Gary C. Vliet

Numerical and experimental results for coupled heat and mass transfer between a desiccant film and air in cross-flow

Abstract Heat and mass transfer with a uniformly distributed falling liquid desiccant film in a cross-flow parallel plate gas-liquid heat exchanger is predicted, and the results compared with experiment. The process air is in cross-flow over the desiccant film. Refrigerant flows through the heat exchanger tubes and removes the heat of absorption for dehumidification and supplies the heat for regeneration. The governing equations, boundary conditions, and interfacial equilibrium conditions for the concentration, temperature and pressure are presented. The problem is more complex than for cocurrent or countercurrent flow, as it is fully three-dimensional.

Designing Solar Hot Water Systems for Scaling Environments

Component failures and system performance degradation in SHW systems due to scaling are common in areas with hard water. It appears that many valve and pump failures on the potable water side are related to scaling, and any scale build-up on heat transfer surfaces will result in performance degradation. Different designs are compared in regard to their susceptibility to problematic scaling. Indirect systems utilizing external and tank wall heat exchangers are compared in regard to the rate of scaling and the consequences of scaling on system performance. The tank wall heat exchanger appears preferable over a doubly pumped external heat exchanger, both in terms of system reliability and resistance to performance degradation.

Identifying and Reducing Scaling Problems in Solar Hot Water Systems

In areas with hard water, scaling can reduce the reliability of solar hot water (SHW) systems. Common reliability problems associated with scaling are both mechanical (collector freeze damage, clogged passages, premature failure of pumps and valves) and thermal (efficiency degradation). A mechanistic and a mathematical scaling rate model are presented. Results from controlled experiments investigating the effect of key water chemistry and heat transfer parameters on the scaling rate are summarized. The implications of these results for designing SHW systems for scaling environments are discussed. Most importantly, indirect systems where the potable water side of the heat exchanger is integrated into the storage tank wall, such as in a wrap-around heat exchanger, are shown to be the most mechanically and thermally reliable systems for scaling environments. A new version of the software SolScale is discussed, which is intended to aid in the design of SHW systems to reduce scaling related reliability problems.Copyright

Effect of an inert gas on heat and mass transfer in a vertical channel with falling films

The effect of inert (noncondensable) gases on the heat and mass transfer (absorption) for channel flow of water vapor in conjunction with falling aqueous LiBr films is investigated. The hydrodynamic flow of the gas in the channel is approximated as fully developed. This is a fair assumption because of the low Reynolds numbers resulting from the low prevailing absorber pressures. The film flow is also assumed to be hydrodynamically developed. This greatly simplifies the problem, as the momentum equation need not be considered. Otherwise the continuity, species, and thermal energy equations govern the problem. Numerical results for a nominal case are presented for the velocity, temperature, and species distributions in the gas and liquid phase regions, and for the interface absorption rate. The effects of varying several parameters (including inerts concentration) on the above variables are also presented. Comparisons are also made with limited data in the literature.

Texas Solar Radiation Database (TSRDB)

Solar radiation data have been acquired over approximately a five year period (1996 to present) at 15 sites in Texas (Texas Solar Radiation DataBase – TSRDB). These data are compared with comparable sites in the National Solar Radiation DataBase (NSRDB). Comparison of the TSRDB and NSRDB data for eleven (11) coincident or nearby locations show reasonably good agreement between the global horizontal values. Relative to the NSRDB, individual monthly average differences between the two sets range from −20 to +13%, and the annual averages varied from −9 to +8.5%, with positive values meaning the TSRDB values are higher. Overall, the TSRDB global horizontal data are about 2% lower than the NSRDB. However, there are considerable differences in the direct normal values, with the TSRDB values generally being higher. The monthly average differences ranged from −18 to +36%, and the average annual difference for the compared locations is about +5%. The greatest deviations for direct normal data are for coastal locations in the winter, with the three compared coastal locations exhibiting an average difference of about +30% for the combined months of December and January. Also, the TSRDB data for the Trans-Pecos region in west Texas exhibits significantly higher direct normal solar radiation throughout the year than does the NSRDB.Copyright