Wet Transfer Effect on the Oxygen Content in the Underground Tunnel. XIE Haimin 1, a, LI Maode 2,b

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vol. 563, pp 9-3 doi:10.408/ 014 Trans Tech Publications, Switzerland Wet Transfer Effect on the Oxygen Content in the Underground Tunnel XIE Haimin 1, a, LI Maode,b 1 Information Center of Ningbo Municipal Civil Defense Office, Zhejiang, China Mechanical Engineering School of Tongji University, Shanghai, China a hilong000@hotmail.com, b limaode@tongji.edu.cn Keywords: porous medium, wet transfer model, oxygen content Abstract. In this article the research methods and theoretical models of moisture transfer in the porous medium material are reviewed and summarized. The optimal theoretical model of moisture transfer in the underground tunnel is found out. Through the comparison among different calculating methods of surface moisture content, the limestone material moisture equilibrium curve and the variation curve of time-dependent moisture transmission coefficient and the amount of moisture transfer are calculated. Based on all the statistics above, using the state equation of ideal gas, the variation trend of time-dependent oxygen content under the condition that the moisture transfer amount increased in the underground tunnel is gained, making ways and offering reference for further related research. Introduction In recent years, with the rapid development of the cities, large amount of rural population pour into the town, resulting in crowded urban space, therefore underground construction especially underground tunnels are greatly developed. As most of the underground tunnel maintenance structure (covered structure and bedrock or soil) is "infinitely thick" (deeply buried) and it has lots of properties, such as defilade, sunshine isolated, less affected by outside air flow, so that the building has a strong underground heat storage capacity, good thermal stability and it is subjected to moisture. Therefore, the "tide" "stuffy" has become the main features of the underground tunnels. This article focuses on the research on the wet transfer from the tunnel wall. Through the simplification of the theoretical model according to the actual conditions, the author obtains the wet transfer effect on the oxygen content in the underground tunnel. Building materials are divided into three categories: (1) non-porous medium, such as glass; () Isothermal and hygroscopic porous medium, such as wood, clay; (3) Capillary porous medium, such as brick, concrete, gypsum boards. Therefore the materials around the tunnel such as soil, rocks and building materials mostly are complicated porous medium. In the holes of these porous medium can be filled with wet air, liquid water even ice. The structural characteristic of the porous medium determines its extremely unstable properties. The oxygen content of the underground tunnel directly affects the health of maintenance personnel as well as the normal operation of the equipment. When the air pressure of the tunnel is reduced, the oxygen content declined. Not only will cause inconvenience to the overhaul of the tunnel, but also affect comfort of passengers. Therefore, the author obtains the air pressure curve and the oxygen content curve through simplification of the theoretical model. Theoretical model of wet transfer for porous medium Scholars from various countries analyzed the wet transfer theoretically, and proposed the theoretical model for wet transfer of porous medium. There are three major categories: the first one is based on the Luikov s phenomenological theory who regards the temperature and volumetric wet All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-1/0/16,06:03:3)

2 30 Sensors and Materials: Advanced Researches content as parameters; the second is based on the Philip and de Vries about heat and mass transfer model in soil mechanics filed to handle the porous medium as continuous medium and to develop the non-isothermal wet transfer theory thereby to establish the heat and mass transfer coupled model in double driving fields; The third is based on Whitaker s average volume theory, who establishes a continuum model of mass, momentum and energy conservation. The models above are mostly achieved in the fields of soil and dryness. I.Budaiwi studies the heat transfer and wet transfer properties of porous medium deeply. They believe that the wet transfer in porous medium includes two parts: gas-state diffusion and liquid-state diffusion. When the quantity of the wet content is less than its inherent one, the liquid-state diffusion could be ignored. YAN Zengfeng modifies the theory that I.Budaiwi presents and obtains a new set of heat and wet diffusion equation by means of simplifying the mass diffusion equation by temperature gradient. WsatMa = DV ρarvt (1) m τ ξρ M m w T * T Ma ρ mcm = λ L( T)( ρadt V m ) () τ Mw Where, W sat is saturated humidity of air, kg/kg. ξ is the slope of the wet balance curve. L (t) is the latent heat of phase change, J/kg. D V is the vapor diffusion coefficient, m /s. ρ a is the air density, kg/m 3. M a is the molar mass of air, g/mol. M W is the molar mass of water molecules, g/mol. C m is the specific heat of rammed earth, J/kg K. λ* is the effective thermal conductivity of the material, W/(m K). The wet transfer from porous medium The wet source of underground construction mainly includes three parts: (1) Wet transfer caused by the remaining water of construction, groundwater seepage through the coating layer and wet air permeability through the wall of the coating layer; () Wet transfer caused by outside air; (3) The internal source. There are three ways of combination to the water and materials: The chemical combination, the chemical-physical combination and the physical combination. The chemical combination which is presented in the materials in molecular crystal type is strong. And it actually does not participate in the wet exchange; Water distributed directly on the particle surface and combined with each other by molecule interactions is physical combination; Physical and chemical binding mode can be defined as such, that it finds its place in the materials and capillary pores, which is quite independent from physical and chemical bond, whereas keeping this bond steady with the help of the surface tension of water and the moist material force. The estimated value of the wet transfer from porous medium Zhang Hualing gives the estimation method for wet transfer from the maintenance structure of the underground plants in her paper Study of the Thermal and Humid Environment in Hydroelectric Station Underground Plants. In order to facilitate the calculation, the remaining water of construction and the seepage are neglected, so that wet transfer from the wall can be calculated according to the following formula in the absence of measured data. W = AJ m (3) Where, W is the wet transfer from the wall (g/h). A is the surface area of the liner layer (m ). J m is wet transfer quantity per unit surface area (g/m h), as shown in Table 1. It should be noted that there will be some error due to the air condition of the building has not be investigated. Table 1. Wet transfer from the material structure type wet transfer(g/m h) structure type wet transfer(g/m h) Limestone unlined 7~8 Concrete adherently lined 1~3 Granite unlined 4~5 Concrete lined ~0.5

