Klaus Spindler

Test and simulation of a solar powered solid sorption cooling machine

Abstract At the Institut fur Thermodynamik und Warmetechnik (ITW) a solar powered cooling machine with no moving parts has been built for demonstration purposes. The main part of the device is an absorber/desorber unit which is mounted inside a concentrating solar collector. The heat of absorption is transported out of the solar collector by means of two horizontally working heatpipes. The working pair consists of NH 3 used as the refrigerant and SrCl 2 as the absorbing medium. The performance of the solar refrigeration unit was measured in a field test. The working principle of a discontinuously working, solid sorption cooling machine will be explained using the demonstration machine as an example. Results obtained from a field test performed in 1995 are presented and discussed. Furthermore, a simulation program for the numerical simulation of a solar powered solid sorption cooling machine has been developed and tested. Finally, to confirm a problem-free continuous operation of a solid sorption system, a long term study over 14 months was carried out using NH 3 /SrCl 2 as the working pair.

The influence of a low viscosity oil on the pool boiling heat transfer of the refrigerant R507

Abstract This paper describes the influence of a low viscosity polyolester based lubricating oil on the pool boiling heat transfer of the refrigerant R507. The pool boiling heat transfer coefficients for this refrigerant–oil mixture are measured on a smooth tube and on an enhanced tube. The investigation is made for oil mass fractions up to 10% and for saturation temperatures between −28.6°C and +20.1°C. For the smooth tube the heat transfer increases for increasing oil mass fractions up to 3% at lower saturation temperatures. At higher saturation temperatures the heat transfer decreases for increasing oil mass fractions for both tubes. For oil mass fractions greater than 1% at the higher saturation temperatures a range of decreasing heat transfer coefficient is found for increasing heat flux. The effect is caused by the different miscibility of the oil and the components of the refrigerant mixture.

An experimental study of the void fraction distribution in adiabatic water-air two-phase flows in an inclined tube

Abstract A fibre optical probe is used to measure the radial distribution of void fraction and bubble frequency of adiabetic water-air two-phase flow in an inclined tube. Three typical void fraction distributions can be established for vertical two-phase flow: sliding bubble flow, coring bubble flow, and an intermediate type. These typical distributions are influenced by the angle of inclination in different ways. In case of coring bubble flow, the void fraction maximum near the tube axis moves to the upper part of the cross-section when the vertical tube changes to the horizontal position. In case of sliding bubble flow, one of the void fraction maxima near the tube wall is increased (upper part of cross-section) and the other one is decreased (lower part of cross-section) when the inclination changes from vertical to horizontal. The flow pattern transition can also be seen in the change of equivalent bubble diameters.

Steady-state and transient temperature field in the absorber tube of a direct steam generating solar collector

Abstract The temperature field in the absorber tube of a direct steam generating parabolic trough collector is calculated. Steady-state and transient operating conditions are considered. A universal program was developed for solving the two-dimensional transient temperature field using a modular nodal point library. The temperature field is extremely asymmetric due to the variation of the heat transfer coefficient at the inner surface and the solar irradiation at the outer surface of the absorber tube. High temperature peaks are found, especially in stratified flow at higher void fractions. The transient behaviour of the absorber tube has been analysed by stepwise increasing or decreasing the solar irradiation. The response time of the absorber tube is between 70 and 140 s for different void fractions inside.

Pool boiling heat transfer of propane from a horizontal wire

Abstract The pool boiling heat transfer of saturated propane (C 3 H 8 ) was investigated in the pressure range between 5 and 20% of the critical pressure (corresponding to a temperature range between −25 and 20°C). The heat flux was varied such that the range from free convection without bubble formation to intensive nucleate boiling was covered. A horizontal electrically heated platinum wire served as both heating element and resistance thermometer. The experimental results are compared with values from VDI-Warmeatlas obtained with a copper tube and with calculated values using a modified equation. The boiling heat transfer coefficients of propane were slightly lower than that of R22 under the same condition.

