HEAT TRANSFER FROM HORIZONTAL CYLINDRICAL SURFACE TO LIQUID POOLS

Abstract

The problem of heat transfer from the outside surface of a horizontal cylinder under the condition of constant heat flux (q^ = constant) to the pool of liquids has been studied in the present investigation. Broadly, the investigation includes the free-convective heat transfer to liquids, heat transfer to liquids with radial flow agitation and nucleate pool boiling heat transfer to the saturated liquids with and without radial flow agitation. The heat transfer surface consisted of a stain less steel cylinder of inside diameter of 18 mm, outside diameter of 32 mm and total heating surface of 1.025xl0~2 2 m . it was oriented horizontally in the pool of liquids in a closed cylindrical vessel with a water-cooled condenser at its top. The liquids constituting the pools included distilled water, benzene and toluene of chemically pure grade. A four-bladed flat turbine type agitator of standard geometric configuration was employed for radial flow agitation in the respective liquids. Calibrated copper-constantan thermocouples measured the heating surface temperature at three positions at the top, the side and the bottom whereas the liquid temperature was measured at two positions in the respective planes passing through the top and the side thermocouples in heating surface. ii The experimental data for the frec-convective heat transfer to distilled v/ater, benzene and toluene have been found to possess an excellent agreement with the predictions fro;:': the B/ikheyev correlation and a satisfactory agreement with the correlation due to McAdams. a least square curve fit for the experimental data of heat transfer to the liquids; distilled water, benzene and toluene with radial flow agitation led to the following equation : 9 Nu A 1.57 ( i\e Gr 0.269 ) (Gr)*0.?13 (/DPr)x0.333 (6.3) The experimental data points showed a maximum deviation of + 25 per cent. The experimental heat transfer data for the saturated nucleate pool boiling of pure liquids; viz., distilled water, benzene ard toluene on the outer surface of the test cylinder at atmospheric pressure have also bee; obtained. The experimental runs wore conducted for heat flux varying from 13,935 d/n2 to 42,604 J/m2. The data points prove the valiiit. of the widely accepted power law relationship of boiling heat transfer ( h ffl q°'7). They are found to possess excellent agreement with some of the investigations. However, the data of Cichclli & Bonilla, Sterpling & Tichacek and Eorishanskii et al do not agree with the present values but there is a general correspondence between the two as a function of heat flux Ill over a whole range of heat flux studied. The discrepancy is due to the difference in the he"ting surfaces employed in these studies. The experimental data were also compa red with the analysis of Alad'ev for wall superheat. As a result of data analysis it is found that the Alad ev equation fits very well with the experimental data if its constant is modified . As a result of data analysis, the following correlation has been found best to represent the oxperiner tal data points on saturated nucleate pool boiling of liquids of this investigation within + 10 per cent error: NuB = 1.391 (PeB)°*7 (P /Pl)0.212 (Pr)-0.212 (fi#6) The experimental data for boiling heat transfer of distilled water, benzene and toluene were also obtained in presence of radial flow agitation., The range of heat flux was from 18,935 W/m to 42,604 W/m2 and that of the impeller speed from 5.235 rad/s to 41.880 rad/s. These experimental data have been correlated by the follow ing correlation within a maximum deviation of + 10 per cent: Nu 0.65 0.50 BA (Pv/\) (Pr)-°-50 (Eoj)0'114 'B V' L' (6.9) Following the Nishikawa model and the adequate equations for nuclcation sites»wall superheat and product iv of bubble break-off volume and its emission frequency, a semi-theoretical analysis has also been carried out for computing nucleate pool boiling heat transfer coefficient from heated surface to saturated pure liquids at atmosphe ric and subatmospheric pressures. The analysis has resulted in the following equations for the calculation of(h/hQ). For distilled water, 2.833 L-= (p_}-0.355 /X_VU5 ( ^ o • T i T" h P.. 0*1.25 Ou (p-V-) (_o } ( i£) pv,o or cl For organic liquids, 2.133 0.7 V L. o 0,7 PL 0.25 ) (pfH L, o (5.23a) ={~J V ( Ts ' kL PL,.' 0.25 h p„ a 1.25 c. . 2.133 q 0.7 (J[ ) (__0) (-Ju£) (—) (5.23b) pV,o 0" c 4o It may be noted that the present analysis, Eqs (5.23a & 5.23b) have succeeded in relating the heat transfer coefficient ratio , h/hQ with the ratios of system pressure, heat flux and physico-thermal properties of boiling liquids. These equations are free from surfaceliquid combination factor and hence cnn be Pr°fitably used for checking the consistency of boiling data V obtained on different heating surfaces. This fact hi been evidentlly proved by the excellent agreement (+, 10 %) between the experimental heat transf coefficient of earlier investigations, which taken on different heating surfaces, and those predicted from the present analysis . Further,this analysis appears to help in predicting the value of heat transfer coefficient for a given heating surface pro vided the pressure ranges from 39,22 kN/m2 to 101.330 kN/m2 and the experimental value of heat transfer coefficient at some pressure within this range. is known#