20120130406013 2

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –

6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME

118

NUMERICAL ANALYSIS OF TURBULENT KINETIC ENERGY PLOTS,

TURBULENT EDDY DISSIPATION PLOTS AND PRESSURE PLOTS OF

FLOW OF THE AIR CURTAIN

Mr. Nitin Kardekar Principal, Jayawantrao Sawant Polytechnic

Dr. V K Bhojwani Professor JSPM’s Jayawantrao Sawant College of Engineering, Pune

Dr. Sane N K Research Supervisor, Singhania University

ABSTRACT

A prototype is developed in the laboratory in order to simulate the conditions of the entrance

of the doorway. The air curtain device is mounted above the doorway. An obstacle of human Shape

(mannequin) is placed in the doorway to simulate the real time situation. The air curtain blows the air

in downward direction. The flow within the air curtain is simulated with commercial computational

Fluid Dynamics (CFD) solver, where the momentum equation is modelled with Reynolds-Average

Navier-Stokes (RANS), K- ε turbulence model. The boundary condition set up is similar to the

experimental conditions. The CFD results are compared and validated against experimental results,

after the validation stage and the air curtain velocity profiles are compared for with obstacle

situations. The Kinetic energy plots, turbulent eddy dissipation plots and pressure variation plots are

generated at predefined planes are analysed and discussed in this paper.

Key words: Air curtain, Reynolds-averaged Navier – Stokes equation, K- ε turbulence Model,

velocity streamlines turbulent kinetic energy

INTRODUCTION

Air curtain devices provide a dynamic barrier instead of physical barrier between two

adjoining spaces (conditioned and unconditioned) thereby allowing physical access between them.

The air curtain consist of fan unit that produces the air jet forming barrier to heat, moisture, dust,

odours, insects etc. The Air curtains are extensively used in cold rooms, display cabinets, entrance

of retail store, banks and similar frequently used entrances. Study found that air curtains are also

finding applications in avoiding smoke propagation, biological controls and explosive detection

INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN

ENGINEERING AND TECHNOLOGY (IJARET)

ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 4, Issue 6, September – October 2013, pp. 118-126 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2013): 5.8376 (Calculated by GISI) www.jifactor.com

IJARET

© I A E M E

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976

6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6,

portals. According to research by US department of energy

by optimising the performance of super market display cabinet air curtain

drinks industry used equivalent of 285 tonnes of oil

most being used in cold storages. In developing countr

markets, retail stores, banks are not only limited to mega cities but they

part of suburban’s and small towns.

no more luxury but are necessary part of business development and economy. Hence study of air

curtain with respect to Indian climate is necessary to ensure optimised

which would leads to energy conservation. The saving of energy

boon for energy starving country like India.

Figure 1 Experimental set up without mannequin

(Photograph)

Figure 3 Meshing Details


6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME

119

portals. According to research by US department of energy 1875 MW energy will be saved

super market display cabinet air curtains. In 2002 t

used equivalent of 285 tonnes of oil per year to power its refrigeration

. In developing countries like India; the rise in cold storages, super

markets, retail stores, banks are not only limited to mega cities but they have also become an

. The effects of globalisation are inevitable. The air curtains are

necessary part of business development and economy. Hence study of air

curtain with respect to Indian climate is necessary to ensure optimised performance of air curtains

leads to energy conservation. The saving of energy (Electrical energy) will be always

boon for energy starving country like India.

without mannequin Figure 2 Geometry Model with obstacle

eshing Details Figure 4 Experimental set up

(Photograph)


October (2013), © IAEME

MW energy will be saved per year

In 2002 the UK food and

to power its refrigeration units with

like India; the rise in cold storages, super

have also become an integral

The effects of globalisation are inevitable. The air curtains are

necessary part of business development and economy. Hence study of air

performance of air curtains

(Electrical energy) will be always

Geometry Model with obstacle

Experimental set up with mannequin

(Photograph)



Figure 5 Plane Definitions

METHODOLOGY

In order to analyse the behaviour of the flow pattern of air curtain flow, it was decided to

analyse the pressure, turbulent kinetic energy and turbulent eddy dissipation plots of

air flow analysis was carried out using commercial software package ANSYS V13.0 Workbench

platform. As shown in Figure 1 the air curtain is mounted on the top of the doorway frame. The

doorway frame chosen is 2270 mm in height and 900 mm

mm. There are two slits opens in the domain; the flow jet is pushed by the blower in the domain

through these slits. The experimentation without insertion of mannequin are carried out first then

repeated after placing the mannequin in the flow of air curtain as shown in figure 4. The entire

experiment is carried out at isothermal conditions; air at 290C (+ 10C) at one atmosphere. The

velocity of leaving air from slits is 9 m/s, similar conditions are used for CFD an

is representative of air curtain flow velocity. As shown in Figure 2 the domain is extended

