Table Of Content要 報
農業気象(J. Agr. Met.)38(4):403-408,1983
Effect of Environmental Temperatures
on the Inflatable Greenhouse
K. O. KESSEY* and P. G. GLOCKNER**
Department of Mechanical Engineering,
The University o£ Calgary, Calgary,
Alberta, Canada
Abstract
A model greenhouse, covered with a transparent plastic material, `Fabrene', was built without
vegetation and tested in Calgary, Canada, with the primary objective o£ determining the effect of
environmental temperatures on its performance characteristics.
It is found that when the temperature of the intake hot air (Ji) and the Reynolds Number (RN)
for air flow through the intake duct are kept constant, the performance of the greenhouse , charac-
terized by surface membrane heat losses, overall heat transfer coefficient, and efficiency, does not
depend a great deal on changes in environmental temperature (Ja) under conditions o£ zero Solar
Parameter (K). This finding is clearly confirmed by the fact that the Supply Air Temperature Drop
(Ji-Je) between the inlet and outlet of the greenhouse varied only slightly in spite of the large
variations in ambient temperatures under conditions of zero Solar Parameter and constant intake
hot air temperature and Reynolds Number.
In view of the above stated objectives of the
1. Introduction
present study, many tests were conducted for
This work is an extension of a study on a model approximately constant inlet hot air temperature
inflatable greenhouse reported in Kessey (1981) and flow rate. Once the controls for these variables
and Kessey and Glockner (1981). Most of it was were set at the desired level, the plant was run
carried out during the coldest period of the continuously for at least twenty-four hours during
1981/82 winter months in Calgary, Canada. which temperatures at twenty-eight locations in
The study was primarily aimed at determining the greenhouse and pressures in the supply duct
the effect of environmental or ambient tempera- were automatically recorded with the aid of a
tures on the operation of the model inflatable Data-Logger.
greenhouse.
2. Equations Used
Equipment and/or instrumentation used in the
earlier study, details of which are fully described A detailed analysis of the energy transfers in the
in Kessey (1981) and Kessey and Glockner (1981), inflatable greenhouse is given in Kessey (1981) and
was suitably modified in order to provide higher Kessey and Glockner (1981). The relevant equa-
air flow rate and greater heating capacity. Thus tions in dimensionless form, taken from these
the size of the interconnecting pipework was made references, are as follows:
larger and the heater capacity was increased from (a) Hot Air Heat Input:
3.0 to 6 kW.
(1)
Received 9 June, 1982. (b) Solar Energy Input:
* Visiting Professor , * * Professor and Head
Report No. 238 (The University of Calgary) (2)
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(c) Ground Loss: Effects of Ambient Temperature:
The dependence of performance characteristics
(3) on ambient or environmental temperature (J
a
(d) Membrane Surface Loss: Ta/To) is shown in Tables 1 and 2 for tests
performed on January 1, 1982.
The tables were drawn from the twenty-four
hour performance characteristics with the aid of
(4) a digital computer by first eliminating those
(e) Greenhouse Efficiency: quantities (including ambient temperatures) for
which the Solar Parameter,K, was non-zero, and
(5) rearranging the remaining ambient temperatures in
(f) Overall Heat Transfer Coefficient: ascending order of magnitude, with corresponding
parameters appended to each ambient temperature.
Hence, the effect of ambient temperature was
determined, exclusive of solar energy effects.
It is observed from Table 1 that when the Solar
3. Results
Parameter is zero, the Supply Air Temperature
This paper presents results based on data for Drop between greenhouse inlet and outlet , (Ji-Je),
the coldest four days of the testing period, seems to decrease insignificantly with increasing
December 29, 1981 to January 1, 1982. These ambient temperature provided that the Reynolds
results are typical of the performance trend of the Number for flow through the intake duct and
greenhouse model for the entire winter period. Inlet Air Temperature are kept approximately
Table 1. Dependence of greenhouse interior temperatures on dimensionless ambient/
environmental temperature for zero solar parameter.
