Open Journal of Fluid Dynamics, 2013, 3, 61-68
doi:10.4236/ojfd.2013.32A010 Published Online July 2013 (
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
Received May 27, 2013; revised June 4, 2013; accepted June 11, 2013
Copyright © 2013 Jianping Yuan et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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-
Copyright © 2013 SciRes. OJFD
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-
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
Ignore the decentralized radiating influence of light-
ing bulb and other appl i ance.
2200 2200 2300 2300
1160 1000
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
Copyright © 2013 SciRes. OJFD
Copyright © 2013 SciRes. OJFD
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
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
Copyright © 2013 SciRes. OJFD
J. P. YUAN ET AL. 65
Figure 5. X = 4.0 cut plane temperature distribution. (a) For MV system; (b) For DV system.
Copyright © 2013 SciRes. OJFD
Figure 6. X = 2.9 cut plane PPD distribution. (a) For MV system; (b) For DV system.
Figure 7. X = 2.9 cut plane CO2 distribution. (a) For MV system; (b) For DV system.
Copyright © 2013 SciRes. OJFD
Copyright © 2013 SciRes. OJFD
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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