The Research of Performance Comparison of Displacement and Mixing Ventilation System in Catering Kitchen

doi:10.4236/ojfd.2013.32A010

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Open Journal of Fluid Dynamics, 2013, 3, 61-68

doi:10.4236/ojfd.2013.32A010 Published Online July 2013 (http://www.scirp.org/journal/ojfd)

The Research of Performance Comparison of Displacement

and Mixing Ventilation System in Catering Kitchen

Jianping Yuan1, Longyan Wang1, Xiaofan Liu1, Z hixia He2

1Research Center of Fluid Machinery and Engineering, Jiangsu University, Zhenjiang, China

2School of Energy Resources and Power Engineering, Jiangsu University, Zhenjiang, China

Email: [email protected], [email protected]

Received May 27, 2013; revised June 4, 2013; accepted June 11, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

A commercial kitchen is a complicated en vironment where multiple compo nents of a ventilation syste m including hood

exhaust, conditioned air supply, and makeup air systems work together but not always in unison. And the application of

an appropriate ventilation system is extremely vital to keep the catering kitchen comfortable, which consequently pro-

motes the productivity and gains. Application of two systems (traditional mixing ventilation system and thermal dis-

placement ventilation system) is compared in a typical kitchen environment using computational fluid dynamics mod-

eling which was used to investigate the difference between mixing and displacement ventilation (DV). It was reported

in two parts, one on thermal comfort and the other one on indoor air quality. The results show that DV can maintain a

thermally comfortable environment that has a low air velocity, a small temperature difference between the head and

ankle level, and a low percentage of dissatisfied people, and may provide better IAQ in the occupied zone. So it was

persuasive that using thermal displacement ventilation in kitchen environment allows for a reduction in space tempera-

ture without increasing the air-condition ing system capacity.

Keywords: Kitchen; Displacement Ventilation; Mixing Ventilation; Numerical Simulation

1. Introduction

It has been well recognized that a modern commercial

kitchen is characterized by high heat loads and big tem-

perature difference. All cooking appliances release heat

into the limited space in the kitchen in the form of con-

vection or radiation. And we are aware that an uncom-

fortable temperature which people work in affects the

productivity. It was reported that a temperature increase

of 10˚F above the comfort level in the space may con-

tribute to a productivity loss as much as 30% (Wyon

1996). And recent regulation o n indoor air quality (IAQ)

for residential building have increased in severity, which

gives us a signal that the living environment with good

breathing air quality is significant for humans, not to

mention in the worse working space (kitchen). And it is

known that mechanical ventilation is the common me-

thod to improve the indoo r air, which can be divided in to

mixing ventilation (MV) and displacement ventilation

(DV) [1]. The goal of this paper is to find out the pros

and cons of both the two ventilation patterns applied in

the catering kitchen.

Most conventional kitchens adopt mixing ventilation

to cool the space. And the cool conditioned air is typi-

cally supplied through ceiling diffusers at a high dis-

charge velocity in a mixing ventilation system. This high

velocity is required to create a high momentum air jet for

efficient mixing of supply air with room air. It shows that

this kind of air distribution may not necessarily be the

best fit for a commercial kitchen, since high discharge

velocity creates unwanted air movement and cross-drafts

in the kitchen which make it hard to capture by hoods

and uncomfortable to feel at the same time. It has been

noted that an alternative air distribution system that sup-

plies air at floor level and return s air near the ceiling has

several advantages over conventional ceiling supply/

return systems [2,3]. Floor-supply systems distribute

conditioned air directly to the occupied zone that is sub-

stantially closer to the occu pants. Rather than mixing the

heat and contaminants in the space, as the mixing system

does, DV stratifies and displaces them out of the occu-

pied zone into the upper part of the space [4]. As a result,

the air velocity in the space is low, with no undesired

cross-drafts, which makes it easier for the hoods to cap-

ture.