(surrounding area) to capture the flow of air leaving frame boundaries in directions of frame

openings. The frame walls are treated as imperme

while choosing the length of extended domain that the direct transverse flow of air curtain will not

cross the boundaries of the domain. Once the configuration is modelled, the mesh is generated

(Figure 3) in the workbench. The structured mesh (hexahedron mesh) is used to build the extended

domain and flow straightener. The frame portion is meshed with unstructured tetra mesh. An effort

was made to mesh the entire domain with structured mesh but due to compl

straightener the frame portion has unstructured mesh. The total mesh count is 385443 of which

59589 are tetrahedral cells and 325854 hexahedral cells. The minimum mesh quality is 0.3, total 708

cells falls within this range, as per

generated in Workbench is internally transferred to CFX

workbench platform. The flow within the air curtain is simulated within commercial Computational

Fluid Dynamics (CFD) solver, where the momentum equation is modelled with Reynolds

Navier-Stokes (RANS), K- ε turbulence model. The default domain is air at 290C. The inlet

boundary condition used is ‘normal speed’ at 9 m/s, since the actual turbulence data a

currently unavailable, for the present simulation the uniform turbulence intensity of 5% (medium

intensity) is used to model the inlet turbulence. The outlet condition is assigned to the extended

domain walls as average static pressure of 0 gaug

Pavilion dv6, with Intel CORE i3, 2.4 GHz processor, 8 GB of RAM. The convergence target was



120

Plane Definitions Figure 6 Result Validation


analyse the pressure, turbulent kinetic energy and turbulent eddy dissipation plots of



doorway frame chosen is 2270 mm in height and 900 mm in width, the breadth of the frame is 290



ing the mannequin in the flow of air curtain as shown in figure 4. The entire


velocity of leaving air from slits is 9 m/s, similar conditions are used for CFD analysis, this velocity



openings. The frame walls are treated as impermeable walls and are ‘no slip’ walls. It is ensured



in the workbench. The structured mesh (hexahedron mesh) is used to build the extended


was made to mesh the entire domain with structured mesh but due to complex geometry at the flow



cells falls within this range, as per the CFD Practices this is a good quality mesh. The mesh is

generated in Workbench is internally transferred to CFX-Pre, a CFD solver available with


ics (CFD) solver, where the momentum equation is modelled with Reynolds

turbulence model. The default domain is air at 290C. The inlet

boundary condition used is ‘normal speed’ at 9 m/s, since the actual turbulence data a



domain walls as average static pressure of 0 gauge magnitude. The computational platform is HP




Result Validation


analyse the pressure, turbulent kinetic energy and turbulent eddy dissipation plots of air curtain. The



in width, the breadth of the frame is 290



ing the mannequin in the flow of air curtain as shown in figure 4. The entire


alysis, this velocity



able walls and are ‘no slip’ walls. It is ensured



in the workbench. The structured mesh (hexahedron mesh) is used to build the extended


ex geometry at the flow



the CFD Practices this is a good quality mesh. The mesh is

Pre, a CFD solver available with


ics (CFD) solver, where the momentum equation is modelled with Reynolds-Average

turbulence model. The default domain is air at 290C. The inlet

boundary condition used is ‘normal speed’ at 9 m/s, since the actual turbulence data at inlet is



e magnitude. The computational platform is HP-




121

set at 1e-4 RMS; with continuity target error is 1e-4 kg/s. The convergence target achieved after 167

iterations.

RESULT AND DISCUSSION

The Figures 7 show the pressure at plane 1 when obstacle is not placed in the doorway. At

plane 1 the pressure is found continuously increasing from top to bottom in all the areas except at the

slits. The pressure at the slit and the pressure at surrounding are found different. There is very little

variation in the pressure at the door way plane 1. The pressure region entirely changes when the

obstacle is introduced in the door way. The high pressure regions are observed at the head, shoulder,

over the hands and above the ground. The reason behind this is stagnation of flow at these areas. The

flow is restricted resulting in increase of the pressure. Because of flow separation from the body the

low pressure region are observed between two high pressure areas around the head of the mannequin.

The expected low pressures are observed in the area below hand and between legs because of

absence of positive flow in this region. If the pressure is improved in this region then the flow patters

will be smooth and it will result in less infiltration.

Figure 9 and figure 10 show the pressure at plane 2 and plane 3 respectively. The pressure

variation, at plane 2 and at plane 3 occurs in similar pattern. The pressure variation is less in the door

way but high pressure zones are observed at the ground because flow hits the ground. The contour of

pressure can be observed in high pressure stagnation region in the Figure 9. Figure 10 and figure 12

show the pressure at plane 2 and plane 3 respectively when the obstacle is introduced in the door

way. The high pressure zones are observed over the head of mannequin and above the ground. The

high pressure region is observed near knee area and the waist of the mannequin. The high pressure

pocket is also observed at a distance of 0.6 m at waist high on the back side of the mannequin.

