E. Hahne

Numerical study of flow and heat transfer characteristics in hot water stores

Abstract The flow and heat transfer characteristics in a cylindrical hot water store during the charging process under adiabatic thermal boundary conditions were studied numerically in the present paper. The charging efficiency was used to evaluate the thermal stratification. The emphasis was put on the effects of charging temperature differences, charging velocities, charging flow rates and length to diameter ratios on the charging efficiency. The results were summarized both in dimensional and dimensionless forms. They indicate that the charging efficiency depends mainly on the modified Richardson number RiH,f and Peclet number PeH,f, which present the combined effects of charging temperature difference and charging velocity on the charging efficiency. If RiH,f is larger than 0.25, the charging efficiency is above 97%. At a given Richardson number the increase of Peclet number leads to a higher charging efficiency. For H/D less than 4, the increase of the height to diameter ratio H/D can improve the charging efficiency as well. The effect of the Fourier number (or charging flow rate) on the charging efficiency, however, is relatively small. A correlation of the numerical results was obtained for the design of effective hot water stores.

Formfaktor und formwiderstand der stationären mehrdimensionalen wärmeleitung

Zusammenfassung Potentialverteilungen sind fur eine Reihe von Anordnungen berechnet und in der Literatur angegeben. Diese Losungen werden kritisch gesichtet und ihre Gemeinsamkeiten herausgearbeitet. Die Abtrennung von anwendungsspezifischen Stoffgrossen fuhrt auf den Formfaktor, der allein von der geometrischen Anordnung abhangt und fur alle Vorgange Gultigkeit hat, denen skalare Potentialfelder zugrundeliegen. Die Berechnung des Warmestromes bei stationarer Warmeleitung stand hier im Vordergrund. Sie ist, bei Kenntnis des Formfaktors auch fur komplizierte geometrische Anordnungen sehr einfach. Da Angaben uber Formfaktoren in der Literatur gelegentlich verwirrend und mehrdeutig siad, ist hier auf eine konsequente Einteilung entsprechend den auf analytische Losungen zuruckgehenden Fallen der planparallelen Platte, der koaxialen Rohre und der Kugel geachtet. Die Charakteristika dieser Losungen sind in jedem angegebenen Formfaktor enthalten. Eine besonders einfache Moglichkeit der Ermittlung von Formfaktoren fur Anordnungen von Einzelkorpern wird gezeigt. Damit und mit der Moglichkeit der Vertauschung von Isothermen und Adiabaten, sowie der symmetrischen Erganzung konnen die hier angegebenen rund 50 Formfaktoren Grundlage einer Vielzahl von geometrischen Anordnungen sein.

Pool boiling heat transfer on finned tubes—an experimental and theoretical study

Pool boiling heat transfer of R11 is investigated experimentally and theoretically on single tubes and twin tube arrangements. Finned tubes with 19 and 26 fins per inch (fpi) are used. The refrigerant evaporates at a saturation state of 1 bar on the outside of the electrically heated tubes. For the twin tube arrangements the tube pitch is varied between, s/dF = 3.0 and 1.05. The heat transfer on the upper tube of the twin tubes is enhanced compared with the single tube, while the heat transfer on the lower tube is not affected by the upper tube. The maximum heat transfer of the upper tube increases with increasing tube pitch.

Test and simulation of a solar-powered absorption cooling machine

Abstract At the Institut fuer Thermodynamik und Waermetechnik (ITW) a solar cooling machine 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 working pair consists of NH3 used as the refrigerant and SrCl2 used as the absorbing medium. Performance of the solar refrigeration unit was measured in a field test. The working principle of a periodically working, dry absorption 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. Further, a simulation program for the numerical simulation of a solar-powered dry absorption cooling machine has been developed and tested.

Thermal conductivity of the new refrigerants R134a, R152a, and R123 measured by the transient hot-wire method

Thermal-conductivity measurements are reported for the new refrigerants R134a, R152a und R123. Transient hot-wire experiments were performed which cover both the liquid and vapor states at temperatures and pressures ranging fromϑ=−20°C to 90°C and fromp=0.1 bar to 60 bar respectively. The results are correlated with density and temperature. In addition temperature dependent correlations are presented for (i) saturated liquid, (ii) saturated vapor, (iii) ideal gas (which equals approximately vapor state at ambient pressure). Finally the results are compared with data from the literature and also with the thermal conductivities of R12 and R11.

