Proceedings of the 6th International Mechanical Engineering Conference & 14th Annual Paper Meet (6IMEC&14APM) 28-29 September 2012, Dhaka, Bangladesh

IMEC&APM-FM-05

© IMEC &APM 2012 FM-05

1. INTRODUCTION HVAC system is one of the important components in a

modern commercial building. In modern time, with the

growth of high density metropolitan city, the need for

proper HVAC design increases rapidly. The function of a

duct system is to transmit air from the air handling

apparatus to each air outlet in the conditioned space. In a

typical case, a known volume of air is treated and

conditioned in the air handling unit and supplied to the

various rooms to maintain the specified human comfort

conditions.

Commercial, industrial and residential air duct system

design must consider, space availability, space air

diffusion, noise levels, air distribution system, duct heat

gains and losses, balancing, fire and smoke control,

initial investment cost and system operating cost [1-2].

Deficiencies in duct design can result in systems that

operate incorrectly or are expensive to own and operate.

Poor design or lack of system sealing can produce

inadequate air flow rates at the terminals, leading to

discomfort, loss of productivity, and even adverse health

effects. Lack of sound attenuation may lead to

objectionable noise levels. Proper duct insulation

eliminates excessive heat gain or loss [2].

In the present investigation, the air flow pattern inside

a HVAC duct used in a local supermarket has been

characterized. The duct chosen for this investigation are

rectangular in shape with varying hydraulic diameter.

The total length of the duct in the present investigation is

21.8m. There are 7 diffusers with an air flow rate of 140

L/s each in the duct system. The designed room

temperature is 23°C and outside temperature is 35°C.

There are 7 SD5-10 type and a single elbow type fittings

in the duct system. The numerical model is based on the

actual dimension of the duct to visualize the effect of

flow friction and other characteristics in the duct system.

These results are then validated using the actual pressure

drop in each inlet diffuser of the each room.

Previously, many other authors investigated different

types of HVAC system and its duct system. Bertagnolio

et al. developed a series of building energy simulation

tools to help the building energy auditor in establishing

his diagnosis and in evaluating the selected energy

conservation opportunities [7]. In their article, the

building HVAC system global model includes simplified

models of building zones and of HVAC equipment.

Wolin et al observed the characteristics of smoke

traveling in an HVAC duct along with response of

selected duct smoke detectors [5]. Their simulated

HVAC system consists of a 9m long duct, 0.45m in

diameter. They have characterized the smoke within the

duct by means of a laser light sheet and charge couple

device (CCD) camera, two white light source and

photocell ensembles, a Pitot tube and an array of eight

thermocouples placed on the vertical plane of symmetry.

A smoke detector was placed at the downstream end of

the test section. Two types of detectors were tested,

ionization and photoelectric, with a single sampling

NUMERICAL INVESTIGATION OF FLOW CHARACTERISTICS INSIDE A HVAC DUCT: THE CASE STUDY IN A COMMERCIAL BUILDING

AA Rezwan, TMH Kabir, AKMH Arefin and MAR Sarkar

Department of Mechanical Engineering

Bangladesh University of Engineering and Technology, BUET

ABSTRACT

The present investigation concentrates in characterizing the air flow pattern inside a

HVAC Duct used in a local supermarket. The duct chosen for this investigation are

rectangular in shape with varying hydraulic diameter. The total length of the duct in the

present investigation is 21.8 m. There are 7 diffusers with an air flow rate of 140 L/s each

in the duct system. The designed room temperature is 23°C and outside temperature is

35°C. There are 7 SD5-10 type and a single elbow type fittings in the duct system. The

numerical model is based on the actual dimension of the duct to visualize the effect of

flow friction and other characteristics in the duct system. The results obtained in the

numerical analysis are then validated using the actual pressure drop in each inlet diffuser

of the each room.

Keywords: HVAC, Air Duct, SD5-10, Elbow

© IMEC &APM 2012 FM-05 2

probe geometry. The fires tested cover a wide range of

fuels. Ji and Lee was experimented many automotive

HVAC in 2009. They had visualized the air flow inside

an automotive HVAC module using a high-resolution

PIV technique with varying temperature control modes

[3-4]. Junior et al. modeled a Thermoelectric HVAC

System for Automobiles [6] 2. DUCT SYSTEM The function of a duct system is to transmit air from

the air handling apparatus to each air outlet in the

conditioned space. The supply air requirement for each

space is first calculated based on the respective cooling

load. The sum total of the air supply requirement of all

the spaces or zones is then computed for the purpose of

calculating the duct sizes. The duct is designed to supply

the proper amount of air at a specified total pressure to

different spaces. The design process involves sizing and

routing of the ducts from the fan to each space within the

constraints of the available space and acceptable noise

level for minimum life-cycle costs.

