Resistance of Monolayer Films to Evaporation from Water Surfaces and Its Effect on Water Temperature

Abstract

In recent years, increasing interest has been taken in reducing evaporation from water and land surfaces, and a number of studies have been published of the effect of fatty alcohols as the evaporation suppressor. But little attention has been paid to the relationships among suppression rate, subsequent increase in temperature and diffusion resistance of monolayer film to evaporation.
In this paper, th rate of evaporation was investigated by employing monolayer films of fatty alcohol's members as shwn in Table 1. Using the instrumentation presented in Figure 1 the diffusian resistance to evaporation was mersured for each substance and was shown in Figure 2 as a function of the number of carbon atoms in fatty alcohols.
The relationship between the number of carbon atoms and the diffusion resistance was found to be approximately
r_f=0.172·N-2.0, N≥11
where, r_f is the diffusion resistance (sec/cm), and N the number of carbon atoms. From the above result, it can be seen that fatty alcohols with carbon atoms less than N=11 are ineffective for reducing the rate of evaporation of water.
On the basis of diffusion theory, the suppression rate of evaporation by monolayer films was expressed by
(1-β)=[D_t/D_f+D_t]
where, (1-β) is the suppression arte (defined as a rate of evaporation from water surface covered by films to that from free water surface under the same weather conditions), D_t and D_f are the integral exchange coefficient (cm/sec.) in the air layer above films and in films (D_f is the reciprocal of r_f), respectively. The dependence of the suppression rate on both D_f and D_t calculated from the above relation is presented in Figure 3. As can be seen in this figure, the rate increases with increasing the integral exchange coefficient in the air layer. This fact seems to indicate that the suppression rate is not a physical quantity for monolayer films.
Temperature increase in shallow water which results from reducing evaporation was found to be approximated by Eq. (11). As has been reported by several authors, monolayer film spread on water surface is swept and is piled upon the leeshore by the action of the wind. Temperature increase expected from Eq. (11) may not be achieved in practice and may be the potential increase in water tempereture. But it provides an interesting upper limit to the increase in water temperature which can be produced by spreading monolayer film on water surface. The results calculated from Eq. (11) are presented as a function of the suppression rate in Figure 4. It is seen in this figure that there is clear dependence of the temperature increase on the suppression rate. When evaporation is completely suppressed, the temperature increase reaches 13.1°C(θ_w′=29.9°C) under assumed weather conditions (S_w=0.5ly/min., θ_a=15°C, e_a=7.7mmHg, D_t=2.0cm/sec.). Bowen's ratio characterizing the heat balance condition at water surface shows considerably increase from about 0.18 at (1-β)=0 to 1.5 at 0.8, indicating that more heat is transferred as sensible heat into air. Figure 4 also shows the decrease in the relative humidity at the upper surface of monolayer films with increasing the suppression rate.