Study On Modification Of Air Permeation Characteristics And Air Permeation Characteristic Constant In Building Door And Window Gaps Under High Altitude And Low Pressure Conditions

The heat load caused by the air permeation in the window and door gaps is an important part of the total heat load of the building,so it is very important to accurately calculate the air permeation through the window and door gaps.Due to the high altitude,low air pressure,low air density and high viscosity of air movement in the Tibetan Plateau area,the air permeability characteristics and permeability volume in the cracks of doors and Windows are different from those in the normal pressure area.The gap method is used to calculate the air seepage volume of window and door gaps,and the key is the characteristic constant of air seepage:At present,the values of air infiltration characteristic coefficient a and air infiltration characteristic index b are determined according to GB50736-2012 Design Code for Heating,Ventilation and Air Conditioning of Civil Buildings,that is,the values of air infiltration characteristic coefficient a are provided by the door and window supplier or according to the current national air tightness standards for doors and Windows.When there is no measured data,the values of air infiltration characteristic index b are determined according to the empirical value0.67.Without considering the influence of the change of air physical parameters on the air seepage characteristic constant under low pressure conditions,it is difficult to ensure the accuracy to calculate the air seepage volume of window gaps under high pressure conditions by directly calculating the value of the air seepage characteristic constant in the existing norms.Therefore,it is very important to master the characteristics of air permeation in the gaps of doors and Windows of buildings under high altitude and low pressure conditions and determine the characteristic constant of air permeation in the gap method under low pressure for accurate calculation of air permeation volume and building heat load.In this paper,three typical building door and window gaps,namely:straight gaps,U-shaped gaps and L-shaped gaps,are selected as the research objects.The influence of changes in air physical parameters on air permeability characteristics in gaps and the determination of air permeability characteristic constants a and b under the condition of plateau and low pressure are studied.First,the mathematical model of air flow in gaps was established to obtain the average permeability velocity v₁and v₂of air in gaps under low pressure and normal pressure.The influences of atmospheric pressure,pressure difference on both sides of gaps and gap size on the average permeability velocity,pressure field and velocity field of air in three typical gaps were analyzed.Secondly,the infiltration air volume Q₁and Q₂in the gap under low pressure and normal pressure were obtained by the calculation formula Q=v F,and the mass flow volume M₁and M₂in the gap under low pressure and normal pressure were obtained by M=ρQ.The relative errorsδ_Qandδ_Mof Q₁and Q₂,M₁and M₂were analyzed respectively,and the variation characteristics of the infiltration air volume and mass flow volume in different gaps with atmospheric pressure were obtained.Furthermore,the relative errorsδ_Q’of Q₃and Q₁are analyzed,and the variation rule and range ofδ_Q’are obtained.Finally,Q₁was substituted into the gap method to calculate the formula Q=a（ΔP）^b,and the values a and b of three kinds of gaps in different sizes under low pressure were obtained.According to the variation law of a and b values with the gap depth to width ratio C,the modified calculation formula of a and b values about C was fitted.The following research conclusions are obtained:（1）The influence laws of atmospheric pressure,gap geometry and pressure difference on both sides of the gap on the average permeability velocity of air in the gap are different.When the pressure difference on both sides of the gap and the gap width increase,the average air permeability velocity relative errorδ_vincreases.When the gap width increases to 6mm,theδ_vtends to be stable,and the effect of pressure difference onδ_vcan be ignored.The simpler the structure of the gap,the greater the change range and change rate of the average air permeation velocity in the gap,that is,the straight gap>L-shaped gap>U-shaped gap.（2）The errors of infiltration air volume and mass flow rate caused by low pressure are large and cannot be ignored.Due to the decrease of air density and the increase of air kinematic viscosity at low pressure,the average permeability velocity of air in the gap v₁is greater than v₂,so Q₁is greater than Q₂and M₁is less than M₂.The variation range ofδ_Qis 14.4%～25.6%for straight gap,11.7%～28%for U-shaped gap and9.6%～32.3%for L-shaped gap,respectively.Theδ_Mranges are as follows:straight gap-36.5%～-20.3%,U-shaped gap-29.1%～-18.8%,L-shaped gap-30.5%～-16.3%.（3）Using the characteristic constants a and b recommended in the specification to calculate the seepage air volume under low pressure,the error is very obvious.Delta Q’is positive when Q₁is greater than Q₃and negative when Q₁is less than Q₃.Theδ_Q’ranges are as follows:straight gap-91.8%～164.3%,U-shaped gap-40%～55.7%,L-shaped gap-42.8%～68.3%.（4）According to Q=a（ΔP）^b,the a values and b values of three kinds of crevice under low pressure under different sizes were obtained,and the modified calculation formula of a values and b values about C was obtained by fitting.The results showed that the R²of straight crevice was greater than 0.90,that of U-shaped crevice was greater than 0.92,and that of L-shaped crevice was greater than 0.92,indicating good goodness of fit and high reliability.In this paper,the comparative analysis ofδ_Qandδ_Q’in three kinds of window and door gaps can provide reference for error correction of the air seepage volume calculated by the gap method at present,and the correction formulas of the air seepage characteristic constants a and b can provide value suggestions for the air seepage volume calculated by the gap method under low pressure.