**1.1 Characteristics of the problem**

The plate exchangers (with the mixed current) and the finned cross-flow heat exchangers, which core has the form of a bunch of pipes with flat plate ribs, have the most important meaning among the currently applied heat exchangers with extended surface. These heat exchangers are usually used for heat transfer between a liquid flowing inside the tubes and a gaseous medium flowing outside the tubes, on the ribs side. Small size, low weight and a high efficiency determine the strong position of such devices. Compact ribbed heat exchangers are commonly used in thermal technique, refrigeration, air-conditioning and automotive industry.

A typical thermodynamic analysis of a cross-flow heat exchanger is usually aimed in determination of the heat transfer surface for the desired design and its capacity. There are several simplifying assumptions made during such calculations, for example neglecting of the heat losses to the environment, uniform flow of media through the exchanger, heat transfer coefficients determined for the average temperatures. These assumptions are fulfilled very rarely in reality and of course it affects the analytical results to some degree.

The subject of this work is evaluation of the impact of a non-uniform flow of media (or flow maldistribution) on very popular finned cross-flow heat exchangers performance. The reasons for such maldistribution occurring in an exchanger include the layout of the exchanger with respect to other components in the system, effects of manufacturing tolerances, the design of the flow circuits in the exchanger and the design of the inlet and outlet headers. In some instances, the maldistribution could also be induced due to temperature effects. These factors become even more critical when the exchangers are applied in compact designs which involve a tortuous flow path for both the fluid streams. This situation may lead to some losses in the total heat flow rates transferred in the heat exchanger and affects its thermal efficiency. There is therefore the obvious question: to what extent inequality of media flows worsens effects of the heat exchanger?

One of the most important parameters describing such heat exchangers is the heat transfer coefficient on the gas side. Usually, this coefficient is determined as an average value for the whole heat transfer surface. This is of course another simplification. Beside of these simplifying assumptions, a variety of constructions being applied causes significant

Impact of a Medium Flow Maldistribution on a Cross-Flow Heat Exchanger Performance 119

improved header configuration can effectively improve the performance of a fin-and-tube type heat exchanger. An experimentally determined flow maldistribution for a plate finand-tube heat exchanger has been also described in (Hoffmann‐Vocke et al., 2009), but the authors have not considered its impact on the heat exchanger thermal efficiency. This group of authors has presented in (Hoffmann‐Vocke et al., 2011) even more detailed, but still only

Experimental analyses considering maldistributions of the agents flow through the heat exchangers and dealing with thermodynamic effects are rare. A. Mueller in (Mueller, 1987) concludes about major significance of flow maldistributions for heat exchangers performance. Based on the study of gross flow maldistribution in an experimental electrical heater the paper (Lalot et al., 1999) presents the effect of flow non-uniformity on the performance of heat exchangers. The original fluid distribution is applied to heat exchangers (condensers, counterflow and cross-flow heat exchangers), and it is shown that gross flow maldistribution leads to a loss of effectiveness of about 7% for condensers and counterflow heat exchangers, and up to 25% for cross-flow exchangers. Similar effects have been observed by the authors of (Luo et al., 2001) indicate that the non-uniformity influences the efficiency of the heat exchangers to a large extent. Berryman and Russell have studied flow maldistribution across tube bundles in air-cooled heat exchangers (Berryman and Russel, 1987). Their experimental results have detected thermal degradation up to 4%, which is much less than in previously cited works. The authors of (Meyer and Kröger, 1998)

Another group of investigations deals with evaporators and condensers, applied in airconditioning and refrigeration. The effects of maldistribution in fin-tube heat exchangers, which takes place on the air-side through the fin passages as well as on the liquid side in the tube circuits, have been investigated by several researchers, for example (Fagan, 1980; Chwalowski et al. 1989; Lee and Domanski, 1997; Aganda et al. 2000). The findings of these works have indicated dependence of the degradation on the mean and standard deviation of

A very complex research has been realized by teams from Indian Institute of Technology – Madras and Lund University of Technology. These works concern plate-type heat exchangers. The numerical model of a one-pass plate heat exchanger has been elaborated first for hydraulic analyses of a flow maldistribution impact (Shrihari et al., 2005) and next it was arranged for multi-pass units (Shrihari and Das, 2008). An experimental investigation has been also carried out to find the flow and the pressure difference across the port to channel in plate heat exchangers (Rao et al., 2006). More recently this research team realized thermal analysis also. The single-blow transient test technique based on axial dispersion model was proposed for the determination of both heat transfer coefficient and axial dispersion coefficient in plate heat exchangers. The experimental analysis presented in (Shaji and Das, 2010) deals with the effect of flow maldistribution on the transient temperature response for U-type plate heat exchangers. The experiments are carried out with uniform and non-uniform flow distributions for various flow rates and two different numbers of

According to (Li-Zhi, 2009) the inlet and outlet duct geometry in an air to air compact heat exchanger is always irregular. Such duct placements usually lead to a non-uniform flow distribution on core surface. The author used a CFD model to predict the flow distribution

hydraulic analysis of the considered heat exchanger.

concluded about minor – up to 5% - effects of this phenomenon also.

the flow maldistribution profile.

plates.

problems with determination of this coefficient. The problem is additionally complicated by a non-uniform flow of a gas. This flow maldistribution induces also some non-uniform distribution of the heat transfer coefficient. So, another important question is how this situation influences the thermodynamic analysis where the average value of this parameter is applied usually?
