Band 37, Heft 3

Abstract. Prediction of heat transfer and pressure drops in flow passages is very essential to heat exchanger and commercial conduit design. It is necessary to know the relation between viscous properties of the fluid and wall shear rate in the conduit. This is done by assuming a constitutive equation which relates local apparent viscosity of fluid to the local shear rate. This research deals with the limits of validity of the power law equation. The useful methodology of the present research involves a consideration of a more general equation which has power law and Newtonian behavior as asymptotes. Use of the power law equation outside of its applicability range can lead to serious errors in predicting the heat transfer and pressure drops. Present computational results of the Nusselt numbers and the friction factors times Reynolds number for pseudo-plastic fluid flows in an isosceles triangular conduit are compared with previous published results, showing agreement with 1.59% in the Newtonian region and 2.85% in the power law region. These pseudo-plastic fluid results also showed the 12% increase of convective heat transfer and 65% reduction of pressure drop at the power law region compared with pressure drop and heat transfer with Newtonian fluid, respectively.

Abstract. Geothermal power plants are currently producing reliable and low-cost, base load electricity. Three basic types of geothermal power plants are currently in operation: single-flashing, dual-flashing, and binary power plants. Typically, the single-flashing and dual-flashing geothermal power plants utilize geothermal water (brine) at temperatures in the range of 550–430 K. Binary units utilize geothermal resources at lower temperatures, typically 450–380 K. The entropy production in the various components of the three types of geothermal power plants determines the efficiency of the plants. It is axiomatic that a lower entropy production would improve significantly the energy utilization factor of the corresponding power plant. For this reason, the entropy production in the major components of the three types of geothermal power plants has been calculated. It was observed that binary power plants generate the lowest amount of entropy and, thus, convert the highest rate of geothermal energy into mechanical energy. The single-flashing units generate the highest amount of entropy, primarily because they re-inject fluid at relatively high temperature. The calculations for entropy production provide information on the equipment where the highest irreversibilities occur, and may be used to optimize the design of geothermal processes in future geothermal power plants and thermal cycles used for the harnessing of geothermal energy.

Abstract. The adsorption properties of monosaccharide glucose and disaccharide isomaltose have been investigated on BEA zeolites with different moduli for a reaction-integrated adsorption system. For BEA-type zeolites from Süd-Chemie the moduli Si/Al  12.5, 75, and 225 were compared and showed a maximum for isomaltose adsorption on Si/Al  75. Adsorption models of Langmuir and Toth were used for fitting of the detected data and compared. Equilibrium measurements at different temperatures allowed the calculation of the adsorption enthalpy. For module Si/Al  75 adsorption enthalpy was using Langmuir and in the case of Toth's model. For glucose only was obtained, indicating a weaker adsorption compared to isomaltose. For an enzymatic production system, BEA 150 (Süd-Chemie) and HSZ-980HOA (Tosoh Corporation) were compared. The adsorption equilibrium constant K is on BEA 150 and on HSZ-980HOA. Hence, HSZ-980HOA shows a three-fold faster desorption, which was determined in desorption kinetics. Despite nearly identical loading under production conditions, BEA 150 reaches a yield of 64% for purification of isomaltose compared to 43% for HSZ-980HOA.

Abstract. A non-equilibrium thermodynamics model able to analyze the combined effect of diffusion and adsorption in porous materials is proposed. The model considers the coupled dynamics of the diffusive phase, described by a diffusion type equation, and the adsorbed phase governed by a generalized Langmuir equation. It is shown that the combination of diffusion and adsorption can be treated as diffusion in an effective medium, with an effective diffusion coefficient depending on the characteristics of the adsorption kinetics. The interplay between these effects leads to the appearance of different regimes in which the effective diffusion coefficient exhibits peculiar behaviors such as a decrease due to the presence of the adsorption process and a non-monotonous behavior resulting from particle interactions. We outline applications of the model developed to diffusion in porous materials such as zeolites and bentonites, and to engineered highly porous materials.

Abstract. The Marine Microbial Food Web (MMFW) includes heterotrophicmicrobes and their protist and viral predators. These microbes consume dissolved organic matter thereby making the MMFW a major component of global biogeochemical and energy cycles. However, quantification of the MMFW contribution to these cycles is dependent on a handful of techniques, all of which require laboratory-derived conversion factors. Here we describe a differential calorimeter capable of measuring the small amounts of heat produced by marine microbes and viruses at natural populations. Using this ultra-sensitive calorimeter, we show that heat production in the presence of viruses is significantly larger than in their absence. This increased heat output occurs despite a net decrease in the number of microbes. This provides direct evidence for top-down control of microbial populations by viruses and shows that there is increased re-mineralization. A comparative statics model was developed to interpret the calorimeter measurements. The spirit of the model is thermodynamic – it restricts its view to net changes in the populations and net heat produced. The model predicts that approximately 25% of the total heat production during the growth phase of a pelagic microbial community is due directly to viral activities. This result has implications for the energy budget of our planet and for climate prediction.