Experimental study on indoor air thermal equilibrium model of the tent

Transcription

1 Experimental study on indoor air thermal equilibrium model of the tent... Lili Zhang 1 *, Yan Yu 1, Long Xu 2,3, Wei Zhang 2 and Qiong Shen 1 1 College of Architectural and Urban Rural Planning, Sichuan Agricultural University, Dujiangyan, Sichuan, China 2 College of Architecture and Environment, Sichuan University, Chengdu, Sichuan, China; 3 Institute of Building Energy Efficiency, Sichuan Academy of Architecture, Chengdu, Sichuan, China... Abstract In this paper, the indoor air thermal equilibrium of the tent was modeled based on the law of conservation of energy, and the model was validated by experimental tests. The results showed that the experimental values coincided well with the theoretical calculation values, and the maximum deviation did not exceed 5%. In addition, we also analyzed the tent indoor air temperature on the sensitivity of the tent inner surface temperature and the air permeability, obtaining the fact that the tent inner surface temperature was the key factor of affecting the tent indoor thermal environment. *Corresponding author: zlili0720@163.com Keywords: tent; thermal equilibrium model; indoor air temperature; inner surface temperature; air permeability Received 26 October 2015; revised 21 December 2015; accepted 29 February INTRODUCTION Because of light weight, rapid putting up, easy dismantling and convenient transportation, tents are very suitable for those who need frequently transform workplaces and temporarily lose residence caused by sudden disasters. For example, as important disaster relief of earthquake resistance, tents played a significant role in the Wenchuan earthquake in 2008 and the Lushan earthquake in But according to the field test results in reconstruction and subsequent studies, we found that the indoor thermal environment of tents was extremely bad, and its formation process was different from that of routine building [1, 2]. So far, the domestic and foreign research focused on three aspects: (i) the performance of the tent fabric to improve the tent of antiultraviolet, color fastness, waterproof, flame retardant and so on [3]; (ii) the structure of the tent to increase the strength of the tent and increase the using area [4]; (iii) the indoor thermal environment of the tent and the measurements to improve its thermal comfort [1, 2, 5 15]. But the formation process and specialty of the tent indoor thermal environment were few studied from the theoretical point, and few results were achieved. As a result, the research on the formation mechanism of the tent indoor thermal environment is quite necessary. The key point of formation mechanism of the building thermal environment is the building thermal process. Thermal process formation of the building is not only the basic issue of architecture, but also the research frontier problems. For the routine building, the reason that its thermal process has always been the focus in this field, and large amount of literature has been published [16 22]. Traditional building envelope thermal process is influenced by internal and external interference quantity. In addition, due to the thermal inertia and the external disturbance, the heat transfer and the indoor temperature fluctuation presents certain attenuation and delay. However, the degree of attenuation and delay depends on the heat storage capacity of the building envelope. The thicker the building envelope is, the bigger the thermal inertia will be, thus performing better resistance to the outer interference, such as stone buildings, earthcovering buildings, cave dwellings etc. [23 26]. Compared with conventional buildings, the barrier of the tent to solar radiation is far less than that of the heavy building envelope. So, the tent is obviously influenced by the external disturbance. Therefore, it is essential to find out the basic characteristics of the tent thermal process. In this paper, the indoor air thermal equilibrium of the tent was modeled based on the law of conservation of energy, and the model was validated by experimental tests. # The Author Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com doi: /ijlct/ctw005 Advance Access Publication 4 April

