Evaluation of heat transfer inside the phloem of a tree during heat treatment of HLB-affected citrus tree by CFD

Thermotherapy, as a non-chemical method has been applied to treat infected plants/trees with heat sensitive pathogen. Huanglongbing (HLB) is one of these heat sensitive pathogens and the required temperature for killing HLB was estimated at 54°C for 60 to 120 s during indoor experimental studies. However, utilizing this method in the field is difficult due to technical difficulties with regards covering a large tree. Recently, we developed a mobile heat treatment machine to treat in-field HLB-infected trees. This device includes a mobile tent that cover the infected tree, and steam is injected into the tent to treat a tree. Although the temperature inside the tent is raising up, the heat should penetrate inside the phloem where the bacteria lives. Therefore, measuring the heat penetration inside the tree is very critical to evaluate the performance of the mobile thermotherapy system. In this study, this mobile thermotherapy system was utilized to in-field treat HLB-affected trees and raise temperature inside the tent to 54°C. To measure the temperature in the phloem, holes were drilled in tree branches, with different diameters, and tree trunks. K-type thermocouples were installed inside and outside of the holes. Furthermore, a thermodynamic model was developed to simulate the heat distribution inside the phloem. Initial experiments showed that the temperature in the phloem was 10-40% lower than the temperature on the bark (near branch's surface). The heat transfer model was developed by comparative analysis of the experimental data. ANSYS software was used to simulate heat transfer; simulation results were consistent with experimental results, with an average relative error less than 5%.