Home Books eBooks Journals References & Proceedings Authors, Editors, Reviewers A-Z Product Index Awards
Proceedings of Symposium on Energy Engineering in the 21<sup>st</sup> Century (SEE2000) Volume I-IV

ISSN:
1-56700-132-7 (Print)

NUMERICAL INVESTIGATION OF THE FLUID FLOW AND HEAT TRANSFER IN A SOLAR DOMESTIC HOT WATER STORE WITH A MANTLE HEAT EXCHANGER FOR THE SOLAR LOOP

Erich Hahne
Institut fur Thermodynamik und Warmetechnik, Universitat Stuttgart, 70550 Stuttgart, Germany

H. Mandel
Institut fur Thermodynamik und Warmetechnik (ITW) Universitat Stuttgart, Germany Pfaffenwaldring 6; 70550 Stuttgart

H. Druck
Institut fur Thermodynamik und Warmetechnik (ITW) Universitat Stuttgart, Germany Pfaffenwaldring 6; 70550 Stuttgart

Abstract

In order to obtain a high solar fraction/with a thermal solar system for domestic hot water supply, it is desirable to build up and maintain as long as possible a thermal stratification in the hot water store. For this purpose, special precautions must be taken while charging the store with hot fluid from the collector loop. One concept to charge the store in a stratified way is to use a store with a mantle heat exchanger. A TRNSYS [1] simulation study with idealized assumptions shows, that this store concept could improve the efficiency of solar domestic hot water systems considerably. In order to investigate in greater detail the possibilities of charging a store in a stratified way by using a mantle heat exchanger, numerical simulations of the fluid flow and heat transfer phenomena inside the annular gap have been carried out. The commercial Computational Fluid Dynamics (CFD)-software FLUENT has been used for this purpose. Two-dimensional as well as three-dimensional models were used. Because of the transient process of charging a long calculation time is required. Hence only the fluid flow and the heat transfer in an annular gap are examined. Thermal stratification inside the gap was assumed as initial condition. Both the area around the inlet pipe as well as the entire geometry are investigated in detail. The results show that charging in a stratified way works well if the incoming fluid is hotter than the fluid at the top of the annular gap. In the case of charging the annular gap with a relatively cold fluid, the stratification is partly destroyed in the upper part of the gap. In the bottom part on the other hand, where the temperatures are lower than that of the charging fluid, the stratification is widely preserved. Concluding it can be said that stores with a mantle heat exchanger can be considered a very interesting alternative to existing store concepts, that make great efforts to obtain and maintain thermal stratification.