Measurement and modeling of heat transfer mechanisms in mulch materials

Crop residues or mulches affect soil temperature influencing plant growth a nd related processes in the soil. A hot/cold plate combination was used to quantify heat transfer through several common dry rest mulch materials (rub ber 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(3) to 33 kg/m( 3), mulch thickness ranged from 61 mm to 140 mm and test temperatures range d from 20 degrees C to 45 degrees C. To determine the effect of thermal rad iation on heat transfer measurements were taken with the test material betw een both a set of low emissivity aluminum (Al) plates and a set of high emi ssivity black painted plates. To quantify free convection, measurements wer e made in a thermally Instable configuration with the hot plate on the bott om and the cold plate on top and in a thermally stable configuration with t he cold plate on the bottom and the hot plate on top. In thermally unstable situations (ie., bottom plate hot, top plate cool), free convection and co nduction mechanisms best explained the heat flux. In thermally stable condi tions, radiation and conduction best explained heat flux. The percentage of heat due to thermal radiation decreased as mulch thickness and density inc reased in both the thermal stable and unstable conditions. The percentage o f heat transfer due to free convection (unstable case) and due to conductio n (stable case) generally increased as mulch thickness and density increase d 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 str aw or wheat straw mulches compared to air Increasing the mulch thickness (p late spacing) resulted in the most difference with low mulch densities or n o mulch. Differences are probably not statistically meaningful at the high mulch densities. For pine straw the average k was 0.11 W m(-1) K-1 and for wheat straw 0.08 W m(-1) K-1 Models were created to develop the radiation, conduction and convection parameters for the mulches tested, with r(2) valu es 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.