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1

Mathematical modelling of the unsteady operation of a plate and fin heat exchanger for the time-varying mass flow rate of liquid and air velocity

Korzeń A., Taler D.

Technical Transactions

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2017

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Vol. 3(114)

183--196

EN

The mathematical simulation of a plate fin and tube heat exchanger is presented in this paper. The simulation of the transient operation of the heat exchanger was carried out using a general numerical model that was previously developed by the authors. The Reynolds number of the water flowing inside the tubes varied in the range from 4000 to 12000. A detailed analysis of the transient response of a heat exchanger to sudden increase in water mass flow rate and the simultaneous reduction in air flow velocity was modelled. Heat transfer correlations for air and water were determined based on the experimental data. Unknown parameters appearing in the relationships for the Nusselt numbers on the airand water-sides were estimated using the least squares method. A set of partial differential equations for the temperature of water, air, tube wall, and fins was solved using the finite volume method. The results of the numerical simulations of a heat exchanger using experimentally determined air and water-side heat transfer formulas for the calculation of heat transfer coefficients were compared with the experimental data. Excellent agreement between computation results (air and water temperatures at the outlet of the heat exchanger) and experimental results was obtained.

PL

Przedstawiona została symulacja matematyczna wymiennika ciepła z rur ożebrowanych. Symulacja nieustalonej pracy wymiennika przeprowadzona została za pomocą modelu matematycznego opracowanego wcześniej przez autorów. Liczba Reynoldsa po stronie wody zmieniała się w zakresie od 4000 do 12 000. Szczegółowa analiza zmian temperatury została przeprowadzona dla przypadku nagłego wzrostu strumienia masowego płynu z jednoczesnym obniżeniem prędkości powietrza. Korelacje na współczynniki wnikania ciepła dla powietrza i wody określono na podstawie danych doświadczalnych. Nieznane parametry, które pojawiają się w równaniach na liczbę Nusselta dla powietrza i wody wyznaczono za pomocą metody najmniejszych kwadratów. Układ równań różniczkowych cząstkowych umożliwiający wyznaczenie temperatury wody, powietrza, ścianki rury i żeber zostały rozwiązane z użyciem metody objętości skończonej. Wyniki numerycznej symulacji pracy wymiennika z użyciem współczynników wnikania ciepła wyznaczonych z korelacji na liczby Nusselta od strony powietrza i wody porównano z danymi eksperymentalnymi. Uzyskano bardzo dobrą zgodność wyników obliczeń i pomiarów.

2

Experimental and computational analysis of ribbing structure modification effect in tube and fin cross-flow heat exchangers operating at non-uniform inflow of media

Bury T., Hanuszkiewicz-Drapała M.

Journal of Power Technologies

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2017

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Vol. 97, nr 5

429--436

EN

Distributions of media streams flowing in a cross-flow tube and fin heat exchanger are usually non-uniform. This could be an effect of the heat exchanger construction, its installation method, design of a flowing channel or all those factors combined. The problem of the non-uniform media flow in heat exchangers of different types is not new, and it has been investigated by many researchers. Early results were sometimes ambiguous. More recent outcomes indicate that the effect of the non-uniform inflow of heat carriers to the heat exchanger could be significant it may adversely affect the device’s efficiency to a large extent. Investigations of tube and fin cross-flow heat exchangers carried out for almost twenty years at the Institute of Thermal Technology of the Silesian University of Technology, by way of experiments and numerical simulations, also confirm these latest conclusions. The reduction in overall heat exchanger capacity, comparing to the uniform inflow of media, may reach up to 18%. This work presents results of experimental and computational investigations of tube, fin, cross-flow, double row heat exchangers air-water. The heat exchangers under consideration are built in the form of two rows of elliptic tubes with rectangular fins. The ribbing structure of the first heat exchanger is uniform. This device was investigated primarily in order to determine its efficiency but also the range and the form of non-uniform inflow of air. The air flow distribution was tested on a special test station during a series of measurements. The results of the analysis of this heat exchanger were used to design a second heat exchanger with a non-uniform structure of fins on individual tubes. It was assumed that by changing the heat transfer surface (thickening the fins) in the region of high air speed, the efficiency of modified heat exchangers could be enhanced. Testing this hypothesis is the main aim of this work. The experimental results generally confirm the hypothesis, showing a rise in efficiency of up to 8%. However, it should be noted that the design of the modified ribbing structure is not optimal and changing this structure impacts the hydraulic resistance and distribution of air mass flow rate at the heat exchanger inflow. This effect should be considered when evaluating the results.

3

Numerical modeling of unsteady state operation of tube and plate fin heat exchangers

Taler D., Korzeń A.

Journal of Energy Science

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2012

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Vol. 1, nr 2

103--117

EN

Cross-flow tubular heat exchangers are applied as condensers and evaporators in air conditioners and heat pumps or as air heaters in heating systems. They are also applied as water coolers in so called 'dry' water cooling systems of power plants, as well as car radiators. There are analytical and numerical mathematical models of heat exchangers of that type to determine the steady state temperature distribution of fluids and the rate of heat transferred between fluids. In view of the wide range of applications in practice, these heat exchangers were experimentally examined in steady-state conditions, mostly to determine the overall heat transfer coefficient or the correlation for the heat transfer coefficients on the air side and on the internal surface of the tubes. There exist many references on the transient response of heat exchangers. Most of them, however, focus on the non-steady-state heat transfer processes in parallel and counter flow heat exchangers. In this paper, the new equation set describing transient heat transfer process in tube and fin cross-flow tube exchanger will be given and subsequently solved using the finite volume method.