2 Indoor air thermal equilibrium model of the tent 2 THE ESTABLISHMENT OF THE THERMAL EQUILIBRIUM MODEL Heat transfer of tents is a complex process which contains convection, thermal conductivity and radiation. On the one hand, there exists basic processes on the tent envelope surface such as heat absorption, heat reflection, heat release and heat conduction of the structure itself. On the other hand, the air temperature in the tent will be also constantly changed with time under the joint action of internal disturbance (including different kinds of furniture and internal personnel) and external disturbance (including the outdoor air temperature and other meteorological conditions such as solar radiation intensity) [27]. 2.1 The building physical model As shown in Figure 1, the heat transfer process of the ordinary tent mainly includes three parts: the heat transfer process of external disturbance through the envelope, the heat transfer process of internal disturbance and air infiltration. Among the heat transfer process, the difference between the tent and routine building mainly lies in: (i) the main material of the tent envelope which is different from the routine building s is porous medium (such as polyester and vinylon canvas) and this material has a certain porosity. So, the outdoor air can get into the tent by infiltration and mix with indoor air even when the tent in a closed condition. Meanwhile, the part of the air infiltration heat can affect the indoor air hot state and then change the indoor air temperature. (ii) According to the measurement of the solar transmittance on the tent envelope, the transmittance of each spectrum of wave bands within the sunlight wavelength range ( nm) shows that the transmittance of solar radiation is almost zero in ultraviolet region and visible region. But when it comes to the infrared region, the transmittance of solar radiation is quite high, which reaches 30%. After this part of the solar radiation heat transmission into the interior, the indoor air temperature of the tent will be eventually affected by the convection of each surface. 2.2 The mathematical model of the thermal equilibrium According to the analysis of the above building physical model, the heat balance equation of the indoor air temperature in the tent can be established as follows [27]: Q 1 ðtþþq 2 ðtþþq 3 ðtþ Q 4 ðtþ ¼0 Where Q 1 (t) denotes the convection heat transfer at t moment with each wall surface, W; Q 2 (t) denotes the convection heat gain at t moment, W; Q 3 (t) denotes the air infiltration heat at t moment, W; Q 4 (t) denotes the value-added of indoor air sensible heat per unit time at t moment, W. (1) Calculation of a variety of convection heat gain at t moment Q 1 ðtþ ¼ XN i A j a n j ½t jðtþ t a ðtþš Here, A j is the inner surface area of the j surface of the envelope, m 2 ; a j n the inner surface convection heat transfer coefficient of the j surface of the envelope, W/m 2.8C; t j (t) the inner surface temperature of the j surface of the envelope at t moment, 8C; t a (t) denotes the indoor air temperature at t moment, 8C. (2) Heat gain by all kinds of convection Q 2 ðtþ ¼q c 1 ðtþ qc 2 ðtþ Where q 1 c (t) denotes the convection heat loss from lighting, sensible heat of the human body and equipment at t moment etc., W; q 2 c (t) denotes the water evaporation sensible heat caused by the absorption of the room heat, W. (3) Heat gain by air infiltration ð1þ ð2þ ð3þ Q 3 ðtþ ¼L w ðtþðcrþ w ½t w ðtþ t a ðtþš ð4þ Where L w (t) denotes the air permeability at t moment, m 3 /h; (cr) w denotes the outdoor air heat capacity per unit volume, KJ/m 3.8C; t w (t) is the outdoor air temperature at t moment, 8C; t a (t) the indoor air temperature at t moment, 8C. (4) Added value of indoor air sensible heat in unit interval at t moment Q 4 ðtþ ¼VðcrÞ a t a ðtþ t a ðt 1Þ ð5þ Figure 1. Architectural physical model. 1, outdoor environment; 2, ground; 3, air heat; 4, IR infiltration; 5, solar heat; 6, air convection heat; 7, the sun reflects heat; 8, ground heat gain; 9, long wave radiation; 10, envelope heat gain. Where V is the room volume, m 3 ;(cr) a denotes the indoor air heat capacity per unit volume, KJ/m 3.8C; t a (t) isthe indoor air temperature at t moment, 8C; t a (t21) the indoor air temperature at t 1 moment, 8C; the time span, h. 37

3 L. Zhang et al. 3 MODEL ANALYSIS AND EXPERIMENTAL VERIFICATION 3.1 Model analysis It can be seen from formula (1) that the tent indoor air temperature is mainly affected by the load (including the heat conduction of the envelope, solar radiation and secondary reflection of the envelope etc.) and the air infiltration heat caused by the building envelope. But the inside heat load such as furniture and the internal heating load related to the people activities is both simplified to 0 so as to facilitate calculation. In this way, formula (1) is simplified to: X N i A j a n j ½t jðtþ t a ðtþš þ L w ðtþðcrþ w ½t w ðtþ t a ðtþš VðcrÞ a t a ðtþ t a ðt 1Þ ¼ 0 Due to the tent structure size, the internal disturbance, the external disturbance and the inner air temperature of the tent before t moment are all known, so formula (6) can be rewritten as: " # X N i A j a n j þ L w ðtþðcrþ w þ VðcrÞ a t a ðtþ L w ðtþðcrþ w t w ðtþþ VðcrÞ a t aðt 1Þ XN i A j a n j t kðtþ ¼0 Based on the calculation of formula (7), the indoor air temperature can be obtained: t a ðtþ ¼ A ja n j t jðtþ A ja n j þ L w ðtþðcrþ w þ½vðcrþ a =Š Make: þ ½L wðtþðcrþ w t w ðtþþ½vðcrþ a =Št a ðt 1ÞŠ A ja n j þ L w ðtþðcrþ w þ½vðcrþ a =Š A j a n j ¼ X j ðcrþ w ¼ Y VðcrÞ a ¼ Z ð6þ ð7þ ð8þ ð9þ ð10þ ð11þ Formula (12) reflects the influence factors of the indoor air temperature in the tent. When X j,y,z,t w (t) and t a (t21) of these factors are known with the size of the tent structure, internal and external disturbance determined, the main factors are the inner surface temperature t j (t) and air permeability L w (t). 3.2 Model experimental verification Throughout the study, the experiment was divided into two parts. The first part is to determine the air permeability of the tent. The indoor air temperature is further affected by air infiltration even when the tent is in a closed condition because of air permeability of tent envelope. Determining the amount of the air infiltration plays a crucial role for analyzing the indoor air heat balance models. In order to determine this part of the air infiltration amount, in this paper, a test platform is built, and the indoor air exchange rate in the tent is determined by CO 2 tracer gas dilution method. Based on the theoretical analysis of the thermal equilibrium model and experimental determination of key parameters (indoor air exchange rate), the second part is to test the indoor air temperature, the outdoor air temperature and the inner surface temperature of the tent in order to validate the thermal equilibrium model and analyze the sensitivity of the indoor air temperature influence factor Experimental apparatus The test apparatus used in the model experiment were Testo480 multifunctional measuring instrument and JTRG-II building thermal temperature and heat flux automatic test instrument. Test data included the indoor and outdoor air temperature, the inner temperature of each inner wall of the tent, the indoor and outdoor pressure and the concentration of carbon dioxide. Testo480 is the most new multifunctional measuring instrument for the air-conditioning system and indoor environment in the office buildings, residential and industrial buildings. Its temperature measurement range is 2200 to þ13708c, and the temperature measurement accuracy is +(0.38C þ 0.1% measurement value), the temperature resolution is 0.18C, the pressure measurement range is 225 to 25 hpa, the pressure measurement accuracy is +(0.02 hpa þ 0.1% measurement value), the pressure measurement resolution is hpa. Temperature measurement range of the JTRG-II is 220 to þ1008c, and the temperature measurement accuracy is +0.58C, the temperature resolution is 0.18C. When testing temperature of the model tent, the tent was placed in the outdoor without shelter and the doors and windows were closed. The experimental apparatus was calibrated before test. There: t a ðtþ ¼ X jt j ðtþ X j þ YL w ðtþþz þ ½YL wðtþt w ðtþþzt a ðt 1ÞŠ X j þ YL w ðtþþz ð12þ Experimental model and experimental scheme This study chose the scaled model tent to test which was proportional to the scale of the actual tent. The experimental model size was: width 600 mm, depth 600 mm, height of the slope roof ridge 600 mm, the front and both sides, respectively, cut two windows (150 mm 150 mm), a door of 100 mm 480 mm in the middle of the frontage. As shown in Figures 2 and 3, 38

4 Indoor air thermal equilibrium model of the tent single PVC-coated canvas was used as the tent envelope, whose thermal conductivity is W (m k) 21 and the thickness of 0.33 mm. The first part of the test is shown in Figures 3 and 4, the indoor test probe was arranged in the middle of the tent with the fan on its side (which made the CO 2 fully mixed inside the tent), and the outdoor test probe was arranged on the outside of the tent. Among them, the probe could record parameters such as temperature and CO 2 concentration at the same time. The position of the probe is 0.3 m high from the ground without obstacles. When testing, the doors and windows of the tent were closed. The second part of the test is shown in Figures 3 and 4. The tent model was tested locating points in the each center of the tent inner wall surface, one indoor point 0.3 m high from the ground in the center of the tent, one outdoor test point was arranged on the outside of the tent and was 0.3 m high from the ground. When testing temperature of the model tent, the tent was placed in the outdoor without shelter and the doors and windows were closed The confirmation of key