Effect of flow obstruction on single-phase heat transfer

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The objective of the proposed investigation is  (i) to provide an experimental data base for  heat transfer enhancement downstream of flow obstacles, (ii)  to compare the data base with current heat transfer prediction methods, and (iii) to present an improved  prediction method.Properly designed turbulence-promoting flow obstacles can enhance the  downstream heat transfer significantly.  Very few experimental studies  have been performed that actually measure the heat transfer enhancement due to obstacles in single phase flow; these heat transfer experiments covered only a narrow range of obstacle shapes and test conditions. The general assumption is that the heat transfer enhancement follows the law of Yao and Hochreiter (1982) which states that Nu = Nuo (1+A exp{-B z/Dhy}where where Nu is the Nusselt number for the obstacle-equipped channel, Nuo is the Nusselt number for the reference channel without any obstacles, A and B are geometry-dependent correlating constants, z is the axial distance downstream from the spacer, and Dhy is the hydraulic-equivalent diameter.  Knowledge of the obstacle effect is of considerable practical importance since flow obstacles such as grids and spacers plates are used  in all nuclear fuel bundles and are used as supports heat exchanger and boiler tubes.  It is proposed to investigate the impact of the following parameters on heat transfer enhancement caused by obstacles: (i)  various shapes of the leading and trailing edges, (ii) flow blockage ratios,  (iii) Reynolds numbers and (iv) Prandtl numbers and (v) distance from the flow obstacle, (vi) number of flow obstacle in series. The tests will be performed in  a directly heated test section,  equipped with various thermocouples along its length.  In order to maintain the same conditions at the measuring station except the distance from the flow obstruction, a unique experimental approach will be used:  the obstacle will be moved  non-intrusively using strong ceramic magnets  located outside the (non-magnetic) Inconel test section.The impact on pressure drop will also be measured.  In the analysis the relationship between pressure drop and heat transfer enhancement will be investigated. The possibility of  introducing a figure of merit, i.e.  expressing the optimum enhancement in terms of minimum increase in pressure drop will be  examined as well.