3 Applied Mechanics and Materials Vol Influence of wet transfer on the content of oxygen. According to the model made by Mendes N, ZHONG Zhihui simulates the wet bulk quantity within one hour by the heat and moisture transfer simulation software, as Fig3. And he obtains the amount of the wet transfer from the wall within one hour, 7.3mg/m s. The relationship between the dry air pressure and humidity are derived in this article. And then we get the dry air pressure curve shown in Fig.4 through the wet transfer curves shown in Fig.3. The components of the normal air calculated by volume include: nitrogen (N ) accounted for about 78%, oxygen (O ) 1%, rare gas 0.94%, carbon dioxide (CO ) 0.03% and other gas and impurities 0.03%. Therefore, it is easy to get the partial pressure of oxygen, as in Fig.5. According to the dry air pressure, the derivation process as follows. The known parameters are the length L=1000m,the diameter D=4m, the ambient temperature T=99K, and the moisture content d=6g/kg. According to the saturated steam table, we can look up P s = MPa. φ=p v /P s (4) P v =φp s =80% =688Pa P v =ρ v R v T (5) ρ v =P v /(R v T)=688/(461 99)=0.0195kg/m m v (t 0 )=ρ v V (6) Therefore we can get the quality of the water vapor contained at the initial moment in the tunnel, that is m v (t 0 )= =44.9kg. The fitting equation getting from Fig.3 is as followings: f(t)=-3e-1t 3 +E-8t -5E-5t (7) By the integral of formula (7), we can get the total wet transfer from the tunnel wall at times. m = t v( t) f( t) Adt + mv( t0) (8) 0 We obtain the partial pressure of the water vapor at seven times, t=500s, 1000s, 1500s, 000s, 500s, 3000s, 3600s, by means of inserting the (7) to (8). Since there is air leakage between the outside and inside the tunnel, the total pressure in the tunnel is equal to atmospheric pressure. Therefore we can get the partial pressure of the dry air according to the total pressure. Table. Partial pressure of each component t(s) P vapor (Pa) P air (Pa) P O (Pa) Fig.1 Limestone equilibrium humidity curve Fig. Limestone wet transfer coefficient

4 3 Sensors and Materials: Advanced Researches Fig.3 Wet transfer profile Fig.4 Partial pressure of each component Conclusions (1) We summarized the wet transfer characteristics of the porous medium materials, and found the theoretical model for the study of the wet transfer characteristics of underground tunnel wall. () We identified the equilibrium moisture curves and wet transfer coefficient of the underground tu nnels lime-stone material and the amount of the moisture transfer versus time. (3) By the deformation and derivation of ideal gas equation and the integral of curve fitting equation of the wet transfer quantity varies with time, we got the total amount of wet transfer of the under ground tunnel versus time. (4) We got the water vapor pressure from the amount of wet transfer by the ideal gas equation. Since there is the air leakage from the tunnel wall, so that the pressure inside and outside the tunnel is the same. (5) According to the partial pressure of dry air, we calculated the oxygen content change with the mo isture increasing, the result showed that with the increase of the wet transfer the oxygen partial pr essure was gradually reduced, that was to say oxygen content of the tunnel was gradually reduced. References [1] Nielsen, D.R. Jackson, et al. Soil Water[M]. American Society of Agronomy and Soil Science Society of American. 197, 0. [] Zheng Liping. Field experimental study of infiltration capacity of layer soil[j]. Journal of Water Resource & Water Engineering. 006, Vol.6, pp:4-6. [3] Gerson Henrique Dos Santos. Heat, air and moisture transfer through hollow porous blocks. International Journal of Heat and Mass Transfer. 5(009), pp: [4] Yan Zengfeng. Dynamic Modeling of the Indoor Thermal and Humidity Environment in the Adobe Buildings[D]. Xian, Xian University of Architecture and Technology, 003. [5] Zhang Hualing. Study of the Thermal and Humid Environment in Hydroelectric Station Underground Plants[D]. Chongqing University, 007. [6] A.V. Luikov. Heat and Mass Transfer. Moscow, Mir Publisher, [7] Zhong Zhihui. Study on Characteristics of Coupled Heat and Moisture in Porous Building Materials[D]. Southwest Jiaotong University, 011.

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