Minimum Reynolds Number 2.30 × 105
Maximum Reynolds Number 2.32 × 105
Mean Reynolds Number2.31 × 105
Spread of Reynolds Number 0.01 × 105
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K.O. Kessey and PG. Glockner: Effect of Environmental Temperatures on the Inflatable Greenhouse
Table 2. Dependence of performance characteristics on
dimensionless ambient temperatures for zero
solar parameter.
Minimum Reynolds Number 2.30 × 105
Maximum Reynolds Number 2.32 × 105
constant. It is noted that the increase in dimen- hot air flows between inlet and outlet.
sionless ambient temperature from 0.892 to 0.912 In fact, reference to Table 2 reveals that, if the
(i.e,-29.6 to -24.1℃)resulted in a change of Solar Parameter is zero, the Thermal Efficiency
Supply Air Temperature Drop from 0.044 to 0.041 of the greenhouse is, like the Supply Air Tempera-
(i.e. 11.2 to 12.1℃). ture Drop, only slightly dependent on environ-
Under these same conditions it is, however, mental temperature changes encountered during
obvious from Tables 1 and 2, that greenhouse tese experiments.
performance, as characterized by its average local Fig. 1 depicts variation of Membrane Surface
temperatures,(Jf), surface film;Jc, core/center; Heat Loss (Ew)and Overall Heat Transfer Co-
Jb, base)appears to depend quite significantly on efficient(H)with Ambient Temperature. It is
ambient temperature changes. Under conditions apparent that the Surface Heat Loss of the green-
of zero Solar Parameter, the relatively large fluc- house does not depend significantly on variations
tuations in dimensionless ambient temperature in the Ambient Temperature when the Reynolds
mentioned above were accompanied by an equally Number and the Temperature of the inlet hot air
large rise in the Average Greenhouse Interior are kept constant and the Solar Parameter is zero.
Temperature from 1.035 to 1.050(i.e. 9.45 to We note from Table 2 that the increase in Ambient
13.76℃). Temperature from -29.6℃ to-24.1℃ brought
The Supply Air Temperature Drop indicates, about a small rise in the dimensionless Surface
directly or indirectly the effectiveness of the Loss from 110 to 119. However, the dependence
greenhouse in utilizing its energy inputs, particular- of the Overall Heat Transfer Coefficient apPears
ly the hot air heat input;for, it represents the to be more pronounced.
amount of stored-up energy or enthalpy of the The observation regarding the approximate con-
stancy of Supply Air Temperature Drop also
hat air, which the greenhouse "consumes" as the
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of solar energy still persisted even though the
Solar Parameter was zero in accordance with the
reading for solar energy.
The hours at which the readings were taken are
shown in appropriate tables in Kessey and Glockner
(1982); they are not given here for the sake of
brevity. If this `divergent' set of readings is ex-
cluded the results for December 30, 1981 are
consistent with those of the remaining three
selected test days. This study strongly supports
all of the main conclusions stated in Kessey (1981)
and Kessey and Glockner (1981), including the
fact that the performance of the inflatable green-
house is highly dependent on the Solar Parameter,
K, or solar energy input, Er. From Fig. 2 observe
the rapid rise in Membrane Surface Heat Loss, Ew ,
and Ground Loss, Eg, with rising solar energy, Er.
Fig.1. Dependence of performance characteristics The dependence of the inflatable greenhouse on
on outside temperature. the important modulus K will be analyzed in a
future paper.
explains why Surface Loss and, consequently,
Overall Heat Transfer Coefficient, vary only slight-
ly despite laxge variations in environmental temper-
ature, with Solar Parameter being zero.
