Pressure drop and heat transfer in a vertical internally heated annulus.
Howard, Charles P.
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The heat transfer and pressure drop of water flowing both upward and downward In an annulus were investigated. The data were obtained from a vertical narrow space annulus which was formed between 1.240 in. I. D. and 1.125 in. O.D, 99.9% copper tubes 142 in. long. By varying the annular fluid flow rate, the Reynolds number, based on an equivalent diameter, (D(2) - D(1)), covers a range from 150 to 10,000. The pressure drop data were obtained for four different conditions: (I) isothermal runs with annular fluid flowing upward; (2) isothermal runs with annular fluid flowing downward; (3) internally heated runs with annular fluid flowing upward; and (4) internally heated runs with annular fluid flowing downward. The results from the four different conditions were practically the same when the average outer annulus wall temperature was employed to evaluate the fluid physical properties for internally heated runs. Within the laminar flow region, the values of friction factor are in excellent agreement with the theoretical predictions. Friction factors for Reynolds numbers greater than 2000 fall on the accepted empirical line for commercial pipes. The heat transfer tests were conducted for two different flow conditions; one was for the annular fluid flowing upward, the other was for the annuler fluid flowing downward. These conditions were used in order to attempt to analyze the effects of natural convection. Steam heated water, circulating at about 10 ft/sec velocity through inner tube from top to bottom was used as heat source. Total heat transfer was based on the annulus side. Surface coefficients of heat transfer between the inner tube and annular fluid were determined by subtracting from the overall resistance to heat transfer the sum of the resistances due to the inner hot water film and the metal wall. The present heat transfer data for the fluid flowing up is compared with the results of several other investigations. In general for the laminar region these results are lower and in the turbulent region they are somewhat higher. The effect of natural convection on the heat transfer coefficients of this annulus is higher than that predicted for infinite width parallel plates. The effect of natural convection on the j-value is about 20% lower for annular fluid flowing down than flowing up within the laminar flow region. The local heat transfer coefficient was determined in this investigation by assuming that the bulk temperature distribution was similar to the outer wall temperature distribution. Upon comparing the averaged value of the local coefficient computed in this manner with the average value as computed using the logarithmic mean temperature differences, discrepancies were noted.
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