NATURAL-CONVECTION ABOVE LINE HEAT-SOURCES IN GREENHOUSE CANOPIES

The characteristics of a plume generated by natural convection above heating pipes in greenhouse row crops is studied. This plume differs from the free plume mainly in two respects: (i) owing to the particular geometry of the cell, a downward flow appears in the edges of the cell and interacts with the plume and (ii) owing to the presence of vegetation heat and momentum sinks appear in the cell crop. In consequence there is no conservation of heat and the evolution of the mass and momentum flows can be different according to the forces balance. An experimental study is first developed in which the air velocity and the temperature excess are measured. It shows that: (i) the similarity hypothesis is relevant in this case while the entrainment hypothesis is not, (ii) the decrease of excess temperature with height is steeper than in a free plume, and (iii) the air velocity increases (decreases) with height at low (high) heights. A theoretical model is also developed. It is based on the mass, momentum and heat transport equations and completed by three closure relations characterizing the drag, the lateral turbulent exchanges and the convective heat exchange with the leaves. The model makes it possible to relate the particular behaviour of the 'crop cell plume' to the presence of heat and momentum sinks in the cell crop. Its predictions are in good agreement with the measurements.