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A high-temperature wide-range humidity sensor has been developed using wetted material temperature measurement. A porous ceramic is used as a sensing element; it has a small tube for supplying water to maintain a wetted surface. However, the water supplied to the elements can affect the surface temperature. In this study, we experimentally investigate the accuracy of the developed sensor and its contributing factors-convective heat transfer, evaporation, radiant heat, and the water supplied to the wetted material-under the conditions of a gas flow temperature of 200 °C, gas flow velocity of 3 or 5 m/s, flow rate of supplied water of 0.01-0.05 g/s, and steam mole fraction of 0.03-1.00. Further, we calculate the heat flux and the surface temperature using a one-dimensional heat transfer model to clarify the effects of the each of the abovementioned factors. Consequently, we conclude that the supplied water strongly affects the accuracy of the developed sensor. We also improve the measurement error of the steam mole fraction from 0.07 to 0.03 by the controlling the flow rate of the supplied water.

For clarification of the mechanism of heat and mass transfer in the early stages of superheated steam drying which accompanies condensation and evaporation, we had done both the experiments and the simple calculation for a semiinfinite heat transfer model. And we found the existence of "reverse point" at which neither condensation nor evaporation occurs in the early stage of superheated steam drying. In this report, in order to investigate the physical reason of the reverse point and its change according to the shapes of heat transfer model, similar analytical and experimental methods were applied for both a spherical and a large slab heat transfer models. As a result, restoration time changes in the range of 3.2∼10times the reverse time with changes of scale, heat transfer rate and initial temperature of the heat transfer models.

A numerical study on a combined radiation and forced convection heat transfer of superheated steam, which is a radiation participating real gas, in thermally developing laminar flow through a parallel-plate channel has been conducted to investigate characteristics of superheated steam drying. The integrodifferential energy equation was solved using an implicit finite-difference technique with a marching solution procedure and an exponential wide-band model for the treatment of the radiative transfer part. Comparison of results with and without gas radiation in various conditions shows that fluid radiation decreases the temperature of the main stream, but increases the total heat flux at a heat transfer surface. Furthermore, the results show that the fluid radiation decreases the inversion point temperature approximately to 150 to 240 °C with the increase of optical thickness. This numerical result agrees in an order of magnitude with the previous experimental studies, but is about 100 K lower than that of former theoretical predictions without considering fluid radiation. © 2000 Scripta Technica, Heat Trans Asian Res, 29(5): 385–399, 2000