MEASUREMENT AND MODELING OF HEAT TRANSFER MECHANISMS IN MULCH MATERIALS

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Crop residues or mulches affect soil temperature influencing plant growth and related processes in the soil. Ahot/cold plate combination was used to quantify heat transfer through several common dry test mulch materials (rubberchips, pine straw, wheat straw) and identify and quantify heat transfer mechanisms with the goal of modeling apparentthermal conductivity of the mulch. Mulch material bulk densities ranged from near 0 kg/m3 to 33 kg/m3 , mulch thicknessranged from 61 mm to 140 mm and test temperatures ranged from 20C to 45C. To determine the effect of thermalradiation 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 athermally unstable configuration with the hot plate on the bottom and the cold plate on top and in a thermally stableconfiguration with the cold plate on the bottom and the hot plate on top. In thermally unstable situations (i.e., bottomplate hot, top plate cool), free convection and conduction mechanisms best explained the heat flux. In thermally stableconditions, radiation and conduction best explained heat flux. The percentage of heat due to thermal radiation decreasedas mulch thickness and density increased in both the thermal stable and unstable conditions. The percentage of heattransfer due to free convection (unstable case) and due to conduction (stable case) generally increased as mulch thicknessand density increased. For a given mulch material, the thermally unstable condition results in an increased apparentthermal conductivity (k) value. The difference between the k values for stable and unstable cases tended to diminish withpine straw or wheat straw mulches compared to air. Increasing the mulch thickness (plate spacing) resulted in the mostdifference with low mulch densities or no mulch. Differences are probably not statistically meaningful at the high mulchdensities. For pine straw the average k was 0.11 W m1 K1 and for wheat straw 0.08 W m1 K1. Models were created todevelop the radiation, conduction and convection parameters for the mulches tested, with r2 values for the estimatedparameter fit ranging from 0.75 to 0.99. These models could be used to estimate the apparent k of dry mulches in the field.