parameters On 21 June July 2013, a total of 10 tests were carried out. The weather conditions were as follows: 21 June, showers, 298C; 24 June, showers overcast, 268C; 26 June, cloudy, 308C; 1 July, moderate rain to cloudy, 288C. Outdoor air pressure between 940 and 951 hpa. The indoor ventilation rate of the tent was obtained under different temperature difference and pressure difference, given in Table 1. Temperature difference and pressure difference in Table 1 are the average of the test period, temperature for indoor temperature minus the outdoor temperature, wind pressure for indoor pressure minus the outside pressure. Test results show that the number of the closed tent ventilation rate is greater than that of closed routine buildings a lot. That partly lies in the indoor and outdoor thermal pressure and wind pressure of the tent, and mainly due to that Figure 2. Tent model structure dimension drawing. Figure 4. The principle diagram of the experimental scheme. 1, multi-function measuring instrument; 2, probe; 3, tent; 4, fan; 5, entrance of gas; 6, gas suction pipe; 7, valve; 8, air bottle; 9, computer. Table 1. The test results of ventilation rate Serial number Experimental time Ventilation rate (times/h) Temperature difference (/8C) Pressure difference (/Pa) Figure 3. The tent field test model. 1 At 10:30 on 21 June At 11:30 on 21 June At 16:30 on 21 June At 17:30 on 21 June At 14:30 on 24 June At 15:30 on 24 June At 16:30 on 24 June At 16:00 on 26 June At 17:00 on 26 June At 10:00 on 1 July

5 L. Zhang et al. tents envelope is a porous medium having a certain permeability and the doors and windows of the tent closed are not strict. And the test results also show that the ventilation rate of closed experimental tents through the action of wind pressure and hot pressing fluctuates between 4 and 7 times/h under the outdoor climate condition Experimental verification When testing temperature of the model tent, it was, respectively, tested each envelope inner surface temperature and the indoor and outdoor air temperature with ignored the equipment load, the lighting load and the staff load in the tent. From the test results in Section 3.2, the ventilation rate fluctuates between 4 and 7 times/h when the tent is closed. In order to simplify the experiment and calculation, we verified this model by assuming the ventilation rate at 4.5, 5.5 and 6.5 times/h, respectively. A set of data was picked up from the test results for comparison, shown in Figures 5 7. It can be seen from the graphs that the theoretical value calculated by the thermal equilibrium Figure 5. Comparison between theoretical value and experimental value of indoor air temperature (the tent ventilation rate at 4.5 times/h). model mainly keep consistent with the test value obtained by experiment. And its relative error is within 5%. 4 THE ANALYSIS OF INFLUENCE FACTORS OF THE INDOOR AIR TEMPERATURE In order to better analysis the influence of the inner surface temperature change and air permeability change on inner temperature of the tent, formula (12) is derived by the following formula (13): t a ðtþ ¼t w ðtþ 8 9 j x j ½t j ðtþ t w ðtþš 1 þ t w ðtþyl w ðtþþt w ðtþ P þ >< N i j x j þ Z >= Z½t a ðt 1Þ t w ðtþš t w ðtþyl w ðtþþt w ðtþ P >: N i j x j þ Z >; ð13þ Formula (13) theoretically displays: When a certain air permeability, the higher the inner surface temperature of the tent, the higher the tent indoor air temperature. And when the tent envelope inner surface temperature is determined as well as the indoor air temperature must be higher than the outdoor temperature, the greater the air permeability, the smaller the indoor air temperature. In order to verify the theoretical analysis and compare the influence of the inner surface temperature and the air permeability on the indoor air temperature, the model tent in Section 3.2 was taken as the research object. And then a set of data was picked up from the test results (outdoor temperature, east, south, west, north, northern top, southern top and ground surface temperature was, respectively, 288C, 37.88C, 36.28C, 38.48C, 38.48C, 39.28C, 38.28C and 34.28C. The indoor air temperature was 35.38C). The inner air temperature of the tent was, respectively, theoretically calculated (shown in Figure 8) in the case of Figure 6. Comparison between theoretical value and experimental value of indoor air temperature (the tent ventilation rate at 5.5 times/h). Figure 7. Comparison between theoretical value and experimental value of indoor air temperature (the tent ventilation rate at 6.5times/h). 40

