The Simulation Of The Traspiration Of The Plant Leaf

Transpiration is one of the basic physiological activities of the plant leaf. It significantly affects leaf temperature. In order to imitate the thermal infrared characteristics of a plant leaf, we must imitate its transpiration.To deeply understand the plant transpiration, the heat and mass transfer processes of the plant leaf were investigated. Stomatal resistance, leaf temperature and transpiration water consumption of7kinds of typical plants in the middle and lower reaches of the Yangtze River Basin were measured. Camphor with large transpiration water consumption was chosen as the investigation object. A general thermophysical model was established for a camphor leaf. The model was verified by the field measured stomatal resistance and temperature of a camphor leaf. Then the leaf temperature, the transpiration water consumption and the heat exchange between the leaf and the environment were calculated throughout the summer using the fitted Jarvis model of the measured stomatal resistance and the weather data of the typical meteorological year in Nanjing. The dynamical simulation revealed that the day-average temperature of the leaf is dominated by the ambient air temperature, diurnal transpiration water consumption is dominated by the solar irradiance; transpiration plays an important role in the cooling of the leaf, it could dissipate more than30%of the total absorbed solar energy; transpiration latent heat has different degree of impact on the leaf temperature under different weather conditions, the impact is more significant in a calm weather than in a windy weather. Based on the above results, a kind of bionic leaf which can imitate the transpiration of the plant leaf and has a similar thermal infrared characteristic to the plant leaf was proposed. It consists of green coating, water holding layer, composite sorbent (CS) layer and adsorption-desorption rate control layer.The CS is the key material of the bionic leaf. Activated carbon fiber cloth (ACFC) and CaCl2CS was prepared by impregnating the ACFC in CaCl2aqueous solution. Expanded graphite (EG) and CaCl2CS was prepared using briquetting method. Then the water vapor sorption performance and thermal conductivity of the CSs were measured. Particular attention was paid to the adsorption mechanism of the ACFC to CaCl2during the impregnating and the water vapor sorption mechanism of the CSs. During the impregnating, Ca2+is absorbed by the surface oxygen functional groups of the ACFC due to the cation exchange, CaCl2is adsorbed by the micropores of the ACFC due to the Van der Waals force. From the quantitative analysis, Ca2+adsorption by the surface functional groups can be ignored for the CS preparation. CaCl2adsorption by the micropores can be described with the Polanyi adsorption potential theory and Dubinin-Radushkevich equation. When all the micropores of the ACFC are fully filled with CaC2, the isotherms of the CS coincide with those of the equivalent CaCl2. When there are still micropores remaining in the ACFC, both the ACFC and dispersed CaCl2contribute to the CS’s water vapor sorption. The physical adsorption quantity of the ACFC of the CS is impacted by the effective micropore volume and can be calculated with the Dubinin-Astakhov equation. During the hydration and deliquescence, the dispersed CaCl2shows a lower hydration and deliquescence pressures than that of bulk CaCl2, and the hydration and deliquescence pressures increase with the CaCl2content of the CS. The sorption quantity of the EG and CaCl2CS equals to that of the equivalent CaCl2. The ACFC and CaCl2CS has a large sorption rate, its mass transfer coefficient is0.027-0.04ls-1. However, its thermal conductivity is low, the maximal thermal conductivity is only0.063W/(m-K). The EG and CaCl2CS has a high thermal conductivity, its thermal conductivity can reach5W/(m·K). However, its mass transfer coefficient is only2×10-5s-1.Based on the measured water vapor sorption property and thermal conductivity of the CS, a thermophysical model was established for the bionic leaf. Then the design calculation of the bionic leaf was completed using the thermophysical model. The dynamical simulation revealed that the influence of the water holding layer on the temperature of the bionic leaf can be ignored, so the water holding layer can be removed; the thermal conductivity of the CS dominates the up surface temperature of the bionic leaf; we should use the EG and CaCl2CS with high thermal conductivity to prepare the bionic leaf; the sorption-desorption rate control layer can be removed due to the low desorption rate of the EG and CaCl2CS; when the CaCl2content of the composite reaches40%, the bionic leaf has the similar temperature to that of the plant leaf throughout the daytime. Based on the above results, we prepared several principle samples of the bionic leaf by compressing the expanded graphite-CaCl2composite and pigment powder together. The radiative temperatures of the bionic and actual leaves were measured. Differeces between them are all less than2℃throughout the whole day.