Effectiveness, fin

The fin effectiveness is then given by Qf/Qfmin and. for case (b), this becomes ... 

The fin surface area will not be as effective as the bare tube surface, as the heat has to be conducted along the fin. This is allowed for in design by the use of a fin effectiveness, or fin efficiency, factor. The basic equations describing heat transfer from a fin are derived in Volume 1, Chapter 9 see also Kern (1950). The fin effectiveness is a function of the fin dimensions and the thermal conductivity of the fin material. Fins are therefore usually made from metals with a high thermal conductivity for copper and aluminium the effectiveness will typically be between 0.9 to 0.95. 

If the thermocouple wires are located in a hole or groove in a metal tube or plate, the fin effect will be remedied, but the heat flow pattern through the solid will be altered. The correct surface temperature can be computed by the relaxation method. This corrected method has been used for boiling studies (S2), but many workers have made no correction for embedded wires. 

Extended surfaces are used to augment heat transfer from the base area. To compare and evaluate these extended surfaces, two performance factors are used fin efficiency and fin effectiveness. In the... 

Fins are used to enhance heat transfer, and the use of fins on a surface cannot be recommended unless the enhancement in heat ivansfer justifies the added cost and complexity associated with the fins. In fact, there is no assurance that adding fins on a surface will enhance heat transfer. The performance of the fins is judged on the basis of the enhancement in heat transfer relative to the no-fin case. The performance of fins is expressed in terms of Ihe fin effectiveness Cf. defined as Fig. 3-44. 

Note that both tlie fin efficiency and fin effectiveness are related to the performance of the fin, but they are different quantities. However, they are related to each other by... 

Therefore, the fin effectiveness can be determined easily when the fin efficiency is known, or vice versa. 

The performance of the fins is judged on the basis of the enhancement in heat transfer relative to the no-fin case and is expressed in terms of the fin effectiveness Cr, , defined as... 

C Two pin fins are identical, except that tbe diameter of one of them is twice the diameter of the other. For which fin is the (a) fin effectiveness and (b) fin efficiency higher Explain. 

A total of 10 rectangular aluminum fins k = 203 W/ni K) arc placed on the outside flat surface of an electronic device. Each fin is 100 mm wide, 20 mm high and 4 mm thick. The fins are located parallel to each other at a cenier-to-cenler distance of 8 mm. The temperature at the outside surface of the electronic device is 60°C. The air is at 20°C, and the heal transfer coefficient is 100 W/in K. Determine (a) the rate of heat loss from Uie electronic device to the surrounding air and (b) the fin effectiveness. 

Aluminum square pin fins k = 237 W/ni C) of 3-cm-long, 2 mm x 2 mm cross-section with a total number of 150 are attached to an 8-cm-long, 6-cm-wide surface, the fin efficiency is 65 percent, the overall fin effectiveness for the surface is... 

Rosa V, Cesar PF, Pereira CFS, Pinto MM, Yoshimura FIN. Effect of test environment and microstructure on the flexural strength of dental porcelains. Journal of Prostho-dontics 2011 20(4) 275-279. 

The enhanced heat transfer coefncient is due to the increase in radial mixing compared with the empty tube and possibly a contribution due to the fin effect associated with the presence of the metal element— particularly if there is good contact between element and tube wall. It is difflcult to relate the heat transfer characteristics of static mixers to empty pipes but low pressure drop mixers (e.g. Kenics, SM X) give increases in heat transfer coefficient of the order of 300% for an increase of pressure drop of the order of 700%. Some simple correlations have been presented for the Kenics mixer ... 

Example 5.4. The inner surface of a tube is machined with 12 fins having a width w of 1,5 mm and height / of 2 mm as shown in the figure. The tube has an inner diameter of 20 mm and an outer diameter of 23 mm. Determine the fin effectiveness and the overall surface effectiveness if air at 0.101 MPa and 205 K with a mass flow rate of 0,1 kg/s is flowing inside the tube. The tube wall has an average temperature of 195 K. The thermal conductivity of the tube and fins may be assumed to be 150 W/m K. 

Solution. To calculate the fin effectiveness we must first determine the heat transfer coefficient. The equivalent diameter, D, for flow in this tube is given by... 

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