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Abstract
The radiant fractions of four currently available orchard heaters (spot, large cone, return stack and auto clean stack) were determined under orchard conditions using observed surface temperatures, emissivity and the Stefan-Boltzmann Law. The contribution of the flame extending above the spot heater stack to its total radiant output was determined. Stack surface temperatures increased with burning rate on the large cone, return stack and auto clean stack heaters but decreased on the spot heater. The position of the highest temperature was near the bottom of the large cone, return stack and auto clean stack heaters and near the top of the spot heater. Surface temperatures were lower in general on the spot heater than on the other three heaters. The radiant fractions of the large cone, return stack and auto clean stack heaters were greater than 20% and relatively constant with burning rate. Even though the flame contribution is purposely overestimated the radiant fraction of the spot heater was less than 16%. For low wind speeds under which most orchard heating occurs the effect of wind speed on the radiant fraction was negligible.
Abstract
Spatial and temporal components of the turbulent heat flux above a heated orchard are discussed. Direct measurements of the temporal component of the turbulent flux at 2 locations are reported. Drag anemometry and fine-wire thermocouples provided measurements of u, v, w, and T at 30 Hz. About 10% of the energy provided by heating escaped the crop zone by the temporal flux. Variation in the flux measured at a given location was caused by changes in wind speed and direction, and heater proximity.