Field Trials of the Tow & Blow Wind Machine

Abstract

Wind machines are used extensively in the horticultural industry for combating the damaging effects of frosts. Tow & Blow (T&B), a Napier based horticultural fan manufacturer have developed a portable frost fan which is comparatively smaller than its fixed-in-place, conventional counterparts. During the year of 2017, a series of five field trials were conducted with the aim of investigating the set up and performance of the frost fan, seeking to gain insight into its frost protection capabilities. An instrumentation system was developed, consisting of three data loggers, thirteen calibrated anemometers and twenty temperature probes. A horizontal traversing technique was used to make measurements of the velocity profile near the exit of the fan (1.2m) and a 10m tall, 8m wide cross-shaped array was used to measure the profile downstream (15m, 20m and 25m). With the novel application of free jet theory to a wind machine, the jet was found to spread at an angle of 15.8° from a virtual origin located 6.74m behind the fan. The velocity measurements were shown to be accurately modelled by a Gaussian distribution allowing integral calculations that proved the jet to conserve momentum, entrain ambient fluid and dissipate kinetic energy through turbulent mixing. An empirical relation for the average velocity decay has been provided which can conveniently be used to estimate the average velocity downstream of the fan. A field trial was conducted during an inversion event to test the effects of the slew rate on the ambient air temperature response. Air temperature was shown to rise at crop level within 6 to 7 minutes of starting the fan. The maximum slew rate of the T&B machine provides the most consistent temperature response during an inversion. By decreasing the slew rate, the jet penetrates a further distance, however, it fluctuates between a maximum a minimum temperature as the area cools after the passage of the jet. Although extended range may be achieved with a slower rate, consideration must be made to the effect of a fluctuating temperature on the specific crop being protected.