A physical approach to piche evaporimeter behavior

In this study a physical approach to Piche evaporimeter behaviour is presented in an attempt to interpret the indications of the instrument. Equations that are normally used in boundary layer calculations of mass and heat transfer from horizontal flat surfaces are outlined and assessed for their validity of application to Piche evaporimeter conditions. Where possible modifications are made. The problems of pure forced and pure free simultaneous convective hat and mass transfer and of mixed convection are discussed and solutions are suggested for the case of application to the Piche instrument. Measurements are reported of evaporation rates from the Piche which were made both indoors (controlled condition) and in a Stevenson screen outdoors (natural conditions as in use in East Africa), indoors for both forced and free convection conditions. A normal table fan was used as a source of wind in the indoor measurements to give a controlled horizontal air stream of speed ranging from 0 (fan off) to 5.0ms-l. Air temperature and humidity were recorded and the temperature distribution over the surface of the evaporating paper was carefully measured by thermocouples. For the outdoor measurements air movement in the screen was also continuously measured using a thermal anemometer. No clear cut correlation appeared to exist between air movement rates inside and outside the screen. A complete description of the instrumentation and experimental procedures is given. The determination of the characteristic dimension for the paper of the Piche., the average surface temperature and the environmental parameters used as input data in the calculation is outlined. Calculated evaporation rates are compared with those obtained experimentally. Calculated indoor evaporation rates were lower than the measured rates by 4% on the average for the forced convection conditions and by 26% for the free convection conditions. These results show a remarkable success in the application of the forced convection flat plate boundary layer equations to the paper of the Piche instrument under pure forced convection conditions (air stream speed between 0.80 and 5.0ms-l) and the need for further research of the free convection heat and mass transfer processes. The outdoor evaporation results for the same horizontal air movement as for the forced convection conditions indoors are comparable with the indoors forced convection results. For lower air movement rates (between 0.55 and 0.25 ms-l) a mixed convection regime appeared to prevail. No turbulence intensity influence appears to exist to enhance the mass transfer in horizontal air flow and apparently an influence of the particular surface temperature distribution over the paper on the transfer is also absent. The place of these results in ongoing literature debates is outlined. A discrepancy between our fan measurements and those of Parlange, et al. Has been attributed to bluff body effects in their experimental set up. A combination of pure and free convection contributions appears to improve the results in mixed convection cases. Finally, consequences regarding the use of the screened Piche evaporimeter and problem in extrapolation of Piche data to those of other evaporating bodies are briefly discussed. Our main conclusion must be that the physical approach to this instrument definitely has disqualified its within screen use for any useful purpose. Suggestions are made on possible applications of the instrument outside a screen.