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Experimental investigations into intra-layer boiling crisis and hysteresis in metal, fibrous, porous, coverings

Wójcik T. M.

Transactions of the Institute of Fluid-Flow Machinery

2003

nr 112

91-102

Capillary-porous structures (CPS) do not only enhance boiling heat transfer, they also may cause the increase in the value of the critical heat flux. Apart from boiling crisis, resulting, as for a smooth surface, in the wall burnout, in CPS there also occurs the so-called "intra-layer- boiling crisis". The results of experimental investigations into boiling heat transfer on heating surfaces covered with fobrous, porous layers were presented. The interest area covered the impact of the increase in heating surface temperature on the values of the heat flux and boiling heat transfer coefficient.

Experimental investigation of boiling heat transfer hystersis on metal fibrous porous coverings

Poniewski M., Wójcik T.

Archives of Thermodynamics

1999

Vol. 20, no 1/2

93-118

Knowledge of thermal characteristics of capillary-porous structures is indispensable for designing and constructing of heat exchangers based on the concept of a heat pipe. The paper presents the results of experimental research in boiling on heating surfaces covered with copper fibrous porous layers that provide a filling for heat pipes. The results of the research pertain to the structures with randam pore size distribution. The research was carried out for pool boiling of distilled water, ethanol and freon-113 for layers with porosity of 40, 70 and 85%. A significant enhancement of heat transfer in comparison to a smooth surface was noted. The heat transfer coefficient and the maximum heat flux depend on the structural parameters of the covering.The boiling curve was determined by increasing and decreasing the heat flux, which for certain sets of structural parameters of the coverings and the physical properties of liquid induce heat transfer hysteresis. Two different types of hysteresis were observed. For For a relatively thick coberings, the hysteresis is characterized by a decrease of the heat transfer coefficient after the flux is reduced (appering after the maximum flux is reached). The second type of hysteresis is characterized by an increase of the heat transfer coefficient when the flux is lowered to below its maximum value. The physical basis of the phenomenon of hysteresis was presented in consideraration of new factors characterizing the mechanism of boiling on a porous covering (intralayer crisis, gradual pore activation, delayed activation of cells within the structure). For the zones of the boiling curve without hysteresis, the experimental results were developed in a form of a correlation for the Nusselt number.