NUCLEATE POOL BOILING HEAT TRANSFER FROM REENTRANT CAVITY SURFACES

Abstract

The present investigation deals with experimental studies related to nucleate boiling heat transfer from reentrant cavity tubes to pool of saturated acetone, isopropanol, ethanol and distilled water at atmospheric pressure. The experimental set-up has been designed, fabricated and commissioned carefully with the objective of obtaining accurate and reliable data. This has been possible through appropriate choice of the dimensions of test vessel, heating tubes as well as reentrant cavities and knock-out condenser. In addition orientation of heating tube, liquid pool height above heating tube, and selection of instruments have also been quite critical. Plain and reentrant cavity heating tubes have been fabricated from the same lot of extruded brass rod. All the reentrant cavity tubes have been identical in their dimensions except the cavity mouth size. Their mouth sizes have been 0.2, 0.25, 0.3, 0.4 and 0.5 mm, respectively. They have been fabricated by rolling a plain integral-fin tube having a fin density of 1024 fins/m, except the one having a mouth size of 0.5 mm. The cavities of the tubes have been made in such a manner that they are inter-connected to form a continuous channel from one end of the tube to the other end . It is emphasized that the interior dimensions of the cavities are the same, irrespective of cavity mouth. size. Before the start of experiments, the experimental set-up has been tested for absence of electrical and mechanical leakages. It has also been ensured that the test vessel and heating surface along with connecting pipe lines are thoroughly cleaned and rinsed before charging the test liquid. Another important step has been • the thermal stabilization of heating tube. Deaeration of liquid pool has been carried out thoroughly. The experimental data have been obtained in the order of increasing heat flux in six steps from 11,962 to 41,866 W/m2. The experimental data for the boiling of liquids on horizontal plain heating tube obey the well-established law, i.e., heat transfer coefficient varies with heat flux raised to the power of 0.7. These data show excellent agreement with the predictions from correlations due to Stephan and Abdelsalam, Cooper, and Alam and Varshney. Most i of the experimental data for the boiling of liquids on horizontal inter-connected reentrant cavity tubes are new. For these data it is established that the heat transfer coefficient changes with heat flux to the power of 0.83. As the experimental data on inter-connected reentrant cavity tubes pertain to liquids of widely differing physico-thermal properties, the conclusions based on them are of a more generalized nature than those from studies attempted by earlier investigators. Another important aspect about the present investigation is that it has studied the effect of cavity mouth size for the nucleate pool boiling heat transfer of liquids other than refrigerants. Thus, it is hoped that the conclusions emerging from the present investigation would form a sound basis for the furtherance of knowledge and necessary future research in this area of interest. An analysis of experimental data shows that the reentrant cavities with the mouth sizes 0.2, 0.25, 0.3, 0.4 and 0.5 mm can be advantageously used to enhance heat transfer coefficient of plain tubes. It is also established that to achieve the best advantage of the inter-connected reentrant cavity tubes in terms of heat transfer coefficient there is a specific mouth size depending upon physico-thermal properties of boiling liquid. This is 0.3 mm for the boiling of acetone and isopropanol at atmospheric pressure. As a matter of fact, this enhancement in heat transfer coefficient leads to reduction in the surface area/wall superheat. Hence the use of inter-connected reentrant cavity tubes in the fabrication of reboilers, and vaporizers used in process industries makes them compact and efficient. Finally, based on experimental data, two correlations have been developed for the prediction of heat transfer coefficient for the transfer of heat from inter-connected reentrant cavity tubes to the pool of saturated acetone and isopropanol on one hand and a separate correlation for ethanol and distilled water on the other hand. These correlations make use of a new dimensionless number, (x/db), recommended by the present investigation, besides some other known dimensionless numbers.