Engineering Review, Vol. 38, Issue 3, 321-327, 2018.

321
________________________________________________________________________________________________________________________

OPTIMIZING AIR VELOCITY FOR THE UNDERFLOOR

FORCED VENTILATION TO PROTECT A REFRIGERATED
WAREHOUSE FROM FROST HEAVING
Jingfu Jia1* – Manjin Hao2 – Jianhua Zhao1

1
Ocean College of Hebei Agricultural University, Qinhuangdao 066003, Hebei, China
2
School of Architecture & Mechanics, Yanshan University, Qinhuangdao 066004, Hebei, China

ARTICLE INFO Abstract:

Article history: Forced or natural ventilation is the most common
Received: 14.04.2016. measure of frost heave protection for refrigerated
Received in revised form: 09.10.2016. warehouse floor. To optimize air velocity for the
Accepted: 03.11.2016. underfloor forced ventilation system of
Keywords: refrigerated warehouse, a steady state three-
Refrigerated warehouse floor dimensional mathematical model of heat transfer is
Forced ventilation set up in this paper. The temperature fields of this
Frost heave protection system are simulated and calculated by CFD
Steady three-dimensional heat transfer software PHOENICS under different air velocity,
Heat transfer performance 1.5m/s, 2.5m/s or 3.5m/s. The results show that the
Air velocity optimized air velocity is 1.5m/s when the tube
spacing is 1.5m.
DOI: https://doi.org/10.30765/er.38.3.9

1 Introduction the base soil could be maintained at a temperature

above 0°C [1-3], and it wouldn’t freeze.
Refrigerated warehouse can provide suitable Several commonly used frost heave protection
humidity and low temperature environment for food, measures consist of electrical heating, pumping of
so it has been an important part of food cold chain. warming liquids, forced or natural ventilation, etc.
At present, it has been widely used in food products Electrical heating measure of frost heave protection
factory, dairy factory, hotel, supermarket, and other refers to burying electric heating wire mesh in
departments. It is mainly used for the constant concrete cushion under the floor insulation layer, and
temperature storage and frozen processing of food supplying power to electric heating wire mesh on
such as dairy products, meat, aquatic products, time. However, it would consume lots of electricity,
poultry, fruits and vegetables, cold drink, and so on. and must be strictly protected from short circuit.
Because the refrigerated warehouse is at low Pumping of warming liquids measure means
temperature environment for years, moisture in the embedding liquid pipelines in concrete cushion under
base soil is liable to be frozen. Once the soil is frozen, the floor insulation layer, supplying heated liquid to
the volume then would expand. This would cause the pipelines and circulating them by pump. However,
ground fracture and the entire building structure to large quantity of liquid pipelines should be used in
deform. The seriously frozen would result in this system, and the hidden danger of corrosion
refrigerated warehouse not being used. Therefore, in leakage in the connection place should not be
addition to adding effective insulating layer to ignored. Forced or natural ventilation measure refers
refrigerated warehouse floor, some frost heave to burying air tubes in concrete cushion under the
protection measures must also be taken. By doing so, floor insulation layer and supplying heat by forced or

*
Corresponding author. Tel.: +86 335 3150030
E-mail address: jjf0369@163.com.
322 J. Jia, M. Hao, J. Zhao: Temperature field simulation…
________________________________________________________________________________________________________________________

natural ventilation. Floor without hidden danger from used as an arithmetic element of the heat-transfer
safety is the most common measure of frost heave model, as shown in Fig. 2. The size of the model is
protection. Due to the stable air velocity, the effect of defined as X*Y*Z=1500mm*3900mm*1000mm.
forced ventilation measure is better than of natural The top-down structure layers of the refrigerated
ventilation. To facilitate centralized management, warehouse floor are [6]: reinforced concrete surface
fans are placed in ventilator room nearby the course, cement mortar protection course, asphalt felt
refrigerating station. The schematic diagram of the damp-proof course, rigid polyurethane foam heat
overall ventilation system is shown in Fig. 1. insulating layer, asphalt felts vapor barrier, cement
mortar leveling course, precast concrete board,
medium sand packing layer(buried concrete
ventilation tube within). A soil layer is below
structure layers of floor.