J. P. YUAN ET AL.

Prior to the laboratory test, it is important to predict

the various performance of these two ventilation systems

using the CFD technique. Airpak software was used to

simulate these two systems numerically and the com-

parison of performances of both ventilations will be re-

ported in both thermal comfort and indoor art quality

(IAQ) two aspects.

2. CFD Model and Boundary Conditions

2.1. Physical Model

The computational model was developed based on a

commercial program, the Chinese restaurant kitchen of

Clancy Hotel (kitchen) Equipment Corporation Ltd. And

the object of research is the operation room of the

kitchen. The kitchen model entity was simplified prop-

erly to establish the physical model for research. The

room size was 9.0 m × 4.92 m × 3.0 m and the displace-

ment ventilation system was floor supply wind with both

sides while the mixing ventilation system was ceiling

diffuser wind. The sizes of supply air ports an d return air

ports are 0.4 × 1.2 and 0.6 × 0.6, respectively. Figure 1

is the plane figures of simplified models for both MV

and DV [5,6].

2.2. Basic Assumptions

The air flow patterns and hear transfer of the indoor

space are usually three-dimension turbulent problems,

moreover the influence of floating lift need to be taken

into consideration necessarily. So the basic hypotheses

for the physical models are shown below:

 The indoor gases are seen as incompressible.

 The gas movement within the space is stable and tur-

bulent.

 It meets the Boussinesq assumption, which means the

density is constant except for the buoyancy polyno-

mial in the momentum equations.

 The kitchen room is well sealed and has no other air

leak.

 Ignore the decentralized radiating influence of light-

ing bulb and other appl i ance.

9000

2200 2200 2300 2300

4200

6500

1160 1000

4920

Figure 1. Simplified size plane of kitchen.

2.3. Ventilation Parameters

The designed environment temperature is 27˚C, and the

boi le r power is 3.5 kW with uniform heat dissi p at io n . Th e

refrigerators are treated as just heat source, as well as for

the freezers, and the powers are 1019 W and 1221 W,

respectively. Cooling load for human bodies is selected

as working adults for 116 W each. The exhaust volume

of return ports is 1.32 m3/s; only carbon dioxide gas was

considered for gas pollution source, and CO2 concentra-

tion of supply air is 300 ppm while which of each boiler

is 900 ppm; the initial indoor pollution lev el is 300 ppm.

Supply air velocity is 0.25 m/s, temperature is 22˚C.

3. Performance Comparison of MV and DV

3.1. Airflow Track

Figures 2 and 3 show the indoor air flow track in 120

seconds for mixing ventilation and displacement ventila-

tion system. We figure out that after the supply air get

into the kitchen interior from both bottom sides wall,

they spread out rapidly in the room and divided into two

separating air streams because of the block of working

bench, consequently come across the hot air plume of

heat source as a result of being sucked to the upper zone

of the space. Part of air is discharged from the exhaust

hood vents, and the other is expelled from the top vents

for DV. As for MV, the supply air enter into the space

from the ceiling vents, an d then, the co ld cond itioning air

sink in due to the natural gravity, spread out at the bot-

tom of the room, encounter the hot plume and carried to

the top space.

3.2. Thermal Comfort

Under all the aspects of ventilation system performances,

the thermal comfort is the most important one. And the

thermal comfort obtained through the calculation of MV

and DV system will be compared in the following sides:

velocity distribution; temperature distribution; predicted

percentage dissatisfied (PPD).

For the airflow velocity distribution (Figure 4), as it

can be seen that the flow within th e space which is close

to the outside windows and the walls rise due to the high

temperature of the separates in the DV system. The heat

source like boiler and fridge freezer will generate mas-

sive heat plumes, which take the conditioned air in the

lower zone to the upper zone. Nevertheless, the down-

draft from the ceilings in the MV system seems like

ejecting flow which forms the relative high-speed area.

They first flow horizontally in all directions after reach-

ing the ground, and then they go up in the same way as

DV system. Generally the velocity magnitude of MV is

higher than DV and the airflow characteristic of spatial

attern is cross-flow, the supply flow goes down while p

J. P. YUAN ET AL.

Figure 2. Airflow track for MV.