In 1937, Taylor and Van Karman proposed the definition of turbulence as ‘an irregular motion which

appears in fluids, gaseous or liquid’. Turbulent kinetic energy means the kinetic energy per unit mass

associated with eddies in the turbulent flow. Using the primed quantities to denote the velocity

fluctuations the Reynolds averaged kinetic energy of turbulent eddies can be written per unit basis as

k = ½ ( u’u’ + v’v’ + w’w’) (1)

Turbulent kinetic energy is produced by the fluid shear, friction or buoyancy or through

external forcing at low frequency eddy scale. Larger the size of turbulent kinetic energy means

higher energy content by eddies. The Turbulent kinetic energy extracted from the main flow to larger

eddies then to smaller the eddies.

Figure 13, Figure 15 and Figure 17 show the variation in turbulent kinetic energy across the

plane 1, plane 2 and plane 3 respectively when the obstacle is not placed in the door way of the air

curtain. Since the air is uniformly distributed and flow is smooth, the turbulent energy in the form of

eddies found on entire flow region with comparatively uniform intensity in flow region and in the

surroundings. Higher values of turbulent kinetic energy are observed at plane 3 in the flow region

below the slit up to 1.2 m from the top. The width of region is maximum 0.065 m at 0.9 m from the

top. This is because the flow from the both slit intermix in this region crossing the mid section area.

The variation in turbulent kinetic energy is found only in upper region when obstacle is introduced in

the door way. Figure 14, Figure 16 and Figure 18 show the variation in turbulent kinetic energy at

plane 1, plane 2 and plane 3 respectively when obstacle is placed in the doorway. Since flow is

unable reach to the ground the turbulent kinetic energy values are found much less below the waist

high area of the mannequin, i.e. below 1.2 m from the top of the doorway.

The dissipation of the kinetic energy of turbulence (the energy associated with turbulent

eddies in a fluid flow) is the rate at which the turbulence energy is absorbed by breaking eddies down



into smaller and smaller eddies until it is ultimately

expressed as the kinetic energy per unit mass per second, with units of velocity squared per second

(m2/s3). The kinetic energy of the turbulence is usually involved directly as a conserved property of

the flow. The rate of dissipation of turbulence kinetic energy is a little more complicated and we

must rely on some empirically based model. The various turbulence models differ mainly in the way

the dissipation rate is derived. In some cases the rate of dissipati

of the flow, for example a length

Figure 7 Pressure variations at plane 1

without obstacle

Figure 9 Pressure variations at plane 2

without obstacle



122

into smaller and smaller eddies until it is ultimately converted into heat by viscous forces. It is



The rate of dissipation of turbulence kinetic energy is a little more complicated and we


the dissipation rate is derived. In some cases the rate of dissipation is derived from other properties

Pressure variations at plane 1 Figure 8 Pressure variations at plane 1

with obstacle (mannequin)

Pressure variations at plane 2 Figure 10 Pressure variations at plane 2




converted into heat by viscous forces. It is



The rate of dissipation of turbulence kinetic energy is a little more complicated and we


on is derived from other properties

Pressure variations at plane 1


Pressure variations at plane 2




123

Figure 11 Pressure variations at plane 3 Figure 12 Pressure variations at plane 3

without obstacle with obstacle (mannequin)

Figure 13 Turbulent Kinetic Energy Variation Figure 14 Turbulent Kinetic Energy Variation

at plane 1 without obstacle at plane 1 with obstacle

Figure 17 Turbulent Kinetic Energy Variation Figure 18 Turbulent Kinetic Energy Variation

at plane 3 without obstacle at plane 3 with obstacle (mannequin)



124

Figure 19 Turbulent Eddy Dissipation Figure 20 Turbulent Eddy Dissipation








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scale. In the widely used k-ε turbulence model the dissipation rate (ε) is modelled as if it were itself a

conserved property. Figure 19, Figure 21 and figure 23 show the turbulent eddy dissipation at plane

1, plane 2 and plane 3 respectively when obstacle is not placed in the door way. The gradually

decreasing turbulent eddy dissipation can be observed at plane 1 with the effect of midsection below

which, the turbulent eddy dissipation values attend higher values as compared in horizontal plane.

The places where higher turbulent energy is observed, the higher turbulent energy dissipation rates

are observed at the same places. Figure 20; figure 22 and Figure 24 shows the turbulent eddy

dissipation at plane 1, plane 2 and plane 3 respectively when obstacle is placed in the door way

reveals similar observation as discussed above only with the difference in the magnitude.

Conclusion ; It is observed from the graph that in addition to velocity profile streamlines turbulent

kinetic energy and turbulent eddy dissipation parameters can also give insight in air curtain flow

performance. The experiments performed during this work confirm higher turbulent kinetic energy

signifies effective barrier where as lower turbulent kinetic energy indicates a weak air barrier.

Turbulent eddy dissipation is higher in the region where turbulent kinetic energy values are high and

are negligible in low turbulent kinetic energy region. Introduction of mannequin showed weak barrier

characteristics under arms and between legs. The turbulent kinetic energy results without mannequin

indicate (figure 17) weak barrier at the centre

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