INVESTIGATION OF POOL BOILING HEAT TRANSFER FROM TUBE BUNDLES IN CROSS-FLOW

Abstract

Cross-flow boiling is one of the efficient means of heat transfer that finds application in variety of industrial appliances such as kettle reboilers, flooded evaporators, steam generators and chemical process equipment. Though boiling heat transfer on a single tube has been widely studied but the results obtained from single tube can not be directly applied to design multitube bundle heat exchangers involving boiling process, because the behaviour of a tube in a bundle is quite different from that of a single tube due to. vapour bubbles generated on lower tubes rising up and causing turbulence around the upper tubes. Very little information is available in the literature concerning boiling heat transfer coefficient in tube bundles under cross-flow conditions. Further, these studies are limited to the use of ideal tube bundles of small size with refrigerants as the working fluid. There is need to investigate the local boiling heat transfer in tube bundles under wide range of system and operating parameters in order to clearly understand the phenomena and for the efficient equipment design. In the present work, an experimental investigation was undertaken to study the boiling heat transfer in horizontal tube bundles of different configurations with low cross-flow velocity and heat flux values using distilled water as the working fluid at atmospheric pressure. Experiments were conducted at pool and cross flow boiling conditions on a single tube ; single column tube bundles comprising of two tubes placed one above the other at four different pitch to 1 tube diameter ratio of 6.0, 4.5, 3.0, and 1.5 ; three tubes one above the other at a pitch to tube diameter ratio of 3.0 ; five tubes one above the other at a pitch to tube diameter ratio of 1.5 ; and a three column 3X5 in-line square tube bundle having a pitch to tube diameter ratio of 1.5. Tubes of the bundle made of stainless steel (AISI 304). of 19.05 mm outer diameter, 17.27 mm inner diameter and 245 mm length were directly heated by means of a high alternating current through a step down transformer. All the tubes of the bundle were heated simultaneously at the same heat flux by connecting them in series. The values of heat flux ranged between 10 to 40 kW/m2 and that of mass flux between 0 to 10 kg/m2s. Eight, 28 BWG copper constantan thermocouples evenly spaced circumferentially at the mid length of the tube were used to measure the surface temperature of the tube wall. From the experimental data on single tube, it is found that the heat transfer coefficient increases with the increase of cross flow velocity and heat flux values. Effect of cross flow velocity is more at low heat flux values and reduces with the increase of heat flux. Heat transfer coefficient of bottom tube of various tube bundle configurations was found to be nearly the same as that of a single tube under similar conditions of heat flux and cross flow velocity, indicating that the heat transfer characteristics of bottom tube are independent of the presence of the bundle. Rohsenow [62] superimposition, Eq. (1), and Kutateladze [60] correlation, Eq. (2), were found to predict with reasonable accuracy, the cross flow boiling heat t