**3.3 Results of numerical simulations**

136 Heat Exchangers – Basics Design Applications

Fig. 17. Calculated (left, K) and measured (right, ºC) temperature field of the first rib surface

The next step in the analysis was the computation and comparison of the total heat flow rates transported from the ribbed surface to the flowing air. The results for measuring series MS-1 to MS-5 and MS-21 to MS-25 are presented in Table 4. The total heat flow rates has

considering the measured values of the volumetric air flow and its temperature

taking into account the computed values of the mentioned parameters (*Q* Fluent in

Measurement No. Nh,W *Q* air, W *Q* Fluent, W *Q* air, % *Q* Fluent, % MS-1 116.5 111.8 104.5 4.03 10.30 MS-2 122.6 117.2 109.9 4.40 10.36 MS-3 128.0 119.0 111.7 7.03 12.73 MS-4 132.4 117.2 109.9 11.48 16.99 MS-5 137.1 120.9 113.6 11.82 17.14 MS-21 192.0 173.1 165.8 9.84 13.65 MS-22 196.5 186.2 178.9 5.24 8.96 MS-23 200.5 190.7 183.4 4.89 8.53 MS-24 207.0 200.0 192.7 3.38 6.91 MS-25 215.8 203.3 196.0 5.79 9.18

measured at the inlet and outlet of the ribs section (*Q* air in Table 4),

Table 4. Comparison of experimental and computational data – heat flow rates.

up to 18% for some cases, but the average difference is somewhat over 10%.

The relative differences (*Q* ) between experimental and numerical results have been calculated. The heat flow rates calculated based on the measured values, as it can be seen, is lower than the measured values of the electric power of the heaters. The obvious reason of this situation is the heat losses through the rear wall of the flow channel. The differences between experimental and computational heat flow rates calculated as the CFD results reach

for experiment MS-4 – air flow direction same as in Fig. 14.

been calculated twice based on the air enthalpy rise:

Table 4).

The analyses presented in subsection 3.2 allowed to withdrawn following conclusions:



Table 5. Selected computational results.

Impact of a Medium Flow Maldistribution on a Cross-Flow Heat Exchanger Performance 139

The author realizes that the combination of experimental tests and numerical simulations to assess the impact of inequality for the work of the heat exchangers may be the subject of some criticism. The best solution would be to do all the analysis by means of measurements. However, to obtain a homogeneous air flow on the described testing rig, while maintaining the appropriate parameters, it is impossible due to technical limitations. Some attempts to implement this idea has been taken in (Bury et al., 2009b), and although it failed to get the

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

cp - specific heat capacity at constant *Q* - heat flow rate, W pressure, J/(kg K) Re - Reynolds number d - heat exchanger pipe diameter, m s - distance between ribs, m

Dh - hydraulic diameter, m S - surface area, m2

l - height of a rib, m t, T - temperature, ºC, K

Nu - Nusselt number - thickness of a rib, m Pr - Prandtl number - mass density, kg/m3

k - thermal conductivity, W/(m K) pipes, m

max - maximum value w - water

No. 6, (June 2000), pp. 499-513, ISSN 1359-4311

ICR0639, Washington DC, USA, August 17-22, 2003

Vol. 23, No. 10, (July 2003), pp. 1235-1246, ISSN 1359-4311

h - heat transfer coefficient, W/(m2K) stp - transverse distance between

*m* - mass flow rate, kg/s *V* - volumetric flow rate, m3/s

a - air p - refer to pipes without ribs in - inlet r - refer to ribbed surface

Aganda, A.A.; Coney, J.E.R.; Farrant, P.E.; Sheppard, C.G.W. & Wongwuttanasatian, T.

Andrecovich, M. & Clarke, R. (2003). Simple Modeling of Flow Maldistribution in Plate-Fin

Anjun, J.; Rui, Z. & Sangkwon J. (2003). Experimental Investigation of Header Configuration

Berryman, R.J. & Russell, C.M.B. (1987). The Effect of Maldistribution of Air Flow on

(2000). A Comparison of the Predicted and Experimental Heat Transfer Performance of a Finned Tube Evaporator. *Applied Thermal Engineering*, Vol. 20,

Exchangers, *Proceedings of the 21st IIR International Congress of Refrigeration*, Paper

on Flow Maldistribution in Plate-Fin Heat Exchanger. *Applied Thermal Engineering*,

Aircooled Heat Exchanger Performance, In: *Maldistribution of Flow and Its Effect on* 

full homogeneity of the flow, it was noted the positive effects.

**5. Acknowledgment** 

**6. Nomenclature** 

Subscripts

out - outlet

**7. References** 

Company is also acknowledged.

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 surfaces to the flowing air during the numerical simulations.

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 with increasing the air flow rate and water inlet temperature.

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.
