**2.4 Selected experimental results**

There were six measuring series carried out for each of the heat exchangers under consideration. The distributions of the air velocity and temperature are one of the most interesting results that may be achieved on the described testing station. These distributions are very important because they allow evaluating the air inflow maldistribution range and form. Sample distributions obtained for the HE-1 heat exchanger are shown in Figs. 5 and 6. These results have been obtained with the total air flow rate of 1.556 m3/s, the water flow rate of 4.5·10-4 m3/s and the water temperature set on the boiler in 50°C.

The form and scope of the air inlet non-uniformity depend on the fan capacity, as shown in Fig. 7. This observation, recorded in (Piątek, 2003) and (Bury et al., 2007a) has been confirmed during actual tests and, moreover, some dependence on the heat exchanger has been also noticed. So, it would be better to say that these parameters depend on the piping and ribbing structures in this certain case.

The investigations accomplished in this work deal with the ribbed cross-flow heat exchangers of the gas-liquid type. There were three water coolers investigated during

HE-1 – typical car cooler (Skoda Favorit 135L) with the core having the form of 2 rows pipe bundle (15 cylindrical pipes ribbed with the plate fins in each row, 380 fins on each pipe);

HE-2 – the cross-flow heat exchanger made by GEA Heat Exchangers Company with the core made of 10 rows of elliptical pipes ribbed with the plate fins (175 on each pipe); steel, HE-3 - the cross-flow heat exchanger made by GEA Heat Exchangers Company with the core having the form of 2 rows pipe bundle (81 fins on each pipe in the first row and 140 fins

Fig. 4. General sketch of the heat exchangers under consideration and the recurrent elements

There were six measuring series carried out for each of the heat exchangers under consideration. The distributions of the air velocity and temperature are one of the most interesting results that may be achieved on the described testing station. These distributions are very important because they allow evaluating the air inflow maldistribution range and form. Sample distributions obtained for the HE-1 heat exchanger are shown in Figs. 5 and 6. These results have been obtained with the total air flow rate of 1.556 m3/s, the water flow

The form and scope of the air inlet non-uniformity depend on the fan capacity, as shown in Fig. 7. This observation, recorded in (Piątek, 2003) and (Bury et al., 2007a) has been confirmed during actual tests and, moreover, some dependence on the heat exchanger has been also noticed. So, it would be better to say that these parameters depend on the piping

rate of 4.5·10-4 m3/s and the water temperature set on the boiler in 50°C.

**2.3 Analysed heat exchanger types** 

realization of this work (see Fig. 4):

on each pipe in the second row); steel.

of three versions of the heat exchangers

and ribbing structures in this certain case.

**2.4 Selected experimental results** 

aluminium,

An attempt for systemizing this non-uniformity has been undertaken in (Malinowski, 2008). The numerical analysis has proved that the reason of the observed air inflow maldistribution is the radial fan. Unfortunately, attempts to describe the measured inequality by using mathematical functions have failed. For this reason, data on the nonuniformity are included in the calculations in tabulated form using rows. This extends the calculation time slightly, but on the other hand allows for accurate recognition of this phenomenon.

Fig. 5. Distribution of the air velocity at the inlet (left) and outlet (right) cross-sectional flow area (210mm x 400mm) of HE-1/1 measurement, m/s.

Fig. 6. Distribution of the air temperature at the inlet (left) and outlet (right) cross-sectional flow area (210mm x 400mm) of HE-1/1 measurement, °C.

Fig. 7. Distribution of the air velocity at the inlet cross-sectional flow area (210mm x 400mm) of HE-2/1 measurement (left – without throttling) and of HE-3/4 measurement (right – maximum throttling), m/s.

Presented in Figs. 5-7 distributions of velocity and temperature of the air were drawn as viewed from the outlet of the heat exchanger.

Impact of a Medium Flow Maldistribution on a Cross-Flow Heat Exchanger Performance 127

The analysed real cross-flow heat exchanger has been replaced with a model rectangular heat exchanger. The model was then divided into elementary fragments (Fig. 8). Each fragment represents a recurrent element of the real heat exchanger - a single tube with the

The energy balance equations for each fragment constitute the mathematical basis of the model. Assuming that the water flows along the X axis and the air flows along the Y axis the

> *w a w pw a pa am a T T dQ m c dydz m c dxdz h T T dA x y*

where ha is an average heat transfer coefficient on the gas side for all the ribbed surface and

The inlet temperatures of the mediums are known so the following boundary conditions

max max *w w <sup>w</sup> g m dm dydz Y Z*

max max *a a <sup>a</sup> g m dm dxdz X Z*

*g*

, *<sup>w</sup> <sup>w</sup> w m w*

(3)

, , (0, , ) ( ,0, ) *Tw w in a a in y z T Tx z T* (4)

(5)

(6)

*<sup>w</sup>* (7)

z

Fig. 8. Model heat exchanger and the recurrent fragment.

energy balance for a recurrent fragment may be written as follow:

The mass flow rates of the fluids are described by the following formulas:

Tm is the average temperature of rib and pipe surface.

The inequality factors gw and ga are defined as follows:

y

rib (Piątek, 2003).

may be used:

dy dx

x

dz


(1): the temperature set at the electric boiler outlet

Table 1. Results of measurements.

The results of the measurements for the three considered heat exchangers are summarized in Table 1. All the measurements have been repeated for three times in order to verify repeatability of results. Presented in the last column heat flow rates, of course, refer to the conditions of non-uniform air flow. In order to determine the impact of this inequality on the efficiency of considered heat exchangers in the next stage the computational analysis was carried out. The measured inlet media parameters were used as input for calculations.
