**1.2 Review of previous studies**

The question of a non-uniform flow of media through heat exchangers is not a new problem. It is the subject of investigations for many years. Results, especially taken from older works, are sometimes very unambiguous.

The first one investigation referred to the heat exchangers with unequal flow of agents was performed at the Institute of Thermal Technology of the Silesian University of Technology (ITT SUT) for gaseous mediums and they had only computational form (Hanuszkiewicz-Drapała, 1996). Investigations of the gas-liquid type cross-flow heat exchanger have been conducted at the ITT SUT since a few years to evaluate an influence of a non-uniform gas inlet on the exchanger functioning (Piątek, 2003). A range and form of the air inflow nonuniformity have been determined on the special testing station - see Fig.1 in the next section. Configuration of the measuring system of the test station allows determining the air velocity and temperature distribution at the heat exchanger inlet and outlet. This test station, in its original arrangement, allowed only for "cold" experiments, it means without presence of the hot medium. Thus, the influence of the measured non-uniformity has been assessed by means of numerical simulations performed by the computer code called HEWES – worked out for thermal analyses of the considered heat exchanger. R. Piątek in his work (Piątek, 2003) concludes that the maldistribution of the air inlet to the investigated car cooler may significantly influence the effectiveness of the heat exchanger.

An unique feature of the investigations realized at the ITT SUT is experimental consideration of the air flow non-uniformity. Similar heat exchangers have been investigated by D. Taler with co-workers (Taler, 2002; Taler and Cebula, 2004) by means of physical experiments and numerical simulations too. Very good compliance of experimental and numerical results has been achieved, but the problem of the non-uniform agents flow is neglected and this fact simplified experimental measurements.

Many researches considering the problem of the non-uniform flow of media have been realized only numerically. Authors of (Ranganayakulu et al., 1997) have simulated the plate fin heat exchanger using the finite elements method and found out that the influence of the non-uniformity of the liquid flow may have significant meaning in some work regimes. A very significant drop of the heat exchanger efficiency has been also observed by authors of (Andrecovich and Clarke, 2003). The opposite results have obtained authors of (Nair et al., 1998) and (Lee and Oh, 2004). Numerical simulations realized for a rotary heat exchanger in the first work and optimization procedure presented in the second one have not shown significant dependence on the agents flow non-uniformity.

There are many works, both experimental and numerical, considering only the flow maldistribution impact on hydraulic efficiency of heat exchangers. Anjun with his coworkers investigated the influence of headers configuration on the non-uniformity range (Anjun et al., 2003). The numerical results presented in (Wen and Li, 2004) indicate that the

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

The question of a non-uniform flow of media through heat exchangers is not a new problem. It is the subject of investigations for many years. Results, especially taken from older works,

The first one investigation referred to the heat exchangers with unequal flow of agents was performed at the Institute of Thermal Technology of the Silesian University of Technology (ITT SUT) for gaseous mediums and they had only computational form (Hanuszkiewicz-Drapała, 1996). Investigations of the gas-liquid type cross-flow heat exchanger have been conducted at the ITT SUT since a few years to evaluate an influence of a non-uniform gas inlet on the exchanger functioning (Piątek, 2003). A range and form of the air inflow nonuniformity have been determined on the special testing station - see Fig.1 in the next section. Configuration of the measuring system of the test station allows determining the air velocity and temperature distribution at the heat exchanger inlet and outlet. This test station, in its original arrangement, allowed only for "cold" experiments, it means without presence of the hot medium. Thus, the influence of the measured non-uniformity has been assessed by means of numerical simulations performed by the computer code called HEWES – worked out for thermal analyses of the considered heat exchanger. R. Piątek in his work (Piątek, 2003) concludes that the maldistribution of the air inlet to the investigated car cooler may

An unique feature of the investigations realized at the ITT SUT is experimental consideration of the air flow non-uniformity. Similar heat exchangers have been investigated by D. Taler with co-workers (Taler, 2002; Taler and Cebula, 2004) by means of physical experiments and numerical simulations too. Very good compliance of experimental and numerical results has been achieved, but the problem of the non-uniform agents flow is

Many researches considering the problem of the non-uniform flow of media have been realized only numerically. Authors of (Ranganayakulu et al., 1997) have simulated the plate fin heat exchanger using the finite elements method and found out that the influence of the non-uniformity of the liquid flow may have significant meaning in some work regimes. A very significant drop of the heat exchanger efficiency has been also observed by authors of (Andrecovich and Clarke, 2003). The opposite results have obtained authors of (Nair et al., 1998) and (Lee and Oh, 2004). Numerical simulations realized for a rotary heat exchanger in the first work and optimization procedure presented in the second one have not shown

There are many works, both experimental and numerical, considering only the flow maldistribution impact on hydraulic efficiency of heat exchangers. Anjun with his coworkers investigated the influence of headers configuration on the non-uniformity range (Anjun et al., 2003). The numerical results presented in (Wen and Li, 2004) indicate that the

is applied usually?

