Permafrost region ventilation pipe foundation design method based on heat balance

Abstract

The invention provides a permafrost region ventilation pipe foundation design method based on a heat balance principle, and aims to keep heat balance of permafrost on the lower portion of a foundation and ensure long-term heat stability of the foundation. According to the method, meteorological data, soil physical parameters and permafrost upper limit and ground temperature distribution information are collected so as to evaluate whether additional protection measures are needed or not and determine the burying height of the ventilation pipe foundation. And based on the selected ventilation pipe configuration, the air speed in the pipe is calculated by using a corresponding air speed calculation formula, and the convective heat transfer coefficient is determined accordingly. And determining the diameter of the pipeline by combining the convective heat transfer coefficient and the economical efficiency. And the cooling radius and the optimal operation mode of the ventilation pipe foundation are further determined, so that heat balance at the bottom of the foundation is ensured, and permafrost at the lower part of the foundation is effectively protected. The design method is simple, convenient, rapid and accurate, and has important engineering application value and wide applicability.

Description

Technical Field

[0001] The invention relates to the technical field of thermal protection design of foundations in permafrost areas, and in particular to a design method for ventilation pipe foundations in permafrost areas based on heat balance. Technical Background

[0002] As the global climate warms, permafrost is gradually degrading. The core reason is that the annual heat absorption of permafrost exceeds the heat release, resulting in net heat accumulation and transfer to the permafrost layer, which in turn causes permafrost melting. Permafrost is widely distributed, and many buildings, roads and pipeline projects have been built in these areas. The construction activities of these projects have destroyed the thermal balance between the surface and the atmosphere, removed vegetation, and increased the heat transferred to the permafrost layer. This additional heat accelerates the degradation of the permafrost layer, exposing projects in permafrost areas to the risk of thawing settlement, which is manifested in uneven settlement and cracking of houses, longitudinal cracks in roads and uneven settlement of slopes, and deformation, uneven settlement, misalignment of airport runways, and slurrying of the road surface in the warm season. [3] Disasters such as fire and fire cause high maintenance costs.

[0003] Ventilation pipe foundation in permafrost areas [4-5] It can effectively reduce the temperature of the permafrost under the foundation, prevent or slow down the degradation of the permafrost, and then slow down or eliminate the foundation settlement deformation caused by the thawing of the permafrost. To achieve the above effect, from the perspective of heat balance, the annual net heat of the ventilation pipe foundation needs to be kept negative. If the annual net heat of the ventilation pipe foundation is positive, the permafrost under the foundation will continue to degenerate, and the foundation settlement deformation cannot be effectively controlled. In road and construction projects in permafrost areas, ventilation pipe foundations have been used to protect the permafrost under the foundation. Although the ventilation pipe foundation has a good cooling effect, the existing specifications only provide the minimum design requirements, and the actual application mostly relies on past engineering experience and field tests, which has the problems of low applicability and long time consumption. In view of the above shortcomings, it is urgent to propose a simple, accurate and fast ventilation pipe foundation design method. Summary of the invention

[0004] In order to make up for the defects of the existing ventilation pipe foundation design in permafrost areas, which is time-consuming and not very targeted, the present invention aims to propose a simple, accurate and fast ventilation pipe foundation design method to slow down or eliminate the settlement deformation caused by the degradation of permafrost under the foundation, so as to ensure the thermal stability of the foundation.

[0005] To achieve the above objectives, the technical solution of the present invention is to design a method to make the annual net heat of the ventilation pipe foundation in permafrost areas lower than 0KJ / m². The core lies in the reasonable design of the ventilation pipe and its configuration, and the accurate calculation of the annual net heat of the foundation. The specific steps are as follows: 1) Data collection: Collect meteorological data such as temperature, wind speed, wind direction, wind and snowfall at the project site. Determine the cross-sectional dimensions of the foundation (width, height, slope), and obtain the physical parameters of the foundation filler to calculate its thermal conductivity and specific heat capacity. At the same time, collect geothermal data of natural soil, the upper limit of permafrost and its physical parameters, and calculate thermal conductivity and specific heat capacity.

[0006] 2) Determine whether anti-sand and snow devices are needed based on snowfall, snow accumulation and wind and sand data. Consider installing inlet and outlet ducts on ventilation pipes to prevent blockage by sand and snow. The air inlet should face the dominant wind direction in winter, the height should not exceed the foundation height, and the air inlet duct should be higher than the air outlet duct to increase the gas flow rate in the duct.

[0007] 3) According to the literature [1-2] The buried height of the ventilation pipe should be 0.5m higher than the natural ground surface.

