Karl Stephan

Heat-transfer correlations for natural convection boiling

Abstract To-date there exists no comprehensive theory allowing the prediction of heat-transfer coefficients in natural convection boiling, in spite of the many efforts made in this field. In order to establish correlations with wide application, the methods of regression analysis were applied to the nearly 5000 existing experimental data points for natural convection boiling heat transfer. As demonstrated by the analysis, these data can best be represented by subdividing the substances into four groups (water, hydrocarbons, cryogenic fluids and refrigerants) and employing a different set of dimensionless numbers for each group of substances, because certain dimensionless numbers important for one group of substances are unimportant to another. One equation valid for all substances could be built up, but its accuracy would be less than that obtained for the individual correlations without adding undesirable complexity.

An investigation of energy separation in a vortex tube

Abstract The process of energy separation in a vortex tube with air as a working medium is studied in detail. Experimental data of the temperatures of the cold and hot air leaving the vortex tube are presented. The variation of the maximum wall temperature along the vortex tube surface provides useful information about the location of the stagnation point of the flow field at the axis of the vortex tube. Experimental results indicate that the Gortler vortex produced by the tangential velocity on the inside wall of the vortex tube is a major driving force for the energy separation. A similarity relation for the prediction of the temperature of the cold exit air, obtained from the dimensional analysis, is presented and confirmed by experimental data.

A model for correlating mass transfer data at gas evolving electrodes

Abstract With reference to models for heat transfer phenomena in nucleate boiling, a model is presented to correlate mass transfer data at gas evolving electrodes. Mathematical treatment of the model results in an equation appropriate for practical use. Comparison with experimental data by several authors shows satisfactory agreement.

High-pressure phase equilibria of the system CO2—water—acetone measured with a new apparatus

Abstract A new apparatus for the rapid determination of fluid phase equilibria up to 300 bar and 120°C is described. VLE measurements of the 37.8°C isotherm of the CO 2 -n-butane system as well as the 40°C isotherm of the CO 2 —acetone system compare well with results obtained by other authors. New VLE and VLLE data are presented for the CO 2 —water—acetone system at 40°C and pressures of 40 bar and 100 bar. The results, correlated with a modified Redlich—Kwong—Soave equation of state with mixing rules proposed by Huron and Vidal, are interpreted with respect to the extractability of acetone from diluted aqueous solutions using compressed CO 2 as the solvent.

A similarity relation for energy separation in a vortex tube

Abstract A general mathematical formulation of the energy separation process taking place in a vortex tube is presented. Based on the governing equations a similarity relation of the variation of the cold gas exit temperature with the cold gas mass ratio for geometrically similar vortex tubes is established and compared with experimental data. The experiments conducted with air, helium and oxygen as working media confirm the theoretical considerations and correspond very well with the similarity relation.

Characterization of a two-phase impinging jet absorber—II. Absorption with chemical reaction of CO2 in NaOH solutions

Absorption of CO2 into NaOH solutions was performed in a two-impinging-jet absorber under conditions of second-order chemical reaction. The enhancement factor, interfacial area and mass transfer coefficient were determined. The values of the specific area a varied between 0.9 and 20.5 cm−1 and those of the mass transfer coefficient kL between 0.029 and 0.066 cm/s corresponding to values of kLa ranging between 0.025 and 1.22s−1, as already reported in the first part of this paper. These values are among the highest encountered for conventional absorbers. The characteristic two-maxima behavior of absorption rate vs internozzle distance repeated itself in this case too.

Analysis of the process characteristics of an absorption heat transformer with compact heat exchangers and the mixture TFE–E181

Abstract In the project described in this paper an experimental rig for a one-stage absorption heat transformer was designed and constructed. One aim of the project was to reduce the investment costs for the apparatus. This incorporates new and less expensive compact brazed plate heat exchangers for generator, evaporator, condenser and solution heat exchanger. The absorber was designed as a helical coil pipe absorber, where the weak solution trickles down as a falling film outside of the coil. The tests of the equipment involved measurements using a mixture of trifluorethanol (TFE) and tetraethyleneglycoldimethylether (E181). The process characteristics were investigated for different temperatures of the rich solution leaving the absorber. Experimental results are presented and compared with the results of a computer simulation model. Additionally the model was used to compare the COP of the heat transformation process with the mixtures lithium bromide–water (LiBr–H2O) and ammonia–water (NH3–H2O). Furthermore, the overall heat and mass transfer coefficients for the plate heat exchangers and the falling film absorber were evaluated and compared with those of shell and tube heat exchangers.

Influence of heat conduction in the wall on nucleate boiling heat transfer

Abstract On the basis of a single bubble model on heat transfer in a wedge shaped micro region the influence of heat conduction in the wall on heat transfer in nucleate boiling is studied. The wall thermal conductivity λW is varied, whereas all other parameters, particularly the bubble site density, are kept constant. Wall conductivity is set to the values of copper, stainless steel and a ceramic material. The results of the parameter study show that even large variations of λW have only moderate influence on heat transfer to a growing vapour bubble. Simulation results for various boiling liquids are then compared with experimental data of others to test the model and examine the influence of liquid properties upon boiling heat transfer. Although the model is based on some simplifying assumptions and thus not all heat transfer mechanisms can be described in detail, the boiling curve can be predicted in the nucleate boiling regime with good accuracy for low to moderate heat fluxes.

Correlations for nucleate boiling heat transfer in forced convection

Abstract As shown in a recent paper [9] the many existing data for natural convection nucleate boiling are well represented by using a different set of dimensionless numbers for different groups of substances. Furthermore a general equation valid for all substances could be built up. The influence of forced convection is now considered. It may be taken into account by an additional term. Existing experimental data could be fairly well correlated by introducing the new pool boiling equation into an equation by Chawla.

Influence of dispersion forces on phase equilibria between thin liquid films and their vapour

Abstract Heat transfer and nucleation processes in nucleate boiling strongly depend on the phase equilibrium at the liquid–vapour interface. In a certain region between heated wall and a vapour bubble where a thin liquid film is adsorbed, phase equilibria are considerably influenced by dispersion forces acting on the liquid film. As shown in the paper in such systems the chemical potential, decisive for phase equilibria between liquid films and their vapour, contains an additional term for the action of dispersion forces, and differs from the chemical potential of dispersion-free systems, though their chemical potential is usually taken in the literature for systems with dispersion forces. With the aid of the chemical potential the Kelvin equations for the pressures at the liquid–vapour interface were derived. It turned out that the Gibbs assumption of a geometrical interface between extremely thin liquid films in equilibrium with its vapour does not hold. Instead, following the ideas of van der Waals junior, the small but finite transition interlayer between both phases had to be introduced. As numerical examples illustrate, the dispersion forces considerably influence the pressures at the liquid–vapour interface. In nucleate boiling processes the driving pressure difference for evaporation undergoes a maximum within a tiny area underneath vapour bubbles. As could be shown the maximum driving pressure difference between gas-side interface and gas-core is a considerable fraction of the vapour pressure itself and contributes significantly to the high heat fluxes in nucleate boiling.