6 Indoor air thermal equilibrium model of the tent Figure 8. Temperature variation of indoor air. ventilation rate changes and the inner surface temperature changes (the inner surface temperature was, respectively, increased by 28C, 48C andreducedby28c, 48C basedonthe selected temperature test data; the ventilation rate changed from1to50times/h). In order to determine the sensitivity of the influence of the two factors on indoor air temperature, formula (14) was used to analyze this sensitivity. S AF ¼ DA=A DF=F ð14þ Where S AF denotes the sensitivity coefficient;4f/f denotes the change rate of uncertain factor F, %;4A/A denotes the corresponding change rate of the evaluation index A when uncertain factor F changes, %. Figure 8 shows that the indoor air temperature decreased with the decrease in the inner surface temperature of the tent while in other conditions unchanged, and the sensitivity coefficient is 1.02, the change is more obvious. However, there are just two ways to reduce the inner surface temperature of the tent. The one is to change the thermal performance of its envelope so as to increase its thermal inertia and retardance. The another way is to change the optical properties by increasing the reflectivity of the solar radiation and reducing its transmission and absorption rate of solar radiation. At the same time, we can also find that the tent indoor air temperature is gradually reduced with the increase in the ventilation rate, and its sensitivity coefficient is 0.001, with no significant effect. That is to say the fluctuation of the tent indoor air temperature is very little, almost negligible when the ventilation rate is at 4 7 times/h. So we can see that the thermal characteristic of tent envelope is essential to improve indoor air temperature. 5 CONCLUSION In this paper: (i) the thermal equilibrium was modeled based on the law of conservation of energy, the calculation method of the tent indoor air temperature changes was obtained through this theoretical model. Meanwhile, we verified the established model by confirming key parameters as well as carrying out experimental tests. The results showed that experimental results of values coincided well with the theoretical calculation, and the maximum deviation did not exceed 5%. Therefore, this model can be used to analyze the tent indoor thermal environmental conditions. (ii) The thermal performance of the tent envelope was the important factor to influence the tent indoor thermal environment. Its sensitivity coefficient is 1.02 and the influence of the air permeation rate can be negligible for its little effects. Its sensitivity coefficient is These above conclusions provide important theoretical support and technical guidance for studying and improving the tent indoor thermal environment. ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (No ) and Sichuan Provincial Education Department of key projects (Natural Science) (No. 15ZA0026). The authors gratefully acknowledge the experimental supports from Architecture and Environment of Sichuan University and Sichuan Academy of Building Research. And the authors gratefully thank all other researchers of the experimental team for their cooperation. REFERENCES [1] Tao W. Research on indoor thermal and humidity environment and simulation of improvement measures for relief tents. MSc Thesis. Chongqing University, [2] Changjiang T. Thermal environment and thermal equilibrium research on the ultra-thin building envelope. MSc Thesis. Sichuan University, [3] Guofan C, inventor. Heat preservation and heat insulation tent. China patent ZL August. [4] Hua X. Research on full-scale folding reticulated shell tent structure. MSc Thesis. Chang An University, June [5] Qi Y. Research on indoor thermal and humidity environment and improvement measures of tents. MSc Thesis. Chongqing University, [6] Jun L. Heating design for a certain tent. HVAC 1999;29:25 6. [7] Shi H, Mei Q, Deng A. Experimental study of radiant heating for a certain tent. J Logistical Eng Univ 2004;4:25 6. [8] Hu S, Meng Q, Wang C. Experimental research of improvement on thermal environment of tents by passive cooling. JLogisticalEngUniv2007;23:81 3. [9] Wanguo Y, Shaoxiang L, Wenfang W, et al. Research on army green heat reflective coatings for tents. Paint and Coatings Industry 2008;38:22 4. [10] Shi L. Large refugee tent design and research. MSc Thesis. Shenyang Architecture University, [11] Shaohan W, Guanqun W. Lower the temperature of tent. J Peking Univ (Health Sciences) 1959;01:52 4. [12] Du X. Canopy structure parameters on temperature and humidity effect of tent. MSc Thesis. Xi an Polytechnic University, [13] Enshen L, Lili Z, Qi Y, et al. inventor. The Portable nomadic tent with the inner top supporting rod. China patent ZL

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