Figure 1. The schematic diagram of the overall

ventilation system.
Figure 2. The calculating element of heat-transfer
Some empirical values are often used to design and model for refrigerated warehouse floor.
construct refrigerated warehouse floor ventilation
system of frost heave protection for years. Deeper 2.2 Model assumptions
researches on this system are not widely known,
unless studied from Zukun GAO and Ran ZHOU et The heat transfer process of the refrigerated
al [4, 5]. So a systematic research is of practical warehouse floor is very complex in fact. In order to
significance to direct design, construction, and better facilitate solving and analyzing, the heat-transfer
operation.. model is simplified and assumed reasonably. These
To optimize air velocity for the underfloor forced assumptions are as follows [7-13]: Firstly, the heat
ventilation system of refrigerated warehouse, a transfer process of the refrigerated warehouse floor is
steady state three-dimensional mathematical model assumed to be a steady state heat three-dimensional
of heat transfer is set up in this paper. The heat- conduction process. Secondly, the materials in the
transfer model is simplified reasonably, and the same structural layer of the floor are assumed to be
calculation conditions are defined according to the homogeneous, isotropic, and with constant physical
heat-transfer process. The temperature fields of this properties. Lastly, the thermal contact resistance
system are simulated and calculated by CFD software between each layer, the thermal resistance of the
PHOENICS under different air velocity, 1.5m/s, asphalt felt layer, and the moisture transfer are all
2.5m/s or 3.5m/s. assumed to be negligible.

2 Materials and methods 2.3 Equation and boundary conditions

2.1 An arithmetic element of heat-transfer model The research object is the cold storage room floor of
refrigerated warehouse in Tianjin area of China in
A tiny part of the cold storage room floor and soil winter.. There is no heat source in the refrigerated
layer of refrigerated warehouse is picked out and warehouse floor, so the steady state three-
Engineering Review, Vol. 38, Issue 3, 321-327, 2018. 323
________________________________________________________________________________________________________________________

dimensional differential equation of heat conduction Where x,y  c ^ d , g ^ h , z  (0 ,1) .

of the arithmetic element of the heat-transfer model The supply air temperature in ventilation tubes of the
may be described as following:
refrigerated warehouse floor is defined as: tin =10°C.
The inner diameter and outer diameter are:
 2t  2 t  2t
  0 (1) d1 =250mm, d 2 =316mm. The boundary condition of
x 2 y 2 z 2
outer surface of ventilation tube is:

V  ρ  c p  t out  t in  
The boundary conditions of the arithmetic element
1 d2 
are defined as following: t  t w2  t f  1  ln  K  l 
The design temperature of the cold storage room K l  2πλ d1 
floor is defined as: t n =-20°C. There are fan blowers (6)
in the cold storage room, therefore the heat
convective coefficient of the upper surface of the cold where, t w2 is the temperature of the outer surface of
storage room floor is defined as: α n = 12W/(m^2•°C) tin  tout
ventilation tube (°C), t f  is mean air
[14]. So the boundary condition of the upper surface 2
of the cold storage room floor is: temperature of ventilation tube (°C), V is the air
flow of the system (m^3/s),  is the air density
t
λ y  L2  αn tn  tnb   12   20  tnb  (2) (kg/m^3), c p is specific heat at constant pressure of
y
air (J/(kg•°C)), t out is the mean temperature of air at
where,  is the heat conduction coefficient of each outlet of the system (°C), K is the coefficient of
kind of material (W/(m•°C)), t nb is the temperature heat-transfer between air and tube wall in ventilation
tube on unit length (W/(m•°C)), l is total length of
of the upper surface of the cold storage room floor
ventilation tube (m).
(°C).
The soil temperature is defined as: t s  10.4 °C. The
3 The results and discussion
soil temperature is derived from the minimum mean
soil temperature of 3.2m deep in Tianjin city during 3.1 Simulation results of the heat-transfer model
the months of March and April, over the years [1].
So the boundary condition of the lower surface of the When the ventilation system is running under
heat-transfer model is: different air velocity, 1.5m/s, 2.5m/s or 3.5m/s, and
tube spacing is 1.5m, and the other simulation
t y＝ 0  t s  10.4 (3) conditions are not changed, numerical simulations of
the heat-transfer model of the refrigerated warehouse
In the refrigerated warehouse floor, ventilation tubes floor are performed using CFD software
are symmetrically placed in terms of certain spacing PHOENICS. The obtained temperature fields of XY
section of the refrigerated warehouse floor model in
in X-direction, so in the arithmetic element, as shown
in Fig. 2, two boundary surfaces of the model in X- each case are shown in Fig. 3, Fig. 4 and Fig. 5. While
direction are approximately seemed as adiabatic air velocity is 3.5m/s, the temperature fields of upper,
surfaces and the boundaries are: middle and lower surface for heating layer are shown
in Fig. 6, Fig. 7 and Fig. 8. The temperature
distribution curves of points at Z=1m on upper,
t t
x 0  x  L1 0 (4) middle and lower surface for heating layer in each
x x case are shown in Fig. 9, Fig. 10 and Fig. 11. The
temperature distribution curves of points at Z=1m on
Similarly, the two boundary surfaces of the model in lower surface for heating layer in each case are shown
Z-direction are also approximately seemed as in Fig. 12.
adiabatic surfaces and the boundaries are:

