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Analytical 1D models of the wall thermal resistance of rectangular minichannels applied in heat exchangers

Rybiński W., Mikielewicz J.

Archives of Thermodynamics

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2016

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Vol. 37, no. 3

63--78

EN

The paper presents four 1-dimensional models of thermal resistance of walls in a heat exchanger with rectangular minichannels. The first model is the simplest one, with a single wall separating two fluids. The second model of the so called equivalent wall takes into account total volume of intermediate walls between layers of minichannels and of side walls of minichannels. The next two more complicated models take separately into account thermal resistance of these walls. In these two models side walls are treated as fins. The results of models comparison are presented. It is shown that thermal resistance may be neglected for metal walls but it should be taken into account for the walls made of plastics. For the case of non-neglected wall thermal resistance the optimum wall thickness was derived. Minichannel heat exchangers made of plastic are larger than those built of metal, but are significantly cheaper. It makes possible to use of such exchangers in inexpensive microscale ORC installations.

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A study of flow boiling heat transfer in a rectangular minichannel using liquid crystal and infrared thermography

Strąk K., Piasecka M.

Transactions of the Institute of Fluid-Flow Machinery

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2015

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nr 128

97--118

EN

This paper presents results of flow boiling heat transfer in two parallel asymmetrically heated vertical minichannels. The heating element for Fluorinert FC-72 flowing in the minichannels was a thin foil with an enhanced surface on the side in contact with the fluid. The channels were observed from both sides through glass panes. Two of the panes allowed us to observe the two phase flow patterns on the enhanced surface of the foil, while the other two panes were used for simultaneous measurement of temperature on the smooth side of the foil. The temperature was measured by applying two contactless methods. One was liquid crystal thermography (LCT), which required treating the foil surface with thermochromic liquid crystals (TLCs), and the other was infrared thermography (IRT), which required coating the foil surface with black paint. Calculations were performed on the basis ofa one-dimensional model to determine the heat transfer coefficient at the foil-fluid interface from the Robin boundary condition. The experimental results were graphically represented as the foil temperature and the heat transfer coefficient against the distance from the minichannel inlet and as LCT and IRT images obtained for saturated boiling.

3

Nierównomierność rozpływu mieszaniny dwufazowej gaz - ciecz w układzie minikanałów współpracujących równolegle

Miłaszewicz B., Ulbrich R.

Inżynieria i Aparatura Chemiczna

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2010

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Nr 3

69-70

PL

Nierównomierna dystrybucja faz do poszczególnych kanałów jest powszechnym zjawiskiem i powoduje istotne obniżenie sprawności aparatów wykorzystujących przepływ w kanałach równoległych. Jest to zjawisko skomplikowane, słabo poznane w zakresie badań w minikanałach. W artykule przedstawiono badania przepływu opadającego mieszaniny dwufazowej do układu ośmiu minikanałów. Zastosowano dwa typy głowicy różniące się geometrią. Wykazano istotne znaczenie zarówno geometrii głowicy jak i strumieni doprowadzanych faz.

EN

Phase maldistribution to particular channels is common phenomena and causes significant efficiency reduction of devices using parallel flow. It is very complicated and poorly examined phenomena in minichannels. Research results of two-phase downward flow of mixture to the system of eight parallel minichannels are presented in the paper. Two kinds of header of various geometry was employed. It was proved that both geometry and volumetric flow rate of gas and liquid had a substantial significance on flow uniformity.