The influence of free-stream turbulence on heat transfer in finned tube bundles
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The objective of this thesis work is to obtain an understanding of the free stream velocity and turbulence intensity upstream of a heat exchanger, and investigate the possible influence on heat transfer through a tube bundle. This thesis is a part of the EFFORT project, where NTNU, in cooperation with SINTEF, focus especially on compact heat exchanger solutions for turbine exhaust gases offshore, and should be seen in relation to the work by Ph.D. candidate Anna Holfeld.A literature survey was performed and available data and correlations regarding uneven heat transfer coefficient distribution, the effect of the number of tube rows and row-to-row development of heat transfer and pressure drop coefficients in finned tube bundles, the effect of free stream turbulence, turbulence generation through grids and hot wire anemometry is presented. The survey revealed limitations and gaps in the literature, especially regarding the effect of free stream turbulence, which substantiated the need for experimental work. The survey also provided a grid design method for generation of homogenous turbulent flow and proved the hot wire to be an appropriate measuring device for turbulent velocity fluctuations. Experiments were executed in a heat transfer test rig at NTNU and three grids were designed in order to generate turbulence upstream of the heat transfer test section. The experimental set up of the rig, grids and test sections, including the calibration equipment, is presented. The velocity distribution and corresponding turbulence intensity was measured upstream of the heat transfer test section for two different geometries. Measurements were performed at two different air mass flows and three different turbulence levels, providing mean air velocities in the range of 15,2-19,2 m/s and 10,5-11,9 m/s for both test sections at high and low mass flow respectively. Subsequent heat transfer measurements were performed for each velocity measurement.Measurements for low turbulence intensity were executed in the open rig, which provided inconsistent velocity distributions and turbulence levels in the range of 1-2% for the two test sections. Turbulence was created by the use of grids and turbulence levels of 13% and 35-38% were obtained. The measurements in grid-generated flow provided more symmetrical and consistent velocity- and turbulence distributions across the test sections than the open rig. The velocity was seen to increase close to the channel walls, while the middle measuring points provided the lowest local velocities in the test section. The opposite trend was observed for the corresponding turbulence intensity distribution.An analysis of the heat transfer results was performed in order to compare the air-side heat transfer coefficient calculated for each tube row to the measured upstream turbulence level. The experimental results of this thesis work show no influence of the upstream flow conditions on the heat transfer coefficient development through the tube bundle, which was surprising compared the significant heat transfer increase with turbulence found in literature.All results are discussed according to existing literature and observations. It is concluded that more experimental data is needed to do a further analysis of the impact of the upstream turbulence level on the row-to-row heat transfer coefficient. Suggestions for further work are presented.