Development of New Estimation Formula for Globe Temperature and Polyethylene Chamber for Research of Heat Exchanges on Vegetation Surface: Experimental Approach Based on Heat Budget

  • Maki OKADA, 2013: Development of New Estimation Formula for Globe Temperature and Polyethylene Chamber for Research of Heat Exchanges on Vegetation Surface: Experimental Approach Based on Heat Budget .

Globe temperature is one of the elements in a heat stress index, Wet Bulb Globe Temperature. Globe temperature is not explicitly predicted in numerical simulation, and its stable measurement is difficult. Therefore, some empirical formulae were introduced to estimate globe temperature using standard meteorological variables, including air temperature, global solar radiation and wind speed. Observations and heat balance analysis of a globe revealed that globe temperature depended curvilinearly on global solar radiation. Additionally, wind speed influenced this dependence. The previous estimation formulae did not consider this physical law of a globe. Therefore, they had a systematic error of globe temperature depending on global solar radiation. Thus, we developed a new estimation formula based on heat balance of a black globe surface. The new formula was able to predict the curvilinear dependence of globe temperature on global solar radiation without any systematic error, and it also showed the globe temperature response to wind speed. In addition, the numerical constants of the newly developed formula were reevaluated from long-term observation data. Therefore, this new formula was applicable to estimate globe temperature under wide global solar radiation and wind speed.

     Urban green area is one of the countermeasures for urban thermal environment. There is no clear answer for optimal vegetation size and its allocation to ameliorate an urban thermal environment. To answer these problems, basic knowledge should be enhanced on heat exchange process between vegetation and surrounding air, and how vegetation morphology affects this process. However, there is hardly a way to investigate how heat exchange changes according to vegetation size and allocation. Then, we developed a new apparatus, “polyethylene (PE) chamber”, to simulate heat exchange between vegetation surface and surrounding air with black Kent papers. The chamber was a 1.51 m-long tube structure with its two ends serving as an air inlet and an air outlet, and it was ventilated in the longitudinal direction using an exhaust fan. Therefore, there is an air temperature difference between the inlet and the outlet of the chamber. Using the air temperature difference between the inlet and the outlet and other heat balance components, we could evaluate the heat exchange process and its dependency to vegetation morphology. The measurement section of the PE chamber was covered with a thin polyethylene film. Such a thin polyethylene film transmits approximately 85% of both shortwave and longwave radiation. Therefore, the PE chamber was applicable to evaluate heat exchange under nocturnal radiative cooling. The outdoor experiments using black Kent papers instead of a real leaf and simple heat balance analysis of the PE chamber revealed the following points: 1) The colder surface did not necessarily produce the larger cooling the surrounding air. 2) Increase of the heat exchange surface resulted in an increase of convective heat transfer, but this relationship did not change linearly and approached a balance point. 3) The scattered heat exchange surface resulted in an increase of convective heat transfer.

The article from a part of the doctoral thesis was published on J.Agric.Meteorol. (2013) Details
The article from a part of the doctoral thesis was published on J. Heat Island Inst. Inter. (2014) Details