Figure 3. Airflow track for DV.

the other upward moves. For DV, the flow pattern goes

up only among the breathing zone, however it moves up

and down for MV, whi ch definite l y induces di scomfort.

The temperature distribution on cut plane X = 4.0 for

MV and DV system are shown in Figures 5(a) and (b)

respectively. For DV, the temperature distribution from

bottom to the top of the sp ace increases gradually and the

temperature stratification is obvious. It can be concluded

that more distance space is from the supply air ports, the

less temperature gradient decreases. Furthermore, the air

spread in horizontal is not so apparent, and the tempera-

ture distribution in that direction is uniform relatively.

For MV, temperature change in horizontal is sligh tly lar-

ger than DV and the temperature distribution pattern

J. P. YUAN ET AL.

(a)

(b)

Figure 4. X = 2.9 cut plane velocity vector. (a) For MV system; (b) For DV system.

is similar to DV in general. To specially mention, the

homogeneity in horizontal direction for MV cannot be

assured if it’s higher speed velocity.

According to the acceptable three thermal environment

grades and thermal comfortable standard analysis applied

for the Chinese area recommended by domestic scholars

for PPD index, we can see the PPD is kept below 20% as

a whole, meanwhile the DV PPD overall is lower than

MV (Figure 6).

3.3. Indoor Air Quality

This paper investigates the IAQ index in the catering

kitchen through the analysis of carbon dioxide distribu-

tion, the average age of air to make the contrast study of

DV and MV system.

Figures 7(a) and (b) show the CO2 distribution of cut

plane X = 2.9 for MV and DV system respectively. It’s

easy to find out that in DV, the airflow around the work-

ers upward move, which means there’s no cross air in-

fection. In the direction of south of workbench and

worker there exist a higher concentration of contaminant

because of the effect of block, meanwhile it won’t affect

the IAQ since it’s beyond the breathing zone of humans.

Yet for MV system around the area next to the vents, the

J. P. YUAN ET AL. 65

(a)

(b)

Figure 5. X = 4.0 cut plane temperature distribution. (a) For MV system; (b) For DV system.

(a)

J. P. YUAN ET AL.

(b)

Figure 6. X = 2.9 cut plane PPD distribution. (a) For MV system; (b) For DV system.

(a)

(b)

Figure 7. X = 2.9 cut plane CO2 distribution. (a) For MV system; (b) For DV system.

J. P. YUAN ET AL.

(a)

(b)

Figure 8. X = 2.9 cut plane mean age of air distribution. (a) For MV system; (b) For DV system.

pollutant concentration is particularly large, which will

consequently affect the breathing air nearby. So we’ve

got the conclusion that the air distribution in MV system

is not so uniform as that in DV system.

The mean age of air from lower zone is less than

higher zone (Figure 8), which indicates the displacement

effect is stronger. One of the advantages for DV system

is that it ensures the stagnation period of the air around

the worker is less. Hence, the IAQ of the DV is better

than that of MV normally with the same magnitude of

flow velocity. Generally speaking, the air in the DV sys-

tem has higher displacement efficiency in comparison

with the air recirculation in the MV system. Moreover, as

demonstrated in the figure the mean ages of air for both

systems are all less than 100 seconds.

4. Summary

This paper analyses index of indoor environment quality

like the distribution of air flow velocity, temperature,

CO2 concentration, mean age of air, PPD in kitchen for

mixing ventilation and displacement ventilation system-

atically for the first time. It is concluded that the DV is

better than MV no matter in the aspect of the thermal

comfort or the IAQ and demonstrates the feasibility of

the application of DV system into kitchen environment.

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[3] E. Mundt, “Convection Flows above Common Heat

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[4] P. O. Fanger, “Thermal Environment Human Require-

J. P. YUAN ET AL.

doi:10.1007/BF02238059

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