Heat transfer to air–water annular flow in a horizontal pipe

Department of Mechanical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
International Journal of Multiphase Flow (Impact Factor: 1.94). 01/2006; 32(1):1-19. DOI: 10.1016/j.ijmultiphaseflow.2005.09.001

ABSTRACT Two-phase air–water flow and heat transfer in a 25mm internal diameter horizontal pipe were investigated experimentally. The water superficial velocity varied from 24.2m/s to 41.5m/s and the air superficial velocity varied from 0.02m/s to 0.09m/s. The aim of the study was to determine the heat transfer coefficient and its connection to flow pattern and liquid film thickness. The flow patterns were visualized using a high speed video camera, and the film thickness was measured by the conductive tomography technique. The heat transfer coefficient was calculated from the temperature measurements using the infrared thermography method. It was found that the heat transfer coefficient at the bottom of the pipe is up to three times higher than that at the top, and becomes more uniform around the pipe for higher air flow-rates. Correlations on local and average Nusselt number were obtained and compared to results reported in the literature. The behavior of local heat transfer coefficient was analyzed and the role of film thickness and flow pattern was clarified.


Available from: G. Hetsroni, Mar 12, 2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: The main objective of the present investigation is to study heat transfer in parallel micro-channels of 0.1 mm in size. Comparison of the results of this study to the ones obtained for two-phase flow in “conventional” size channels provides information on the complex phenomena associated with heat transfer in micro-channel heat sinks. Two-phase flow in parallel micro-channels, feeding from a common manifold shows that different flow patterns occur simultaneously in the different micro-channels: liquid alone (or single-phase flow), bubbly flow, slug flow, and annular flow (gas core with a thin liquid film, and a gas core with a thick liquid film). Although the gas core may occupy almost the entire cross-section of the triangular channel, making the side walls partially dry, the liquid phase always remained continuous due to the liquid, which is drawn into the triangular corners by surface tension. With increasing superficial gas velocity, a gas core with a thin liquid film is observed. The visual observation showed that as the air velocity increased, the liquid droplets entrained in the gas core disappeared such that the flow became annular. The probability of appearance of different flow patterns should be taken into account for developing flow pattern maps. The dependence of the Nusselt number, on liquid and gas Reynolds numbers, based on liquid and gas superficial velocity, respectively, was determined in the range of ReLS = 4–56 and ReGS = 4.7–270. It was shown that an increase in the superficial liquid velocity involves an increase in heat transfer (NuL). This effect is reduced with increasing superficial gas velocity, in contrast to the results reported on two-phase heat transfer in “conventional size” channels.
    International Journal of Heat and Mass Transfer 08/2009; 52(17-18-52):3963-3971. DOI:10.1016/j.ijheatmasstransfer.2009.03.027 · 2.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Using thermocouples and a particle tracking velocimetry technique, temperature and velocity measurements are conducted to investigate flow and heat transfer characteristics of turbulent natural convection from a vertical heated plate in water with sub-millimeter-bubble injection. Hydrogen-bubbles generated by the electrolysis of water are used as the sub-millimeter-bubbles. In the turbulent region, the heat transfer deterioration occurs for a bubble flow rate Q=33mm3/s, while the heat transfer enhancement occurs for Q=56mm3/s. Temperature and velocity measurements suggest that the former is caused by a delay of the transition due to the bubble-induced upward flow. On the other hand, the latter is mainly due to two factors: one is the enhancement of the rotation of eddies in the outer layer, and the other is the increase in the gradient of the streamwise liquid velocity at the heated wall. These are caused by bubbles, which are located in the inner layer, rising at high speed.
    Experiments in Fluids 09/2010; 49(3):613-622. DOI:10.1007/s00348-010-0838-8 · 1.91 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, the heat transfer performance of the alternating discrete two phase flow in micro-tubes has been numerically investigated. Two immiscible fluids are injected into a micro-tube alternately, generating a plug flow pattern with two completely separated phases. The overall Nusselt number could be increased by sixfold over the single phase fully developed flow when the ratio of primary phase to the tube inner diameter (L1/D) is kept at about 7. Characteristic of the average Nusselt number is found to be almost independent of the secondary phase. The friction factor is much dependent on the viscosity of the secondary phase. Secondary phase with higher viscosity would induce higher pressure loss and large pressure fluctuation is observed at regions close to the interfaces. The flow field that is far from the interfaces could be approximated as the single phase fully developed flow. Weber number of the primary phase and Capillary number of the secondary phase should be kept below certain critical values in order to maintain the discrete flow patterns. Frictional loss is a more important consideration than the heat capacity when choosing a particular secondary phase. Secondary phase with lower viscosity is preferred due to the overall heat transfer enhancement will become less beneficial if the pressure loss induced by the secondary phase becomes significant. Liquid–gas discrete flow is found to have better heat transfer performance than the liquid–liquid discrete flow.
    International Journal of Heat and Mass Transfer 07/2014; 74:333–341. DOI:10.1016/j.ijheatmasstransfer.2014.03.041 · 2.52 Impact Factor