Table 3 gives a summary of the variations in the
performance characteristics for the four days when
ambient temperatures were at their lowest during
the 1981/82 winter in Calgary. It is apparent that,
while the dimensionless ambient temperature
fluctuated significantly between 0.892 and 0.925
(i.e.-29.60℃ and -20.50℃)for December 31,
1981 and January 1, 1982, respectively, the
performance characteristics, remained relatively
constant. Dimensionless Membrane Surface Loss,
Fig. 2. Hourly energy transfer rates.
for example, varied between 110.10 and 124.90
while the Dimensionless Overall Heat Transfer
Coefficient changed from 730.3 to 930.2. The 4. Conclusions
Supply Air Temperature Drop varied between
The following conclusions, already discussed
0.410 and 0.469 (i. e. 10.65℃ and 11.81℃) in
above, are recapitulated for convenience;
spite of this wide variation in the environmental
1. Average Greenhouse Temperatures (surface
temperature.
film, centre/core and base) appear to be quite
It is also obvious from Table 3 that Ew, H and
dependent on ambient or environmental tem-
ηγ show a wide variation for the experiment from
December 30,1981(rows b). A careful study of peratures. They decrease with decreasing
ambient temperatures.
more comprehensive tables in Kessey and Glockner
2. Membrane Surface Heat Loss and, consequent-
(1982)indicates that the Supply Air Temperature
ly, the Overall Heat Transfer Coefficient are
Drop, for a particular set of readings, is almost
not affected to the same extent as greenhouse
twice as large as for any of the other fourteen sets
temperatures by changes in the environmental
of readings. This`divergent' set of readings was
temperatures.
taken at 17:00 hours when, apparently, the effect
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KU. Kessey and PG. Glockner: Effect of Environmental Temperatures on the Inflatable Greenhouse
Table 3. Dependence of performance characteristics on dimensionless
ambient temperature.
(a)Gives parameters for 29:12:81 Minimum Reynolds Number:1.877 × 105
(b)Gives parameters for 30:12:81 Maximum Reynolds Number:2.308 × 105
(c)Gives parameters for 31:12:81 Mean Reynolds Number:2.120 × 105
(d)Gives parameters for O1:01:82 Spread in Reynolds Number:0.431 × 105
Nomenclature D Diameter of greenhouse
d Diameter of blower discharge duct
Italic Letters
h Convective heat transfer coefficient
A Area I Intensity of radiation
Cp Specific heat at constant pressure k Thermal conductivity
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L Length of greenhouse f Surface film
l Thickness of insulation g Ground
m Mass flow rate i Inlet or insulation
Q Rate of heat or energy transfer in Interior
T Absolute temperature o Reference/datum
To 273.2°K, reference absolute temperature r Solar radiation
t Ordinary temperature s Supply air
U Overall heat transfer coefficient t Total
u Local velocity of air w Membrane wall
Greek Symbols
Acknowledgements
ρ Air density
The authors would like to thank the staff of
μ absolute viscosity
the Mechanical Engineering Workshop, especially
τ Transmissivity
Mr. Ben Sanders, for constructing the experimental
ηγ Efficiency involving solar energy
rig.
Dimensionless Moduli This research was made possible by a Strategic
Dimensionless energy flow Grant to the second author from the Natural
rate Sciences and Engineering Research Council of
Dimensionless overall heat Canada, Grant No. G0106.
transfer coefficient
References
Solar parameter
1. Kessey, K. O., 1981: Experimental investiga-
Prandtl Number tion of energy transfers including solar energy
in the inflatable greenhouse. July, 1981.
Reynolds Number
2. Kessey, K. O., and Glockner, P. G., 1981:
Dimensionless temperature Energy transfers in the inflatable greenhouse -
Part I. Dept. of Mech. Engg., 214, The
Subscripts University of Calgary, September 1981.
a Ambient/air 3. Kessey, K. O. and Glockner, P. G., 1982:
Energy transfers in the inflatable greenhouse -
b Base
Part II. Dept. of Mech. Engg., 221, The
c Core
University of Calgary, April 1982.
e Exit/outlet
空気吹込型温室 に関す る周囲の温度の影響
ケ ィ ・オ ー ・ケ ッ セ イ ・ ピー ・ジ ー ・グ ロ ッ ク ナ ー
(カルガリー大学機械工学部)
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