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.

Solar-Assisted District Heating Plants: Status of the German Programme Solarthermie-2000

Within the 10 year Programme “Solarthermie-2000” of the German Federal Ministry for Economics and Technology (BMWi), large-scale solar-assisted heating plants have been funded in Germany. Long-term monitoring programmes are carried out to prove the technical and economic feasibility of various solar system concepts with and without seasonal storage. The paper presents a summary and review of the present status and results of the activities in the Programme. Long-term monitoring programmes have shown that the design data of the solar plants can be achieved if realistic assumptions are made. The main obstacle for the implementation of the concepts on a broader scale without public subsidies is the high investment costs of the solar systems. The main goal of the Programme is the further improvement of the cost-effectiveness of the solar concepts.

Natural convection heat transfer on finned tubes in air

Abstract In an experimental investigation temperature fields and mean heat transfer coefficients were obtained on a finned tube. The diameter of the fin was varied in order to find out about the effect of fin height. Such an effect on the heat transfer coefficient vanishes in a correlation given in the literature. By thermovision the usefulness of the fins can be observed: the smaller fin proved to be better than the larger one under given conditions. The heat transfer coefficient depends on fin height, with better heat transfer for smaller fins. The respective correlation based on thermovisually obtained fin temperatures is presented. For the more practical case, with the Grashof number being based on the temperature difference between central tube and ambient air, an improved correlation is presented with less than 6% scatter.

Monitoring and simulation of the thermal performance of solar heated outdoor swimming pools

Abstract Based on detailed measurements of two outdoor swimming pools (at Leonberg and Mohringen) a computer model has been developed and validated for the simulatin of the thermal behaviour of such pools. The subroutine is compatible to TRNSYS 13.1. Correlations for the heat losses due to evaporation, convection, and radiation were taken from literature and tested in the model. It was not possible to select one optimal correlation for the description of the evaporative heat losses of both swimming pools due to the different exposure to wind. Using the most suitable correlation for the evaporative heat losses of each pool allowed for the simulation of the pool temperature with less than 0.5 K standard deviation between measured and simulated temperature. The major problem was the measurement of the relevant wind speed to be used in the correlations describing the evaporative heat losses under real outdoor conditions. A method is described detailing how to calibrate the model using the heating energy requirement and the measured pool temperature during actual operation periods. The analysis of the measured data of two different outdoor swimming pools under the same climatic conditions showed differences of a factor 2 and more in the heat demand per unit pool area. This was mainly caused by the difference in local wind speed which differed by more than a factor 4. The two pools investigated were heated by solar energy with a fraction of 28% and 14%, respectively, and the seasonal efficiency of the solar systems was 37.7% and 33.4%. Simulations show that a reduction of the water temperature from 24°C to 22°C during periods low with outdoor temperatures and few visitors, reduces the fuel consumption to less than half and increases the solar fraction from 28% to 50% in one pool.

The ITW solar heating system: an oldtimer fully in action

Abstract Public awareness of energy in the early 1970s stimulated a number of projects on alternative ways of heating. The ‘Institut fur Thermodynamik und Warmetechnik’ (ITW) of the University of Stuttgart has been operating a solar heating system since 1985. Ever since, this system has been minutely monitored. In particular, the storage was painstakingly considered as it was intended to serve as a pilot facility for the much discussed problem of seasonal storage. This storage unit should be simple and cheap but heavily instrumented in order to obtain many and accurate data and it should be versatile in order to gain knowledge for operation. The solar heating is provided by collectors that are unglazed, so a heat pump is required for appropriate heating temperatures. However, the heat pump allowed for a combination of heating and cooling in our system and this proved to be very advantageous, as cooling energy is more expensive and more in demand in our building than heating energy. The system was used in various seasonal cycles with changing conditions. It has now been operating for almost 15 years. During this period, neither storage nor collectors caused any trouble. Some difficulties were experienced with the heat pump. The first one had to be replaced; we made suggestions for improvement of the second one. This provided good COPs but there has been an occasional defect. The experience with our solar heating system was so satisfactory that, based on the knowledge gained from it, large housing projects in Friedrichshafen and Hamburg and an office building project in Chemnitz were conceived and built and are now being monitored.