[0008] 4) Wind speed calculation: The wind speed in winter affects the convective heat exchange between the air in the ventilation duct and the foundation, which in turn affects the temperature distribution of the foundation and the underlying frozen soil. The higher the wind speed, the higher the convective heat exchange efficiency and the better the winter cooling effect.

[0009] According to fluid dynamics theory, the gas flow in the ventilation pipe needs to overcome the longitudinal resistance and local resistance. The longitudinal resistance is the resistance caused by the friction in the air when the air flows in the pipe. The expression is as follows: (1) Where: L is the length of the pipeline; D is the inner diameter of the pipe; u is the average flow velocity in the pipe section; is the friction coefficient along the pipeline; is the fluid density.

[0010] The friction coefficient of air flow along the ventilation duct is calculated using the Colebrook-White formula: (2) Where: is the Reynolds number, v is the dynamic viscosity coefficient of air flow; k is the equivalent roughness of the pipeline.

[0011] The local resistance loss of the ventilation pipe mainly includes the local resistance loss at the pipe air inlet, pipe bend and pipe outlet. The calculation formula is as follows: (3) Where: is the local resistance loss coefficient of the pipeline.

[0012] For horizontal ventilation pipes, only the resistance loss along the pipe needs to be considered. The wind speed calculation formula in the pipe is as follows: (4) Where: K 1 , K 2 is the aerodynamic coefficient at the air inlet and outlet; L is the length of the pipeline; D is the pipe diameter.

[0013] For ventilation pipes equipped with inlet and outlet ducts, the resistance along the pipe and the local resistance loss must be considered. The wind speed calculation formula in the pipe is as follows: (5) Where: is gravity; H is the air inlet height.

[0014] According to the design in 2), the corresponding wind speed calculation formula is selected based on the wind speed data collected in 1) to obtain the wind speed distribution of different pipe diameters.

[0015] 5) Calculation of convective heat transfer coefficient: Based on the wind speed in the duct obtained in 4), the convective heat transfer coefficient of the ventilation duct is calculated using the following formula: (6) Where: Air velocity in ventilation duct.

[0016] 6) Determine the optimal diameter of the ventilation duct based on the calculation results of the convective heat transfer coefficient and economic efficiency.

[0017] 7) Cooling radius calculation: The cooling radius refers to the area where the ventilation pipe can effectively reduce the temperature when cooling the frozen soil layer. This radius has no fixed value and is affected by many factors such as diameter, buried height, extension length and ambient wind speed.

[0018] The cooling radius of the ventilation duct foundation can be approximately regarded as a cylindrical area centered on the ventilation duct. Using the meteorological parameters, foundation parameters and ventilation duct design parameters collected in 1), a numerical model is established, and the cooling radius is determined by simulation. When constructing the numerical model of the cooling radius of the ventilation duct foundation, the uniformity of the fluid in the calculation area is ensured. The distance between the top of the calculation area and the ground is set to 6H, the distance between the inlet boundary and the foundation is at least 5H, and the distance between the outlet boundary and the bottom of the foundation is greater than 15H. The model only needs to construct the foundation area and the fluid area. After the model is established, the temperature of the calculation area is set according to the data collected in 1), and the temperature in the middle of the foundation is monitored. When the temperature data stabilizes and reaches the lowest point, the test ends.

[0019] 8) Based on the cooling radius calculated in 7), determine the laying spacing of the ventilation duct foundation pipes.

[0020] 9) Combine the determined design parameters with local meteorological, geological data and design documents to perform numerical calculations and monitor the annual net heat at the bottom of the foundation ( Q ). The annual net heat is calculated as follows: (7) Where: Q is the net heat (KJ / m 2 ) ; q is the soil heat flux density (W / m 2 ); A is the base bottom area (m2); t is time (days); and The interval is 1 year.

[0021] If the design service life Q If it is less than 0, it means that the permafrost below the foundation has been effectively protected and has not degraded. Q If the value is greater than 0, the operation mode of the ventilation pipe foundation should be adjusted and the annual net heat should be recalculated to ensure that the annual net heat is negative within the design service life. When constructing the annual net heat numerical model, the upper area is based on the settings in 7), the lower soil is constructed based on the geological data in 1), and the model width is set according to the laying spacing in 8). The numerical calculation is divided into two steps: First, the temperature field distribution of the natural soil is calculated under no warming conditions, and the permafrost upper limit and ground temperature distribution data collected in 1) are used for verification to ensure accuracy. After the initial temperature field calculation is completed, the normal operation of the ventilation pipe foundation within the design service life is calculated under warming conditions.

