tailieunhanh - Computational Fluid Mechanics and Heat Transfer Third Edition_11

Tham khảo tài liệu 'computational fluid mechanics and heat transfer third edition_11', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Heat transfer during cross flow over cylinders 379 Figure Comparison of Churchill and Bernstein s correlation with data by many workers from several countries for heat transfer during cross flow over a cylinder. See for data sources. Fluids include air water and sodium with both qw and Tw constant. All properties in eqns. to are to be evaluated at a film temperature Tf Tw T 2. Example An electric resistance wire heater m in diameter is placed perpendicular to an air flow. It holds a temperature of 40OC in a 20 C air flow while it dissipates W m of heat to the flow. How fast is the air flowing Solution. h W m n m 40 - 20 K 2833 W m2K. Therefore Nup 2833 where we have evaluated k at T 30 C. We now want to find the Rep for which Nup is . From Fig. we see that Rep is around 300 380 Forced convection in a variety of configurations when the ordinate is on the order of 10. This means that we can solve eqn. to get an accurate value of Rep Rep - Nup 1 04 2 3 l1 4 Pr1 3 V Pr _ but Pr so Rep 4 1 2 3 1 3 . 463 Then V X 10 5 u p Rep I--------10 4---I 463 m s The data scatter in Rep is quite small less than 10 it would appear in Fig. . Therefore this method can be used to measure local velocities with good accuracy. If the device is calibrated its accuracy is improved further. Such an air speed indicator is called a hot-wire anemometer as discussed further in Problem . Heat transfer during flow across tube bundles A rod or tube bundle is an arrangement of parallel cylinders that heat or are being heated by a fluid that might flow normal to them parallel with them or at some angle in between. The flow of coolant through the fuel elements of all nuclear reactors being used in this country is parallel to the heating rods. The flow on the shell side of most shell-and-tube heat exchangers is generally normal to the tube bundles.