STUDIES OF POOL BOILING HEAT TRANSFER FROM A HEATING TUBE IN HORIZONTAL AND VERTICAL ORIENTATIONS

Abstract

The present investigation was undertaken to obtain experimental data for nucleate pool boiling of saturated distilled water and isopropanol under subatmospheric and atmospheric pressures on the electrically heated tube in its both horizontal and vertical orientations with distinct aim to compare heat transfer coefficients for these orientations. The pressure for the boiling of `distilled water ranged from 16.00 kN/m2 to 97.20 kN/m2 whereas 26.67 kN/m2 to 97.04 kN/m2 for isopropanol. The heat flux was varied from 14,117 Wm' to 35,882 Wm' for both the liquids. The heating tube was made of brass having outside and inside diameters of 35mm and 18mm respectively with effective heating length of 155mm. The electric heater, to provide constant heat flux to the tube was a porcelain rod with nichrome wire wound on it. It was placed inside the test heating tube. The 24 gauge copper-constantan thermocouples were used to measure the wall temperatures of the heating tube and that of, the boiling liquid at the places corresponding to the places where wall temperatures were measured. For the horizontal orientation of the tube, the wall temperatures were measured at the top-,the side-,and the bottom- positions at a cross-section which was midway of the heated tube length. Corresponding to these places, liquid bulk temperatures were also measured.For the vertical orientation the wall temperatures were measured at cross-sections at heights of 82,102,137,168 and 187mm from the bottom end of the tube. Liquid bulk temperatures were also measured corresponding to these heights. From the experimental data for the pool boiling of liquids on the tube in its horizontal orientation it is observed that the wall temperature varies circumferentially. It increases from top- to side- to bottom- positiork continuously for a given value of heat flux and pressure. But the bulk temperature of the liquid did not vary appreciably. They are almost equal to saturation temperature corresponding to pressure on the boiling liquid.When the value of heat flux is raised without changing the pressure, the behaviour of the temperature distribution remains the same but the temperatures at the top-, the side-and the bottom- positions assume higher values. With further increase in heat flux the values of wall temperatures increase correspondingly. As regards the effects of change in pressure fromatmospheric to .various subatmospheric pressures at a given value of heat flux, there is a consistent progressive decrease in wall temperatures at the top-,the side- and the bottom- positions for the boiling of 'both the liquids but the behaviour that the wall temperature increases from top -to bottom- positions remain unaltered. The bulk temperature of the liquid assumed values approximately corresponding to the values of the pressures under which agiven liquid boiled. iii The data analysis ofaverage wall temperature of the tube in its horizontal orientation has resulted in a generalised correlation relating the wall superheat, (ATW)Ii to heat flux, and the physico-thermal properties of the boiling liquid, namely; latent heat of vapourisation, specific-heat, thermal conductivity and saturation temperature.