International Journal of Heat and Mass Transfer, Vol.130, 9-24, 2019
Experimental study on fluid flow and heat transfer characteristics of falling film over tube bundle
The fluid flow characteristics, film thickness and heat transfer of falling liquid films over an array of horizontal tubes are experimentally studied. The effects of the flow rate, the heat flux and nozzles-holes arrangements on the averaged convective heat transfer coefficient are addressed. An experimental setup was constructed including a water circulation system, isothermal water bath, a feeder nozzle, three horizontal test tubes, aligned vertically, the setup structure frame and measuring system. The tubes wall temperatures were measured using K-type thermocouples embedded in the test tube walls. The fluid flow and the film thickness were captured using a high-speed camera. The measurements were performed at 20 degrees C for two kinds of nozzles-holes configurations of single space (nozzles pitch of 12 mm) and double space (nozzles pitch of 24 mm), three flow rates of 50, 100 and 150 mLPM and four heat fluxes of 5, 10, 15 and 20 kW/m(2 )degrees C. The measured results are reported in the form of flow-behavior photos in various time consequences, and the steady state film thickness in photos with marked measures. The tubes wall temperatures and heat flux are reported in tables and graphs. The measured results demonstrate that the arrangement of nozzles induces significant impact on the fluid flow and heat transfer characteristics of the test tubes. However, in the case of low flow rate (50 mLPM), the differences on the thermal performance are not notable. The results also show that the type and shape of film formation over the test tubes are very important on the heat transfer performance. When the liquid film extends over the test tube or covers it, the heat transfer rate increases. The heat transfer coefficient (h) for the test tube 1, just below the nozzles, is the highest; then test tube 2 in the middle receives better heat transfer rate, and test tube 3 at bottom is the last with lowest heat transfer rate. The nozzles pitch is a key parameter, which allows film bonding and can be considered as a controlling parameter for heat transfer enhancement and the flow structure. (C) 2018 Elsevier Ltd. All rights reserved.