Research regarding thermal and hydrodynamic performance levels of surfaces with sinuous fins for automotive heat exchangers.

Nagi, Mihai ; Carabas, Ioan Daniel ; Ilies, Paul 等

1. INTRODUCTION

Compact aluminium heat exchangers were studied during the experimental research, with sinuous fins of different thickness, 0,14; 0,16; 0,3; 0,5 mm. The influence of "entry effect" on thermal and hydrodynamic performance for heat exchangers was identified.

2. THEORETICAL CONSIDERATION

If a small distance x (under 25% of the fin pitch) at fluid entry is considered, this portion can be considered a flat wall surface and fluid speed [w.sub.w] to be parallel to thee wall. A laminar flow layer is observed (fig. 1), also called a hydraulic limit layer. This layer has a very low thickness [section]x at fluid entry and increases as the distance x is increased. This is known in literature as the "entry effect"( Arjanikov & Maltev, 1954).

[FIGURE 1 OMITTED]

The thickness of the two layers can be considered to show a parabolic variation, but different for the two, [[delta].sub.x]--hydraulic limit layer thickness, [[delta].sub.t]--thermal limit layer thickness.

These two dimensions can be correlated with the following equation, as given by Plhausen (Ilies P et at., 2009):

[[partial derivative].sub.t] = [partial derivative]x/[cube root Pr] (1)

Heat exchange between the fluid and the wall is most effective at the entry end of the wall, where the thickness of the limit layer is very small. A maximum value for the heat transfer coefficient "[[alpha].sub.x]" is noticed where the layer thickness is close to zero. This is called as the "entry effect", and can be induced on any surface, for a small given length (Bejan, 1973).

3. EXPERIMENTAL SETUP

A special installation was used for testing heat exchangers, a rig that can simulate working conditions as close to reality as possible. Multiple parameters were recorded at a very high level of precision. For studying thermal and hydrodynamic performance testing, aluminium water-air type heat exchangers were used, (Nagi et al., 1994) with plates and bars (fig.2.) (Leca, 1983).

[FIGURE 2 OMITTED]

These heat exchangers had identical frontal areas, with rectangular section water canals also with identical dimensions. Air passage ways feature sinuous fins with a constant height of 8,8 mm, a pitch p of 3,5; 4; 5; 6,5 mm and matrix width G of 30; 45; 65; 95 and 115 mm (fig.3) (Nagi, 1996).

[FIGURE 3 OMITTED]

4. PROCESSING EXPERIMENTAL DATA

Each device was tested in at least 30 functioning work regimes.

Data processing was done using the LabView 7.0 software, that monitors testing parameters with an error of <2%, can plot graphs of these parameters and controls the valve that regulates cold air flow. Recorded data was stored in Excel files.

5. RESULTS

Diagrams in figures 3, 4 and 5 were plotted based on the results obtained during experimental trials. Colborn j criteria for Reynolds (Re) number were plotted in figure 3 and 4 for heat exchangers with p =3.5 and 4 mm pitch, while figure 5 shows plotted results of the friction coefficient cf for a Reynolds number ranging from 600 to 3600, for sinuous fins compared to Kays and London type fins(Kays &London, 1984) (Nagi et al., 2006).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

6. CONCLUSIONS

Heat exchange coefficient "[[alpha].sub.x]" shows a maximum value when the thickness of limit layer is close to zero.

By plotting the variation of the Colborn j criteria as a function of Re number, the obtained results were generalized. The obtained results can be analyzed from the point of view of their quality as well as quantity and the diagrams can be used with a high precision to design similar sinuous fins without any regard to the condition of the experimental trials. Colborn values are higher for short passage ways. The differences in Colborn values for the same family of heat exchangers and the same Re number are explained by the "entry effect". In this manner, the theory regarding this effect was proven by experimental trials.

Figure 5 shows that friction coefficient values for sinuous fins on the air side are dependent only on the hydraulic diameter of the passage ways. Hydrodynamic performance is better for sinuous fins (for the same hydraulic diameter) compared to those tried by Kays and London (the Kays and London fins are named sinuous but in fact they are discontinuous fins). This was to be expected, as thermal performance levels for sinuous fins are slightly lower regarding those tested by Kays and London (Nagi et al., 2008).

7. ACKNOWLEDGEMENTS

This work was partially supported by the strategic grant POSDRU 6/1.5/S/13, (2008) of the Ministry of Labour, Family and Social Protection, Romania, co-financed by the European Social Fund--Investing in People.

8. REFERENCES

Arjanikov, N.S. & Maltev, V.N. (1954). Aerodynamics (original title in Romanian), Ed.Tehnica

Bejan, I. (1973). Contributions for the study of heat transfer for piping with internal fins (original title in Romanian), PhD thesis I.P.Timisoara, 1973.

Ilies P., Nagi M., Mare, C. & Sucila, M.(2009). Determination of the optimal design parameters for the wavy air fins used in the construction of automotive radiators. DAAAM International pag. 771-772, ISBN 978-3-901509-70-4, ISSN 1726-9679

Kays W. M. & London A. L. (1984). Compact Heat Exchanger-Third edition, ISBN 1-57524-060-2

Leca, I.(1983). Heat and mass transfer. Theory and applications (original title in Romanian). Ed. Didactica si Pedagogica, Bucuresti

Nagi, M., Negru, L.D., Laza, I. & Lelea, D.(1994). Theoretical and experimental studies regarding the possibility of comparing oil coolers used for internal combustion engines (original title in Serbian). OSMI Strucni skup o opremi u procesnoj industriji Beograd, 26-27 oct. Procesna tehnika Nr.3-4, pag.30-32.

Nagi, M.(1996). Determination of the prediction equation for the fluid laminar flowing through sinusoidal fins. Buletin Stiintific Tehnic, U.T.T. Tom 41(55) Mecanica, pag. 110-115.

Nagi, M., Iorga, D., Laza, I., Mihon, L. & Ostoia, D. (2006). Heat exchangers (original title in Romanian), vol.1, Ed. Mirton, Timisoara

Nagi, M., Ilies, P. & Negoitescu, A. (2008). The detrmination of the optimal circulation fluids speeds through the heat Exchangers. DAAAM International, VIENA, AUSTRIA pag. 943-944, ISSN 1726-9679, ISBN 978-3-901509-68-1