For the air conditioning application, the supply and

return duct system may be categorized with respect to the

velocity and pressure of the air within the duct.

Conventional or low velocity systems would have main

duct velocities up to about 10 m/s (Ameen, April, 2010).

While low velocity systems are more common, in large

central air conditioning installations where a

considerable quantity of air must be circulated, the use of

low velocity flow may result in prohibitively large ducts.

The use of a high velocity flow system, in which the main

duct velocities are as high as 30 m/s, permits the use of a

smaller duct for a given air flow. However, high

velocities in ducts result not only in greater pressure

losses in systems but also in greater noise levels.

Generally, a low noise level is maintained by limiting the

air velocity, b using sound absorbing duct liners, and by

avoiding drastic restrictions in the duct such as nearly

closed dampers. A low velocity duct system will

generally have a pressure loss of less than 1.23 Pa/m,

whereas, high velocity systems may have pressure losses

up to about 5.7 Pa/m.

In the present case, the HVAC systems use a low

velocity duct systems where flow rate is about 140 L/s.

The losses occurs in different diffuser are on the scale of

2 Pa/m.

Fig. 1: Duct System

3. NUMERICAL CALCULATION In the present investigation ANSYS CFX has been

used to solved the numerical model. The set of equation

solved by ANSYS CFX are the unsteady Navier-Stokes

equation in their conservation form.

3.1 Transport Equations The instantaneous equations of mass, momentum and

energy conservation can be written as follows in a

stationary frame:

The Continuity Equation

(1)

The Momentum Equations

(2)

where the stress tensor, τ, is related to the strain rate by

(3)

The Total Energy Equation

(4)

where htot is the total enthalpy, related to the static

enthalpy, h(T, p) by,

(5)

The term represents the work due to viscous

stresses and is called the viscous work term. This models

the internal heating by viscosity in the fluid, and is

negligible in most flows. The term U.SM represents the

work due to external momentum sources and is currently

neglected.

3.2 Turbulence Models k-ε turbulence model has been used for the present

analysis. k is the turbulence kinetic energy and is defined

as the variance of the fluctuations in velocity. It has

dimensions of (L2T

-2); for example, m

2/s

2. ε is the

turbulence eddy dissipation, and has dimensions of k per

unit time (L2T

-3); for example, m

2/s

3.

© IMEC &APM 2012 FM-05 3

The k-ε model introduces two new variables into the

system of equations. The continuity equation is then

(6)

and the momentum equation becomes

(7)

where SM is the sum of body forces, μeff is the effective

viscosity accounting for turbulence, and p’ is the

modified pressure.

The k-ε model, like the zero equation model, is based

on the eddy viscosity concept, so that:

(8)

where μt is the turbulence viscosity. The k-ε model

assumes that the turbulence viscosity is linked to the

turbulence kinetic energy and dissipation via the relation:

(9)

where Cμ is a constant.

The values of k and ε come directly from the

differential transport equations for the turbulence kinetic

energy and turbulence dissipation rate

(10)

where Cε1, Cε2, σk and σε are constants.

Pkb and Pεb represent the influence of the buoyancy

forces, which are described below. Pk is the turbulence

production due to viscous forces, which is modeled using

(11)

For incompressible flow, is small and the

second term second term on the right side of Equation

does not contribute significantly to the production. For

compressible flow, is only large in regions

with high velocity divergence, such as at shocks.

4. NUMERICAL CALCULATION PROCEDURE 4.1 Geometry & Boundary Conditions The duct chosen for the present investigation are

rectangular in shape with varying hydraulic diameter.

The total length of the duct in the present model is 21.8m.

There are 7 diffusers with an air flow rate of 140 L/s each

in the duct system. The designed room temperature is

23°C and outside temperature is 35°C. There are seven

SD5-10 type and a single elbow type fittings in the duct

system.

Fig. 2: Geometry of the HVAC Duct

4.2 Boundary Conditions There are three types of boundaries model in the

present HVAC duct. Boundary conditions are as follows:

Table 1: Boundary Physics for HVAC Duct

Boundary - Inlet

Type INLET

Location Inlet

Settings

Flow Direction Normal to Boundary Condition

Flow Regime Subsonic

Mass And

Momentum Mass Flow Rate

Mass Flow Rate 1.66 [kg s^-1]

Turbulence Medium Intensity and Eddy

Viscosity Ratio

Boundary - Outlet

Type OPENING

Location Outlet

Settings

Flow Direction Normal to Boundary Condition

Flow Regime Subsonic

Mass And

Momentum Opening Pressure and Direction

Relative Pressure 1 [atm]

Turbulence Medium Intensity and Eddy

Viscosity Ratio

© IMEC &APM 2012 FM-05 4

Boundary - Wall

Type WALL

Location Wall

Settings

Mass And

Momentum No Slip Wall

Wall Roughness Smooth Wall

Fig. 3: Different Boundaries of the System

4.3 Mesh Generation Mesh Generation has been done using CFX-Mesh.

The following figure shows the mesh generation for the

geometry.

