Measurement and modeling of heat transfer mechanisms in mulch materials

摘要

Crop residues or mulches affect soil temperature influencing plant growth and related processes in the soil. A hot/cold plate combination was used to quantify heat transfer through several common dry test mulch materials (rubber chips, pine straw wheat straw) and identify and quantify heat transfer mechanisms with the goal of modeling apparent thermal conductivity of the mulch. Mulch material bulk densities ranged from near 0 kg/m{sup}3 to 33 kg/m{sup}3, mulch thickness ranged from 61 mm to 140 mm and test temperatures ranged from 20℃ to 45℃. To determine the effect of thermal radiation on heat transfer measurements were taken with the test material between both a set of low emissivity aluminum (Al) plates and a set of high emissivity black painted plates. To quantify free convection, measurements were made in a thermally unstable configuration with the hot plate on the bottom and the cold plate on top and in a thermally stable configuration with the cold plate on the bottom and the hot plate on top. In thermally unstable situations (i.e., bottom plate hot, top plate cool), free convection and conduction mechanisms best explained the heat flux. In thermally stable conditions, radiation and conduction best explained heat flux. The percentage of heat due to thermal radiation decreased as mulch thickness and density increased in both the thermal stable and unstable conditions. The percentage of heat transfer due to free convection (unstable case) and due to conduction (stable case) generally increased as mulch thickness and density increased. For a given mulch material, the thermally unstable condition results in an increased apparent thermal conductivity (k) value. The difference between the k values for stable and unstable cases tended to diminish with pine straw or wheat straw mulches compared to air Increasing the mulch thickness (plate spacing) resulted in the most difference with low mulch densities or no mulch. Differences are probably not statistically meaningful at the high mulch densities. For pine straw the average k was 0.11 W m{sup}(-1) K{sup}(-1) and for wheat straw 0.08 W m{sup}(-1) K{sup}(-1). Models were created to develop the radiation, conduction and convection parameters for the mulches tested, with r{sup}2 values for the estimated parameter fit ranging from 0.75 to 0.99. These models could be used to estimate the apparent k of dry mulches in the field.