Thermal conductivity and predictive modeling of microencapsulated phase change materials for improved silt sands
Objective In regions with seasonal frozen ground,frost damage in channels is a common issue due to significant temperature fluc-tuations affecting the foundation soil.To address this challenge,phase change materials(PCMs)are being integrated into founda-tion soil to regulate soil temperature dynamics and mitigate frost damage.Understanding the thermal conductivity of PCM-modified soil is crucial for accurately analyzing temperature distribution.Experimental studies have highlighted several factors influencing soil thermal conductivity,including temperature,moisture content,dry density,salt content,mineral composition,and fine particle content.The sensitivity of these factors varies depending on whether the soil is frozen or thawed.Due to the complex interplay of these factors,developing accurate predictive models is essential to assess their impact on thermal conductivity.Empirical models employing artificial intelligence algorithms are gaining traction due to their high accuracy and adaptability,par-ticularly in thermal conductivity prediction.However,significant progress has been made in analyzing and predicting thermal con-ductivity in typical geological materials.Despite advances in thermal conductivity analysis for standard soils,research on atypical soils like PCM-modified soils remains relatively limited.To bridge this gap,studies are investigating microencapsulated phase change materials(mPCM)as amendments for sandy soil foundations in Ningxia.They are examining how factors such as mPCM content,moisture,temperature,and dry density affect thermal conductivity,supplemented by scanning electron microscopy(SEM)to analyze internal pore structures.To accurately assess the temperature distribution in the drainage base of pulverized sandy soil improved by microencapsulated phase change materials(mPCM),it is essential to study the thermal conductivity of this modified soil and establish a reliable prediction model.This research will provide crucial references for the application of PCM-modified soils in engineering projects within regions with seasonal frozen ground.Methods Samples of soil were collected from the Yinchuan section of the West Canal in Ningxia Hui Autonomous Region,with a sampling depth of approximately 1.0 meter.The soil,characterized by a yellow-brown color,exhibited plastic limit mass fraction and liquid limit mass fraction of 11.79％and 22.67％,respectively.The optimum moisture mass fraction and maximum dry den-sity were determined to be 13.54％and 1.93 g/cm3,respectively.To investigate the impact of microencapsulated phase change materials(mPCM)on thermal conductivity,experiments were conducted using sandy soil samples amended with mPCM with a phase change temperature of(5±1)℃.The mPCM used had a paraffin core and a poly(methyl methacrylate)(PMMA)shell,with a latent heat of 120 kJ/kg,density of 0.88 g/cm3,and thermal conductivity of 0.21 w/(m·K).Thermal conductivity measurements were performed using transient line heat source method with a portable high-precision thermal conductivity meter.Three experi-mental schemes were designed to study the influence of dry density,moisture content,temperature,and mPCM content on the thermal conductivity of mPCM-modified sandy soil.The experiments included investigating the impact of mPCM content and experimental temperature at specific dry density and moisture content conditions,as well as evaluating the thermal conductivity variations under different moisture and dry density conditions for both sandy soil and mPCM-modified sandy soil.Sample prepara-tion involved adding water and varying concentrations(5％,8％,10％)of mPCM to sandy soil,followed by thorough mixing and sealing for 24 hours.Cylindrical specimens were then prepared using a compaction method and subjected to thermal conductivity measurements in a temperature-controlled chamber.Results and Discussion The thermal conductivity of mPCM-modified sandy soil exhibited temperature-dependent behavior,with mPCM content significantly influencing this relationship.Increasing mPCM content enhanced thermal conductivity by facili-tating denser particle packing and increased contact between particles.SEM analysis confirmed that higher mPCM content resulted in improved soil structure integrity and densification,as evidenced by filled pores and enhanced overall compactness compared to pure sandy soil.These findings illustrate the effective role of mPCM in modifying soil properties for channel base applications in regions susceptible to seasonal freezing.The thermal conductivity of silt sand amended with mPCM was observed to be influenced by the test temperature and the phase transition temperature of the mPCM.Notably,the thermal conductivity demonstrated a pronounced temperature dependency,characterized by three distinct phases:a rapid decrease(-10 to 0 ℃),a slow decrease(0 to 5 ℃),and a gradual increase(5 to 10 ℃).Furthermore,the coefficient of thermal conductivity of mPCM-amended silt loam exceeded that of unamended silt loam and exhibited augmentation with increasing water content,dry density,and mPCM mass fraction.Both multiple linear regression and Support Vector Machine(SVM)models effectively predicted the thermal conductivity of mPCM-amended silt loam.Nevertheless,the SVM model proved to be more adept at capturing the nonlin-ear relationship among the influencing factors of thermal conductivity in mPCM-amended silt loam.Conclusion The thermal conductivity of mPCM-amended silty sand soil is influenced by several key factors,including water con-tent,dry density,and mPCM content.Furthermore,it is significantly impacted by the relative proportions of ice and water,the latent heat of phase change for ice and water,the latent heat of phase change of mPCM,as well as the role of mPCM filling and densification.These factors exhibit notable temperature-dependent effects.For accurate prediction of thermal conductivity,the SVM model proves to be effective.The findings of this study can provide valuable insights for the application and exploration of phase change materials in regions characterized by seasonal permafrost.
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