A new pressure drop correlation for subcooled flow boiling of refrigerants

Abstract Pressure drop data from experiments on subcooled flow boiling of refrigerant R12 and R134a have been correlated. A wide range of pressures (8–20 bar), mass fluxes (750–3000 kg s−1 m−2), subcooling temperatures (2–47.6 K) and heat fluxes (up to 207 500 W m−2) has been considered. The new correlation for the reduced pressure drop (sum of fractional and accelerational component) is a function of the boiling number and the Jakob number. The ratio of heated and wetted perimeter of the flow channel is included in the correlation so that it is applicable for both tubes and annuli. With the exception of six out of a total of 326 experimental data points all data lie within ±25% around the correlation.

Fully developed nucleate boiling in upflow and downflow

Abstract Heat transfer coefficients from experiments on flow boiling with R12 are obtained for a wide range of pressure, mass flux, heat flux and flow quality. The experimental results are compared with seven different correlations given in the literature. Good agreement is found only for some correlations. Best agreement is found for the modified Chen correlation using the pool boiling equation by Gorenflo and the suppression factor by Bennett et al.. In the correlation proposed by Bjorge et al. a numerical value for the constant BM for R12 is recommended. With a minimum liquid velocity of 0.25 m s−1, a clear effect of flow direction—upwards or downwards—cannot be observed.

Pool boiling heat transfer of refrigerant mixtures R32/R125

Abstract Nucleate pool boiling heat transfer of new, environmentally harmless refrigerant mixtures R32/R125 has been systematically investigated in a wide range of pressure and heat flux under saturation conditions using a horizontal platinum wire (d = 0.1 mm). The platinum wire served as both heating element and resistance thermometer. The experimental results are compared with calculated values using a modified Jungnickel correlation. The comparison between all calculated data and experimental data shows good agreement. Most of the data fall within ±20% of the correlation. The mean average deviation is ±8.25%.

Vergleich von Korrelationen zum Wärmeübergang beim unterkühlten Sieden

Experimental results of subcooled boiling heat transfer of refrigerant R12 (CCl2F2) in a vertical copper tube in upflow and downflow are reported. The results are compared with different correlations given in literature. The constants of some correlations are adjusted to yield better agreement with the experimental data of R12. A clear effect of flow direction — upwards or downwards — cannot be observed for liquid flow velocities of 0.5 to 1.75 m/s.ZusammenfassungEs werden Experimente zum Wärmeübergang beim unterkühlten Sieden von R12 (CCl2F2) in einem senkrechten Kupferrohr bei Aufwärts- und Abwärtsströmung vorgestellt. Die Wärmeübergangskoeffizienten werden mit Korrelationen aus der Literatur verglichen. Für einige Korrelationen werden Änderungen in den Konstanten vorgeschlagen, um eine bessere Übereinstimmung mit den experimentellen Daten zu erreichen. Ein deutlicher Einfluß der Strömungsrichtung — aufwärts oder abwärts — auf den Wärmeübergangskoeffizienten konnte für Flüssigkeitsgeschwindigkeiten von 0,5 bis 1,75 m/s nicht festgestellt werden.

Fibre Optical Sensors for Measuring Local Quantities in Gas Liquid Two-Phase Flows

For measuring the local void fraction, bubble frequency, bubble diameter and bubble velocity in two-phase flows, many types of fibre optical sensors are known. They use the difference of the refractive indices of the liquid and the gas phase as measuring principle. The power of the light emitting device has been reduced from 500 W to a few mW. The sensitive diameter of the fibre optical probe has been reduced from a few millimetres to a few micrometres. Single probes, double probes and arrangements of several probes up to bundles with 500 fibres have been developed. In the past the pure signals were displayed on a pen recorder or an oscilloscope. Nowadays high sophisticated signal processing units using transputers or FFT- analyzers give additional information about the two-phase flow pattern. Looking on their historical development an overview of the various types of fibre optical sensors will be given.