling to floor. The velocity is high at the fan tip and low around the fan

center because of the distance from fan center. The working space is specified at 6 ft from the

room floor. At that working level the velocity is 0.5 – 1 m/s. This is acceptable comfort level at

the working place. The total pressure of air is provided in Error! Reference source not found.

(right). Again, the total pressure is highest at the fan tip and reduces to the fan center. Please note

that only axial and radial velocities are defined in this fan model and the swirl velocity is not

specified.


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Figure:3 Velocity contour (left) and total pressure contour in basic room

The most important parameter to study is temperature distribution in room. The

temperature contour is illustrated in

Figure 4. Due to thermal stratification the temperature is high near ceiling (80°F) and low at floor

(60°F). However, when the ceiling fans are installed, a strong air circulation mixes temperature in

the room and thermal stratification significantly reduces.

Figure 4 presents temperature difference between floor and ceiling as less than 5°F. At the working

place the temperature is remained about 72°F, which is comfortable for working people in the small

and medium-scale industrial facilities.

Figure 4: Temperature contour in basic room

Using CFD we can determine the optimal number of fans for various scenarios of different

sized buildings. In this section only the room’s dimensions are changed and other factors are kept

the same, i.e. there are two previous 10ft-diameter fans, ceiling temperature is 80°F and floor

temperature is 60°F. The velocity contour of air in room dimension: 150 ft x 50 ft x 30 ft is


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presented in Figure 5. In this case these fans still work well and they are able to reduce the thermal

stratification in the room.

Figure 5: Velocity contour in room dimension: 150 ft x 50 ft x 30 ft

However, when the room dimension becomes larger the number of fans needs to be

increased. Figure 6 presents the velocity contour for the room dimension: 200 ft x 50 ft x 30 ft. It

is clear that the ceiling fans are not able to force more air in the space between these fans. In this

case more fans must be added into this room, or they need to be spaced out more effectively.

Figure 6: Velocity contour in room dimension: 200 ft x 50 ft x 30 ft

The various scenarios of different sized buildings with various sizes and placement of fans

are modeled in CFD simulation to determine the optimal configuration for these fans. Table 1

presents the optimum number of fans for various scenarios.

Table 1: Number of fans for various scenarios

Q = 48882 CFM

Width

Length Aspect Ratio #Fans

70 ft 1.4:1 2

100 ft 2.0:1 2

125 ft 2.5:1 3

150 ft 3.0:1 3

175 ft 3.5:1 3

200 ft 4.0:1 4

50 ft

AirVolution 10

Q = 90695 CFM

Width

Length Aspect Ratio #Fans

70 ft 1.4:1 2

100 ft 2.0:1 2

125 ft 2.5:1 2

150 ft 3.0:1 2

175 ft 3.5:1 3

200 ft 4.0:1 4

AirVolution 14

50 ft


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For this case, the temperature difference was reduced to less than 5ºF from 20ºF. For a

typical warehouse space, the average BTU/ft² is 13,400, for a non-refrigerated space1. According

to industry, for a room with 30 ft ceilings, the percentage of energy savings when ceiling fans are

introduced is roughly 29%2. Based on a room that is 100,000 ft², which is a typical size room for

a manufacturing facility in New York State, and the energy content per square foot of a typical

non-refrigerated warehouse, the potential energy savings is 3,886 CCF Natural gas. The emissions

reductions from this natural gas savings are displayed in Table 2. Values for the emission factors

of each element in the table were taken from EPA standards3. Calculations for this can be seen in

the Appendix of this report.

Reductions in Pollutants and Greenhouse Gas Emissions (lbs)

CO2 NOx PM SO2 CH4 TOC VOC

171,474 3.14 10.86 0.86 3.29 15.72 7.86 Table 2: Reduction in Pollutants and Greenhouse Gas Emissions

As previously stated, there are social economic and environmental implications associated

with the installation and optimization of ceiling fans. By utilizing ceiling fans for heating and

cooling, there are many benefits. This project only examined the benefits associated with small

and medium scale industrial facilities, but this could be used to determine and quantify the savings

associated with installation of ceiling fans. For the presentation the results will be presented in a

poster format.

Conclusions

It is evident from this study that the use of ceiling fans effectively decreases thermal

stratification. Through the use of CFD modeling, it is shown that the temperature difference

between the ceiling and floor could be reduced from 20°F to less than 5°F. Such reduction in the

extent of thermal stratification results in near floor temperature of approximately 72°F, leading to

a more comfortable work environment during the heating season. Reducing thermal stratification


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is also economically beneficial, since much less natural gas will be needed to provide fuel for the

heating systems. Reduced natural gas use during winter, in turn, reduces the amount of pollutants

and greenhouse gases released to the atmosphere.

Future work on this project includes introducing new heat or flow sources into the room

for a more accurate depiction of the flow around people and objects. As previously stated, the

traditional methods of installing ceiling fans depend upon certain fan specifications and room size.

This project provides a methodology on how to best place ceiling fans. This project examined a

room with no heat sources, machinery, nor people. Comparing traditional ceiling fan calculations

with the CFD modeling, as was done in this project, does not show significant differences between

the two approaches. However, when a heat source is present in a room, the CFD modeling will

likely be very effective, as the placement of fans would be more important.

This CFD modeling approach to ceiling fan placement could be used in other applications

as well, aside from warehouse space. On college and university campuses any room, such as large

lecture halls and auditoriums, that has a large open floor plan could benefit from this type of

advanced CFD modeling, identify fan placement to reduce thermal stratification. Similarly, school

buildings, such as gymnasiums, that have high ceilings could also benefit from CFD model based

fan placement to reduce the thermal stratification.

This project provides an insight into the extent of cost and environmental benefits that

could be realized, through the use of CFD modeling tools, by appropriate fan placement to reduce

the extent of thermal stratification typically present in large heated open space buildings. The

impact of the dual benefits of reduced fuel use and reduced atmospheric emissions could be

substantial, when we consider the large number of such heated open space buildings currently in

use throughout New York State.


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Appendix

13,400 Btu/ft² Energy content per square foot

100,000 ft² Typical building size

1,340,000,000 Btu

100,000 Btu/CCF NG Energy content per CCF Natural Gas

13,400 CCF NG

29% Percent energy savings

3,886 CCF NG Natural gas savings

1,428,949 SCF Based on 10CCF NG = 3677.172051106 SCF

End Notes

1 "Managing Energy Costs in Warehouses." Managing Energy Costs in Warehouses. E Source Companies LLC, 21

Mar. 2013. Web. 08 Apr. 2015. 2 "HVAC Energy Savings | Destratification Fans by Airius." HVAC Energy Savings | Destratification Fans by

Airius. Airius LLC, n.d. Web. 08 Apr. 2015. 3 U.S. Environmental Protection Agency, Office Of Air Quality Planning and Standards, Compilation of Air

Pollution Emission Factors: Volume 1: Stationary Point And Area Sources, (North Carolina, 1995), 1.4:5-6.

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Using CFD to Efficiently Reducing Thermal … CFD to Efficiently Reducing Thermal Stratification at Small and Medium-scale Industrial Facilities 2 conditions. A convective heat transfer