Today engineering world is using many simulation packages in order to design or solve problems. This report outlines the temperature distribution along the fin and tube of heat exchanger by using finite element analysis approach on ANSYS. It depicts the choice of fin length of the most efficient design. However this variation has been clearly shown in the table and graph forms. While later in the report this chosen design is carried to show the steady state temperature distribution along its length when the hot fluid temperature is decreasing inside along the tube.

All the findings have been thoroughly discussed, referred to figures and appended log file. This log file shows all the steps taken to run the analysis.

Beside these asked findings many extra tasks were undertaken to analyse the situation in depth. Evidences to these findings can be found in the appendix attached at the end.

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3: Literature Survey

Many online papers along with the handouts helped to understand the working of ANSYS. All the exercises of Engineering Analysis 3 were beneficial to design the model and run the analysis. Apart from ANSYS, Heat and Flow notes helped to learn the basic working of heat exchanger and importance of fins.

4: Theory

4.1 Steady state temperature distribution for different fin lengths

Analysis was carried in three basic steps pre-processing, solution and general post-processing keeping different aspects of heat exchanger principles in mind.

PRE-PROCESSING

To make this problem simple 1/8 symmetry of the heat exchanger shown in figure 1 is chosen as this would make no difference and will save time. In order to analyse the temperature distribution along the fin 2-D element type was chosen, it’s called solid thermal PLANE55 having four nodes. After this thermal conductivity K= 60 W/moC was defined under material modelling.

Fig. 1

In order to model 1/8 symmetric section of the model eight key points were then defined to model the whole section as an area can be seen in fig.2a. All the elements were meshed as an area for smart size 6 in Quad shape shown in fig. 2b. At this time model is ready to apply temperatures at the inner and outer side of the tube

Fig. 2a Fig. 2b

SOLUTION

To apply thermal loads inner and outer surfaces were chosen to define the bulk temperatures of hot Ti (200oC) and cold To (25oC) fluid passing inside and outside the tube respectively due to convection. With regards to convection heat transfer coefficient ‘h’ was fed to be 750 W/m2 oC. By assuming adiabatic end conditions this problem becomes independent of boundary conditions. After this solve command executed the solution.

POST-PROCESSING

Before looking at the nodal solution for different fin lengths log file was retrieved that can be seen in the appendix. In this log file different values of fin lengths Lf were taken ranging between 0.005m and 0.025m, graphs corresponding to these values can be seen in fig 3.

Fig. 3

The values shown in the graph are properly tabulated and can be found in the Appendix.

4.2 Steady state temperature distribution along heat exchangers length

On comparing various fin lengths it was discovered that the most effective design would be to go for maximum fin length as this would increase the cooling.

Log file from the last case was used to design the 3-D model of the very same heat exchanger. But the few things were needed to be changed like element type as now model is 3-D so it needs more nodes for designing and calculation purpose. This time the element type is Brick 8 node 70. Apart from this everything was done the same way but making meshing a little coarser due to program license limitation. In order to determine this temperature distribution along tubes length temperature gradient dT/dLf of -100 oC/m was defined. All the necessary issues and results are discussed in the results section.

5: Results

5.1 Steady state temperature distribution for different fin lengths

Fig. 4

As it can be seen from fig. 3 that higher the fin length lesser the tip temperature which means by increasing the fin length to certain value cooling could be effective.

While fig. 4 shows the evaluation of steady state temperature along the length of the fin in which minimum value of the tip temperature drops down to 42oC. The linear relation of the fin length and minimum fin temperature can be justified through the graph shown in fig. 3. Rest of the ANSYS pictures for different fin lengths are appended at the end.

5.2 Steady state temperature distribution along heat exchangers length

Fig. 5 Fig. 6

It can be seen that temperature of the hot fluid inside the tube is falling along its length and also decreasing radially. After running the analysis it was learnt that the best design would be to go for the longest fin. The whole process of heat transfer between these two fluids can be divided into two parts radial conduction and convection. Longer the fin length more heat from inside will be conducted to the tip of the fin which would increase the fin efficiency and is clearly depending on its length.

By increasing the fin length more and more cold fluid is coming into the contact with the fin and that is increasing the effective contact are between cold fluid and fin. As discussed in the first part that the minimum temperature of the temperature of fin would be at the end. But after applying the gradient along the length in z- direction reduces mean temperature of the material along the length. In fig. 5 mean temperature is gradually decreasing along the length and its cross section. However, fig. 6 shows the inner surface of the tube where the temperature is also gradually decreasing.

Before concluding the report it is important to analyse that areas selected for the convection through cold fluid didn’t include the end cross sections of the tube and fin. In order to learn that how different would be the analysis after including these areas, is shown in the appendix.

5: Conclusion

This analysis helped in understanding the basic concepts of heat transfer for this type of heat exchanger. Different ANSYS tools were used to evaluate the temperature distribution along the tube length and fin length.

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