[0022] The design method of the present invention is based on the heat balance of the foundation system, and proposes a simple, accurate and fast ventilation pipe foundation design method. The core is to ensure that the annual net heat at the bottom of the foundation is negative within the design service life. This method provides a theoretical basis for the design and calculation of ventilation pipe foundations and has important application value in engineering. By collecting meteorological, geological and foundation size data, the type and location of the ventilation pipe are scientifically designed, the wind speed and convection heat transfer coefficient are calculated, and the optimal diameter is determined. The cooling radius is calculated to determine the laying spacing, and its operation mode is determined according to the principle of heat balance. This method is simple, scientific, and accurate, and effectively controls the settlement deformation caused by frozen soil degradation.

[0023] Compared with the prior art, the present invention has the following advantages: it provides a clear ventilation pipe foundation design process, abandons the method of relying on engineering experience and field tests, shortens the design cycle, improves engineering efficiency, and is highly targeted. At the same time, the present invention considers the impact of temperature changes on the design, and uses actual data to verify the numerical model, ensuring the accuracy of the calculation results under different environmental conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of a design method for ventilation pipe foundation in permafrost areas based on heat budget balance Figure 2 Changes in annual net heat at the bottom of the ventilation pipe foundation Figure 3 Changes in annual net heat at the bottom of the ventilation duct foundation after installing a temperature control damper DETAILED DESCRIPTION The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0025] This example selects a secondary highway section in the permafrost area of ​​the Qinghai-Tibet Plateau for design calculation. The road surface of this section is a black high heat absorption road surface. This high heat absorption characteristic aggravates the thawing and settlement of the permafrost below the roadbed, which in turn leads to serious settlement of the roadbed and longitudinal cracks on the road surface. To solve this problem, we use a ventilation pipe foundation to protect the permafrost to achieve the goal of controlling the settlement and deformation of the roadbed.

[0026] This example uses a highway section in a permafrost area on the Qinghai-Tibet Plateau as an example for design calculation. The black road surface of this section has high heat absorption, which accelerates the melting of permafrost under the roadbed, resulting in roadbed settlement and longitudinal cracks on the road surface. To solve these problems, this example uses a ventilation pipe foundation to protect permafrost and control roadbed settlement and deformation.

[0027] The specific implementation steps are as follows: Step 1: Data Collection The cross-sectional dimensions of the roadbed were obtained through on-site measurements: road surface width 13m, slope ratio 1:1.5, and fill height 3m. The design documents and geological survey results provided the parameters of the roadbed soil and natural soil (see Table 1). At the same time, wind speed and snowfall data were collected based on the data from the local meteorological station (see Tables 2 and 3).

[0028] Table 1 Physical parameters of roadbed and natural soil Table 2 Average wind speed per month (m / s) Table 3 Snowfall in the past ten years (mm) Step 2: Analyzing the snowfall data in the past ten years provided in Table 3, we judge that the snowfall in the area will not block the pipes. Therefore, no additional snow prevention measures are required, and horizontal ventilation pipes can be used directly.

[0029] Step 3: Use formula (4) to calculate the wind speed in the ventilation duct. In this case, the ventilation duct is set at 0.5m above the natural ground, where K1 is 0.3 and K2 is -0.2. According to the specification, three ventilation ducts with different inner diameters (0.4m, 0.5m, 0.6m) are selected for calculation, and the length of the duct is 21m. The calculated wind speed data is detailed in Table 4.

[0030] Table 4 Average wind speed in ventilation ducts of different diameters (m / s) Step 4: Use formula (6) to calculate the convective heat transfer coefficient. The calculation results are shown in Table 5.

[0031] Table 5 Convective heat transfer coefficient of ventilation pipes with different diameters (m 2 / k) Step 5: The results in Table 5 show that the convective heat transfer coefficients of the 0.5m and 0.6m diameter ventilation pipes are slightly different. Considering the economic efficiency, the 0.5m inner diameter ventilation pipe is selected.

[0032] Step 6: Using the collected basic parameters, a numerical model with dimensions of 30m (height) × 10m (width) × 221m (length) was established, and two ventilation pipes with a centerline spacing of 5m were arranged in it to reduce the error. Considering the temperature rise of 2.6℃ in 50 years, the cooling radius of the ventilation pipe was calculated, and it was found that the cooling effect was best within 2m on both sides of the centerline of the pipe, so the optimal laying spacing was 4m.

[0033] Step 7: Based on the previous ventilation pipe design parameters and the data in Table 1, a numerical model with a depth of 30 m and a width of 4 m was constructed. The remaining dimensions were the same as those in Step 6. After obtaining a stable temperature field that matches the ground temperature and the upper limit of permafrost, a 30-year service life was set, and the heat at the bottom of the foundation was calculated using the warming boundary condition. The annual net heat was calculated using formula (7). The results are shown in Figure 2 .