Table 2: Mesh Information for HVAC Duct

Domain Nodes Elements

Default Domain 187223 1012422

Fig. 4: Meshing of the System

4.4 Physics Report The physics that model the HVAC duct has the

following particulars.

Table 3: Domain Physics for HVAC Duct

Domain - Default Domain

Type Fluid

Location B592

Materials

Air at 25 C

Fluid Definition Material Library

Morphology Continuous Fluid

Settings

Buoyancy Model Non Buoyant

Domain Motion Stationary

Reference Pressure 1 [atm]

Heat Transfer Model Isothermal

Fluid Temperature 23[C]

Turbulence Model k epsilon

Turbulent Wall Functions Scalable

4.5 Validation The numerical result has been validated by comparing

the result of pressure drop for various outlets with a

measured value of the actual duct.

© IMEC &APM 2012 FM-05 5

5. RESULT AND DISCUSSION 5.1 Pressure Distribution The pressure distribution inside the duct has been

depicted in the figure 5. The pressure distribution shows

that with the increase of duct length the pressure

gradually decreased. From the fig. it can also be noted

that the pressure suddenly decreased at the diffuser

section.

Fig. 5: Pressure Distribution inside the HVAC Duct

Fig. 6: Pressure Variation

5.2 Velocity Distribution Figure 9 shows the velocity distribution inside the

duct. Generally, the velocity inside the duct has been

decreased with the increasing duct length. But at the

diffuser, some of the air escape, decreasing the air flow

rate. This causes a sudden decrease in velocity after the

diffuser section. But this also result in a constant air

velocity through the duct.

Fig. 7: Velocity Variation

Fig. 8: Velocity Streamline

© IMEC &APM 2012 FM-05 6

Fig. 9: Velocity Distribution

6. SUMMARY A commercial building HVAC duct has been

numerically modeled and simulated for characterizing

the air flow pattern. It has been noticed that the pressure

inside the duct has been gradually decreased along the

duct length. Generally, the velocity inside the duct has

been decreased with the increase of duct length. But as

the air passes through the diffuser, some of the air has

been escape through the diffuser. As a result there have

been a loss of mass flow rate, and the velocity after the

diffuser has been decreased significantly. As a result, the

velocity at diffuser section for all 7 diffuser remains

almost constant. The result shows a consistent design for

the HVAC duct. These results can be modified for more

complex system of the HVAC duct and can be helped in

designing duct for any residential and commercial HVAC

system.

7. REFERENCES 1. Ameen, Ahmadul. April, 2010. Refrigeration and

Air Conditioning . New Delhi : PHI Learning

Private Ltd., April, 2010.

2. ASHRAE. 2009. Duct Design. ASHRAE Handbook.

s.l. : ASHRAE, 2009.

3. Experimental Study of The Flow Characteristics in

Automotive HVAC System using a PIV Technique. Ji,

S., H. and Lee, S., J. 2009. 5, s.l. : KSAE, 2009,

International Journal of Automotive Technology,

Vol. 10. DOI 10.1007/s12239−009−0065−6.

4. Investigation on the Flow Charactersitics inside an

Automotive HVAC System with Varying Ventilation

Mode. Kang, J., H. and Lee, S., J.,. 4, s.l. : The

Visualization Society of Japan, Journal of

Visualization , Vol. 12.

5. Measurements of Smoke Characteristics in HVAC

Ducts. Wolin, S., D., Ryder, N., L., Leprince, F.,

Milke, J., A., Mowrer F., W., and Torero, J., L.

2001. s.l. : Kluwer Academic Publisher, 2001, Fire

Technology, Vol. 37.

6. Modeling a Thermoelectric HVAC System for

Automobiles. Junior, C., S., Strupp, N.,C., Lemke,

N.,C., and Koehler, J. 2009. 7, s.l. : TMS, 2009,

Journal of Electronic Materials, Vol. 38. DOI:

10.1007/s11664-009-0749-8.

7. Simulation of a building and its HVAC system with

an Equation Solver: Application to AUDIT.

Bertagnolio, S., Andre, P. and Lemort, V. 2010. Berlin Heidelberg : Tsinghua University Press, 2010,

Build Simulation, Vol. 3. DOI

10.1007/s12273-010-0204-z.