Study On Thermal Resistance Optimization Of Train’s Impervious Envelope

The transportation is developing rapidly,whose energy consumption share is increasing more and more.The energy consumption amount and the comfort of vehicles mainly depend on the thermal performance of their envelope,and the high-speed train is especially true,which has been studied by many institutions and researchers.At present,the facts are 1)The heat transfer coefficient of the whole train body is larger than 1.456 W/(m~2·K),2)there is still some error between the numerical study and the experimental study.Compared to the transparent envelope,the construction of the opaque envelope is more complex because a large number of narrow cavities(horizontal and vertical).The heat transfer in the narrow cavity is considered as one-dimensional and both faces vertical to heat flux are bigger than other four faces.Not only these cavities increase the thermal resistance of the train envelope,but also create a chance to further strengthen the thermal resistance of the cavity and reduce the heat transfer coefficient of the vehicle envelope according to the radiation mechanism.Based on above reasons,this paper uses CRH2 high-speed train as the research object.By pasting aluminum foils on the cavity inner faces(horizontal and vertical)to change their emissivity,to reduce the cavity radiation heat transfer share,and finally,to reduce the heat transfer coefficient of the body.The research works and the achievements of this paper are described below.1)First of all,the cavity structure and its area share of the CRH2 high-speed train body are calculated and analyzed.The results show that the cavity area share of the opaque envelope is more than 80%,the maximum cavity thickness is about 100 mm,and the cavity thickness of the rest body is generally about 5-50 mm.2)By pasting aluminum foils on the single-sided inner face which is vertical to the heat flux and on the double-sided inner face respectively,the narrow closed vertical cavity surface emissivity is changed.And then,these narrow closed vertical cavities with the thickness of 5mm,10 mm and 15 mm were tested respectively.Based on the test data,their radiation heat transfer share and equivalent thermal conductivity were calculated and analyzed.The results show that the radiation heat transfer share in the vertical cavity made of the conventional material is dominate,and the radiation heat transfer share increase up to about 80% with the cavity thickness in the test range.The radiation heat transfer share of the cavity can effectively be reduced by pasting aluminum foil on the inner faces of the cavity.Because the thickness increases the thermal resistance,the average equivalent heat transfer coefficient ofthe narrow vertical cavity decreases with its thickness.The equivalent thermal conductivity of the cavity can be effectively reduced by pasting aluminum foil on its inner face up to 62% as the thickness.The effect of pasting double-sided aluminum foils is about 5% better than that of pasting single-sided aluminum foil.3)Through the experimental method,the thermal resistance of the narrow closed horizontal cavities with thickness of 10 mm,15 mm and 20 mm and different emissivity of were tested and analyzed respectively.The results are tha same as that of the vertcal cavities,the horizontal cavity radiation heat transfer share can reach about 79% in the test scope,and the effect of pasting double-sided aluminum foisl is about 2% better than that of pasting single-sided aluminum foil.4)The thermal characteristics of the cavities tested above are studied respectively by the numerical simulation method,and the comparsion between the tested result and the numerical result are done.The results show that the numerical model and the method used in this paper are reasonable and effective,and so the numerical study on the thermal characteristics of the cavity and the train body can be carried out.On the other hand,The test result is verified based on the numerical result too.5)According to the structural characteristics of CRH2 high-speed train body,a numerical physical model of the train opaque envelope is established,which includes some narrow closed cavities.The temperature field and the velocity field of the opaque envelope were analyzed by numerical simulation.6)Considering both the cavities and the cold bridge,the heat transfer coefficients K of CRH2 high-speed train body before and after improving cavity thermal resistance are calculated by numerical simulation respectively,and the results are 1.453 W /(m~2·K)and1.444 W /(m~2·K).Aluminum foil tape is inexpensive and easy to paste.The study of this paper shows that it can effectively reduce the heat transfer coefficient of the vehicle body by pasting aluminum foils on the cavity inner faces of the vehicle body,and thus reduce the energy consumption of the train heating and air conditioning system and improve the vehicle environmental thermal comfort.