Paraphrase for Discussion - Essay Example

Discussion The main objective of this experiment was to determine the effect of fluid velo (turbulence) on the heat transfer coefficient for a cross flow heat exchanger. As demonstrated above, the heat transfer coefficient increased with an increase in Reynolds number of the air across the heat exchanger. This is attributed to the cylindrical geometry of the heat exchanger as well as the effects of boundary layer. Notably, the effect of separation is deemed crucial at higher Reynolds number. Because of separation, air becomes more turbulent than before thereby enhancing the conduction of heat away from the tubes.

In the case where multiple tubes are placed in a staggered pattern, just as in the experiment, each additional row progressively creates more turbulence on the other proceeding rows. Thus, this increase in turbulence leads to an increase in heat transfer for each tube rows.A keen look at the rate of increase in heat transfer from one row to another will reveal a decreasing trend with more decrease realized in those rows farthest from the first row of the heat exchangers. This is attributable to the fact that all the flow in the in the first row of heat exchangers is at first laminar before being converted into turbulent.

On the other hand, the subsequent rows function as a turbulence generating grid, which therefore increases the turbulence in the entire heat exchanger. Still, for each additional row encountered, a larger percentage of the flow hitting a particular row is already turbulent before the hit leading to a release of just a small of percentage of the flow for conversion to turbulent flow. For instance, if 60% of laminar flow is converted to turbulent for each row that the flow encounters, the returns will be diminishing.

This implies that for the first row, 60% of the flow will be turbulent whereas for the second and third rows only 78% and 84% respectively will be converted. Finally, the other factor that affects the heat transfer coefficient is the difference in temperature between the surface of the element and air. Since the heat transfer coefficient varies inversely with the difference in temperature (see equation 5), this implies that transfer coefficient for a given heat transfer increases as the temperature difference decreases.

The second experiment was conducted to examine the effect of heat transfer as a function of location on a cylinder. A cylindrical probe was used to measure the heat transfer as a function of angle. The measurements were taken for uniform flow in an air tunnel. A graph of heat transfer coefficient as a function of angle was created (see Fig. 10). From the graph, it is evident that the greatest heat transfer occurs when the probe is normal to flow. However, there is a decrease in the heat transfer when the probe is rotated to 1800 from the flow.

This is caused by the fluid dynamic of boundary layer. The boundary layer is at its largest when the probe is parallel to the flow of air. Such location is known as the forward stagnation point. Further rotation of the probe to 90o and 1800 makes the boundary layer to become smaller thereby allowing the fluid to move faster near the surface of the cylinder. Our data indicates that the angle at which the flow separates from the cylinder is approximately 900. As seen in fig. 10, up to that point heat transfer coefficient had been decreasing quite rapidly.

After that point, the value of “h” showed some variance with the range of +5%.