Interferometric research of the flow within a tube bank at low Reynolds number values.

Cernecky, Jozef ; Koniar, Jan ; Brodnianska, Zuzana 等

1. INTRODUCTION

During the flow across lateral boundaries of a cylinder a boundary layer is formed at its front surface, that always has its final thickness in the area between the leading and separation points. The separation point position depends on the Reynolds number. The shifting of the separation point influences the dependence of the mean heat transfer coefficient as well as the aerodynamic resistance coefficient on Re. The heat transfer intensity changes non-uniformly along the circumference of the cylinder (Barborak, 2007).

During the circumfluence of the tube bank the conditions for the first row of tubes are only a little different from those around one tube. In the following rows the heat transfer intensity increases under the influence of more intensive flow turbulence. As a result of the aerodynamic wake behind the first row of tubes the circumfluence character of the second and the following rows changes as well. The flow character stabilizes only in the fourth row of tubes at their alternating arrangement.

In the contribution we focused on the flow visualization at low Re values. For the experiment a model of a bank of 20 mmdiameter tubes heated by a heat-transfer medium (water) was used. Taking into consideration the viewing field area of the objective only the surrounding space of three tubes of alternating arrangement was studied at both the free and the forced convections (Fig. 1).

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The interferometric method of research was chosen as it is an exact, objective and non-contact method that makes it possible to visualize non-homogeneities of transparent objects (changes of refractive index under the influence of a temperature field) in the whole studied space at the same time, which makes it possible to reveal the substance of the studied phenomena better. The results of interferometric visualization of thermal fields within transparent objects are represented by interferograms that are possible to consider qualitatively and evaluate quantitatively.

At quality consideration of the two-dimensional temperature field interferograms where the interferometer is adjusted to infinite fringe width, the interference fringes represent isotherms of a temperature field (at free convection or low Re values (Bird et al., 1960).

The quality consideration of interferograms makes it possible to reveal the shapes of temperature fields and their influence on local as well as mean heat transfer parameters.

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From the study of interferograms in real time or evaluation of dynamic records it is possible to make conclusions about the influence of parasite flow in the system, behaviour of dynamic temperature fields and dynamic development of heat transfer parameters.

2. DISCUSSION

With the aid of a holographic variant of the Mach-Zehnder interferometer interferograms of the temperature fields in the surroundings of the heated horizontal tube bank were obtained (Oosthuizen & Naylor 1999). The arrangement of the first three tubes and their diameters is displayed in Fig. 1. During the measurements the interferometer was adjusted to infinite fringe width. Considering the small viewing field area the tube bank must be shifted horizontally so that the interferograms from another row of tubes can be recorded as well.

From the obtained interferograms the interference order depending on state quantities of the measured environment and the wave length of the used light was determined (Taraba et al., 2004). From the interference images the positions of interference fringes were also determined and the respective temperature values for each fringe were calculated.

The temperature field in the surroundings of the tubes at free convection is illustrated in Fig. 2. The first picture (Fig. 2a) represents a holographic interferogram of a temperature field at the tube surface temperature of [t.sub.w] = 60[degrees]C and the temperature of surrounding environment of [t.sub.[infinity]] = 20[degrees]C. In Fig. 2b its quantitative analysis is shown and in Fig. 2c the result of numerical simulation is displayed.

From the interferograms the thermal boundary layer at free convection is visible, in which the air flows upwards. Outside the boundary layer there is surrounding environment with constant air temperature. The expansion of the thermal boundary layer above the tube causes a decrease of the heat transfer gradient.

At forced convection the temperature conditions in the tube surroundings are changing. In Fig. 3 the temperature field in the surroundings of the tubes of surface temperature of [t.sub.w] = 60[degrees]C at the temperature of surroundings of [t.sub.[infinity]] = 20[degrees]C and the forced air flow with the value of Re = 255 is illustrated. As it follows from Fig. 3a, the thermal boundary layer is influenced by air flow, which influences the temperature field of the tubes in the second row.

Considering the fact, that the Re values are low, the forced convection influence is relatively small. In Fig. 3b quantitative evaluation of the holographic interferogram from Fig. 3a is displayed. The obtained results are possible to compare with the values calculated in the FLUENT programme (Fig. 3c).

3. CONCLUSION

In the present contribution it is indicated that interferometry has proved competent in the given research and, therefore, it can be applied to solve further research tasks. The effective utilization of interferometry, however, is conditioned by the development of image computer processing, which is also the subject of our contemporary research. By complementation of the interferometric method with computer image processing we will obtain a highly effective experimental tool for different research fields. Interferometry is a prospective method not only in the field of research, but also in education where it can complement the physical problems explanation with suitable and interesting image material. The obtained values can be used for further processing, i.e. calculation of heat transfer coefficients. The experimentally obtained results are also possible to compare with the simulations in the FLUENT programme use them for the CFD models verification. The results achieved can also be used in optimizing the construction of tube heat exchangers in relation to their energetic effectiveness improvement.

The contribution was elaborated within the research project /KEGA No. 3/6431/08 Determination of the Characteristics of Emission and Indicator Quantification According to European Legislation

4. REFERENCES

Barborak, O. (2007). Technical drive, Dubnica nad Vahom, ISBN 978-80-969615-6-6, pp. 146

Bird, R.B.; Steward, W.E. & Lighfood, E.N. (1960). Transport phenomena, J. Wiley, New York, CSVTS, Praha

Oosthuizen, P.H. & Naylor, D. (1999). An Introduction to Convective Heat Transfer Analysis, WBC/McGraw-Hill, New York, ISBN 258-458-73-02

Taraba, B. ; Behulova, M. & Kravarikova, H. (2004). Liquid drive, STU Bratislava, ISBN 80-227-2041-0, pp. 214

Taraba, B. ; Behulova, M. & Kravarikova, H. (1999). Hydromechanic Thermomechanic, ES STU Bratislava, ISBN 80-227-2041-0, pp. 118