Miniature wind tunnel for measuring all components of surface and leaf energy balance

The effect of wind speed on evaporation can be measured in the field and using standard wind tunnels in the lab. The effect of wind on sensible heat flux is usually inferred from observations with heated plates and their energy throughput while monitoring surface and air temperature. Here we are developing a circular, insulated wind tunnel to measure latent and sensible heat flux independently by energy and mass balance of the incoming and outgoing air stream. The insulation serves to minimise uncontrolled heat flux across the channel walls. The objective is to measure all components of the surface energy balance of a leaf or porous surface, including radiative, sensible and latent heat fluxes.

We designed a circular wind tunnel in order to decouple wind speed from the inflow rate of cool, dry air, allowing to control chamber air temperature and humidity by adjusting inflow rate, and to compute the added moisture and heat by the difference in temperature and humidity of incoming and outgoing air. For high sensitivity of measurement, the chamber volume was held small (1 l) and the inflow rate of cool, dry air is controlled at constant rate, varying between 1 and 10 l/min. The wind tunnel has a transparent sample chamber, where an artificial leaf or an evaporating porous material can be placed in the light.

The wind tunnel is equipped with copper-constantan thermocouples, miniature CTA wind sensors, miniature net radiometers, a fan with externally controlled power throughput and a wing to modify wind speed without modifying the fan power. For controlling the inflow rate, temperature and humidity of cool, dry air, we use a humidifier and cooler outside of the wind tunnel, while we monitor evaporation rate by recording liquid water flow rate into the sample using micro flow meters.

Link to the publication on external page Hydrology and Earth System Sciences - HESSD

Team: Stan Schymanski, Dani Or, Dani Breitenstein
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