t t
z 0  z 1 0 (5)
z z
324 J. Jia, M. Hao, J. Zhao: Temperature field simulation…
________________________________________________________________________________________________________________________

Figure 6. The temperature field of upper surface for

heating layer while air velocity is 3.5m/s.

Figure 3. The temperature field of XY section of

refrigerated warehouse floor while air
velocity is 1.5m/s.

Figure 7. The temperature field of middle surface

for heating layer while air velocity is
3.5m/s.

Figure 4. The temperature field of XY section of

refrigerated warehouse floor while air
velocity is 2.5m/s.

Figure 8. The temperature field of lower surface for

heating layer while air velocity is 3.5m/s.

When underfloor forced ventilation system is running

and air velocity is 1.5m/s, the obtained numerical
results are shown in Fig. 3, Fig. 9 and Fig. 12. The
average temperature of the upper surface of heating
layer is 1.290°C. The average temperature of the
lower surface of heating layer is 2.322°C. The
temperature fluctuation amplitudes of each point at
Z=1m on upper surface of heat layer are
7.723°C(from -2.186 to 5.537°C).
Figure 5. The temperature field of XY section of
refrigerated warehouse floor while air
velocity is 3.5m/s.
Engineering Review, Vol. 38, Issue 3, 321-327, 2018. 325
________________________________________________________________________________________________________________________

Figure 12. The temperature curves of points on

lower surface for heating layer while air
Figure 9. The temperature curves of points on velocity is 1.5m/s, 2.5m/s and 3.5m/s.
upper, middle, and lower surface for
heating layer while air velocity is 1.5m/s. The temperature fluctuation amplitudes of each point
at Z=1m on lower surface of heat layer are
4.424°C(from 0.387 to 4.811°C). The average
temperature of the section at Z=1m is 4.54°C.
When underfloor forced ventilation system is running,
and air velocity is 2.5m/s, the obtained numerical
results are shown in Fig. 4, Fig. 10 and Fig. 12. The
average temperature of the upper surface of heating
layer is 2.190°C. The average temperature of the
lower surface of heating layer is 3.077°C. The
temperature fluctuation amplitudes of each point at
Z=1m on upper surface of heat layer are
7.499°C(from -1.164 to 6.335°C). The temperature
fluctuation amplitudes of each point at Z=1m on
lower surface of heat layer are 4.330°C(from 1.187 to
Figure 10. The temperature curves of points on 5.517°C). The average temperature of the section at
upper, middle, and lower surface for Z=1m is 5.124°C.
heating layer while air velocity is 2.5m/s. When underfloor forced ventilation system is running
and air velocity is 3.5m/s, the obtained numerical
results are shown in Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig.
11 and Fig. 12. The average temperature of the upper
surface of heating layer is 2.371°C. The average
temperature of the lower surface of heating layer is
3.266°C. The temperature fluctuation amplitudes of
each point at Z=1m on upper surface of heat layer are
7.338°C(from -0.912 to 6.426°C). The temperature
fluctuation amplitudes of each point at Z=1m on
lower surface of heat layer are 4.234°C(from 1.418 to
5.652°C). The average temperature of the section at
Z=1m is 5.242°C.

3.2 Numerical results analysis

Figure 11. The temperature curves of points on
upper, middle, and lower surface for In terms of the obtained temperature distribution
heating layer while air velocity is 3.5m/s. figures shown in Fig. 3 to Fig. 12, the numerical
results are discussed. In each model, the temperatures
326 J. Jia, M. Hao, J. Zhao: Temperature field simulation…
________________________________________________________________________________________________________________________