**1.2 Review of previous studies** 

are sometimes very unambiguous.

significantly influence the effectiveness of the heat exchanger.

neglected and this fact simplified experimental measurements.

significant dependence on the agents flow non-uniformity.

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 hydraulic analysis of the considered heat exchanger.

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

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 the flow maldistribution profile.

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 plates.

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

Impact of a Medium Flow Maldistribution on a Cross-Flow Heat Exchanger Performance 121

 are own results consistent with data published by other authors stating an important meaning of the flow maldistribution (considering the range of observed heat exchanger

The whole analytical procedure (experiments and numerical simulations) has been performed for three cross flow heat exchangers with different ribbing structure in order to answer these questions. The experimental and numerical procedures are presented in this

The test station consists of two main modules: the air supply module (see Fig. 1) and the hot water supply module (Fig. 2). The air supply module originally was a special testing station constructed during realization of the project (Piątek, 2003) for determination of a form and

Fig. 1. Test station - the air supply module (1 – support plate, 2 – heat exchanger, 3 – thermoanemometric sensor, 4 – measuring probe, 5 – diffuser, 6 – channel, 7 – control

The air is supplied by the radial fan of the maximum capacity of 6900 m3/h. The fan capacity can be controlled by the throttling valve installed before the fan. Then the air flows through the 1.7 m long channel (rectangular cross-section 190x240 mm). The channel ends with the filter section. Usually this section is empty and only during special tests filters having the form of wire nets or perforated metal sheets are used. Actually, filter is not a good word describing the purpose of these elements – they are installed in order to make the air flow more uniform. The diffuser dimensions have been fit to the first examined heat

The main element of the measuring system is the V1T-type thermoanemometric sensor installed onto the measuring probe which shifting is controlled by a computer. It allows

**567 8**

chapter, as well as the most important results and conclusions.

efficiency drop)? are these results repeatable?

**2.1 Test station** 

computer, 8 – fan).

exchanger: they are 280x490 mm.

**2. Experimental investigations** 

scope of the air inflow non-uniformity.

and next calculated the heat exchange effectiveness and the thermal performance deterioration factor with finite difference scheme. Experiments were performed to validate the flow distribution and heat transfer model. The results indicate that when the channel pitch is below 2.0 mm, the flow distribution is quite homogeneous and the thermal deterioration due to flow maldistribution can be neglected. However, when the channel pitch is larger than 2 mm, the maldistribution is quite large and a 10–20% thermal deterioration factor could be found.

This literature review of the selected positions shows, as already mentioned, that the problem of the non-uniform fluid inflow to the heat exchangers has been the subject of many computational and experimental investigations, but the results obtained are unambiguous in terms of thermal performance. Many investigations are limited to the hydraulic analysis only and they deal with liquid-liquid type heat exchangers. Most researchers are consistent in finding that the non-uniformity of the flow significantly strikes the hydraulic efficiency of heat exchangers. Thermal analyses refer first of all to the heat exchanger effectiveness, but they are not very numerous. It is lack of complete investigations of the finned cross-flow heat exchangers of the gas-liquid type with unequal inflow of the agents, especially of unequal inflow of the gas.

#### **1.3 Aim and scope of presented studies**

The degradation effects of flow maldistribution on the performance of a heat exchanger are well-known. Not only does the thermal performance decrease but the fluid pressure drop across the exchanger core also increases simultaneously. Analyzing the results of (Piątek, 2003) the obvious question has appeared: how reliable are these results? The HEWES code validation procedure has to be carried out in order to answer this question. It became possible after modernization of the experimental rig and installation of the hot water supply module. In (Bury et al., 2007b) there have been presented the only initial results obtained by use of the modified testing station, and the results of initial and detailed validation and sensitivity analysis have been presented in (Bury et al., 2008a)) and (Bury et al., 2008b). Significant differences have been recorded between experimental and numerical data after the initial validation of the model. Minor changes have been put into the code and the validation procedure was then repeated with usage of the infra-red thermography measurements results also. The last stage of the research was the sensitivity analysis. This analysis has shown that the heat transfer coefficient from ribbed surfaces to a gas may be a reason for recorded discrepancies between numerical and experimental results. An additional testing station, in the lab-scale, has been designed and constructed in order to check the numerical procedure responsible for determination of the heat transfer coefficient from the ribs to the gas. The papers (Bury et al., 2009a; Bury and Składzień,2010) and recently also (Składzień and Bury, 2011) present results of this analysis.

Applying the validated version of the HEWES code and modified testing station the analysis of the above mentioned car cooler has been repeated and the results allowed to sustain the conclusions withdrawn by Piątek – the air inflow maldistribution may significantly affect the heat exchanger performance (Bury et al., 2009b).

The following questions have emerged after analysis the experience gained so far:


The whole analytical procedure (experiments and numerical simulations) has been performed for three cross flow heat exchangers with different ribbing structure in order to answer these questions. The experimental and numerical procedures are presented in this chapter, as well as the most important results and conclusions.
