**5. Acknowledgment**

138 Heat Exchangers – Basics Design Applications

Considering the abomentioned facts it was decided to apply the CFD approach with the recurrent elements models for determination of the heat transfer coefficient from ribbed

Simulations were aimed in determination of the non-uniform air inlet impact on the heat exchangers efficiency and have been realized using the described earlier model and the computer code HEWES. All these simulation have been performed applying the uniform air inflow to the exchanger. The uniform mass flow rate of the air has been derived assuming that the total mass flow rate of the air spreads equally on the all measuring fields. The selected results of computations are gathered in Table 5 and, as expected, they shown quite significant improvement of the efficiency of the heat exchanger. The efficiency growth raises

The numbers in the last column of Table 5 give an average value of 15%. This should be considered as significant deterioration of the cross-flow heat exchanger thermal efficiency due to the medium flow maldistribution. Moreover, these results obtained for three units with different ribbing structure are similar. So, it seems that the air inlet non-uniformity affects the performance of the heat exchangers under consideration to the same extent.

The experiments performed for three considered cross-flow heat exchangers have shown that the air inflow non-uniformity range may be significant and its form depends on the air volumetric flow rate in the considered configuration. The experimental data allowed for determination of the total heat flow rates transported between the agents in the heat

The computational results, as it was expected, have shown significant decrease in the heat flow rates comparing with the exchanger with fully uniform air inflow. The average deterioration factor is about 15%. Two aspects should be taken into account while evaluating the numbers from Table 5: the measurements errors and the accuracy of the code HEWES. Taking into account accuracy of the measuring instruments the maximum measurements error has been determined to be of ±4%. The uncertainty of numerical results has been assessed during the validation of the code - see (Bury et al., 2008a; Bury et al., 2008b) for more details - and the differences between numerical and experimental results may reach almost 11%. These two numbers and the fact that the numerical results are always underestimated allow to conclude that the air inlet maldistribution has significant

Following final conclusions and remarks can be pointed for summarizing this study:

flow maldistribution for cross-flow heat exchanger thermal performance,

in this work remain in the range achieved by the other researchers,

models of ribs referring to HE-2 and HE-3 heat exchangers.

 experimental and numerical analyses accomplished within the framework of investigations confirmed the earlier observations about significant meaning of media

results concerning the increase of the efficiency due to uniformization of flow obtained

 application of CFD tools for computational analyses of heat exchangers may be useful and reliable but models should be thoroughly validated first; further validation of the numerical models described in subsection 3.2.2 is planned in the nearest future for

surfaces to the flowing air during the numerical simulations.

with increasing the air flow rate and water inlet temperature.

impact on a cross-flow heat exchanger performance.

**4. Conclusions** 

exchangers.

This investigation was supported by the Polish Ministry of Science and Higher Education under the project No. N N512 458836. Technical support of the GEA Heat Exchangers Company is also acknowledged.