of each point on lower surface of heat layer are more to increase. The cold load of refrigerated warehouse
uniform, and the temperature fluctuation amplitude is will increase accordingly , too. This will lead to more
smaller than which of points on upper and middle serious waste of energy. At the same time, the storage
surface. The closer the distance to the tube center is, quality of cargos near the refrigerated warehouse
the temperatures of each point on the upper, middle, floor will be affected. Chibin YU [6] pointed out that
and lower surface of the heat layer are higher. better average temperature of the heating layer is
When air velocity is larger, the temperature between 1°C and 2°C. Therefore, based on the
fluctuation amplitudes of points on the upper and simulation conditions and calculated results of this
lower surface of heating layer are both smaller, and paper, when the tube spacing is 1.5m, the optimized
the average temperatures of the upper and lower air velocity is 1.5m/s.
surfaces are both higher. If air velocity is enlarged,
the heat convective coefficient of the inner surface of References
pipe would increase, and the heat exchange capacity
would enhance. Therefore, when air velocity is larger, [1] SBIT: Code for design of cold store, China
the heat exchange amount is larger, too, and the cold Planning Press, Beijing, 2001.
energy transferred from refrigerated warehouse floor [2] RPEAOMC: Refrigeration technology for cold
to soil layer would be substantially reduced. For the storage, China Financial and Economic
higher temperature of the lower surface of heating Publishing House, Beijing, 1980.
layer, the moisture of soil can effectively prevent [3] Tan, X.D.: Cold storage construction, China
from freezing. So the refrigerated warehouse floor Light Industry Press, Beijing, 2006.
can refrain from frost heaving. However, [4] Gao, Z.K.: Heat transfer calculation of the cold
accompanied by the increase of air velocity, the heat store mechanically ventilated for preventing the
exchange capacity of tube would not infinitely ground from frost heaving, Heating Ventilation
enhance. According to the numerical results, the & Air Conditioning, 26 (1996) 4, 68-71.
difference of heat exchange capacity of tube is [5] Zhou, R. et al: Numerical simulation of
inconspicuous when air velocity is respectively temperature field of cold storage ventilation
2.5m/s and 3.5m/s. ground structure layer, Cold Storage
Technology, 30 (2007) 4, 37-40.
4 Conclusion [6] Yu, C.B.: Practical handbook of refrigeration
and air-Conditioning engineering, China
Based on the developed model of refrigerated Machine Press, Beijing, 2001.
warehouse floor, by performing numerical [7] Jia, J.F., He, W.: Effect of Ventilation Tube
simulations and analyzing the obtained temperature Diameter on Thermal Performance of Food
distributions, conclusions are summarized as Refrigerated Warehouse Floor Antifreezing
following: Mechanical Ventilation System, Advance
When the ventilation system is running under Journal of Food Science and Technology, 7
different air velocity, 1.5m/s, 2.5m/s or 3.5m/s, and (2015) 5, 320-325.
tube spacing is consistently 1.5m, the average [8] Li, H.M., Jin, W.L.: Temperature field
temperature of the lower surface of heating layer in simulation of building envelops, Journal of
each case will be always above 0°C, and the soil Building Structures, 25 (2004) 6, 93-98.
under heating layer will not be frozen for ever. The [9] Tsilingiris, P.T.: Thermal flywheel effects on the
effectiveness of frost heave protection in each case is time varying conduction heat transfer through
better. structural walls, Energy and Buildings, 35
The air velocity has certain influence on the (2003) 10, 1037-1047.
temperature distributions of heating layer of the [10] Jia, J.F., He, W.: Effect of thickness of heat
refrigerated warehouse floor. However, when air insulating material in bulkhead on cabin
velocity increases to a certain extent, the influence of temperature of reefer ship for agricultural
it on the heat transfer performance of floor is products, Journal of Chemical and
obviously weakened. Pharmaceutical Research, 5 (2013) 11, 403-408.
When other simulation conditions are not changed, [11] Sui, X.M., Han, G.H., Chen, F.: Numerical
the larger air velocity is, the better effectiveness of simulation on air distribution of a tennis hall in
frost heave protection there is. But the larger air winter and evaluation on indoor thermal
velocity may cause the temperature of heating layer
Engineering Review, Vol. 38, Issue 3, 321-327, 2018. 327
________________________________________________________________________________________________________________________

environment, Engineering Review, 34 (2014) 2, [13] Lv, Z.: Analysis on the advantages and
109-118. disadvantages of several kinds of building
[12] Wang, H.X.: Technical requirements and external wall thermal insulation technology,
measures of energy-saving insulation wall, Jiangsu Building Materials, 26 (2006), 40-42.
Energy Conservation Technology, 3 (2002), 36- [14] Li, J.H., Wang, C.: Refrigerating house design,
37. China Machine Press, Beijing, 2003.