[0034] Step 8: Figure 2 It shows that within the design service life of 30 years, the annual net heat of the ventilation pipe foundation is negative in the first two years and positive in the third year. According to the principle of heat balance, the ventilation pipe running all year round will cause soil degradation and does not meet the design requirements. Therefore, a temperature control damper is installed to adjust the operation mode, which will automatically start when the outside temperature is below zero degrees. Figure 3 It shows that after installing the damper, the annual net heat remains negative throughout the service life.

[0035] In this specific embodiment, a ventilation pipe foundation with a temperature-controlled damper and a buried height of 0.5m, a laying spacing of 4m, and a pipe diameter of 0.5m can ensure that the permafrost under the foundation is stable and does not degrade or thaw.

[0036] The embodiments are intended to illustrate the present invention, but not to limit its application. Technicians may make adjustments, substitutions or improvements without violating the core concept and spirit of the present invention, and these modifications all fall within the scope of protection of the present invention.

[0037] [1] CCCC First Highway Survey and Design Institute Co., Ltd., ed. Technical Specifications for Design and Construction of Highways in Permafrost Areas[M]. Beijing: People's Communications Press, 2023; [2] JGJ 118-2011; Code for Design of Building Foundations in Frozen Soil Areas[S]; 2011.08.29; [3] Liu Weibo, Yu Wenbing, Chen Lin, Yi Xin, Han Fenglei, Hu Da. Current status of airport runway construction technology in permafrost areas[J]. Journal of Glaciology and Geocryology, 2015, 37(6): 1599-1610; [4] Su Wanxin, Xu Xueyan, Qiu Mingguo. Calculation of ventilation area of ​​ventilation duct foundation under heating conditions [J]; Low Temperature Building Technology, 2004, (5): 74-76; [5] Cheng Guodong, Wu Qingbai, Ma Wei; Engineering effect of active cooling subgrade on Qinghai-Tibet Railway[J]; Chinese Science (Series E: Technical Sciences), 2009, 39(1): 16-22.

Claims

1. A design method for ventilation pipe foundation in permafrost areas based on heat budget balance, with the following specific features: 1) Collect data on temperature, wind speed, snowfall, sandstorm, wind direction and foundation dimensions at the project site; collect physical parameters of backfill soil and natural soil, as well as data on the natural upper limit depth and ground temperature distribution of permafrost; 2) Based on the wind, sand and snow data collected in 1), determine whether it is necessary to take measures to protect against sand and snow; 3) Based on the wind speed information in 1) and references [1-2] , determine the laying height of the ventilation pipe foundation; 4) Use the wind speed calculation formula that takes into account the pipe resistance loss to calculate the wind speed in the ventilation duct. The wind speed calculation formula for the horizontal ventilation duct is as follows: Where: K 1 , K 2 is the aerodynamic coefficient at the air inlet and outlet; L is the length of the pipeline; D is the pipe diameter; is the friction coefficient along the pipeline; For ventilation ducts equipped with inlet and outlet ducts, the wind speed calculation formula is as follows: Where: g is gravity; H is the height of the air inlet; is the local resistance loss coefficient of the pipeline; 5) Based on the wind speed result calculated in 4), calculate the convection heat transfer coefficient and select the appropriate ventilation pipe diameter; 6) Based on the results in 5), determine the cooling radius of the ventilation duct; 7) Based on the previous design results, determine the operation mode of the ventilation duct foundation to ensure the balance of heat input and expenditure.

2. The method according to claim 1, characterized in that: The buried height of the ventilation pipe foundation should exceed the natural ground by 0.5m.

3. The method according to claim 1, characterized in that: If the anti-sand and snow measure is to install air inlet and outlet ducts, the air inlet should face the direction with higher wind speed in winter, and the height should not exceed the foundation height. At the same time, the air inlet duct should be higher than the air outlet duct.

4. The method according to claim 1, characterized in that: The convective heat transfer coefficient is calculated based on the wind speed in the ventilation duct.

5. The method according to claim 1, characterized in that: The cooling radius of the ventilation duct is determined through numerical simulation, taking into account multiple factors such as the ventilation duct diameter, burial height and ambient wind speed.

6. The method according to claim 1, characterized in that: The operation mode of the ventilation duct foundation is determined by numerical simulation to ensure that the annual net heat at the bottom of the foundation is negative within the design service life; when the annual net heat at the bottom of the ventilation duct foundation is positive, the operation mode of the ventilation duct can be adjusted to ensure that the annual net heat is negative within the design service life.

7. The method according to claim 1, characterized in that: The effect of temperature change is taken into account in the numerical calculation, where the temperature change rate is determined based on the data collected in 1).

8. The numerical calculation method according to claim 7, characterized in that: When calculating the annual net heat at the bottom of the foundation, this method uses the permafrost upper limit and ground temperature distribution data collected in 1) to verify the numerical model to ensure the accuracy of the calculation.

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